vmcp composition - Template Expander Enhancement

Successfully implemented the ExpandOutputFormat() method in
the template expander to support output aggregation for
multi-step vMCP composite tool workflows.
Also integrate with the workflow

Author: taskbot

pkg/vmcp/composer/composer.go

@@ -324,6 +324,20 @@ type TemplateExpander interface {
 
 	// EvaluateCondition evaluates a condition template to a boolean.
 	EvaluateCondition(ctx context.Context, condition string, workflowCtx *WorkflowContext) (bool, error)
+
+	// ExpandOutputFormat evaluates the output_format template with workflow context.
+	// Returns a map representing the expanded JSON output structure.
+	// The template has access to:
+	//   - .params.*          - Input parameters
+	//   - .steps.*.output    - Step outputs
+	//   - .steps.*.status    - Step status
+	//   - .workflow.*        - Workflow metadata (id, duration, timestamps, step_count)
+	ExpandOutputFormat(
+		ctx context.Context,
+		outputFormatTemplate string,
+		workflowCtx *WorkflowContext,
+		startTime, endTime int64,
+	) (map[string]any, error)
 }
 
 // WorkflowContext contains the execution context for a workflow.

pkg/vmcp/composer/template_expander.go

@@ -215,6 +215,81 @@ func (e *defaultTemplateExpander) EvaluateCondition(
 	}
 }
 
+// ExpandOutputFormat evaluates the output_format template with workflow context.
+// Returns a map representing the expanded JSON output structure.
+func (e *defaultTemplateExpander) ExpandOutputFormat(
+	ctx context.Context,
+	outputFormatTemplate string,
+	workflowCtx *WorkflowContext,
+	startTime, endTime int64,
+) (map[string]any, error) {
+	// Check context cancellation before expensive operations
+	if err := ctx.Err(); err != nil {
+		return nil, fmt.Errorf("context cancelled before output format expansion: %w", err)
+	}
+
+	// Build template context with params, steps, and workflow metadata
+	tmplCtx := map[string]any{
+		"params":   workflowCtx.Params,
+		"steps":    e.buildStepsContext(workflowCtx),
+		"workflow": e.buildWorkflowMetadata(workflowCtx, startTime, endTime),
+	}
+
+	// Parse template
+	tmpl, err := template.New("output").Funcs(e.funcMap).Parse(outputFormatTemplate)
+	if err != nil {
+		return nil, fmt.Errorf("invalid output format template: %w", err)
+	}
+
+	// Execute template to string
+	var buf bytes.Buffer
+	buf.Grow(4096) // Pre-allocate reasonable buffer size
+
+	if err := tmpl.Execute(&buf, tmplCtx); err != nil {
+		return nil, fmt.Errorf("output format expansion failed: %w", err)
+	}
+
+	// Enforce output size limit
+	if buf.Len() > maxTemplateOutputSize {
+		return nil, fmt.Errorf("output format result too large: %d bytes (max %d)",
+			buf.Len(), maxTemplateOutputSize)
+	}
+
+	// Parse JSON result
+	var output map[string]any
+	if err := json.Unmarshal(buf.Bytes(), &output); err != nil {
+		return nil, fmt.Errorf("output format must produce valid JSON: %w (template output: %s)",
+			err, buf.String())
+	}
+
+	return output, nil
+}
+
+// buildWorkflowMetadata creates workflow metadata for template expansion.
+func (*defaultTemplateExpander) buildWorkflowMetadata(
+	workflowCtx *WorkflowContext,
+	startTime, endTime int64,
+) map[string]any {
+	// Acquire read lock to safely access Steps map
+	workflowCtx.mu.RLock()
+	stepCount := len(workflowCtx.Steps)
+	workflowCtx.mu.RUnlock()
+
+	// Calculate duration in milliseconds
+	durationMs := int64(0)
+	if endTime > 0 && startTime > 0 {
+		durationMs = endTime - startTime
+	}
+
+	return map[string]any{
+		"id":          workflowCtx.WorkflowID,
+		"step_count":  stepCount,
+		"duration_ms": durationMs,
+		"start_time":  startTime,
+		"end_time":    endTime,
+	}
+}
+
 // jsonEncode is a template function that encodes a value as JSON.
 func jsonEncode(v any) (string, error) {
 	b, err := json.Marshal(v)

pkg/vmcp/composer/template_expander_test.go

@@ -2,6 +2,7 @@ package composer
 
 import (
 	"context"
+	"fmt"
 	"testing"
 	"time"
 
@@ -218,3 +219,261 @@ func TestWorkflowContext_Clone(t *testing.T) {
 	assert.NotEqual(t, original.Params, clone.Params)
 	assert.NotEqual(t, len(original.Steps), len(clone.Steps))
 }
+
+func TestTemplateExpander_ExpandOutputFormat(t *testing.T) {
+	t.Parallel()
+
+	expander := NewTemplateExpander()
+	startTime := time.Now().UnixMilli()
+	endTime := startTime + 1500 // 1.5 seconds later
+
+	tests := []struct {
+		name           string
+		template       string
+		params         map[string]any
+		steps          map[string]*StepResult
+		workflowID     string
+		expectedFields map[string]any // Fields to check in output
+		wantErr        bool
+		errContains    string
+	}{
+		{
+			name:     "simple step output aggregation",
+			template: `{"logs": {{json .steps.fetch_logs.output}}, "metrics": {{json .steps.fetch_metrics.output}}}`,
+			steps: map[string]*StepResult{
+				"fetch_logs":    {Status: StepStatusCompleted, Output: map[string]any{"count": 100}},
+				"fetch_metrics": {Status: StepStatusCompleted, Output: map[string]any{"cpu": "50%"}},
+			},
+			expectedFields: map[string]any{
+				"logs":    map[string]any{"count": float64(100)}, // JSON unmarshal converts to float64
+				"metrics": map[string]any{"cpu": "50%"},
+			},
+		},
+		{
+			name: "with workflow metadata",
+			template: `{
+				"data": {{json .steps.fetch_data.output}},
+				"metadata": {
+					"workflow_id": "{{.workflow.id}}",
+					"duration_ms": {{.workflow.duration_ms}},
+					"step_count": {{.workflow.step_count}}
+				}
+			}`,
+			workflowID: "test-wf-123",
+			steps: map[string]*StepResult{
+				"fetch_data": {Status: StepStatusCompleted, Output: map[string]any{"result": "ok"}},
+			},
+			expectedFields: map[string]any{
+				"data": map[string]any{"result": "ok"},
+				"metadata": map[string]any{
+					"workflow_id": "test-wf-123",
+					"duration_ms": float64(1500),
+					"step_count":  float64(1),
+				},
+			},
+		},
+		{
+			name: "with parameters",
+			template: `{
+				"incident_id": "{{.params.incident_id}}",
+				"data": {{json .steps.fetch.output}}
+			}`,
+			params: map[string]any{"incident_id": "INC-12345"},
+			steps: map[string]*StepResult{
+				"fetch": {Status: StepStatusCompleted, Output: map[string]any{"status": "resolved"}},
+			},
+			expectedFields: map[string]any{
+				"incident_id": "INC-12345",
+				"data":        map[string]any{"status": "resolved"},
+			},
+		},
+		{
+			name: "multi-step with status",
+			template: `{
+				"results": {
+					"step1": {
+						"status": "{{.steps.step1.status}}",
+						"output": {{json .steps.step1.output}}
+					},
+					"step2": {
+						"status": "{{.steps.step2.status}}",
+						"output": {{json .steps.step2.output}}
+					}
+				}
+			}`,
+			steps: map[string]*StepResult{
+				"step1": {Status: StepStatusCompleted, Output: map[string]any{"a": 1}},
+				"step2": {Status: StepStatusCompleted, Output: map[string]any{"b": 2}},
+			},
+			expectedFields: map[string]any{
+				"results": map[string]any{
+					"step1": map[string]any{
+						"status": "completed",
+						"output": map[string]any{"a": float64(1)},
+					},
+					"step2": map[string]any{
+						"status": "completed",
+						"output": map[string]any{"b": float64(2)},
+					},
+				},
+			},
+		},
+		{
+			name: "nested data structures",
+			template: `{
+				"pages": {
+					"overview": {{json .steps.fetch_overview.output}},
+					"details": {{json .steps.fetch_details.output}}
+				},
+				"summary": {
+					"total_pages": 2,
+					"completed_at": {{.workflow.end_time}}
+				}
+			}`,
+			steps: map[string]*StepResult{
+				"fetch_overview": {
+					Status: StepStatusCompleted,
+					Output: map[string]any{"title": "Overview", "content": "..."},
+				},
+				"fetch_details": {
+					Status: StepStatusCompleted,
+					Output: map[string]any{"title": "Details", "sections": []any{"intro", "body"}},
+				},
+			},
+			expectedFields: map[string]any{
+				"pages": map[string]any{
+					"overview": map[string]any{"title": "Overview", "content": "..."},
+					"details":  map[string]any{"title": "Details", "sections": []any{"intro", "body"}},
+				},
+				"summary": map[string]any{
+					"total_pages":  float64(2),
+					"completed_at": float64(endTime),
+				},
+			},
+		},
+		{
+			name:        "invalid template syntax",
+			template:    `{"data": {{.steps.fetch.output}`,
+			wantErr:     true,
+			errContains: "invalid output format template",
+		},
+		{
+			name:        "template references missing field",
+			template:    `{"data": {{.nonexistent.field}}}`,
+			wantErr:     true,
+			errContains: "output format must produce valid JSON",
+		},
+		{
+			name:        "non-JSON output",
+			template:    `This is not JSON`,
+			wantErr:     true,
+			errContains: "output format must produce valid JSON",
+		},
+		{
+			name:        "invalid JSON structure",
+			template:    `{"unclosed": "bracket"`,
+			wantErr:     true,
+			errContains: "output format must produce valid JSON",
+		},
+		{
+			name: "empty steps",
+			template: `{
+				"workflow_id": "{{.workflow.id}}",
+				"step_count": {{.workflow.step_count}}
+			}`,
+			workflowID: "empty-wf",
+			steps:      map[string]*StepResult{},
+			expectedFields: map[string]any{
+				"workflow_id": "empty-wf",
+				"step_count":  float64(0),
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			ctx := newWorkflowContext(tt.params)
+			if tt.workflowID != "" {
+				ctx.WorkflowID = tt.workflowID
+			} else {
+				ctx.WorkflowID = "test-workflow"
+			}
+			if tt.steps != nil {
+				ctx.Steps = tt.steps
+			}
+
+			result, err := expander.ExpandOutputFormat(context.Background(), tt.template, ctx, startTime, endTime)
+
+			if tt.wantErr {
+				require.Error(t, err)
+				if tt.errContains != "" {
+					assert.Contains(t, err.Error(), tt.errContains)
+				}
+				return
+			}
+
+			require.NoError(t, err)
+			require.NotNil(t, result)
+
+			// Verify expected fields
+			for key, expectedValue := range tt.expectedFields {
+				actualValue, exists := result[key]
+				require.True(t, exists, "expected key %q not found in result", key)
+				assert.Equal(t, expectedValue, actualValue, "mismatch for key %q", key)
+			}
+		})
+	}
+}
+
+func TestTemplateExpander_ExpandOutputFormat_SizeLimits(t *testing.T) {
+	t.Parallel()
+
+	expander := NewTemplateExpander()
+	ctx := newWorkflowContext(nil)
+	ctx.WorkflowID = "test"
+
+	// Create a large step output that will exceed 10MB size limit
+	// Each entry is ~100 bytes, so we need >100k entries
+	largeString := make([]byte, 100)
+	for i := range largeString {
+		largeString[i] = 'x'
+	}
+
+	largeData := make(map[string]any)
+	for i := 0; i < 120000; i++ { // 120k entries * ~100 bytes > 10MB
+		largeData[fmt.Sprintf("key_%d", i)] = string(largeString)
+	}
+
+	ctx.Steps = map[string]*StepResult{
+		"large_step": {Status: StepStatusCompleted, Output: largeData},
+	}
+
+	template := `{"data": {{json .steps.large_step.output}}}`
+
+	_, err := expander.ExpandOutputFormat(context.Background(), template, ctx, 0, 0)
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "too large")
+}
+
+func TestTemplateExpander_ExpandOutputFormat_ContextCancellation(t *testing.T) {
+	t.Parallel()
+
+	expander := NewTemplateExpander()
+	ctx := newWorkflowContext(nil)
+	ctx.WorkflowID = "test"
+	ctx.Steps = map[string]*StepResult{
+		"step1": {Status: StepStatusCompleted, Output: map[string]any{"result": "ok"}},
+	}
+
+	// Create cancelled context
+	cancelledCtx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	template := `{"data": {{json .steps.step1.output}}}`
+
+	_, err := expander.ExpandOutputFormat(cancelledCtx, template, ctx, 0, 0)
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "context cancelled")
+}