Classification: layer=E (verification) + H (weak-reader); mechanism=scaffold H.

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.

Also Known As: reference implementation; in-memory fake; executable spec; oracle model; test double/simulator; MSW handler; `.d.ts` shape model.

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).

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.

Structure & Participants: *Reference model* (runnable, small) · *In-memory fake* (MSW for network, fake seam implementations) · *`.d.ts` shape model* · *Stepping interface* (inspect intermediate state).

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).

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.

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.

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.

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.

Evidence & Transfer-strength: R2C-008 (simulator in the transformative library, benchmark), DR2-019 (execution-prediction weakness, benchmark). Class: benchmark. Tag: **[E-strong]**.