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API Error Handling Guidelines

Every REST error this server returns is an RFC 9457 application/problem+json body produced by a @RestControllerAdvice. A controller throws a meaningful exception. An advice class translates it into HTTP semantics and decides exactly what detail leaves the server. No controller formats an error itself, and no stack trace reaches a client.

One problem model​

A problem has the RFC 9457 members type, title, status and detail.

  • type is the only machine-readable code. A client branches on type, never on another member or on the detail text.
  • ProblemType (core/exception) is the registry of every type that a client can branch on. Each constant has a slug. The type is https://docs.hephaestus.build/contributor/api-error-handling#<slug>, which points to the slug in Problem types.
  • A problem without a distinct kind keeps type: about:blank. Its status is then its kind.
  • Other members carry named data, such as maxAgeSeconds, errors or withholdingReason. Do not add a second code member such as code, reason, kind, reasonCode or error.
  • Build a typed problem with ProblemType.X.problem(status, title, detail). For a ResponseStatusException, set getBody().setType(ProblemType.X.uri()).
  • A slug is part of the API. Renaming or removing one is a breaking change. Add the slug to Problem types in the same change.
  • The webapp reads the type through isProblemType and stepUpChallengeOf in webapp/src/lib/problem-detail.ts. It shows detail, or title when detail is absent.

How WorkspaceControllerAdvice works​

  1. @RestControllerAdvice(basePackages = "de.tum.cit.aet.hephaestus.workspace") scopes the advice to workspace endpoints only, but covers the entire workspace package (controllers for repositories, labels, lifecycle, etc.). One mapper handles every route in that package, which guarantees consistency.
  2. Each @ExceptionHandler method matches a domain exception (for example EntityNotFoundException) and returns a configured ProblemDetail object. Spring automatically serializes it to JSON with the proper status code.
  3. The private helpers sanitize exception messages through LoggingUtils before they return them. This prevents leaks of stack traces or SQL fragments. The server logs any unexpected IllegalStateException as a server error. Domain-specific violations, such as slug conflicts and lifecycle issues, have their own exception classes. Thus, the advice can return the correct HTTP semantics.
  4. Controllers simply throw exceptions (or let services throw) and never build ad-hoc Map.of("error", ...) responses again.

Rolling the pattern out elsewhere​

When you touch another REST controller:

  1. Identify the domain-specific exceptions the controller or underlying services already throw.
  2. Create a dedicated advice class in the same package. If multiple controllers share the same error model, use a shared module instead. Annotate it with @RestControllerAdvice(basePackages = "your.package"). Thus, every controller in that bounded context reuses the mapper.
  3. Map exceptions to HTTP semantics:
    • Validation or invariant violations → 400 Bad Request
    • Missing resources → 404 Not Found
    • Conflicts (duplicates, state transitions) → 409 Conflict
    • Authorization issues should still bubble up to Spring Security so we get 401/403 uniformly.
  4. Return ProblemDetail everywhere. Include a user-actionable title plus a detail sourced from the exception where it is safe to expose. Give a problem a ProblemType only when a client must tell it apart from other problems with the same status.
  5. Document the mapping (update this file or the feature README) so clients know what to expect.

Shared advice for multiple controllers​

If controllers reuse the same exception hierarchy, point @RestControllerAdvice at the shared base package. Git provider controllers are an example. Every endpoint in that bounded context then inherits the mapper. Keep advice classes focused. Too many unrelated handlers in one class make maintenance difficult.

Validation errors deserve structure, too​

Spring surfaces method-argument validation failures as MethodArgumentNotValidException (body binding) or ConstraintViolationException (query/path parameters). Both become structured JSON, so a client can highlight the field that failed rather than print a sentence. See RFC 9457 for the media type. Our convention:

  • Use the same advice class to @ExceptionHandler both exception types.
  • Return ProblemDetail with title = "Validation failed", status = 400, and a short descriptive detail.
  • Attach a machine-friendly errors map through problem.setProperty("errors", …). Each key is the field or parameter that failed. We remove method prefixes so only the field name remains. Each value is a list of human-readable messages.

Path-variable validation​

Slug parameters across WorkspaceController use the shared @WorkspaceSlug constraint, which WorkspaceSlugValidator implements. The controller carries @Validated. Invalid slugs raise a ConstraintViolationException. The advice handles this exception and returns the same errors map shape as body validation failures.

Testing expectations​

  • Add WebTestClient or MockMvc tests that trigger the exception path and assert the ProblemDetail body.
  • Cover the advice indirectly through integration tests so we catch serialization regressions when upgrading Spring Boot.

Problem types​

Each heading is the slug at the end of a problem type.

agent-binding-slot-mismatch​

The selected model does not fill this agent binding row. declaredTier names the model's data-handling tier.

agent-job-state-conflict​

The agent job is not in a state that allows the request.

concurrent-modification​

Another request changed the record first. Repeat the request.

csrf-token-rejected​

The CSRF filter refused the request before the endpoint ran. Both workspace address modes use this problem response. Fetch a new token once, then retry the request once.

feedback-withheld​

The publication context does not permit approval. withholdingReason names the reason and state the feedback state.

llm-call-pool-in-use​

A model still uses the call pool.

llm-call-pool-slug-conflict​

Another call pool has this slug.

llm-connection-in-use​

A model or binding still uses the LLM connection.

llm-connection-slug-conflict​

Another LLM connection has this slug.

llm-model-in-use​

A binding still uses the LLM model.

llm-model-slug-conflict​

Another LLM model has this slug.

llm-model-upstream-id-conflict​

Another LLM model on this connection has this upstream id.

mentor-lease-lost​

A Hephaestus host wrote for a turn attempt that lost its lease.

mentor-refused​

Hephaestus does not reply to this member. The detail names the setting that changes it.

mentor-rejected​

The conversation refused a write that conflicts with its state.

oauth-callback-failed​

An OAuth callback failed. callbackError names the failure with the same value as the browser redirect's reason.

outline-rate-limited​

Outline limits the requests. Retry-After tells when to try again.

outline-unreachable​

The server could not reach Outline.

passkey-required​

The action needs a passkey verification.

review-batch-conflict​

The request conflicts with a review batch that is already open.

review-schedule-conflict​

The request conflicts with the current review schedule.

step-up-required​

The action needs a recent sign-in. maxAgeSeconds gives the allowed age of the sign-in.

sync-not-supported​

This connection does not support a manual sync.

unknown-outline-collection​

Outline does not know the collection.

workspace-budget-exceeded​

The sandbox workspace is larger than its byte budget. workspaceBytes and workspaceByteBudget give the sizes.