feat: add MCP server scaffold with stdio transport

[dtsong]

Summary

• MCP server implementation with stdio transport
• Entry point pr-review-mcp added to pyproject.toml
• Credential helpers for GitHub token and Anthropic key

Changes

• src/pr_review_agent/mcp/server.py — Server instance, stdio runner, main entry point
• src/pr_review_agent/mcp/__init__.py — Package init
• pyproject.toml — mcp>=1.0.0 dependency + pr-review-mcp script entry point
• tests/test_mcp_server.py — 6 tests

Test plan

• 6 unit tests passing
• Lint clean
• uv sync installs mcp dependency

Closes #11

🤖 Generated with Claude Code

[github-actions]

AI Code Review

Summary

This PR adds a basic MCP server implementation with stdio transport and credential helpers. The implementation is minimal but functional, with good test coverage for the core functionality.

Confidence: 0.00 (low)
This PR may require human review

Strengths

• Good separation of concerns with credential helpers
• Proper async/await usage for server operations
• Clean entry point configuration in pyproject.toml
• Good test coverage for credential helper functions
• Proper error handling with descriptive error messages

Issues Found

Major (3)

src/pr_review_agent/mcp/server.py:12 - logic

The get_github_token function returns an empty string when GITHUB_TOKEN is not set, but then checks if the token is falsy. This creates inconsistent behavior - it should either return None or use a different check.
Suggestion: Either return None when the environment variable is not set, or check specifically for empty strings

src/pr_review_agent/mcp/server.py:20 - logic

Same issue as get_github_token - inconsistent handling of missing environment variables
Suggestion: Use consistent pattern for environment variable checking

tests/test_mcp_server.py:1 - testing

Missing test coverage for the actual server functionality - run_server() and main() functions are not tested. The server instance is created but never actually tested for MCP protocol compliance.
Suggestion: Add integration tests that verify the MCP server can handle basic protocol messages and that run_server() and main() work correctly

Minor (2)

src/pr_review_agent/mcp/server.py:7 - logic

The server instance is created at module level but the credential functions are never called, so credentials aren't validated until runtime
Suggestion: Consider validating credentials when the server starts or documenting that credentials are only validated on demand

src/pr_review_agent/mcp/server.py:26 - documentation

The run_server function lacks documentation about what it does and how it integrates with the MCP protocol
Suggestion: Add docstring explaining the server lifecycle and stdio transport usage

Concerns

• The MCP server doesn't implement any actual tools or capabilities - it's just a skeleton
• No integration testing of the actual MCP protocol functionality
• Environment variable handling pattern is inconsistent
• Missing validation that the server can actually communicate via MCP protocol

Questions

• What tools or capabilities will this MCP server implement for PR review functionality?
• Should the credential validation happen at server startup or on-demand?
• Are there plans to add configuration options beyond environment variables?
• How will this integrate with the existing PR review agent functionality?

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_{Model: claude-sonnet-4-20250514 | Cost: $0.0836 |
Tokens: 22712 in / 1032 out}