Add request validation API

engine.go

@@ -346,6 +346,29 @@ func (e *Engine) InspectProfile(ctx context.Context, profileID string) (*Profile
 // relationship to ctx.Err is promised. Prepare returns no partial result on
 // error. It validates RunRequest.TransientRetries but makes no model attempts.
 func (e *Engine) Prepare(ctx context.Context, req RunRequest) (*PreparedRun, error) {
+	prepared, err := e.prepareRequest(ctx, req)
+	if err != nil {
+		return nil, err
+	}
+	return fromDomainPreparedRun(prepared), nil
+}
+
+// ValidateRequest reports whether req passes the same preparation path used by
+// [Engine.Prepare] at the time of this call. A nil error means this concrete
+// request passed preparation. The method returns no prepared snapshot or
+// executable handle. It does not call an LLM, but preparation may still perform
+// resource I/O, including loading supplied inputs and schemas.
+//
+// Its errors and context handling follow [Engine.Prepare], including an error
+// matching ErrInvalidConfig when the Engine is nil. Success does not guarantee
+// that a provider will accept a later request. Use [Engine.PrepareExecution]
+// when execution must use the snapshot produced by preparation.
+func (e *Engine) ValidateRequest(ctx context.Context, req RunRequest) error {
+	_, err := e.prepareRequest(ctx, req)
+	return err
+}
+
+func (e *Engine) prepareRequest(ctx context.Context, req RunRequest) (*domain.PreparedRun, error) {
 	ctx = observation.WithoutExecutionState(ctx)
 	if e == nil || e.internal == nil {
 		return nil, fmt.Errorf("%w: engine is nil", ErrInvalidConfig)
@@ -360,7 +383,7 @@ func (e *Engine) Prepare(ctx context.Context, req RunRequest) (*PreparedRun, err
 	if err != nil {
 		return nil, mapPublicError(err)
 	}
-	return fromDomainPreparedRun(prepared), nil
+	return prepared, nil
 }
 
 // PrepareExecution completely prepares a prompt request without calling the