feat(mcp): advertise output schemas for data tools

[felixgeelhaar]

What

Adopts mcp-go's structured-output support (OutputSchema + structuredContent) for the data-returning MCP tools in the mcp package. A typed struct return plus .OutputSchema(T{}) makes mcp-go advertise an outputSchema in tools/list and promote the marshaled result to typed structuredContent alongside the existing text content.

Adopted (7 tools)

Tool	Output type	Note
list_machines	MachineListOutput{items, total}	enveloped the bare []MachineInfo so structuredContent is a JSON object
get_state	StateOutput	already an object
get_context	Ctx (context object)	already an object
get_machine_data	*viz.VizMachine	refactored return from json.RawMessage to the typed struct; identical JSON payload
validate_machine	ValidateOutput	already an object
<prefix>.get_state	ExposeStateOutput	ExposeInterpreter surface
<prefix>.get_context	ExposeContextOutput[C]	generic wrapper {context: ...}

Skipped (with reasons)

• create_machine, send_event, reset_machine, delete_machine, <prefix>.send_event — mutations, not read/query tools.
• export_machine — returns a formatted string (json / mermaid / ascii), not a typed object.
• <prefix>.matches — returns a scalar boolean predicate {matches: bool}, not genuine structured data.

Behavior preserved

Text content for every tool is unchanged except list_machines, whose result is now the envelope object {items, total} (the sanctioned technique for making a list tool's structuredContent a valid JSON object). get_machine_data marshals to the same JSON as before.

Guard test

OutputSchema runs schema.Generate at registration and silently drops the tool if it errors. Added TestOutputSchemasGenerate asserting schema.Generate succeeds for every advertised output type (including the generic ExposeContextOutput[C] with both map and struct contexts). Updated the list_machines and get_machine_data handler tests for the new return shapes.

Verification

• gofmt -l on changed files: clean
• go build ./...: ok
• go vet ./mcp/...: clean
• go test ./... -count=1: all pass
• golangci-lint run ./mcp/: 0 issues

https://claude.ai/code/session_01LCyhyAffdzBmzG3yPPqzTT

[felixgeelhaar]

Closing — per maintainer decision, statekit doesn't need an MCP server, so this structured-output enhancement is moot. See follow-up on whether to remove the MCP package entirely.