Add direct per-run session overrides

internal/usecase/runner.go

@@ -160,6 +160,7 @@ func (r *Runner) Run(ctx context.Context, req domain.RunRequest) (*domain.RunRes
 		PromptID:             prepared.PromptID,
 		PromptVersion:        prepared.PromptVersion,
 		PromptHash:           prepared.PromptHash,
+		SessionID:            prepared.SessionID,
 		RenderedPromptHash:   prepared.RenderedPromptHash,
 		SelectedProfileID:    prepared.SelectedProfileID,
 		SelectedBackendID:    prepared.SelectedBackendID,
@@ -178,6 +179,10 @@ func (r *Runner) Prepare(ctx context.Context, req domain.RunRequest) (*domain.Pr
 	if strings.TrimSpace(req.PromptID) == "" {
 		return nil, fmt.Errorf("%w: prompt id is required", ErrInvalidRequest)
 	}
+	directSessionID, err := domain.NormalizeSessionID(req.SessionID)
+	if err != nil {
+		return nil, fmt.Errorf("%w: session_id: %v", ErrInvalidRequest, err)
+	}
 
 	start := time.Now().UTC()
 
@@ -251,10 +256,19 @@ func (r *Runner) Prepare(ctx context.Context, req domain.RunRequest) (*domain.Pr
 		inputHashes[name] = art.Hash
 	}
 
-	renderedPrompt, err := r.renderer.Render(ctx, def, resolvedInputs, req.Vars)
+	definitionToRender := def
+	if directSessionID != "" {
+		definitionCopy := *def
+		definitionCopy.SessionID = ""
+		definitionToRender = &definitionCopy
+	}
+	renderedPrompt, err := r.renderer.Render(ctx, definitionToRender, resolvedInputs, req.Vars)
 	if err != nil {
 		return nil, fmt.Errorf("%w: %w", ErrPromptRender, err)
 	}
+	if directSessionID != "" {
+		renderedPrompt.SessionID = directSessionID
+	}
 
 	end := time.Now().UTC()
 	return &domain.PreparedRun{