fix: match vendor media type case-insensitively

fast_version/accept.py

@@ -42,7 +42,9 @@ def parse_accept_version(accept_header: str, vendor_media_type: str) -> AcceptVe
     except ValueError:
         return Ignore()
 
-    if media_type.strip() != vendor_media_type:
+    # Media types are case-insensitive (RFC 9110 8.3.1); the header side is already
+    # lowercased by get_accept_header_from_scope, so lowercase the vendor to match.
+    if media_type.strip() != vendor_media_type.lower():
         return Ignore()
 
     version_key = ""

planning/changes/2026-07-04.02-vendor-media-type-case-insensitive/change.md

@@ -0,0 +1,43 @@
+---
+summary: Match the vendor media type case-insensitively per RFC 9110/6838, so a mixed-case configured vendor no longer silently falls through to Ignore.
+---
+
+# Change: Case-insensitive vendor media-type matching
+
+**Lane:** lightweight — one-line behavior fix in `accept.py` plus one seam test;
+no new file, no public-API change.
+
+## Goal
+
+`parse_accept_version` compared the request media type (already lowercased by
+`get_accept_header_from_scope`) against `vendor_media_type` as configured. A
+mixed-case vendor (e.g. `application/vnd.Some.Name+json`) never matched and the
+request silently fell through to `Ignore`, defaulting to v1.0. Media types are
+case-insensitive (RFC 9110 §8.3.1, RFC 6838 §4.2), so the match must be too.
+Resolves the item recorded in [`../../deferred.md`](../../deferred.md).
+
+## Approach
+
+Lowercase the vendor in the comparison only:
+`media_type.strip() != vendor_media_type.lower()`. This is the robustness
+principle — liberal in what we accept (case-insensitive match), conservative in
+what we send: the response `content-type` still echoes the exact casing the user
+configured (that string is unchanged, so no emitted-header behavior changes).
+Normalizing at `init` was rejected because it would also rewrite the emitted
+content-type casing — a change beyond the bug, with no spec basis.
+
+## Files
+
+- `fast_version/accept.py` — `.lower()` on the vendor in the media-type check.
+- `tests/test_accept.py` — `test_parse_matches_vendor_case_insensitively`: a
+  mixed-case vendor matches a lowercased request media type → `ParsedVersion`.
+- `planning/deferred.md` — remove the now-resolved entry.
+
+## Verification
+
+- [x] Failing test first — `pytest tests/test_accept.py::test_parse_matches_vendor_case_insensitively`
+      → `AssertionError: Ignore() == ParsedVersion(version=(1, 0))`.
+- [x] Apply the `.lower()` fix.
+- [x] Test passes.
+- [ ] `just test-ci` — full suite green, coverage 100%.
+- [ ] `just lint-ci` — ruff + ty clean, `planning: OK`.

planning/deferred.md

@@ -2,9 +2,4 @@
 
 Real-but-unscheduled items, each with a revisit trigger.
 
-- **Vendor media-type case-sensitivity** — `parse_accept_version` compares a
-  lowercased header media-type against a non-lowercased `vendor_media_type`,
-  so a mixed-case vendor never matches (silently falls through to `Ignore`).
-  Revisit trigger: a user configures a vendor type with uppercase letters, or
-  we decide to guarantee case-insensitive media-type matching. Fix is a
-  failing `tests/test_accept.py` case plus a `.lower()` on the vendor.
+_(none)_

tests/test_accept.py

@@ -56,6 +56,14 @@ def test_parse_bad_version_format(version: str) -> None:
     assert result == ParseError(detail="Version should be in <major>.<minor> format")
 
 
+def test_parse_matches_vendor_case_insensitively() -> None:
+    # Media types are case-insensitive (RFC 9110 8.3.1, RFC 6838 4.2): a mixed-case
+    # configured vendor must still match the lowercased request media type.
+    header: typing.Final = f"{VENDOR}; version=1.0"
+    mixed_case_vendor: typing.Final = "application/vnd.Some.Name+json"
+    assert parse_accept_version(header, mixed_case_vendor) == ParsedVersion(version=(1, 0))
+
+
 def test_get_accept_header_from_scope_normalizes() -> None:
     scope: typing.Final = {"type": "http", "headers": [(b"accept", b"  Foo/Bar  ")]}
     assert get_accept_header_from_scope(scope) == "foo/bar"