Test foundations (safety nets)

[maxisbey]

The safety net the core reworks rely on: pin today's behaviour so nothing can silently break it, and give every other ticket a way to write tests against the modern path. Zero dependencies — anyone can start today.

What's in it

• Legacy wire-protection golden tests: pin the 2025 stateful and stateless_http=True paths byte-for-byte (session lifecycle, GET stream, resumability, the literal HTTP 400 error bodies the current transport returns, and the behaviour when no MCP-Protocol-Version header is sent), plus negative checks that no 2026 vocabulary leaks onto a 2025 wire.
• In-memory / DirectDispatcher 2026 parity so other tickets can write end-to-end tests without HTTP.
• Add the modern sessionless path as a tests/interaction connect-fixture leg, so every existing transport-agnostic test re-proves over the 2026 wire mode.
• Optional: a public-API snapshot test (so accidental surface breaks are loud).

Conformance

• (test infrastructure — proven by the suite itself staying green through the core reworks)

Dependencies

• Depends on: none

References

• python-sdk tests/interaction
• expected-failures baseline

[maoxin1234]

Hi! I'd like to help with this. I've been reading through the 2026-07-28 spec tracking issue (#2891) and this test-foundations piece looks like a good place to start building safety nets before the larger refactors land.

Could a maintainer share which areas should be prioritized (e.g. transport layer, version negotiation, capabilities handling)? Happy to start with a focused PR on whichever sub-area is most useful and iterate from there.