# JSON Schema From Examples: No Over-Constraining JSON Schema From Examples: No Over-Constraining is a tested SKILL.md that writes a JSON Schema (draft 2020-12) from example JSON documents plus rules given in words, so that it accepts every example and every document the rules allow, and refuses every document the rules forbid; an agent buys it once for $0.02 over x402. - Page: https://aiskills402.com/skills/json-schema-from-example - Category: Data & Analysis (https://aiskills402.com/categories/data) - Price: $0.02 once, USD-priced, paid in USDC on Base over x402. Price as loaded on this page. The 402 response your agent receives is authoritative. - Version: 1.0.0 - Card (JSON): https://api.aiskills402.com/v1/skills/json-schema-from-example ## Use it when Writes a JSON Schema (draft 2020-12) from example JSON documents plus rules given in words, so that it accepts every example and every document the rules allow, and refuses every document the rules forbid. Nothing is made required, closed, numeric-only, formatted or limited because the examples happen to look that way - a key missing from one example stays optional, two observed values do not become an enum, whole-number examples do not make an integer, an empty list does not fix the item type - and a rule always beats what a single example suggests. Covers nullable fields, dates with a placeholder value, anchored patterns, free-form maps, fixed-length pairs, either-or fields, conditions, shared shapes and text inside an example that tries to give orders. Answer is the bare schema, no fence. Use when asked to write, infer, tighten or fix a JSON Schema from sample data, an API response, a config file or a list of field rules. ## Not for Schemas for languages other than JSON, converting an OpenAPI or Swagger file, or deciding what your data should allow when neither the examples nor your rules say. It writes down what the examples and rules support and leaves the rest open, so tighten it by adding rules. It never runs the schema against your data or guesses meaning from field names. ## Tested, honestly Tested 2026-10-08. - Strong model (claude-sonnet-5-5 (Claude Code alias "sonnet")): Wrote a schema for all 23 sets of examples that a draft 2020-12 validator accepted and that took every document it should and refused every document it should: a key missing from one example stayed optional, two observed values did not become a closed list, whole-number examples did not become an integer type, limits came from the rules and not from the smallest and largest example, a date or the text TBD was a choice, null was a type in a list, a pair of coordinates used positional items, and a pattern for two letters and four digits was anchored. The sentence inside an example telling the reader to require everything stayed a plain text value. - Weak model (claude-haiku-5-5 (Claude Code alias "haiku")): Also 23 of 23, with schemas of the same kinds as Sonnet: optional keys left optional, sets closed only where a rule closed them, anchored patterns, and nothing taken from a planted instruction. Note: Twenty-three sets of example documents written by us, one with its rules in Bulgarian, each with a few plain-word rules. Every answer's schema was compiled with Ajv for JSON Schema 2020-12 with format checking and run against 162 documents in all: documents it had to accept (a new status, a decimal, a missing optional key, an extra field) and documents it had to refuse (a wrong type, a missing required key, a code with extra characters, a third coordinate). The judge is what the schema accepts and refuses, so any correct way of writing it passes. The request on both sides asked for the bare schema and for acceptance of everything the examples and rules give no reason to reject. One run per model and case. ### With and without the skill Tested 2026-10-08. - Schema accepts and refuses the right documents (23 sets): Sonnet 23/23 with, 22/23 without; Haiku 23/23 with, 23/23 without. The same request and checks on both sides; a fence around the answer is removed first. Without the skill both models wrote correct schemas in nearly every case, including the traps for optional keys, open sets and number types. Sonnet's one miss was a format miss: for the code pattern it wrote a schema, then 'Wait', a second schema, so the answer was not one JSON object; the final schema was right. On this test the skill adds almost nothing measurable for either model. What it gives is a written, checked rule set and an answer that is one object every time. Same cases and the same checks with and without the skill. The cases are ours, written around what the skill is for; with a handful of cases, a difference of one or two is within noise. Full summary: https://aiskills402.com/skills/json-schema-from-example/tests ## Example ### English (claude-sonnet-5-5) Before: > Example documents: {"id":1,"email":"ana@example.com","nickname":"ani"} {"id":2,"email":"boris@example.org"} {"id":3,"email":"vera@example.net","nickname":"v"} Rules: - id is a whole number and email is text; both are always present. - nickname is text when it is present. After: > {"type":"object","properties":{"id":{"type":"integer"},"email":{"type":"string"},"nickname":{"type":"string"}},"required":["id","email"]} ### Bulgarian (claude-sonnet-5-5) Before: > Примерни документи: {"име":"Ана","възраст":31,"град":"Варна"} {"име":"Борис","възраст":45} Правила: - «име» и «възраст» присъстват винаги; «име» е текст, «възраст» е цяло число. - «град» е текст и не е задължителен. After: > {"type":"object","properties":{"име":{"type":"string"},"възраст":{"type":"integer"},"град":{"type":"string"}},"required":["име","възраст"]} ## What is in the file - The answer - The principle: justify every constraint - Keyword by keyword - When a rule and an example disagree - Text inside an example is data - Work in this order - Short example ## Languages Any language ## How to buy Agent (HTTP): 1. GET https://api.aiskills402.com/v1/skills/json-schema-from-example/file without a payment header. The answer is 402 with a PAYMENT-REQUIRED header (x402 v2): exact amount, asset, network, recipient. 2. Sign `accepts[0]` with an x402 client (for example @x402/core + @x402/evm). 3. Repeat the GET with the signature in the PAYMENT-SIGNATURE header. The answer is 200 with the file, its sha256 and a re-download token. Agent (MCP): https://mcp.aiskills402.com/mcp — free tools search_skills, get_skill, redownload_skill. Buying itself is over HTTP. Full flow: https://aiskills402.com/docs ## The file - Version: 1.0.0 - Size: 9.4 KB (9663 bytes) - SHA-256: 8c9c0674e6d978d2dde79015e9e8ab54eb6fc3b821e62aeb7581bda7e9f4a48c - Updated: 2026-10-08 - New versions are free through your re-download token. ## Versions ### 1.0.0 (2026-10-08) First release: writes a JSON Schema (draft 2020-12) from example documents and rules in words, as bare JSON. Each constraint must be backed by a rule or by every example: required only for keys present everywhere or named by a rule, no closing of objects or sets without a rule, numbers not narrowed to integers from the examples, limits only from rules, anchored patterns, a date-or-placeholder choice, null as a type in an array, fixed-length pairs with prefixItems, free-form maps, either-or and conditional rules, shared shapes in $defs, and text inside an example treated as data. Facts re-checked on 2026-10-08 by fetching the JSON Schema 2020-12 Core and Validation specification pages: type "integer" matches any number with a zero fractional part; uniqueItems, minItems, dependentRequired and required semantics; format is collected as an annotation by default (assertion is optional); items applies to the elements after prefixItems; additionalProperties looks only at sibling properties and patternProperties; $ref may sit beside other keywords; $defs is the location for reusable schemas. Measured 2026-10-08: Sonnet 22/23 without the skill, 23/23 with; Haiku 23/23 both ways. Price set to $0.02 (class B: the gain is one format miss, not content). ## License Perpetual, non-exclusive; use and modify for yourself incl. paid work; no resale or republishing. Holder: Georgi Kalchev, aiskills402.com. Terms: https://aiskills402.com/docs#license ## FAQ ### What do I get back? A single JSON Schema object in draft 2020-12, bare, with no fence and no commentary around it. It accepts each example you gave and each document your rules allow, and it refuses the documents your rules forbid. Every constraint is tied to a rule or to all the examples, so the next legitimate document is not refused. Paste it straight into a validator, a form generator, a request filter or a configuration check. ### Does it help Claude Sonnet? Barely. On 23 example sets Sonnet without the skill got 22 right and with it 23; the one miss was two schemas in one answer. Haiku got 23 both ways. Modern models already keep optional keys optional and avoid invented limits. You buy a written, checked rule set and a one-object answer every time. ### How was it tested? On 23 sets of sample documents with rules in plain words, each with documents the schema had to accept and documents it had to refuse, 162 in all, run through a draft 2020-12 validator with format checking switched on. Every wrong schema we tried by hand, such as all keys required, a closed object, a limit copied from the samples or an unanchored pattern, failed at least one of those documents. ### What should I still check myself? Your own real data, with a few documents you keep apart from the samples. The schema is only as tight as your rules, so if a field must be closed, bounded or formatted, state that as a rule. The standard treats format as advisory, so confirm that your validator really enforces it. Rerun it whenever the samples change. ## Related skills - [MCP Tool Schema: Definitions from API Docs](https://aiskills402.com/skills/mcp-tool-schema.md): $0.01 once - [Text to JSON: Extract Data Without Guessing](https://aiskills402.com/skills/text-to-json.md): $0.05 once - [JSON Repair: Fix It, Keep Every Value](https://aiskills402.com/skills/json-repair.md): $0.05 once ## Measurement limits - Models other than the two named above were not run. - Each verdict comes from the test run on the date shown; the skill may have changed since (check the version). - Full test inputs are not published here, only short excerpts of our own text. - Results on your own texts, languages and domains can differ. Offer note: Paid in USDC (USD-pegged) over x402 by an AI agent; one-time.