Run a seeded what-if scenario against a stored model: marking overrides, rate overrides, piecewise rate schedules, and params assignments to the model's declared structural parameters (arc weights, capacities — batch sizes and shelf sizes). Pure read — asking cannot change the model. Returns trajectory, final marking, metrics, depleted, contended, waitingForWork, neverFired (all three described below), caveats and assumptions. "samples" (default 60) is the trajectory's resolution: the number of evenly spaced points from 0 to hours inclusive at which times and every place's series (mean and std_dev per point) are reported — it sizes the answer, not the run, since metrics (throughput, mean, p95, utilization, inFlight) are time-weighted over every firing and do not change with the grid. "summary": true omits the times and series arrays entirely (the keys are absent, not null) and returns just final, finalStdDev, metrics, depleted, contended, waitingForWork, neverFired, caveats and assumptions — the verdict without the chart data, and the right form when nothing will be plotted. finalStdDev, summarized or not, is each place's spread across realizations at the horizon beside final's mean (absent at one realization and on the ode engine). Transitions declaring stages (phase-type durations) run with the declared lower spread — the engine expands them structurally and reports in the model's own vocabulary. A model-declared schedule (the day shape on a transition) is honored by every run; the scenario's own schedule or rate override still wins for that transition. "engine" picks the reading: "ssa" (default, discrete Gillespie — the right choice whenever counts are small enough that variance is the answer, or a schedule is in play) or "ode" (continuous mass-action; refuses a schedule, and refuses outright rather than silently misread a model carrying a read arc, inhibitor, reached capacity, guard or non-kinetic arc — Forecast's caveats name which). See docs/engine-selection.md for the full decision rule, including why an arc weight above 1 gets a genuinely different rate law from each engine, and sim_crosscheck to run both readings side by side. Contention arrives split three ways. contended holds only what the run was short of, something an operator could buy more of: kind conserved (a fixed pool such as staff) or bounded (a shelf with a declared capacity), each entry with fraction (the share of the run, across every realization, in which that place was the only input missing for the transitions under blocking), longest wait first. waitingForWork holds the engine's other entries in the same shape, and neither of its kinds is a shortage: queue (filled only by the model's own flow, such as waiting customers or work in service) means the work had not arrived, a statement about demand; state (a marker of which phase of a cycle the model is in, such as a stoplight's colour) means the model was in another phase. neverFired is one note per entry whose place never held a token in any run and whose transitions never fired, e.g. "work" never fired in any run: its input "q" never held a token. A queue or state entry like that appears only as its note; a conserved or bounded one keeps its contended row as well, since the run was short of it. The run cannot tell why a place stayed empty. Either it starts empty and nothing fills it (check its initial count and what produces it), or its producer was switched off (a rate of 0, or a scenario marking of 0 on it or upstream of it), or what fills it is rarer than this run's hours and realizations can sample (raise either; once the place fills, the note goes). sim_receipt and the HTTP scenario routes return the engine's unsplit contended list instead, each entry with its kind.