switchboard-relay 0.2.3

A local MCP server that gives independent Claude Code sessions a shared, durable messaging channel.

🎛️ switchboard-relay

One session asks, another answers — no copy‑pasting between terminals.

A tiny local MCP server that gives independent Claude Code sessions a shared, durable messaging channel.

Claude Code's built‑in session channel is injected only by the desktop app, so terminal sessions can't use it. A config‑level MCP server, by contrast, loads on every surface — terminal CLI, desktop, and IDE — and its tools are allowlistable. switchboard-relay is exactly that: it routes named messages between any set of Claude Code sessions on one machine, backed by SQLite so mailboxes survive restarts.

        register("worker:auth")                       register("lead")
                  │                                           │
   ┌──────────────▼──────────────┐    ask()    ┌──────────────▼─────────────┐
   │       worker session        │ ──────────▶ │        lead session        │
   │  "how do we refresh JWTs?"  │             │  parked in a wait() loop,  │
   │           …blocks…          │ ◀────────── │  answers with reply_to set │
   └──────────────┬──────────────┘    reply    └──────────────┬─────────────┘
                  │                                            │
                  └─────────────────────┬──────────────────────┘
                                        ▼
                     ┌──────────────────────────────────────┐
                     │  board  ·  myrepo-3f9c1a  (SQLite)    │
                     │  durable mailboxes, survive restarts  │
                     └──────────────────────────────────────┘

The canonical pattern is a long‑running lead that short‑lived workers ask questions of. But switchboard just routes named messages, so any addressing scheme works.

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Contents

• Quickstart — zero to working in about a minute
• Concepts — the five words that explain everything
• Install — Claude Code, Claude Desktop, allowlisting
• Tools — the eight tools, at a glance
• Boards: one switchboard per project
• The lead / worker pattern — recipes + terminal inspection
• Configuration — environment variables
• How it works — and its one honest limitation
• Turn injection (push) — make an open session react to a message, no daemon needed
• Troubleshooting — the common "huh?" moments
• Development

---

Quickstart

Three steps, no configuration.

1. Install (pick one):

brew install mgd43b/taps/switchboard-relay   # macOS/Linux (Homebrew)
uv tool install switchboard-relay            # or uv
pipx install switchboard-relay               # or pipx

2. Add it to Claude Code at user scope, so it loads in every project:

claude mcp add --scope user -- switchboard-relay

3. Try it. Open two Claude Code sessions in the same repo. Paste into the first:

Register me on switchboard-relay as "lead", then wait() for a message and reply to its sender.

…and into the second:

Register me on switchboard-relay, then ask() "lead": what should I work on next?

The second session gets its answer back inline — no window‑switching. That's the whole loop. 🎉

Why did that just work? Both sessions share the same board (this repo), so they found each other with zero setup. The second session didn't even need a name — it registered under its session title. Read on for how names, roles, durability, and boards fit together.

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Concepts

Five words cover the whole model:

Term	What it is
Participant	A registered session. Any Claude Code session becomes one by calling register().
Name	Your address that others send() to — e.g. "lead", "worker:auth". Optional: omit it and Claude registers under your session title.
Role	An optional shared address for a group (e.g. "worker"). A message to a role goes to whichever member reads it first.
Board	One isolated switchboard — its own participants and mailboxes. Defaults to one per project, so repos don't cross wires.
Durable	Messages wait in the recipient's mailbox until read — even if the recipient hasn't registered yet, or the process restarted.

---

Install

The fastest path is the Claude Code plugin — two commands, no Python setup (the server runs via uvx). Prefer explicit config? Jump to installing the package manually.

Claude Code plugin (recommended)

Run these in any Claude Code session:

/plugin marketplace add mgd43b/switchboard-relay
/plugin install switchboard-relay@mgd43b

The mcp__switchboard-relay__* tools are wired up automatically — the plugin declares an MCP server that runs via uvx switchboard-relay, so there's nothing to pip install. Verify with /plugin or claude mcp list. (Installing the plugin covers Claude Code on every surface; the manual steps below are the alternative, not an addition.)

Install the package manually

switchboard-relay is a standard Python package (Python ≥ 3.10). Install it so the switchboard-relay command is on your PATH:

# Homebrew (macOS/Linux)
brew install mgd43b/taps/switchboard-relay

# with uv (recommended)
uv tool install switchboard-relay

# or pipx
pipx install switchboard-relay

# or from a checkout of this repo
uv tool install .

Add it to Claude Code

Register it at user scope so it loads in every project, on every surface (terminal CLI, desktop, and IDE):

claude mcp add --scope user -- switchboard-relay

That's it — open any Claude Code session and the eight switchboard-relay tools are available. Verify with claude mcp list.

No install step? Point Claude Code at uvx and skip installing anything:

claude mcp add --scope user -- uvx switchboard-relay

Add it to Claude Desktop

Open Settings → Developer → Edit Config (or edit claude_desktop_config.json directly — ~/Library/Application Support/Claude/ on macOS, %APPDATA%\Claude\ on Windows) and add a switchboard-relay entry:

{
  "mcpServers": {
    "switchboard-relay": {
      "command": "switchboard-relay",
      "env": { "SWITCHBOARD_BOARD": "desktop" }
    }
  }
}

Then restart Claude Desktop. Notes:

• Use the full path (which switchboard-relay) if the binary isn't on Claude Desktop's PATH, or swap in "command": "uvx", "args": ["switchboard-relay"] to skip installing.
• Claude Desktop isn't project‑scoped, so pin an explicit SWITCHBOARD_BOARD (see Boards) to keep its sessions on a predictable board.
• Claude Desktop can't self‑react via Channels (no launch flag), but it can be a turn‑injection reactor via ccd_session_mgmt with one approval click per message (SWITCHBOARD_CCD_INJECT=1). Otherwise it uses the durable tools and polls.

Run the tools without a confirmation prompt (optional)

By default Claude asks before each tool call. To let switchboard's tools fire silently, allowlist them in your user settings (~/.claude/settings.json). The easy way — one entry for the whole server:

{ "permissions": { "allow": ["mcp__switchboard-relay"] } }

Or allowlist each tool individually

{
  "permissions": {
    "allow": [
      "mcp__switchboard-relay__register",
      "mcp__switchboard-relay__participants",
      "mcp__switchboard-relay__send",
      "mcp__switchboard-relay__inbox",
      "mcp__switchboard-relay__wait",
      "mcp__switchboard-relay__ask",
      "mcp__switchboard-relay__broadcast",
      "mcp__switchboard-relay__unregister"
    ]
  }
}

---

Tools

Eight tools, grouped by what you reach for:

Presence — join and see who's around

Tool	Signature	What it does
register	register(name?, role?)	Claim an address for this session. name is optional — omit it and Claude registers under the session title. role is an optional shared group address. Re‑call to heartbeat or change role. Returns the board you joined and the live participants.
participants	participants()	List sessions seen within the TTL window: name, role, idle_seconds.
unregister	unregister()	Leave the switchboard (drop out of participants()). Your mailbox is preserved for when you return.

Send — put a message in someone's mailbox

Tool	Signature	What it does
send	send(to, body, reply_to?)	Append a message to to's durable inbox. to matches a participant name or role. reply_to threads a reply to a message id. Returns the new message id — plus no_live_recipient: true and a warning if nobody is currently registered as to (usually a typo; the message is still queued).
broadcast	broadcast(body)	Send body to every currently‑live participant except yourself. Returns the per‑recipient message ids.

Receive — read your mailbox

Tool	Signature	What it does
inbox	inbox(peek?, since?)	Read messages addressed to you. Drains by default (each delivered once); peek=true reads without removing; since=<id> returns only messages newer than that id.
wait	wait(timeout_s?)	Block up to timeout_s seconds (default 30, max 3600) until a message arrives, then drain and return it. Returns timed_out: true on timeout.

Ask — send and block for the answer in one call

Tool	Signature	What it does
ask	ask(to, body, timeout_s?)	Send body to to, then block until a reply threaded to it comes back (reply_to = the returned question_id). Leaves other inbox messages untouched; returns timed_out: true if no reply in time — with no_live_recipient: true when nobody was registered as to at send time, so you can tell a wrong address from a slow reply.

Durability & addressing. A message sent to a name that hasn't registered yet simply waits in that mailbox until it's read. Addressing by role fans a message out to whichever participant reads with that role first — for reliable one‑to‑one delivery, use unique names.

---

Boards: one switchboard per project

A board is one isolated switchboard — its own participants and its own mailboxes. By default the board is derived from your project, so sessions in different repos don't see each other and each project gets a private bus for free. All of a repo's git worktrees (and any subdirectory) resolve to the same board, because the board is keyed off the repository's shared .git, not the working directory.

The board a session joins is resolved in this order:

1. $SWITCHBOARD_BOARD — an explicit board name (any string), used verbatim. The special value project forces the project‑derived board below.
2. The current project (the default) — keyed off the git repo (via CLAUDE_PROJECT_DIR, which Claude Code injects into the server), falling back to the launch directory when it isn't a git repo. The resulting board name looks like myrepo-3f9c1a.

register() returns the board you joined, so a session can always see which switchboard it's on. Each board is its own SQLite file under ~/.claude/switchboard/<board>.db. (Setting SWITCHBOARD_DB to a raw path still overrides everything — handy for pointing several sessions at one exact file.)

Sharing a board across projects

Want the classic cross‑repo setup where a worker in repo A asks a lead in repo B? Put both sessions on the same named board:

# lead, in repo B
claude mcp add --scope user --env SWITCHBOARD_BOARD=team -- switchboard-relay
# worker, in repo A — same board name
claude mcp add --scope user --env SWITCHBOARD_BOARD=team -- switchboard-relay

Any shared string works; pick one name and use it everywhere those sessions should talk.

Upgrading from ≤ 0.2? The default used to be a single global board (~/.claude/switchboard.db). It's now per‑project. To get the old global behavior back, set SWITCHBOARD_BOARD to a shared name (as above), or point SWITCHBOARD_DB at the old file.

---

The lead / worker pattern

Within one project this works with zero board configuration — every session in the repo shares the project's board automatically. For a lead and workers spread across different repos, first put them on a shared board (see Boards).

The lead (coordinator)

Keep one session open as the long‑running lead. Paste this and let it run:

Register me on switchboard-relay as "lead", then keep calling wait() in a loop:
whenever a message arrives, answer the question by sending a reply back to its
sender (use reply_to), then wait() again.

Claude will register(name="lead") and park in wait(). A lead keeps a well‑known name so workers can address it; anyone who doesn't need a fixed address can just say "register me on switchboard-relay" and Claude registers under the session title. Keep it going hands‑free with the /loop skill:

/loop wait for a switchboard-relay message, answer it, and reply to the sender

A worker

In any other session, ask the lead and get the answer inline — one call, no explicit name needed:

Register me on switchboard-relay with role "worker", then use ask() to ask the
lead how our auth middleware refreshes tokens.

The worker registers under its session title and calls ask("lead", "…") — one call that sends the question and blocks for the answer. The lead's loop picks it up, replies with reply_to set, and the worker's ask() returns the reply. No window‑switching, no manual wait().

Tip: launch a worker pre‑addressed via environment variables so it doesn't even need an explicit register call — set SWITCHBOARD_NAME=worker:auth and SWITCHBOARD_ROLE=worker in that session's MCP server env.

More copy‑paste recipes live in examples/.

Peek at the traffic from your terminal

switchboard-relay doubles as a small inspection CLI over the same database — handy for seeing who's connected and what's queued, without an MCP client. It targets the current project's board by default; add --board <name> (or --db <path>) to inspect another:

switchboard-relay doctor                         # ⭐ "why isn't this working?" — resolution, peers, hints
switchboard-relay boards                         # every local board + its live participant count
switchboard-relay participants                   # live participants on this board (name, role, idle)
switchboard-relay tail                           # queued (undelivered) messages on this board
switchboard-relay tail --follow                  # …and keep watching
switchboard-relay prune                          # delete old dead-letter messages + expired participants
switchboard-relay participants --board team      # …a specific board instead

doctor is the one-shot diagnostic: it prints which board you resolved to (and how — --board, $SWITCHBOARD_BOARD, project-derived, …), the relevant env vars, live participants, the queued-message count, and a plain-English hint when something looks off (you're alone on a board, or messages are piling up against a name nobody reads).

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Configuration

All optional. Set as environment variables (e.g. via claude mcp add --env KEY=value):

Variable	Default	Meaning
SWITCHBOARD_BOARD	(project)	Board to join. An explicit name (any string) puts these sessions on a shared bus; project forces per‑project derivation. See Boards.
SWITCHBOARD_DB	(the board's file)	Raw SQLite path override — wins over SWITCHBOARD_BOARD. Point several sessions at one exact file to share it.
SWITCHBOARD_TTL	300	Seconds of inactivity before a participant drops out of participants().
SWITCHBOARD_MSG_TTL	604800 (7 days)	Undelivered messages older than this are pruned automatically during normal operation. Set 0 to disable age‑out.
SWITCHBOARD_MAX_BODY	262144 (256 KiB)	Reject a send() whose body exceeds this many UTF‑8 bytes. Set 0 to disable the cap.
SWITCHBOARD_NAME	—	Auto‑register this session under this address (skips an explicit register; also the fallback when register() is called without a name).
SWITCHBOARD_ROLE	—	Role to pair with SWITCHBOARD_NAME.
SWITCHBOARD_PUSH	0	Enable turn‑injection push over Channels (CLI reactors) — runs the background self‑watch loop that nudges this session's own client. Off by default (a small background poll cost); set 1 to enable.
SWITCHBOARD_CCD_INJECT	0	Enable turn injection on Claude Desktop: send() returns an inject hint so the sender's Claude can call ccd_session_mgmt.send_message. Off by default (leans on a Desktop tool + a per‑message approval).
SWITCHBOARD_CCD_SESSION_ID	(local_<CLAUDE_CODE_SESSION_ID>)	Override this session's CCD id used for Desktop injection. Needed only when the default derivation is wrong (e.g. an agent/child session). Used verbatim.

Example — a longer liveness window:

claude mcp add --scope user --env SWITCHBOARD_TTL=600 -- switchboard-relay

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How it works

Each Claude Code session spawns its own switchboard-relay process (stdio transport). Those processes don't talk to each other directly — they share state through a SQLite database (one file per board), which gives you durability and cross‑session delivery for free. wait() long‑polls that database.

The one honest limitation: switchboard cannot wake a closed session. In the baseline (poll) model a recipient learns about a message by calling inbox() or wait() — on its next turn, or while parked in a /loop. An open but idle session can be pushed into a turn with turn injection, but nothing reaches a session that isn't running. That's a property of how Claude Code sessions work, not a switchboard limitation.

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Turn injection (push)

By default a recipient only learns about a message when it next polls (inbox()/wait()). Turn injection removes that wait: a send() makes the recipient's open session react on the spot — the message arrives as a turn, and the session drains its inbox and acts, with no manual poll. It's the "responsive‑lead" setup — a lead that answers the instant a worker asks.

There are two mechanisms, by recipient surface — set up whichever matches where your reactor runs (the sender can be any surface):

Reactor runs on…	Mechanism	Feel
Claude Code (CLI/terminal)	Channels — the recipient self‑injects	zero‑touch, fully automatic
Claude Desktop	ccd_session_mgmt.send_message — the sender injects	one approval click per message

Both are best‑effort accelerators on top of durable poll, and both preserve drain‑once. Below: the CLI setup first, then the Desktop setup.

Reacting on the CLI (Channels) — the three hard constraints

Turn injection over Channels works only when all three hold. Miss any one and delivery silently falls back to the durable poll — nothing breaks, the message just waits in the inbox as usual:

1. Push is enabled. It's a background convenience with a small polling cost, so it's off by default — set SWITCHBOARD_PUSH=1 to turn it on.
2. The recipient session is open. Channels inject into a running session on its next turn. Nothing can wake a fully idle or closed session — see How it works.
3. The recipient subscribed to switchboard as a channel — launched with the channel flag below. A session that connected normally still works; it just polls instead of reacting.

Setup

Because MCP stdio is bidirectional, each session's own switchboard process can push a notification to its own client. With push enabled, that process runs a background watcher that polls the shared board and self‑nudges when a message lands — so you get turn injection with no daemon, no launchd, no reboot story, and per‑project boards keep working.

# 1. Register the stdio server with push enabled (one extra env var):
claude mcp add --scope user --env SWITCHBOARD_PUSH=1 -- switchboard-relay

# 2. Launch each session that should REACT with switchboard subscribed as a
#    channel. A config‑level MCP server is a `server:` channel, which during the
#    research preview is never on the first‑party allowlist — so it needs the
#    development flag (plain `--channels server:…` is skipped as "not on the
#    approved channels allowlist"):
claude --dangerously-load-development-channels server:switchboard-relay

That last flag is the whole subscription recipe. On start you'll see a dim confirmation like Channels (experimental) messages from server:switchboard-relay inject directly in this session. Now park the lead in a wait() /loop and have workers ask() as usual — the lead reacts the moment a question lands, no poll required.

Org policy. On Claude.ai Team/Enterprise (and Console orgs with managed settings) an admin must set channelsEnabled: true first, or channels are blocked (the server still connects and its tools still work — only the push is suppressed). Pro/Max users without an org skip that check.

Reacting on Claude Desktop (ccd_session_mgmt)

Claude Desktop has no channel launch flag, so a Desktop session can't self‑inject the way a CLI session does. But Desktop exposes a built‑in ccd_session_mgmt.send_message tool that injects a turn into another session — so switchboard can broker it: it can't call that tool itself (an MCP server can't invoke another server's tools), but it can hand the sender's Claude everything needed to make the call.

# Register the stdio server with Desktop injection enabled:
claude mcp add --scope user --env SWITCHBOARD_CCD_INJECT=1 -- switchboard-relay

How it flows: each session's switchboard auto‑captures its own CCD id at register() (local_<CLAUDE_CODE_SESSION_ID>, which Claude Code puts in the server's env) and stores it on the board. When you send(to=X), the result carries an inject field with X's session_id and a ready‑to‑send message; the sender's Claude then calls ccd_session_mgmt.send_message(...) and X reacts. No session ever discloses its id to another — each records only its own.

The honest caveats:

• One approval click per message. send_message is on a hardcoded list in the Desktop client (alongside AskUserQuestion/ExitPlanMode) that always prompts you to approve — it's the guardrail against one session silently driving another, and there is no setting to disable it.
• The sender must be a Desktop/Cowork session (only those have the ccd_session_mgmt tools). The reactor is Desktop; a pure terminal claude can't be the injecting sender. So this is the mirror image of the CLI path.
• ask() isn't covered — it blocks the sender while waiting, so the sender can't inject mid‑call. Use send() for a Desktop reactor (it replies with its own send()), or make the reactor a CLI session.
• The CCD id derivation (local_<CLAUDE_CODE_SESSION_ID>) holds for a normal top‑level session; in an agent/child context the env id isn't the addressable one, so set SWITCHBOARD_CCD_SESSION_ID=<full id> to override. Confirm once with the two‑session check below.
• Best‑effort and idempotent: the injected turn is a body‑less nudge (the real message stays in the durable inbox and drains exactly once), so even if a recipient also has the CLI watcher, a double nudge can't double‑deliver.

What's actually sent (and why it stays exactly‑once)

On the CLI path, switchboard emits a notifications/claude/channel notification that Claude Code wraps into the recipient's next turn as <channel source="switchboard-relay" msg_from="…" msg_id="…">…</channel>. Each session's own watcher polls the board for messages addressed to itself and self‑nudges; the cross‑session hop is the shared SQLite board.

The notification is a nudge that says "drain your inbox and handle it", not the message body. The durable SQLite row stays the single source of truth, which is what preserves drain‑once: when a role is addressed, every connected member is nudged, but only the one that wins the atomic inbox() drain receives the message — the others find an empty inbox (the nudge says so) and do nothing. (Inlining the body for a unique‑name/single‑reader target was considered and deliberately declined: it would split the source of truth and risk double‑handling.)

Two things stay true no matter what: push never replaces durable delivery (disable it and everything still works by polling), and it can't wake a closed session. Channels is a research preview whose contract may change, so treat push as the fast path on top of the durable poll, never a dependency.

Verify it end‑to‑end (two real sessions)

The research‑preview reaction can't be unit‑tested, so confirm it by hand once:

1. Register the server with push on: claude mcp add --scope user --env SWITCHBOARD_PUSH=1 -- switchboard-relay.
2. Terminal 1 — session B (the reactor): launch subscribed, register, and park it:

   claude --dangerously-load-development-channels server:switchboard-relay
   

   In B: register(name="lead"), then run /loop with wait(timeout_s=600) (or just leave it idle). Do not call inbox() by hand.
3. Terminal 2 — session A (the sender): launch (subscription optional for a pure sender), register(name="worker"), then send(to="lead", body="ping — what's 2+2?").
4. Watch B react with no manual poll: within ~a second a <channel source="switchboard-relay" …> turn appears in B, and B calls inbox() on its own, reads "ping — what's 2+2?", and handles it (e.g. replies with send(to="worker", body="4", reply_to=<id>)). If instead B does nothing, re‑check the three constraints — most often push wasn't enabled (SWITCHBOARD_PUSH=1), B wasn't launched with --dangerously-load-development-channels, or an org policy has channelsEnabled off.

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Troubleshooting

Start here: run switchboard-relay doctor. It resolves your board, lists live peers and queued messages, and prints a hint for the two most common failures below — usually enough to spot the problem in one shot.

"I sent a message but nothing happened." switchboard can't wake an idle session — a recipient only sees a message when it calls inbox() or wait(). Keep your lead parked in a /loop so it's always listening. (See How it works.)

"The other session can't see me / participants() is empty." You're probably on different boards — each project gets its own by default. Run switchboard-relay boards to list them, and make sure both sessions are on the same one: the same repo, or the same SWITCHBOARD_BOARD. (See Boards.)

"ask() timed out, or a send() came back with no_live_recipient." Nobody is registered under that name/role right now — usually a typo ("leed" vs "lead") or the recipient is offline. Check live addresses with participants() or switchboard-relay participants. The message is still queued durably, so a correctly‑named recipient gets it later.

"register() gave me a session-… name I didn't choose." No name was passed and none could be derived, so one was assigned. That's fine for a worker that only asks questions; for anything others need to address (like a lead), pass an explicit name — register(name="lead").

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Development

uv venv && uv pip install -e '.[dev]'
uv run pytest                 # runs all tiers; coverage gate is enforced (fail-under 95%)
uv run pytest -m unit         # or a single tier: unit | feature | integration
uv run ruff check .           # lint
uv run ruff format .          # format

Source layout — each module is small and single‑purpose:

• store.py — the durable SQLite store (registry + mailboxes). Pure and clock‑free.
• board.py — board resolution: which switchboard a session joins (env / git worktree → DB path). Pure and transport‑free.
• server.py — the FastMCP server: identity binding, the eight tools, the wait() poll loop, and best‑effort push.

Tests are split into three tiers:

• tests/unit/ — the SQLite store and board resolution in isolation.
• tests/feature/ — tool behavior through the server layer (identity, push, roles, hygiene bounds, the CLI) with a fake Context.
• tests/integration/ — the tools over a real MCP transport, including two real stdio subprocesses, plus an N‑process exactly‑once stress test that reconciles sent‑vs‑received ids under contention (including shared‑role drains).

The Claude Code plugin/marketplace manifests live in .claude-plugin/; validate them with claude plugin validate ..

CI (Python 3.10–3.14) runs ruff + the full suite with the coverage gate on every push and PR. Releases and Homebrew packaging are documented in RELEASING.md.

Non‑goals (v1)

Single machine only (no cross‑machine bus), no auth/encryption, no transcript search, no GUI.

License

MIT