yaacov/mcp-tool-development

MCP Tool Development

The MCP server exposes kubectl-mtv commands as structured tools for AI assistants. CLI commands are auto-discovered; MCP tools rarely need changes.

Architecture

cmd/mcpserver/mcpserver.go       -> Server startup, tool registration
pkg/mcp/discovery/registry.go   -> Command discovery from help --machine
pkg/mcp/tools/mtv_read.go       -> Read-only tool (get, describe, health)
pkg/mcp/tools/mtv_write.go      -> Write tool (create, delete, patch, start)
pkg/mcp/tools/mtv_help.go       -> Help/documentation tool
pkg/mcp/util/util.go            -> Command execution, response parsing

Auto-Discovery (Most Common Case)

New CLI commands are automatically exposed via MCP. No MCP code changes needed if:

1. The command is under a known verb (get, describe, create, delete, patch, start, cancel, archive, unarchive, cutover, health).
2. The verb maps to a category in pkg/cmd/help/generator.go getCategory().
3. The command is runnable (leaf node with RunE).

The discovery flow: help --machine -> JSON schema -> Registry -> tool descriptions.

Tool Input Pattern

Each tool has an input struct with jsonschema tags:

type MTVReadInput struct {
    Command string         `json:"command" jsonschema:"Command path (e.g. get plan, get inventory vm)"`
    Flags   map[string]any `json:"flags,omitempty" jsonschema:"All parameters as key-value pairs"`
    DryRun  bool           `json:"dry_run,omitempty" jsonschema:"If true, returns CLI command instead of executing"`
    Fields  []string       `json:"fields,omitempty" jsonschema:"Limit JSON to these top-level keys only"`
}

Tool Definition Pattern

func GetMTVReadTool(registry *discovery.Registry) *mcp.Tool {
    return &mcp.Tool{
        Name:         "mtv_read",
        Description:  registry.GenerateReadOnlyDescription(),
        OutputSchema: mtvOutputSchema,
    }
}

Handler Pattern

func HandleMTVRead(registry *discovery.Registry) func(context.Context, *mcp.CallToolRequest, MTVReadInput) (*mcp.CallToolResult, any, error) {
    return func(ctx context.Context, req *mcp.CallToolRequest, input MTVReadInput) (*mcp.CallToolResult, any, error) {
        ctx = extractKubeCredsFromRequest(ctx, req)
        if err := validateCommandInput(input.Command); err != nil {
            return nil, nil, err
        }
        cmdPath := normalizeCommandPath(input.Command)
        if !registry.IsReadOnly(cmdPath) {
            return nil, nil, fmt.Errorf("unknown or write command '%s'", input.Command)
        }
        if input.DryRun {
            ctx = util.WithDryRun(ctx, true)
        }
        args := buildArgs(cmdPath, input.Flags)
        result, err := util.RunKubectlMTVCommand(ctx, args)
        if err != nil {
            return nil, nil, fmt.Errorf("command failed: %w", err)
        }
        data, err := util.UnmarshalJSONResponse(result)
        if err != nil {
            return nil, nil, err
        }
        if errResult := buildCLIErrorResult(data); errResult != nil {
            return errResult, nil, nil
        }
        return nil, data, nil
    }
}

Tool Registration

In cmd/mcpserver/mcpserver.go createMCPServerWithHeaderCapture():

// Read tools (always registered)
tools.AddToolWithCoercion(server, tools.GetMTVReadTool(registry),
    wrapWithHeaders(tools.HandleMTVRead(registry), capturedHeaders))
mcp.AddTool(server, tools.GetMTVHelpTool(),
    wrapWithHeaders(tools.HandleMTVHelp, capturedHeaders))

// Write tool (skipped in read-only mode)
if !readOnlyMode {
    tools.AddToolWithCoercion(server, tools.GetMTVWriteTool(registry),
        wrapWithHeaders(tools.HandleMTVWrite(registry), capturedHeaders))
}

• AddToolWithCoercion handles boolean coercion (string "true"/"false" -> bool). Use for tools with boolean flags.
• mcp.AddTool is standard registration. Use when no coercion needed.
• wrapWithHeaders injects HTTP auth headers for SSE mode.

Adding a New MCP Tool

1. Create pkg/mcp/tools/mtv_newtool.go with input struct, GetTool(), and HandleTool().
2. Register in cmd/mcpserver/mcpserver.go createMCPServerWithHeaderCapture().
3. Add tests in pkg/mcp/tools/mtv_newtool_test.go.

Key Utilities

| Function | Location | Purpose | |----------|----------|---------| | util.RunKubectlMTVCommand() | pkg/mcp/util/util.go | Execute kubectl-mtv subprocess | | util.UnmarshalJSONResponse() | pkg/mcp/util/util.go | Parse JSON, strip command field, truncate | | buildCLIErrorResult() | pkg/mcp/tools/mtv_read.go | Convert non-zero exit to IsError response | | validateCommandInput() | pkg/mcp/tools/mtv_read.go | Reject garbled LLM input early | | normalizeCommandPath() | pkg/mcp/tools/mtv_read.go | "get plan" -> "get/plan" | | buildArgs() | pkg/mcp/tools/mtv_read.go | Flags map -> CLI args | | help.MarkMCPHidden() | pkg/cmd/help/generator.go | Hide flags from MCP schema |

Shared Output Schema

All tools return the same envelope:

{
  "return_value": 0,
  "data": {},
  "output": "text output",
  "stderr": "error output"
}

Registry

The Registry (pkg/mcp/discovery/) splits commands into ReadOnly and ReadWrite maps keyed by path ("get/plan", "create/provider"). It also holds GlobalFlags, Parents (non-runnable nodes like get/inventory), and generates tool descriptions.