> ## Documentation Index > Fetch the complete documentation index at: https://docs.reducto.ai/llms.txt > Use this file to discover all available pages before exploring further. # Schema Keyword Support > Which JSON Schema keywords Extract guarantees in the output, and which it treats as guidance Extract accepts a JSON Schema and returns JSON that matches it. Not every keyword carries the same guarantee. Some keywords are enforced: the output always satisfies them. Others are best effort: Extract reads them as guidance and follows them in most cases, but you should validate them in your own code. This page applies to the `v4` extract model. See [Model Versions](/reference/model-versions) for how to select it. *** ## How Extract uses your schema Your schema does two jobs at once: 1. **Output shape.** Extract constrains the response so it always has the fields and types you declared. 2. **Guidance.** Field names, descriptions, and constraints tell Extract what to look for and how to format it. Enforced keywords do both jobs. Best-effort keywords only do the second one. For example, `"pattern": "^[A-Z]{2}-\\d{4}$"` on a string guarantees the output matches. `"format": "email"` does not guarantee a valid address, but it does steer Extract toward one. *** ## Enforced keywords The output satisfies these keywords, with the exceptions noted in each row. | Keyword | Notes | | ------------------------------------------------------------ | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `type` | Includes type lists such as `["string", "null"]`. Scalar fields can also be `null` when the value is not in the document. | | `enum` | The output value is always one of the listed values, or `null` when the value is not in the document. | | `const` | The output value always equals the declared value. Declare `type` next to `const` so the field is never `null`. | | `properties` | Every declared field is always present. | | `required` | Treated as all fields required. Every declared field is always in the output. | | `additionalProperties` | Objects are closed by default: Extract only returns declared fields. | | `items` | Every array item matches the item schema. | | `prefixItems` | Positional item schemas are honored. | | `minItems`, `maxItems` | Array length stays inside the bounds during generation. Extract then drops empty items (`null`, `[]`, or objects with only `null` values), so a returned array can be shorter than `minItems`. | | `minLength`, `maxLength` | String length stays inside the bounds. | | `pattern` | The whole string must match. Patterns are treated as anchored even without `^` and `$`. | | `minimum`, `maximum`, `exclusiveMinimum`, `exclusiveMaximum` | Exact for integers. For non-integer numbers, exclusive bounds can behave like inclusive bounds. | | `anyOf`, `oneOf` | The output matches at least one branch. | | `patternProperties`, `propertyNames`, `minProperties` | Object key constraints are honored. | | `$ref` to `#/$defs/...` | Local references are resolved before extraction. | | `nullable: true` | Treated as adding `null` to `type`. | | `uniqueItems` | Checked after generation. A response with duplicate items fails validation and is never returned as a success. Arrays reached through `$ref` are not checked. | ### Every field is always returned Extract returns every declared field on every object, even if you list only some of them in `required`. Scalar fields (string, number, integer, boolean, enum) with no supporting value in the document come back as `null`. Object and array fields keep their declared shape: an array with no matches is `[]`, and an object has all of its own fields set to `null`. If a field must not be `null` in your system, check for it in your code. ```json theme={null} { "type": "object", "properties": { "invoice_number": { "type": "string" }, "po_number": { "type": "string" } }, "required": ["invoice_number"] } ``` Output for a document with no PO number: ```json theme={null} { "invoice_number": "INV-2041", "po_number": null } ``` *** ## Best-effort keywords Extract reads these keywords as guidance. Output usually follows them, but it is not guaranteed. Validate these in your code if they matter. | Keyword | What to expect | | ------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------- | | `description`, `title`, `examples` | Guidance only. Descriptions are the strongest lever you have for extraction quality. See [Best Practices](/extraction/best-practices-extract). | | `format` | Steers formatting for values such as `date`, `date-time`, `email`, and `uuid`. Use `pattern` when the exact shape must be guaranteed. | | `multipleOf` | Not guaranteed. | | `allOf` | Not guaranteed. Merge the branches into a single schema when possible. | | `not` | Not guaranteed. | | `if`, `then`, `else`, `dependentRequired`, `dependentSchemas` | Conditional rules are not guaranteed. | | `contains`, `minContains`, `maxContains` | Not guaranteed. | ### Prefer `pattern` over `format` `format` improves how Extract writes a value, but only `pattern` guarantees the shape. ```json theme={null} { "type": "string", "description": "Invoice date in ISO 8601 format", "pattern": "^\\d{4}-\\d{2}-\\d{2}$" } ``` *** ## Ignored keywords These keywords are accepted but have no effect on extraction: `default`, `$comment`, `$schema`, `deprecated`, `readOnly`, `writeOnly`. *** ## Schema validation Extract validates your schema before it runs. Invalid schemas fail with a `400` and a message that points at the problem. Common causes: * The schema is not valid JSON Schema (draft 2020-12). * An object declares no `properties`. * An array declares no `items`. * `$ref` points outside `#/$defs`, or forms a cycle. * A `required` entry names a field that is not in `properties` on a closed object. * An `enum` or `const` value does not match the declared `type` or violates a sibling constraint, such as `"enum": ["ab"]` with `"minLength": 3`. *** ## Troubleshooting **A value comes back `null` even though it is in the document.** Extract works from the Parse output, so your Parse configuration affects extraction accuracy. Use [Parse r-1](/parse/r-1) or [agentic modes](/configs/parse/agentic-modes) for the best results. If the value is still missing, tighten the field `description` to say where the value appears. **A string does not match my `format`.** `format` is best effort. Add a `pattern` for the exact shape. **An extra field I did not declare is missing.** Extract only returns declared fields. Declare the field in `properties`. **My `allOf` constraint is not respected.** `allOf` is best effort. Combine the branches into one object schema with the union of their properties and constraints.