Mission Rebuild the capability of Phase 13 without following the exact code line-by-line. The goal is transfer: can you use the ideas when the variable names, UI, or route shape changes?
Required outcome Create a stable application error model that separates expected domain/transport failures from unexpected faults and keeps diagnostics observable.
Required artifact Error taxonomy + NestJS mapping layer/filters + structured error response with request IDs.
Constraints • Start from the previous phase checkpoint or a fresh branch. • Use the course architecture boundaries, but do not copy the reference implementation until after your first attempt. • Keep changes scoped to the capability. • Preserve validation, authorization, error handling, and observability expectations already earned in earlier phases. • Record any design decision where more than one implementation is reasonable.
Acceptance criteria • Can distinguish expected vs unexpected errors. • Can map failures centrally. • Can trace a request by ID. • Can design frontend recovery from stable error codes.
Independent build sequence 1. Inventory current TaskFlow failures and group them into validation, auth, forbidden, missing, conflict, infrastructure, and unexpected categories. 2. Create explicit application errors for expected business outcomes rather than throwing arbitrary strings. 3. Map errors to one stable HTTP envelope at a central boundary. 4. Add request/correlation IDs to responses and logs. 5. Trigger each error class intentionally and verify both client-safe response and server diagnostic context. 6. Prove an unexpected exception returns a generic 500 while the real stack/context remains in logs.
Self-review Compare your implementation to the reference only after the acceptance criteria pass. Differences are not automatically mistakes. For each difference, ask: does it preserve the same contract, security boundary, correctness, maintainability, and operational behavior? Keep the alternative when you can justify it with evidence.
Stretch challenge Change one assumption: more users, a second tenant, a slow network, an expired session, a concurrent mutation, a deployment restart, or a failed dependency. Predict whether your design still behaves correctly. If not, document the next architectural improvement rather than adding complexity blindly.
Phase exit Run the relevant checks, inspect Git diff, and write a three-sentence summary: what the system can do now, which boundary owns the new responsibility, and which failure you now know how to diagnose. Only then mark the phase complete.
Phase Synthesis & Independent Challenge 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 “Phase Synthesis & Independent Challenge” 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: Error taxonomy + NestJS mapping layer/filters + structured error response with request IDs.. 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 “Phase Synthesis & Independent Challenge” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Inventory current TaskFlow failures and group them into validation, auth, forbidden, missing, conflict, infrastructure, and unexpected categories.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Returning raw exception messages/stacks
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.