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Development Guidelines

This document contains critical information about working with this codebase. Follow these guidelines precisely.

Core Development Rules

  1. Package Management

    • ONLY use uv, NEVER pip
    • Installation: uv add <package>
    • Running tools: uv run <tool>
    • Upgrading: uv lock --upgrade-package <package>
    • FORBIDDEN: uv pip install, @latest syntax

  2. Code Quality

    • Type hints required for all code
    • Public APIs must have docstrings
    • Functions must be focused and small
    • Follow existing patterns exactly
    • Line length: 120 chars maximum
    • FORBIDDEN: imports inside functions. THEY SHOULD BE AT THE TOP OF THE FILE.

  3. Testing Requirements

    • Framework: uv run --frozen pytest
    • Async testing: use anyio, not asyncio
    • Do not use Test prefixed classes, use functions
    • Coverage: test edge cases and errors
    • New features require tests
    • Bug fixes require regression tests
    • IMPORTANT: The tests/client/test_client.py is the most well designed test file. Follow its patterns.
    • IMPORTANT: Be minimal, and focus on E2E tests: Use the mcp.client.Client whenever possible.
    • Coverage: CI requires 100% (fail_under = 100, branch = true).

      • Full check: ./scripts/test (~23s). Runs coverage + strict-no-cover on the default Python. Not identical to CI: CI also runs 3.10–3.14 × {ubuntu, windows}, and some branch-coverage quirks only surface on specific matrix entries.

      • Targeted check while iterating (~4s, deterministic):

        uv run --frozen coverage erase
uv run --frozen coverage run -m pytest tests/path/test_foo.py
uv run --frozen coverage combine
uv run --frozen coverage report --include='src/mcp/path/foo.py' --fail-under=0
UV_FROZEN=1 uv run --frozen strict-no-cover

        Partial runs can't hit 100% (coverage tracks tests/ too), so --fail-under=0 and --include scope the report. strict-no-cover has no false positives on partial runs — if your new test executes a line marked # pragma: no cover, even a single-file run catches it.

    • Coverage pragmas:
      • # pragma: no cover — line is never executed. CI's strict-no-cover fails if it IS executed. When your test starts covering such a line, remove the pragma.
      • # pragma: lax no cover — excluded from coverage but not checked by strict-no-cover. Use for lines covered on some platforms/versions but not others.
      • # pragma: no branch — excludes branch arcs only. coverage.py misreports the ->exit arc for nested async with on Python 3.11+ (worse on 3.14/Windows).
    • Avoid anyio.sleep() with a fixed duration to wait for async operations. Instead:
      • Use anyio.Event — set it in the callback/handler, await event.wait() in the test
      • For stream messages, use await stream.receive() instead of sleep() + receive_nowait()
      • Exception: sleep() is appropriate when testing time-based features (e.g., timeouts)
    • Wrap indefinite waits (event.wait(), stream.receive()) in anyio.fail_after(5) to prevent hangs

Test files mirror the source tree: src/mcp/client/streamable_http.py → tests/client/test_streamable_http.py Add tests to the existing file for that module.

  • For commits fixing bugs or adding features based on user reports add:

    git commit --trailer "Reported-by:<name>"

    Where <name> is the name of the user.

  • For commits related to a Github issue, add

    git commit --trailer "Github-Issue:#<number>"

  • NEVER ever mention a co-authored-by or similar aspects. In particular, never mention the tool used to create the commit message or PR.

Pull Requests

  • Create a detailed message of what changed. Focus on the high level description of the problem it tries to solve, and how it is solved. Don't go into the specifics of the code unless it adds clarity.

  • NEVER ever mention a co-authored-by or similar aspects. In particular, never mention the tool used to create the commit message or PR.

Breaking Changes

When making breaking changes, document them in docs/migration.md. Include:

  • What changed
  • Why it changed
  • How to migrate existing code

Search for related sections in the migration guide and group related changes together rather than adding new standalone sections.

Python Tools

Code Formatting

  1. Ruff

    • Format: uv run --frozen ruff format .
    • Check: uv run --frozen ruff check .
    • Fix: uv run --frozen ruff check . --fix
    • Critical issues:
      • Line length (88 chars)
      • Import sorting (I001)
      • Unused imports
    • Line wrapping:
      • Strings: use parentheses
      • Function calls: multi-line with proper indent
      • Imports: try to use a single line

  2. Type Checking

    • Tool: uv run --frozen pyright
    • Requirements:
      • Type narrowing for strings
      • Version warnings can be ignored if checks pass

  3. Pre-commit

    • Config: .pre-commit-config.yaml
    • Runs: on git commit
    • Tools: Prettier (YAML/JSON), Ruff (Python)
    • Ruff updates:
      • Check PyPI versions
      • Update config rev
      • Commit config first

Error Resolution

  1. CI Failures

    • Fix order:
      1. Formatting
      2. Type errors
      3. Linting
    • Type errors:
      • Get full line context
      • Check Optional types
      • Add type narrowing
      • Verify function signatures

  2. Common Issues

    • Line length:
      • Break strings with parentheses
      • Multi-line function calls
      • Split imports
    • Types:
      • Add None checks
      • Narrow string types
      • Match existing patterns

  3. Best Practices

    • Check git status before commits
    • Run formatters before type checks
    • Keep changes minimal
    • Follow existing patterns
    • Document public APIs
    • Test thoroughly

Exception Handling

  • Always use logger.exception() instead of logger.error() when catching exceptions
    • Don't include the exception in the message: logger.exception("Failed") not logger.exception(f"Failed: {e}")
  • Catch specific exceptions where possible:
    • File ops: except (OSError, PermissionError):
    • JSON: except json.JSONDecodeError:
    • Network: except (ConnectionError, TimeoutError):
  • FORBIDDEN except Exception: - unless in top-level handlers