pig can be the server side of the Model Context Protocol as well as the client: --mode mcp serves one conversation to another agent — Claude Code, Claude Desktop, Cursor, another pig — as a single tool, ask. The caller hands pig a question or a job; pig runs a turn with its own tools, hooks and extensions in its working directory, and hands the final answer back.

For connecting pig to MCP servers, see Connect MCP Servers.

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# Streamable HTTP (default): http://127.0.0.1:8089/mcp
pig --mode mcp

# Elsewhere on the machine, or on the LAN
pig --mode mcp --mcp-host 0.0.0.0 --mcp-port 9000

# stdio, for a host that spawns its servers itself
pig --mode mcp --mcp-stdio

# Over an existing conversation (`-c` for the most recent one)
pig --mode mcp --session <id>

Everything that applies to a terminal session applies here — --model, --thinking, --tools, -e, project trust, settings.json. The server is the same session with a different front.

Option	Behavior
--mcp-host <h>	Address to listen on (default 127.0.0.1)
--mcp-port <n>	Port to listen on (default 8089)
--mcp-stdio	One JSON-RPC message per line on stdin/stdout instead of HTTP; the host closing stdin stops it

--mode mcp takes no prompt on the command line: the questions arrive as tool calls.

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Claude Code:

# HTTP
claude mcp add --transport http pig http://127.0.0.1:8089/mcp

# stdio — Claude Code starts pig itself, in the project directory it is run from
claude mcp add pig -- pig --mode mcp --mcp-stdio

Any other MCP client takes the same URL or the same command in its own configuration file.

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{ "name": "ask", "arguments": { "prompt": "What does bin/pig do on startup?" } }

• One conversation. Every call is a turn in the same session, so a follow-up can refer to an earlier answer ("now fix the second one"). Start pig with --session to continue a conversation you already have, or --no-session for one that is not written down.
• The answer is the final text, as pig -p prints it: the last assistant message's text blocks, with thinking and tool calls left out. A turn that fails (quota, refused request) comes back as a result with isError: true and the reason as its text.
• Calls queue. A call made while a turn is running waits for it and then takes its own turn, in arrival order. Nothing is refused and nothing is dropped.
• Progress while it runs. A caller that sends _meta.progressToken gets notifications/progress with each piece of text as the model produces it. Over HTTP the stream also carries a comment line every 15 seconds so nothing in between decides it is idle.

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Over HTTP this is Streamable HTTP (MCP 2025-03-26 and later) in its smallest reading:

• Every message is a POST /mcp. initialize, ping and tools/list are answered as one JSON document; tools/call as a text/event-stream that ends with the response and then closes.
• initialize issues an Mcp-Session-Id, which the client sends back on every request. An id the server does not know — after a restart, say — is answered 404, which the specification defines as "initialize again", so a client that outlives pig recovers on its own. DELETE /mcp forgets the id.
• GET /mcp is 405: pig has nothing to say that is not an answer to a request.

Over stdio there is no session id and no keep-alive, because a pipe needs neither.

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• Not an exposure of pig's RPC commands or built-in tools. One tool, on purpose: the other agent gets a colleague to ask, not a second copy of the filesystem.
• Not authenticated. --mcp-host 127.0.0.1 is the default for that reason; anything that can reach the port can run ask, and ask can run bash. Put it on a LAN or a public address only behind something that checks who is calling.