fix: ensure valid parameters schema for parameter-less functions in OpenAiApi

models/spring-ai-openai/src/main/java/org/springframework/ai/openai/api/OpenAiApi.java

@@ -935,7 +935,7 @@ private Function() {
 			public Function(String description, String name, Map<String, Object> parameters, Boolean strict) {
 				this.description = description;
 				this.name = name;
-				this.parameters = parameters;
+				this.parameters = ensureValidParametersSchema(parameters);
 				this.strict = strict;
 			}
 
@@ -946,7 +946,39 @@ public Function(String description, String name, Map<String, Object> parameters,
 			 * @param jsonSchema tool function schema as json.
 			 */
 			public Function(String description, String name, String jsonSchema) {
-				this(description, name, ModelOptionsUtils.jsonToMap(jsonSchema), null);
+				this.description = description;
+				this.name = name;
+				this.parameters = ensureValidParametersSchema(
+						jsonSchema != null ? ModelOptionsUtils.jsonToMap(jsonSchema) : null);
+				this.strict = null;
+			}
+
+			/**
+			 * Ensures that the parameters schema is valid for OpenAI API. OpenAI requires
+			 * that the parameters object must have a "properties" field, even if it's
+			 * empty.
+			 * @param parameters the parameters map from JSON schema
+			 * @return a valid parameters map with required fields
+			 */
+			private static Map<String, Object> ensureValidParametersSchema(Map<String, Object> parameters) {
+				if (parameters == null) {
+					parameters = new java.util.HashMap<>();
+					parameters.put("type", "object");
+					parameters.put("properties", new java.util.HashMap<>());
+					return parameters;
+				}
+
+				// Ensure "type" field exists
+				if (!parameters.containsKey("type")) {
+					parameters.put("type", "object");
+				}
+
+				// Ensure "properties" field exists for object types
+				if ("object".equals(parameters.get("type")) && !parameters.containsKey("properties")) {
+					parameters.put("properties", new java.util.HashMap<>());
+				}
+
+				return parameters;
 			}
 
 			public String getDescription() {

models/spring-ai-openai/src/test/java/org/springframework/ai/openai/api/FunctionToolParametersTest.java

@@ -0,0 +1,156 @@
+/*
+ * Copyright 2023-2025 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.ai.openai.api;
+
+import java.util.Map;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for FunctionTool.Function parameters validation.
+ *
+ * @author Liu Guodong
+ */
+class FunctionToolParametersTest {
+
+	private final ObjectMapper objectMapper = new ObjectMapper();
+
+	@Test
+	void testFunctionWithNoParameters() throws Exception {
+		// Test case 1: JSON schema with no properties field
+		String jsonSchemaNoProperties = """
+				{
+					"type": "object"
+				}
+				""";
+
+		OpenAiApi.FunctionTool.Function function = new OpenAiApi.FunctionTool.Function(
+				"Test function with no parameters", "test_function", jsonSchemaNoProperties);
+
+		assertThat(function.getParameters()).isNotNull();
+		assertThat(function.getParameters().get("type")).isEqualTo("object");
+		assertThat(function.getParameters().get("properties")).isNotNull();
+		assertThat(function.getParameters().get("properties")).isInstanceOf(Map.class);
+
+		// Verify serialization produces valid JSON
+		String json = this.objectMapper.writeValueAsString(function);
+		assertThat(json).contains("\"properties\"");
+	}
+
+	@Test
+	void testFunctionWithEmptyProperties() throws Exception {
+		// Test case 2: JSON schema with empty properties
+		String jsonSchemaEmptyProperties = """
+				{
+					"type": "object",
+					"properties": {}
+				}
+				""";
+
+		OpenAiApi.FunctionTool.Function function = new OpenAiApi.FunctionTool.Function(
+				"Test function with empty properties", "test_function", jsonSchemaEmptyProperties);
+
+		assertThat(function.getParameters()).isNotNull();
+		assertThat(function.getParameters().get("type")).isEqualTo("object");
+		assertThat(function.getParameters().get("properties")).isNotNull();
+
+		// Verify serialization produces valid JSON
+		String json = this.objectMapper.writeValueAsString(function);
+		assertThat(json).contains("\"properties\"");
+	}
+
+	@Test
+	void testFunctionWithParameters() throws Exception {
+		// Test case 3: JSON schema with actual parameters
+		String jsonSchemaWithParams = """
+				{
+					"type": "object",
+					"properties": {
+						"param1": {
+							"type": "string",
+							"description": "First parameter"
+						}
+					},
+					"required": ["param1"]
+				}
+				""";
+
+		OpenAiApi.FunctionTool.Function function = new OpenAiApi.FunctionTool.Function("Test function with parameters",
+				"test_function", jsonSchemaWithParams);
+
+		assertThat(function.getParameters()).isNotNull();
+		assertThat(function.getParameters().get("type")).isEqualTo("object");
+		assertThat(function.getParameters().get("properties")).isNotNull();
+		assertThat(function.getParameters().get("properties")).isInstanceOf(Map.class);
+
+		@SuppressWarnings("unchecked")
+		Map<String, Object> properties = (Map<String, Object>) function.getParameters().get("properties");
+		assertThat(properties).containsKey("param1");
+
+		// Verify serialization produces valid JSON
+		String json = this.objectMapper.writeValueAsString(function);
+		assertThat(json).contains("\"properties\"");
+		assertThat(json).contains("\"param1\"");
+	}
+
+	@Test
+	void testFunctionWithNullSchema() throws Exception {
+		// Test case 4: null JSON schema (edge case)
+		String nullSchema = null;
+		OpenAiApi.FunctionTool.Function function = new OpenAiApi.FunctionTool.Function("Test function with null schema",
+				"test_function", nullSchema);
+
+		// Should create a valid empty parameters object
+		assertThat(function.getParameters()).isNotNull();
+		assertThat(function.getParameters().get("type")).isEqualTo("object");
+		assertThat(function.getParameters().get("properties")).isNotNull();
+
+		// Verify serialization produces valid JSON
+		String json = this.objectMapper.writeValueAsString(function);
+		assertThat(json).contains("\"properties\"");
+	}
+
+	@Test
+	void testFunctionWithVoidTypeSchema() throws Exception {
+		// Test case 5: Schema generated for Void.class (common case for no-param
+		// functions)
+		// This simulates what JsonSchemaGenerator would produce
+		String voidSchema = """
+				{
+					"$schema": "https://json-schema.org/draft/2020-12/schema",
+					"type": "object",
+					"additionalProperties": false
+				}
+				""";
+
+		OpenAiApi.FunctionTool.Function function = new OpenAiApi.FunctionTool.Function("Test function for Void type",
+				"test_void_function", voidSchema);
+
+		assertThat(function.getParameters()).isNotNull();
+		assertThat(function.getParameters().get("type")).isEqualTo("object");
+		assertThat(function.getParameters().get("properties")).isNotNull();
+		assertThat(function.getParameters().get("properties")).isInstanceOf(Map.class);
+
+		// Verify serialization produces valid JSON for OpenAI API
+		String json = this.objectMapper.writeValueAsString(function);
+		assertThat(json).contains("\"properties\"");
+	}
+
+}