diff --git a/README.md b/README.md
index 6b24359..51a708a 100644
--- a/README.md
+++ b/README.md
@@ -206,6 +206,9 @@ file with `--config`.
 
 ## How it works
 
+For a step-by-step trace of a clean sweep, see the
+[execution happy-path sequence diagram](docs/execution-happy-path.md).
+
 The source lives under `src/`, split into an entry point plus logic
 modules:
 
@@ -358,6 +361,7 @@ move --help
 | `src/device.ts`     | `Device` I/O seam over nut.js (`Point`, `createNutDevice`); the only nut.js importer. |
 | `src/strategies.ts` | Pure movement-pattern generators, the strategy registry, and name validation. |
 | `src/executor.ts`   | `executePath` driver: bounds policy, pacing, interrupt detection, restore.     |
+| `docs/execution-happy-path.md` | Sequence diagram + invariants for a clean sweep.                  |
 | `package.json`      | Bun project manifest. Single runtime dep: `@nut-tree-fork/nut-js`.            |
 | `tsconfig.json`     | Strict TypeScript config tuned for Bun (ESNext, bundler resolution).          |
 | `bun.lock`          | Bun's lockfile. Commit this.                                                  |
diff --git a/docs/execution-happy-path.md b/docs/execution-happy-path.md
new file mode 100644
index 0000000..57f0f16
--- /dev/null
+++ b/docs/execution-happy-path.md
@@ -0,0 +1,88 @@
+# Execution: the happy path
+
+This traces one full idle-triggered sweep that completes cleanly — the
+"happy path" where the machine is idle long enough to fire, the configured
+pattern runs to exhaustion, and no real user activity interrupts it.
+
+For the module breakdown and the three seams (`device` / `strategies` /
+`executor`), see the "How it works" section of the [README](../README.md).
+
+```mermaid
+sequenceDiagram
+    autonumber
+    participant Entry as move.ts
+    participant Keeper as runKeeper
+    participant Sim as simulateActivity
+    participant Strat as Strategy<br/>(e.g. line)
+    participant Exec as executePath
+    participant Dev as Device<br/>(nut.js)
+
+    Note over Entry: startup (args → config)
+    Entry->>Entry: parseCliArgs()
+    Entry->>Entry: loadConfigFile()
+    Entry->>Entry: resolveConfig(file, cli)
+    Entry->>Keeper: runKeeper(config)
+
+    Keeper->>Dev: createNutDevice()
+    Note right of Dev: sets mouse.config.autoDelayMs = 0
+    Keeper->>Keeper: log.info(banner)
+    Keeper->>Dev: getPosition()
+    Dev-->>Keeper: lastPos
+    Note over Keeper: lastActivity = now
+
+    loop every checkInterval (until idle long enough)
+        Keeper->>Dev: sleep(checkInterval)
+        Keeper->>Dev: getPosition()
+        Dev-->>Keeper: pos
+        Note over Keeper: pos == lastPos (no user movement)<br/>now - lastActivity ≥ moveInterval → fire
+    end
+
+    Keeper->>Sim: simulateActivity(config, log, dev)
+    Sim->>Dev: getPosition()
+    Dev-->>Sim: start
+    Sim->>Dev: width()
+    Dev-->>Sim: width
+    Sim->>Dev: height()
+    Dev-->>Sim: height
+    Note over Sim: strategy = STRATEGIES[config.pattern]<br/>ctx = { start, width, height, rng }
+    Sim->>Exec: executePath(strategy, ctx, dev, log, config)
+
+    Exec->>Strat: path(ctx)
+    Strat-->>Exec: Iterable<Point>
+
+    loop for each target point (clean run)
+        Exec->>Exec: resolveTarget(bounds, target) → point
+        Exec->>Dev: setPosition(point)
+        Exec->>Dev: sleep(stepDelay)
+        Exec->>Dev: getPosition()
+        Dev-->>Exec: current
+        Note over Exec: |current - point| ≤ 2px → not the user, continue
+    end
+
+    Note over Exec: path exhausted, no interruption
+    Exec->>Dev: setPosition(round(start))
+    Note right of Exec: restore cursor to origin
+    Exec-->>Sim: "completed"
+    Sim-->>Keeper: (done)
+
+    Keeper->>Dev: getPosition()
+    Dev-->>Keeper: lastPos (== start; re-sync)
+    Note over Keeper: lastActivity = now<br/>loop continues
+```
+
+## Invariants this path relies on
+
+- **`createNutDevice()` is the only nut.js touchpoint.** It disables nut.js's
+  100ms auto-delay so `executePath` owns cadence via `stepDelay`.
+- **The strategy is pure.** `path(ctx)` yields ideal points from geometry
+  alone (`start` / `width` / `height` / `rng`); it never touches the device,
+  which is what makes every pattern unit-testable without a screen.
+- **Every step re-reads the cursor** and compares it against the *commanded*
+  point (not the strategy's ideal, possibly fractional target) within a 2px
+  tolerance. On the happy path each check passes, so the loop runs to
+  exhaustion. A mismatch beyond tolerance is real user activity and returns
+  `"interrupted"` without restoring — the branch this diagram omits.
+- **Clean completion restores the cursor to `round(start)`.** That is why the
+  follow-up `getPosition()` in `runKeeper` re-syncs `lastPos` to the origin as
+  a no-op, and the next idle check sees no net movement (so the synthetic
+  sweep is never mistaken for the user returning).