# CARD: scaffold-h-simulators — Local Simulators / Reference Models (TypeScript) {#root}

@status:spec/done

[p01] @fact:status-line **Discipline v0.2 · BETA · T2 · TypeScript** @status:impl/done

## Band 1 — Identity & Recognition {#band-one-identity}

[p02] @fact:CLASSIFICATION Classification: layer=E (verification) + H (weak-reader); mechanism=scaffold H. @status:impl/done

[p03] @fact:INTENT Intent: Ship a small runnable model of a subsystem's behavior the reader can EXECUTE to understand or predict — offloading the execution-prediction that weak models fail at, without running the whole system. @status:impl/done

[p04] @fact:ALSO-KNOWN-AS Also Known As: reference implementation; in-memory fake; executable spec; oracle model; test double/simulator; MSW handler; `.d.ts` shape model. @status:spec/done

[p05] @fact:APPLICABILITY-RECOGNITION Applicability / Recognition: Apply when — a subsystem has non-obvious dynamics (a state machine, a protocol, an async reducer/fixpoint); understanding requires mentally simulating execution; an external dependency (HTTP, a queue) must be reasoned about offline. *Detector seed:* a subsystem whose behavior is documented in prose-describing-execution, with no runnable model or fake → recognition fires (execution-prediction is weak models' weakest point — DR2-019, CRUXEval ~63% even for strong models). @status:impl/done

## Band 2 — Justification & Tradeoffs {#band-two-justification}

[p06] @fact:MOTIVATION Motivation: A weak agent must modify a Redux-style reducer or an async state machine driving a UI flow. It cannot mentally simulate the dispatch→effect→re-render convergence. A runnable reference model it can step through (feed actions, watch state converge), plus MSW fakes for the network the flow depends on, replaces mental simulation with execution — the EsoLang library shipped exactly this idea (a local simulator) and it carried the weak-agent gain. @status:spec/done

[p07] @fact:STRUCTURE-AND-PARTICIPANTS Structure & Participants: *Reference model* (runnable, small) · *In-memory fake* (MSW for network, fake seam implementations) · *`.d.ts` shape model* · *Stepping interface* (inspect intermediate state). @status:impl/done

[p08] @fact:COLLABORATIONS Collaborations: Provides the comparator for Class D oracles; backs Class C contracts (the model defines expected behavior); pairs with Class G (the model's usage is Twoslash-exampled). @status:impl/done

[p09] @fact:GOALS-AND-NON-GOALS Goals / Non-Goals: *Goals:* make non-obvious dynamics executable, not just described. *Non-Goals:* NOT a second production implementation (a reference model, kept simple); NOT for trivially-obvious subsystems. @status:impl/done

[p10] @fact:CONSEQUENCES Consequences: (+) the reader runs instead of simulates; (+) doubles as a Class D comparator and an MSW-backed test fixture. (−) a model is code to keep in sync — drift detection or a conformance test against production; (−) over-modeling wastes effort — only non-obvious dynamics. @status:spec/done

[p11] @fact:ALTERNATIVES Alternatives: prose describing behavior (weak readers can't execute prose); reading the production code directly (the thing too complex to simulate). The model is the offload. @status:spec/done

[p12] @fact:RISKS-AND-ASSUMPTIONS Risks & Assumptions: assumes the subsystem's behavior is modelable simply; a model that drifts from production misleads — conformance-test it. *Sunset:* if the production code becomes simple enough to read directly, the model retires. @status:spec/done

[p13] @fact:EVIDENCE-AND-TRANSFER-STRENGTH Evidence & Transfer-strength: R2C-008 (simulator in the transformative library, benchmark), DR2-019 (execution-prediction weakness, benchmark). Class: benchmark. Tag: **[E-strong]**. @status:spec/done

## Band 3 — Operation {#band-three-operation}

[p14]
```card-ops
trigger: WHEN a subsystem with non-obvious dynamics has no runnable reference model or fake THEN apply
mode: gate
routine:
  1. Identify the dynamics a reader must predict (states, transitions, async convergence).
  2. Write a small runnable reference model with a stepping/inspection interface.
  3. Provide in-memory fakes (MSW for network, fake seam implementations) for external dependencies.
  4. Add a conformance test: model vs production agree on representative inputs.
  5. Twoslash the model's usage (Class G).
checker: @typescript-eslint/conform `nonobvious-subsystem-has-model` + model-vs-production conformance test (vitest)
raid_role: layer=cells; order=after:contracts; batch=cell
budget: active_rules=1; first_signal=conformance test (<60s)
```