Tool input validation

Treelang validates fully evaluated tool arguments against the provider's JSON Schema immediately before invocation. The same Draft 2020-12 validator is used by schema v1 and v2 execution.

Validated constraints include:

• required and optional fields;
• JSON scalar, array, object, and nullable union types;
• enums and constants;
• numeric ranges and multiples;
• string length, patterns, and recognized formats;
• array length, uniqueness, and item schemas;
• nested object properties and additional-property policy; and
• local $ref definitions.

External schema references are rejected during tool discovery so argument validation never retrieves remote content. Invalid metadata raises ProviderResponseError. Invalid evaluated arguments raise ASTValidationError containing only the tool name, safe field path, and failed constraint—not the argument value. Rejected calls do not reach the provider or consume tool-call budget.

MCP providers preserve the complete inputSchema. Custom providers can return:

{
    "name": "register",
    "properties": {
        "age": {"type": "integer", "minimum": 18},
        "nickname": {"type": "string"},
    },
    "input_schema": {
        "type": "object",
        "properties": {
            "age": {"type": "integer", "minimum": 18},
            "nickname": {"type": "string"},
        },
        "required": ["age"],
        "additionalProperties": False,
    },
}

For compatibility, providers that return only properties retain the historical contract: every listed property is required and additional properties are rejected. JSON Schema default remains an annotation; Treelang does not inject default values. Schema v1 can omit trailing optional parameters but cannot skip a positional property to provide a later one. Schema v2 uses named arguments and can omit any field not listed in required.