additional-properties-object

Category: Semantic — schema semantics trigger this rule

Error Codes

Profile	Code	Severity
openai.so.2026-04-30	OAI-S-additional-properties-object	Forbid
anthropic.so.2026-04-30	ANT-S-additional-properties-object	Forbid

Description

additionalProperties must be set to false, not an object schema

Provider Evidence

Profile	Status	Source	Basis
anthropic.so.2026-04-30	sdk_transform	Anthropic SDK transformation	Official SDK transformation sets additionalProperties to false for object schemas.
openai.so.2026-04-30	inferred	OpenAI additionalProperties	The documented subset requires additionalProperties to be false.

Rationale

LLM structured-output providers require additionalProperties: false to guarantee schema compliance. An object value indicates intent to define allowed extras, which most providers do not support.

Bad Example

{
  "type": "object",
  "additionalProperties": { "type": "string" },
  "properties": {}
}

Good Example

{
  "type": "object",
  "additionalProperties": false,
  "properties": {}
}