hermes-agent/website/docs/user-guide/features/api-server.md

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14	API Server	Expose hermes-agent as an OpenAI-compatible API for any frontend

API Server

The API server exposes hermes-agent as an OpenAI-compatible HTTP endpoint. Any frontend that speaks the OpenAI format — Open WebUI, LobeChat, LibreChat, NextChat, ChatBox, and hundreds more — can connect to hermes-agent and use it as a backend.

Your agent handles requests with its full toolset (terminal, file operations, web search, memory, skills) and returns the final response. When streaming, tool progress indicators appear inline so frontends can show what the agent is doing.

Quick Start

1. Enable the API server

Add to ~/.hermes/.env:

API_SERVER_ENABLED=true
API_SERVER_KEY=change-me-local-dev
# Optional: only if a browser must call Hermes directly
# API_SERVER_CORS_ORIGINS=http://localhost:3000

2. Start the gateway

hermes gateway

You'll see:

[API Server] API server listening on http://127.0.0.1:8642

3. Connect a frontend

Point any OpenAI-compatible client at http://localhost:8642/v1:

# Test with curl
curl http://localhost:8642/v1/chat/completions \
  -H "Authorization: Bearer change-me-local-dev" \
  -H "Content-Type: application/json" \
  -d '{"model": "hermes-agent", "messages": [{"role": "user", "content": "Hello!"}]}'

Or connect Open WebUI, LobeChat, or any other frontend — see the Open WebUI integration guide for step-by-step instructions.

Endpoints

POST /v1/chat/completions

Standard OpenAI Chat Completions format. Stateless — the full conversation is included in each request via the messages array.

Request:

{
  "model": "hermes-agent",
  "messages": [
    {"role": "system", "content": "You are a Python expert."},
    {"role": "user", "content": "Write a fibonacci function"}
  ],
  "stream": false
}

Response:

{
  "id": "chatcmpl-abc123",
  "object": "chat.completion",
  "created": 1710000000,
  "model": "hermes-agent",
  "choices": [{
    "index": 0,
    "message": {"role": "assistant", "content": "Here's a fibonacci function..."},
    "finish_reason": "stop"
  }],
  "usage": {"prompt_tokens": 50, "completion_tokens": 200, "total_tokens": 250}
}

Streaming ("stream": true): Returns Server-Sent Events (SSE) with token-by-token response chunks. When streaming is enabled in config, tokens are emitted live as the LLM generates them. When disabled, the full response is sent as a single SSE chunk.

Tool progress in streams: When the agent calls tools during a streaming request, brief progress indicators are injected into the content stream as the tools start executing (e.g. `💻 pwd`, `🔍 Python docs`). These appear as inline markdown before the agent's response text, giving frontends like Open WebUI real-time visibility into tool execution.

POST /v1/responses

OpenAI Responses API format. Supports server-side conversation state via previous_response_id — the server stores full conversation history (including tool calls and results) so multi-turn context is preserved without the client managing it.

Request:

{
  "model": "hermes-agent",
  "input": "What files are in my project?",
  "instructions": "You are a helpful coding assistant.",
  "store": true
}

Response:

{
  "id": "resp_abc123",
  "object": "response",
  "status": "completed",
  "model": "hermes-agent",
  "output": [
    {"type": "function_call", "name": "terminal", "arguments": "{\"command\": \"ls\"}", "call_id": "call_1"},
    {"type": "function_call_output", "call_id": "call_1", "output": "README.md src/ tests/"},
    {"type": "message", "role": "assistant", "content": [{"type": "output_text", "text": "Your project has..."}]}
  ],
  "usage": {"input_tokens": 50, "output_tokens": 200, "total_tokens": 250}
}

Multi-turn with previous_response_id

Chain responses to maintain full context (including tool calls) across turns:

{
  "input": "Now show me the README",
  "previous_response_id": "resp_abc123"
}

The server reconstructs the full conversation from the stored response chain — all previous tool calls and results are preserved.

Named conversations

Use the conversation parameter instead of tracking response IDs:

{"input": "Hello", "conversation": "my-project"}
{"input": "What's in src/?", "conversation": "my-project"}
{"input": "Run the tests", "conversation": "my-project"}

The server automatically chains to the latest response in that conversation. Like the /title command for gateway sessions.

GET /v1/responses/{id}

Retrieve a previously stored response by ID.

DELETE /v1/responses/{id}

Delete a stored response.

GET /v1/models

Lists hermes-agent as an available model. Required by most frontends for model discovery.

GET /health

Health check. Returns {"status": "ok"}. Also available at GET /v1/health for OpenAI-compatible clients that expect the /v1/ prefix.

System Prompt Handling

When a frontend sends a system message (Chat Completions) or instructions field (Responses API), hermes-agent layers it on top of its core system prompt. Your agent keeps all its tools, memory, and skills — the frontend's system prompt adds extra instructions.

This means you can customize behavior per-frontend without losing capabilities:

• Open WebUI system prompt: "You are a Python expert. Always include type hints."
• The agent still has terminal, file tools, web search, memory, etc.

Authentication

Bearer token auth via the Authorization header:

Authorization: Bearer ***

Configure the key via API_SERVER_KEY env var. If you need a browser to call Hermes directly, also set API_SERVER_CORS_ORIGINS to an explicit allowlist.

:::warning Security The API server gives full access to hermes-agent's toolset, including terminal commands. If you change the bind address to 0.0.0.0 (network-accessible), always set API_SERVER_KEY and keep API_SERVER_CORS_ORIGINS narrow — without that, remote callers may be able to execute arbitrary commands on your machine.

The default bind address (127.0.0.1) is for local-only use. Browser access is disabled by default; enable it only for explicit trusted origins. :::

Configuration

Environment Variables

Variable	Default	Description
API_SERVER_ENABLED	false	Enable the API server
API_SERVER_PORT	8642	HTTP server port
API_SERVER_HOST	127.0.0.1	Bind address (localhost only by default)
API_SERVER_KEY	(none)	Bearer token for auth
API_SERVER_CORS_ORIGINS	(none)	Comma-separated allowed browser origins

config.yaml

# Not yet supported — use environment variables.
# config.yaml support coming in a future release.

Security Headers

All responses include security headers:

• X-Content-Type-Options: nosniff — prevents MIME type sniffing
• Referrer-Policy: no-referrer — prevents referrer leakage

CORS

The API server does not enable browser CORS by default.

For direct browser access, set an explicit allowlist:

API_SERVER_CORS_ORIGINS=http://localhost:3000,http://127.0.0.1:3000

When CORS is enabled:

• Preflight responses include Access-Control-Max-Age: 600 (10 minute cache)
• SSE streaming responses include CORS headers so browser EventSource clients work correctly
• Idempotency-Key is an allowed request header — clients can send it for deduplication (responses are cached by key for 5 minutes)

Most documented frontends such as Open WebUI connect server-to-server and do not need CORS at all.

Compatible Frontends

Any frontend that supports the OpenAI API format works. Tested/documented integrations:

Frontend	Stars	Connection
Open WebUI	126k	Full guide available
LobeChat	73k	Custom provider endpoint
LibreChat	34k	Custom endpoint in librechat.yaml
AnythingLLM	56k	Generic OpenAI provider
NextChat	87k	BASE_URL env var
ChatBox	39k	API Host setting
Jan	26k	Remote model config
HF Chat-UI	8k	OPENAI_BASE_URL
big-AGI	7k	Custom endpoint
OpenAI Python SDK	—	OpenAI(base_url="http://localhost:8642/v1")
curl	—	Direct HTTP requests

Limitations

• Response storage — stored responses (for previous_response_id) are persisted in SQLite and survive gateway restarts. Max 100 stored responses (LRU eviction).
• No file upload — vision/document analysis via uploaded files is not yet supported through the API.
• Model field is cosmetic — the model field in requests is accepted but the actual LLM model used is configured server-side in config.yaml.