diff --git a/README.md b/README.md
index ae62defa..4953c173 100644
--- a/README.md
+++ b/README.md
@@ -33,8 +33,8 @@ coding agents (Claude Code / Codex) compose and evolve **typed dora-rs
 dataflows** against a **Genesis** physics scene, with frozen
 verification/reset and scoped safety structure. The scene contract is
 engine neutral; two optional engines run the same graphs for development,
-[Nexus](docs/decisions/ADR-67.md) on the GPU and
-[rapier](docs/decisions/ADR-68.md) on the CPU (`--sim-engine nexus|rapier`),
+[Nexus](docs/simulation-backends.md#nexus) on the GPU and
+[rapier](docs/simulation-backends.md#rapier) on the CPU (`--sim-engine nexus|rapier`),
 while Genesis stays the default and the only engine behind the measured
 record.
 <!-- claim:typed-dataflow-causal/readme -->
@@ -236,6 +236,38 @@ uv run --extra sim --locked python tools/dora_runtime.py verify --prefix "$AISLE
 uv run --extra sim --locked python tools/quickstart.py --runtime-prefix "$AISLE_DORA_PREFIX"
 ```
 
+### Choose a simulation backend
+
+Genesis is installed by the `sim` extra and remains the default. Select it
+explicitly, or choose an installed optional backend, with the same rollout
+flag:
+
+```bash
+# Default, measured backend
+uv run --extra sim --locked harness rollout --graph graphs/expert_t0.yaml --tier T0 \
+    --episodes 2 --seeds 0..1 --no-idea-gate --env-baseline local \
+    --sim-engine genesis
+
+# Optional GPU backend: install after `uv sync --extra sim --locked`
+uv run --no-sync python tools/nexus_runtime.py install
+uv run --no-sync python tools/nexus_runtime.py verify
+uv run --no-sync harness rollout --graph graphs/expert_t0.yaml --tier T0 \
+    --episodes 2 --seeds 0..1 --no-idea-gate --env-baseline local \
+    --sim-engine nexus
+
+# Optional CPU solver with the Nexus renderer
+uv run --no-sync python tools/rapier_runtime.py install
+uv run --no-sync python tools/rapier_runtime.py verify
+uv run --no-sync harness rollout --graph graphs/expert_t0.yaml --tier T0 \
+    --episodes 2 --seeds 0..1 --no-idea-gate --env-baseline local \
+    --sim-engine rapier
+```
+
+See the [simulation backend guide](docs/simulation-backends.md) for platform
+support, source receipts, backend overrides, direct graph declarations, and
+current limitations. Nexus and rapier are development backends; do not compare
+their results with the Genesis measured record.
+
 The Python API remains pinned to 1.0.1; the CLI is built from the corrected,
 immutable source revision in `dora-runtime.json`. CLI version output alone does
 not identify that correction: the installer and quickstart verify its receipt.
diff --git a/docs/getting-started.md b/docs/getting-started.md
index 3dbfc584..b03e8d2e 100644
--- a/docs/getting-started.md
+++ b/docs/getting-started.md
@@ -33,6 +33,11 @@ Things to know before anything else:
   (#516). Install the corrected source pin rather than relying on version
   output, which is also 1.0.1 for the corrected binary.
 
+Genesis is the default simulation engine. Nexus and rapier are optional source
+builds installed after this locked environment; see the
+[simulation backend guide](simulation-backends.md) for a comparison and the
+shortest install, verify, and selection path.
+
 On Ubuntu 24.04, install the CPU quickstart's rendering prerequisites. Genesis
 constructs an offscreen renderer even when physics runs on CPU:
 
@@ -201,11 +206,11 @@ argument switches the whole build to local sources, so you never get a
 half-pinned, half-local mix. Then:
 
 ```bash
-uv run --extra sim --locked pytest -m sim tests/sim/test_nexus_scene.py
+uv run --no-sync pytest -m sim tests/sim/test_nexus_scene.py
 ```
 
 ```bash
-uv run --extra sim --locked harness rollout --graph graphs/expert_t0.yaml --tier T0 \
+uv run --no-sync harness rollout --graph graphs/expert_t0.yaml --tier T0 \
     --episodes 2 --seeds 0..1 --no-idea-gate --env-baseline local --sim-engine nexus
 ```
 
@@ -236,15 +241,23 @@ uv run --no-sync python tools/rapier_runtime.py install
 
 `--rapier ../rapier` builds from a local checkout instead.
 
-`--no-sync` matters here: a syncing `uv run` would reinstall the locked
-environment first and take the Nexus wheel back out, which is the renderer
-this engine needs.
+Keep `--no-sync` on the installer and subsequent commands so no sync step runs
+or changes the environment containing the out-of-lock Nexus wheel that this
+engine needs for rendering.
 
 `tools/rapier_runtime.py verify` reports the solver's receipt and the
 renderer's together, so a half-installed environment is visible before a run
 rather than at the first render. Its engine constants live in
 `src/aisle/sim/rapier_physics.toml`, alongside the Nexus ones.
 
+Run the same graph through rapier without another sync step:
+
+```bash
+uv run --no-sync python tools/rapier_runtime.py verify
+uv run --no-sync harness rollout --graph graphs/expert_t0.yaml --tier T0 \
+    --episodes 2 --seeds 0..1 --no-idea-gate --env-baseline local --sim-engine rapier
+```
+
 ## 4. Where to go next
 
 - `docs/physical-ai-primer.md` — the concepts behind the project
diff --git a/docs/harness-guide.md b/docs/harness-guide.md
index 1bb9f9fa..0a10ec41 100644
--- a/docs/harness-guide.md
+++ b/docs/harness-guide.md
@@ -54,7 +54,7 @@ the scene (ADR-67, ADR-68); it defaults to `genesis` and is recorded in the
 manifest as `sim_engine`, beside a digest of the engine realization and the
 wheel's build receipt. `nexus` (GPU) and `rapier` (CPU, rendering through
 the Nexus viewer) refuse before launch when their wheels are not installed
-(see [getting started](getting-started.md) §3b and §3c), and results are not
+(see the [simulation backend guide](simulation-backends.md)), and results are not
 comparable across engines. A graph whose bridge node declares
 `AISLE_SIM_ENGINE` owns the choice: passing a different `--sim-engine` is
 refused rather than silently overridden. The same flag is on
@@ -62,7 +62,7 @@ refused rather than silently overridden. The same flag is on
 and `harness fleet`.
 
 ```bash
-uv run harness rollout --graph graphs/expert_t0.yaml --tier T0 \
+uv run --no-sync harness rollout --graph graphs/expert_t0.yaml --tier T0 \
     --episodes 2 --seeds 0..1 --no-idea-gate --env-baseline local --sim-engine nexus
 ```
 
diff --git a/docs/simulation-backends.md b/docs/simulation-backends.md
new file mode 100644
index 00000000..09a8900a
--- /dev/null
+++ b/docs/simulation-backends.md
@@ -0,0 +1,124 @@
+# Simulation backends
+
+AISLE can realize the same scene and bridge contract with three physics
+engines. Select one with `harness rollout --sim-engine`; the harness injects
+the choice into the bridge and records it in the run manifest.
+
+| Engine | Physics | Renderer | Installation | Status |
+|---|---|---|---|---|
+| `genesis` | Genesis | Genesis | Included in the `sim` or `cuda` extra | Default; only engine behind the measured record |
+| `nexus` | Nexus GPU | Nexus | Build the pinned `nexus3d` wheel | Development only |
+| `rapier` | Rapier CPU | Nexus | Build both `nexus3d` and `rapier3d` | Development only |
+
+Results from different engines are not directly comparable. Each run records
+the engine, resolved backend and device, the engine realization digest, and
+the optional wheel build receipt.
+
+## Genesis
+
+Genesis is installed with the normal simulation environment:
+
+```bash
+uv sync --extra sim --locked
+```
+
+It is the default when `--sim-engine` is omitted. An explicit development
+rollout is:
+
+```bash
+uv run --extra sim --locked harness rollout --graph graphs/expert_t0.yaml --tier T0 \
+    --episodes 2 --seeds 0..1 --no-idea-gate --env-baseline local \
+    --sim-engine genesis
+```
+
+On NVIDIA Linux, use `uv sync --extra cuda --locked`, use `--extra cuda` on
+the `uv run`, and add `--sim-extra cuda`. The CUDA request fails closed if no
+CUDA device is available.
+
+## Nexus
+
+Nexus is outside `uv.lock`. The installer fetches the source commit pinned in
+`engine-runtime.json`, builds the wheel with maturin, installs it into the
+project environment, and writes `.nexus-runtime-receipt.json`.
+
+Start from the normal `sim` environment, then install and verify Nexus:
+
+```bash
+uv sync --extra sim --locked
+uv run --no-sync python tools/nexus_runtime.py install
+uv run --no-sync python tools/nexus_runtime.py verify
+```
+
+Run a graph with Nexus:
+
+```bash
+uv run --no-sync harness rollout --graph graphs/expert_t0.yaml --tier T0 \
+    --episodes 2 --seeds 0..1 --no-idea-gate --env-baseline local \
+    --sim-engine nexus
+```
+
+The default Nexus build feature is `metal` on macOS and `webgpu` elsewhere.
+Use `tools/nexus_runtime.py install --feature <feature>` when the wheel needs a
+different compiled feature. `AISLE_SIM_BACKEND` accepts `metal`, `webgpu`,
+`cuda`, or `cpu`, but ordinary rollouts should let the harness resolve and
+attest the backend from `--sim-extra` and the host.
+
+To build a local Nexus checkout instead of the pinned source:
+
+```bash
+uv run --no-sync python tools/nexus_runtime.py install --nexus ../nexus
+```
+
+## Rapier
+
+Rapier steps physics on the CPU and uses Nexus for rendering. Install Nexus
+first, then build and verify the pinned Rapier Python bindings:
+
+```bash
+uv sync --extra sim --locked
+uv run --no-sync python tools/nexus_runtime.py install
+uv run --no-sync python tools/rapier_runtime.py install
+uv run --no-sync python tools/rapier_runtime.py verify
+```
+
+Run a graph with Rapier:
+
+```bash
+uv run --no-sync harness rollout --graph graphs/expert_t0.yaml --tier T0 \
+    --episodes 2 --seeds 0..1 --no-idea-gate --env-baseline local \
+    --sim-engine rapier
+```
+
+`tools/rapier_runtime.py install --rapier ../rapier` builds a local checkout.
+Add `--determinism` to build Rapier's enhanced determinism feature when the
+experiment requires cross-platform math consistency.
+
+## Selection rules
+
+- `--sim-engine {genesis,nexus,rapier}` is available on `harness rollout`,
+  `harness fleet`, `harness monolith run`, `harness fault calibrate`, and
+  `harness skill register`.
+- A graph can declare `AISLE_SIM_ENGINE` in its simulation bridge node. A
+  conflicting CLI selection is refused instead of silently overriding the
+  graph.
+- For a direct `dora run`, set `AISLE_SIM_ENGINE` in the bridge node's `env`
+  mapping. Prefer the harness for recorded work because it validates, injects,
+  hashes, and records the engine choice.
+- Nexus and Rapier require clean build receipts. A missing, malformed, or dirty
+  source receipt fails the environment gate before launch.
+
+## Keep optional wheels installed
+
+`uv sync` removes the out-of-lock Nexus and Rapier wheels. After either
+optional installer runs, use `uv run --no-sync ...` for verification, tests,
+and rollouts so no sync step runs or changes the installed environment. If a
+command reports that an optional engine is not installed, rerun its installer
+and verifier.
+
+More detail:
+
+- [Getting started](getting-started.md), including platform prerequisites
+- [Harness CLI guide](harness-guide.md)
+- [Troubleshooting](troubleshooting.md)
+- [ADR-67: Nexus engine](decisions/ADR-67.md)
+- [ADR-68: Rapier engine](decisions/ADR-68.md)
diff --git a/docs/troubleshooting.md b/docs/troubleshooting.md
index 7df8d0f2..049ed8fe 100644
--- a/docs/troubleshooting.md
+++ b/docs/troubleshooting.md
@@ -19,6 +19,8 @@ That sync also removes the optional engine wheels, silently: neither
 lock ([getting started](getting-started.md) §3b and §3c), so any `uv sync`
 drops them and `--sim-engine nexus` then refuses at the `sim_engine` gate
 with `simulation engine 'nexus' is not installed in this environment`.
+The complete install and selection matrix is in the
+[simulation backend guide](simulation-backends.md).
 Reinstall them from the commits `engine-runtime.json` pins:
 
 ```bash
@@ -29,8 +31,8 @@ uv run --no-sync python tools/nexus_runtime.py install
 uv run --no-sync python tools/rapier_runtime.py install
 ```
 
-Keep `--no-sync` on those: a syncing `uv run` reinstalls the locked
-environment first and takes the other engine wheel back out.
+Keep `--no-sync` on the installers and subsequent commands so no sync step
+runs or changes the environment containing the out-of-lock wheels.
 
 A pinned install that fails with `upload-pack: not our ref` means the pinned
 commit is not on the remote, usually because a local engine commit has not