<!-- Generated by crates/swarmops-cli/src/reference/mcp_page.rs from the code: do not edit. Change the code, then run SWARMOPS_REGEN_REFERENCE=1 with the regen_reference test. -->

# undo

Moves every ref an operation of op\_log moved back to what it named before, by compare-and-swap in one transaction: all of them or none. A ref moved since stops it (E\_CAS\_MISMATCH, nothing written: undo the later operations first); an undo run again is a replay (already\_applied). Changesets are never removed.

```yaml
title: "undo"
description: "Undo an operation."
since: "0.1.0"
mutates: true
identity_required: true
cli_command: "swarmops op undo"
envelope_op: "undo"
request_type: "UndoReq"
annotations: {"destructiveHint":true,"idempotentHint":true,"openWorldHint":false,"readOnlyHint":false}
max_result_chars: null
```

## Input schema

`arguments` is a `UndoReq`: `swarmops schema UndoReq` prints the type. A field the type does not know is ignored.

| Field | Type | Required | Default | Description |
| --- | --- | --- | --- | --- |
| `op` | integer | yes |  | The operation to undo: its `op` in the operation log. |

### JSON Schema

As `tools/list` gives it: each `$ref` replaced by its definition, so that it stands alone.

```json
{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "description": "`undo` (CLI `op undo`): restore every ref an operation moved to what it named before, by\ncompare-and-swap, in one transaction: all of them or none. Changesets are never removed.",
  "properties": {
    "op": {
      "description": "The operation to undo: its `op` in the operation log.",
      "format": "uint64",
      "minimum": 0,
      "type": "integer"
    }
  },
  "required": [
    "op"
  ],
  "title": "UndoReq",
  "type": "object"
}
```

## Annotations

It moves or deletes refs by compare-and-swap, changing what they name. Run again with the same arguments, it finds them as it asked (`already_applied`) or not as it expected (`E_CAS_MISMATCH`), and changes nothing more.

| Annotation | Value | Meaning |
| --- | --- | --- |
| `destructiveHint` | `true` | True: a tool that changes the store may overwrite or delete what is there, not only add to it. |
| `idempotentHint` | `true` | True: calling it again with the same arguments changes nothing more. |
| `openWorldHint` | `false` | True: it reaches beyond a closed set of things. Every tool here stays inside the store. |
| `readOnlyHint` | `false` | True: the tool does not change its environment, the store. |

Its entry in `tools/list` carries no `_meta`.

## CLI command

[`swarmops op undo`](../commands/op-undo.md) runs the same operation and prints the same envelope.

## Examples

### MCP

```json
{
  "id": 20,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "op": 6
    },
    "name": "undo"
  }
}
```

```json
{
  "id": 20,
  "jsonrpc": "2.0",
  "result": {
    "content": [
      {
        "text": "{\"ok\":true,\"op\":\"undo\",\"api\":1,\"result\":{\"undone\":6,\"op\":7,\"ref_moves\":[{\"name\":\"release/v1\",\"new\":\"[c1]\",\"actor\":\"agent/worker-17\",\"at\":\"[at]\"}],\"already_applied\":false},\"meta\":{\"repo\":\"[tmp]/store\",\"elapsed_ms\":\"[elapsed_ms]\",\"resolved\":{},\"auth\":\"trust\"}}",
        "type": "text"
      }
    ],
    "isError": false,
    "structuredContent": {
      "api": 1,
      "meta": {
        "auth": "trust",
        "elapsed_ms": "[elapsed_ms]",
        "repo": "[tmp]/store",
        "resolved": {}
      },
      "ok": true,
      "op": "undo",
      "result": {
        "already_applied": false,
        "op": 7,
        "ref_moves": [
          {
            "actor": "agent/worker-17",
            "at": "[at]",
            "name": "release/v1",
            "new": "[c1]"
          }
        ],
        "undone": 6
      }
    }
  }
}
```

The server ran as `swarmops mcp --repo [tmp]/store --as agent/worker-17`, after `initialize`. Recorded by the MCP tests. A name in square brackets, such as `[tmp]` or `[c1]`, stands for what differs from run to run: the test's temporary directory, an id, a time, the version.

## Details

`op` is an operation's number from `op_log`.
It moves every ref that operation moved back to what it named before, by compare-and-swap in
one transaction: all of them or none.
A ref moved since stops it (`E_CAS_MISMATCH`, nothing written): undo the later operations first.
An undo run again is a replay (`already_applied`).
Changesets are never removed.

## Return information

A tool result: the envelope, whose `op` is `undo`, as `structuredContent` and again as the text of its one `content` block. `isError` is true exactly when the envelope's `ok` is false. On success the envelope's `result` is what [`swarmops op undo`](../commands/op-undo.md#return-information) returns.

## Errors

A failed call is a tool result with `isError: true`; the envelope's `error.code` is one of the [error codes](../errors.md).

- [`E_INVALID_ARG`](../errors.md#e_invalid_arg): arguments that do not fit `UndoReq`, a field named twice among them included, or that break one of the server's own rules.
- [`E_IDENTITY_REQUIRED`](../errors.md#e_identity_required): the server runs with no identity.
- [`E_NOT_A_REPO`](../errors.md#e_not_a_repo): the server's store is not there; the server serves on.
- [`E_INTERNAL`](../errors.md#e_internal): the operation panicked, a bug.

Then the errors of the operation, as [`swarmops op undo`](../commands/op-undo.md#errors) lists them.

## See also

- [`swarmops op undo`](../commands/op-undo.md)
- [MCP tools](README.md)
- [MCP integration](../../develop/mcp.md)
- [Error codes](../errors.md)