Decide where to enforce structured LM output (constrain at decode time with Outlines vs validate-and-retry with Instructor vs grammar with Guidance) and which failure stance to take (Assert/hard-fail vs Suggest/soft-retry). Use when an LM's output is parsed or typed by downstream code and you must pick one enforcement library plus its failure handling, when malformed output is burning tokens on retries, or when choosing between decode-time vs validation-time constraints for local vs API models.
Decide where to enforce structured LM output (constrain at decode time with Outlines vs validate-and-retry with Instructor vs grammar with Guidance) and which failure stance to take (Assert/hard-fail vs Suggest/soft-retry). Use when an LM's output is parsed or typed by downstream code and you must pick one enforcement library plus its failure handling, when malformed output is burning tokens on retries, or when choosing between decode-time vs validation-time constraints for local vs API models.
domain
choosing and configuring a structured-output enforcement layer when LM output is consumed by code
overlay
true
enhances
["outlines","instructor","guidance"]
source
Local lib skills outlines / instructor / guidance (frontmatter + "When to Use");
DSPy Assert/Suggest constraint primitives (dspy-sop-skill, arXiv 2312.13382);
sibling Phase-D skill output-format-by-model (cross-link, not duplicate);
provider docs for OpenAI structured outputs + Anthropic tool_use.
audience
Coder-agents and pipeline authors wiring an LM whose output is parsed/typed by
downstream code; anyone who has outlines / instructor / guidance installed and
must pick one + decide failure handling.
status
tool-skill
Structured-Output-Picker — 在解码处约束，还是在校验处约束？
One-liner: Three local libraries (Outlines, Instructor, Guidance) plus
provider-native structured outputs all "make the model emit valid structure",
but they enforce at different points and fail differently. Pick by how
costly a malformed output is and whether you control the decoder. Then
pick the failure stance — Assert (hard-fail + retry) vs Suggest (soft
nudge, degrade gracefully) — borrowed from DSPy's constraint primitives.
This is an ENHANCE overlay. The four enforcement mechanisms each have a
working local skill; what no single one provides is the cross-library *which-one
how-to-handle-failure* decision. That gap is hit every time an LM output is
consumed by code. For what shape the content should take (code vs JSON vs
prose), descend first to [[agentsop-output-format-by-model]] — this skill assumes the
shape is already chosen and asks only how to enforce it.
1. 何时激活 (When to activate)
Activate after you have decided the output shape (via
[[agentsop-output-format-by-model]]) and the answer was "a typed/validated object", and
before you write the parsing code.
Trigger
Signal
LM output feeds a parser
json.loads(resp) / Model.model_validate(...) is in the next line of code
You picked a typed shape
format-by-model said "JSON / Pydantic / typed field" — now: who enforces it?
Repeated parse failures
JSONDecodeError, ValidationError, truncated/extra-prose responses in logs
Library is already installed
outlines, instructor, or guidance is in the env and you must choose between them
An enum / regex / range must hold
output must be one of N labels, a valid date, a bounded int
You must decide failure stance
"if the model returns garbage, do I crash, retry, or accept-and-flag?"
Anti-triggers (skip this skill):
The content is — go back to
; enforcing a JSON grammar on code is the headline
anti-pattern there (Aider 61%→20%). Enforcement strength is the wrong question
when the is wrong.
code / multi-step reasoning / long prose
[[agentsop-output-format-by-model]]
shape
No code consumes the output yet (one-shot exploration).
The shape is one token / one number — any parser works; no library needed.
2. 核心心智模型 (Core mental model)
2.1 The axis: where is the constraint applied?
PROMPT ──────► DECODE ──────► RAW TEXT ──────► VALIDATE ──────► TYPED OBJECT
│ │ │
│ constraint at DECODE constraint at VALIDATE
│ (grammar masks tokens) (parse, check, retry on fail)
│ │ │
ask nicely Outlines / Guidance / Instructor / DSPy Suggest /
(weakest) provider strict-mode hand-rolled retry loop
↑ ↑
CANNOT emit invalid CAN emit invalid, then
structure — masked at catches it and re-asks
the logit level with the error injected
The pick is governed by one question: how costly is a malformed output?
A malformed output is cheap to recover from (one extra API round-trip is
fine; the model is strong; failures are rare) → constrain at validate
(Instructor-style retry). Simpler, model writes more naturally, no decoder
access needed.
A malformed output is expensive or impossible to recover from (no retry
budget, hard real-time, the output must be in a fixed enum/grammar, a single
bad token corrupts a batch job) → constrain at decode (Outlines / Guidance
grammar, or provider strict-mode). The model cannot emit invalid structure.
2.2 Two prerequisites that gate the choice
Do you control the decoder? Grammar/token-masking (Outlines, Guidance)
requires logit access — i.e. local/open weights (Transformers, vLLM,
llama.cpp) [[outlines]] [[guidance]]. Closed API models (GPT, Claude) expose
only their own native structured-output / tool_use; you cannot bolt
Outlines onto them. Instructor works on top of the API by parse-and-retry
[[instructor]].
Is the content code-shaped? If yes, stop — see §1 anti-triggers and
[[agentsop-output-format-by-model]]. Grammar-constraining code yields valid JSON
containing degraded code: enforcement cannot buy back content quality.
2.3 Failure stance is orthogonal — Assert vs Suggest
Independent of which library, you choose what happens on a constraint
violation. DSPy names the two stances [dspy-sop-skill §Constraint primitives;
arxiv.org/pdf/2312.13382]:
Stance
Behavior on violation
Use when
Assert (hard)
retry up to N; then raise / halt
Dev-time bug-catching; downstream cannot tolerate a bad value; correctness > availability
Suggest (soft)
retry with error injected; then log + continue with best effort
Production; partial result beats no result; availability > strict correctness
Decode-time grammar (Outlines/Guidance) is itself a hard guarantee on
shape — but semantic checks (range, cross-field, business rules) still need
an Assert/Suggest stance layered on top.
3. SOP 工作流 (Decision workflow)
Three ordered steps. Each presupposes [[agentsop-output-format-by-model]] already said
"typed/validated object".
Step 1 — Decide enforcement strength (how costly is malformed?)
malformed output cost?
│
├── cheap to recover (retry OK, strong model, rare failures)
│ └──► VALIDATE-time enforcement (weakest sufficient — prefer this)
│
├── must never escape (enum/regex/grammar is a hard contract)
│ └──► DECODE-time grammar (only if you control the decoder)
│
└── no retry budget / hard real-time / batch where 1 bad token poisons many
└──► DECODE-time grammar (or provider strict-mode if API)
Heuristic: start at the weakest layer that meets the cost constraint. Validate
retry is cheaper to build, keeps the model's generation natural, and needs no
decoder access. Escalate to decode-time grammar only when retry is too costly or
the contract is absolute.
Step 2 — Pick the library (gated by decoder access + provider)
Guidance — decode-time + Pythonic control flow [[guidance]]
Any model + only semantic checks needed (shape already valid)
range / cross-field / business rules
retry loop with Assert/Suggest stance (DSPy or hand-rolled)
Step 3 — Choose the failure stance (Assert vs Suggest)
Default Suggest in production (degrade gracefully; log the violation).
Use Assert in dev/test and on values whose corruption is unacceptable
downstream (IDs that index a DB, money amounts, irreversible actions).
Decode-time grammar covers shape for free; still wrap semantic checks in a
stance. Set a finite retry cap on either stance — an unbounded retry loop is an
outage.
Case A — "Instructor keeps retrying and burning tokens; should I switch to Outlines?"
Trigger: An extraction service on GPT-4o uses Instructor with max_retries=5.
Latency p95 spiked; logs show 15% of calls retry ≥2× on the same nested schema.
Constraints:
Closed API model — no decoder access, so Outlines is not available for
GPT-4o [[outlines]] [[instructor]].
The schema is deeply nested with several free-text fields.
The cost being paid is retries, i.e. malformed output is currently expensive.
Decision steps:
Outlines is off the table for an API model (Step-1 prerequisite §2.2(1)). The
real lever is reducing the validate-time failure rate.
Inspect what fails. If the model emits valid JSON but wrong content, no
enforcement layer helps — that's a prompt/format problem; check whether a
free-text field is being squeezed into JSON ([[agentsop-output-format-by-model]]
mixed-content trap).
If the failure is shape (extra prose, truncation): switch the API call to
provider native structured outputs / tool_use (strict-mode guarantees
schema validity at the API), then keep Instructor only for the Pydantic typing
layer. Retries collapse because shape is now guaranteed upstream.
Lower max_retries to 2 and flip the stance to Suggest with a logged
fallback object, so a stubborn case degrades instead of inflating p95.
Outcome: Don't "switch to Outlines" (impossible here). Move shape-enforcement
to the provider's native layer, keep Instructor for typing, cap retries, Suggest.
Case B — "Local model, 200k-row batch extraction; a few rows come back malformed"
Trigger: Nightly batch over a local vLLM-served model. ~0.3% of rows produce
JSON that fails to parse, poisoning the downstream load.
Constraints:
Local weights → decoder access available (Outlines/Guidance both viable).
Batch job, no interactive retry budget — re-running the whole batch is the
only "retry", which is hugely expensive. Malformed output is very costly.
The structure is a flat JSON schema; no interleaved free text.
Decision steps:
Step-1: malformed output is expensive and there is no per-row retry budget →
decode-time grammar. This is exactly the case validate-time loses.
Step-2: local weights + pure JSON shape + no interleaving → Outlines
JSON-schema. (Guidance would be the pick only if rows needed interleaved
free-text + constrained spans, Case D-style.) [[outlines]] [[guidance]]
Shape is now guaranteed per row — the 0.3% parse failures go to zero by
construction. Stance for shape becomes moot.
Layer a Suggest semantic check (e.g. amount >= 0) and write violators to
a quarantine table rather than failing the batch — availability of the 99.7%
beats halting on outliers.
Outcome: Decode-time Outlines grammar removes the shape failures the retry
model couldn't afford to catch; a Suggest semantic check quarantines the rest.
6. 反模式与边界 (Anti-patterns & boundaries)
Anti-patterns
Grammar-constraining when a retry suffices. Reaching for Outlines/Guidance
on a strong API model with rare failures buys complexity (and may be
impossible — no decoder access) when an Instructor retry would have been two
lines. Start at the weakest sufficient layer (§3 Step 1).
Assert where Suggest is enough → over-rejection. Hard-failing the whole
request because one optional field violated a soft preference throws away a
usable answer. In production, default to Suggest; reserve Assert for values
whose corruption is unacceptable [dspy 2312.13382].
Enforcing structure on code/prose content. The headline cross-skill
anti-pattern: a JSON grammar produces valid JSON containing degraded code.
Enforcement cannot recover content quality — fix the shape first via
[[agentsop-output-format-by-model]].
Assuming provider strict-mode fixes content. Native structured outputs
guarantee schema validity, not field correctness — same lesson as Aider's
strict-mode JSON test [[agentsop-output-format-by-model]].
Unbounded retry loop. Validate-time enforcement without a finite cap turns
a flaky field into an availability outage. Always cap N.
Stacking Outlines on a closed API model. Token-masking needs logits;
GPT/Claude expose none. Mixing the mental models wastes a debugging cycle
(§2.2(1)).
Using a grammar for cross-field/semantic rules. Grammars constrain token
shape, not relationships like start < end or "id exists in DB". Those need a
validate-time check + stance, regardless of how shape was enforced.
Picking the library before deciding strength + stance. The library is the
third decision, after "how costly is malformed" and "Assert or Suggest".
Boundaries (when this skill doesn't apply)
Output is code / reasoning / prose → [[agentsop-output-format-by-model]], not here.
One token / one number / yes-no → any parser; no enforcement library.
No code consumes the output → one-shot exploration, defer.
Provider contract is fixed (must emit a specific webhook JSON) → enforcement
is mandatory by definition; the only remaining choice is the failure stance.
The fix is prompt-level (model emits valid-but-wrong values) → no
enforcement layer helps; this is a prompt/format problem.
7. 跨框架对照 (Ecosystem cross-reference)
Where each mechanism sits on the decode vs validate axis, and what it
guarantees. Cross-link [[agentsop-output-format-by-model]] for the prior shape decision.
Mechanism
Constraint point
Needs decoder access?
Works on closed API?
Native failure model
Best for
Outlines [[outlines]]
Decode (token mask: regex / CFG / JSON-schema)
Yes (Transformers/vLLM/llama.cpp)
No
Cannot emit invalid shape — no failure to handle for shape
Local models; hard enum/regex/JSON guarantee; batch w/o retry budget
Instructor [[instructor]]
Validate (parse → Pydantic → auto-retry)
No
Yes
Catches ValidationError, re-asks with error; streaming partials
API models; Pydantic typing + graceful retry
Guidance [[guidance]]
Decode (grammar) + interleaved control flow
Yes
No
Constrained spans can't be invalid; free spans unconstrained
Local; interleaved text + constrained blocks; multi-step programs
[[agentsop-output-format-by-model]] already lists Outlines/Guidance under "grammar libs"
and provider tool_use under its §7 — it says choose the format first, then let a
lower layer enforce it. This skill is that lower layer made into a decision.
Quick reference card
┌────────────────────────────────────────────────────────────────────┐
│ STRUCTURED-OUTPUT ENFORCEMENT CARD │
├────────────────────────────────────────────────────────────────────┤
│ 0. Shape already "typed object"? If not → [[agentsop-output-format-by-model]]│
│ 1. How costly is a malformed output? │
│ cheap to recover → VALIDATE-time (Instructor / retry) │
│ must never escape → DECODE-time grammar (need decoder access) │
│ 2. Pick library: │
│ API model + Pydantic + retry ok → Instructor │
│ API model, zero deps → provider native │
│ local + hard enum/regex/JSON → Outlines │
│ local + interleaved text+constr. → Guidance │
│ only semantic/cross-field checks → retry loop + stance │
│ 3. Failure stance: │
│ prod default → Suggest (log + continue, capped retries) │
│ dev / unrecoverable value → Assert (raise after N) │
├────────────────────────────────────────────────────────────────────┤
│ NEVER: │
│ • Bolt Outlines/Guidance onto a closed API model (no logits) │
│ • Grammar-constrain code/prose (valid JSON, degraded content) │
│ • Assume strict-mode fixes content correctness │
│ • Run an uncapped retry loop │
│ • Assert where Suggest suffices (over-rejection) │
└────────────────────────────────────────────────────────────────────┘
引用源 (Citations)
Source lib skills (local):
[[outlines]] — decode-time grammar/regex/JSON-schema; local models
(Transformers/vLLM/llama.cpp); ~/.claude/skills/outlines/SKILL.md.
