Expose structured generation errors to consumers

engine.go

@@ -69,8 +69,9 @@ var (
 	// request, an LLM or provider rate-limit response, or ErrLLMGenerate.
 	ErrCapacityExceeded = errors.New("backend capacity exceeded")
 	// ErrLLMGenerate identifies a model-client failure or a nil successful
-	// response. Errors returned by an injected LLMClient remain available
-	// through errors.Is.
+	// response. A built-in OpenAI-compatible non-2xx response is available as a
+	// [GenerationError]. Errors returned by an injected LLMClient remain
+	// available through errors.Is.
 	ErrLLMGenerate = errors.New("failed to generate output")
 	// ErrValidation identifies an operational failure to load or compile a
 	// schema or validate output. A completed validation whose Status is
@@ -646,11 +647,13 @@ func (e *Engine) PrepareExecution(ctx context.Context, req RunRequest) (*Prepare
 // discoverable as [CapacityError] and still matches ErrCapacityExceeded. It
 // occurs before artifacts, schemas, rendering, or model generation because the
 // selected backend's admission capacity is full; it does not match
-// ErrInvalidRequest or ErrLLMGenerate. Errors from injected clients remain
-// available through errors.Is. Cancellation while waiting for model-generation
-// capacity matches both ErrLLMGenerate and the context error. Cancellation
-// otherwise follows the active collaborator's documented behavior. A nil
-// Engine returns ErrInvalidConfig. Run returns no partial result on error.
+// ErrInvalidRequest or ErrLLMGenerate. A built-in OpenAI-compatible non-2xx
+// response is discoverable as [GenerationError]. Errors from injected clients
+// remain available through errors.Is. Cancellation while waiting for
+// model-generation capacity matches both ErrLLMGenerate and the context error.
+// Cancellation otherwise follows the active collaborator's documented
+// behavior. A nil Engine returns ErrInvalidConfig. Run returns no partial
+// result on error.
 func (e *Engine) Run(ctx context.Context, req RunRequest) (*RunResult, error) {
 	if e == nil || e.runner == nil {
 		return nil, fmt.Errorf("%w: engine is nil", ErrInvalidConfig)
@@ -688,9 +691,10 @@ func (e *Engine) Run(ctx context.Context, req RunRequest) (*RunResult, error) {
 // ErrCapacityExceeded, ErrLLMGenerate, or ErrValidation as applicable while
 // preserving documented collaborator and context identities. An engine
 // admission rejection is discoverable as [CapacityError] and still matches
-// ErrCapacityExceeded. A completed content-validation rejection is returned
-// in RunResult, not as an operational error. An operational error returns no
-// partial RunResult.
+// ErrCapacityExceeded. A built-in OpenAI-compatible non-2xx response is
+// discoverable as [GenerationError]. A completed content-validation rejection
+// is returned in RunResult, not as an operational error. An operational error
+// returns no partial RunResult.
 func (e *Engine) RunPrepared(ctx context.Context, prepared *PreparedExecution) (*RunResult, error) {
 	if e == nil || e.runner == nil {
 		return nil, fmt.Errorf("%w: engine is nil", ErrInvalidConfig)