pidp10-mcp

An MCP server for driving an ITS session on a PiDP-10 (simh KA10) emulator over its raw TCP terminal line.

It exists because generic telnet MCP servers do not work against this target: they cannot transmit raw control bytes, they tie the TCP connection's lifetime to tool-call cadence, and they reconnect dead sessions in the background — which, on a port that maps to a single terminal line, produces zombie connections fighting each other for it.

What it does differently

• Raw control bytes get through. A ~-escape syntax in send puts exact bytes on the wire: ~z for the Ctrl-Z that calls ITS, ~e for the ESC that DDT prints as $, ~xNN for anything else.

• The connection belongs to the server process, not to tool calls. A background reader drains the socket continuously into a 256 KB scrollback. Nothing cares how long the client spends thinking between calls; a three-minute gap is invisible to the session.

• It never reconnects on its own. If the socket dies, the session is marked dead and the next tool response says so. Reopening is an explicit decision.

• Closes hard. close() sets SO_LINGER to zero so the socket is reset rather than left in a half-closed state that keeps the line marked busy, and the same teardown runs from atexit plus SIGTERM/SIGHUP handlers.

• Terse responses. Only output produced since the last call, VT52 noise stripped, followed by one trailer line. No banners, no echoed inputs, no re-dumping the session log.

Install

Requires Python 3.11+ and the official mcp SDK 2.x.

uv sync                # or: pip install -e .
uv run pidp10-mcp      # stdio transport (default)

Register it with an MCP client — for Claude Code:

claude mcp add pidp10 -- uv --directory /path/to/pidp10/mcp run pidp10-mcp

or by hand, in an mcpServers config block:

{
  "mcpServers": {
    "pidp10": {
      "command": "uv",
      "args": ["--directory", "/path/to/pidp10/mcp", "run", "pidp10-mcp"],
      "env": { "PIDP10_HOST": "pidp10.local", "PIDP10_PORT": "10018" }
    }
  }
}

Streamable HTTP

uv run pidp10-mcp --http --http-host 127.0.0.1 --http-port 8010

Configuration

open(host, port) can override the host and port per call.

Escape syntax

Escapes are expanded in send's input. An unknown escape is an error rather than being passed through as text — silently sending ~q to DDT is worse than a rejection.

Line endings

send appends a CR (0x0D) automatically, because that is what the line editor wants, and normalises any literal LF or CRLF in the input to CR. A bare LF is not a newline on this system — it is a DDT command. If you genuinely want to send one, ~n is exempt from normalisation.

raw=true sends the expanded bytes verbatim: no CR appended, no normalisation.

Tools

How send and read decide to return

They return on whichever comes first:

• expect (a regex) matches the new output → reason matched

• the line has been silent for quiet_ms → reason quiet

• timeout_ms elapses → reason timeout

The quiet timer only starts after the first byte arrives, so a program that takes five seconds to say anything is not cut off at 700 ms. A call that sees no output at all runs to timeout_ms and returns timeout.

With auto_more (default on), a trailing --More-- (Space=yes, Rubout=no) prompt is answered with a space and collection continues, up to 20 pages; the answered prompts are removed from the returned text. Hitting the cap returns reason more_limit, and read continues from there. To flush a pager instead of paging through it, send ~d (rubout).

send and read return only output produced since the last call. peek does not move that cursor.

Output filtering

Raw output carries VT52 escape sequences, NUL padding and occasional telnet IAC bytes. The filter drops NULs, ESC+letter sequences, ESC Y <row> <col> cursor addressing, IAC negotiation (without implementing any telnet stack), and other nonprinting bytes; it keeps text, tabs and newlines. CR, LF and CRLF all become a single \n. Trailing whitespace and runs of blank lines are collapsed in returned text to save tokens; the scrollback keeps the unabridged text.

Typical session

open()
send("~z", expect="Happy hacking|ITS")   # ^Z calls ITS -> DDT banner
send(":login rms")
send(":listf")                           # pages collected automatically
send("foo~ej")                           # f-o-o ESC j CR  (DDT's foo$j)
close()

A detached but logged-in ITS session gets auto-logged-out after about five minutes; ITS itself never times out an idle line, so a held-open session is stable indefinitely.

Tests

uv run pytest                             # offline: filter, escapes, session, tools
PIDP10_LIVE=1 uv run pytest -m live       # acceptance tests on a real emulator
PIDP10_LIVE=1 PIDP10_LIVE_SLOW=1 uv run pytest -m live   # ...including the 3-minute idle test

The offline tests run the whole stack — including the MCP tool layer, via an in-process client — against a fake TCP line, so no emulator is needed.

Live tests are skipped unless PIDP10_LIVE=1; they take the single terminal line for their duration. They honour PIDP10_HOST / PIDP10_PORT, plus PIDP10_USER (default guest) and PIDP10_LISTF_DIR (default sys;, which needs to be a directory big enough to make the pager appear).

The test_three_minute_gap_does_not_drop_the_session case sits idle for 190 seconds on purpose — it is the regression test for the failure that motivated this server — so it needs PIDP10_LIVE_SLOW=1 as well.