The HTTP transport seeded `MCP-Protocol-Version` from LATEST_PROTOCOL_VERSION,
which on mcp 2.x is 2026-07-28 — a revision that replaced the `initialize`
handshake with a per-request envelope. But this transport connects through
`ClientSession.initialize()`, which sends LATEST_HANDSHAKE_VERSION (2025-11-25)
in the body. Header and body therefore disagreed by construction, and a
conforming 2.x server honours the header: it routed the request onto its
per-request-envelope ladder and rejected the legacy body with
params._meta is missing the required envelope key(s):
io.modelcontextprotocol/protocolVersion,
io.modelcontextprotocol/clientCapabilities
Observed against a live MCP endpoint, and confirmed by probing the same
endpoint three ways: the header at 2026-07-28 is rejected, at 2025-11-25 it
succeeds, and with no header at all it succeeds.
Third defect in this migration from one cause: the 2.x bump changed what an
existing constant *means* without revisiting its uses. The header seed was
written when LATEST_PROTOCOL_VERSION was 2025-03-26 and was correct then.
Seeded from LATEST_HANDSHAKE_VERSION, imported with a fallback to
LATEST_PROTOCOL_VERSION for SDKs predating the split, where the two are the
same thing and header and body agree either way. An explicitly configured
header still wins — that override is why servers demanding a specific revision
can have one, and a test pins it.
`streamable_http_client` yields `(read, write, get_session_id)` on mcp 1.x and
`(read, write)` on 2.x. `_run_http` unpacked a fixed 3-tuple, so on 2.x every
HTTP and SSE MCP server failed its handshake with `ValueError: not enough
values to unpack (expected 3, got 2)` and parked after exhausting its retry
ladder. Only stdio servers kept working.
This is the same defect as the import gating fixed earlier in this branch, one
layer further in. That fix's own comment claimed reaching
`streamable_http_client` was "the path that does work" — reaching it was
necessary and not sufficient, and the comment asserted the half that was never
exercised. Corrected along with the code.
Unpacked positionally rather than by arity, since this file deliberately
supports both SDK generations and `get_session_id` was never used here.
The reason this survived review is worth the test it now has: the existing
coverage in test_mcp_client_cert.py fakes the transport with a 3-tuple, so it
encoded 1.x's shape into the assertion and passed on 2.x regardless. The new
test drives `_run_http` once per arity the supported SDK range actually yields,
and asserts the streams handed to ClientSession are the first two — positional,
because 1.x's third element is not a stream. Verified it fails on the 2.x case
without this change.
Found while pointing a real HTTP MCP server at a live deployment running this
branch: the server parked at startup and no tool from it ever registered.
elphamale