> ## Documentation Index
> Fetch the complete documentation index at: https://docs.zhentan.me/llms.txt
> Use this file to discover all available pages before exploring further.

# Co-Sign Transaction

> Add the backup key's signature to a queued screening-off SafeTx, then relay-execute.

## POST /transactions/:id/cosign

Completes a **user-proposed screening-off SafeTx** queued below the threshold: the backup key signs the transaction's `safeTxHash`, the server verifies the signature recovers to a **distinct non-agent owner** of that Safe, appends it, and triggers execution — **relay-only**, since the user's signatures now meet the threshold and the agent signs nothing it didn't screen.

This is the mirror image of `POST /transactions/:id/sign`, which completes an *agent-proposed* (pre-signed) transaction with the user's primary signature.

The same transaction can always be completed in the Safe app instead — confirm with the backup key at app.safe.global and Zhentan's sync worker reconciles the execution.

## Request

```json theme={null}
{
  "signature": "0x..."
}
```

| Field       | Type     | Required | Description                                                        |
| ----------- | -------- | -------- | ------------------------------------------------------------------ |
| `signature` | `string` | Yes      | Backup key's EIP-712 signature over the transaction's `safeTxHash` |

## Validation

* The transaction must be a queued SafeTx with a proposer signature and no terminal state
* The signature must recover to an owner of the Safe that is **not the agent** and has **not already signed**
* Ownership: authenticated calls must belong to the transaction's Safe

## Response

```json theme={null}
{
  "status": "cosigned",
  "txId": "tx-1a2b3c4d",
  "execution": { "status": "executed", "txHash": "0x..." }
}
```

| Field       | Description                                               |
| ----------- | --------------------------------------------------------- |
| `status`    | `cosigned` — the signature was verified and stored        |
| `execution` | Result of the relay execution triggered immediately after |