REVIEWABLE PREVIEW · 15 MEASURED CHECKS

Starlette TestClient: unexpected keyword argument app

Reproduced TestClient error: Client.__init__() got an unexpected keyword argument 'app'. Starlette 0.37.2 passes ASGI checks with HTTPX 0.28.1.

What this material covers

Client.__init__() got an unexpected keyword argument 'app'

Starlette 0.36.3 → 0.37.2; HTTPX 0.28.1 unchanged

Historical compatibility regression executed in an isolated dependency venv on the measured Windows CPython 3.12 environment. Starlette 0.36.3 to 0.37.2 was the only package change; HTTPX remains 0.28.1. These old exact pins are for reproduction, not a current production upgrade recommendation. The in-process ASGI contract verifies lifespan, routing, middleware, JSON mutation/readback, rejection without mutation, background tasks, HTTP errors, server exception modes, and one WebSocket exchange. It does not verify a network server, production concurrency, FastAPI dependency constraints, authentication, or other platforms. A venv is not a security sandbox. Customer final verification remains required. This is engineering evidence, not external demand or AI-visit evidence.

Measured 2026-09-09T10:46:48.637045+00:00 · Windows / AMD64. Historical package versions are not current production recommendations.

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