Run several scenarios against one model on one shared seed and return them side by side — the seed sharing is server-enforced, so differences are the scenarios, not the dice. Returns a summary by default (finals with finalStdDev, each final count's spread across realizations, throughput/mean/P95 metrics, depletion and contention — no time series); pass full=true for the complete trajectories, which run to hundreds of KB. A scenario carrying "summary": true stays summarized even under full=true, so one comparison can chart some scenarios and only read the rest. Unset hours default to 8, samples to 60 (the trajectory grid, which only matters under full=true — metrics are time-weighted and do not depend on it) and realizations to 16 per scenario. Each scenario can set its own "engine" (see sim_scenario / docs/engine-selection.md); comparing an "ode" run against an "ssa" one is legitimate but the shared seed only removes dice from scenarios using the same engine. Costs 1 token per scenario from the compute bucket (per address, or per account when signed in). Each result's contention is split exactly as sim_scenario's is: contended holds only what the run was short of (kind conserved or bounded), waitingForWork the entries of kind queue (the work had not arrived) or state (the model was in another phase of its cycle), neither of them a shortage, and neverFired one note per place that never held a token in any run while its transitions never fired; sim_scenario's description lists what the run cannot tell apart behind such a note.