tests/interaction: era-axis machinery for the requirements manifest

tests/interaction/README.md

@@ -95,6 +95,14 @@
   would require real-time waits the suite refuses.
 - **`transports`** names the transports a behaviour applies to; omitted means transport-independent.
 - **`issue`** carries the tracking link for a recorded gap once one is filed.
+- **`note`** carries free-form context that does not fit `divergence` or `deferred`.
+- **`added_in`** / **`removed_in`** bound the spec versions the behaviour exists in, as a half-open
+  `[added_in, removed_in)` window.
+- **`supersedes`** / **`superseded_by`** link a retired entry to its replacement; the link is
+  bidirectional and both ends must be versioned.
+- **`arm_exclusions`** carve specific `(transport, spec_version)` matrix cells out with a typed
+  `ArmExclusionReason`.
+- **`known_failures`** mark specific `(transport, spec_version)` cells as strict xfail.
 
 Tests link themselves to the manifest with a decorator:
 
@@ -126,15 +134,51 @@
 divergence note (the rewrite accidentally fixed a known gap — decide whether to keep the fix) or
 it does not (the rewrite broke something that was correct — fix the rewrite).
 
-### When a new spec revision is released
-
-1. Update `SPEC_REVISION` and walk the new revision's changelog.
-2. For each changed interaction, find its requirements (the IDs use the wire method strings the
-   changelog speaks in), re-audit the tests against the new text, and update `source` links and
-   assertions where behaviour legitimately changed.
-3. New interactions get new requirements and new tests; removed interactions get their
-   requirements deleted along with their tests.
-4. A behaviour that is correct under both revisions needs no change beyond the `source` link.
+### Spec versions and the era axis
+
+`SPEC_VERSIONS` in `_requirements.py` is the ordered tuple of protocol revisions the suite
+exercises; `SPEC_REVISION = SPEC_VERSIONS[-1]` is the newest. The `connect` fixture fans out over
+`CONNECTABLE_TRANSPORTS × SPEC_VERSIONS`, but the grid is filtered per test:
+`pytest_generate_tests` reads the test's stacked `@requirement` marks and calls `compute_cells()`,
+which intersects the admissible cells across every cited requirement — a cell survives only if
+**all** of the test's requirements admit it.
+
+`streamable-http-stateless` is the fourth connectable transport: the 2025-era unofficial stateless
+mode where each request opens a fresh transport, no session id is issued, and there is no standalone
+GET stream. Requirements that need a server→client back-channel or persisted session state are
+excluded from that arm via `arm_exclusions` (reasons `server-initiated-request` and
+`requires-session`).
+
+What admits or excludes a cell:
+
+- **`added_in` / `removed_in`** gate which spec versions a requirement exists in, as a half-open
+  `[added_in, removed_in)` window. A test runs only on versions inside every cited requirement's
+  window.
+- **`arm_exclusions`** carve specific `(transport, spec_version)` cells out with a typed
+  `ArmExclusionReason`. The reason vocabulary doubles as a re-admission checklist: when the gap
+  closes, grep for the reason string to find every cell to re-admit.
+- **`known_failures`** keep a cell in the grid but mark it as a strict xfail — the test runs and
+  must fail; an unexpected pass fails the suite.
+- **`transports`** is descriptive metadata for the non-`connect` transport-specific suites under
+  `transports/` and does **not** drive cell generation. Only `arm_exclusions`, `added_in`, and
+  `removed_in` filter the grid.
+- **`supersedes` / `superseded_by`** link a retired entry to its replacement. `test_coverage.py`
+  enforces that links are bidirectional and versioned: the retired entry carries `removed_in`, the
+  replacement carries `added_in`.
+
+Node IDs stay `[transport]` while `len(SPEC_VERSIONS) == 1`, so today's test IDs are
+byte-identical to before the era axis existed. They become `[transport-version]` the moment a
+second version is appended to `SPEC_VERSIONS`.
+
+When a new spec revision lands:
+
+1. Append the version string to `SPEC_VERSIONS` (and to the `SpecVersion` `Literal`).
+2. Walk the new revision's changelog.
+3. For each affected requirement: set `removed_in` on retired behaviour, add a new entry with
+   `added_in` for its replacement, and link the pair with `supersedes` / `superseded_by`.
+   Behaviour that survives unchanged needs nothing beyond a re-audit of its `source` URL.
+4. For requirements that cannot run on the new era's path, add an `arm_exclusions` entry with the
+   appropriate `ArmExclusionReason`.
 
 ## Writing a test
 

tests/interaction/_connect.py

@@ -9,6 +9,7 @@
 
 from collections.abc import AsyncIterator, Awaitable, Callable, Iterable
 from contextlib import AbstractAsyncContextManager, asynccontextmanager
+from functools import partial
 from typing import Any, Protocol
 
 import httpx
@@ -69,6 +70,7 @@ def __call__(
         message_handler: MessageHandlerFnT | None = None,
         client_info: Implementation | None = None,
         elicitation_callback: ElicitationFnT | None = None,
+        protocol_version: str = LATEST_PROTOCOL_VERSION,
     ) -> AbstractAsyncContextManager[Client]: ...
 
 
@@ -83,6 +85,7 @@ async def connect_in_memory(
     message_handler: MessageHandlerFnT | None = None,
     client_info: Implementation | None = None,
     elicitation_callback: ElicitationFnT | None = None,
+    protocol_version: str = LATEST_PROTOCOL_VERSION,
 ) -> AsyncIterator[Client]:
     """Yield a Client connected to the server over the in-memory transport."""
     async with Client(
@@ -113,13 +116,15 @@ async def connect_over_streamable_http(
     message_handler: MessageHandlerFnT | None = None,
     client_info: Implementation | None = None,
     elicitation_callback: ElicitationFnT | None = None,
+    protocol_version: str = LATEST_PROTOCOL_VERSION,
 ) -> AsyncIterator[Client]:
     """Yield a Client connected to the server's streamable HTTP app, entirely in process.
 
-    With the defaults this is the matrix leg (stateful sessions, SSE responses); the
-    transport-specific tests pass `stateless_http` or `json_response` to select the other
-    server modes, and the resumability tests pass an `event_store` (with `retry_interval=0` so
-    the client's reconnection wait is a no-op).
+    With the defaults this is the matrix leg (stateful sessions, SSE responses); the stateless
+    matrix arm binds `stateless_http=True` (see `connect_over_streamable_http_stateless`);
+    transport-specific tests pass `json_response` to select the other server mode, and the
+    resumability tests pass an `event_store` (with `retry_interval=0` so the client's
+    reconnection wait is a no-op).
     """
     app = server.streamable_http_app(
         stateless_http=stateless_http,
@@ -145,6 +150,12 @@ async def connect_over_streamable_http(
         yield client
 
 
+connect_over_streamable_http_stateless: Connect = partial(connect_over_streamable_http, stateless_http=True)
+"""The streamable-http matrix arm with the server in stateless mode (fresh transport per request,
+no session id, no standalone GET stream). The same shared Server instance backs every request --
+stateless mode does not require a server factory."""
+
+
 @asynccontextmanager
 async def mounted_app(
     server: Server | MCPServer,
@@ -326,6 +337,7 @@ async def connect_over_sse(
     message_handler: MessageHandlerFnT | None = None,
     client_info: Implementation | None = None,
     elicitation_callback: ElicitationFnT | None = None,
+    protocol_version: str = LATEST_PROTOCOL_VERSION,
 ) -> AsyncIterator[Client]:
     """Yield a Client connected to the server's legacy SSE transport, entirely in process."""
     app, _ = build_sse_app(server)