Copilot generated:
The Architect notes the ['run', '${file}'] config literal now lives in two places (here and dynamicdebugConfigurationService.ts), and the ['run'] default is encoded both in tryResolveFastApiArgs and as ?? ['run'] in the dynamic caller. Hoist to a shared constant / small helper to prevent drift. Low priority.

[verified]

I tried this but extracting ['run', '${file}'] into a readonly string[] constant required spreading at every call site ([...FASTAPI_RUN_FILE_ARGS]), which was longer than the inline literal.

Copilot generated:
Unresolved from prior review (Medium): path.relative emits OS-native separators, so a single subdir match persists "args": ["run", "backend\\app\\main.py"] on Windows. A launch.json shared to macOS/Linux then runs fastapi run backend\app\main.py, which treats backslashes literally and fails despite correct detection. The Skeptic notes the central fix actually broadened exposure here (root single-matches previously emitted separator-free ['run']). Normalize to POSIX in the persisted arg (e.g. relative.split(path.sep).join('/')) and confirm fastapi run accepts forward slashes on Windows.

[verified]

Savannah Ostrowski (@savannahostrowski) what did you think of this idea? Does fastapi work with forward slashes on Windows?

Copilot generated:
The Skeptic notes the nested-match expectations use path.join('backend','app','main.py'), which produces the resolver's own OS-specific output on every platform — the separator assertion is tautological and will never catch the portability bug above. Once the persisted arg is normalized to POSIX, assert the literal 'backend/app/main.py' instead.

[verified]

Of course that other change would affect this test if you modified it to use forward slashes.

module: 'fastapi' runs python -m fastapi run …, which requires fastapi-cli (fastapi[standard]) or FastAPI ≥ 0.111. The previous module: 'uvicorn' worked with a bare uvicorn install, so environments that debugged fine before can now fail immediately with No module named fastapi.__main__. Consider detecting CLI availability or surfacing an actionable message, and document the minimum FastAPI/fastapi-cli requirement.

path.relative yields OS-native separators (e.g. backend\app\main.py on Windows) that get baked into args and persisted to launch.json. A committed launch.json should be portable; consider normalizing to POSIX separators (fastapi run accepts / on Windows) so a shared config works cross-platform.

Multi-root workspace edge case: getFastApiPaths uses workspace-wide workspace.findFiles, so tryResolveFastApiArgs(folder, paths) can compute path.relative(folder, fileInAnotherFolder) → ..\otherFolder\main.py, which is then passed straight to fastapi run and points outside the folder. Consider filtering detected paths to the target folder, or treating cross-folder matches as "not a single match" and falling back to plain run.

The "FastAPI File" (${file}) variant is only pushed inside if (fastApiPaths?.length), so a user whose app doesn't match the app\s*=\s*FastAPI\( regex (e.g. application = FastAPI(), a factory pattern, or app in __init__.py) gets neither config — even though fastapi run ${file} would work on their open file. The ${file} variant doesn't depend on project detection; consider offering it independently of the detection gate.

tryResolveFastApiArgs returns undefined for BOTH "0 matches" and ">1 matches", and the two callers resolve that ambiguity differently (dynamic provider → ?? ['run']; interactive builder → prompt). This is intentional and tested, but undocumented. Add a one-line comment stating it returns args only for an unambiguous single match and that callers decide how to handle 0-or-many, so a future maintainer doesn't "fix" it and break one path.

The prefill computed at fastapiLaunch.ts (path.relative(workspaceRoot, fastApiPaths[0].fsPath)) is never asserted — the multi-match test stubs showInputBox but doesn't check the value (prefill) argument, so a wrong prefill would pass CI. Also untested: the telemetry fields autoDetectedFastAPIMainPyPath/manuallyEnteredAValue and the state.folder === undefined branch. Consider adding assertions for the prefill value and telemetry payload.