fix: include implicit HEAD in allow header

fastapi/routing.py

@@ -1166,6 +1166,12 @@ class APIRoute(routing.Route):
             and "HEAD" not in route_methods
         )
 
+    def _allow_methods(self, methods: Collection[str]) -> set[str]:
+        allow_methods = set(methods)
+        if "GET" in allow_methods and "HEAD" not in allow_methods:
+            allow_methods.add("HEAD")
+        return allow_methods
+
     def matches(self, scope: Scope) -> tuple[Match, Scope]:
         effective_context = _get_scope_effective_route_context(scope)
         if effective_context is not None and effective_context.original_route is self:
@@ -1191,7 +1197,7 @@ class APIRoute(routing.Route):
                 and scope["method"] not in methods
                 and not self._is_head_for_get(scope, methods)
             ):
-                headers = {"Allow": ", ".join(methods)}
+                headers = {"Allow": ", ".join(self._allow_methods(methods))}
                 if "app" in scope:
                     raise HTTPException(status_code=405, headers=headers)
                 response = PlainTextResponse(
@@ -1206,6 +1212,19 @@ class APIRoute(routing.Route):
                 _effective_route_context_var.reset(token)
             await app(scope, receive, send)
             return
+        if (
+            self.methods
+            and scope["method"] not in self.methods
+            and not self._is_head_for_get(scope)
+        ):
+            headers = {"Allow": ", ".join(self._allow_methods(self.methods))}
+            if "app" in scope:
+                raise HTTPException(status_code=405, headers=headers)
+            response = PlainTextResponse(
+                "Method Not Allowed", status_code=405, headers=headers
+            )
+            await response(scope, receive, send)
+            return
         # Allow HEAD requests through to the GET handler.
         if self._is_head_for_get(scope):
             await self.app(scope, receive, send)

tests/test_auto_head_for_get.py

@@ -62,6 +62,20 @@ def test_head_not_in_openapi_schema():
     assert list(schema["paths"]["/items/{item_id}"].keys()) == ["get"]
 
 
+def test_allow_header_includes_implicit_head_for_get_route():
+    app = FastAPI()
+
+    @app.get("/items")
+    def read_items():
+        return []
+
+    client = TestClient(app)
+    response = client.post("/items")
+
+    assert response.status_code == 405
+    assert set(response.headers["allow"].split(", ")) == {"GET", "HEAD"}
+
+
 def test_head_not_added_to_non_get_routes():
     app = FastAPI()
 

tests/test_router_include_context.py

@@ -532,7 +532,7 @@ def test_included_partial_match_returns_405_when_no_later_full_match_exists():
     response = TestClient(app).post("/api/items")
 
     assert response.status_code == 405
-    assert response.headers["allow"] == "GET"
+    assert set(response.headers["allow"].split(", ")) == {"GET", "HEAD"}
 
 
 def test_included_slash_redirect_does_not_block_later_exact_match():
@@ -682,7 +682,10 @@ async def test_included_api_route_without_app_scope_returns_405_response():
 
     assert messages[0]["type"] == "http.response.start"
     assert messages[0]["status"] == 405
-    assert dict(messages[0]["headers"])[b"allow"] == b"GET"
+    assert set(dict(messages[0]["headers"])[b"allow"].decode().split(", ")) == {
+        "GET",
+        "HEAD",
+    }
 
 
 def test_effective_api_route_context_does_not_match_websocket_scope():