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Composite tools and workflows

Composite tools let you define multi-step workflows that execute across multiple backends with parallel execution, conditional logic, approval gates, and error handling.

Overview​

A composite tool combines multiple backend tool calls into a single workflow. When a client calls a composite tool, Virtual MCP orchestrates the execution across backends, handling dependencies and collecting results.

Key capabilities​

  • DAG-based execution: Steps run in parallel when dependencies allow
  • Template expansion: Dynamic arguments using step outputs
  • Elicitation: Request user input mid-workflow (approval gates, choices)
  • Error handling: Configurable abort, continue, or retry behavior
  • Timeouts: Workflow and per-step timeout configuration

Use cases​

Incident investigation​

Gather data from multiple monitoring systems in parallel:

Deployment with approval​

Human-in-the-loop workflow for production deployments:

Cross-system data aggregation​

Collect and correlate data from multiple backends:

Workflow definition​

Parameters​

Define input parameters with types and defaults:

parameters:
  required_param: { type: 'string' }
  optional_param: { type: 'integer', default: 10 }

Steps​

Each step can be a tool call or an elicitation:

steps:
  - id: step_name # Unique identifier
    tool: backend.tool # Tool to call
    arguments: # Arguments with template expansion
      arg1: '{{.params.input}}'
    depends_on: [other_step] # Dependencies for DAG execution
    condition: '{{.steps.check.output.approved}}' # Optional condition
    timeout: '30s' # Step timeout
    on_error:
      action: abort # abort | continue | retry

Elicitation (user prompts)​

Request input from users during workflow execution:

- id: approval
  type: elicitation
  message: 'Proceed with deployment?'
  schema:
    type: object
    properties:
      confirm: { type: boolean }
  timeout: '5m'
  on_decline:
    action: skip_remaining
  on_cancel:
    action: abort

Error handling​

Configure behavior when steps fail:

ActionDescription
abortStop workflow immediately
continueLog error, proceed to next step
retryRetry with exponential backoff
on_error:
  action: retry
  retry_count: 3
  retry_delay: 5s

Template syntax​

Access workflow context in arguments:

TemplateDescription
{{.params.name}}Input parameter
{{.steps.id.output}}Step output
{{.steps.id.content}}Elicitation response content
{{.steps.id.action}}Elicitation action (accept/decline/cancel)

Complete example​