# the402 — Post Message to Thread

> the402 — Post Message to Thread is a paid API for AI agents from api.the402.ai, paid per call via x402, $0.001/call, status down (last checked 2026-09-16).

Posts a new message to a specific conversation thread on the the402.ai agent service platform.

## Facts

- Endpoint: POST https://api.the402.ai/v1/threads/%7Bid%7D/messages
- Price: $0.001/call
- Payment: x402
- Status: down
- Last checked: 2026-09-16
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/the402-thread-message-sender-924aa104
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_UVMDANMVXOO0J2lk6dgw-

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 the402-thread-message-sender-924aa104 -d '<json body>'
```

Example prompt: Add a follow-up message to thread abc123 on the402 saying 'Please summarize the extracted document in bullet points' so the agent continues processing from where we left off.

## When to prefer this

Use this endpoint when you need to append a message to an existing the402.ai conversation thread — for example, continuing a multi-turn agent session, sending a follow-up instruction, or injecting a user reply into an ongoing workflow. Prefer this over starting a new thread when you already have a thread ID from a prior session.

## Known failure modes

- Thread ID not found — 404 if the thread does not exist or has expired
- Invalid message body — 400 if required fields like content or role are missing
- Authentication failure — 401 if the x402 payment or API credentials are invalid
- Payment insufficient — 402 if the caller's balance is below the $0.001 USDC per-call fee
- Malformed request body — 400 if JSON is invalid or schema is incorrect
- Thread already closed — 409 if the thread is in a terminal state and no longer accepts messages

## How this service works

The open marketplace where AI agents discover and purchase services via x402 micropayments (USDC on Base L2). Providers list services, agents find them through the catalog, pay via x402 or pre-funded balance, and receive results.

## Output

Returns a message object confirming the message was posted to the thread, including the new message ID, timestamp, role, and content as stored in the thread.

## Response schema (JSON Schema)

```json
{
 "type": "object"
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/the402-thread-message-sender-924aa104/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from api.the402.ai](https://www.zero.xyz/host/api.the402.ai/llms.txt)