Summary

Fixes #15503

Custom APIRoute subclasses that define an explicit __init__ matching the pre-strict_content_type parameter list crash with TypeError when APIRouter.add_api_route passes strict_content_type as a keyword argument. This regression was introduced when strict_content_type was added to APIRoute.__init__ and propagated through add_api_route.

Root Cause

APIRouter.add_api_route (line 1386 of fastapi/routing.py) unconditionally passes strict_content_type=get_value_or_default(...) to route_class(...). Custom APIRoute subclasses that defined constructors before strict_content_type was added don't accept this keyword, causing a TypeError at route registration time.

Fix

Wrap the route_class(...) call in a try-except that catches TypeError and retries without the strict_content_type keyword argument. This maintains backward compatibility with existing custom route classes while preserving the strict_content_type functionality for default APIRoute instances and subclasses that accept it.

Changes

• fastapi/routing.py: 12 lines modified (+10 -2) — wrap route_class() call with try-except fallback
• tests/test_custom_route_class.py: 82 lines added — LegacyAPIRoute class and regression test

Testing

• Custom route class without strict_content_type in constructor: router registers routes without TypeError ✅
• Default APIRoute routes: strict_content_type still passed and enforced ✅
• Existing custom route class tests: all 5 existing tests pass unchanged ✅