Motivation

Part of #1678

The Streamable HTTP test fixtures launch servers via multiprocessing.Process. Since coverage.py cannot track code in child processes by default, server-side code paths appear uncovered despite being exercised by tests, requiring # pragma: no cover workarounds in both test and source files.

Changes

Test refactoring

Replace multiprocessing.Process with in-process threading.Thread + uvicorn.Server for the three main server fixtures (basic_server, json_server, resumable_server). coverage.py tracks all threads in the same process by default, so these code paths are now properly covered.

• Add _start_server_thread(): launches uvicorn.Server on a daemon thread with graceful shutdown (server.should_exit = True)
• Remove dead run_server() function and unused traceback import
• Remove 8 # pragma: no cover markers from test server code
• Add 13 new tests covering previously-unreachable branches:
  • replay_events_after (non-existent event ID, None-message skipping)
  • slow:// resource read
  • long_running_with_checkpoints tool
  • Sampling with non-text content
  • multi_close_tool / tool_with_standalone_stream_close with and without event store
  • POST with invalid Content-Type (400 from transport security)
  • POST/GET/DELETE with mismatched session ID (404 from session manager)
  • PUT unsupported HTTP method (405)
• Remove unused http_client fixture

Source file pragma adjustment

Since the test servers now run in-process, coverage.py now tracks source code that was previously unreachable. This requires adjusting coverage annotations to satisfy both coverage --fail-under=100 and strict-no-cover:

• Remove # pragma: no cover from lines now deterministically covered
• Replace with # pragma: lax no cover on non-deterministic except handlers (e.g. race-condition-dependent BrokenResourceError, ClosedResourceError) — these are sometimes covered depending on thread timing; lax is excluded by coverage.py (via exclude_lines) but not flagged by strict-no-cover
• Add # pragma: no branch for branch partials that are never taken in tests (e.g. mcp_session_id checks that are always True in the stateful manager architecture)

Files modified (annotation changes only, no logic changes):

• src/mcp/server/streamable_http.py
• src/mcp/server/transport_security.py
• src/mcp/server/session.py

Results

• All 74 Streamable HTTP tests pass
• pyright 0 errors, ruff check + ruff format clean
• Coverage: 100.00%
• strict-no-cover: ✅ no wrongly marked lines