Pick an LM output format per (task x consumer x model) rather than by reflex: different formats carry different cognitive load (e.g. code-in-JSON makes the same model write worse code than plain-text+diff, while asking for prose when you need a typed object fails the other way). Use when designing or debugging an LM's output schema, choosing between plain text / diff / JSON / tool-call / grammar-constrained output, or when a model's quality drops after wrapping its output in a structured format.
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Pick an LM output format per (task x consumer x model) rather than by reflex: different formats carry different cognitive load (e.g. code-in-JSON makes the same model write worse code than plain-text+diff, while asking for prose when you need a typed object fails the other way). Use when designing or debugging an LM's output schema, choosing between plain text / diff / JSON / tool-call / grammar-constrained output, or when a model's quality drops after wrapping its output in a structured format.
domain
LM output schema design for coder-agents and any LM workflow whose output is consumed by code
Anyone designing an LM output format — coder-agent authors, RAG pipeline builders,
tool-use harness designers, prompt engineers shipping typed objects.
status
tool-skill
Output-Format-by-Model — 让格式服务于任务，而不是反过来
One-liner: Different output formats carry different cognitive load for the model. Code-in-JSON is the canonical proof: the same model writes worse code when wrapped in a JSON tool-call than when emitted as plain text + diff. The reverse failure (asking for prose when you need a typed object) is just as common. Pick format per (task × consumer), not by reflex.
1. 何时激活 (When to activate)
Activate this skill before committing to an output schema in any of these situations:
Trigger
Signal
Designing a coder-agent
"should the model return a apply_patch tool call or plain-text diff?"
Adding a tool to an existing agent
"tool input has a code / query / sql / regex field — should I nest it in JSON or leave it as a string?"
Building extraction / classification
"should I use dspy.Predict typed fields, Pydantic + response_format=json_schema, or just markdown?"
Wiring an evaluator
"the metric needs a number — but the model also has to reason to produce it"
Migrating a working prompt to "structured outputs"
someone said "let's make it safer with JSON schema"
The format is fixed by an external API (e.g. you must return OpenAI function-call JSON — no choice).
One-shot exploratory prompting where no consumer parses the output yet.
The task is itself about JSON (e.g. "fix this malformed JSON") — see §6.
2. 核心心智模型 (Core mental model)
2.1 Three-layer claim
┌──────────────────────────────────────────────────┐
│ FORMAT FOLLOWS FUNCTION │
│ │
│ Some formats add cognitive load to the model │
│ and measurably degrade quality on the │
│ *content* the format is supposed to wrap. │
└──────────────────────────────────────────────────┘
▲ ▲
│ │
What's being consumed? Who consumes it?
(code? prose? entities? (human reader? parser?
number? action selection?) downstream LM? compiler?)
│ │
└──────────┬───────────────┘
▼
FORMAT SELECTION
(text+diff | markdown | JSON | tool_use
| grammar-constrained | typed field)
Aider's code-in-json benchmark is the load-bearing empirical anchor:
GPT-4 Turbo refactor benchmark with unified diff: 61% pass.
Same model with JSON-wrapped code: 20% pass.
That is a 3× swing from format alone, not from model choice or prompt rewording. [aider.chat/2023/12/21/unified-diffs.html, aider.chat/2024/08/14/code-in-json.html]
The pattern holds across every major model tested in 2024 (Claude Sonnet, DeepSeek Coder, GPT-4o). Even with OpenAI strict-mode JSON validity enforced, the code inside the JSON degraded — more SyntaxError / IndentationError. Sonnet kept syntax clean but still scored lower overall. [aider.chat/2024/08/14/code-in-json.html]
Academic generalization, same year:
"Let Me Speak Freely?" (arXiv 2408.02442) — format-restricted reasoning drops 10–15% on math and complex analysis vs free-form-then-convert. [arxiv.org/abs/2408.02442]
StructEval (arXiv 2505.20139) — even o1-mini only scores 75.58 average on structural-output generation. [arxiv.org/html/2505.20139v1]
2.2 The two reflexes to unlearn
Reflex
When it's wrong
"Structured output is always safer."
False for code, multi-step reasoning, free-form prose. Strictness ≠ quality of contents.
"Markdown is only for humans."
False — markdown is also the highest-fidelity wire format for many LM-to-LM hand-offs (Aider uses it; DSPy's default chat adaptor uses field-marked markdown over JSON for many signatures).
2.3 Mental model: format as a tax on the generation surface
Every formatting requirement consumes some of the model's attention budget. The tax is:
Low for formats the model has seen millions of times in pretraining (markdown, unified diff, Python).
Medium for JSON containing simple scalar fields (entity extraction, dates, booleans).
High for JSON containing code, multi-line strings with escapes, or deeply nested reasoning. Each \n becomes \\n; each quote becomes \"; the model has to track this while also solving the actual problem.
⇒ Heuristic: the more semantically dense the content, the cheaper the format must be.
3. SOP 工作流 (Decision workflow)
Three questions, in order:
Step 1 — What's being consumed?
Content type
Default format
Why
Source code edits
plain text + diff format (SEARCH/REPLACE, unified diff, or str_replace tool with code as a single string field)
Aider 20%→61% on GPT-4 Turbo; same direction across all models. [aider.chat/2024/08/14/code-in-json.html]
Source code (full file rewrite)
plain text or markdown fenced block
Same reason. JSON-wrap adds escaping tax.
Structured data extraction (entities, dates, IDs, classifications)
JSON / Pydantic / typed OutputField
Schema helps here — fields are the task.
Action selection (which tool to call)
JSON tool_use
Tool name + scalar args. The decision is structured by definition.
Action body (SQL, regex, code, file contents)
single string field inside tool_use — do not sub-structure
Same code-in-JSON penalty applies to any code-shaped payload.
Reasoning / intermediate steps
markdown or Python-style scratchpad
CoT in JSON measurably degrades; see "Let Me Speak Freely?" [arxiv.org/abs/2408.02442]
Numeric answer with reasoning
markdown reasoning + final answer in fenced block or final-line convention
Don't force the reasoning into a JSON reasoning field — it shortens and stiffens.
What's being emitted?
│
┌───────────────────┼───────────────────┐
│ │ │
code structured free-form
edits data fields prose/reasoning
│ │ │
▼ ▼ ▼
diff/SEARCH- JSON / Pydantic / markdown
REPLACE/ tool_use scalar
str_replace, args
code as string
│ │ │
▼ ▼ ▼
pick per-model wrap any code- do NOT
edit format shaped payload in force into
(Aider table) a single string field JSON schema
4. 操作模型 (Task → format pairs)
Ten concrete operations. Format choice is not arbitrary — each citation is the empirical anchor.
#
Task
Recommended format
Rationale / Evidence
1
"Edit auth.py to use JWT" (coder-agent core loop)
Plain-text SEARCH/REPLACE diff in markdown fence
Aider's measured 3× on GPT-4 Turbo, generalizes across models. [aider.chat/2024/08/14/code-in-json.html]
2
"Apply this patch to a file via Claude's text-editor tool"
Anthropic str_replace tool — old_str / new_str as string fields, no JSON sub-structure inside the code
Matches Anthropic's published tool-design guidance: fields should be high-signal scalars, not low-level identifiers. [docs.anthropic.com text-editor-tool]
3
"Extract invoice fields (vendor, amount, date, line items)"
JSON / Pydantic / DSPy typed OutputField
Schema is the task. Structured-output benchmarks show high accuracy here. [arxiv.org/html/2505.20139v1]
4
"Classify ticket priority (P0/P1/P2/P3)"
Single-token output or JSON {"priority": "..."}
Trivial; structure helps determinism.
5
"Answer a math word problem"
Markdown reasoning + final answer in \boxed{} or fenced final line
"Let Me Speak Freely?" — 10–15% degradation when locked into JSON reasoning. [arxiv.org/abs/2408.02442]
6
"Decide which tool to call next"
JSON tool_use with tool name + scalar args
Action selection is intrinsically structured.
7
"Generate the SQL query for that tool"
Tool args contain {"sql": "SELECT ..."} — SQL as a raw single string, no further JSON sub-structure
Same family as code-in-JSON; SQL is code-shaped.
8
"Write a customer-support reply"
Markdown
Native to instruction tuning; JSON-wrap costs nothing useful.
9
"Summarize a paper and return key claims as a list"
Markdown prose + a fenced claims: YAML or JSON block at the end (two-zone output)
Best of both: prose flows freely, downstream parser reads the trailing block.
10
"Rewrite a long document AND extract entities"
Two passes: pass 1 rewrite (markdown), pass 2 extract (JSON, fed the pass-1 output)
One call cannot serve both contents at peak quality.
5. 困境决策案例 (Dilemma cases / worked examples)
Case A — "I'm writing an extractor for invoice fields. Plain text or JSON?"
Trigger: New extraction pipeline. Fields = vendor_name, total_amount, invoice_date, line_items[].
Constraints:
Output is consumed by a downstream Python parser.
Some line items contain free-text descriptions with newlines and quotes.
Pydantic validation is desired.
Decision steps:
The content is structured data, not code. JSON tax is low here — fields are short scalars or short strings.
Use Pydantic + OpenAI response_format=json_schema or DSPy typed OutputFields. Field names carry the semantic load (DSPy's "Signatures carry semantic load" principle [dspy.ai/learn/programming/signatures]).
For line_items[].description containing free-text: still inside JSON — descriptions are prose data, not code. Escaping cost is real but bounded.
Watch for: extraction-then-rewrite mission creep. If a future spec says "also rewrite the invoice text in cleaner language" → switch to Case B's two-pass approach.
Outcome: JSON wins. Schema validation catches missing fields cheaply; no measurable degradation expected on this content shape.
Case B — "Same task but the extractor also rewrites the invoice text"
Trigger: PM adds requirement — "and produce a cleaned-up version of the invoice document body."
Constraints:
"Cleaned text" is multi-paragraph prose with embedded numbers, possibly tables.
Putting cleaned_text: "..." inside the same JSON forces multi-paragraph escaping and competes with the extraction reasoning for attention.
Decision steps:
Recognize this as the mixed-content trap. One LM call cannot serve both contents at peak.
Two solutions, choose by latency budget:
Two-pass (preferred): Call 1 returns {vendor, amount, date, line_items} as JSON. Call 2 receives the original doc + extracted JSON, returns markdown rewrite.
Two-zone single call: Prompt requests markdown rewrite first, then ===EXTRACTION=== separator, then a fenced JSON block. Parser splits on the separator. Cheaper but more fragile.
Do not nest the markdown rewrite as a JSON string field. The model will compress it and lose structure.
Outcome: Plain text + JSON split, not unified JSON. Format follows content, even within one logical task.
Case C — "Tool call decision, but the tool's input is SQL"
Trigger: Building a data-analyst agent. One tool is run_query(sql: str).
Constraints:
The agent must decide which tool to call (structured choice).
The agent must write SQL (code-shaped content).
Provider is OpenAI / Anthropic tool_use.
Decision steps:
The tool-selection layer is intrinsically structured — use JSON tool_use. The model picks run_query vs read_file vs list_tables.
The tool body is code. Per Aider's findings, code in JSON degrades. Mitigations:
Make sql a single top-level string field. Don't sub-structure it ({from: ..., where: ...}). Let the model write idiomatic SQL.
Keep other args as scalars (e.g. dry_run: bool).
Expect some quality hit vs returning SQL as plain text; budget for retry/repair.
If quality is unacceptable: switch the architecture. Generate SQL in a separate plain-text LM call (markdown fenced ```sql block), then a thin downstream agent wraps it into the tool call. This is the Aider "architect" pattern transplanted to data tools. [aider.chat/2024/09/26/architect.html]
Outcome: JSON for the decision; single string for the SQL body; consider a two-step generate-then-dispatch if quality matters.
Case D — "Reviewer says 'use structured output everywhere for safety'"
Trigger: Eng-org-wide push to add response_format=json_schema to every LM call.
Constraints:
The org has been bitten by json.loads failures.
Some calls produce code; some produce prose; some produce typed objects.
The reviewer is conflating "schema-valid" with "quality."
Decision steps:
Surface the empirical finding: JSON validity ≠ content quality. Cite Aider's 20%→61% (unified diff vs JSON) [aider.chat/2024/08/14/code-in-json.html] and "Let Me Speak Freely?" [arxiv.org/abs/2408.02442].
Audit each call by content type using §3 Step 1 table.
For code-emitting calls: keep markdown; add a defensive parser (regex extract from fenced block). The robustness cost is small; the quality gain is large.
For typed-object calls: yes, add json_schema. This is where structured output earns its keep.
Document the policy: "Structured output is mandatory for typed-data extraction; forbidden for code edits and multi-step reasoning; optional for everything else."
Outcome: Reject the blanket policy; replace it with a content-type-driven one. This is the inverse of the "JSON everywhere" reflex.
6. 反模式与边界 (Anti-patterns & boundaries)
Anti-patterns
"JSON everywhere" reflex. Structured ≠ better. Code, multi-step reasoning, long-form prose all degrade inside JSON envelopes — Aider measured 3× on code; arXiv 2408.02442 measured 10–15% on reasoning. Use JSON where the content itself is structured.
"Markdown only when a human reads it" reflex. Markdown is also an excellent LM-to-LM wire format and often the highest-quality format for the producing model. DSPy's default chat adaptor uses field-marked markdown for many signatures even with no human in the loop. [dspy.ai/learn/programming/signatures/]
Wrapping code in JSON tool calls without budgeting quality loss. If your harness requires tool calls, fine — but acknowledge the tax and put the code in a single string field. Don't sub-structure it into {language, lines: [...], imports: [...]}. Every level of nesting compounds the escape tax.
Forcing reasoning into a reasoning JSON field. Models compress and stiffen when reasoning is inside JSON — they treat it as a label rather than as thinking. Prefer free markdown reasoning followed by a structured tail.
Using JSON schema as a substitute for prompt engineering. Schema enforces shape. It does nothing for correctness of contents. Write the prompt as if there were no schema, then add the schema for safety.
Asking for typed data as freeform prose, then post-hoc parsing with regex. The inverse mistake. If the consumer is a parser and the fields are scalars, use a schema — the model is happy to oblige.
Mixing code and prose in one JSON document. Use two-zone output (prose followed by fenced block) or two passes.
Ignoring per-model edit-format defaults. Aider's defaults exist because the same edit format performs differently across models. Don't override without an A/B. [aider.chat/docs/more/edit-formats.html]
Treating tool-use schemas as documentation-free zones. Anthropic's tool-writing guide emphasizes high-signal field names (name, file_type) over technical identifiers (uuid, mime_type). The model reads your schema as part of its prompt. [anthropic.com/engineering/writing-tools-for-agents]
"The task is fixing JSON, so format doesn't matter." Counter-edge case: when the content itself is JSON (e.g. "fix this malformed JSON"), output it as a fenced ```json block in markdown — not as a JSON tool-call wrapping JSON-as-string. Even here, the rule holds: code-shaped content goes in a plain string surface, not nested escapes. [arxiv.org/html/2510.04717v1 — JSON Whisperer]
Boundaries (when format choice doesn't matter much)
The model is overwhelmingly capable for the task (GPT-5-class on entity extraction with 4 scalar fields). The format tax is dwarfed by headroom.
The output is one token / one number / yes-no. Any format works; pick by parser convenience.
The pipeline is one-shot exploration with no downstream consumer.
You're forced into a format by an external contract (e.g. publishing to a webhook expecting JSON). Then your job is mitigation (single string fields for code-shaped payloads), not format choice.
7. 跨框架对照 (Ecosystem cross-reference)
How major frameworks implement format selection. Use this to translate the principles into the stack you're already in.
DSPy's Signature → Module pipeline ships with multiple adaptors that materialize the same logical I/O contract into different wire formats:
ChatAdapter (default): field-marked markdown with [[ ## field_name ## ]] headers. Used even for typed outputs because models reliably emit it.
JSONAdapter: schema-enforced JSON. Used when the consumer must parse strictly (e.g., feeding another typed pipeline).
TwoStepAdapter: free-form generation, then a second cheaper call reformats into JSON. Direct application of the "Let Me Speak Freely?" finding — generate freely, format separately. [arxiv.org/abs/2408.02442]
DSPy's recommendation: stay on ChatAdapter unless a downstream consumer demands JSON. The framework itself encodes the principle of this skill.
Outlines / Guidance / LMQL (grammar libs)
Token-level grammar enforcement (regex, CFG, JSON schema). Guarantees validity, not quality of content.
Best used for: forced enums, numeric ranges, valid JSON shells around scalar fields.
Worst used for: forcing code or long prose through a JSON grammar — you'll get valid JSON containing degraded code.
Pair with this skill: choose format using §3, then use Outlines to enforce whichever format you chose.
OpenAI tool_use / function calling
Tool selection: structured by definition. Use it.
Tool arguments: keep scalar where possible; wrap code-shaped payloads in a single string field.
response_format=json_schema for typed extraction: yes. For code generation: avoid; if forced, single string field with code as the type.
"Strict mode" (strict: true) guarantees schema validity but does not fix content degradation — Aider tested this explicitly. [aider.chat/2024/08/14/code-in-json.html]
Built-in text_editor tool: commands view, str_replace, create, insert, undo_edit. str_replace takes old_str and new_str as single string fields — explicitly avoiding the JSON-nests-code anti-pattern. This is the principle of this skill made into a first-party API.
Tool-writing guidance: high-signal scalar field names; avoid low-level identifiers in tool schemas. [anthropic.com/engineering/writing-tools-for-agents]
Cross-framework summary
Layer | Format mechanism
-----------------------|-----------------------------------
Generation grammar | Outlines / Guidance — enforce
Per-task adaptor | DSPy ChatAdapter vs JSONAdapter
Per-model edit format | Aider's edit-format table
Tool envelope | OpenAI / Anthropic tool_use
Wire serialization | markdown vs JSON vs YAML
Every layer can independently make a wrong format choice. This skill operates at the per-task layer: decide first what format the content wants, then let each lower layer enforce it.
Quick reference card
┌──────────────────────────────────────────────────────────────────────┐
│ FORMAT DECISION CARD │
├──────────────────────────────────────────────────────────────────────┤
│ Code edits → text + diff (per-model: udiff/SEARCH/whole) │
│ [Aider: 20% → 61% on GPT-4 Turbo] │
│ Code generation → markdown fenced block │
│ SQL / regex / shell → string field inside tool_use, NOT sub-JSON │
│ Entity extraction → JSON / Pydantic / typed OutputField │
│ Classification → single token or JSON {label} │
│ Action selection → JSON tool_use │
│ Reasoning + answer → markdown CoT + fenced final answer │
│ [Format-restricted reasoning: -10 to -15%] │
│ Prose / explanation → markdown │
│ Mixed content → two passes OR two-zone output │
├──────────────────────────────────────────────────────────────────────┤
│ NEVER: │
│ • Nest code inside JSON sub-structure │
│ • Force CoT reasoning into a JSON "reasoning" field │
│ • Use schema as substitute for prompt engineering │
│ • Assume strict-mode JSON fixes content quality │
└──────────────────────────────────────────────────────────────────────┘
引用源 (Citations)
Primary empirical anchors:
Aider code-in-JSON benchmark: [aider.chat/2024/08/14/code-in-json.html] — "All of the models did worse on the benchmark when asked to return code in a structured JSON response."
Aider unified diff vs JSON: [aider.chat/2023/12/21/unified-diffs.html] — GPT-4 Turbo refactor 20% → 61% with udiff; "lazy comments" cut to 1/3.
