# Language fallback simulator

- Human page: https://happytails.ai/en/developer-tools/language-fallback-simulator/
- Markdown: https://happytails.ai/en/developer-tools/language-fallback-simulator.md
- Structured JSON: https://happytails.ai/agents/en/developer-tools/language-fallback-simulator/index.json
- Visibility: public

## Tool capabilities

| Property | Value |
| --- | --- |
| Tool ID | `language-fallback-simulator` |
| Category | developer-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 | Not callable through HTTP. |
| Browser-agent interface | run_current_tool |

### Implementation notes

Supply resources and explicit requests with locale, chain and keys. Keys are JSON Pointers. The first present value wins, including null and empty strings. There are no automatic parent-locale rules or network resource loads. At most 64 bundles, 100 requests, 16 fallback steps and 2,000 keys per request. Processing stays on your device. JSON supports up to 1,000,000 input characters, 50,000 values and 64 nesting levels. Duplicate keys and numbers that cannot survive JSON serialization exactly are rejected. Operations are limited to 100,000 comparisons, 4,000,000 JSON output characters and 8,000,000 CSV output characters. Tables show the first 200 rows; downloads preserve the complete report. CSV uses exact JSON Pointer paths and typed JSON value envelopes for spreadsheet-safe reporting.

### Inputs and settings

Text input: JSON input.

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

```text
{
  "resources": {
    "pt-BR": {
      "greeting": "Olá"
    },
    "pt": {
      "farewell": "Adeus"
    },
    "en": {
      "farewell": "Goodbye"
    }
  },
  "requests": [
    {
      "locale": "pt-BR",
      "chain": [
        "pt-BR",
        "pt",
        "en"
      ],
      "keys": [
        "/greeting",
        "/farewell",
        "/missing"
      ]
    }
  ]
}
```

Supply resources and explicit requests with locale, chain and keys. Keys are JSON Pointers. The first present value wins, including null and empty strings. There are no automatic parent-locale rules or network resource loads. At most 64 bundles, 100 requests, 16 fallback steps and 2,000 keys per request. Up to 1,000,000 characters. Review the settings before running.

No additional settings are declared for this tool.

### 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

Supply resources and explicit requests with locale, chain and keys. Keys are JSON Pointers. The first present value wins, including null and empty strings. There are no automatic parent-locale rules or network resource loads. At most 64 bundles, 100 requests, 16 fallback steps and 2,000 keys per request. Processing stays on your device. JSON supports up to 1,000,000 input characters, 50,000 values and 64 nesting levels. Duplicate keys and numbers that cannot survive JSON serialization exactly are rejected. Operations are limited to 100,000 comparisons, 4,000,000 JSON output characters and 8,000,000 CSV output characters. Tables show the first 200 rows; downloads preserve the complete report. CSV uses exact JSON Pointer paths and typed JSON value envelopes for spreadsheet-safe reporting.

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

### Complete machine-readable capability record

```json
{
  "id": "language-fallback-simulator",
  "category": "developer-tools",
  "built": true,
  "requirements": "Supply resources and explicit requests with locale, chain and keys. Keys are JSON Pointers. The first present value wins, including null and empty strings. There are no automatic parent-locale rules or network resource loads. At most 64 bundles, 100 requests, 16 fallback steps and 2,000 keys per request. Processing stays on your device. JSON supports up to 1,000,000 input characters, 50,000 values and 64 nesting levels. Duplicate keys and numbers that cannot survive JSON serialization exactly are rejected. Operations are limited to 100,000 comparisons, 4,000,000 JSON output characters and 8,000,000 CSV output characters. Tables show the first 200 rows; downloads preserve the complete report. CSV uses exact JSON Pointer paths and typed JSON value envelopes for spreadsheet-safe reporting.",
  "implementation": {
    "fields": [],
    "presentation": "metrics",
    "inputMaxLength": 1000000,
    "inputErrorCode": "DATA_DIAGNOSTIC_INPUT",
    "note": "Supply resources and explicit requests with locale, chain and keys. Keys are JSON Pointers. The first present value wins, including null and empty strings. There are no automatic parent-locale rules or network resource loads. At most 64 bundles, 100 requests, 16 fallback steps and 2,000 keys per request. Processing stays on your device. JSON supports up to 1,000,000 input characters, 50,000 values and 64 nesting levels. Duplicate keys and numbers that cannot survive JSON serialization exactly are rejected. Operations are limited to 100,000 comparisons, 4,000,000 JSON output characters and 8,000,000 CSV output characters. Tables show the first 200 rows; downloads preserve the complete report. CSV uses exact JSON Pointer paths and typed JSON value envelopes for spreadsheet-safe reporting.",
    "experience": {
      "automatic": false,
      "single": false,
      "inputMaxLength": 1000000,
      "inputMode": "text",
      "label": "JSON input",
      "placeholder": "{\n  \"resources\": {\n    \"pt-BR\": {\n      \"greeting\": \"Olá\"\n    },\n    \"pt\": {\n      \"farewell\": \"Adeus\"\n    },\n    \"en\": {\n      \"farewell\": \"Goodbye\"\n    }\n  },\n  \"requests\": [\n    {\n      \"locale\": \"pt-BR\",\n      \"chain\": [\n        \"pt-BR\",\n        \"pt\",\n        \"en\"\n      ],\n      \"keys\": [\n        \"/greeting\",\n        \"/farewell\",\n        \"/missing\"\n      ]\n    }\n  ]\n}",
      "help": "Supply resources and explicit requests with locale, chain and keys. Keys are JSON Pointers. The first present value wins, including null and empty strings. There are no automatic parent-locale rules or network resource loads. At most 64 bundles, 100 requests, 16 fallback steps and 2,000 keys per request. Up to 1,000,000 characters. Review the settings before running.",
      "action": null
    },
    "mode": "explicit",
    "interactive": false
  },
  "execution": "Browser interface",
  "api": null,
  "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

See which locale resource wins for each key under explicit fallback rules, including missing resources and unresolved values.

How can we improve this tool?

Report a problem or share an idea.

Please leave out private information. Your message and the tool name go to our team via PostHog.

### How to use Language fallback simulator

1. Paste the documented input and choose the relevant settings.
2. Run the local diagnostic and inspect its findings.
3. Download the complete output and reports. Tables preview the first 200 rows.

## Useful next steps

- [JSON path inventory](https://happytails.ai/en/data-tools/json-path-inventory/) ([Markdown](https://happytails.ai/en/data-tools/json-path-inventory.md))

- [JSON validator](https://happytails.ai/en/developer-tools/json-validator/) ([Markdown](https://happytails.ai/en/developer-tools/json-validator.md))

## Control the fallback chain explicitly

List the resources and their exact lookup order for each request. A present null or empty string wins. No regional parent chain, framework behavior or network resource is assumed.

### Keep the complete report

JSON records the full result, settings and assumptions. CSV contains exact JSON Pointer paths, value types and JSON value envelopes for every report node, including typed object and array markers. Preview limits do not truncate downloadable reports.