@@ -137,8 +137,10 @@ it does not (the rewrite broke something that was correct — fix the rewrite).
 ### 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:
+exercises. `SPEC_BASE_URL` (and `SPEC_2026_BASE_URL`) are pinned literals — not derived from
+`SPEC_VERSIONS` — so growing the active axis never repoints existing `source` links. 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.
 
@@ -48,8 +48,13 @@
 SPEC_VERSIONS: tuple[SpecVersion, ...] = ("2025-11-25",)
 """The active spec-version matrix axis, ordered oldest to newest. Every entry must be in KNOWN_PROTOCOL_VERSIONS."""
 
-SPEC_REVISION = SPEC_VERSIONS[-1]
-SPEC_BASE_URL = f"https://modelcontextprotocol.io/specification/{SPEC_REVISION}"
+SPEC_BASE_URL = "https://modelcontextprotocol.io/specification/2025-11-25"
+"""Deep-link base for entries citing the 2025-11-25 revision (the bulk of the manifest). Pinned --
+not derived from SPEC_VERSIONS -- so adding a newer revision to the active axis does not silently
+repoint existing source URLs."""
+
+SPEC_2026_BASE_URL = "https://modelcontextprotocol.io/specification/2026-07-28"
+"""Deep-link base for entries citing the 2026-07-28 revision."""
 
 Transport = Literal["in-memory", "stdio", "streamable-http", "streamable-http-stateless", "sse"]
 
 
@@ -20,6 +20,8 @@
 from tests.interaction._requirements import (
     CONNECTABLE_TRANSPORTS,
     REQUIREMENTS,
+    SPEC_2026_BASE_URL,
+    SPEC_BASE_URL,
     SPEC_VERSIONS,
     ArmExclusion,
     KnownFailure,
@@ -116,6 +118,12 @@ def test_spec_versions_are_known_and_include_latest() -> None:
     assert LATEST_PROTOCOL_VERSION in SPEC_VERSIONS
 
 
+def test_spec_base_urls_are_pinned_to_their_revision() -> None:
+    """SPEC_BASE_URL constants are pinned literals, so growing SPEC_VERSIONS cannot repoint existing source links."""
+    assert SPEC_BASE_URL == "https://modelcontextprotocol.io/specification/2025-11-25"
+    assert SPEC_2026_BASE_URL == "https://modelcontextprotocol.io/specification/2026-07-28"
+
+
 def test_connectable_transports_match_connect_factories() -> None:
     """CONNECTABLE_TRANSPORTS and the conftest factory map name exactly the same transports."""
     assert set(CONNECTABLE_TRANSPORTS) == set(_FACTORIES)
@@ -260,8 +268,8 @@ def _req(
 
 
 def test_compute_cells_with_no_requirements_yields_full_grid() -> None:
-    """An empty requirement list yields one cell per connectable transport at the single active spec version."""
-    cells = compute_cells([])
+    """With a single-version axis, an empty requirement list yields one cell per connectable transport."""
+    cells = compute_cells([], spec_versions=("2025-11-25",))
     assert [c.id for c in cells] == ["in-memory", "sse", "streamable-http", "streamable-http-stateless"]
     assert [c.values for c in cells] == [
         (("in-memory", "2025-11-25"),),
@@ -301,7 +309,10 @@ def test_compute_cells_drops_era_locked_transport_outside_its_versions() -> None
 
 def test_compute_cells_honours_arm_exclusion_from_any_stacked_requirement() -> None:
     """An arm exclusion on any stacked requirement drops the matching cell even when other requirements have none."""
-    cells = compute_cells([_req(), _req(arm_exclusions=(ArmExclusion(reason="requires-session", transport="sse"),))])
+    cells = compute_cells(
+        [_req(), _req(arm_exclusions=(ArmExclusion(reason="requires-session", transport="sse"),))],
+        spec_versions=("2025-11-25",),
+    )
     assert [c.id for c in cells] == ["in-memory", "streamable-http", "streamable-http-stateless"]
 
 
@@ -313,7 +324,10 @@ def test_compute_cells_wildcard_arm_exclusion_drops_every_cell() -> None:
 
 def test_compute_cells_marks_known_failure_as_strict_xfail() -> None:
     """A known failure attaches a strict xfail mark to exactly the matching cell and leaves others unmarked."""
-    cells = compute_cells([_req(known_failures=(KnownFailure(note="broken on sse", transport="sse"),))])
+    cells = compute_cells(
+        [_req(known_failures=(KnownFailure(note="broken on sse", transport="sse"),))],
+        spec_versions=("2025-11-25",),
+    )
     by_id = {c.id: c for c in cells}
     assert set(by_id) == {"in-memory", "sse", "streamable-http", "streamable-http-stateless"}
     assert by_id["sse"].marks[0].name == "xfail"
@@ -325,21 +339,21 @@ def test_compute_cells_marks_known_failure_as_strict_xfail() -> None:
 
 def test_compute_cells_wildcard_known_failure_marks_every_cell() -> None:
     """A known failure with both transport and spec_version unset marks every emitted cell as strict xfail."""
-    cells = compute_cells([_req(known_failures=(KnownFailure(note="all broken"),))])
+    cells = compute_cells([_req(known_failures=(KnownFailure(note="all broken"),))], spec_versions=("2025-11-25",))
     assert len(cells) == 4
     assert all(c.marks[0].name == "xfail" for c in cells)
     assert all(c.marks[0].kwargs == {"reason": "all broken", "strict": True} for c in cells)
 
 
 def test_compute_cells_ignores_transports_field() -> None:
     """Requirement.transports is descriptive metadata only and does not filter the cell grid."""
-    cells = compute_cells([_req(transports=("stdio",))])
+    cells = compute_cells([_req(transports=("stdio",))], spec_versions=("2025-11-25",))
     assert [c.id for c in cells] == list(CONNECTABLE_TRANSPORTS)
 
 
 def test_cell_id_omits_version_when_single_spec_version() -> None:
-    """With one active spec version the cell id is just the transport name, keeping today's node ids byte-identical."""
-    assert cell_id("sse", "2025-11-25") == "sse"
+    """With a single-version axis the cell id is just the transport name."""
+    assert cell_id("sse", "2025-11-25", spec_versions=("2025-11-25",)) == "sse"
 
 
 def test_cell_id_appends_version_when_multiple_spec_versions() -> None: