# Arch Tools — Session Message

> Arch Tools — Session Message is a paid API for AI agents from archtools.dev, paid per call via x402, $0.02/call, status unknown (last checked 2026-09-14).

Sends a user message to an existing AI session and retrieves a response, enabling multi-turn conversational interactions.

## Facts

- Endpoint: POST https://archtools.dev/v1/tools/session-message
- Price: $0.02/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-14
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/arch-tools-session-message-94018ac0
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_yyeDjStUFU8SDIKJxJvPY

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability arch-tools-session-message-94018ac0 -d '<json body>'
```

Example prompt: Continue my Arch Tools session (session ID 'abc-123') and send the message: 'What are the next steps based on what we discussed?'

## When to prefer this

Use this endpoint when you have an active session created via session-create and need to send follow-up messages in a stateful, multi-turn conversation. It is ideal for workflows that require context continuity across multiple interactions, such as iterative data extraction, guided reasoning, or conversational agents. Prefer this over stateless endpoints when session history and context must be preserved between calls.

## Known failure modes

- Invalid or expired session_id returns an error indicating the session does not exist
- Missing required 'message' or 'session_id' fields returns a 400-level validation error
- Session timeout or expiry may return a session-not-found or unauthorized error
- Empty or malformed message string may result in an error or empty response
- Payment failure (insufficient USDC balance) blocks the request with a 402 error

## How this service works

Arch Tools — session-message

## Output

A JSON object containing the AI-generated response to the user message within the specified session, reflecting the current conversational context accumulated across session turns.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "message": {
   "type": "string",
   "description": "User message to send"
  },
  "session_id": {
   "type": "string",
   "description": "Session ID from session-create"
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/arch-tools-session-message-94018ac0/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from archtools.dev](https://www.zero.xyz/host/archtools.dev/llms.txt)