agents_and_robots/cmd/devicemesh-mcp/bridge.go

// bridge.go — adapter that registers every devicemesh.ToolSpec from a
// ToolRegistry as an MCP tool on a mcp-go server.MCPServer.
//
// Tool name preservation: we register tools under their dotted devicemesh
// name verbatim ("exec", "shell.eval", "fs.read"). claude exposes them to
// the model as `mcp__<server_name>__<tool_name>` (the MCP transport prefixes
// automatically).
//
// Schema: ToolSpec.InputSchema is already a JSON-Schema-lite map. We
// marshal it to a json.RawMessage and feed it via mcp.WithRawInputSchema so
// the LLM sees the full structure (required fields, enums, descriptions).
//
// Handler: each tool's handler invokes reg.Call(ctx, name, args). The
// registry runs ValidateInput → ArgMapping → HTTP dispatch → ResultMapping
// just like the in-process tool-use path. The result is JSON-encoded into
// an MCP text-content block. Errors become NewToolResultError so the model
// can self-correct on the next turn.
package main

import (
	"context"
	"encoding/json"
	"fmt"
	"log/slog"

	"github.com/mark3labs/mcp-go/mcp"
	"github.com/mark3labs/mcp-go/server"

	"github.com/enmanuel/agents/pkg/tools/devicemesh"
)

// RegisterToolBridge walks reg and registers each spec on srv. Returns the
// first registration error, if any. Pure data adapter except for the slog
// debug events.
func RegisterToolBridge(srv *server.MCPServer, reg *devicemesh.ToolRegistry, logger *slog.Logger) error {
	if srv == nil {
		return fmt.Errorf("RegisterToolBridge: srv is nil")
	}
	if reg == nil {
		return fmt.Errorf("RegisterToolBridge: reg is nil")
	}
	for _, spec := range reg.List() {
		tool, err := buildMCPTool(spec)
		if err != nil {
			return fmt.Errorf("build MCP tool %q: %w", spec.Name, err)
		}
		handler := makeHandler(reg, spec, logger)
		srv.AddTool(tool, handler)
		if logger != nil {
			logger.Debug("registered MCP tool",
				"name", spec.Name,
				"capability", spec.Capability,
				"requires_approval", spec.RequiresApproval,
			)
		}
	}
	return nil
}

// buildMCPTool transforms a devicemesh.ToolSpec into an mcp.Tool with the
// raw input schema attached. The description is augmented with the
// capability marker so the model knows the tool is remote.
//
// We use mcp.NewToolWithRawSchema (not NewTool + WithRawInputSchema) because
// NewTool initialises a default ToolInputSchema with Type="object", which
// then conflicts at marshal time with our RawInputSchema (the SDK rejects
// having both set — see mcp/tools.go ::Tool.MarshalJSON).
func buildMCPTool(spec devicemesh.ToolSpec) (mcp.Tool, error) {
	desc := spec.Description
	if spec.Capability != "" {
		desc = fmt.Sprintf("%s [device_mesh: %s]", desc, spec.Capability)
	}
	if spec.RequiresApproval {
		desc += " (approval required)"
	}

	if spec.InputSchema == nil {
		// Fall back to a minimal "no params" schema so the tool is still
		// callable. Should not happen for the builtins (they all set
		// InputSchema), but the adapter must not panic on third-party specs.
		return mcp.NewToolWithRawSchema(spec.Name, desc,
			json.RawMessage(`{"type":"object","properties":{}}`)), nil
	}
	raw, err := json.Marshal(spec.InputSchema)
	if err != nil {
		return mcp.Tool{}, fmt.Errorf("marshal input schema: %w", err)
	}
	return mcp.NewToolWithRawSchema(spec.Name, desc, raw), nil
}

// makeHandler returns a server.ToolHandlerFunc bound to a single spec. The
// closure captures the registry so the HTTP dispatch goes through the same
// validate → map → call pipeline as the in-process path.
func makeHandler(reg *devicemesh.ToolRegistry, spec devicemesh.ToolSpec, logger *slog.Logger) server.ToolHandlerFunc {
	return func(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolResult, error) {
		args := req.GetArguments()
		if args == nil {
			args = map[string]any{}
		}
		if logger != nil {
			logger.Debug("tools/call received",
				"tool", spec.Name,
				"capability", spec.Capability,
				"arg_keys", keysOf(args),
			)
		}

		result, err := reg.Call(ctx, spec.Name, args)
		if err != nil {
			if logger != nil {
				logger.Warn("tools/call failed",
					"tool", spec.Name,
					"err", err.Error(),
				)
			}
			// NewToolResultError returns a CallToolResult with isError=true.
			// Returning (result, nil) lets the model see and self-correct
			// instead of treating it as a transport-level failure.
			return mcp.NewToolResultError(err.Error()), nil
		}

		text := encodeResult(result)
		if logger != nil {
			logger.Debug("tools/call ok",
				"tool", spec.Name,
				"result_len", len(text),
			)
		}
		return mcp.NewToolResultText(text), nil
	}
}

// encodeResult converts a tool result (any) to the string payload the model
// will see. Mirrors devicemesh.AdaptTool's formatToolResult so MCP and the
// in-process path produce consistent transcripts.
//
//   - nil    → ""
//   - string → returned as-is (avoids double-encoding JSON strings)
//   - other  → json.Marshal; on failure fall back to fmt.Sprintf so we never
//     drop data on the floor.
func encodeResult(v any) string {
	if v == nil {
		return ""
	}
	if s, ok := v.(string); ok {
		return s
	}
	b, err := json.Marshal(v)
	if err != nil {
		return fmt.Sprintf("%v", v)
	}
	return string(b)
}

// keysOf returns the sorted keys of a map for log context. Pure helper.
func keysOf(m map[string]any) []string {
	if len(m) == 0 {
		return nil
	}
	out := make([]string, 0, len(m))
	for k := range m {
		out = append(out, k)
	}
	return out
}