# Underline text

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

## Tool capabilities

| Property | Value |
| --- | --- |
| Tool ID | `underline-text` |
| Category | text-tools |
| Implementation | Registered implementation. Registration alone is not a production-quality guarantee. |
| Browser execution | Automatic updates after valid input or file selection. |
| Browser processing | Browser; see tool notes for network behavior. |
| HTTP API | /api/v1/tools/underline-text/run |
| Browser-agent interface | run_current_tool |

### Inputs and settings

Text input: Your text.

Example or placeholder shown in the interface (not a validated fixture):

```text
Hello world!
```

Change the input or settings to update your result. Results update as you type. Up to 1,000,000 characters.

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/underline-text/run
Content-Type: application/json
```

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

#### Limits

| Limit | Value |
| --- | --- |
| request_bytes | 8388608 |
| input_characters | 1000000 |
| files_bytes | 5242880 |
| max_files | 0 |
| 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": {}
    }
  }
}
```

#### 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"
              }
            }
          }
        }
      }
    }
  }
}
```

#### Recorded request and response

Saved fixture from 2026-09-28T18:13:35Z. This is historical evidence, not a new validation or a guarantee for every input.

```json
{
  "input": "Hi",
  "options": {}
}
```

```json
{
  "tool": "underline-text",
  "result": {
    "text": "H̲i̲",
    "files": [],
    "name": "result.txt",
    "mime": "text/plain;charset=utf-8"
  }
}
```

#### 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

Combining underline versus HTML output option; handle whitespace explicitly.

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

### Complete machine-readable capability record

```json
{
  "id": "underline-text",
  "category": "text-tools",
  "built": true,
  "requirements": "Combining underline versus HTML output option; handle whitespace explicitly.",
  "implementation": {
    "fields": [],
    "experience": {
      "automatic": true,
      "single": false,
      "label": "Your text",
      "placeholder": "Hello world!",
      "help": "Change the input or settings to update your result. Results update as you type. Up to 1,000,000 characters.",
      "action": null
    },
    "mode": "automatic",
    "interactive": false
  },
  "execution": "POST /api/v1/tools/underline-text/run",
  "api": {
    "id": "underline-text",
    "name": "Underline text",
    "category": "text-tools",
    "description": "Add combining underlines to non-whitespace characters.",
    "notes": "",
    "human_path": "/en/text-tools/underline-text/",
    "method": "POST",
    "endpoint": "/api/v1/tools/underline-text/run",
    "schema_url": "/api/v1/tools/underline-text",
    "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": {}
        }
      }
    },
    "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": 0,
      "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

Add combining underlines to non-whitespace characters.

## How to use Underline text

1. Enter your text in the input field.
2. The result updates as you type. Adjust the available settings if needed.
3. Review the result, then use Copy result or a download link when available.

## What to expect

Add combining underlines to non-whitespace characters. This produces Unicode text, not HTML. Rendering and screen-reader support vary.

### Try a small example

```
Hi
```

## Useful next steps

- [Bold text](https://happytails.ai/en/text-tools/bold-text/): Turn letters and digits into bold Unicode characters. ([Markdown](https://happytails.ai/en/text-tools/bold-text.md))

- [Italic text](https://happytails.ai/en/text-tools/italic-text/): Turn Latin letters into italic Unicode characters. ([Markdown](https://happytails.ai/en/text-tools/italic-text.md))

- [Strikethrough text](https://happytails.ai/en/text-tools/strikethrough-text/): Add a strikethrough mark to visible characters. ([Markdown](https://happytails.ai/en/text-tools/strikethrough-text.md))

- [Small text](https://happytails.ai/en/text-tools/small-text/): Convert supported letters into small-cap Unicode characters. ([Markdown](https://happytails.ai/en/text-tools/small-text.md))