diff --git a/AGENTS.md b/AGENTS.md
index 9da9ecd1..78ca9698 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -2,7 +2,7 @@
 
 ## Overview
 
-Ralph is an autonomous AI agent loop that runs AI coding tools (Amp or Claude Code) repeatedly until all PRD items are complete. Each iteration is a fresh instance with clean context.
+Ralph is an autonomous AI agent loop that runs AI coding tools (Amp, Claude Code, or GitHub Copilot) repeatedly until all PRD items are complete. Each iteration is a fresh instance with clean context.
 
 ## Commands
 
@@ -18,13 +18,16 @@ cd flowchart && npm run build
 
 # Run Ralph with Claude Code
 ./ralph.sh --tool claude [max_iterations]
+
+# Run Ralph with GitHub Copilot
+./ralph.sh --tool copilot [max_iterations]
 ```
 
 ## Key Files
 
-- `ralph.sh` - The bash loop that spawns fresh AI instances (supports `--tool amp` or `--tool claude`)
-- `prompt.md` - Instructions given to each AMP instance
--  `CLAUDE.md` - Instructions given to each Claude Code instance
+- `ralph.sh` - The bash loop that spawns fresh AI instances (supports `--tool amp`, `--tool claude`, or `--tool copilot`)
+- `prompt.md` - Instructions given to each Amp and GitHub Copilot instance
+- `CLAUDE.md` - Instructions given to each Claude Code instance
 - `prd.json.example` - Example PRD format
 - `flowchart/` - Interactive React Flow diagram explaining how Ralph works
 
@@ -41,7 +44,7 @@ npm run dev
 
 ## Patterns
 
-- Each iteration spawns a fresh AI instance (Amp or Claude Code) with clean context
+- Each iteration spawns a fresh AI instance (Amp, Claude Code, or GitHub Copilot) with clean context
 - Memory persists via git history, `progress.txt`, and `prd.json`
 - Stories should be small enough to complete in one context window
 - Always update AGENTS.md with discovered patterns for future iterations
diff --git a/README.md b/README.md
index d79d8b62..b86cb789 100644
--- a/README.md
+++ b/README.md
@@ -2,7 +2,7 @@
 
 ![Ralph](ralph.webp)
 
-Ralph is an autonomous AI agent loop that runs AI coding tools ([Amp](https://ampcode.com) or [Claude Code](https://docs.anthropic.com/en/docs/claude-code)) repeatedly until all PRD items are complete. Each iteration is a fresh instance with clean context. Memory persists via git history, `progress.txt`, and `prd.json`.
+Ralph is an autonomous AI agent loop that runs AI coding tools ([Amp](https://ampcode.com), [Claude Code](https://docs.anthropic.com/en/docs/claude-code), or [GitHub Copilot](https://github.com/features/copilot)) repeatedly until all PRD items are complete. Each iteration is a fresh instance with clean context. Memory persists via git history, `progress.txt`, and `prd.json`.
 
 Based on [Geoffrey Huntley's Ralph pattern](https://ghuntley.com/ralph/).
 
@@ -13,6 +13,7 @@ Based on [Geoffrey Huntley's Ralph pattern](https://ghuntley.com/ralph/).
 - One of the following AI coding tools installed and authenticated:
   - [Amp CLI](https://ampcode.com) (default)
   - [Claude Code](https://docs.anthropic.com/en/docs/claude-code) (`npm install -g @anthropic-ai/claude-code`)
+  - [GitHub Copilot CLI](https://docs.github.com/en/copilot) - Requires VS Code with [Copilot Chat extension](https://marketplace.visualstudio.com/items?itemName=GitHub.copilot-chat) installed; the CLI binary is bundled at `~/.vscode/extensions/github.copilot-chat-*/copilotCli/copilot` (note: this is different from the `gh copilot` extension)
 - `jq` installed (`brew install jq` on macOS)
 - A git repository for your project
 
@@ -28,9 +29,9 @@ mkdir -p scripts/ralph
 cp /path/to/ralph/ralph.sh scripts/ralph/
 
 # Copy the prompt template for your AI tool of choice:
-cp /path/to/ralph/prompt.md scripts/ralph/prompt.md    # For Amp
+cp /path/to/ralph/prompt.md scripts/ralph/prompt.md      # For Amp and GitHub Copilot
 # OR
-cp /path/to/ralph/CLAUDE.md scripts/ralph/CLAUDE.md    # For Claude Code
+cp /path/to/ralph/CLAUDE.md scripts/ralph/CLAUDE.md      # For Claude Code
 
 chmod +x scripts/ralph/ralph.sh
 ```
@@ -115,9 +116,12 @@ This creates `prd.json` with user stories structured for autonomous execution.
 
 # Using Claude Code
 ./scripts/ralph/ralph.sh --tool claude [max_iterations]
+
+# Using GitHub Copilot
+./scripts/ralph/ralph.sh --tool copilot [max_iterations]
 ```
 
-Default is 10 iterations. Use `--tool amp` or `--tool claude` to select your AI coding tool.
+Default is 10 iterations. Use `--tool amp`, `--tool claude`, or `--tool copilot` to select your AI coding tool.
 
 Ralph will:
 1. Create a feature branch (from PRD `branchName`)
@@ -133,8 +137,8 @@ Ralph will:
 
 | File | Purpose |
 |------|---------|
-| `ralph.sh` | The bash loop that spawns fresh AI instances (supports `--tool amp` or `--tool claude`) |
-| `prompt.md` | Prompt template for Amp |
+| `ralph.sh` | The bash loop that spawns fresh AI instances (supports `--tool amp`, `--tool claude`, or `--tool copilot`) |
+| `prompt.md` | Prompt template for Amp and GitHub Copilot |
 | `CLAUDE.md` | Prompt template for Claude Code |
 | `prd.json` | User stories with `passes` status (the task list) |
 | `prd.json.example` | Example PRD format for reference |
@@ -162,7 +166,7 @@ npm run dev
 
 ### Each Iteration = Fresh Context
 
-Each iteration spawns a **new AI instance** (Amp or Claude Code) with clean context. The only memory between iterations is:
+Each iteration spawns a **new AI instance** (Amp, Claude Code, or GitHub Copilot) with clean context. The only memory between iterations is:
 - Git history (commits from previous iterations)
 - `progress.txt` (learnings and context)
 - `prd.json` (which stories are done)
@@ -223,7 +227,7 @@ git log --oneline -10
 
 ## Customizing the Prompt
 
-After copying `prompt.md` (for Amp) or `CLAUDE.md` (for Claude Code) to your project, customize it for your project:
+After copying `prompt.md` (for Amp and GitHub Copilot) or `CLAUDE.md` (for Claude Code) to your project, customize it for your project:
 - Add project-specific quality check commands
 - Include codebase conventions
 - Add common gotchas for your stack
@@ -237,3 +241,4 @@ Ralph automatically archives previous runs when you start a new feature (differe
 - [Geoffrey Huntley's Ralph article](https://ghuntley.com/ralph/)
 - [Amp documentation](https://ampcode.com/manual)
 - [Claude Code documentation](https://docs.anthropic.com/en/docs/claude-code)
+- [GitHub Copilot documentation](https://docs.github.com/en/copilot)
diff --git a/ralph.sh b/ralph.sh
index baff052a..2e9af304 100755
--- a/ralph.sh
+++ b/ralph.sh
@@ -1,6 +1,6 @@
 #!/bin/bash
 # Ralph Wiggum - Long-running AI agent loop
-# Usage: ./ralph.sh [--tool amp|claude] [max_iterations]
+# Usage: ./ralph.sh [--tool amp|claude|copilot] [max_iterations]
 
 set -e
 
@@ -29,8 +29,8 @@ while [[ $# -gt 0 ]]; do
 done
 
 # Validate tool choice
-if [[ "$TOOL" != "amp" && "$TOOL" != "claude" ]]; then
-  echo "Error: Invalid tool '$TOOL'. Must be 'amp' or 'claude'."
+if [[ "$TOOL" != "amp" && "$TOOL" != "claude" && "$TOOL" != "copilot" ]]; then
+  echo "Error: Invalid tool '$TOOL'. Must be 'amp', 'claude', or 'copilot'."
   exit 1
 fi
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
@@ -90,6 +90,9 @@ for i in $(seq 1 $MAX_ITERATIONS); do
   # Run the selected tool with the ralph prompt
   if [[ "$TOOL" == "amp" ]]; then
     OUTPUT=$(cat "$SCRIPT_DIR/prompt.md" | amp --dangerously-allow-all 2>&1 | tee /dev/stderr) || true
+  elif [[ "$TOOL" == "copilot" ]]; then
+    PROMPT_CONTENT=$(cat "$SCRIPT_DIR/prompt.md" | sed "s|Read the PRD at \`prd.json\` (in the same directory as this file)|Read the PRD at \`$PRD_FILE\`|" | sed "s|Read the progress log at \`progress.txt\`|Read the progress log at \`$PROGRESS_FILE\`|" | sed "s|APPEND to progress.txt|APPEND to $PROGRESS_FILE|")
+    OUTPUT=$(copilot -i "$PROMPT_CONTENT" --yolo --no-ask-user 2>&1 | tee /dev/stderr) || true
   else
     # Claude Code: use --dangerously-skip-permissions for autonomous operation, --print for output
     OUTPUT=$(claude --dangerously-skip-permissions --print < "$SCRIPT_DIR/CLAUDE.md" 2>&1 | tee /dev/stderr) || true