# Merge subtitles

- Human page: https://happytails.ai/en/subtitle-tools/merge-subtitles/
- Markdown: https://happytails.ai/en/subtitle-tools/merge-subtitles.md
- Structured JSON: https://happytails.ai/agents/en/subtitle-tools/merge-subtitles/index.json
- Visibility: public

## Tool capabilities

| Property | Value |
| --- | --- |
| Tool ID | `merge-subtitles` |
| Category | subtitle-tools |
| Implementation | Registered implementation. Registration alone is not a production-quality guarantee. |
| Browser execution | Use the visible action or interactive controls. |
| Browser processing | Browser; see tool notes for network behavior. |
| HTTP API | /api/v1/tools/merge-subtitles/run |
| Browser-agent interface | run_current_tool |

### Implementation notes

Merges on the existing timeline; files are not concatenated end-to-end.

### Inputs and settings

No free-text input is required. Use the file inputs and/or settings below.

Accepted browser files: `.srt,.vtt`. Multiple files are supported.

No additional settings are declared for this tool.

### HTTP contract

API calls process supplied data on the server. The website origin is `https://happytails.ai`. Server API hosting is not connected on the public website yet. Use your local server origin to test these requests.

```http
POST https://happytails.ai/api/v1/tools/merge-subtitles/run
Content-Type: application/json
```

[Detailed operation reference](https://happytails.ai/en/api/tools/merge-subtitles.md) · [Shared API guide](https://happytails.ai/en/api.md)

#### Limits

| Limit | Value |
| --- | --- |
| request_bytes | 8388608 |
| input_characters | 1000000 |
| files_bytes | 5242880 |
| max_files | 20 |
| timeout_seconds | 12 |

#### Complete input schema

```json
{
  "type": "object",
  "additionalProperties": false,
  "properties": {
    "input": {
      "type": "string",
      "maxLength": 1000000,
      "default": "",
      "description": "Plain text input. File tools use files instead unless otherwise documented."
    },
    "options": {
      "type": "object",
      "additionalProperties": false,
      "properties": {}
    },
    "files": {
      "type": "array",
      "maxItems": 20,
      "items": {
        "type": "object",
        "required": [
          "name",
          "base64"
        ],
        "additionalProperties": false,
        "properties": {
          "name": {
            "type": "string",
            "maxLength": 200,
            "description": "Filename only, no path."
          },
          "mime": {
            "type": "string",
            "maxLength": 150
          },
          "bytes": {
            "type": "integer",
            "minimum": 0,
            "description": "Optional decoded byte count; must match content if supplied."
          },
          "base64": {
            "type": "string",
            "contentEncoding": "base64",
            "description": "File bytes as padded base64. Use the tool-specific limits.files_bytes value for the total decoded input size."
          }
        }
      }
    }
  }
}
```

#### Complete output schema

```json
{
  "type": "object",
  "required": [
    "tool",
    "result"
  ],
  "properties": {
    "tool": {
      "type": "string"
    },
    "result": {
      "type": "object",
      "required": [
        "text",
        "files"
      ],
      "properties": {
        "text": {
          "type": [
            "string",
            "null"
          ]
        },
        "data": {
          "description": "Parsed JSON when the textual result is JSON."
        },
        "name": {
          "type": "string"
        },
        "mime": {
          "type": "string"
        },
        "files": {
          "type": "array",
          "items": {
            "type": "object",
            "required": [
              "name",
              "mime",
              "bytes",
              "base64"
            ],
            "properties": {
              "name": {
                "type": "string"
              },
              "mime": {
                "type": "string"
              },
              "bytes": {
                "type": "integer"
              },
              "base64": {
                "type": "string",
                "contentEncoding": "base64"
              }
            }
          }
        }
      }
    }
  }
}
```

#### Error and retry handling

Failures return a non-200 status and `error.code` plus `error.message`. Correct invalid input before retrying; retry server-busy responses with bounded backoff. Returned files contain Base64 data, not persistent download URLs. Decode and inspect the output before treating conversion as successful. See the shared API guide for the complete status and timeout rules.

### Browser-agent workflow

1. Open the human page in a browser that supports WebMCP.
2. Discover the interface exposed by that page; use its actual schema.
3. Select files on the page first when the tool requires files.
4. Call `run_current_tool` with valid input and settings.
5. Inspect the returned result or error and the displayed output. Retrieve files from the displayed download links.

Calls do not automatically copy or download results. Browser permissions still apply. Interface availability alone is not proof of successful execution.

### Planning specification

Merge timelines with offset and overlap handling; preserve original ordering options.

This describes intended requirements. Use the implementation notes, interface schema and observed output to determine current support.

### Complete machine-readable capability record

```json
{
  "id": "merge-subtitles",
  "category": "subtitle-tools",
  "built": true,
  "requirements": "Merge timelines with offset and overlap handling; preserve original ordering options.",
  "implementation": {
    "fields": [],
    "files": true,
    "multiple": true,
    "input": false,
    "accept": ".srt,.vtt",
    "note": "Merges on the existing timeline; files are not concatenated end-to-end.",
    "experience": {
      "automatic": false,
      "single": false,
      "label": "Your text",
      "placeholder": "Type or paste your input…",
      "help": "Up to 1,000,000 characters. Review the settings before running.",
      "action": null
    },
    "mode": "explicit",
    "interactive": false
  },
  "execution": "POST /api/v1/tools/merge-subtitles/run",
  "api": {
    "id": "merge-subtitles",
    "name": "Merge subtitles",
    "category": "subtitle-tools",
    "description": "Merge timelines with offset and overlap handling.",
    "notes": "Merges on the existing timeline; files are not concatenated end-to-end.",
    "human_path": "/en/subtitle-tools/merge-subtitles/",
    "method": "POST",
    "endpoint": "/api/v1/tools/merge-subtitles/run",
    "schema_url": "/api/v1/tools/merge-subtitles",
    "input_schema": {
      "type": "object",
      "additionalProperties": false,
      "properties": {
        "input": {
          "type": "string",
          "maxLength": 1000000,
          "default": "",
          "description": "Plain text input. File tools use files instead unless otherwise documented."
        },
        "options": {
          "type": "object",
          "additionalProperties": false,
          "properties": {}
        },
        "files": {
          "type": "array",
          "maxItems": 20,
          "items": {
            "type": "object",
            "required": [
              "name",
              "base64"
            ],
            "additionalProperties": false,
            "properties": {
              "name": {
                "type": "string",
                "maxLength": 200,
                "description": "Filename only, no path."
              },
              "mime": {
                "type": "string",
                "maxLength": 150
              },
              "bytes": {
                "type": "integer",
                "minimum": 0,
                "description": "Optional decoded byte count; must match content if supplied."
              },
              "base64": {
                "type": "string",
                "contentEncoding": "base64",
                "description": "File bytes as padded base64. Use the tool-specific limits.files_bytes value for the total decoded input size."
              }
            }
          }
        }
      }
    },
    "output_schema": {
      "type": "object",
      "required": [
        "tool",
        "result"
      ],
      "properties": {
        "tool": {
          "type": "string"
        },
        "result": {
          "type": "object",
          "required": [
            "text",
            "files"
          ],
          "properties": {
            "text": {
              "type": [
                "string",
                "null"
              ]
            },
            "data": {
              "description": "Parsed JSON when the textual result is JSON."
            },
            "name": {
              "type": "string"
            },
            "mime": {
              "type": "string"
            },
            "files": {
              "type": "array",
              "items": {
                "type": "object",
                "required": [
                  "name",
                  "mime",
                  "bytes",
                  "base64"
                ],
                "properties": {
                  "name": {
                    "type": "string"
                  },
                  "mime": {
                    "type": "string"
                  },
                  "bytes": {
                    "type": "integer"
                  },
                  "base64": {
                    "type": "string",
                    "contentEncoding": "base64"
                  }
                }
              }
            }
          }
        }
      }
    },
    "processing": "Server; same transformation code as browser tools",
    "limits": {
      "request_bytes": 8388608,
      "input_characters": 1000000,
      "files_bytes": 5242880,
      "max_files": 20,
      "timeout_seconds": 12
    }
  },
  "agent_processing": "Server; same transformation code as browser tools",
  "browser_agent": {
    "supported": true,
    "name": "run_current_tool",
    "discovery": "WebMCP on the human page in a supporting browser",
    "verification": "See audit/API-AUDIT.md; availability is not verification"
  }
}
```

## Page guide

Merge timelines with offset and overlap handling.

## Useful next steps

- [SRT to VTT](https://happytails.ai/en/subtitle-tools/srt-to-vtt/): Convert timestamp punctuation and cue structure. ([Markdown](https://happytails.ai/en/subtitle-tools/srt-to-vtt.md))

- [VTT to SRT](https://happytails.ai/en/subtitle-tools/vtt-to-srt/): Warn when VTT styling/position metadata is lost. ([Markdown](https://happytails.ai/en/subtitle-tools/vtt-to-srt.md))

- [Shift subtitles](https://happytails.ai/en/subtitle-tools/shift-subtitles/): Apply positive/negative offset. ([Markdown](https://happytails.ai/en/subtitle-tools/shift-subtitles.md))

## How to use Merge subtitles

1. Choose one or more files in SRT or VTT.
2. Review the settings, then choose Run Merge subtitles.
3. Review the result, then use Copy result or a download link when available.

## Working with merge subtitles

A subtitle file combines text with time ranges. Converting SRT to VTT changes its representation; shifting subtitles changes timing; translation changes the language. Generating subtitles from speech adds a transcription step before the captions can be reviewed.

## Common questions

### Does converting subtitle formats fix synchronization?

No. A format conversion generally retains the source timing. Constant offsets can be adjusted with a shift; drift across a recording may require timing alignment or manual correction.

### Why do translated subtitles need another review?

Translated text may be longer than the original even when its meaning is correct. Review line breaks and reading speed without assuming that the original cue duration suits every language.

Continue your workflow: [SRT to VTT](https://happytails.ai/en/subtitle-tools/srt-to-vtt/), [VTT to SRT](https://happytails.ai/en/subtitle-tools/vtt-to-srt/), [Shift subtitles](https://happytails.ai/en/subtitle-tools/shift-subtitles/).