Agent Streaming Protocol

A protocol for AI agent sessions with durable message streaming and resumable connections.

What is Agent Streaming Protocol?

The Agent Streaming Protocol enables AI agent interactions over HTTP with full resumability. Unlike traditional WebSocket-based chat, sessions survive disconnections - clients can reconnect and replay any missed messages from the exact point they left off.

Built on top of Unbroken Protocol, it combines:

Claude Agent SDK for AI model interaction
Unbroken Protocol for message persistence
SSE for real-time streaming

Key Features

Feature	Description
Resumable	Reconnect and continue from any point
Persistent	Full conversation history preserved
Tool Use	Stream tool calls and results
Web Native	Works in browsers via SSE

Architecture

┌─────────────────────────────────────────────────────────────┐
│                         Client                               │
│  ┌─────────────┐    ┌─────────────┐    ┌─────────────┐     │
│  │   Message   │    │    Tool     │    │   Offset    │     │
│  │    Input    │    │  Executor   │    │   Storage   │     │
│  └──────┬──────┘    └──────┬──────┘    └──────┬──────┘     │
└─────────┼──────────────────┼──────────────────┼─────────────┘
          │ POST             │ POST             │
          ▼                  ▼                  │
┌─────────────────────────────────────────────────────────────┐
│                    Agent Server                              │
│  ┌─────────────┐    ┌─────────────┐    ┌─────────────┐     │
│  │   Claude    │◀───│   Session   │───▶│   Durable   │     │
│  │     API     │    │   Manager   │    │   Stream    │     │
│  └─────────────┘    └─────────────┘    └─────────────┘     │
└─────────────────────────────────────────────────────────────┘

Quick Example

# Create a session
curl -X PUT http://localhost:9999/agent/my-chat \
  -H "Content-Type: application/json" \
  -d '{"model": "claude-sonnet-4-5-20250929"}'

# Stream messages (SSE)
curl "http://localhost:9999/agent/my-chat?offset=-1&live=sse"

# Send a prompt
curl -X POST http://localhost:9999/agent/my-chat/prompt \
  -H "Content-Type: application/json" \
  -d '{"content": [{"type": "text", "text": "Hello!"}]}'

Protocol at a Glance

Endpoints

Method	Path	Description
`PUT`	`/agent/{id}`	Create session
`GET`	`/agent/{id}?offset=X&live=sse`	Stream messages
`POST`	`/agent/{id}/prompt`	Send user message
`POST`	`/agent/{id}/tool-result`	Submit tool result
`POST`	`/agent/{id}/cancel`	Cancel current turn
`DELETE`	`/agent/{id}`	End session
`HEAD`	`/agent/{id}`	Check session exists

Headers

Header	Direction	Description
`Stream-Next-Offset`	Response	Offset for resumption
`Stream-Up-To-Date`	Response	At end of messages

SSE Event Types

Event	Description
`user`	User message sent to agent
`assistant`	Agent response text
`tool_use`	Agent requests tool execution
`tool_result`	Tool result submitted
`turn_complete`	Turn finished with stop reason
`error`	Error occurred
`control`	Stream metadata/offset

Use Cases

AI Assistants: Persistent chat with tool use
Code Agents: Autonomous coding with file/terminal tools
Workflows: Long-running AI tasks with checkpoints
Multi-client: Share agent sessions across devices

Specification

For the complete protocol specification, see PROTOCOL.md.

Default Port

The agent server uses port 9999/tcp by default (same as Unbroken Protocol).

Requirements

Server: Bun runtime, ANTHROPIC_API_KEY environment variable
Client: Any HTTP client (browser, curl, custom)