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schema and config changes for vMCP: Composition - Output Aggregation for Multi-Step Workflows
Successfully added the OutputFormat / outputFormat field to all configuration layers for vMCP composite tool workflows. This PR lays the foundation for implementing output aggregation in multi-step workflows as described in issue #174.
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‎cmd/thv-operator/api/v1alpha1/virtualmcpcompositetooldefinition_types.go‎

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@@ -46,6 +46,32 @@ type VirtualMCPCompositeToolDefinitionSpec struct {
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// +kubebuilder:default=abort
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// +optional
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FailureMode string `json:"failureMode,omitempty"`
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// OutputFormat is an optional Go template for constructing the workflow output.
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// If specified, the template is evaluated with access to:
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// - .params.* - Input parameters
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// - .steps.*.output - Step outputs
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// - .steps.*.status - Step status
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// - .workflow.* - Workflow metadata (id, duration, timestamps)
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//
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// The template must produce valid JSON. If omitted, defaults to returning
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// the last step's output (backward compatible behavior).
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//
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// Example:
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// outputFormat: |
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// {
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// "data": {
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// "logs": {{.steps.fetch_logs.output}},
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// "metrics": {{.steps.fetch_metrics.output}}
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// },
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// "metadata": {
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// "workflow_id": "{{.workflow.id}}",
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// "duration_ms": {{.workflow.duration_ms}}
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// }
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// }
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//
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// +optional
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OutputFormat string `json:"outputFormat,omitempty"`
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}
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// VirtualMCPCompositeToolDefinitionStatus defines the observed state of VirtualMCPCompositeToolDefinition

‎deploy/charts/operator-crds/crds/toolhive.stacklok.dev_virtualmcpcompositetooldefinitions.yaml‎

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@@ -92,6 +92,31 @@ spec:
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minLength: 1
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pattern: ^[a-z0-9]([a-z0-9_-]*[a-z0-9])?$
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type: string
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outputFormat:
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description: |-
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OutputFormat is an optional Go template for constructing the workflow output.
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If specified, the template is evaluated with access to:
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- .params.* - Input parameters
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- .steps.*.output - Step outputs
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- .steps.*.status - Step status
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- .workflow.* - Workflow metadata (id, duration, timestamps)
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The template must produce valid JSON. If omitted, defaults to returning
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the last step's output (backward compatible behavior).
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Example:
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outputFormat: |
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{
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"data": {
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"logs": {{.steps.fetch_logs.output}},
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"metrics": {{.steps.fetch_metrics.output}}
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},
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"metadata": {
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"workflow_id": "{{.workflow.id}}",
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"duration_ms": {{.workflow.duration_ms}}
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}
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}
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type: string
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parameters:
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additionalProperties:
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description: ParameterSpec defines a parameter for a composite tool

‎docs/operator/crd-api.md‎

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‎pkg/vmcp/composer/composer.go‎

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@@ -59,6 +59,17 @@ type WorkflowDefinition struct {
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// Options: "abort" (default), "continue", "best_effort"
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FailureMode string
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// OutputFormat is an optional Go template for constructing the workflow output.
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// If specified, the template is evaluated with access to:
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// - .params.* - Input parameters
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// - .steps.*.output - Step outputs
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// - .steps.*.status - Step status
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// - .workflow.* - Workflow metadata (id, duration, timestamps)
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//
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// The template must produce valid JSON. If omitted, defaults to returning
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// the last step's output (backward compatible behavior).
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OutputFormat string
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// Metadata stores additional workflow information.
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Metadata map[string]string
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}

‎pkg/vmcp/config/config.go‎

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@@ -339,6 +339,30 @@ type CompositeToolConfig struct {
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// Steps are the workflow steps to execute.
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Steps []*WorkflowStepConfig `json:"steps"`
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// OutputFormat is an optional template for constructing the workflow output.
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// If specified, the template is evaluated with access to:
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// - .params.* - Input parameters
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// - .steps.*.output - Step outputs
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// - .steps.*.status - Step status
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// - .workflow.* - Workflow metadata (id, duration, timestamps)
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//
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// The template must produce valid JSON. If omitted, defaults to returning
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// the last step's output (backward compatible behavior).
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//
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// Example:
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// output_format: |
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// {
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// "data": {
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// "logs": {{.steps.fetch_logs.output}},
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// "metrics": {{.steps.fetch_metrics.output}}
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// },
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// "metadata": {
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// "workflow_id": "{{.workflow.id}}",
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// "duration_ms": {{.workflow.duration_ms}}
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// }
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// }
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OutputFormat string `json:"output_format,omitempty" yaml:"output_format,omitempty"`
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}
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// ParameterSchema defines a workflow parameter.

‎pkg/vmcp/server/workflow_converter.go‎

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@@ -75,12 +75,13 @@ func ConvertConfigToWorkflowDefinitions(
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// Create workflow definition
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def := &composer.WorkflowDefinition{
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Name: ct.Name,
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Description: ct.Description,
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Parameters: params,
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Steps: steps,
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Timeout: timeout,
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Metadata: make(map[string]string),
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Name: ct.Name,
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Description: ct.Description,
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Parameters: params,
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Steps: steps,
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Timeout: timeout,
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OutputFormat: ct.OutputFormat,
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Metadata: make(map[string]string),
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}
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workflowDefs[ct.Name] = def

‎pkg/vmcp/server/workflow_converter_test.go‎

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@@ -264,3 +264,89 @@ func TestConvertSteps_ComplexWorkflow(t *testing.T) {
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assert.NotEmpty(t, result[2].Condition)
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assert.Equal(t, []string{"confirm"}, result[2].DependsOn)
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}
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func TestConvertConfigToWorkflowDefinitions_WithOutputFormat(t *testing.T) {
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t.Parallel()
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tests := []struct {
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name string
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input []*config.CompositeToolConfig
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wantOutputFormat string
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}{
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{
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name: "workflow without output_format",
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input: []*config.CompositeToolConfig{{
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Name: "simple",
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Steps: []*config.WorkflowStepConfig{
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{ID: "s1", Type: "tool", Tool: "backend.tool"},
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},
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}},
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wantOutputFormat: "",
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},
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{
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name: "workflow with output_format",
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input: []*config.CompositeToolConfig{{
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Name: "aggregated",
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Steps: []*config.WorkflowStepConfig{
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{ID: "fetch_logs", Type: "tool", Tool: "splunk.fetch"},
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{ID: "fetch_metrics", Type: "tool", Tool: "datadog.fetch"},
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},
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OutputFormat: `{
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"logs": {{.steps.fetch_logs.output}},
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"metrics": {{.steps.fetch_metrics.output}}
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}`,
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}},
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wantOutputFormat: `{
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"logs": {{.steps.fetch_logs.output}},
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"metrics": {{.steps.fetch_metrics.output}}
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}`,
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},
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{
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name: "workflow with complex output_format",
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input: []*config.CompositeToolConfig{{
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Name: "investigation",
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Steps: []*config.WorkflowStepConfig{
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{ID: "fetch_data", Type: "tool", Tool: "backend.fetch"},
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{ID: "analyze", Type: "tool", Tool: "backend.analyze"},
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},
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OutputFormat: `{
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"data": {{.steps.fetch_data.output}},
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"analysis": {{.steps.analyze.output}},
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"metadata": {
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"workflow_id": "{{.workflow.id}}",
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"duration_ms": {{.workflow.duration_ms}}
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}
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}`,
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}},
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wantOutputFormat: `{
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"data": {{.steps.fetch_data.output}},
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"analysis": {{.steps.analyze.output}},
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"metadata": {
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"workflow_id": "{{.workflow.id}}",
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"duration_ms": {{.workflow.duration_ms}}
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}
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}`,
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},
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}
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for _, tt := range tests {
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t.Run(tt.name, func(t *testing.T) {
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t.Parallel()
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result, err := ConvertConfigToWorkflowDefinitions(tt.input)
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require.NoError(t, err)
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require.Len(t, result, 1)
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// Get the first (and only) workflow definition
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var workflowDef *composer.WorkflowDefinition
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for _, def := range result {
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workflowDef = def
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break
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}
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require.NotNil(t, workflowDef)
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assert.Equal(t, tt.wantOutputFormat, workflowDef.OutputFormat)
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})
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}
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}

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