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

# preview_back

> Preview backing an opinion (step 1 of 2)

Returns the seat the key owner would take in an opinion's backing relay, what
that seat costs, and the conditional payback — plus the `confirm_token`
[`back_opinion`](/mcp/tools/back-opinion) needs.

**Scope**: `write` · **Annotations**: `readOnlyHint`
**REST equivalent**: `POST /open/v1/opinions/{id}/back-preview`

<Note>
  Backing spends coins to endorse someone else's opinion. This tool **charges
  nothing** and counts against the read window only.
</Note>

## Parameters

| Name         | Type                                     | Required | Default | Notes                                                        |
| ------------ | ---------------------------------------- | -------- | ------- | ------------------------------------------------------------ |
| `topic_id`   | string (uuid)                            | ✅        | —       | The topic the opinion belongs to                             |
| `opinion_id` | string (uuid)                            | ✅        | —       | From [`list_topic_opinions`](/mcp/tools/list-topic-opinions) |
| `language`   | see [languages](/mcp/overview#languages) |          | `en`    |                                                              |

## Returns

`my_seq_if_now` is the seat the owner would take; the price rises along the
relay, so the quoted `price_coins` belongs to that seat only.

Captured live:

```json theme={null}
{
  "can_back": true,
  "summary": {
    "topic_title": "Will the Fed cut rates at the November 2026 meeting?",
    "opinion_excerpt": "Disagree. Two governors have signalled they want another quarter of data before moving, and November is early.",
    "author": {
      "id": "33333333-3333-4333-8333-333333333333",
      "handle": "kenji",
      "nickname": "Kenji Sato",
      "avatar_url": "https://cdn.seesaw.fun/avatars/20260901/kenji.png",
      "account_type": "user",
      "url": "https://seesaw.fun/profile/33333333-3333-4333-8333-333333333333"
    },
    "backer_count": 0,
    "my_seq_if_now": 1,
    "price_coins": "1.0000",
    "balance_before": "4900.0000",
    "balance_after": "4899.0000",
    "max_relay_payback_sapphires": "2.0000",
    "close_time": "2026-10-06T23:34:02.46058Z"
  },
  "confirm_token": "oc1.eyJhIjoiYmFja19vcGluaW9u…OjcAlET72O_MU5T4",
  "expires_at": "2026-09-08T23:40:33.745248Z"
}
```

<Warning>
  `max_relay_payback_sapphires` is **conditional, not a promise**: it is the most
  this seat can receive **if two more backers follow**. If nobody follows, it pays
  nothing. Say so when you present it.
</Warning>

<Warning>
  **Show the summary to the user and get their explicit confirmation before
  calling the execute tool.** The token is what proves a preview happened — it is
  not a substitute for asking.
</Warning>

## When backing isn't allowed

| `reason`               | Meaning                                                                                      |
| ---------------------- | -------------------------------------------------------------------------------------------- |
| `OPINION_INACTIVE`     | Hidden or not through moderation                                                             |
| `ALREADY_BACKED`       | The owner already backed it — one backing per person; `my_seq_if_now` is their existing seat |
| `TOPIC_CLOSED`         | Trading closed, so there is no quote                                                         |
| `INSUFFICIENT_BALANCE` | Not enough coins for this seat                                                               |
| `OPINION_BACK_FULL`    | The price ladder is exhausted                                                                |

## Example prompts

* "What would it cost to back that take?"
* "How much do I pay to be the next backer?"

## Related

[`back_opinion`](/mcp/tools/back-opinion) ·
[`list_topic_opinions`](/mcp/tools/list-topic-opinions) ·
[`list_my_backed_opinions`](/mcp/tools/list-my-backed-opinions)