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Add Google GenAI Sampling Handler#2977

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strawgate wants to merge 3 commits intomainfrom
claude/issue-2976-20260121-2112
Open

Add Google GenAI Sampling Handler#2977
strawgate wants to merge 3 commits intomainfrom
claude/issue-2976-20260121-2112

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Summary

Adds a Google GenAI sampling handler to FastMCP, enabling users to use Google's Gemini models with the Model Context Protocol. This handler follows the same pattern as the existing Anthropic and OpenAI handlers, providing full tool support and comprehensive message conversion.

Key Features

  • Full tool calling support with Google's function calling API
  • Proper JSON schema to Google schema conversion
  • Handles nullable types, nested objects, and arrays
  • Test coverage that gracefully skips when google-genai is not installed

Closes #2976

Generated with Claude Code

@strawgate strawgate changed the title Add Google GenAI Sampling Handler [Draft] Add Google GenAI Sampling Handler Jan 21, 2026
@marvin-context-protocol marvin-context-protocol bot added enhancement Improvement to existing functionality. For issues and smaller PR improvements. client Related to the FastMCP client SDK or client-side functionality. DON'T MERGE PR is not ready for merging. Used by authors to prevent premature merging. labels Jan 21, 2026
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/marvin can you compare this implementation to the openai and anthropic ones and then to the google docs and make sure there's no funny business going on

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marvin-context-protocol bot commented Jan 21, 2026

Claude finished @strawgate's task in 3m 2s —— View job


Google GenAI Handler Review Complete

I've thoroughly compared the Google GenAI handler implementation against the Anthropic and OpenAI handlers, and verified it against Google's official documentation. Here's what I found:


🎯 Overall Assessment

The implementation is solid and follows the existing patterns well. However, there are 3 critical issues and several improvements to consider:


🚨 Critical Issues

1. ThinkingConfig Used Inappropriately (Line 105)

Severity: High | src/fastmcp/client/sampling/handlers/google_genai.py:105

thinking_config=ThinkingConfig(thinking_budget=200),

Problem: ThinkingConfig with thinking_budget is ONLY supported by Gemini 2.5+ models (specifically gemini-2.5-flash and gemini-2.5-pro). According to Google's documentation, models like gemini-2.0-flash-exp (the example default) do not support thinking budgets and will cause syntax errors.

Impact: This will break for most users since the example uses gemini-2.0-flash-exp which doesn't support thinking.

Fix: Either:

  • Remove ThinkingConfig entirely (safest approach)
  • Only add it conditionally if the model name contains 2.5 or 3
  • Add it as an optional parameter users can opt into

Fix this →


2. Tool Result ID Parsing Logic is Fragile (Lines 276-282)

Severity: Medium-High | src/fastmcp/client/sampling/handlers/google_genai.py:276-282

# Extract function name from toolUseId
# Our IDs are formatted as "{function_name}_{uuid8}", so extract the name
tool_use_id = content.toolUseId
if "_" in tool_use_id:
    # Split and rejoin all but the last part (the UUID suffix)
    parts = tool_use_id.rsplit("_", 1)
    function_name = parts[0]
else:
    # Fallback: use the full ID as the name
    function_name = tool_use_id

Problem: This assumes the tool use ID follows the format {function_name}_{uuid8} that this handler itself creates on line 395. However:

  1. OpenAI and Anthropic handlers do NOT generate IDs - they use IDs provided by their APIs
  2. This creates a circular dependency - you're extracting the function name from an ID you generated, but you already had the function name when you generated the ID
  3. If a tool name contains underscores (e.g., get_user_info), rsplit("_", 1) will return get_user_info correctly, but this is fragile
  4. The comment "Our IDs are formatted as..." is misleading - these are IDs YOU created in _response_to_result_with_tools

Comparison:

  • Anthropic: Uses tool.id from ToolUseBlock (API-provided)
  • OpenAI: Uses tool_call.id (API-provided)
  • Google: Generates IDs locally and then tries to extract the name back out

The Real Issue: Google's API doesn't provide a tool use ID in the FunctionCall, but FastMCP's ToolResultContent.toolUseId requires one. The current approach works but is convoluted.

Better Approach: Store a mapping of generated IDs to function names, or include the original FunctionCall object to preserve the name directly.

Fix this →


3. Schema Type Mapping Missing "null" Type (Lines 185-193)

Severity: Medium | src/fastmcp/client/sampling/handlers/google_genai.py:185-193

type_map: dict[str, str] = {
    "string": "STRING",
    "integer": "INTEGER",
    "number": "NUMBER",
    "boolean": "BOOLEAN",
    "array": "ARRAY",
    "object": "OBJECT",
}

Problem: The type map doesn't handle "null" types, even though the code handles nullable types via anyOf. According to Google's schema documentation, you can include "null" in type arrays.

Impact: If a schema contains {"type": "null"} directly (not in anyOf), it will map to the default "STRING" which is incorrect.

Fix: Add "null": "STRING" as a fallback OR raise a ValueError for unsupported null types (since Google handles nullability differently).

Fix this →


⚠️ Pattern Inconsistencies (vs Anthropic/OpenAI)

4. Different Context Type Annotation

File: google_genai.py:84 vs anthropic.py:75 and openai.py:60

Google:

async def __call__(
    self,
    messages: list[SamplingMessage],
    params: SamplingParams,
    context: Any,  # ❌ Too generic
) -> CreateMessageResult | CreateMessageResultWithTools:

Anthropic:

async def __call__(
    self,
    messages: list[SamplingMessage],
    params: SamplingParams,
    context: Any,  # Same
) -> CreateMessageResult | CreateMessageResultWithTools:

OpenAI:

async def __call__(
    self,
    messages: list[SamplingMessage],
    params: SamplingParams,
    context: RequestContext[ServerSession, LifespanContextT]
        | RequestContext[ClientSession, LifespanContextT],  # ✅ Properly typed
) -> CreateMessageResult | CreateMessageResultWithTools:

Finding: Google handler matches Anthropic's loose typing with Any, but OpenAI uses proper typing. For consistency, all three should either:

  • Use proper RequestContext typing (preferred)
  • Or all use Any (acceptable since context isn't used)

Note: This is actually an existing inconsistency between OpenAI and Anthropic - the Google handler just followed Anthropic's pattern.


5. Missing TYPE_CHECKING Imports for Context Types

The Google handler imports TYPE_CHECKING but doesn't use it for the proper context types (which OpenAI does). This is fine since context: Any is used, but it's inconsistent with OpenAI's approach.


✅ What's Correct

  1. Schema Conversion is Solid

    • Properly handles basic types (string, integer, number, boolean, array, object)
    • Correctly handles nullable types via anyOf
    • Recursive handling of nested objects and arrays works well
    • Preserves descriptions and required fields
  2. Tool Choice Mapping is Correct

    • "auto"FunctionCallingConfigMode.AUTO
    • "required"FunctionCallingConfigMode.ANY
    • "none"FunctionCallingConfigMode.NONE
    • Matches Google's ToolConfig documentation
  3. Message Conversion Follows Patterns

    • Properly converts user/assistant roles to UserContent/ModelContent
    • Handles list content (tool calls + results)
    • Follows the same structure as Anthropic/OpenAI handlers
  4. Stop Reason Mapping is Appropriate

    • STOP"endTurn"
    • MAX_TOKENS"maxTokens"
    • Function calls → "toolUse"
  5. Test Coverage is Good

    • Gracefully skips when google-genai not installed
    • Tests schema conversion with nullable types, arrays, objects
    • Tests tool choice conversion
    • Tests basic message conversion

📝 Minor Improvements

6. Model Selection Logic is Simpler than Anthropic/OpenAI

File: google_genai.py:117-122

def _get_model(self, model_preferences: ModelPreferences | None) -> str:
    if model_preferences and model_preferences.hints:
        for hint in model_preferences.hints:
            if hint.name:
                return hint.name
    return self.default_model

Comparison:

  • Anthropic (lines 289-297): Uses _iter_models_from_preferences and filters for models starting with "claude"
  • OpenAI (lines 290-298): Uses _iter_models_from_preferences and validates against ChatModel type args
  • Google: Takes the first hint name without validation

Impact: Google's approach accepts ANY model name from hints, even invalid ones. This could lead to API errors.

Suggestion: Consider adding model validation or at least following the same iterator pattern for consistency.


7. Image/Audio Content Not Supported but Not Documented

File: google_genai.py:246-252

The handler imports ImageContent and AudioContent types but then raises ValueError for them:

if isinstance(content, ToolUseContent):
    ...
elif isinstance(content, ToolResultContent):
    ...

msg = f"Unsupported content type: {type(content)}"
raise ValueError(msg)

Note: Google's API DOES support image content (multimodal), but this handler doesn't implement it yet. Consider:

  • Adding a comment explaining this is a future enhancement
  • Or implementing basic image support (the API supports it)

8. No __all__ Export Consistency

All three handlers export their main class in __all__, which is good and consistent.


🔍 Security & Edge Cases

9. No Input Validation on Tool Arguments

When converting tool results (line 287), the handler creates a FunctionResponse with:

response={"result": result_text}

This is fine, but there's no validation that result_text is properly extracted. If content.content is empty or malformed, result_text will be an empty string, which might not be what the API expects.

Comparison: Anthropic and OpenAI handlers have similar edge case handling.


10. Error Messages Could Be More Descriptive

Line 335: msg = "No candidate in response from completion."
Line 402: raise ValueError("No content in response from completion")

These could include more context (like the actual response object or finish reason) for debugging.


📚 Documentation Quality

The docstring example (lines 59-71) is clear and follows the pattern of Anthropic/OpenAI handlers. Good job!


🎓 Summary

Found "Funny Business"?

No malicious code or security issues
No major logic flaws
One critical bug: ThinkingConfig will break most models
⚠️ Two design issues: ID parsing fragility and missing null type handling
📋 Several minor inconsistencies: Context typing, model validation

Recommendation: Fix the ThinkingConfig issue before merging (it's a blocker). The other issues can be addressed in follow-up PRs if needed.


Sources

--- | Branch: claude/issue-2976-20260121-2112

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Test Failure Analysis

Summary: The static analysis job failed due to unused imports in the test file for the Google GenAI sampling handler.

Root Cause: The test file imports several types at the top level within the block (lines 8, 12, 19 in ) that are not actually used in the test code. These imports were included but then re-imported within individual test functions:

  1. Candidate (line 8) - unused, imported but never referenced
  2. Part (line 12) - unused, imported but never referenced
  3. SamplingMessage (line 19) - unused at module level, re-imported in test functions (lines 42, 63)

Suggested Solution: Remove the three unused imports from the module-level import block.

In tests/client/sampling/handlers/test_google_genai_handler.py, change lines 7-14 from:

from google.genai.types import (  # type: ignore[import-untyped]
    Candidate,
    FunctionCallingConfigMode,
    GenerateContentResponse,
    ModelContent,
    Part,
    UserContent,
)

to:

from google.genai.types import (  # type: ignore[import-untyped]
    FunctionCallingConfigMode,
    GenerateContentResponse,
    ModelContent,
    UserContent,
)

And change lines 15-22 from:

from mcp.types import (
    CreateMessageResult,
    ModelHint,
    ModelPreferences,
    SamplingMessage,
    TextContent,
    ToolChoice,
)

to:

from mcp.types import (
    CreateMessageResult,
    ModelHint,
    ModelPreferences,
    TextContent,
    ToolChoice,
)

The imports are re-imported locally where needed (e.g., SamplingMessage and TextContent in individual test functions), so removing them from the module level won't break anything.

Detailed Analysis

Ruff Check Output

F401 \`google.genai.types.Candidate\` imported but unused
F401 \`google.genai.types.Part\` imported but unused  
F401 \`mcp.types.SamplingMessage\` imported but unused

Why These Are Unused

  • Candidate: Never referenced in any test
  • Part: Never referenced in any test
  • SamplingMessage: Imported locally in test functions (lines 42, 63) rather than using the module-level import

Additional Formatting Changes

Ruff also applied some formatting fixes to src/fastmcp/client/sampling/handlers/google_genai.py:

  • Reformatted multi-line list comprehensions and function calls for better readability
  • Changed TYPE_CHECKING block to use pass instead of unused imports

These are cosmetic changes and don't affect functionality.

Related Files
  • tests/client/sampling/handlers/test_google_genai_handler.py - Test file with unused imports (needs fixing)
  • src/fastmcp/client/sampling/handlers/google_genai.py - Implementation file (formatting applied automatically)

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Test Failure Analysis

Summary: The static analysis job failed due to unused imports in the test file for the Google GenAI sampling handler.

Root Cause: The test file imports several types at the top level within the try block (lines 8, 12, 19 in tests/client/sampling/handlers/test_google_genai_handler.py) that are not actually used in the test code. These imports were included but then re-imported within individual test functions:

  1. Candidate (line 8) - unused, imported but never referenced
  2. Part (line 12) - unused, imported but never referenced
  3. SamplingMessage (line 19) - unused at module level, re-imported in test functions (lines 42, 63)

Suggested Solution: Remove the three unused imports from the module-level import block.

In tests/client/sampling/handlers/test_google_genai_handler.py, change lines 7-14 from:

from google.genai.types import (  # type: ignore[import-untyped]
    Candidate,
    FunctionCallingConfigMode,
    GenerateContentResponse,
    ModelContent,
    Part,
    UserContent,
)

to:

from google.genai.types import (  # type: ignore[import-untyped]
    FunctionCallingConfigMode,
    GenerateContentResponse,
    ModelContent,
    UserContent,
)

And change lines 15-22 from:

from mcp.types import (
    CreateMessageResult,
    ModelHint,
    ModelPreferences,
    SamplingMessage,
    TextContent,
    ToolChoice,
)

to:

from mcp.types import (
    CreateMessageResult,
    ModelHint,
    ModelPreferences,
    TextContent,
    ToolChoice,
)

The imports are re-imported locally where needed (e.g., SamplingMessage and TextContent in individual test functions), so removing them from the module level will not break anything.

Detailed Analysis

Ruff Check Output

F401 `google.genai.types.Candidate` imported but unused
F401 `google.genai.types.Part` imported but unused  
F401 `mcp.types.SamplingMessage` imported but unused

Why These Are Unused

  • Candidate: Never referenced in any test
  • Part: Never referenced in any test
  • SamplingMessage: Imported locally in test functions (lines 42, 63) rather than using the module-level import

Additional Formatting Changes

Ruff also applied some formatting fixes to src/fastmcp/client/sampling/handlers/google_genai.py:

  • Reformatted multi-line list comprehensions and function calls for better readability
  • Changed TYPE_CHECKING block to use pass instead of unused imports

These are cosmetic changes and do not affect functionality.

Related Files
  • tests/client/sampling/handlers/test_google_genai_handler.py - Test file with unused imports (needs fixing)
  • src/fastmcp/client/sampling/handlers/google_genai.py - Implementation file (formatting applied automatically)

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💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 544ef46f34

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Comment on lines 112 to 115
# Return appropriate result type based on whether tools were provided
if params.tools:
return _response_to_result_with_tools(response, self.default_model)
return _response_to_create_message_result(response, self.default_model)

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P2 Badge Report the model actually requested

The handler chooses the request model with _get_model(params.modelPreferences) but always returns self.default_model in the CreateMessageResult/CreateMessageResultWithTools. This mislabels responses whenever a caller provides ModelPreferences hints (or any other override), which can break downstream caching, routing, or telemetry keyed by model name because the response metadata no longer reflects the real model that was used.

Useful? React with 👍 / 👎.

Comment on lines +275 to +279
tool_use_id = content.toolUseId
if "_" in tool_use_id:
# Split and rejoin all but the last part (the UUID suffix)
parts = tool_use_id.rsplit("_", 1)
function_name = parts[0]

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P2 Badge Don’t infer tool name from toolUseId

Google function responses require the declared function name, but ToolResultContent only carries toolUseId (which is opaque in MCP). This code assumes the ID embeds the function name (splitting on _), so any tool results whose IDs don’t follow that convention—e.g., persisted conversations or tool calls from a different handler that uses random IDs—will send an invalid function name and cause generate_content to reject or ignore the tool response.

Useful? React with 👍 / 👎.

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Walkthrough

Adds a new GoogleGenaiSamplingHandler at src/fastmcp/client/sampling/handlers/google_genai.py. The handler converts MCP SamplingMessage sequences and MCP Tool definitions into Google GenAI Content, FunctionDeclarations, ToolConfig, and ThinkingConfig when provided; selects the model from ModelPreferences or a default; calls google.genai.Client.generate_content; and translates responses into MCP CreateMessageResult or CreateMessageResultWithTools, including function_call/tool-use mapping, stopReason derivation, schema conversion, and validation/error handling.

🚥 Pre-merge checks | ✅ 1 | ❌ 4
❌ Failed checks (1 warning, 3 inconclusive)
Check name Status Explanation Resolution
Docstring Coverage ⚠️ Warning Docstring coverage is 72.73% which is insufficient. The required threshold is 80.00%. Write docstrings for the functions missing them to satisfy the coverage threshold.
Description check ❓ Inconclusive The description covers key features and references the closed issue (#2976), but the Contributors Checklist boxes are not checked and the description deviates from the template structure. Check all boxes in the Contributors Checklist to confirm testing, documentation updates, and workflow compliance before merging.
Linked Issues check ❓ Inconclusive The implementation appears to address core requirements: Google GenAI handler class, MCP message/tool conversion, model selection, and GenerateContentResponse parsing; however, review comments highlight potential issues with schema conversion, thought_signature bypass, tool ID handling, and import conventions that suggest incomplete alignment with project standards. Address reviewer concerns: verify schema translation necessity, remove thought_signature bypass, standardize tool ID handling, move imports to module level, and ensure all error handling and validations meet project conventions before final approval.
Out of Scope Changes check ❓ Inconclusive Changes are within scope: Google GenAI handler implementation and v3-features documentation update are directly tied to issue #2976; however, websockets version change in lock file warrants verification for unintended side effects. Verify that the websockets lock file change is intentional and does not introduce unintended dependencies or side effects on the project.
✅ Passed checks (1 passed)
Check name Status Explanation
Title check ✅ Passed The title clearly and concisely summarizes the main change: adding a Google GenAI sampling handler to FastMCP.

✏️ Tip: You can configure your own custom pre-merge checks in the settings.

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  • Commit unit tests in branch claude/issue-2976-20260121-2112

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Actionable comments posted: 4

🧹 Nitpick comments (1)
src/fastmcp/client/sampling/handlers/google_genai.py (1)

74-78: Make thinking_config opt‑in instead of hard‑coding a budget.

Line 105 hard-codes ThinkingConfig(thinking_budget=200) for every request, which overrides model defaults and may cause failures. Gemini 2.5 models default to ~8,192 tokens for thinking when unspecified, and Gemini 3+ models use thinking_level (enum) instead of thinking_budget—hard-coded values outside the model's supported range trigger errors. Add a thinking_budget parameter to __init__ (defaulting to None) and use it conditionally, allowing the model's default behavior when not specified.

♻️ Suggested change
     def __init__(
         self, default_model: str, client: GoogleGenaiClient | None = None
+        thinking_budget: int | None = None,
     ) -> None:
         self.client: GoogleGenaiClient = client or GoogleGenaiClient()
         self.default_model: str = default_model
+        self.thinking_budget: int | None = thinking_budget
@@
         response: GenerateContentResponse = (
             await self.client.aio.models.generate_content(
                 model=self._get_model(model_preferences=params.modelPreferences),
                 contents=contents,
                 config=GenerateContentConfig(
                     system_instruction=params.systemPrompt,
                     temperature=params.temperature,
                     max_output_tokens=params.maxTokens,
                     stop_sequences=params.stopSequences,
-                    thinking_config=ThinkingConfig(thinking_budget=200),
+                    thinking_config=(
+                        ThinkingConfig(thinking_budget=self.thinking_budget)
+                        if self.thinking_budget is not None
+                        else None
+                    ),
                     tools=google_tools,
                     tool_config=tool_config,
                 ),
             )
         )

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/marvin do we really need to translate the schema for gemini? is this really comprehensive?

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marvin-context-protocol bot commented Jan 22, 2026

Claude finished @strawgate's task in 2m 32s —— View job


Analysis: Google GenAI Schema Translation

I've analyzed whether schema translation is necessary and assessed the comprehensiveness of the implementation. Here are my findings:


⚠️ Critical Issue: Schema Translation is INCORRECT

The schema translation is implementing the WRONG format. The code converts JSON Schema types to uppercase (e.g., "STRING", "INTEGER", "OBJECT"), but Google's Gemini API actually expects lowercase types ("string", "integer", "object").

Evidence

From the code (src/fastmcp/client/sampling/handlers/google_genai.py:185-193):

type_map: dict[str, str] = {
    "string": "STRING",      # ❌ WRONG
    "integer": "INTEGER",    # ❌ WRONG
    "number": "NUMBER",      # ❌ WRONG
    "boolean": "BOOLEAN",    # ❌ WRONG
    "array": "ARRAY",        # ❌ WRONG
    "object": "OBJECT",      # ❌ WRONG
}

From Google's official documentation (Function calling with the Gemini API):

  • Parameters use "type": "string" (lowercase)
  • Parameters use "type": "integer" (lowercase)
  • Parameters use "type": "object" (lowercase)
  • Parameters use "type": "array" (lowercase)
  • Parameters use "type": "boolean" (lowercase)
  • Parameters use "type": "number" (lowercase)

The documentation consistently shows lowercase type specifications across all examples in Python, JavaScript, and REST API formats.


📊 Why Translation IS Necessary

Yes, schema translation is necessary, but not in the way currently implemented. Here's why:

Comparison with Anthropic and OpenAI Handlers

Handler Schema Translation Reason
Anthropic (anthropic.py:300-316) No translation Anthropic accepts JSON Schema directly (input_schema=dict(tool.inputSchema))
OpenAI (openai.py:301-320) No translation OpenAI accepts JSON Schema directly (parameters=dict(tool.inputSchema))
Google GenAI Needs translation Google uses a different schema format with uppercase type names... OR DOES IT?

The Real Requirement

Looking at the Google GenAI SDK documentation and migration guide, the FunctionDeclaration expects a parameters field that follows JSON Schema format but with potential differences:

  1. Type names: The API docs show lowercase type names ("string", "integer", etc.)
  2. Structure: Same as JSON Schema (properties, required, description, etc.)
  3. Additional features: Supports nullable field for optional values

So translation is needed, but the current implementation has the type case backwards!


🔍 Comprehensiveness Assessment

Assuming the types were corrected to lowercase, here's what the translation covers:

✅ What's Handled Well

  1. Basic types: string, integer, number, boolean, array, object
  2. Nullable types: Converts anyOf with null type to nullable: true
  3. Nested objects: Recursively converts nested schemas
  4. Arrays with items: Handles array item schemas
  5. Descriptions and enums: Preserves these fields
  6. Required fields: Maintains required arrays

⚠️ What's Missing or Problematic

  1. Type arrays not handled (lines 183-193):

    schema_type: str | None = schema.get("type")

    JSON Schema allows "type": ["string", "null"], but this code assumes type is always a string, which will cause a TypeError when used with .get() on a list.

  2. Format field ignored:
    JSON Schema supports format (e.g., "format": "date-time"), but this is not preserved.

  3. Constraints ignored:

    • minimum, maximum for numbers
    • minLength, maxLength for strings
    • minItems, maxItems for arrays
    • pattern for string regex validation

    These are valid JSON Schema fields that might be useful for the model.

  4. $ref not handled:
    JSON Schema $ref references are not resolved or converted.

  5. oneOf/allOf not handled:
    Only anyOf is handled (for nullable types). Other combinators are ignored.

  6. Default values not preserved:
    The default field from JSON Schema is not carried over.


🎯 Is It Comprehensive Enough?

For basic tool usage: YES (with the type case fix)

  • Covers the most common cases: simple types, objects, arrays, nullability
  • Sufficient for typical function calling scenarios
  • Matches the patterns shown in Google's examples

For advanced schemas: NO

  • Missing type arrays (which can cause crashes)
  • Missing validation constraints (min/max, patterns, etc.)
  • Missing JSON Schema combinators (oneOf, allOf)
  • Missing format specifications

🧪 Test Coverage Analysis

The tests (tests/client/sampling/handlers/test_google_genai_handler.py:129-170) verify:

  • ✅ Basic type conversion (string → STRING, integer → INTEGER)
  • ✅ Nullable types via anyOf
  • ✅ Arrays with items
  • ✅ Objects with properties and required fields
  • ✅ Descriptions

But the tests are testing the WRONG behavior! They expect uppercase types:

assert _convert_json_schema_to_google_schema({"type": "string"}) == {"type": "STRING"}

This should be:

assert _convert_json_schema_to_google_schema({"type": "string"}) == {"type": "string"}

🚨 Action Required

  1. Fix the type mapping to use lowercase types
  2. Update all tests to expect lowercase types
  3. Consider adding support for type arrays to prevent crashes
  4. Document known limitations (missing constraints, combinators, etc.)

📚 Sources


Summary

Schema translation IS necessary for Google GenAI, but the current implementation has a critical bug: it uses uppercase type names (STRING, INTEGER) when Google's API expects lowercase (string, integer). The translation logic is otherwise reasonably comprehensive for basic use cases but misses some edge cases (type arrays, validation constraints, schema combinators).

Fix type case issue →

--- | Branch: claude/issue-2976-20260121-2112

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Test Failure Analysis

Summary: The static analysis workflow failed due to unused imports in the test file and a formatting issue in the handler file.

Root Cause: Three unused imports in tests/client/sampling/handlers/test_google_genai_handler.py that were imported in the conditional block but never used:

  • Candidate (line 8)
  • Part (line 12)
  • SamplingMessage (line 19)

Additionally, src/fastmcp/client/sampling/handlers/google_genai.py needs formatting.

Suggested Solution:

  1. Remove unused imports from tests/client/sampling/handlers/test_google_genai_handler.py:7-22:

    • Remove Candidate (line 8)
    • Remove Part (line 12)
    • Remove SamplingMessage from the mcp.types import (line 19)
  2. Run ruff format to fix formatting:

    uv run ruff format src/fastmcp/client/sampling/handlers/google_genai.py
  3. Run prek to verify all checks pass:

    uv run prek run --all-files
Detailed Analysis

Ruff Check Errors

F401 `google.genai.types.Candidate` imported but unused
F401 `google.genai.types.Part` imported but unused
F401 `mcp.types.SamplingMessage` imported but unused

The imports were likely added for type hints or mocking but are not actually used in the test functions. The test file imports SamplingMessage again at the function level (line 42) where it's actually used.

Ruff Format Issue

One file needs reformatting:

  • src/fastmcp/client/sampling/handlers/google_genai.py

Type Check

Type checking passes once the unused imports are removed.

Related Files

Modified files in this PR:

  • tests/client/sampling/handlers/test_google_genai_handler.py:7-22 - Contains unused imports
  • src/fastmcp/client/sampling/handlers/google_genai.py - Needs formatting

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Test Failure Analysis

Summary: The static analysis (ty check) failed with type errors in src/fastmcp/utilities/skills.py due to improper type narrowing when using hasattr() checks.

Root Cause: The type checker cannot infer that content.text is a str when using hasattr(content, "text") checks. After the hasattr() check, the type of content.text is inferred as object rather than str, causing type errors when passing it to functions that expect str (like json.loads() and Path.write_text()).

Suggested Solution:

Replace hasattr() checks with isinstance() checks using proper type narrowing:

In src/fastmcp/utilities/skills.py:104-110, change:

if hasattr(content, "text"):
    try:
        manifest_data = json.loads(content.text)
    except json.JSONDecodeError as e:
        raise ValueError(f"Invalid manifest JSON for skill: {skill_name}") from e

to:

if isinstance(content, mcp.types.TextResourceContents):
    try:
        manifest_data = json.loads(content.text)
    except json.JSONDecodeError as e:
        raise ValueError(f"Invalid manifest JSON for skill: {skill_name}") from e

In src/fastmcp/utilities/skills.py:200-209, change:

if hasattr(content, "text"):
    file_path.write_text(content.text)
elif hasattr(content, "blob"):
    # Handle base64-encoded binary content
    import base64

    file_path.write_bytes(base64.b64decode(content.blob))

to:

if isinstance(content, mcp.types.TextResourceContents):
    file_path.write_text(content.text)
elif isinstance(content, mcp.types.BlobResourceContents):
    # Handle base64-encoded binary content
    import base64

    file_path.write_bytes(base64.b64decode(content.blob))

Additionally, remove unused type: ignore comments:

  • tests/server/test_dependencies.py:70 - Remove # type: ignore[assignment]
  • tests/server/test_dependencies.py:201 - Remove # type: ignore[assignment]
Detailed Analysis

Type Checker Errors

error[invalid-argument-type]: Argument to function `loads` is incorrect
     --> src/fastmcp/utilities/skills.py:106:40
      |
  106 |             manifest_data = json.loads(content.text)
      |                                        ^^^^^^^^^^^^ Expected `str | bytes | bytearray`, found `object`

error[invalid-argument-type]: Argument to bound method `write_text` is incorrect
     --> src/fastmcp/utilities/skills.py:201:34
      |
  201 |             file_path.write_text(content.text)
      |                                  ^^^^^^^^^^^^ Expected `str`, found `object`

Why This Happens

The read_resource() method returns list[mcp.types.TextResourceContents | mcp.types.BlobResourceContents]. When the code uses hasattr(content, "text"), the type checker:

  1. Recognizes that objects with a text attribute exist
  2. But cannot infer that content.text is specifically a str - it only knows the attribute exists as object
  3. This breaks type safety since json.loads() and Path.write_text() require str

Using isinstance() with the proper types allows the type checker to narrow the type correctly and preserve that TextResourceContents.text is a str.

Note About PR Changes

Important: The files with type errors (src/fastmcp/utilities/skills.py and tests/server/test_dependencies.py) were NOT modified in this PR. These are pre-existing type issues that need to be fixed. The PR itself only adds Google GenAI sampling handler support.

Related Files

Files requiring fixes:

  • src/fastmcp/utilities/skills.py:104-110, 200-209 - Replace hasattr() with isinstance()
  • tests/server/test_dependencies.py:70, 201 - Remove unused type: ignore comments

Import needed:

import mcp.types  # Add to imports in src/fastmcp/utilities/skills.py if not present

Updated: 2026-01-28 04:51 UTC - Analysis reflects latest workflow run

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Test Failure Analysis

Summary: The static analysis (ty check) failed due to type checking errors in src/fastmcp/utilities/skills.py and unused type ignore comments in test files.

Root Cause: The code uses hasattr(content, "text") to check if a resource content object has a .text attribute, but this doesn't narrow the type for the type checker. The type checker sees content.text as having type object instead of str, causing errors when passing it to functions expecting str | bytes | bytearray (like json.loads) or str (like file_path.write_text).

Suggested Solution:

  1. Fix type narrowing in skills.py: Replace hasattr checks with isinstance checks to properly narrow types:

    • Line 104-106: Change hasattr(content, "text") to isinstance(content, mcp.types.TextResourceContents)
    • Line 200-201: Same change
    • Add import: import mcp.types at the top of the file
  2. Remove unused type ignore comments: These comments are no longer needed:

    • tests/server/test_dependencies.py:70 - Remove # type: ignore[assignment]
    • tests/server/test_dependencies.py:201 - Remove # type: ignore[assignment]
Detailed Analysis

Type Checking Errors in skills.py

The issue occurs at two locations where hasattr is used to check for attributes:

Line 106:

if hasattr(content, "text"):
    manifest_data = json.loads(content.text)  # ❌ Type checker sees content.text as 'object'

Line 201:

if hasattr(content, "text"):
    file_path.write_text(content.text)  # ❌ Type checker sees content.text as 'object'

The variable content has type mcp.types.TextResourceContents | mcp.types.BlobResourceContents. When using hasattr, the type checker doesn't narrow the union type, so content.text is typed as object rather than str.

The fix is to use isinstance checks which properly narrow types:

if isinstance(content, mcp.types.TextResourceContents):
    manifest_data = json.loads(content.text)  # ✅ Type checker knows content.text is str

Unused Type Ignore Comments

Two # type: ignore[assignment] comments in test files are no longer needed and should be removed. These were likely added for previous type checking issues that have since been resolved.

Log Excerpts

error[invalid-argument-type]: Argument to function `loads` is incorrect
     --> src/fastmcp/utilities/skills.py:106:40
  106 |             manifest_data = json.loads(content.text)
      |                                        ^^^^^^^^^^^^ Expected `str | bytes | bytearray`, found `object`

error[invalid-argument-type]: Argument to bound method `write_text` is incorrect
     --> src/fastmcp/utilities/skills.py:201:34
  201 |             file_path.write_text(content.text)
      |                                  ^^^^^^^^^^^^ Expected `str`, found `object`

warning[unused-ignore-comment]: Unused blanket `type: ignore` directive
    --> tests/server/test_dependencies.py:70:83
Related Files
  • src/fastmcp/utilities/skills.py (lines 104-106, 200-201): Contains the type checking errors that need to be fixed with isinstance checks
  • tests/server/test_dependencies.py (lines 70, 201): Contains unused type ignore comments that should be removed
  • src/fastmcp/client/mixins/resources.py: Shows the return type of read_resource is list[mcp.types.TextResourceContents | mcp.types.BlobResourceContents]

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marvin-context-protocol bot commented Jan 28, 2026

Test Failure Analysis

Summary: ✅ RESOLVED - The "lowest-direct dependencies" test failure has been fixed by bumping the minimum google-genai version to 1.18.0.

Root Cause: The commit b6ad6997 added thought_signature=b"skip_thought_signature_validator" to bypass thought signature validation when creating Part objects with function calls (src/fastmcp/client/sampling/handlers/google_genai.py:290). However, the thoughtSignature parameter was only added to the Part type in google-genai version 1.18.0, while the minimum version constraint was google-genai>=1.0.0. When CI ran with --resolution lowest-direct, it installed version 1.7.0, which doesn't support this parameter, causing a Pydantic validation error.

Solution Applied: Updated the minimum version constraint for google-genai in pyproject.toml from >=1.0.0 to >=1.18.0 in commit 6733bee.


Note: This analysis was updated on 2026-01-31 to reflect that the issue has been resolved. The original test failure occurred on workflow run 21550787783 at commit 0233464, and the fix was applied shortly after.

Original Detailed Analysis

Test Failure Logs

The tests failed with:

pydantic_core._pydantic_core.ValidationError: 1 validation error for Part
thought_signature
  Extra inputs are not permitted [type=extra_forbidden, input_value=b'skip_thought_signature_validator', input_type=bytes]

This occurred in two test cases:

  • test_sampling_content_to_google_genai_part_tool_use
  • test_convert_messages_with_tool_use

Version Investigation

Through binary search testing, I confirmed:

  • google-genai==1.17.0: Does NOT have thoughtSignature parameter
  • google-genai==1.18.0: DOES have thoughtSignature parameter
  • Previous minimum: google-genai>=1.0.0 (pyproject.toml:54)
  • Lowest-direct resolution was installing: 1.7.0

The Bypass Mechanism

The code uses thought_signature=b"skip_thought_signature_validator" which is a documented approach for bypassing thought signature validation when manually constructing function call history. According to Google's documentation, this should be used as a last resort but is valid for scenarios where function calls are created programmatically rather than generated by the Gemini API.

Related Files
  • src/fastmcp/client/sampling/handlers/google_genai.py:290 - Where thought_signature parameter is used
  • pyproject.toml:54 - Where the google-genai version constraint was updated
  • tests/client/sampling/handlers/test_google_genai_handler.py - Tests that were failing with older versions

@strawgate
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/marvin let's bump the minimum version of google genai please

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marvin-context-protocol bot commented Jan 31, 2026

Claude finished @strawgate's task in 3m 32s —— View job


Bumping google-genai minimum version ✓

Task: Update minimum version constraint for google-genai to fix test failures with thought_signature parameter.

Context

The code uses thought_signature=b"skip_thought_signature_validator" which was added in google-genai 1.18.0. The current minimum version (>=1.0.0) causes test failures when CI runs with --resolution lowest-direct.

Changes

  • Updated pyproject.toml line 54: google-genai>=1.0.0google-genai>=1.18.0
  • Synced dependencies (now using google-genai 1.60.0)
  • All tests pass (3814 passed)
  • All linting/type checks pass

Tasks

  • Read repository guidelines (CLAUDE.md)
  • Update pyproject.toml to require google-genai>=1.18.0
  • Sync dependencies with uv sync
  • Run test suite (3814 passed ✓)
  • Run linting/type checks (all passed ✓)
  • Commit and push changes

Summary

Successfully bumped the minimum google-genai version to 1.18.0. This ensures that the thought_signature parameter used in the Google GenAI handler is always available, fixing the test failures that occurred when CI resolved to older versions of the package.

Commit: 6733bee - chore: bump google-genai minimum version to 1.18.0

--- | Branch: claude/issue-2976-20260121-2112

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@coderabbitai please review

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coderabbitai bot commented Feb 8, 2026

@strawgate: I'll review the changes in this PR.

✅ Actions performed

Review triggered.

Note: CodeRabbit is an incremental review system and does not re-review already reviewed commits. This command is applicable only when automatic reviews are paused.

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Test Failure Analysis

Summary: Two tests failed in the latest workflow run (#21807550906), but both failures are unrelated to this PR's changes (Google GenAI handler). These are pre-existing test flakiness issues in other parts of the codebase.


Failure 1: OpenAPI Performance Test (Ubuntu, lowest-direct deps)

Test: tests/server/providers/openapi/test_comprehensive.py::TestOpenAPIComprehensive::test_server_performance_no_latency

Root Cause: Performance assertion is too strict. The test expects OpenAPI provider initialization to complete in under 100ms, but it took 233ms on this run.

# tests/server/providers/openapi/test_comprehensive.py:736
assert initialization_time < 0.1  # Should be under 100ms
# AssertionError: assert 0.23323297500610352 < 0.1

Why this is unrelated to this PR: This PR only adds a Google GenAI sampling handler (src/fastmcp/client/sampling/handlers/google_genai.py). It doesn't touch OpenAPI provider code.

Suggested Solution: This is a flaky performance test. The initialization time varies based on CI runner performance. Options:

  1. Increase the threshold to 0.25s or 0.3s to account for slower CI runners
  2. Use @pytest.mark.xfail(strict=False) to mark as expected to occasionally fail
  3. Skip this test in lowest-direct dependency runs where performance may be different

Failure 2: Azure Auth Test Timeout (Windows)

Test: tests/server/auth/providers/test_azure.py::test_prepare_scopes_for_upstream_refresh_empty_scopes

Root Cause: Test timed out during DiskStore initialization on Windows. The test hung while initializing the SQLite-backed disk cache used by the OAuth proxy.

# Stack trace shows timeout at:
File "D:\a\fastmcp\fastmcp\src\fastmcp\server\auth\oauth_proxy\proxy.py", line 426
    key_value=DiskStore(directory=settings.home / "oauth-proxy"),
File "D:\a\fastmcp\fastmcp\.venv\lib\site-packages\diskcache\core.py", line 2438
    sql('PRAGMA %s = %s' % (pragma, value)).fetchall()

Why this is unrelated to this PR: This PR only adds Google GenAI sampling handler code. It doesn't modify Azure auth providers or disk storage.

Suggested Solution: This appears to be a Windows-specific issue with SQLite/diskcache, possibly due to:

  • File locking contention on Windows
  • Slow disk I/O on Windows CI runners
  • SQLite PRAGMA operations hanging

Possible fixes:

  1. Increase test timeout for Windows runs
  2. Use an in-memory cache for tests instead of DiskStore
  3. Investigate if parallel test execution is causing SQLite locking issues
  4. Mock the DiskStore in Azure provider tests

PR Impact

This PR's changes are not causing these failures

The Google GenAI handler implementation:

  • Adds new files: src/fastmcp/client/sampling/handlers/google_genai.py and tests
  • Does not modify OpenAPI provider code
  • Does not modify Azure auth provider code
  • Does not modify disk storage or caching code

These failures are pre-existing flaky tests that need to be addressed separately.


Related Files (for fixing the flaky tests)

OpenAPI Performance Test:

  • tests/server/providers/openapi/test_comprehensive.py:736 - Performance assertion that's too strict

Azure Auth Timeout:

  • tests/server/auth/providers/test_azure.py:587 - Test that times out on Windows
  • src/fastmcp/server/auth/oauth_proxy/proxy.py:426 - Where DiskStore is initialized
  • Consider using @pytest.fixture with scope="session" for DiskStore to avoid repeated initialization

Updated: 2026-02-08 23:45 UTC - Workflow run #21807550906

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Actionable comments posted: 3

🧹 Nitpick comments (3)
src/fastmcp/client/sampling/handlers/google_genai.py (3)

300-302: Use TypeError instead of ValueError for unsupported content types.

Per Ruff TRY004, TypeError is more appropriate when the issue is an unexpected type.

Proposed fix
                 else:
-                    msg = f"Unsupported tool result content type: {type(item).__name__}"
-                    raise ValueError(msg)
+                    msg = f"Unsupported tool result content type: {type(item).__name__}"
+                    raise TypeError(msg)

404-436: Thought parts are silently dropped when thinking_budget is set.

When a thinking budget is configured (Lines 108-112), the model may return thought parts alongside text/function_call parts. The content-building loop (Lines 422-436) only handles text and function_call, silently dropping thought parts. This is likely the desired behavior (MCP consumers don't need internal chain-of-thought), but a brief comment documenting this intentional omission would prevent future confusion.

Also, the hasattr checks on Lines 424/426 are unnecessary for typed Part objects—these attributes always exist (as None). Using part.text is not None and part.function_call is not None would be more idiomatic.

Suggested cleanup
         for part in candidate.content.parts:
-            if hasattr(part, "text") and part.text:
+            # Skip thought parts (from thinking_config) — not relevant for MCP responses
+            if part.text is not None and part.text:
                 content.append(TextContent(type="text", text=part.text))
-            elif hasattr(part, "function_call") and part.function_call:
+            elif part.function_call is not None:

284-291: Add a comment explaining the thought_signature bypass.

The magic byte string b"skip_thought_signature_validator" lacks inline documentation. The thought_signature field is part of Gemini's tool-calling flow where the model returns an encrypted reasoning state that must be echoed back in subsequent turns; for Gemini 3, omitting required signatures causes HTTP 400 errors.

Since this code injects tool-use Parts from the MCP protocol (not from Gemini's response), they lack legitimate signatures. The bypass is documented in the Gemini API as a last resort to allow such injected tool calls, but the code should note this reason:

thought_signature=b"skip_thought_signature_validator",  # MCP tool calls bypass Gemini's thought signature validation

Or, more explicitly: "This tool call originates from MCP, not from a Gemini response, so it has no real thought signature. The bypass value allows validation to pass but may impact tool-calling quality."

Comment on lines 143 to 168
def _convert_tool_to_google_genai(tool: MCPTool) -> GoogleTool:
"""Convert an MCP Tool to Google GenAI format."""
input_schema: dict[str, Any] = tool.inputSchema
properties: dict[str, Any] = input_schema.get("properties", {})
required: list[str] = input_schema.get("required", [])

# Build parameters schema with Google's type format
google_properties: dict[str, Any] = {}
for prop_name, prop_schema in properties.items():
google_properties[prop_name] = _convert_json_schema_to_google_schema(
dict(prop_schema)
)

return GoogleTool(
function_declarations=[
FunctionDeclaration(
name=tool.name,
description=tool.description or "",
parameters_json_schema={
"type": "OBJECT",
"properties": google_properties,
"required": required,
},
)
]
)
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⚠️ Potential issue | 🔴 Critical

🧩 Analysis chain

🌐 Web query:

google-genai Python FunctionDeclaration parameters_json_schema documentation

💡 Result:

In the google-genai (python-genai) Python SDK, types.FunctionDeclaration.parameters_json_schema is documented as the JSON-Schema way to describe a function’s input parameters (the schema must be an object whose properties are the function parameters), and it is mutually exclusive with parameters (the SDK’s Schema model). (googleapis.github.io)

Where it’s documented (and example):

  • Python SDK docs (Function calling example) shows types.FunctionDeclaration(..., parameters_json_schema={...}) used when declaring a tool function. (googleapis.github.io)
  • Python SDK API reference lists FunctionDeclaration.parameters_json_schema and describes it (including the “mutually exclusive with parameters” note). (googleapis.github.io)

Related note: there’s a reported/closed issue indicating parameters_json_schema worked for “regular” generate_content models but not for some Gemini Live models (empty args), with the suggested workaround being to use parameters instead. (github.com)

Citations:


Fix schema format mismatch: remove conversion when using parameters_json_schema.

parameters_json_schema expects standard JSON Schema format (lowercase "string", "object", etc.), but the code converts to Google's schema format (uppercase "STRING", "OBJECT"). Since tool.inputSchema is already in standard JSON Schema format, pass it directly instead of converting:

Recommended fix
 def _convert_tool_to_google_genai(tool: MCPTool) -> GoogleTool:
     """Convert an MCP Tool to Google GenAI format."""
     return GoogleTool(
         function_declarations=[
             FunctionDeclaration(
                 name=tool.name,
                 description=tool.description or "",
-                parameters_json_schema={
-                    "type": "OBJECT",
-                    "properties": google_properties,
-                    "required": required,
-                },
+                parameters_json_schema=tool.inputSchema,
             )
         ]
     )
📝 Committable suggestion

‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.

Suggested change
def _convert_tool_to_google_genai(tool: MCPTool) -> GoogleTool:
"""Convert an MCP Tool to Google GenAI format."""
input_schema: dict[str, Any] = tool.inputSchema
properties: dict[str, Any] = input_schema.get("properties", {})
required: list[str] = input_schema.get("required", [])
# Build parameters schema with Google's type format
google_properties: dict[str, Any] = {}
for prop_name, prop_schema in properties.items():
google_properties[prop_name] = _convert_json_schema_to_google_schema(
dict(prop_schema)
)
return GoogleTool(
function_declarations=[
FunctionDeclaration(
name=tool.name,
description=tool.description or "",
parameters_json_schema={
"type": "OBJECT",
"properties": google_properties,
"required": required,
},
)
]
)
def _convert_tool_to_google_genai(tool: MCPTool) -> GoogleTool:
"""Convert an MCP Tool to Google GenAI format."""
input_schema: dict[str, Any] = tool.inputSchema
properties: dict[str, Any] = input_schema.get("properties", {})
required: list[str] = input_schema.get("required", [])
# Build parameters schema with Google's type format
google_properties: dict[str, Any] = {}
for prop_name, prop_schema in properties.items():
google_properties[prop_name] = _convert_json_schema_to_google_schema(
dict(prop_schema)
)
return GoogleTool(
function_declarations=[
FunctionDeclaration(
name=tool.name,
description=tool.description or "",
parameters_json_schema=tool.inputSchema,
)
]
)

Comment on lines +305 to +316
# Extract function name from toolUseId
# Our IDs are formatted as "{function_name}_{uuid8}", so extract the name.
# Note: This is a limitation of MCP's ToolResultContent which only carries
# toolUseId, while Google's FunctionResponse requires the function name.
tool_use_id = content.toolUseId
if "_" in tool_use_id:
# Split and rejoin all but the last part (the UUID suffix)
parts = tool_use_id.rsplit("_", 1)
function_name = parts[0]
else:
# Fallback: use the full ID as the name
function_name = tool_use_id
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⚠️ Potential issue | 🟡 Minor

Fragile function-name extraction from toolUseId.

The ID format {name}_{uuid4().hex[:8]} is an internal convention set at Line 432, but the reverse parsing here assumes that convention holds for all incoming ToolResultContent. If a tool name contains no underscores, or the ID originates from a different MCP participant that uses a different format, rsplit("_", 1) will silently produce incorrect results.

Consider carrying the function name through a more reliable channel (e.g., a lookup map keyed by toolUseId), or at minimum document this coupling prominently at both the generation site (Line 432) and the parsing site here.

Comment on lines +375 to +389
def _response_to_create_message_result(
response: GenerateContentResponse,
model: str,
) -> CreateMessageResult:
"""Convert Google GenAI response to CreateMessageResult (no tools)."""
if not (text := response.text):
candidate = _get_candidate_from_response(response)
msg = f"No content in response: {candidate.finish_reason}"
raise ValueError(msg)

return CreateMessageResult(
content=TextContent(type="text", text=text),
role="assistant",
model=model,
)
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⚠️ Potential issue | 🟡 Minor

🧩 Analysis chain

🌐 Web query:

google-genai python GenerateContentResponse text property raises ValueError exception

💡 Result:

In google-genai (aka googleapis/python-genai), GenerateContentResponse.text is a convenience property that only works when the model output contains only text parts. If the response includes any non-text part (e.g., function_call, executable_code, etc.), accessing .text can raise:

ValueError: GenerateContentResponse.text only supports text parts, but got ... [1], [2]

This commonly happens when you enable tools (Google Search, function calling, code execution): the model may emit a tool/call/code part (sometimes before the text part), and .text refuses to “skip past” those parts. [1], [2]

Workarounds

1) Safely extract just the text parts

texts = []
for part in (response.candidates[0].content.parts or []):
    if part.text:
        texts.append(part.text)
print("".join(texts))

Issue #359 shows a response whose first part is executable_code and second part is text, which triggers the exception if you call response.text. [1]

2) Handle tool/function-call responses explicitly

  • Check response.function_calls and act on them instead of calling .text. The SDK README documents response.function_calls and how to disable automatic function calling if you want manual control. [3]

3) If streaming, expect tool/function_call chunks
When streaming with tools enabled, individual chunks may be function-call parts, so chunk.text can raise the same ValueError—see issue #106. [2]

Sources: GitHub issue demonstrating response.text failing with executable_code parts [1]; GitHub issue demonstrating the same failure with function_call parts (incl. streaming) [2]; SDK README documenting function calling patterns and response.function_calls / disabling automatic function calling [3].


🏁 Script executed:

# Find the google-genai dependency and version
fd -t f "requirements" | xargs grep -l "google-genai\|google-ai-generativelanguage" 2>/dev/null || fd -t f "pyproject.toml" | xargs cat | grep -A5 -B5 "google-genai"

Repository: jlowin/fastmcp

Length of output: 529


🏁 Script executed:

# Search for other usages of response.text in the codebase to see patterns
rg "response\.text" --type py -B2 -A2

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Handle response.text raising ValueError in non-tools path.

The google-genai SDK's GenerateContentResponse.text raises ValueError when the response is blocked by safety filters, empty, or otherwise contains non-text content. The current code at line 380 using the walrus operator only handles falsy returns, not exceptions. If .text raises, the custom error message is bypassed and a less informative traceback is shown.

Wrap the access in try/except:

Proposed fix
 def _response_to_create_message_result(
     response: GenerateContentResponse,
     model: str,
 ) -> CreateMessageResult:
     """Convert Google GenAI response to CreateMessageResult (no tools)."""
-    if not (text := response.text):
+    try:
+        text = response.text
+    except (ValueError, AttributeError):
+        text = None
+    if not text:
         candidate = _get_candidate_from_response(response)
         msg = f"No content in response: {candidate.finish_reason}"
         raise ValueError(msg)

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/marvin please fix merge conflicts and make sure precommit checks pass

@strawgate strawgate changed the title [Draft] Add Google GenAI Sampling Handler Add Google GenAI Sampling Handler Feb 11, 2026
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Claude finished @strawgate's task in 3m 49s —— View job


Fixing merge conflicts and precommit checks ✓

Tasks

  • Check git status and identify any merge conflicts
  • Resolve merge conflicts with main branch (rebased to origin/main)
  • Run uv sync to update dependencies
  • Run uv run prek run --all-files for static checks (all passed!)
  • Run test suite to verify changes (40 tests passed!)
  • Commit and push fixes

Summary

Successfully resolved merge conflicts and ensured all precommit checks pass:

  1. Resolved conflicts: Reset the branch to origin/main (commit efcc12b) and reapplied the Google GenAI handler changes cleanly
  2. Updated dependencies: Added google-genai>=1.18.0 to pyproject.toml and ran uv sync
  3. Static checks: All prek checks passed (ruff, prettier, ty, loq)
  4. Tests: All 40 sampling tests pass, including 16 Google GenAI-specific tests
  5. Pushed: Force-pushed to claude/issue-2976-20260121-2112 with --force-with-lease

The PR is now ready for review with a clean rebase on the latest main branch.


Branch: claude/issue-2976-20260121-2112 | View job

@strawgate strawgate removed the DON'T MERGE PR is not ready for merging. Used by authors to prevent premature merging. label Feb 11, 2026
@strawgate strawgate added this to the 3.0 milestone Feb 11, 2026
@github-actions github-actions bot force-pushed the claude/issue-2976-20260121-2112 branch from 7cd5690 to 47a0cae Compare February 11, 2026 05:34
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Actionable comments posted: 2

Comment on lines 171 to 240
def _convert_json_schema_to_google_schema(schema: dict[str, Any]) -> dict[str, Any]:
"""Convert JSON Schema to Google GenAI Schema format.

Handles:
- Basic types (string, integer, number, boolean, array, object)
- Nullable types via anyOf with null type
- Nested objects and arrays
"""
result: dict[str, Any] = {}

# Handle anyOf for nullable types (e.g., anyOf: [{type: string}, {type: null}])
if "anyOf" in schema:
any_of_types = schema["anyOf"]
non_null_types = [t for t in any_of_types if t.get("type") != "null"]
has_null = len(non_null_types) < len(any_of_types)

if non_null_types:
# Recursively convert the non-null type
non_null_schema = non_null_types[0]
result = _convert_json_schema_to_google_schema(non_null_schema)

if has_null:
result["nullable"] = True

# Preserve description from parent schema
if "description" in schema:
result["description"] = schema["description"]

return result

schema_type = schema.get("type")

# Handle type arrays (e.g., ["string", "null"]) for nullable types
if isinstance(schema_type, list):
non_null_types = [t for t in schema_type if t != "null"]
has_null = len(non_null_types) < len(schema_type)
schema_type = non_null_types[0] if non_null_types else None
if has_null:
result["nullable"] = True

if isinstance(schema_type, str):
type_map: dict[str, str] = {
"string": "STRING",
"integer": "INTEGER",
"number": "NUMBER",
"boolean": "BOOLEAN",
"array": "ARRAY",
"object": "OBJECT",
}
result["type"] = type_map.get(schema_type, "STRING")

if "description" in schema:
result["description"] = schema["description"]

if "enum" in schema:
result["enum"] = schema["enum"]

if "items" in schema:
result["items"] = _convert_json_schema_to_google_schema(dict(schema["items"]))

if "properties" in schema:
result["properties"] = {
str(k): _convert_json_schema_to_google_schema(dict(v))
for k, v in dict(schema["properties"]).items()
}

if "required" in schema:
result["required"] = schema["required"]

return result
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🛠️ Refactor suggestion | 🟠 Major

This function becomes unnecessary if parameters_json_schema is used correctly.

If the fix above is applied (passing tool.inputSchema directly), this entire function can be removed. If you instead switch to using the parameters field (Google's Schema type), then this conversion logic would be needed — but should use the Schema model rather than a raw dict with uppercase type strings.

Comment on lines 284 to 291
if isinstance(content, ToolUseContent):
return Part(
function_call=FunctionCall(
name=content.name,
args=content.input,
),
thought_signature=b"skip_thought_signature_validator",
)
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⚠️ Potential issue | 🟡 Minor

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🌐 Web query:

google-genai SDK Part thought_signature documentation function_call validation

💡 Result:

In the google-genai (Google GenAI) SDK / Gemini API, thought_signature (aka thoughtSignature) is an opaque, encrypted “save state” the model returns to preserve its reasoning context—especially for tool / function calling. If you get a thought signature back, you should send it back exactly as-is in the next request’s history. [1][2]

Where it appears (as “Parts”)

The API can return a thought signature on response parts such as text or functionCall. The Python SDK explicitly exposes Part.thought_signature / PartDict.thought_signature. [2][3]

Function-call validation rule (the 400 you’re seeing)

For Gemini 3 models, function calling enforces strict validation: if the model produced a functionCall part with a thought_signature, you must include that same signature in that same part when you send the conversation back along with the tool result—otherwise you can get a 4xx / 400 validation error complaining a function call is missing thought_signature. [2]

Notes from the docs:

  • Single function call: the functionCall part includes a thought_signature. [2]
  • Parallel function calls: only the first functionCall part contains the signature; you must return it in the same place. [2]
  • Validation is enforced for function calls in the current turn (the tool-execution loop). [2]

Do you need to handle it manually?

Usually no: the official Google GenAI SDK says it handles thought signatures automatically if you use its chat/history patterns (or append the full model response back into history without modifying parts). You typically only manage signatures manually if you’re editing/truncating history or using raw REST. [1][2]

Sources: Gemini “Thinking” / thought signatures overview [1], Thought Signatures requirements & function-call validation [2], Google GenAI Python SDK API reference showing Part.thought_signature [3].


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🌐 Web query:

google-genai SDK Part ModelContent thought_signature required validation function_call

💡 Result:

In the Google GenAI / Gemini APIs, Gemini 3 (and some “thinking” models) attach a thought_signature to response Parts (especially **functionCall parts). If you later send conversation history back without that same thought_signature in the same Part, the server can fail request validation with a 400 like “function call … is missing a thought_signature”. (ai.google.dev)

What’s going on (why it’s “required”)

  • For Gemini 3 function calling, the API strictly validates that, within the current turn, the first functionCall part in each step includes its thought_signature. If you omit it, you get a 400 error. (ai.google.dev)
  • With parallel function calls, only the first functionCall part may carry the signature; you must still return that exact part. (ai.google.dev)

The fix (google-genai SDK)

If you’re using the official SDK, don’t rebuild ModelContent/Content manually for history. Instead:

  1. Append the model’s returned content object (with its parts) to your contents / history as-is, then
  2. Append your functionResponse, then call the model again.
    This preserves thought_signature automatically. (docs.cloud.google.com)

Google also notes that if you use the SDK’s normal chat/history flow (or append the full model response content object), you shouldn’t need to manage signatures yourself. (ai.google.dev)

If you must construct history manually (ModelContent / Part)

  • Copy the thought_signature back exactly where it appeared (same Part, same position). Don’t merge/split parts in a way that moves/removes the signature. (docs.cloud.google.com)
  • For multiple tool calls in a step, keep ordering rules: all functionCalls, then all functionResponses (don’t interleave), or you can also get a 400. (docs.cloud.google.com)
  • Last resort: set thought_signature to skip_thought_signature_validator (Google warns it can reduce performance). (docs.cloud.google.com)

Citations:


Add comment explaining the thought_signature bypass.

The hardcoded thought_signature=b"skip_thought_signature_validator" is a Google-documented fallback for reconstructed message history. When Gemini 3 function calls are included in ModelContent built from scratch (rather than preserved from the model's original response), this bypass prevents validation errors. Add a comment explaining this pattern to prevent future maintainers from treating it as an undocumented hack.

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jlowin commented Feb 11, 2026

/marvin please add a small note about this functionality to a new "RC1" section of the v3-features document, as we'll release 3.0 RC1 shortly with this

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Claude finished @jlowin's task in 4m 27s —— View job


Adding Google GenAI handler documentation to v3-features ✓

Tasks

  • Locate the v3-features document
  • Read existing content to understand format
  • Add new "RC1" section with Google GenAI handler note
  • Run formatting checks
  • Commit and push changes

Summary

Successfully added documentation for the Google GenAI sampling handler to the v3-features document in a new "3.0.0rc1" section. The documentation includes:

  • Overview of the new Google GenAI handler functionality
  • Code example showing basic usage
  • Key features list (tool schema conversion, model support, type handling, etc.)
  • Context about how it fits with existing Anthropic and OpenAI handlers

All formatting checks passed and the changes have been committed and pushed to the PR branch.


View job | Branch: claude/issue-2976-20260121-2112

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jlowin commented Feb 11, 2026

Review from Claude - not sure how much is valid and how much is google idiosyncracy:


Review: Add Google GenAI Sampling Handler

The PR adds a Google GenAI sampling handler following the pattern of the existing Anthropic and OpenAI handlers. It's a solid contribution structurally, but there are several issues
worth addressing — some of which Marvin already flagged in the comments, and some new ones.

The schema conversion is probably wrong (or at least unnecessary)

The biggest concern: the handler uses parameters_json_schema on FunctionDeclaration but converts types to uppercase (STRING, INTEGER, etc.). parameters_json_schema is the
JSON-Schema-compatible field — it should accept standard lowercase types. The uppercase convention (STRING, NUMBER) is for the separate parameters field which takes Google's
proto-based Schema format.

This means _convert_json_schema_to_google_schema is likely doing the wrong transformation. Compare with how the Anthropic and OpenAI handlers handle this — both pass
tool.inputSchema through essentially as-is (just ensuring "type": "object" is present). The Google handler could probably do the same with parameters_json_schema, which would
eliminate ~70 lines of code and the entire schema conversion function.

Tool use ID round-tripping is fragile

Google's API doesn't provide tool use IDs the way Anthropic (tool.id) and OpenAI (tool_call.id) do. The handler works around this by generating IDs as {function_name}{uuid8} and
then parsing the name back out of toolUseId via rsplit("
", 1). This works because the UUID suffix doesn't contain underscores, but it's a convoluted pattern — you're encoding
information into a string just to decode it later.

A cleaner approach: store a dict[str, str] mapping tool use IDs to function names on the handler instance (or in a local variable during the tool-calling loop), and look up from
that when processing results. This also avoids the "what if the format changes" fragility.

hasattr usage violates project conventions

_response_to_result_with_tools uses hasattr(part, "function_call") and hasattr(part, "text") to inspect response parts. The project CLAUDE.md explicitly says not to use
hasattr/getattr when type guards or narrowing would work. The Part type from google-genai has typed attributes — isinstance checks or direct attribute access with None checks would
be more appropriate.

thought_signature=b"skip_thought_signature_validator" is a red flag

This is bypassing an internal SDK validation mechanism. It was added to make Part construction work when building function call history, and it's the reason the minimum version had
to be bumped to 1.18.0 (where the field was introduced). This feels like a workaround for constructing Parts incorrectly. The SDK likely has an intended way to build function call
Parts without triggering thought signature validation — worth investigating whether Part.from_function_call() or similar factory exists.

Tests have function-level imports

Several test functions re-import SamplingMessage, TextContent, etc. inside the function body instead of at module level. Project rules require all imports at module level.

Minor issues

  • Model selection is simpler than other handlers: Anthropic and OpenAI both use _iter_models_from_preferences with provider-specific validation (checking for "claude" prefix or
    ChatModel type args). The Google handler just takes the first hint name without any validation — any garbage string will be passed to the API.
  • websockets downgrade in lock file: The lock file shows websockets going from 16.0 to 15.0.1. This is presumably because google-genai pins to an older websockets. Worth verifying
    this doesn't cause issues elsewhere.
  • Image/audio content unsupported: The handler imports ImageContent and AudioContent but raises ValueError for them. Google's API supports multimodal input, so this is a missed
    opportunity, though it's fine to defer.
  • Context parameter typed as RequestContext[...] but inconsistent with Anthropic: This is a pre-existing inconsistency (Anthropic uses Any, OpenAI uses the full type). The Google
    handler actually uses the proper type, which is correct.

Bottom line

The core structure is right and follows the existing patterns well. The main things I'd want addressed before merging are the schema conversion (which might be actively wrong) and
the thought_signature bypass (which couples the handler to SDK internals). The tool ID round-tripping is worth improving but less urgent.

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jlowin commented Feb 11, 2026

/marvin sorry we updated v3-features on main, please pull those changes and put your rc1 note in the righ tplace to resolve the conflict. also fix the conflict on uv.lock by regenerating it

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Resolving merge conflicts ✓

Tasks

  • Read repository guidelines (CLAUDE.md)
  • Fetch latest changes from origin/main
  • Identify and understand merge conflicts
  • Resolve v3-features.mdx conflict (place RC1 note in correct location)
  • Regenerate uv.lock with uv lock
  • Sync dependencies with uv sync
  • Run test suite (16 tests passed!)
  • Run linting/type checks (all passed!)
  • Commit and push resolved changes

Summary

Successfully resolved all merge conflicts and updated the PR:

  1. Branch rebase: Reset to origin/main (263e0bf) and reapplied the Google GenAI handler changes cleanly
  2. Documentation conflict: Added the Google GenAI handler section to the RC1 portion of v3-features.mdx in the correct location (after the Breaking change section, before beta2)
  3. Lock file regenerated: Ran uv lock to regenerate uv.lock with the new google-genai dependencies
  4. All checks passing: 16 tests passed, all linting and type checks passed
  5. Pushed: Force-pushed with --force-with-lease to update the PR branch

The PR is now up to date with main and ready for review.

Commit: 358e08b - chore: resolve merge conflicts with main


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@github-actions github-actions bot force-pushed the claude/issue-2976-20260121-2112 branch from 1573ca8 to 358e08b Compare February 11, 2026 15:32
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Actionable comments posted: 1

🧹 Nitpick comments (2)
src/fastmcp/client/sampling/handlers/google_genai.py (2)

325-326: Use TypeError instead of ValueError for unsupported content types.

This branch handles an invalid type of content, making TypeError the more appropriate exception per Python conventions.

Proposed fix
-    msg = f"Unsupported content type: {type(content)}"
-    raise ValueError(msg)
+    msg = f"Unsupported content type: {type(content).__name__}"
+    raise TypeError(msg)

404-436: Replace hasattr checks with attribute-level None checks on Part.

Part from google.genai.types has text, function_call, etc. as typed optional attributes. Using hasattr bypasses type checking and is fragile if the SDK changes its attribute model. Use direct None checks instead.

Proposed fix
     if candidate.content and candidate.content.parts:
         for part in candidate.content.parts:
-            if hasattr(part, "function_call") and part.function_call:
+            if part.function_call is not None:
                 has_function_calls = True
                 break
 
     # ...
 
     if candidate.content and candidate.content.parts:
         for part in candidate.content.parts:
-            if hasattr(part, "text") and part.text:
+            if part.text:
                 content.append(TextContent(type="text", text=part.text))
-            elif hasattr(part, "function_call") and part.function_call:
+            elif part.function_call is not None:
                 fc = part.function_call

Comment on lines 77 to 94
```python
from fastmcp.client.sampling.handlers import GoogleGenaiSamplingHandler
from google.genai import Client as GoogleGenaiClient

# Initialize the handler
handler = GoogleGenaiSamplingHandler(
default_model="gemini-2.0-flash-exp",
client=GoogleGenaiClient(), # Optional - creates one if not provided
)

# Use with MCP sampling
async with Client("http://server/mcp") as client:
result = await client.sample(
messages=[...],
params=SamplingParams(tools=[...]),
handler=handler,
)
```
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⚠️ Potential issue | 🟡 Minor

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Code example has incorrect API usage and missing imports.

The example shows handler=handler and params=SamplingParams(...) passed to client.sample(), but the sampling handler is configured at Client construction time, not per-call. The sample() method accepts individual parameters like tools= instead of a params= object.

Correct the example:

  1. Add missing imports: from fastmcp import Client and from fastmcp.client.sampling import SamplingParams
  2. Move sampling_handler to the Client constructor
  3. Replace params=SamplingParams(tools=[...]) with tools=[...] in the sample() call
Corrected example
from fastmcp import Client
from fastmcp.client.sampling import SamplingParams
from fastmcp.client.sampling.handlers import GoogleGenaiSamplingHandler
from google.genai import Client as GoogleGenaiClient

# Initialize the handler
handler = GoogleGenaiSamplingHandler(
    default_model="gemini-2.0-flash-exp",
    client=GoogleGenaiClient(),  # Optional - creates one if not provided
)

# Use with MCP sampling
async with Client("http://server/mcp", sampling_handler=handler) as client:
    result = await client.sample(
        messages=[...],
        tools=[...],
    )

@strawgate
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/marvin please address the pr feedback

@marvin-context-protocol
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marvin-context-protocol bot commented Feb 11, 2026

Claude finished @strawgate's task in 3m 40s —— View job


Addressing PR Feedback ✓

Tasks

  • Read repository guidelines (CLAUDE.md)
  • Review all PR feedback and current implementation
  • Fix schema conversion (removed unnecessary conversion, pass JSON schema directly)
  • Fix model selection to return actual model used (already done in current code!)
  • Add documentation for thought_signature bypass
  • Replace hasattr checks with proper type guards
  • Fix documentation example in v3-features.mdx
  • Run tests and linting
  • Commit and push changes

Summary

Successfully addressed all PR feedback by implementing the following changes:

Key Changes

  1. Removed unnecessary schema conversion (~70 lines deleted)

    • The _convert_json_schema_to_google_schema function was converting JSON Schema types to uppercase (STRING, INTEGER, etc.)
    • Google's parameters_json_schema accepts standard JSON Schema format directly
    • Now passes tool.inputSchema directly without conversion, simplifying the code and fixing the type case issue
  2. Added comprehensive documentation for thought_signature bypass

    • Added detailed comments explaining why skip_thought_signature_validator is necessary
    • Documents that manually constructed tool calls from MCP protocol lack legitimate signatures
    • Links to Google's official documentation for reference
  3. Replaced hasattr with proper type guards

    • Changed hasattr(part, "text") and part.text to part.text
    • Changed hasattr(part, "function_call") and part.function_call to part.function_call is not None
    • Added comment explaining thought parts are intentionally skipped
  4. Fixed documentation example

    • Corrected API usage to show handler configured at Client construction
    • Fixed parameter passing (removed incorrect params=SamplingParams(...))
    • Added missing imports (from fastmcp import Client)
  5. Cleaned up tests

    • Removed obsolete test_convert_json_schema_to_google_schema test
    • Removed import of deleted function
    • All 15 Google GenAI handler tests pass

Test Results

  • ✅ All 15 Google GenAI handler tests pass
  • ✅ All 39 sampling tests pass
  • ✅ All static checks pass (ruff, prettier, ty)

Notes

  • The model selection was already correctly implemented (using selected_model throughout)
  • Ruff automatically removed the now-unused from typing import Any import

Commit: ce226e3 - Address PR feedback: simplify schema handling and improve code quality

View job | Branch: claude/issue-2976-20260121-2112

@strawgate
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/marvin fix merge conflicts plz

@marvin-context-protocol
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marvin-context-protocol bot commented Feb 12, 2026

Claude finished @strawgate's task in 5m 23s —— View job


Merge conflicts resolved ✓

Tasks

  • Read repository guidelines (CLAUDE.md)
  • Check git status and identify issue
  • Analyze branch divergence (found: unrelated histories)
  • Create clean branch from origin/main
  • Port Google GenAI handler files
  • Update pyproject.toml with google-genai dependency
  • Update v3-features.mdx documentation
  • Run uv sync to generate uv.lock
  • Run uv run prek run --all-files for static checks (all passed!)
  • Run test suite (15 tests passed!)
  • Commit changes
  • Force-push to existing PR branch with --force-with-lease

Solution Applied

The repository history was completely rewritten on main (commit 359575b adds all files as new), creating "unrelated histories" between main and this PR branch. Standard merge/rebase was impossible.

Fixed by:

  1. Created a clean branch from origin/main
  2. Cherry-picked the Google GenAI handler changes:
    • src/fastmcp/client/sampling/handlers/google_genai.py
    • tests/client/sampling/handlers/test_google_genai_handler.py
    • docs/python-sdk/fastmcp-client-sampling-handlers-google_genai.mdx
    • Updated pyproject.toml with google-genai>=1.18.0 dependency
    • Updated docs/development/v3-notes/v3-features.mdx with feature documentation
  3. Ran uv sync to regenerate uv.lock
  4. Verified all static checks pass (ruff, prettier, ty, loq)
  5. Verified all 15 Google GenAI handler tests pass
  6. Force-pushed to PR branch

The PR is now cleanly based on the latest main and ready for review!


View job | Branch: claude/issue-2976-20260121-2112

Co-authored-by: Bill Easton <strawgate@users.noreply.github.com>
@github-actions github-actions bot force-pushed the claude/issue-2976-20260121-2112 branch from ce226e3 to bb34db3 Compare February 12, 2026 17:44
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Add Google GenAI Sampling Handler

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