regression-gate — Eval Set + Metric + Threshold, Wired Into CI

"Every subsequent change must be gated on these numbers." — Synthesized from [[llamaindex]] Stage 2 (eval loop before optimizing) [llamaindex-sop-skill/SKILL.md:114-126]

"Compiled program beats baseline on a held-out test set (not the val set used in optimization)." — [[dspy]] Stage 3 exit criterion [dspy-sop-skill/SKILL.md:101]

This is an enhancement overlay. The regression-gate SOP exists only as fragments scattered across base skills — [[llamaindex]] OP-10 EvalLoop ("gate every change"), [[dspy]] train/dev/test split + metric — and is never assembled as a standalone cross-framework discipline. It is the discipline that turns a one-off eval into a gate: a test suite that runs in CI on every prompt/model/retriever change and fails the build on regression. It consumes a metric from [[agentsop-metric-design]] and, for domain-specific held-out sets, hands off to [[agentsop-domain-eval-set]].

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1. 何时激活 (When to Activate)

Activate when any of these is true:

• Any prompt change you want to ship safely: a prompt edit, a system-message tweak, a few-shot-demo swap is about to merge and you have no automated way to know if it made things worse.
• Any model change: swapping GPT-4o → a cheaper/newer model, a temperature change, a provider migration. An LM change silently shifts the whole output distribution. [[dspy]] Case B: "If you optimize a complex pipeline for GPT-4, it usually breaks on a smaller model" [dspy-sop-skill/SKILL.md:184].
• Any retriever/chunking/reranker change in a RAG pipeline: every such change needs a quantitative gate. [[llamaindex]] OP-10: "Quantitative regression test for every chunking / embedding / retriever / prompt change" [llamaindex-sop-skill/SKILL.md:235].
• Recurring "it got worse" surprises: the team keeps shipping changes that users report as regressions after the fact. The fix is a gate, not more careful review.
• Setting up CI for an LLM app and there is no eval job in the pipeline.

Do NOT activate for:

• One-shot tasks with no production surface — there is nothing to regress. ([[dspy]] boundary: "Summarize this email once → raw API call" [dspy-sop-skill/SKILL.md:259].)
• The signature/task is still changing daily — gate only after the I/O contract stabilizes, else you re-baseline every commit. ([[dspy]] boundary [dspy-sop-skill/SKILL.md:260].)
• No willingness to define any success criterion — without a metric there is nothing to gate. Route to [[agentsop-metric-design]] first; if the user refuses, this skill cannot help.

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2. 核心心智模型 (Core Mental Model)

"An LM change is a code change. Gate it with a test suite: eval set + metric + threshold."

You already gate code with unit tests in CI: a change that breaks a test fails the build. A prompt edit, a model swap, a chunk-size tweak are also changes to the system's behavior — but they slip through review because their effect is statistical, not a stack trace. The regression gate is the missing unit test for LM behavior.

The gate is exactly three artifacts plus a wiring step:

   eval set        metric           threshold          CI wiring
  (held-out QA)  (ex,pred)->score   (fail if <X / drop>Y)  (block merge)
        │              │                  │                    │
        └──────────────┴──────────────────┴────────────────────┘
                          REGRESSION GATE

Three load-bearing principles:

1. The eval set is held out and frozen. It is a labelled, version-controlled fixture that the prompt/model under test has never seen. [[dspy]] is explicit: the compiled program must beat baseline on a held-out test set "not the val set used in optimization" [dspy-sop-skill/SKILL.md:101]. The split is train / dev / test; the gate runs on test only. If the eval set leaks into the prompt (few-shot demos, instructions), the gate measures memorization, not quality.

2. The metric comes from [[agentsop-metric-design]], not invented here. This skill does not design metrics — it consumes one. A bad metric makes the gate theatre: it will pass changes that hurt users and block changes that help them. The metric must be human-calibrated before it gates anything ([[agentsop-metric-design]] OP-M05).

3. The threshold is a policy, not a number you guess. Two common shapes: an absolute floor (fail if score < X) and a relative no-regression (fail if score drops > Y from the committed baseline). Relative is the regression gate proper; absolute is a quality bar. Most teams use both: a floor for "never ship below this," plus a no-regression delta for "this PR must not make it worse."

Build the eval loop before you optimize anything

[[llamaindex]] Stage 2 is named "Build the eval loop before optimizing anything" [llamaindex-sop-skill/SKILL.md:114]. The anti-pattern it names is A3: "No eval loop; debug by anecdote" [llamaindex-sop-skill/SKILL.md:348]. The gate is the institutional form of that loop — once it exists, every change is debugged by number, not by vibe.

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3. SOP (Standard Operating Procedure)

0. Confirm activation (§1); confirm a metric exists or invoke [[agentsop-metric-design]]
1. BUILD eval set:  generate candidates -> curate to a golden set -> freeze + version
2. SPLIT:           train / dev / test; the GATE runs on TEST only
3. PICK metric:     consume from [[agentsop-metric-design]] (do not invent here)
4. SET threshold:   absolute floor AND/OR relative no-regression delta
5. WIRE into CI:    run eval on every prompt/model/retriever PR; fail on regression
6. HANDLE flakiness: pin seeds/temp, average N runs, separate flaky from real drops

Stage 1 — Build the eval set (generate + curate)

Two stages, never one. Generation gives coverage cheaply; curation gives trust.

• Generate candidates from your corpus. [[llamaindex]] DatasetGenerator.from_documents(docs).generate_dataset_from_nodes(num=50) produces labelled QA pairs from the documents themselves [llamaindex-sop-skill/SKILL.md:121]. promptfoo and synthetic-data generators do the same for non-RAG tasks.
• Curate the generated set into a golden set: a human reviews, fixes wrong labels, drops ambiguous items, and adds known hard/edge cases the generator missed. A purely generated set inherits the generator model's blind spots and tends to be too easy (Dilemma 1).
• Freeze and version. The golden set is a committed fixture (eval/golden_v1.jsonl), tagged with the date and the generator model. Changing it is a versioned event, not an edit.

Size: [[dspy]] documents the sweet spot — "30 examples = minimum useful, 300 = recommended" [dspy-sop-skill/SKILL.md:87]. For a held-out gate, 50–200 curated domain examples is the working range; descend to [[agentsop-domain-eval-set]] for domain-specific construction.

Stage 2 — Split: train / dev / test

The gate runs on test only. Keep test sealed from anything that touches the prompt:

• train — feeds optimizers / few-shot demo selection.
• dev — tuning and threshold-setting.
• test — the gate. Never used to author prompts, pick demos, or tune. ([[dspy]] held-out exit criterion [dspy-sop-skill/SKILL.md:101].)

Stage 3 — Pick the metric (cross-link [[agentsop-metric-design]])

Do not invent a metric here. Consume one from [[agentsop-metric-design]]:

• RAG → the Faithfulness + Relevancy + Retriever(MRR/hit-rate) triad ([[llamaindex]] OP-10 [llamaindex-sop-skill/SKILL.md:234]).
• Exact-answer → exact-match ([[dspy]] §4.3 [dspy-sop-skill/SKILL.md:137]).
• Open-ended → decomposed sub-judges, bool-in-compile/float-in-eval, length penalty ([[agentsop-metric-design]] OP-M01/OP-M02/OP-M03).

A metric that has not been human-calibrated must not gate ([[agentsop-metric-design]] OP-M05). An uncalibrated gate is worse than no gate — it gives false confidence.

Stage 4 — Set the threshold

Threshold shape	Rule	Use when
Absolute floor	fail if score(test) < X	"never ship below this quality bar"
Relative no-regression	fail if baseline − score > Δ	"this PR must not make it worse" (the gate proper)
Per-slice floor	fail if any slice (e.g. lexical-query subset) drops > Δ	aggregate hides a regressed minority

Set Δ above measured run-to-run noise (Stage 6), else the gate flaps. Commit the current test score as baseline.json next to the eval set; the gate compares against it.

Stage 5 — Wire into CI

• Add an eval CI job that runs on every PR touching prompts, model config, retriever/chunking config, or the program graph.
• Job: load frozen eval set → run pipeline at PR's config → compute metric → compare to baseline.json → exit non-zero on threshold breach → post the before/after table as a PR comment.
• On merge to main with an intended improvement, bump baseline.json in the same PR (reviewed, not silent).

Stage 6 — Handle flaky evals

LLM outputs are nondeterministic; a naive gate flaps and gets disabled. Mitigations:

• Pin temperature=0 and seeds where the provider supports them; disable response caching in CI ([[dspy]] AP-10: "Forgetting cache=False in stateless deploys" [dspy-sop-skill/SKILL.md:255]).
• Average N runs (e.g. 3) and gate on the mean; report variance.
• Separate flaky from real: if the same config scores differently across reruns by more than Δ, the gate (Δ too tight) or the metric (noisy judge) is the problem — fix those before trusting a single red build. A judge-based metric with high variance should be hardened in [[agentsop-metric-design]], not papered over by widening Δ.

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4. 操作模型 (Operations)

OP-01 — GenerateEvalCandidates

• Trigger: Need a held-out eval set; no labelled fixture exists yet.
• Action: Generate QA candidates from the corpus — [[llamaindex]] DatasetGenerator.generate_dataset_from_nodes(num=50), promptfoo synthetic generation, or task-specific synthesis. Tag with generator model + date.
• Output: A raw candidate set (unvalidated), ready for curation.
• Evidence: [[llamaindex]] Stage 2 [llamaindex-sop-skill/SKILL.md:121]; OP-10 [llamaindex-sop-skill/SKILL.md:234]; external "eval set generation".

OP-02 — CurateGoldenSet

• Trigger: A generated candidate set exists; it has not been human-reviewed.
• Action: Human reviews every item: fix wrong labels, drop ambiguous/duplicate items, inject known hard cases and past production failures. Freeze as a versioned fixture (golden_vN.jsonl).
• Output: A trusted, frozen golden eval set that the gate runs against.
• Evidence: [[dspy]] metric human-validation discipline [dspy-sop-skill/SKILL.md:212]; [[agentsop-metric-design]] OP-M05; Dilemma 1.

OP-03 — SplitTrainDevTest

• Trigger: Eval set built; about to use it for both tuning and gating.
• Action: Partition into train/dev/test. Seal test from all prompt-authoring. The gate reads only test. (Note [[dspy]]'s reversed 20/80 train/val split for prompt optimizers [dspy-sop-skill/SKILL.md:96] — that is an optimizer concern; the gate still needs an untouched test slice.)
• Output: Three disjoint splits; a sealed test set for the gate.
• Evidence: [[dspy]] Stage 3 held-out exit criterion [dspy-sop-skill/SKILL.md:101]; AP-9 [dspy-sop-skill/SKILL.md:254].

OP-04 — SetRegressionThreshold

• Trigger: Have a test score; need a pass/fail policy.
• Action: Commit current test score as baseline.json. Define absolute floor X and/or relative no-regression Δ (Δ > measured noise). Optionally per-slice floors.
• Output: A versioned threshold policy the CI job enforces.
• Evidence: [[dspy]] Stage 3 exit "by ≥ task-relevant delta" [dspy-sop-skill/SKILL.md:101]; [[llamaindex]] gate-every-change [llamaindex-sop-skill/SKILL.md:124].

OP-05 — WireCIGate

• Trigger: Eval set + metric + threshold exist; not yet enforced automatically.
• Action: Add a CI job triggered by prompt/model/retriever/graph changes: run eval at PR config → compute metric → compare to baseline → fail on breach → comment the before/after table.
• Output: Merges that regress quality are blocked; every change carries a number.
• Evidence: [[llamaindex]] OP-10 regression-test framing [llamaindex-sop-skill/SKILL.md:233-235]; external "llm regression testing CI", "promptfoo".

OP-06 — BumpBaselineOnIntendedWin

• Trigger: A PR intentionally raises quality; the gate would otherwise pin the old baseline forever.
• Action: In the same reviewed PR, update baseline.json to the new test score. Never let CI auto-bump silently.
• Output: Baseline ratchets upward deliberately; future regressions are caught against the new bar.
• Evidence: [[dspy]] "keep both program.gpt4o.json and program.llama8b.json; A/B" [dspy-sop-skill/SKILL.md:191] (versioned-artifact discipline).

OP-07 — StabilizeFlakyEval

• Trigger: The gate flaps — same config, different verdicts across reruns.
• Action: Pin temperature/seed, disable CI caching, average N runs and gate on the mean, report variance. If variance > Δ, fix the metric (harden judge via [[agentsop-metric-design]]) or widen Δ — never disable the gate.
• Output: A stable gate whose red builds are trustworthy.
• Evidence: [[dspy]] cache=False AP-10 [dspy-sop-skill/SKILL.md:255]; [[agentsop-metric-design]] judge-bias hardening; [[dspy]] Stage 2 exit "stable across two runs" [dspy-sop-skill/SKILL.md:91].

OP-08 — SliceTheEvalSet

• Trigger: Aggregate score is flat but a subpopulation (lexical queries, a tenant, a topic) silently regressed.
• Action: Tag eval items by slice; compute and gate per-slice. A win on the majority must not mask a regression on a minority slice.
• Output: Slice-level regression detection; aggregate no longer hides harm.
• Evidence: [[llamaindex]] per-query-type taxonomy [llamaindex-sop-skill/SKILL.md:280]; [[agentsop-metric-design]] triad (no single number).

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5. 困境决策案例 (Dilemma Cases)

Dilemma 1 — Generated eval set vs hand-curated golden set

困境: Team needs an eval set fast. DatasetGenerator produces 200 QA pairs in minutes ([[llamaindex]] Stage 2 [llamaindex-sop-skill/SKILL.md:121]). Hand-curating 200 examples costs days of human time. Ship the generated set as the gate, or pay for curation?

约束: The generator is the same model family that powers the pipeline → its questions are answerable by exactly the kind of reasoning the pipeline already does (self-preference leakage). Generated sets skew easy and miss the long-tail failures users actually hit. But zero eval set means shipping blind (anti-pattern).

决策步骤:

1. Generate for coverage, never gate on raw generation. Use the generated set as a candidate pool, not the gate.
2. Curate a golden subset (OP-02): a human keeps the good items, fixes labels, drops the trivially-easy and the ambiguous, and injects known production failures and adversarial/edge cases the generator never proposes.
3. Cross-family generation where possible: generate with a different model family than the task model to reduce self-preference leakage (mirrors [[agentsop-metric-design]] OP-M04).
4. Size to budget: 50 curated > 200 raw. [[dspy]] floor is 30 useful examples [dspy-sop-skill/SKILL.md:87]; a tight, curated, edge-case-loaded 50 gates better than a bloated easy 200.
5. Version both: keep the raw generated pool (regeneratable) and the frozen golden set (the gate fixture).

结果: The golden set is the gate; the generated pool is scaffolding. Teams that gate on raw generated sets ship regressions that the easy set never exercised — the gate was green while users churned.

可提取的操作: OP-01 GenerateEvalCandidates, OP-02 CurateGoldenSet. Lesson: generation buys coverage, curation buys trust. A gate needs trust — never gate on an uncurated generated set.

Dilemma 2 — Threshold too strict blocks good changes

困境: A no-regression gate is set at Δ = 0 (any drop fails). A genuinely good refactor — simpler prompt, 40% cheaper model — scores 0.81 vs the 0.83 baseline: a 2-point drop within run-to-run noise. The gate blocks a change that is net-positive (equal quality, far cheaper). The team starts overriding the gate, and soon ignores it entirely.

约束: Run-to-run noise on this judge-based metric is ±1.5 points (measured across 3 reruns). The 2-point "drop" is statistically indistinguishable from noise. A gate that flags noise as regression trains the team to bypass it — a bypassed gate is worse than none.

决策步骤:

1. Measure noise first (OP-07): rerun the baseline config N times; compute the standard deviation. Here σ ≈ 1.5pp.
2. Set Δ above noise: Δ = 2σ ≈ 3pp, not 0. A drop must clear the noise band to count as a regression.
3. Average N runs and gate on the mean to shrink the noise band, rather than just widening Δ.
4. Separate cost from quality: this PR is a cost win at equal quality. The quality gate should pass (drop within Δ); cost is tracked on its own axis (cross-link [[agentsop-cost-tiered-models]]). Don't let a quality gate block a cost win that doesn't hurt quality.
5. If the metric is too noisy to set a sane Δ, the metric is the bug — harden it in [[agentsop-metric-design]] (decompose, length penalty, cross-family judge), don't widen Δ to infinity.

结果: Δ tuned to ~2σ passes the cheaper-equal-quality change, still catches real regressions (a 6pp drop), and the team keeps trusting the gate. A gate calibrated to noise survives; a Δ=0 gate gets disabled.

可提取的操作: OP-04 SetRegressionThreshold, OP-07 StabilizeFlakyEval. Lesson: the threshold must clear measured noise. A gate that flags noise as failure gets bypassed, and a bypassed gate protects nothing.

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6. 反模式与边界 (Anti-Patterns & Boundaries)

Anti-patterns

#	Anti-pattern	Why it's wrong	Fix
AP-1	No eval set; ship blind	Every prompt/model change is an uncontrolled experiment on users; "it got worse" is discovered in production	Build a held-out gate ([[llamaindex]] A3 [llamaindex-sop-skill/SKILL.md:348])
AP-2	Eval set leaks into the prompt (few-shot demos / instructions drawn from test)	The gate measures memorization, not generalization; green build, real regression	Seal test; demos come from train only (OP-03; [[dspy]] AP-9 [dspy-sop-skill/SKILL.md:254])
AP-3	Gate on a raw generated set	Inherits generator blind spots; too easy; misses real failures	Curate a golden set (OP-02; Dilemma 1)
AP-4	Gate on an uncalibrated metric	A wrong metric passes harmful changes and blocks good ones — gate is theatre	Calibrate via [[agentsop-metric-design]] OP-M05 before gating
AP-5	Δ = 0 / threshold below noise	Gate flaps on noise, team bypasses it	Set Δ > 2σ measured noise (OP-04, OP-07; Dilemma 2)
AP-6	Run the gate on the val/dev set used for tuning	Optimistic, leaks tuning into evaluation	Gate on held-out test only ([[dspy]] [dspy-sop-skill/SKILL.md:101])
AP-7	Caching on in CI	Stale cached outputs mask the change under test	cache=False ([[dspy]] AP-10 [dspy-sop-skill/SKILL.md:255])
AP-8	Aggregate-only gate	A win on the majority hides a regressed minority slice	Per-slice gating (OP-08)
AP-9	Silent baseline auto-bump	Quality can ratchet down unnoticed if CI rewrites baseline	Bump baseline only in a reviewed PR (OP-06)
AP-10	Disabling the gate when it flakes	Removes the only protection; flakiness is a metric/Δ bug, not a gate bug	Stabilize (OP-07), never disable

Boundaries (when this skill does not apply)

• One-shot / throwaway tasks — no production surface to regress ([[dspy]] [dspy-sop-skill/SKILL.md:259]).
• Unstable signature — gate only after the I/O contract stabilizes ([[dspy]] [dspy-sop-skill/SKILL.md:260]); otherwise you re-baseline every commit.
• No metric possible and none willing to be built — without a metric there is nothing to gate; route to [[agentsop-metric-design]] first.
• Public-benchmark evaluation (MMLU, HumanEval) — that is a capability benchmark, not a domain regression gate; use lm-evaluation-harness. For a domain-specific held-out set, descend to [[agentsop-domain-eval-set]].

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7. 跨框架对照 (Cross-Framework Mapping)

Concept	LlamaIndex	DSPy	promptfoo	LangSmith	This skill
Eval set generation	DatasetGenerator.generate_dataset_from_nodes(num=N) [llamaindex-sop-skill/SKILL.md:121]	bring labelled examples; BootstrapFewShot self-generates demos (not the test set)	tests: synthesis / generate from prompts	Datasets created from traces / uploads	OP-01 GenerateEvalCandidates
Golden / curated set	manual review of generated QA	hand-labelled trainset/devset	curated tests YAML with assert	curated Dataset + reference outputs	OP-02 CurateGoldenSet
Train/dev/test split	manual	explicit; reversed 20/80 for prompt optimizers, held-out test for gate [dspy-sop-skill/SKILL.md:96,101]	n/a (test set is the suite)	dataset splits	OP-03 SplitTrainDevTest
Metric	Faithfulness/Relevancy/RetrieverEvaluator(mrr,hit_rate) [llamaindex-sop-skill/SKILL.md:234]	def metric(ex,pred,trace=None)->bool|float [dspy-sop-skill/SKILL.md:88]	assert (equals/contains/llm-rubric/javascript)	evaluator fns / LLM-as-judge	consumed from [[agentsop-metric-design]]
Threshold / gate	manual (gate every change [llamaindex-sop-skill/SKILL.md:124])	held-out beats baseline "by ≥ delta" [dspy-sop-skill/SKILL.md:101]	assert pass + --fail-on thresholds	rules + alerts on eval scores	OP-04 SetRegressionThreshold
CI wiring	not built-in (DIY job around Evaluate)	not built-in (DIY around dspy.Evaluate)	first-class: promptfoo eval in CI, non-zero exit	CI integration + regression alerts	OP-05 WireCIGate
Flaky handling	run multiple times	cache=False; "stable across two runs" [dspy-sop-skill/SKILL.md:91,255]	repeat + threshold	run aggregation	OP-07 StabilizeFlakyEval

Combination patterns:

• LlamaIndex + this skill: DatasetGenerator for OP-01, the Faithfulness/Relevancy/Retriever triad as the metric, wired into a DIY CI job. The base skill names the loop ("gate every change"); this skill makes it a CI gate.
• DSPy + this skill: DSPy's split + metric is the eval-loop substrate; this skill adds the CI enforcement DSPy leaves to you. Gate the compiled artifact on the held-out test set; bump baseline when recompiling for a new model (Case B).
• promptfoo as the engine: promptfoo is the most CI-native option — promptfoo eval exits non-zero on failed asserts; it is the closest off-the-shelf realization of OP-05. Use it as the runner; still bring a curated set (OP-02) and a calibrated metric.
• LangSmith for managed datasets + monitoring: managed datasets and regression alerts; pairs with this skill's curation/threshold discipline.

Opinionated default: build the eval set with the base framework's generator (OP-01), curate by hand (OP-02), keep the metric in [[agentsop-metric-design]], and run the gate with promptfoo (CI-native) or a thin script around dspy.Evaluate / LlamaIndex evaluators. The gate, the metric, and the eval set are three separable, version-controlled artifacts — never one tangled blob.

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References

• references/R1-source-evidence.md — every cited claim resolved to a source line
• intermediate/operation_candidates.json — machine-readable operation registry