Error Taxonomy & API Contract Expected failures, unexpected faults, HTTP semantics, machine codes, and public error design
Phase 13 - Professional Error Handling Volume 04 - Professional Backend
Architecture milestone Failure occurs
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Phase 13 creates one professional exception boundary and a correlation story that spans client response and server logs.
Not every failure is the same Professional error handling begins by classifying failures before choosing a status code or message. Treating every exception as a generic 500 makes the API hard to use. Treating every failure as a client mistake hides real server faults.
Invalid request blank title, malformed UUID 400 with validation details
Unexpected fault bug, unavailable dependency, unknown exception 500 sanitized to client
Figure 1 - Expected failure vs unexpected fault Request enters system
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Can the application explain the failure safely?
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A client-facing API should expose actionable contract information without exposing server internals.
Status code is not the whole contract Two 404 responses can mean different application situations. A stable application error code lets clients distinguish them without parsing English text.
{ "statusCode": 404, "code": "TASK_NOT_FOUND", "message": "Task ... was not found.", "requestId": "7", "timestamp": "2026-09-18T...Z", "path": "/tasks/..." }
statusCode HTTP semantics; proxies, clients and tooling understand it
code Stable machine-readable application meaning
message Human-readable explanation
details Optional structured validation information
requestId Correlation between client failure and server log
timestamp When the server produced the error
path Which endpoint failed
Frontend code should branch on statusCode or code, not on exact English wording such as message.includes("not found"). Messages can change for clarity; stable codes are the contract.
Error handling is a boundary concern Business services should explain domain failures. The global exception boundary should format those failures for HTTP. Database adapters should not dictate the public API. This separation prevents infrastructure-specific error text from becoming a client contract.
Error Taxonomy and API Contract 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 “Error Taxonomy and API Contract” 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 “Error Taxonomy and API Contract” 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.