Isimud MCP Server

isimud

isimud is a macOS menu bar app and MCP server that lets AI agents speak. An agent sends text to an MCP tool; isimud resolves a named voice, synthesizes it through a local or cloud provider, and plays it aloud through a single serialized speech queue.

It is the functional inverse of muninn (speech-to-text). Where muninn turns your voice into text for an agent, isimud turns an agent's text into voice. Named after Isimud, the two-faced messenger god of Enki.

isimud is:

• a local macOS menu bar app whose tray pulses while speaking, with a --headless mode for a pure server
• an MCP streamable-HTTP server exposing speak, stop, list_voices, and status tools to any MCP client
• BYOK by design: you bring provider keys; isimud routes across Apple local TTS, OpenAI, and Google with local-first fallback

What isimud Does

High-level flow:

agent calls isimud.speak -> resolve named voice -> pick first available provider -> synthesize audio -> enqueue on serialized speech worker -> play aloud -> broadcast lifecycle events

A single speech worker plays one utterance at a time. speak is fire-and-forget by default (returns a job id immediately) or blocking (wait=true). Every job transition is broadcast as a SpeechEvent to the tray (for the pulse animation) and to connected MCP peers (as notifications).

The current app supports:

• a live menu bar tray that pulses while speaking, or headless server-only operation
• an MCP streamable-HTTP endpoint on 127.0.0.1:3654/mcp (3654 = T9 for ENKI)
• named voices that hide provider-specific voice identifiers behind friendly names
• ordered provider routing with availability checks and silent fallback (with loud logging)
• a bounded, serialized speech queue with backpressure and degraded-health supervision
• macOS LaunchAgent autostart

MCP Tools

Tool	Description	Key params
isimud.speak	Enqueue speech; returns a job_id and queue_depth.	text, optional voice, rate, wait
isimud.stop	Cancel the active utterance and clear the queue.	—
isimud.list_voices	List configured named voices plus per-provider voices.	—
isimud.status	Report speech state, active job, queue depth, and health.	—

isimud.speak returns { job_id, queue_depth }. With wait=true it also returns an outcome (completed / failed / cancelled / timeout) and, on failure, an error. When the queue is full the call is rejected with JSON-RPC error code -32010 and a { queue_depth, capacity } data payload.

isimud.status returns { state, job_id?, voice?, provider?, queue_depth, degraded }, where state is idle or speaking and degraded flags that the speech subsystem needs attention.

Speech lifecycle events are forwarded to connected MCP peers as isimud/speech_event custom notifications. Event variants: enqueued, started, finished, failed, stopped, and degraded.

Named Voices & Providers

Named voices are the primary abstraction. A [voices.<name>] block maps a friendly name to a provider plus that provider's voice id, so agents never deal with provider-specific identifiers.

[voices.default]
provider = "apple"
voice = "Samantha"

[voices.narrator]
provider = "openai"
voice = "onyx"

[voices.googler]
provider = "google"
voice = "en-US-Neural2-C"
language = "en-US"

default_voice is a voice name, not a provider name. [tts].default_voice (and the voice argument to isimud.speak) must match one of the [voices.<name>] keys above — e.g. default, narrator, or googler. Setting default_voice = "openai" fails with unknown voice 'openai' unless a [voices.openai] block exists. To make OpenAI the default for a plain speak(text), point default_voice at a voice whose provider = "openai" (such as narrator), or add your own [voices.<name>] block mapped to that provider.

Providers and their routing:

• Apple — local, no API key. Synthesizes through the macOS say CLI (cancellable, headless-safe) and enumerates installed voices natively via AVSpeechSynthesisVoice. It plays its own audio, so it honors rate but cannot apply volume or pitch (logged once when requested).
• OpenAI — POST /v1/audio/speech; set OPENAI_API_KEY.
• Google — Google Cloud TTS text:synthesize; set GOOGLE_API_KEY.

[tts].providers is the availability / fallback order. When a named voice's provider is unavailable (for example a missing key), isimud falls back to the next available provider with a loud log line rather than failing silently.

OpenAI voices

Set the voice of a provider = "openai" block to one of the following ids. Styles are approximate character descriptions:

Voice	Style
alloy	neutral, balanced
ash	clear, articulate
ballad	smooth, melodic
cedar	warm, grounded
coral	vibrant, warm
echo	precise, resonant
fable	expressive, storyteller
marin	natural, conversational
nova	energetic, bright
onyx	deep, authoritative
sage	calm, measured
shimmer	cheerful, light
verse	poetic, expressive

BYOK & Provider Setup

isimud loads ./.env from the current working directory by default (disable with ISIMUD_LOAD_DOTENV=0). Shell environment variables win over .env and config values.

Concern	Variables	Notes
Apple TTS	none	Local macOS say + AVSpeechSynthesisVoice. No key required.
OpenAI TTS	OPENAI_API_KEY	Falls back to [providers.openai].api_key. Endpoint and model are configurable.
Google TTS	GOOGLE_API_KEY	Falls back to [providers.google].api_key. Endpoint and language are configurable.
MCP auth	ISIMUD_AUTH_TOKEN	Optional bearer token; falls back to [server].auth_token. Required for remote binds.
Config path	ISIMUD_CONFIG	Overrides the default config path resolution.

Quick Start

1) Build the app

cargo build --release

2) Resolve the config path

Config file precedence:

• ISIMUD_CONFIG
• $XDG_CONFIG_HOME/isimud/config.toml
• ~/.config/isimud/config.toml

On first run isimud writes a launchable default config to the resolved path. To start from the sample explicitly:

cp configs/config.sample.toml ~/.config/isimud/config.toml

3) Set provider env vars (optional)

export OPENAI_API_KEY="sk-..."   # only if you use the openai provider
export GOOGLE_API_KEY="..."      # only if you use the google provider

Apple voices work with no keys at all.

4) Run

./target/release/isimud            # menu bar tray + MCP server
./target/release/isimud --headless # MCP server only

Point your MCP client at http://127.0.0.1:3654/mcp and call isimud.speak.

Configuration

See configs/config.sample.toml for the full schema.

[tts]
providers = ["apple", "openai", "google"] # availability / fallback order
default_voice = "default"                  # a [voices.<name>] key below, NOT a provider name
rate = 1.0                                  # neutral multiplier (1.0 = normal)
max_queue_depth = 64                        # jobs allowed behind the active one; 0 = unbounded
wait_timeout_secs = 0                       # seconds wait=true blocks; 0 = wait forever

[server]
host = "127.0.0.1"
port = 3654
path = "/mcp"
allow_remote = false                        # non-loopback bind requires an auth token

[indicator.colors]
idle = "#636366"                            # systemGray (matches muninn)
speaking_bright = "#30D158"                 # systemGreen pulse, bright phase
speaking_dim = "#208A3A"                    # ~66% systemGreen, off phase

The bind address is loopback-only unless [server].allow_remote = true and an auth token is set.

Live config reload

isimud watches the config file and applies changes without a restart. Saving the file hot-swaps the running configuration via an ArcSwap, so tray icon colors ([indicator.colors], validated as #RRGGBB hex), named voices, rates, and provider credentials all take effect on the next utterance. Invalid edits are rejected with a logged warning and the previous config is kept.

Backpressure & Observability

isimud serializes all speech through one worker and bounds the queue:

• [tts].max_queue_depth caps jobs waiting behind the active utterance. When full, isimud.speak is rejected with JSON-RPC code -32010 and a { queue_depth, capacity } payload. Set 0 for unbounded fire-and-forget.
• [tts].wait_timeout_secs bounds how long a wait=true call blocks; 0 waits forever. On timeout the result outcome is timeout.
• A supervisor marks the engine degraded if the speech worker panics or exits unexpectedly, broadcasting a degraded event and surfacing degraded: true in isimud.status.
• Lifecycle events (enqueued / started / finished / failed / stopped / degraded) are broadcast to the tray and to MCP peers.

Tracing logs go to stderr and are controlled with RUST_LOG (targets: runtime, server, provider, config, speech). Set the base level with [logging].level.

Packaging (macOS)

TARGET=aarch64-apple-darwin scripts/build-release.sh
scripts/package-macos-app.sh   # builds dist/Isimud.app (+ .zip)

Enable login autostart with [app].autostart = true; isimud writes a LaunchAgent using the current executable path. When running the packaged .app, prefer macOS Login Items over the raw LaunchAgent.

Local Pre-commit

This repo ships a native prek.toml for fast local gates before you commit.

prek run --all-files
prek install

The hooks stay intentionally small: cargo fmt --all -- --check and cargo clippy --all-targets --all-features -- -D warnings.

Current Limits

• isimud currently supports macOS only.
• The Apple say provider honors rate but cannot apply volume or pitch.
• Provider fallback is silent substitution with loud logging; there is no per-call provider pinning beyond the named voice.
• The MCP server is streamable-HTTP only; there is no stdio transport.

License

MIT — see LICENSE.