Structured Output (--json-schema)

Constrain the model’s final answer to a JSON Schema you supply. Qwen Code registers a synthetic terminal tool the model is required to call, parses the call’s arguments against your schema, and exposes the validated payload on stdout (or in the JSON / stream-json result envelope). The first valid call ends the run.

Headless only — works with qwen -p, a positional prompt, or a prompt piped via stdin.

Quick start

qwen --prompt "Summarize the changes in HEAD with risk_level" \
  --json-schema '{
    "type": "object",
    "properties": {
      "summary":    { "type": "string" },
      "risk_level": { "type": "string", "enum": ["low", "medium", "high"] }
    },
    "required": ["summary", "risk_level"],
    "additionalProperties": false
  }'

Output on stdout (default --output-format text):

{ "summary": "…", "risk_level": "low" }

The line is exactly the JSON-stringified payload + newline — no envelope, no event log. Pipe it straight into jq or another consumer.

In text mode, stdout is reserved for the JSON payload on success and is empty on failure; error messages and log lines go to stderr. That makes $(qwen --json-schema …) || exit 1 capture patterns safe under text mode — failures land in stderr, not mixed into the captured variable. The model’s incidental prose during planning is not mirrored to stderr either — text mode discards it; reach for --output-format json or stream-json if you need to see it.

In --output-format json and stream-json, the failure result message is emitted on stdout alongside the success path (as the final element of the JSON array, or the terminating result line on the JSONL stream). Not all failure modes emit a result to stdout — max-session-turns (exit 53) and signal interrupts (exit 130) exit with stderr output only. Check the exit code first; is_error on the result object disambiguates within the subset of failures that do produce a result event.

Empty schema: Passing {} produces {} (an empty JSON object) on stdout. The model calls structured_output with no arguments; the upstream argument-normalisation path turns the empty function call into an empty-object payload, which passes validation against the empty schema and is emitted verbatim.

Supplying the schema

Two equivalent forms:

# Inline JSON literal
qwen -p "…" --json-schema '{"type":"object", "properties":{…}}'
 
# Read from a file
qwen -p "…" --json-schema @./schemas/summary.json

The @path form expands ~, normalizes the path, and reads the file with utf8 encoding.

Latency note: Successful runs incur a shutdown holdback capped at ~500 ms while in-flight background agents flush their final notifications before the result is emitted. The holdback exits early if no background tasks are pending, so simple runs barely notice it; batch pipelines that fan out hundreds of --json-schema invocations against busy agents should account for this upper bound.

Security note: Schemas may contain user-supplied regular expressions in pattern keywords. Ajv compiles these with the ECMAScript regex engine, which is vulnerable to catastrophic backtracking. Because tool arguments are always objects, the pattern keyword only fires inside string properties — a malicious schema like {"type":"object","properties":{"value":{"type":"string","pattern":"(a+)+b"}}} can hang the CLI when the model supplies a moderately long matching value. Only run --json-schema with schemas from sources you trust.

Validation at parse time:

• The file must be a regular file (no FIFOs, character devices, or directories).
• File size is capped at 4 MiB. Real-world JSON schemas are well under this; multi-MiB files almost always indicate a wrong-path mistake.
• The schema must be valid JSON. For @path input, the parse error is generic (“content of <path> is not valid JSON”) rather than echoing the SyntaxError detail, so a wrapping process that surfaces stderr can’t read a prefix of the file’s contents back from the error.
• The schema must compile under the strict Ajv configuration — typos like propertees are surfaced, but spec-valid patterns (e.g. required without listing every key in properties) are accepted.
• The schema root must accept object-typed values. Function-calling APIs (Gemini, OpenAI, Anthropic) all require tool arguments to be JSON objects, so a non-object root would register an unusable tool.

The root-acceptance check walks type, const, enum, anyOf, oneOf, allOf, not, and if/then/else (best-effort for the decidable cases). When in doubt it defers to Ajv at runtime.

Root $ref is rejected by the parse-time check. If your schema reuses a definition via $ref, wrap it in allOf:

// Rejected:
{ "$ref": "#/$defs/MyObj", "$defs": { "MyObj": { "type": "object", "properties": { "name": { "type": "string" } } } } }
 
// Accepted (root accepts objects via the allOf branch):
{ "allOf": [{ "$ref": "#/$defs/MyObj" }], "$defs": { "MyObj": { "type": "object", "properties": { "name": { "type": "string" } } } } }

$ref inside anyOf / oneOf / allOf is deferred to Ajv at runtime, so the wrapped form passes the root-acceptance check.

Output shape per format

In both JSON formats, prefer reading structured_result over result when you want the object; result is the stringified form provided for consumers that always expect a string in that field. For --output-format json, read the last element of the array and pull structured_result from there (e.g. jq '.[-1].structured_result'); for stream-json, read the final type: "result" line on the stream.

Restrictions

Retry and failure modes

Cost note. Two things multiply token spend in a --json-schema run, both worth designing for:

• Schema embedded in every turn. The schema ships as the structured_output function declaration’s parameters block on every model request, not just the first. Large schemas (up to the 4 MiB parse cap) proportionally increase per-turn input tokens for the entire run.
• Each validation retry is a full model turn. A schema the model misses repeatedly is multiplied per failure (request + inference + response). Keep schemas constrained enough to guide the model and simple enough to nail on the first try; raise --max-session-turns when retries are expected.

The session ends on the first valid call. Until then:

• Args fail validation. structured_output returns a tool-result error with Ajv’s message, the model sees it on the next turn, and may correct the arguments and call again.
• Model calls a side-effecting tool in the same turn as structured_output. The pre-scan suppresses the sibling — it never runs, regardless of whether the structured call ultimately validates. The two paths split on what the model sees next:
  • Validation succeeds: the run ends immediately, and the model never gets another turn — the suppressed sibling is silently discarded.
  • Validation fails: the model gets another turn and sees a synthesised “Skipped:” tool_result for the suppressed call, so it can re-issue that call in a separate turn (one that does not include structured_output).
• Model emits plain text instead of calling structured_output. Exit code 1. The error message includes the turn count and a truncated preview of the model’s output so you can see what it actually said.
• Run reaches maxSessionTurns. Exit code 53. Standard “Reached max session turns” exit, plus a --json-schema-specific hint that points at the three common stuck-run causes: model never called the tool, structured_output is denied by permission rules, or the schema is unsatisfiable.
• Run is interrupted (SIGINT / Ctrl-C). Exit code 130. The structured result is normally not emitted, but the shutdown holdback loop does not poll the abort signal, so a SIGINT that arrives after a successful call has been captured but before the result reaches stdout may still land on stdout. Treat the exit code as the source of truth.

Privacy

The args you submit through structured_output ARE the structured payload — already emitted on stdout. To avoid persisting the same payload a second time into on-device surfaces that may be exported off the machine, args are redacted with the placeholder { __redacted: 'structured_output payload (see stdout result)' } on:

• The ToolCallEvent telemetry path (OTLP exports, QwenLogger, ui-telemetry stream, chat-recording UI event mirror).
• The on-disk chat-recording JSONL at ~/.qwen/projects/<sanitized-cwd>/chats/<sessionId>.jsonl (re-fed into model context on --continue / --resume), including every validation-failure retry.

Tool-call metrics (duration, success, decision) and surrounding event metadata are preserved.

Schema is sent to the model provider. Redaction covers the call arguments on local surfaces only. The schema itself rides on every model request as the structured_output function declaration’s parameters block — so any literal values you put inside it (enum, const, default, examples, description, $comment, etc.) reach the provider in cleartext just like prompt text. Schemas should describe shape and constraints; treat them as public toward the provider and keep secrets, customer records, and other sensitive payloads out of the schema body.

Hooks see raw args. The redaction described above only applies to telemetry and chat-recording. PreToolUse, PostToolUse, and PostToolUseFailure hooks (including HTTP hooks that can forward payloads off-device) receive the unredacted tool_input for structured_output, since the hook contract is “see what the tool sees.” If you operate audit-style catch-all hooks, either disable them for structured_output (filter on tool_name) or add hook-side redaction before running --json-schema against sensitive data.

Session resumption (--continue / --resume)

--json-schema is a per-run flag, not a per-session property. The synthetic tool is registered when the CLI parses its arguments, so:

• Re-pass --json-schema on every --continue / --resume you want the terminal contract to apply to. The same schema as the original run is the safe default — a mid-session schema swap is allowed but changes the contract the model is being held to.
• If you --continue without --json-schema, the resumed run is an ordinary headless session: structured_output simply doesn’t exist as a tool, and the model will respond in free-form text.
• The __redacted placeholder in the resumed chat-recording does not affect resumability in practice. A successful structured_output call terminates the session immediately, so the only redacted args a resumed run could see are from failed attempts. The model still has each attempt’s Ajv validation error in the recorded tool_result and the live parameter schema (re-registered from --json-schema), which is enough to retry.

Permission gating

structured_output deliberately bypasses the --core-tools allowlist: the tool only exists when --json-schema is set, so excluding it would leave the run with no terminal contract.

Explicit permissions.deny rules and --exclude-tools settings DO take effect — both use the same deny mechanism and both prevent structured_output from being registered, so the model never sees the tool declaration. The typical result is that the model answers in plain text (exit 1). If the model loops through other tools without ever producing text, it will eventually hit maxSessionTurns (exit 53) and the --json-schema hint in the error message tells you where to look.

--bare caveat. Bare mode ignores most settings-derived inputs, including settings-level permissions.deny and tools.exclude. The synthetic tool stays registered, so a settings-only deny of structured_output will silently no-op under --bare. Argv-level --exclude-tools structured_output still applies in bare mode — use the flag rather than settings if you need to lock down a bare run.

Conflict with MCP tools

If an MCP server registers a tool literally named structured_output, the tool-registry collision check renames the MCP tool to mcp__<server-name>__structured_output so the synthetic tool keeps the bare name. The user-supplied schema is always the one the model sees.

Example: gating a multi-step run on the structured output

RESULT=$(qwen --prompt "Audit this diff and rate its risk." \
  --json-schema @./schemas/audit.json) || exit 1
 
risk=$(jq -r '.risk_level' <<<"$RESULT")
if [ "$risk" = "high" ]; then
  echo "High-risk diff; pausing pipeline." >&2
  exit 2
fi

See also

• Headless Mode — the -p-based flow --json-schema builds on.
• Dual Output — records a JSON-event sidecar alongside the TUI (a different approach to machine-readable output; does not require --json-schema).