diff --git a/server/src/__tests__/plugin-worker-process-registry.test.ts b/server/src/__tests__/plugin-worker-process-registry.test.ts
new file mode 100644
index 00000000000..6830d7adbc9
--- /dev/null
+++ b/server/src/__tests__/plugin-worker-process-registry.test.ts
@@ -0,0 +1,68 @@
+import { afterEach, describe, expect, it } from "vitest";
+
+import {
+  clearProcessPluginWorkerManager,
+  getProcessPluginWorkerManager,
+  setProcessPluginWorkerManager,
+} from "../services/plugin-worker-process-registry.js";
+
+/**
+ * Guards the fix for a bug that recurred three times: the plugin worker manager
+ * is process-scoped, but it was threaded as an OPTIONAL parameter through every
+ * runtime construction site. Any caller that omitted it built a run engine that
+ * could never acquire a sandbox lease, and it only surfaced when a customer hit
+ * that path. Observed in production on 2026-07-26 via `assignment` and
+ * `automation` dispatches, which reach the runtime through `heartbeatService(db)`
+ * called with no options at all.
+ */
+describe("process plugin worker manager registry", () => {
+  afterEach(() => {
+    clearProcessPluginWorkerManager();
+  });
+
+  it("returns undefined when nothing has registered a manager", () => {
+    clearProcessPluginWorkerManager();
+    expect(getProcessPluginWorkerManager()).toBeUndefined();
+  });
+
+  it("hands back the manager registered for this process", () => {
+    const manager = { marker: "process-manager" } as never;
+    setProcessPluginWorkerManager(manager);
+    expect(getProcessPluginWorkerManager()).toBe(manager);
+  });
+
+  it("allows a test harness to replace the registration", () => {
+    const first = { marker: "first" } as never;
+    const second = { marker: "second" } as never;
+    setProcessPluginWorkerManager(first);
+    setProcessPluginWorkerManager(second);
+    expect(getProcessPluginWorkerManager()).toBe(second);
+  });
+
+  it("treats an undefined registration as no manager rather than throwing", () => {
+    // Contexts that legitimately have no worker (migrations, CLI entrypoints)
+    // must keep working; the sandbox driver produces the accurate error instead.
+    setProcessPluginWorkerManager(undefined);
+    expect(getProcessPluginWorkerManager()).toBeUndefined();
+  });
+});
+
+describe("sandbox driver manager resolution", () => {
+  afterEach(() => {
+    clearProcessPluginWorkerManager();
+  });
+
+  it("prefers an explicitly passed manager over the process registration", () => {
+    // The explicit parameter has to keep winning, because tests inject their own
+    // manager and must not be affected by whatever the process registered.
+    const registered = { marker: "registered" } as never;
+    const explicit = { marker: "explicit" } as never;
+    setProcessPluginWorkerManager(registered);
+
+    const resolve = (options: { pluginWorkerManager?: unknown }) =>
+      options.pluginWorkerManager ?? getProcessPluginWorkerManager();
+
+    expect(resolve({ pluginWorkerManager: explicit })).toBe(explicit);
+    expect(resolve({})).toBe(registered);
+  });
+});
diff --git a/server/src/app.ts b/server/src/app.ts
index 32e3d6e2ad7..add199ed54c 100644
--- a/server/src/app.ts
+++ b/server/src/app.ts
@@ -64,6 +64,7 @@ import { applyUiBranding, BRAND_DIR_PUBLIC_PATH, getBrandDir } from "./ui-brandi
 import { logger } from "./middleware/logger.js";
 import { DEFAULT_LOCAL_PLUGIN_DIR, pluginLoader } from "./services/plugin-loader.js";
 import { createPluginWorkerManager, type PluginWorkerManager } from "./services/plugin-worker-manager.js";
+import { setProcessPluginWorkerManager } from "./services/plugin-worker-process-registry.js";
 import { createPluginJobScheduler } from "./services/plugin-job-scheduler.js";
 import { pluginJobStore } from "./services/plugin-job-store.js";
 import { createPluginToolDispatcher } from "./services/plugin-tool-dispatcher.js";
@@ -239,6 +240,11 @@ export async function createApp(
 
   const hostServicesDisposers = new Map<string, () => void>();
   const workerManager = opts.pluginWorkerManager ?? createPluginWorkerManager();
+  // Register the manager for this process so that any code path which dispatches
+  // a run can find it without being handed it explicitly. Without this, a
+  // construction site that omits the manager builds a run engine that can never
+  // acquire a sandbox lease, and it only shows up when a customer hits that path.
+  setProcessPluginWorkerManager(workerManager);
 
   // Mount API routes
   const api = Router();
diff --git a/server/src/services/environment-runtime.ts b/server/src/services/environment-runtime.ts
index dfca69817c4..481c43590ad 100644
--- a/server/src/services/environment-runtime.ts
+++ b/server/src/services/environment-runtime.ts
@@ -40,6 +40,7 @@ import {
 } from "./sandbox-provider-runtime.js";
 import { pluginRegistryService } from "./plugin-registry.js";
 import type { PluginWorkerManager } from "./plugin-worker-manager.js";
+import { getProcessPluginWorkerManager } from "./plugin-worker-process-registry.js";
 import type { PluginStreamBus } from "./plugin-stream-bus.js";
 import {
   destroyPluginEnvironmentLease,
@@ -730,7 +731,15 @@ function createSandboxEnvironmentDriver(
     pluginWorkerReadyPollMs?: number;
   } = {},
 ): EnvironmentRuntimeDriver {
-  const pluginWorkerManager = options.pluginWorkerManager;
+  // Fall back to the process-scoped manager when a caller did not pass one.
+  // Every path that dispatches a run funnels through here, so this single
+  // fallback covers all of them: the six runtime construction sites, and the
+  // `heartbeatService(db)` calls that pass no options at all (which is how
+  // `assignment` and `automation` dispatches used to reach a manager-less
+  // runtime and fail with "sandbox plugin workers are unavailable in this
+  // server process"). An explicitly passed manager still wins, so tests can
+  // inject their own.
+  const pluginWorkerManager = options.pluginWorkerManager ?? getProcessPluginWorkerManager();
   const pluginWorkerReadyTimeoutMs = options.pluginWorkerReadyTimeoutMs ?? DEFAULT_PLUGIN_SANDBOX_WORKER_READY_TIMEOUT_MS;
   const pluginWorkerReadyPollMs = options.pluginWorkerReadyPollMs ?? DEFAULT_PLUGIN_SANDBOX_WORKER_READY_POLL_MS;
   const environmentsSvc = environmentService(db);
diff --git a/server/src/services/plugin-worker-process-registry.ts b/server/src/services/plugin-worker-process-registry.ts
new file mode 100644
index 00000000000..9209a68c300
--- /dev/null
+++ b/server/src/services/plugin-worker-process-registry.ts
@@ -0,0 +1,52 @@
+import type { PluginWorkerManager } from "./plugin-worker-manager.js";
+
+/**
+ * Process-scoped access to the plugin worker manager.
+ *
+ * There is exactly ONE plugin worker manager per server process, which is why a
+ * run that cannot find one fails with "sandbox plugin workers are unavailable in
+ * this server process". Despite being process-scoped, it used to be threaded as
+ * an OPTIONAL parameter through six runtime construction sites and dozens of
+ * route mounts, so any caller that omitted it built a run engine that could
+ * never acquire a sandbox lease. The omission only surfaced when a customer hit
+ * that particular path, and it recurred three times:
+ *
+ *   - routes mounted without it (fixed per-route in #304)
+ *   - `heartbeatService(db)` called with no options at all, which is how
+ *     `assignment` and `automation` dispatches reached a manager-less runtime
+ *
+ * Registering it once at boot removes the whole class: a caller can still pass
+ * one explicitly (tests do), but forgetting to is no longer a failure mode.
+ *
+ * Deliberately not a general service locator. It holds this one process-scoped
+ * dependency, because the alternative was making it a required parameter on
+ * every construction site, which is both a much larger change and still
+ * bypassable with `undefined as any`.
+ */
+let processPluginWorkerManager: PluginWorkerManager | undefined;
+
+/**
+ * Register the manager for this process. Called once during app construction.
+ * Idempotent: registering the same manager twice is a no-op, and replacing it
+ * is allowed so a test harness can install its own.
+ */
+export function setProcessPluginWorkerManager(manager: PluginWorkerManager | undefined): void {
+  processPluginWorkerManager = manager;
+}
+
+/**
+ * The manager for this process, or undefined when nothing has registered one.
+ *
+ * Returning undefined rather than throwing is deliberate: contexts that
+ * legitimately have no worker (migrations, CLI entrypoints, unit tests that
+ * never dispatch a run) must keep working, and the sandbox driver already
+ * produces an accurate, non-retryable error when it genuinely has no manager.
+ */
+export function getProcessPluginWorkerManager(): PluginWorkerManager | undefined {
+  return processPluginWorkerManager;
+}
+
+/** Test hook: drop the registration so a test can assert the no-manager path. */
+export function clearProcessPluginWorkerManager(): void {
+  processPluginWorkerManager = undefined;
+}