MCP server adapter (Rust)

[douglasjordan2]

The spec notes "Built-in MCP server" as out of scope with the suggestion to "build this as a separate thin adapter." This is that adapter.

What it does

A ~380-line Rust binary (t4a-mcp) that exposes t4a's Unix socket protocol as an MCP stdio server. 9 tools mapping 1:1 to the protocol commands: t4a_create, t4a_list, t4a_send, t4a_screenshot, t4a_text, t4a_cursor, t4a_resize, t4a_kill, t4a_wait.

The main thing that justifies the MCP layer over just using the CLI via shell: t4a_screenshot returns the PNG inline as an MCP image content block, so the LLM gets the screenshot in a single tool call without save-to-file-then-read. t4a_wait also provides a clean blocking event wait that maps better to MCP's request/response model than piping t4a events.

Tech stack

• Rust, same as t4a itself
• rmcp 1.2 (official MCP Rust SDK from modelcontextprotocol/rust-sdk)
• Shares dependencies with t4a: tokio, serde, serde_json
• Connects to /tmp/t4a.sock (or $T4A_SOCKET), auto-starts daemon if not running
• Single-threaded current_thread tokio runtime
• Release binary: ~1.7MB with LTO + strip

Structure

src/
  main.rs      # MCP server, tool definitions (~230 lines)
  client.rs    # t4a Unix socket client (~150 lines)

Design decisions

• t4a_send does escape parsing (\n, \t, \r, \e, \xHH) matching the CLI's behavior, since MCP clients send these as literal two-character sequences in JSON strings. Also supports input_base64 for raw bytes.
• Event type validation against the spec's 6 known types (command_done, idle, activity, exit, bell, title) to fail fast on typos instead of silently timing out.
• Screenshot hardcoded to pad=1, scale=66 (the empirically optimal config from the vision tests).

Questions for maintainers

1. Would you accept this as a PR? If so, should it live as a mcp/ subdirectory in this repo or as a separate crate/repo?
2. Any preferences on tool naming? Currently prefixed with t4a_ to avoid collisions when the MCP server is registered alongside others.

Happy to open a PR if there's interest. Source: https://github.com/douglasjordan2/t4a-mcp