Structured output

Lime can constrain a turn’s response to a JSON Schema. Useful when you’re piping Lime into another tool, building a pipeline, or want a model to emit a specific data shape.

How it works

Pass --output-schema FILE at launch. The schema is loaded, validated, and forwarded to the active provider as a structured-output constraint (or, where the provider does not natively support schema constraints, enforced by Lime via the structured_output tool).

lime --output-schema ./schema.json --resume latest /…

The exact wire mechanism depends on the provider:

Provider	How the schema is enforced
OpenAI	Forwarded as response_format.json_schema on the Responses API.
Anthropic	Enforced via the structured_output tool in the system prompt.
Gemini	Forwarded as responseSchema on generateContent.
OpenAI-compatible	Forwarded as response_format if the preset declares schema support; otherwise enforced via structured_output.

Schema file shape

The file must be a single JSON document containing a JSON Schema (Draft 2020-12). Top-level type is typically "object" with "required" and "properties".

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "FailingTests",
  "type": "object",
  "required": ["count", "tests"],
  "properties": {
    "count": { "type": "integer", "minimum": 0 },
    "tests": {
      "type": "array",
      "items": {
        "type": "object",
        "required": ["name", "file"],
        "properties": {
          "name": { "type": "string" },
          "file": { "type": "string" },
          "line": { "type": "integer" }
        },
        "additionalProperties": false
      }
    }
  },
  "additionalProperties": false
}

Tip

Set "additionalProperties": false on objects to give the model a strict shape. This is the single most effective constraint for getting machine-parseable output.

Worked example

cat > /tmp/failing-tests.json <<'EOF'
{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "required": ["tests"],
  "properties": {
    "tests": {
      "type": "array",
      "items": {
        "type": "object",
        "required": ["name", "file"],
        "properties": {
          "name": { "type": "string" },
          "file": { "type": "string" }
        },
        "additionalProperties": false
      }
    }
  },
  "additionalProperties": false
}
EOF

lime --output-schema /tmp/failing-tests.json

The streamed response is incremental but converges to a single JSON document that validates against the schema.

Caveats

• Schema enforcement is a constraint on the final answer, not a guarantee at every streaming chunk. Do not parse partial chunks as JSON.
• Some openai-compat services do not advertise schema support; Lime falls back to the tool-based path automatically.
• Reasoning summaries and tool calls are not constrained — the schema applies to the model’s final visible output only.