stacklok / toolhive / 18910702901

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github

Committed by web-flow

Commit Message

Implement capability discovery and querying for Virtual MCP Server (#2354)

* Implement capability discovery and querying for Virtual MCP Server

This implements the core capability discovery and querying functionality
for the Virtual MCP Server feature (based on proposal THV-2106).

Virtual MCP aggregates multiple MCP servers from a ToolHive group into a
single unified interface, enabling complex workflows spanning multiple
tools and services. This PR implements the first phase: discovering
backends and querying their capabilities.

SDK Choice - mark3labs/mcp-go:
We use mark3labs/mcp-go instead of the official modelcontextprotocol/go-sdk
because:
- Already battle-tested in ToolHive (pkg/transport/bridge.go)
- Better client-side flexibility via WithHTTPHeaderFunc for per-backend auth
- Direct http.Handler implementation (no wrapper layer)
- Zero migration risk from existing code
- Both SDKs support standard Go middleware, but mark3labs provides simpler
  integration patterns for our per-backend authentication requirements

Changes:
- Add BackendClient for MCP protocol communication with backends
  - Uses mark3labs/mcp-go SDK for streamable-HTTP and SSE transports
  - Implements ListCapabilities, CallTool, ReadResource, GetPrompt
  - Proper handling of MCP Content interfaces (AsTextContent, AsImageContent)
  - Converts ToolInputSchema structs to map[string]any for domain layer

- Add CLI Backend Discoverer for Docker/Podman workloads
  - Discovers backends from ToolHive groups using existing managers
  - Filters for healthy/running workloads only
  - Converts core.Workload to vmcp.Backend domain types
  - Preserves metadata (group, labels, transport type)

- Add Default Aggregator for capability aggregation
  - Parallel backend queries using errgroup (limit: 10 concurrent)
  - Graceful failure handling (continues with remaining backends)
  - Five-method interface: QueryCapabilities, QueryAllCapabilities,
    ResolveConflicts, MergeCapabilities, Aggrega... (continued)

Run Details

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