Jambonz

jambonz Voice Application Skill

jambonz is an open-source CPaaS (Communications Platform as a Service) for building voice and messaging applications. It handles telephony infrastructure — SIP, carriers, phone numbers, media processing — so you can focus on application logic. Your application controls calls by returning arrays of verbs — JSON instructions that execute sequentially.

This skill covers decision-making: which verb to pick, which transport to use, what mistakes to avoid. For implementation code, load the jambonz-recipes skill. For runnable starter apps, load jambonz-starters. For schema lookups, use the jambonz MCP server.

Using This Skill with the MCP Server

1. Before writing any jambonz code, call jambonz_developer_toolkit for the complete SDK guide and schema index.
2. For a verb's exact properties, call get_jambonz_schema with the verb, component, or callback name.
3. For a working example, call get_sdk_example with the example name.

If MCP tools are unavailable, read AGENTS.md from the repo root and schema files from schema/verbs/, schema/components/, schema/callbacks/.

Workflow: Use this skill to decide what to build → use MCP tools to look up how to build it → load jambonz-recipes for the implementation pattern.

Server Versions

jambonz has two editions: v0.9.x (open source) and v10.x (commercial). Each verb schema includes a minVersion field. Only ask the user about their server version if a verb requires minVersion higher than 0.9.6. If all verbs needed have minVersion: "0.9.6", the code works on both editions.

Transport Selection

Choose the transport based on what the application needs.

Use WebSocket when:

• Using any speech-to-speech verb (openai_s2s, google_s2s, deepgram_s2s, ultravox_s2s, elevenlabs_s2s, s2s) or the cascaded agent verb — mandatory
• Streaming raw audio (stream verb with bidirectional audio)
• Using TTS token streaming
• Building complex conversational flows with session state
• Needing bidirectional real-time events or async mid-call control from the app

Use Webhook when:

• Simple IVR menus (gather + say)
• Call routing (dial to a number or SIP endpoint)
• Voicemail or basic record-and-hang-up
• Any straightforward request-response pattern

Rule: If ANY verb in the application requires WebSocket, the entire application must use WebSocket transport. The verb JSON structure is identical in both modes — only the transport differs.

Verb Selection by Task

Use this decision tree to pick the right verb(s) for the task. For implementation details on any verb listed here, load the jambonz-recipes skill.

"Build a voice AI agent / chatbot"

The user wants a caller to have a conversation with an LLM. There are two main patterns:

• use a speech-to-speech model, where the entire conversation is orchestrated by one model (openai_s2s, google_s2s, deepgram_s2s, ultravox_s2s, elevenlabs_s2s, or generic s2s)
• use a cascaded voice pipeline by combining separate STT + LLM + TTS verbs (agent)

Never use llm in generated code — it is a legacy name. Use either a vendor shortcut or s2s.

"Build an IVR menu / collect input"

Use gather with nested say (for TTS prompt) or play (for audio file). Supports speech recognition, DTMF digits, or both.

"Transfer / connect / bridge a call"

• Bridge to another party: dial with a target (phone number, SIP URI, registered user, Teams user)
• Blind SIP transfer: sip-refer
• Transfer to a different application/webhook: redirect

"Queue calls / hold music"

• Put caller in queue: enqueue (with waitHook for hold music)
• Picks up caller from queue: dequeue

"Record the call"

Two options:

• Enable recording at the account level in the jambonz portal (recordings are sent to user's configured cloud storage).
• Use a third-party SIPREC server via inject commands (WebSocket) or REST API (webhook).

"Stream raw audio for custom processing"

Use stream (preferred name; listen is a synonym — always use stream). This is also known as "bidirectional streaming". Customers that have their built-out back end for media processing and AI often prefer this low-level access to the audio.

"Transcribe the call in real-time"

Use transcribe with a transcriptionHook.

"Play audio or speak text"

• TTS: say (supports SSML, multiple voices)
• Audio file: play (from URL)
• Background audio track: dub (mixes alongside the call)

"Reject an incoming call"

Use sip-decline with a SIP status code.

Common Gotchas

1. Using llm verb name — Always use vendor shortcuts (openai_s2s, etc.) or s2s. Never llm.
2. Using listen verb name — Always use stream. They are synonyms; stream is preferred.
3. .send() vs .reply() confusion — .send() is the initial response only. .reply() is for all actionHook responses. Using .send() in an actionHook handler will fail.
4. Using process.env — jambonz apps should generally use application environment variables (session.data.env_vars / req.body.env_vars), not process.env.
5. env_vars only on initial call — The env_vars object is only present in the first webhook POST or session:new. Store values in a variable if needed in actionHook handlers.
6. Webhook transport for s2s/agent apps — These verbs require WebSocket. Always use createEndpoint from @jambonz/sdk/websocket.
7. ElevenLabs: passing model or messages — ElevenLabs uses agent_id auth. The model and prompt are configured in the ElevenLabs dashboard. Pass llmOptions: {}.
8. Marks silently failing — Marks require bidirectionalAudio: { enabled: true, streaming: true } on the listen/stream verb. Without streaming mode, marks are accepted but never fire.
9. Not binding actionHook listeners before .send() — In WebSocket mode, if no listener is bound for an actionHook, the SDK auto-replies with an empty verb array, which usually means the call hangs up unexpectedly.
10. Forgetting the /call-status handler — Webhook apps must handle call status POST requests. Missing this causes errors in the jambonz logs.

Project Scaffolding

• Language: Default to TypeScript. Use JavaScript only when the user explicitly asks for it. Both work with @jambonz/sdk.
• Simple apps (1-2 routes): Single file. This is the default and works fine for production.
• Complex apps (3+ routes): Standard src/app.ts + src/routes/ layout (use .js for JavaScript projects).
• Dependencies: @jambonz/sdk always. Add express for webhook apps. WebSocket apps need no additional deps.
• TypeScript config: "module": "nodenext", "moduleResolution": "nodenext".

For complete runnable scaffolds, load the jambonz-starters skill — it indexes 17+ ready-to-clone starter apps.

Sibling Skills

This skill is part of the jambonz-skills plugin. Load these companion skills based on the task:

• jambonz-recipes — Load when implementing a specific feature. Contains @jambonz/sdk patterns and raw-JSON examples for voice AI agents, IVR menus, call control (dial/transfer/queue/conference/recording), and env_vars configuration.
• jambonz-starters — Load when the user wants to clone a complete runnable app rather than assemble one from recipes.
• jambonz-setup-mcp — Load when wiring the jambonz MCP server into Claude Code, Cursor, Codex, Windsurf, or VS Code Copilot.