The SSE and StreamableHTTP security test files have been flaking in CI. Two recent examples: test_sse_security_custom_allowed_hosts failing with 421 for an explicitly allowed host, and test_idle_session_is_reaped failing with 406 instead of 404 on Windows.

Motivation and Context

The security tests had a port-allocation race. Each test picked an ephemeral port by bind-then-close, spawned a uvicorn subprocess to re-bind it, and polled until the port accepted connections. Under pytest-xdist, two workers can pick the same port in that window: the second server fails to bind (silently — log_level="error" in a subprocess), the readiness poll succeeds against the other worker's server, and the test asserts against a server configured with different security settings. That's how a test that allows custom.host observes a 421.

The fix removes the network entirely: both files now drive the same Starlette apps in process through the interaction suite's StreamingASGITransport (re-exported from tests.interaction.transports as the sanctioned import point for code outside that suite). No sockets, no subprocesses, no ports to race over. Assertions are unchanged — the diff is a harness swap. Each file also drops from seconds of subprocess churn to ~0.25s.

The idle-reap test had a timing race. It slept 0.1s after a 0.05s idle timeout and asserted the session was gone — on a slow runner the reaper hasn't fired yet, the request routes into the still-live transport, and the missing Accept header yields the 406. Polling for the 404 instead would never converge: each request to a live session pushes its idle deadline forward. The test now waits (bounded by anyio.fail_after(5)) on the manager's own "idle timeout" log record, which is emitted synchronously with the session being unregistered — once observed, the 404 is guaranteed.

One src change: the new in-process GET test covers the validation-failure return in _handle_get_request, so the now-stale # pragma: no cover on that line is removed (the success path was already driven in process by the interaction suite).

Two subtleties worth knowing as a reviewer:

• The SSE rejection path sends a second http.response.start through the bridge (connect_sse sends the 421/403 itself, then the handler's trailing Response() is sent by Starlette). This is deterministic because anyio's MemoryObjectSendStream.send() checkpoints on the non-empty error body before the overwrite can happen (holds down to the anyio>=4.9 floor); there's a comment at the spot documenting the assumption.
• The SSE client uses cancel_on_close=False so the bridge drains the app's disconnect handling instead of cancelling it, matching how the interaction suite's own SSE leg uses the bridge.

tests/shared/test_sse.py, tests/shared/test_streamable_http.py, and tests/client/test_http_unicode.py still use the subprocess pattern and are follow-up candidates.

How Has This Been Tested?

• ./scripts/test green: 1526 passed, 100% line+branch coverage, strict-no-cover clean
• The three changed files stress-tested heavily: thousands of executions across repeated sequential runs, pytest-xdist at several -n values, shuffled test orders, and under sustained CPU load — zero failures
• The deflaked idle-reap test passed 50/50 consecutive solo runs

Breaking Changes

None — test-only, plus one pragma removal in src.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Test names keep their existing feature-label style (rather than the interaction suite's behaviour-sentence names) so the "assertions unchanged" claim stays auditable in the diff; happy to rename in a follow-up if preferred.

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