JSON Schema for Tool Parameters: A Reference Guide

Tool parameters are almost always a JSON Schema object describing one JSON object of arguments.

Providers differ on which keywords they honor strictly. This cheatsheet covers the conventions that work well for agent tools across OpenAI-compatible APIs, OpenRouter, and common frameworks (verify vendor support at build).

How to Use This Cheatsheet

Start from the minimal object skeleton, then add constraints only where they prevent bad calls.
Prefer closed enums and required arrays over prose-only rules.
Validate the same schema in the host before execution; never trust the model alone.
When a keyword is ignored by a provider, enforce it in host code anyway.

A - Minimal Skeleton

{
  "type": "object",
  "properties": {
    "order_id": {
      "type": "integer",
      "description": "Numeric order id, e.g. 1042"
    }
  },
  "required": ["order_id"],
  "additionalProperties": false
}

Field	Role
`type: "object"`	Root arguments are always an object for tools
`properties`	Named parameters
`required`	Must-present keys
`additionalProperties: false`	Reject surprise keys (support varies; enforce in host)
per-property `description`	Steers argument fill quality

B - Scalar Types

JSON Schema type	Use for	Notes
`string`	Names, ids-as-text, free text	Add `enum`, `pattern`, or format notes in description
`integer`	Counts, numeric ids	Models sometimes emit strings; coerce carefully in host
`number`	Floats, money if not integer cents	Prefer integer cents for currency
`boolean`	Flags	Avoid tri-state; use optional bool or enum
`null`	Rare	Many tool stacks dislike nullable unions; prefer omit optional

{
  "email": {
    "type": "string",
    "description": "Customer email, e.g. ada@example.com"
  },
  "limit": {
    "type": "integer",
    "description": "Page size",
    "minimum": 1,
    "maximum": 100
  },
  "dry_run": {
    "type": "boolean",
    "description": "If true, validate only and do not commit"
  }
}

C - Enums and Closed Sets

{
  "unit": {
    "type": "string",
    "enum": ["celsius", "fahrenheit"],
    "description": "Temperature unit"
  }
}

Guidance	Why
Keep enums small	Large enums bloat tokens and confuse selection
Use stable wire values	Display labels belong in descriptions, not alternate enum spellings
Prefer enum over free string	Cuts invented synonyms (`C`, `celcius`)

D - Arrays and Nested Objects

{
  "type": "object",
  "properties": {
    "item_ids": {
      "type": "array",
      "description": "Product ids to include",
      "items": { "type": "string" },
      "minItems": 1,
      "maxItems": 20
    },
    "address": {
      "type": "object",
      "description": "Shipping address",
      "properties": {
        "line1": { "type": "string" },
        "city": { "type": "string" },
        "country": {
          "type": "string",
          "description": "ISO 3166-1 alpha-2, e.g. PT"
        }
      },
      "required": ["line1", "city", "country"],
      "additionalProperties": false
    }
  },
  "required": ["item_ids"],
  "additionalProperties": false
}

Pattern	Prefer when
Flat parameters	Simple tools; easier for models
One nested object	Natural record (address, money amount+currency)
Deep trees	Avoid; split tools or accept host-side assembly

E - Optional Fields, Defaults, and Required

Approach	Schema	Host behavior
Required	list key in `required`	Reject if missing
Optional omit	not in `required`	Apply default in code
Optional with default keyword	`"default": 5`	Helpful docs; still default in host
Sentinel empty string	discouraged	Prefer omit or null policy you document

Do not list a key as required if the model rarely has the value. Inject server-side context (tenant, actor id) in the host instead of asking the model to guess.

F - Formats, Patterns, and Constraints

Keyword	Example	Caveat
`minimum` / `maximum`	integers, numbers	Enforce in host too
`minLength` / `maxLength`	strings	Good guard against dumps
`minItems` / `maxItems`	arrays	Cap blast radius
`pattern`	`"^[A-Z]{2}$"`	Models may still fail; return clear errors
`format`	`date-time`, `email`	Support varies widely

{
  "sku": {
    "type": "string",
    "description": "SKU like ABC-1234",
    "pattern": "^[A-Z]{3}-[0-9]{4}$"
  },
  "ship_by": {
    "type": "string",
    "description": "ISO-8601 date (YYYY-MM-DD), UTC calendar date"
  }
}

G - Unions and anyOf (use sparingly)

{
  "identifier": {
    "description": "Order id as int or external string key",
    "anyOf": [
      { "type": "integer" },
      { "type": "string", "minLength": 1 }
    ]
  }
}

Advice	Reason
Prefer two tools or one clear type	Unions increase invalid args
If you must union, document decision rule	"Use int when numeric; else external key"
Validate host-side exhaustively	Model-produced unions are messy

H - Provider Packaging Wrapper

Many OpenAI-compatible APIs wrap the schema:

tool = {
    "type": "function",
    "function": {
        "name": "get_order_status",
        "description": "Fetch live status for one order id.",
        "parameters": {
            "type": "object",
            "properties": {
                "order_id": {"type": "integer", "description": "e.g. 1042"}
            },
            "required": ["order_id"],
            "additionalProperties": False,
        },
    },
}

Other stacks (Anthropic-style tool specs, MCP inputSchema, Pydantic AI, LangGraph) carry the same object schema under slightly different keys. Keep an internal canonical schema and adapt at the edge.

I - Generate From Pydantic (optional)

from pydantic import BaseModel, Field
 
class GetOrderArgs(BaseModel):
    order_id: int = Field(description="Numeric order id, e.g. 1042")
 
    model_config = {"extra": "forbid"}
 
schema = GetOrderArgs.model_json_schema()
# use schema as the tool parameters object; verify $defs inlining for your provider

Benefit	Watch-out
Single source for validate + schema	Some exporters emit `$refs` providers dislike
Field descriptions stay close to code	Still write tool-level description separately

J - Host Validation Checklist

Check	On failure
JSON parse of arguments string	tool error `invalid_json`
Schema validate	tool error with path + expected type
Business rules	tool error `invalid_argument` with fix hint
Authz	tool error or hard refuse without leaking secrets
Size caps	truncate or reject oversized strings/arrays

FAQs

Is full JSON Schema supported for tools?

No universal guarantee. Stick to a conservative subset: object, properties, required, type, enum, basic numeric/string bounds. Verify at build for each model/provider.

Should I set additionalProperties to false?

Yes when the provider honors it, and always enforce in host validation. Surprise keys are a common model failure mode.

Integer vs string ids?

Use the type your backend truly uses. If systems are mixed, document one preferred type and normalize in the host.

How do I represent money?

Prefer integer minor units (amount_cents) plus a currency enum. Floating number invites rounding bugs.

Can parameters be empty objects?

Yes for no-arg tools: "properties": {} with "required": []. Some APIs still want type: object.

What about output schemas?

Tool parameters are inputs. Output shape is your observation contract; some platforms also support structured final answers separately.

Why did the model omit a required field?

Weak description, overloaded context, or model limits. Return a clear error and tighten the schema/description; consider forcing the tool only when appropriate.

Are $ref and $defs safe?

Often problematic for tool calling. Inline small nested objects unless your provider documents $ref support.

Tool Use & Function Calling Basics - first schema in a loop
Writing Tool Descriptions the Model Will Actually Use Correctly - wording that fills args well
How Function Calling Actually Works Under the Hood - where schemas sit in the protocol
Handling Tool Errors Gracefully Inside an Agent Loop - invalid args as observations
Tool Use & Function Calling Best Practices - naming and scope habits
Versioning and Deprecating Agent Tools Without Breaking Agents - schema evolution

Stack versions: Pins from the category manifest (verify at build): OpenRouter (~315+ models, July 2026 pricing/fees); LangGraph 1.0+; CrewAI 1.14+; Microsoft Agent Framework 1.0; Vercel AI SDK 6; Pydantic AI (latest); LlamaIndex (latest); OpenAI Agents SDK (latest + MCP); MCP (Linux Foundation governance); A2A (HTTP+SSE+JSON-RPC 2.0); Solana @solana/web3.js + @solana/spl-token.

JSON Schema for Tool Parameters: A Reference Guide

Tool parameters are almost always a JSON Schema object describing one JSON object of arguments.

How to Use This Cheatsheet

Start from the minimal object skeleton, then add constraints only where they prevent bad calls.
Prefer closed enums and required arrays over prose-only rules.
Validate the same schema in the host before execution; never trust the model alone.
When a keyword is ignored by a provider, enforce it in host code anyway.

A - Minimal Skeleton

{
  "type": "object",
  "properties": {
    "order_id": {
      "type": "integer",
      "description": "Numeric order id, e.g. 1042"
    }
  },
  "required": ["order_id"],
  "additionalProperties": false
}

Field	Role
`type: "object"`	Root arguments are always an object for tools
`properties`	Named parameters
`required`	Must-present keys
`additionalProperties: false`	Reject surprise keys (support varies; enforce in host)
per-property `description`	Steers argument fill quality

B - Scalar Types

JSON Schema type	Use for	Notes
`string`	Names, ids-as-text, free text	Add `enum`, `pattern`, or format notes in description
`integer`	Counts, numeric ids	Models sometimes emit strings; coerce carefully in host
`number`	Floats, money if not integer cents	Prefer integer cents for currency
`boolean`	Flags	Avoid tri-state; use optional bool or enum
`null`	Rare	Many tool stacks dislike nullable unions; prefer omit optional

{
  "email": {
    "type": "string",
    "description": "Customer email, e.g. ada@example.com"
  },
  "limit": {
    "type": "integer",
    "description": "Page size",
    "minimum": 1,
    "maximum": 100
  },
  "dry_run": {
    "type": "boolean",
    "description": "If true, validate only and do not commit"
  }
}

C - Enums and Closed Sets

{
  "unit": {
    "type": "string",
    "enum": ["celsius", "fahrenheit"],
    "description": "Temperature unit"
  }
}

Guidance	Why
Keep enums small	Large enums bloat tokens and confuse selection
Use stable wire values	Display labels belong in descriptions, not alternate enum spellings
Prefer enum over free string	Cuts invented synonyms (`C`, `celcius`)

D - Arrays and Nested Objects

{
  "type": "object",
  "properties": {
    "item_ids": {
      "type": "array",
      "description": "Product ids to include",
      "items": { "type": "string" },
      "minItems": 1,
      "maxItems": 20
    },
    "address": {
      "type": "object",
      "description": "Shipping address",
      "properties": {
        "line1": { "type": "string" },
        "city": { "type": "string" },
        "country": {
          "type": "string",
          "description": "ISO 3166-1 alpha-2, e.g. PT"
        }
      },
      "required": ["line1", "city", "country"],
      "additionalProperties": false
    }
  },
  "required": ["item_ids"],
  "additionalProperties": false
}

Pattern	Prefer when
Flat parameters	Simple tools; easier for models
One nested object	Natural record (address, money amount+currency)
Deep trees	Avoid; split tools or accept host-side assembly

E - Optional Fields, Defaults, and Required

Approach	Schema	Host behavior
Required	list key in `required`	Reject if missing
Optional omit	not in `required`	Apply default in code
Optional with default keyword	`"default": 5`	Helpful docs; still default in host
Sentinel empty string	discouraged	Prefer omit or null policy you document

Do not list a key as required if the model rarely has the value. Inject server-side context (tenant, actor id) in the host instead of asking the model to guess.

F - Formats, Patterns, and Constraints

Keyword	Example	Caveat
`minimum` / `maximum`	integers, numbers	Enforce in host too
`minLength` / `maxLength`	strings	Good guard against dumps
`minItems` / `maxItems`	arrays	Cap blast radius
`pattern`	`"^[A-Z]{2}$"`	Models may still fail; return clear errors
`format`	`date-time`, `email`	Support varies widely

{
  "sku": {
    "type": "string",
    "description": "SKU like ABC-1234",
    "pattern": "^[A-Z]{3}-[0-9]{4}$"
  },
  "ship_by": {
    "type": "string",
    "description": "ISO-8601 date (YYYY-MM-DD), UTC calendar date"
  }
}

G - Unions and anyOf (use sparingly)

{
  "identifier": {
    "description": "Order id as int or external string key",
    "anyOf": [
      { "type": "integer" },
      { "type": "string", "minLength": 1 }
    ]
  }
}

Advice	Reason
Prefer two tools or one clear type	Unions increase invalid args
If you must union, document decision rule	"Use int when numeric; else external key"
Validate host-side exhaustively	Model-produced unions are messy

H - Provider Packaging Wrapper

Many OpenAI-compatible APIs wrap the schema:

tool = {
    "type": "function",
    "function": {
        "name": "get_order_status",
        "description": "Fetch live status for one order id.",
        "parameters": {
            "type": "object",
            "properties": {
                "order_id": {"type": "integer", "description": "e.g. 1042"}
            },
            "required": ["order_id"],
            "additionalProperties": False,
        },
    },
}

I - Generate From Pydantic (optional)

from pydantic import BaseModel, Field
 
class GetOrderArgs(BaseModel):
    order_id: int = Field(description="Numeric order id, e.g. 1042")
 
    model_config = {"extra": "forbid"}
 
schema = GetOrderArgs.model_json_schema()
# use schema as the tool parameters object; verify $defs inlining for your provider

Benefit	Watch-out
Single source for validate + schema	Some exporters emit `$refs` providers dislike
Field descriptions stay close to code	Still write tool-level description separately

J - Host Validation Checklist

Check	On failure
JSON parse of arguments string	tool error `invalid_json`
Schema validate	tool error with path + expected type
Business rules	tool error `invalid_argument` with fix hint
Authz	tool error or hard refuse without leaking secrets
Size caps	truncate or reject oversized strings/arrays

FAQs

Is full JSON Schema supported for tools?

No universal guarantee. Stick to a conservative subset: object, properties, required, type, enum, basic numeric/string bounds. Verify at build for each model/provider.

Should I set additionalProperties to false?

Yes when the provider honors it, and always enforce in host validation. Surprise keys are a common model failure mode.

Integer vs string ids?

Use the type your backend truly uses. If systems are mixed, document one preferred type and normalize in the host.

How do I represent money?

Prefer integer minor units (amount_cents) plus a currency enum. Floating number invites rounding bugs.

Can parameters be empty objects?

Yes for no-arg tools: "properties": {} with "required": []. Some APIs still want type: object.

What about output schemas?

Tool parameters are inputs. Output shape is your observation contract; some platforms also support structured final answers separately.

Why did the model omit a required field?

Weak description, overloaded context, or model limits. Return a clear error and tighten the schema/description; consider forcing the tool only when appropriate.

Are $ref and $defs safe?

Often problematic for tool calling. Inline small nested objects unless your provider documents $ref support.

Tool Use & Function Calling Basics - first schema in a loop
Writing Tool Descriptions the Model Will Actually Use Correctly - wording that fills args well
How Function Calling Actually Works Under the Hood - where schemas sit in the protocol
Handling Tool Errors Gracefully Inside an Agent Loop - invalid args as observations
Tool Use & Function Calling Best Practices - naming and scope habits
Versioning and Deprecating Agent Tools Without Breaking Agents - schema evolution

Stack versions: Pins from the category manifest (verify at build): OpenRouter (~315+ models, July 2026 pricing/fees); LangGraph 1.0+; CrewAI 1.14+; Microsoft Agent Framework 1.0; Vercel AI SDK 6; Pydantic AI (latest); LlamaIndex (latest); OpenAI Agents SDK (latest + MCP); MCP (Linux Foundation governance); A2A (HTTP+SSE+JSON-RPC 2.0); Solana @solana/web3.js + @solana/spl-token.

JSON Schema for Tool Parameters: A Reference Guide

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JSON Schema for Tool Parameters: A Reference Guide