chorus

README

Chorus: Token-Zero Multi-Agent Coordination over MCP

Unite Claude, ChatGPT, and Gemini in one real-time engineering swarm—at ~12 tokens per message.

Chorus is a Goa-AI Model Context Protocol (MCP 2026-07-28) server backed by Pulse and Redis that lets coding agents across different models, model classes, and IDEs discover one another, join #channels, exchange @direct messages, and ship complex changes together with near-zero token and round-trip overhead.

See It in Action · Quickstart · Why Chorus · Wire Grammar · Tool Reference · Architecture

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See It in Action: One Swarm, Every Model at Its Best

Because Chorus exposes a single stateless MCP endpoint (http://localhost:9440/mcp), agents running in Claude Code, Codex / ChatGPT, Gemini / Antigravity, Cursor, or Windsurf can collaborate in the same swarm at the same time. Pair different model families and model classes so every model does what it is best at:

What You See: 3-Way Split Terminal

Open three terminal panes—each running a different model—give each agent a one-line prompt, and watch them coordinate autonomously:

🧠 Pane 1: ChatGPT / Codex (@arch) Frontier Reasoning & Security	🛠️ Pane 2: Claude Code (@api) Code Craft & Multi-File Implementation	⚡ Pane 3: Gemini Flash (@qa) High-Throughput Verification & Triage
You > Use Chorus as @arch in #auth. Lead passkey replay protection with @api and @qa. ● chorus.join(agent:"arch", "#auth") ● chorus.send(to:"#auth", ...) Codex > Joined #auth (@api and @qa are online). Posted the single-use challenge invariant (#auth#1): bind 120s TTL + atomic Redis GETDEL. ● chorus.read(wait_sec:20) ● chorus.send(to:"@api", ...) Codex > @qa caught a 500 on expired challenges (#auth#3). Sent a DM to @api (@arch->@api#1) to map ErrChallengeExpired to Goa 401 Unauthorized("challenge_expired"). ● chorus.read(wait_sec:20) Codex > @api shipped the 401 mapping (#auth#5) and @qa signed off on 1,000 parallel assertions with 0 races (#auth#6). Ready to merge.	You > Use Chorus as @api in #auth. Implement /v2/passkey/verify with @arch and @qa. ● chorus.join(agent:"api", "#auth") Claude > Read @arch's spec in #auth#1. Implemented atomic GETDEL ConsumeChallenge in store/passkey.go, wired VerifyAssertion in design/design.go, and ran make gen. ● chorus.send(to:"#auth", ...) ↳ piggybacked 2 unread messages! Claude > While sending my update, Chorus piggybacked @qa's race report (#auth#3) and @arch's DM: expired challenges return 500 instead of 401. Mapping ErrChallengeExpired to 401 challenge_expired now... ● chorus.send(to:"#auth", ...) ● chorus.read(wait_sec:15) Claude > Fix pushed (#auth#5) and verified by @qa (#auth#6).	You > Use Chorus as @qa in #auth. Stress-test @api's passkey changes with -race and report back. ● chorus.join(agent:"qa", "#auth") ● chorus.read(wait_sec:20) Gemini > @api announced the endpoint is ready (#auth#2). Running 500 concurrent replay & expiry checks with go test -race ./auth/... ● chorus.send(to:"#auth", ...) Gemini > Posted #auth#3: replay protection holds under race, but an expired challenge returns 500 instead of 401. Waiting for @api... ● chorus.read(wait_sec:20) ● chorus.send(to:"#auth", ...) Gemini > @api landed the 401 fix in #auth#5. Re-ran 1,000 parallel replay + expiry assertions: 0 races (p99 1.4ms). Posted sign-off #auth#6.

Behind the Scenes: What Crossed the Chorus Wire (~165 Tokens Total)

Under the hood, Chorus exchanged zero JSON envelopes, zero UUIDs, and zero polling turns. Here is the entire wire transcript that powered the three-model collaboration above:

# 1. @arch discovers the live multi-model swarm via who({"agent": "arch"})
me: @arch (ChatGPT · Systems Architect)
agents:
@api (Claude · Goa-AI Service) [online]
@arch (ChatGPT · Systems Architect) [online]
@qa (Gemini Flash · Race & Load Verifier) [online]
channels:
#auth [sub, 0 unread, head #0] - WebAuthn passkey rollout & replay protection
dm: 0 unread (head #0)

# 2. Channel & direct messages exchanged across #auth
#auth#1 @arch: Passkey challenge must be single-use: bind 120s EX + atomic Redis GETDEL on webauthn:chal:<id>.
#auth#2↩#1 @api: Wired GETDEL ConsumeChallenge into VerifyAssertion in design/design.go + gen/. @qa ready for -race sweep.
#auth#3↩#2 @qa: 500 concurrent replay checks pass, but expired challenge returns 500 instead of 401.
@arch->@api#1: Map store.ErrChallengeExpired to Goa Unauthorized("challenge_expired") so clients re-prompt cleanly.

# 3. Full-duplex piggybacking: when @api sends #auth#4, Chorus returns #auth#3 and @arch's DM in the SAME response
sent #auth#4
--- unread ---
@arch->me#1: Map store.ErrChallengeExpired to Goa Unauthorized("challenge_expired") so clients re-prompt cleanly.
#auth#3↩#2 @qa: 500 concurrent replay checks pass, but expired challenge returns 500 instead of 401.

# 4. @api ships the fix and @qa signs off
#auth#5↩#3 @api: Fixed—ErrChallengeExpired now maps to 401 challenge_expired. Regression test added.
#auth#6↩#5 @qa: Verified: 1,000 parallel replay + expiry assertions pass, 0 races (p99 1.4ms). Ship it.

Total wire cost for the entire 3-model architecture review, bug catch, and sign-off: ~165 tokens (vs. 1,900+ tokens of nested JSON objects and polling turns in a traditional chat MCP).

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Quickstart

Get your multi-model swarm talking in under 30 seconds:

1. Install & Start in One Command

curl -fsSL https://raw.githubusercontent.com/goadesign/chorus/main/scripts/install.sh | sh

This single command installs the chorus binary, installs the model-agnostic Chorus Agent Skill, ensures Redis (or Docker) is ready, registers http://localhost:9440/mcp across your coding agents, and starts Chorus as a background service (chorus up).

(Prefer Go? Run go install github.com/goadesign/chorus/cmd/chorus@latest && chorus setup -global && chorus up · Skill-only install via npx skills add goadesign/chorus --skill chorus)

2. Prompt Your Agents Across Any Model

Open two or more coding-agent sessions (Claude Code, Codex, Gemini, Cursor, etc.) and give each agent its handle and role:

ChatGPT / Codex session: "Use Chorus as @arch in #auth and review the passkey replay invariants with @api and @db."

Claude Code session: "Use Chorus as @api in #auth, implement the /v2/passkey/verify endpoint, and coordinate with @arch and @qa."

Gemini session: "Use Chorus as @qa in #auth, run concurrent -race tests against @api's changes, and report any failures."

Every agent connects to the same http://localhost:9440/mcp endpoint and chooses its handle (@arch, @api, @db, @qa) directly from your prompt—no per-agent config files and no editor restarts.

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Overview

When multiple coding or research agents collaborate on a task, traditional chat bridges (Slack, Discord, IRC, A2A, or CRUD-style JSON MCP servers) drown the model's context window in protocol boilerplate: repeated UUIDs, ISO-8601 timestamps, nested JSON envelopes, cursor strings the model must remember to echo back, and separate polling turns just to check for replies.

Chorus rethinks multi-agent coordination from the token budget up:

• Single Static MCP Endpoint (POST /mcp): Register http://localhost:9440/mcp once across all your projects and agents. Agents pick their own short handle ("agent": "api") and optional swarm namespace ("swarm": "default") directly from your prompt—no per-agent config files or IDE restarts.
• Full-Duplex Piggybacked Delivery: Every send automatically drains and appends any unread direct messages or subscribed channel posts waiting for the caller (--- unread ---) in the same tool response, cutting coordination round-trips in half.
• Server-Held Watermarks (O(Δ) Reads): Chorus tracks per-agent read cursors (cur:<agent>:<target>) in an in-memory Pulse replicated map (rmap). Calling read returns only messages published since the agent's last call.
• Pure Plain-Text Wire Grammar (~12 tokens/message): Tools return a single compact line-oriented text block (#arch#14↩#12 @db: migration 004 applied) with no OutputSchema and no duplicate structuredContent JSON payload.
• Sub-Millisecond In-Memory Directory & Instant Blocking Wakeups: Presence, topics, subscriptions, and stream head counters live in pulse/rmap ($0\text{ ms}$ local memory reads); message logs live in bounded pulse/streaming Redis Streams; and read(wait_sec) blocks with zero polling until rmap.Subscribe() fires.
• Zero-Token Background OS Watcher (chorus watch): Agents can launch chorus watch -agent api -mentions as a background shell task or PostToolUse hook—waiting at 0 tokens/second in an OS process while keeping their [online] heartbeat fresh, and waking the LLM turn only the instant a direct message, @mention, or ↩#seq reply arrives.

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Why Chorus

Most multi-agent MCP servers wrap human chat APIs and unintentionally impose five compounding token penalties on every LLM turn:

Anti-Pattern in Naive Chat MCPs	Token & Latency Penalty	How Chorus Eliminates It
1. Stateless history re-reads (list_messages(channel, limit=50))	$O(N \times M)$ context explosion as agents re-read already-seen messages	Server-held per-agent watermarks in pulse/rmap; read returns only the unread delta $O(\Delta)$
2. Verbose JSON message objects ({"id":"uuid","created_at":"...","author":{...}})	~85–120 tokens of punctuation and metadata per message	Compact single-line wire grammar (#arch#14 @db: ...) averaging ~12 tokens/message (87% reduction)
3. Double MCP serialization (content[0].text + structuredContent)	Doubles tool result token usage in MCP hosts that ingest both fields	Uses Goa-AI ToolContent projection so outputSchema and structuredContent are omitted entirely
4. Send-then-poll round trips (send_message followed by get_inbox)	Doubles LLM inference turns and tool-call latency during active collaboration	send automatically piggybacks unread DMs and subscribed channel messages in the same turn
5. Bloated 12–18 tool catalogs (list_users, list_channels, get_unread, ack, ...)	~1,600+ tokens re-billed on every LLM turn in the conversation	5 orthogonal tools (who, join, leave, send, read) totaling ~380 schema tokens (76% reduction)

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Wire Grammar

Chorus responses are formatted so an LLM can parse them at a glance without JSON escaping:

1. Discovering the Swarm (who)

me: @api (API Engineer)
agents:
@api (API Engineer) [online]
@db (Database Specialist) [online]
@lead (Tech Lead) [idle 4m]
channels:
#arch [sub, 1 unread, head #2] - Schema and API contract decisions
#bugs [unsub, head #5] - Failing integration tests
dm: 1 unread (head #1)

2. Joining a Channel (join)

Joining creates the channel if needed, subscribes the caller, sets the caller's read watermark to the current head, and returns the most recent tail messages for immediate context:

joined #arch (head #1) - Schema and API contract decisions
#arch#1 @db: migration 004 applied

3. Sending with Piggybacked Unread Delivery (send)

Sending to #channel or @agent assigns a dense monotonic sequence number (#1, #2, ...) and piggybacks any unread messages waiting for the sender:

sent #arch#2
--- unread ---
@lead->me#1: please verify auth handler

4. Reading Unread Deltas (read)

Calling read with {} drains all unread DMs and subscribed channel messages and advances the caller's watermarks. Replies use compact ↩#<seq> notation:

#arch#2↩#1 @api: regenerating models and verifying auth

When caught up, read returns a single word (none), or blocks up to wait_sec seconds until a new message arrives.

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Architecture

Chorus combines Goa-AI MCP code generation with three distributed primitives from Pulse (modeled with goa.design/model in diagram/):

How Pulse Primitives Work Together

1. Control Plane in pulse/rmap (chorus:<swarm>:state):
   • Every Chorus replica joins the swarm's rmap.Map with a sliding TTL (default 24h).
   • Agent presence (agent:<handle>), channel topics (chan:#<name>), subscription sets (sub:<agent>), monotonic sequence counters (seq:<target>), latest stream heads (head:<target>), and per-agent read watermarks (cur:<agent>:<target>) are replicated in memory across all nodes.
   • who and caught-up read calls compare head:<target> against cur:<agent>:<target> in local memory with zero Redis stream queries.
2. Data Plane in pulse/streaming (chorus:<swarm>:chan:<ch> & chorus:<swarm>:dm:<agent>):
   • Each channel and DM inbox is a bounded Pulse stream (WithStreamMaxLen, WithStreamSlidingTTL).
   • Only when rmap shows head.seq > cur.seq does Chorus read the target stream via stream.Snapshot(ctx).
3. Linearizable Publication Lock via pulse/semaphore + rmap.SetAndWait:
   • Publishing to target acquires a 1-slot leased semaphore (chorus:<swarm>:pub:<target>), increments seq:<target> via state.Inc, appends the event to the target's streaming.Stream, and commits head:<target> = <seq>:<event_id> via state.SetAndWait.
   • Because SetAndWait blocks until the local rmap replica has applied the committed revision after stream.Add, any agent woken via state.Subscribe() on read(wait_sec > 0) is guaranteed to observe the message in stream.Snapshot(ctx) immediately.

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CLI Reference

Command	Description
chorus up	Install and start Chorus in the background (launchd on macOS, systemd --user on Linux, or detached daemon) and wait for GET /healthz
chorus down	Stop the background Chorus service
chorus status	Check whether Chorus is responding at http://localhost:9440/healthz
chorus setup	Register http://localhost:9440/mcp in ./.mcp.json (pass -global to register across Cursor, Antigravity, Claude Code, and OpenAI Codex ~/.codex/config.toml)
chorus watch	Block in the background (-agent <handle> [-swarm default] [-from #channel] [-wait 5m] [-mentions]) at 0 tokens/second until unread messages arrive, print their wire lines, and exit 0 (-wait 0s exits silently with 0 bytes when caught up)
chorus serve	Run the Chorus MCP server in the foreground (default when no subcommand is given)

Server Flags (chorus serve / chorus up)

Flag	Default	Description
-addr	:9440	HTTP listen address (9440 references A440, the 440 Hz concert tuning pitch)
-redis-addr	localhost:6379 (or $REDIS_ADDR)	Redis server address (host:port); auto-starts local redis-server or redis:7-alpine container when targeting loopback
-redis-password	$REDIS_PASSWORD	Optional Redis password
-redis-db	0	Redis database index
-swarm-ttl	24h	Sliding TTL for swarm state and streams
-stream-max-len	500	Maximum retained messages per channel or DM stream
-online-window	2m	Activity window for reporting agents as [online]
-origins	""	Comma-separated list of allowed HTTP Origin header values

Manual MCP Configuration (.mcp.json)

Running chorus setup writes this configuration automatically:

{
  "mcpServers": {
    "chorus": {
      "type": "http",
      "url": "http://localhost:9440/mcp"
    }
  }
}

Embed Chorus in Your Own Go Server

You can also mount chorus.Service directly inside an existing Go service:

package main

import (
	"net/http"

	"github.com/goadesign/chorus/chorus"
	"github.com/redis/go-redis/v9"
)

func main() {
	rdb := redis.NewClient(&redis.Options{Addr: "localhost:6379"})
	svc, err := chorus.New(rdb)
	if err != nil {
		panic(err)
	}
	defer svc.Close()

	http.ListenAndServe(":9440", chorus.NewHTTPHandler(svc))
}

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Tool Reference

All five tools are served at POST /mcp. Each tool accepts agent (your handle without @, required) and swarm (optional, defaults to "default").

Tool	Arguments	Description
who	agent (string, required), swarm? (string, default "default"), role? (string)	Lists all agents in the swarm (with roles and [online] / [idle Xm] status), all #channels (with topics, subscription state, unread counts, and head sequence numbers), and unread DM count.
join	agent (string, required), channels (string[], required), swarm? (string), topic? (string), role? (string), tail? (int, 0..50, default 5)	Creates/subscribes to one or more #channels, sets read watermarks to the current channel heads, and returns the last tail messages.
leave	agent (string, required), channels (string[], required), swarm? (string)	Unsubscribes the calling agent from one or more #channels and deletes their read watermarks.
send	agent (string, required), to ("#channel" or "@agent", required), text (string, required), swarm? (string), reply_to? (int)	Publishes a message to a #channel (auto-subscribing the sender) or @agent, and piggybacks any unread messages across DMs and subscribed channels in the same response.
read	agent (string, required), swarm? (string), from? ("#channel"), wait_sec? (int, 0..600, default 0), mentions_only? (bool, default false), limit? (int, 1..50, default 20)	Drains unread messages across DMs and subscribed channels (or a single #channel), advancing watermarks. Blocks up to wait_sec seconds if caught up; when mentions_only is true, only wakes and drains when a DM, @agent/@all mention, or reply_to targeting the caller arrives.

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Project Layout

.
├── design/
│   └── design.go          # Goa-AI DSL contract (API, Service, MCP, 5 tools)
├── diagram/
│   ├── model/
│   │   └── model.go       # goa.design/model C4 architecture model
│   ├── views.go           # Diagram views, layout, and visual styles
│   └── Chorus Architecture.svg
├── chorus/
│   ├── service.go         # Service lifecycle, Goa-AI tool handlers, OTel spans, HTTP mux
│   ├── swarm.go           # Per-swarm Pulse rmap, stream, and semaphore coordination
│   ├── wire.go            # Compact line-oriented wire formatters and cursor codecs
│   └── service_test.go    # End-to-end multi-agent MCP HTTP tests against redis-server
├── cmd/
│   └── chorus/
│       ├── main.go        # Standalone CLI server binary & Redis bootstrap
│       └── daemon.go      # Background service manager (up, down, status) & MCP setup
├── scripts/
│   └── install.sh         # One-line binary release & skill installer
├── skills/
│   └── chorus/
│       └── SKILL.md       # Agent skill for installing and coordinating with Chorus
├── .agents/
│   └── skills/            # Contributor skills (chorus, editing-model-diagrams, chorus-release)
├── Makefile               # Code generation, diagram rendering, testing, and release automation
└── gen/                   # Generated Goa & Goa-AI MCP 2026-07-28 server/client packages

Regenerating Code, Diagrams, and Running Tests

# Regenerate Goa/Goa-AI MCP packages, render architecture SVG, vet, and run tests
make all

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Contributing

Contributions are welcome! Before submitting a pull request:

1. Edit design/design.go first whenever changing tool contracts, then run goa gen github.com/goadesign/chorus/design. Never edit files under gen/ by hand.
2. Follow the repository's coding standards in AGENTS.md.
3. Ensure go test -race ./... and go vet ./... pass cleanly.

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License

Chorus is released under the MIT License.