Composite Tools Support Structured Content

[jerm-dro]

Stacked on #3006 because of when I discovered this gap.

Summary

• Fix issue where composite tool steps could not access nested fields from backend tool responses
• Backend HTTP client now checks for StructuredContent before falling back to Content array processing (legacy behavior)
• Add integration tests for structured content access and nested defaultResults
• Document step output format and numeric comparison behavior
• I slightly deviated from the solution proposed in 2994, because I felt namespacing all data behind structuredContent would be verbose/clunky. It also does not match much of the existing documentation.

Fixes #2994

Changes

Core Fix (pkg/vmcp/client/client.go):

• Check result.StructuredContent before processing result.Content array
• If StructuredContent is an object (map[string]any), use it directly as step output
• Fall back to existing Content processing for text/image content

Test Helpers (test/integration/vmcp/helpers/backend.go):

• Add StructuredHandler field to BackendTool for structured content responses
• Add NewBackendToolWithStructuredResponse() constructor
• Update CreateBackendServer to use mcp.NewToolResultStructuredOnly() when appropriate

Integration Tests (test/integration/vmcp/vmcp_integration_test.go):

• TestVMCPServer_StructuredContent: Tests nested field access via {{.steps.X.output.field.nested}}
• TestVMCPServer_DefaultResults_NestedStructure: Tests nested defaultResults fallback

Documentation:

• docs/operator/virtualmcpcompositetooldefinition-guide.md: Added "Step Output Format" and "Numeric Values in Templates" sections
• docs/operator/advanced-workflow-patterns.md: Added "Step Output Format" and "Numeric Comparisons" sections

[codecov]

Codecov Report

❌ Patch coverage is 0% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.29%. Comparing base (cbf413d) to head (53bcb9d).

Files with missing lines	Patch %	Lines
test/integration/vmcp/helpers/backend.go	0.00%	12 Missing ⚠️
pkg/vmcp/client/client.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## jerm/composite-tools-default-results #3009 +/- ## ======================================================================== - Coverage 56.31% 56.29% -0.03%  ======================================================================== Files 332 332 Lines 32641 32659 +18 ======================================================================== + Hits 18383 18385 +2  - Misses 12696 12712 +16  Partials 1562 1562 

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