401 Error Semantics Making authentication failures predictable and safe
Course principle: a user ID becomes trusted only after the access token has been cryptographically verified by the backend.
Authentication failure matrix Scenario HTTP Public meaning
Do not leak verification internals A client does not need to know whether signature verification failed because of a key ID, algorithm, or JWK cache detail. Keep precise diagnostics in safe server logs and return a stable public 401 contract.
The guard can throw UnauthorizedException. The existing global exception filter then adds TaskFlow fields such as requestId, timestamp, path, and stable error code. { "statusCode": 401, "code": "UNAUTHORIZED", "message": "Authentication required.", "requestId": "42", "path": "/tasks", "timestamp": "2026-09-18T..." }
Frontend behavior • 401: refresh/re-establish session or redirect to login.
• 403: show an authorization/permission message; logging in again may not help.
• Network failure: do not mislabel it as authentication failure.
401 and 403 must remain distinct. Authentication and authorization are different security decisions.
Source note: verified against current Supabase Auth/getClaims/signing-key guidance and NestJS authentication/authorization docs. Full URLs and version notes are included in SOURCE_NOTES.txt.
401 Error Semantics and Client Recovery matters because it changes a real boundary in TaskFlow. The goal is to understand the inputs, owner, observable output, and failure modes before adding more abstraction.
Restate the goal of “401 Error Semantics and Client Recovery” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: failure → typed/domain error → exception mapping → HTTP envelope → client handling. Mark the one boundary this lesson changes.
Inspect the current repository state and record the exact files, route, table, or runtime involved. Avoid creating an abstraction before you can name the existing owner.
Implement the smallest vertical slice. Add one coherent change, run the narrowest verification command, and read the output before continuing.
Connect the slice to TaskFlow’s phase artifact: Authentication guard + verification service + current-user helper + protected /auth/me and task routes.. Verify behavior from the outside as a user or API client would.
Create one controlled failure that is safe to reproduce. Predict the symptom first, capture evidence, locate the earliest broken boundary, then fix only that cause.
Close the reference and rebuild or explain the core change from memory. Record what you would monitor or test before trusting the same change in production.
{
"error": {
"code": "TASK_NOT_FOUND",
"message": "Task was not found",
"requestId": "req_..."
}
}Do not copy this as a finished answer. Predict what each boundary does, type the smallest useful slice, run it, then compare the result with your prediction.
Return to the nearest observable boundary: input, network, process log, database, or rendered UI. Compare actual evidence with the predicted path.
Change one variable at a time and reproduce the original failure after the fix; otherwise you have correlation, not diagnosis.
Without copying the final source, create a second small scenario that applies “401 Error Semantics and Client Recovery” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Add Authorization: Bearer token handling in the API client without putting tokens in URLs/logs.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Returning 403 when no valid authentication exists
Predict the symptom before causing the failure. Then locate the earliest boundary where reality diverges from expectation.Why does this responsibility live here, what input does it trust, what output proves success, and what would go wrong if another layer owned it?
If your explanation is vague, repeat the boundary trace before moving on.Close the reference. Reproduce the core idea from memory or explain the exact request/data flow aloud. Then check your diff or notes for accidental shortcuts that bypass validation, authorization, error handling, or verification earned in earlier phases.