# JSON schema type mismatches

`json-schema-type-mismatches` · version 1.0.0 · JSON & JSONL · free, no key needed

List every value whose JSON type fails a JSON Schema 2020-12 "type" keyword, with expected and actual types.

**Use when you need to: find wrong types in llm json output · which values have the wrong json type for the schema · list type mismatches with expected and actual type.**

## Decide before calling

Read the [versioned contract](/v1/tools/json-schema-type-mismatches/versions/1.0.0) and the supported scope below. Reuse `json-schema-type-mismatches@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-schema-type-mismatches@1.0.0` for find wrong types in llm json output. 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

- find wrong types in llm json output
- which values have the wrong json type for the schema
- list type mismatches with expected and actual type
- json pointer of every value failing the type keyword
- detect numbers returned as strings in structured output
- is 1.0 an integer under json schema 2020-12

## Not supported

- coercing values to the expected type (for example "42" to 42)
- other assertions such as enum, const, minimum, minLength, pattern or format
- schemas using allOf/anyOf/oneOf/not/if/patternProperties/prefixItems, remote or anchor $ref (unsupported_input)

## Behavior

- Validation 6.1.1: type is one name or an array of unique names from null, boolean, object, array, number, string, integer; a value passes when its type matches any listed name. An empty type array matches nothing.
- integer matches any number with a zero fractional part, so 1.0 is an integer; number matches every number. The test applies to the IEEE-754 double the value arrives as (JSON text such as 1.0000000000000001 has already become 1). actual reports "integer" for an integer-valued number and "number" otherwise.
- expected lists the keyword's names in schema order (a single string becomes a one-element array). A mismatching value is still traversed: properties, additionalProperties and items keep applying to its children.
- Applicability follows Core 2020-12: properties, additionalProperties as a subschema, items as a single subschema, same-document fragment $ref with sibling keywords, $defs and boolean subschemas. Other keywords are ignored (Core 6.5) except the unsupported applicators, which throw unsupported_input.
- instance_pointer is an RFC 6901 JSON string representation; keyword_location is the evaluation path to the type keyword including "$ref" steps (Core 12.3.1); schema_pointer is that keyword's location in the schema document. Order: pre-order over instance locations (members by ascending UTF-16 code units, elements by index), then schemas in application order.
- count is the total number of failures; at most max_results are listed and truncated is true when count exceeds it; valid is true exactly when count is 0. Lone surrogates in any string or property name, non-finite numbers and non-JSON values throw invalid_input; a $ref cycle with no instance progress throws not_computable.

## Input

- `schema` (object or boolean, required): A JSON Schema 2020-12 schema (object or boolean).
- `instance` (any JSON value, required): The JSON value to diagnose (any JSON value).

## Output

- `valid` (boolean, required)
- `count` (integer, required): min 0
- `truncated` (boolean, required)
- `mismatches` (array of object, required): max items 256

## Limits

- max input bytes: 262144
- max depth: 64
- max nodes: 20000
- max schema evaluations: 100000
- max results: 256
- max output bytes: 1048576

## Example

Request input:

```json
{
  "schema": {
    "type": "object",
    "properties": {
      "age": {
        "type": "integer"
      },
      "tags": {
        "type": "array",
        "items": {
          "type": "string"
        }
      }
    }
  },
  "instance": {
    "age": "42",
    "tags": [
      "a",
      3
    ]
  }
}
```

Response:

```json
{
  "result": {
    "valid": false,
    "count": 2,
    "truncated": false,
    "mismatches": [
      {
        "instance_pointer": "/age",
        "expected": [
          "integer"
        ],
        "actual": "string",
        "keyword_location": "/properties/age/type",
        "schema_pointer": "/properties/age/type"
      },
      {
        "instance_pointer": "/tags/1",
        "expected": [
          "string"
        ],
        "actual": "integer",
        "keyword_location": "/properties/tags/items/type",
        "schema_pointer": "/properties/tags/items/type"
      }
    ]
  }
}
```

## How to call it

### MCP

Connect `https://computefirst.net/mcp` ([setup](/docs#connect)), then call `execute` with:

```json
{
  "id": "json-schema-type-mismatches",
  "version": "1.0.0",
  "input": {
    "schema": {
      "type": "object",
      "properties": {
        "age": {
          "type": "integer"
        },
        "tags": {
          "type": "array",
          "items": {
            "type": "string"
          }
        }
      }
    },
    "instance": {
      "age": "42",
      "tags": [
        "a",
        3
      ]
    }
  }
}
```

### HTTP (no key)

```sh
curl -X POST https://computefirst.net/v1/tools/json-schema-type-mismatches/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"schema":{"type":"object","properties":{"age":{"type":"integer"},"tags":{"type":"array","items":{"type":"string"}}}},"instance":{"age":"42","tags":["a",3]}}'
```

The machine-readable contract is at [/v1/tools/json-schema-type-mismatches/versions/1.0.0](/v1/tools/json-schema-type-mismatches/versions/1.0.0).

### CLI

```sh
node cli.mjs run json-schema-type-mismatches 1.0.0 --input input.json --base-url https://computefirst.net
```

Get the client at [/clients/cli/](/clients/cli/).

## Related tools

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- [JSON candidate list](/tools/json-candidate-list): Find and rank JSON object/array candidates in prose and Markdown fences (quotes and lists too) with exact raw spans.
- [JSON schema additional find](/tools/json-schema-additional-find): Find every member a JSON Schema 2020-12 "additionalProperties": false forbids, with the member's JSON Pointer.
- [JSON schema output validate](/tools/json-schema-output-validate): Validate JSON (e.g. LLM output) against a Draft 2020-12 schema subset; returns spec output units with JSON Pointers.
- [JSON schema batch validate](/tools/json-schema-batch-validate): Validate many JSON records against one Draft 2020-12 subset schema; per-record validity, error pointers, counts.
- [JSON fence extract](/tools/json-fence-extract): List Markdown fenced code blocks (CommonMark 0.31.2) with exact spans, unwrapped content and an RFC 8259 JSON check.