Enum ResponseFormat 

pub enum ResponseFormat {
    Text,
    JsonObject,
    JsonSchema {
        schema: Value,
        name: String,
        strict: bool,
    },
}

Desired output shape for an LLM call.

Default Text matches the historical free-form text behaviour. JsonObject constrains output to syntactically valid JSON with no schema enforcement; JsonSchema adds strict-shape enforcement when the provider supports it.

Message::extract_json::<T>() is the recommended way to parse the resulting assistant message back into a typed value — it handles both native JSON-mode output (text content is JSON) and tool-call emulation (arguments JSON of a well-known synthetic tool) uniformly.

Variants

Text

Free-form text. Default; providers ignore the field entirely.

JsonObject

Constrain output to valid JSON; no schema enforcement.

Maps to:

• OpenAI Completions / Responses / Azure: response_format: { type: "json_object" }
• Google GenAI / Vertex: responseMimeType: "application/json"
• Anthropic / Bedrock-Anthropic: a synthetic respond_json tool with an empty-shape schema; the LLM is forced to call it with its answer
• Bedrock non-Anthropic: not supported — provider returns SchemaMismatch

JsonSchema

Strict JSON Schema enforcement. The schema is forwarded to the provider when supported natively; otherwise emulated via tool-call shape (Anthropic).

Fields

schema: Value

JSON Schema (Draft 2020-12 compatible) describing the expected output.

name: String

Human-readable schema name (some providers use this in error messages).

strict: bool

Whether the provider should enforce strict shape (no extra fields). Some providers’ strict mode disables defaults — fall back to non-strict if unsupported.

Trait Implementations

impl Clone for ResponseFormat

fn clone(&self) -> ResponseFormat

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for ResponseFormat

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for ResponseFormat

fn default() -> ResponseFormat

Returns the “default value” for a type.