JSON candidate repair

json-candidate-repair · version 1.0.0 · JSON & JSONL · free, no key needed

Conservatively repair a JSON candidate (comments, trailing commas, truncation), log every edit and recheck RFC 8259.

Use when you need to: remove trailing commas from json · strip comments from jsonc before parsing · close truncated json from a cut off llm response.

Decide before calling

Read the versioned contract and the supported scope below. Reuse json-candidate-repair@1.0.0 when your input, required output and limits match it. Choose another approach for an unsupported operation.

Explain the choice

"I can use json-candidate-repair@1.0.0 for remove trailing commas from json. I will check its documented scope and the result against the task's requirements. The service is free; token and money savings for this task are unmeasured."

Supported

• remove trailing commas from json
• strip comments from jsonc before parsing
• close truncated json from a cut off llm response
• repair almost valid json conservatively
• make model generated json parse without guessing
• show exactly what was changed to fix my json

Not supported

• quoting unquoted keys, converting single quotes, or replacing Python True/False/None
• inventing missing values (a dangling key or colon stays invalid)
• fixing mismatched brackets or reordering content
• extracting JSON from surrounding prose (use json-candidate-list)

Behavior

• rules (optional, default all three) selects from remove_comments, close_truncated, remove_trailing_commas; they always run in one left-to-right pass outside JSON strings regardless of array order. An empty array applies no repair and only validates.
• remove_comments deletes '//' through the end of its line (the LF/CR is kept) and '/*' through the next '*/' (or the end of the text).
• remove_trailing_commas deletes a ',' whose next significant character (skipping JSON whitespace and removed comments) is '}' or ']'. Only that last comma is deleted, so ',,]' stays invalid. With close_truncated it also deletes a ',' left dangling at the end of a truncated text.
• close_truncated appends, at the end of the text, '"' if the text ends inside a string, then the closer of every still-open bracket (innermost first). It is skipped when any closer did not match the innermost open bracket, when the text ends inside a string escape, or when nothing is open.
• edits lists every change sorted by offset: {rule, offset (UTF-16 offset in the input), removed, inserted}. text is the result; changed is true when edits is non-empty. valid_json applies RFC 8259 strictly to text, and json_error is null or {code, offset} into text.
• No value is ever invented: a text that is still invalid after these rules is returned with valid_json false (not an error).

Input

• text (string, required): max length 131072
• rules (array of one of "remove_comments", "close_truncated", "remove_trailing_commas", optional): max items 3

Output

• text (string, required)
• changed (boolean, required)
• valid_json (boolean, required)
• json_error (object or null, required)
• edits (array of object, required): max items 512

Limits

• max text bytes: 131072
• max edits: 512

Example

Request input:

{
  "text": "{\"a\": [1, 2,], // note\n \"b\": tru"
}

Response:

{
  "result": {
    "text": "{\"a\": [1, 2], \n \"b\": tru}",
    "changed": true,
    "valid_json": false,
    "json_error": {
      "code": "invalid_literal",
      "offset": 24
    },
    "edits": [
      {
        "rule": "remove_trailing_commas",
        "offset": 11,
        "removed": ",",
        "inserted": ""
      },
      {
        "rule": "remove_comments",
        "offset": 15,
        "removed": "// note",
        "inserted": ""
      },
      {
        "rule": "close_truncated",
        "offset": 32,
        "removed": "",
        "inserted": "}"
      }
    ]
  }
}

How to call it

MCP

Connect https://computefirst.net/mcp (setup), then call execute with:

{
  "id": "json-candidate-repair",
  "version": "1.0.0",
  "input": {
    "text": "{\"a\": [1, 2,], // note\n \"b\": tru"
  }
}

HTTP (no key)

curl -X POST https://computefirst.net/v1/tools/json-candidate-repair/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"text":"{\"a\": [1, 2,], // note\n \"b\": tru"}'

The machine-readable contract is at /v1/tools/json-candidate-repair/versions/1.0.0.

CLI

node cli.mjs run json-candidate-repair 1.0.0 --input input.json --base-url https://computefirst.net

Get the client at /clients/cli/.

Related tools

• JSON text repair: Repair six common lexical JSON defects (comments, trailing commas, single quotes, ...) and list every edit made.
• JSON candidate list: Find and rank JSON object/array candidates in prose and Markdown fences (quotes and lists too) with exact raw spans.
• JSON repair diagnose: Classify JSON text as valid, repairable or unrepairable and locate each lexical defect and refusal by line and column.
• JSON schema instance locate: Resolve RFC 6901 instance locations (string or URI fragment) from schema errors to values, or say why they fail.
• JSON fence extract: List Markdown fenced code blocks (CommonMark 0.31.2) with exact spans, unwrapped content and an RFC 8259 JSON check.
• JSON schema output validate: Validate JSON (e.g. LLM output) against a Draft 2020-12 schema subset; returns spec output units with JSON Pointers.