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+# ADR-0007: Runtime API and Simulation Engine Boundaries
+
+## Status
+
+Accepted
+
+## Context
+
+The simulator consists of multiple layers with distinct responsibilities:
+
+- a host-facing API layer used by benchmarks and user code,
+- a discrete-event simulation engine that executes requests,
+- device components that model hardware behavior.
+
+Without strict boundaries, orchestration logic can leak into components,
+or simulation internals can become entangled with user-facing APIs.
+
+This ADR defines clear responsibility boundaries between:
+
+- runtime API,
+- simulation engine (sim_engine),
+- hardware components.
+
+---
+
+## Decision
+
+### D1. Runtime API is host-facing orchestration only
+
+The runtime API represents host/driver-level behavior and MUST:
+
+- expose high-level operations (tensor deployment, kernel launch),
+- submit requests only to endpoint components (e.g., IO_CPU),
+- await completion via futures/handles,
+- own and persist host-side metadata (tensor allocation maps, kernel bindings).
+
+The runtime API MUST NOT:
+
+- hardcode hop-by-hop routing or fan-out,
+- directly invoke internal components (M_CPU, PE_CPU, engines),
+- embed topology- or routing-specific assumptions.
+
+---
+
+### D2. Simulation engine executes and schedules requests
+
+The simulation engine (sim_engine) MUST:
+
+- inject requests into the compiled topology graph,
+- schedule and execute events using a discrete-event model,
+- manage correlation ids and completion tracking,
+- decompose operations into low-level requests when required
+  (e.g., MemoryWrite events).
+
+The simulation engine MUST NOT:
+
+- define tensor semantics,
+- define kernel execution policies,
+- expose internal graph details to the runtime API.
+
+---
+
+### D3. Components own fan-out and aggregation
+
+Device-side components MUST:
+
+- fan-out requests to downstream domains
+  (IO_CPU → M_CPU → PE_CPU → schedulers/engines),
+- aggregate completion and failure signals,
+- propagate results deterministically upstream.
+
+Neither the runtime API nor the simulation engine may orchestrate
+component-level fan-out explicitly.
+
+---
+
+## Consequences
+
+- Runtime APIs remain stable as topology and routing evolve.
+- Simulation internals can change without affecting user-facing code.
+- Component implementations remain swappable via DI.
+
+---
+
+## Links
+
+- SPEC R4, R7, R8
+- ADR-0008 (Tensor deployment)
+- ADR-0009 (Kernel execution)