diff --git a/AGENTS.md b/AGENTS.md
index 8bfc5fd17d..d9e2f87f5f 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -56,6 +56,7 @@
 
 - Only write high quality tests that verify where there could be potential bugs. Avoid testing trivial getters/setters, framework wiring, or other code that is unlikely to break.
 - Pipe slow test output to a file, then read the file. Example: `pnpm exec turbo run test --filter=@bb/integration-tests --force > /tmp/test-out.txt 2>&1`.
+- Package `vitest.config.ts` files build their `projects` with `sharedWorkerProjects` from `vitest.shared.ts`. It runs node-environment test files in shared workers (`isolate: false`) and gives a file its own worker when it runs in a DOM environment (`jsdom`) or when the file, or a test helper it imports, mutates worker-global state (`vi.mock`, `vi.stubGlobal`, `process.env`, `globalThis.*` assignments, `Object.defineProperty` on a global). Re-importing the module graph per file was 80–90% of the big suites' CPU. Restore what a test changes anyway; the scan is a safety net, not a license.
 
 ## GitHub Issues And Pull Requests
 
diff --git a/apps/app/vitest.config.ts b/apps/app/vitest.config.ts
index 3082049898..4a1f12347f 100644
--- a/apps/app/vitest.config.ts
+++ b/apps/app/vitest.config.ts
@@ -1,5 +1,8 @@
 import path from "path";
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 import react from "@vitejs/plugin-react";
 import tailwindcss from "@tailwindcss/vite";
 import { sharedUiEnvSeam } from "./vite-shared-ui-seam.js";
@@ -14,8 +17,16 @@ export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
     environment: "node",
-    include: ["src/**/*.test.ts", "src/**/*.test.tsx"],
     setupFiles: ["src/test/setup.ts"],
     testTimeout: 15_000,
+    // Per-file module-graph import and setup were ~85% of this suite's CPU.
+    // Node-environment files that do not mock share a worker context; jsdom
+    // files keep their own worker (see vitest.shared.ts).
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      aliases: { "@": path.resolve(__dirname, "./src") },
+      name: "@bb/app",
+      include: ["src/**/*.test.ts", "src/**/*.test.tsx"],
+    }),
   },
 });
diff --git a/apps/cli/vitest.config.ts b/apps/cli/vitest.config.ts
index 3f13ba1a19..3649ce826d 100644
--- a/apps/cli/vitest.config.ts
+++ b/apps/cli/vitest.config.ts
@@ -1,13 +1,19 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
-    name: "@bb/cli",
-    exclude: ["dist/**", "node_modules/**"],
     env: {
       BB_SERVER_URL: "http://127.0.0.1:49161",
       BB_HOST_DAEMON_PORT: "49162",
     },
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "@bb/cli",
+      include: ["src/**/*.test.ts"],
+    }),
   },
 });
diff --git a/apps/desktop/vitest.config.ts b/apps/desktop/vitest.config.ts
index a479ab4b02..1135e8b001 100644
--- a/apps/desktop/vitest.config.ts
+++ b/apps/desktop/vitest.config.ts
@@ -1,9 +1,15 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   test: {
     environment: "node",
-    include: ["test/**/*.test.ts"],
-    name: "@bb/desktop",
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "@bb/desktop",
+      include: ["test/**/*.test.ts"],
+    }),
   },
 });
diff --git a/apps/host-daemon/vitest.config.ts b/apps/host-daemon/vitest.config.ts
index 9b7bdc85de..c92035d1f8 100644
--- a/apps/host-daemon/vitest.config.ts
+++ b/apps/host-daemon/vitest.config.ts
@@ -1,16 +1,21 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
-    name: "@bb/host-daemon",
-    include: ["src/**/*.test.ts", "test/**/*.test.ts"],
-    exclude: ["dist/**", "node_modules/**"],
     env: {
       BB_DATA_DIR: "/tmp/bb-host-daemon-test",
       BB_SERVER_URL: "http://127.0.0.1:49161",
       BB_HOST_DAEMON_PORT: "49162",
     },
     testTimeout: 15_000,
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "@bb/host-daemon",
+      include: ["src/**/*.test.ts", "test/**/*.test.ts"],
+    }),
   },
 });
diff --git a/apps/mobile/vitest.config.ts b/apps/mobile/vitest.config.ts
index b798123a6d..25422dbbaa 100644
--- a/apps/mobile/vitest.config.ts
+++ b/apps/mobile/vitest.config.ts
@@ -1,5 +1,8 @@
 import path from "node:path";
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 // Pure-logic tests only (node environment). Screen behavior is covered by
 // Maestro flows under e2e/flows. Modules under test must not import
@@ -13,8 +16,13 @@ export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
     environment: "node",
-    include: ["src/**/*.test.ts"],
     passWithNoTests: true,
     testTimeout: 15_000,
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      aliases: { "@": path.resolve(__dirname, "./src") },
+      name: "@bb/mobile",
+      include: ["src/**/*.test.ts"],
+    }),
   },
 });
diff --git a/apps/server/test/helpers/provider-registry.ts b/apps/server/test/helpers/provider-registry.ts
index 90e5280b86..d70f512c76 100644
--- a/apps/server/test/helpers/provider-registry.ts
+++ b/apps/server/test/helpers/provider-registry.ts
@@ -177,34 +177,55 @@ export function stubHostArtifact(pluginId: string): PluginHostArtifactSnapshot {
   };
 }
 
+const firstPartyBridgeArtifactBuilds = new Map<
+  string,
+  Promise<PluginHostArtifactSnapshot | null>
+>();
+
+async function buildFirstPartyBridgeArtifact(
+  pluginId: string,
+): Promise<PluginHostArtifactSnapshot | null> {
+  const rootDir = pluginRootDir(pluginId);
+  // Pi has no `bb.host`: its bridge stays in the daemon bundle.
+  if (!(await hasHostEntry(rootDir))) {
+    return null;
+  }
+  const toolchain = await resolvePluginBuildToolchain(
+    join(tmpdir(), "bb-plugin-build-toolchain"),
+  );
+  const build = await buildPluginHost(rootDir, "0.0.0-test", toolchain);
+  const bytes = await readFile(build.jsPath);
+  return {
+    digest: build.artifactDigest,
+    byteLength: bytes.byteLength,
+    path: build.jsPath,
+    generation: `test-${pluginId}`,
+  };
+}
+
 /**
  * Builds and records the first-party provider bridge artifacts, exactly as the
  * plugin runtime does on load. Without this a graduated provider has no
  * `bridgeLaunch`, so the daemon has no bridge for it at all — which is the
  * whole point of the artifact route and therefore worth exercising rather
  * than stubbing. Bridges are rebuilt from source so a stale `dist/` cannot
- * make a test pass against yesterday's bridge.
+ * make a test pass against yesterday's bridge — once per worker process:
+ * the sources do not change during a run, and the ~0.6s esbuild pass was
+ * paid by every integration harness, one per test.
  */
 export async function recordFirstPartyProviderBridgeArtifacts(
   artifacts: PluginHostArtifactRegistry,
 ): Promise<void> {
-  const toolchain = await resolvePluginBuildToolchain(
-    join(tmpdir(), "bb-plugin-build-toolchain"),
-  );
   for (const pluginId of FIRST_PARTY_PROVIDER_PLUGIN_IDS) {
-    const rootDir = pluginRootDir(pluginId);
-    // Pi has no `bb.host`: its bridge stays in the daemon bundle.
-    if (!(await hasHostEntry(rootDir))) {
-      continue;
+    let build = firstPartyBridgeArtifactBuilds.get(pluginId);
+    if (!build) {
+      build = buildFirstPartyBridgeArtifact(pluginId);
+      firstPartyBridgeArtifactBuilds.set(pluginId, build);
+    }
+    const snapshot = await build;
+    if (snapshot !== null) {
+      artifacts.set(pluginId, snapshot);
     }
-    const build = await buildPluginHost(rootDir, "0.0.0-test", toolchain);
-    const bytes = await readFile(build.jsPath);
-    artifacts.set(pluginId, {
-      digest: build.artifactDigest,
-      byteLength: bytes.byteLength,
-      path: build.jsPath,
-      generation: `test-${pluginId}`,
-    });
   }
 }
 
diff --git a/apps/server/test/helpers/test-app.ts b/apps/server/test/helpers/test-app.ts
index 7189968047..912d2b3da8 100644
--- a/apps/server/test/helpers/test-app.ts
+++ b/apps/server/test/helpers/test-app.ts
@@ -3,7 +3,7 @@ import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { serve } from "@hono/node-server";
 import type { AddressInfo } from "node:net";
-import type { DbConnection } from "@bb/db";
+import { createConnection, type DbConnection } from "@bb/db";
 import { defaultFeatureFlags, type HostType } from "@bb/domain";
 import { initDb } from "../../src/db.js";
 import { createApp } from "../../src/server.js";
@@ -119,6 +119,22 @@ export function createTestDaemonHostKey(
   });
 }
 
+let migratedTemplate: Buffer | null = null;
+
+/**
+ * A fresh in-memory database with every migration applied and the personal
+ * project seeded, exactly as `initDb` leaves it. The first call migrates for
+ * real and keeps the serialized image; every later call opens an independent
+ * copy of that image. Replaying the 100+ migrations was ~57ms of the ~61ms a
+ * harness cost, paid by nearly two thousand tests.
+ */
+export function createTestDb(): DbConnection {
+  if (migratedTemplate === null) {
+    migratedTemplate = initDb(":memory:").$client.serialize();
+  }
+  return createConnection(migratedTemplate);
+}
+
 export async function createTestAppHarness(
   overrides: TestAppHarnessConfigOverrides = {},
 ): Promise<TestAppHarness> {
@@ -129,7 +145,7 @@ export async function createTestAppHarness(
     ...configOverrides
   } = overrides;
   const dataDir = await mkdtemp(join(tmpdir(), "bb-server-test-"));
-  const db = initDb(":memory:");
+  const db = createTestDb();
   const hub = new NotificationHubImpl();
   const watchInterests = new WatchInterestCoordinator({ db, hub });
   const sharedPorts = new HostSharedPortCoordinator({ db, hub });
diff --git a/apps/server/test/internal/internal-skill-trees.test.ts b/apps/server/test/internal/internal-skill-trees.test.ts
index fedeb305a1..3332f8fa3a 100644
--- a/apps/server/test/internal/internal-skill-trees.test.ts
+++ b/apps/server/test/internal/internal-skill-trees.test.ts
@@ -1,4 +1,4 @@
-import { mkdir, writeFile } from "node:fs/promises";
+import { chmod, mkdir, writeFile } from "node:fs/promises";
 import path from "node:path";
 import { describe, expect, it } from "vitest";
 import { readSkillTreeManifest } from "../../src/services/skills/injected-skills.js";
@@ -13,6 +13,9 @@ describe("internal skill tree routes", () => {
       const rootPath = path.join(harness.config.dataDir, "tree-route-skill");
       await mkdir(rootPath, { recursive: true });
       await writeFile(path.join(rootPath, "SKILL.md"), "tree route bytes\n");
+      // The manifest reports the on-disk mode; pin it so the process umask
+      // cannot change the expected entry.
+      await chmod(path.join(rootPath, "SKILL.md"), 0o644);
       const manifest = readSkillTreeManifest(rootPath);
       harness.deps.skillTreeRegistry.register(manifest.treeHash, rootPath);
 
diff --git a/apps/server/vitest.config.ts b/apps/server/vitest.config.ts
index 8638b2d567..720e91e7c8 100644
--- a/apps/server/vitest.config.ts
+++ b/apps/server/vitest.config.ts
@@ -1,10 +1,8 @@
 import {
   defineWorkspaceTestConfig,
-  findIsolationRequiringTests,
+  sharedWorkerProjects,
 } from "../../vitest.shared.js";
 
-const isolationTests = findIsolationRequiringTests(__dirname, ["src", "test"]);
-
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
@@ -13,23 +11,10 @@ export default defineWorkspaceTestConfig({
       BB_SERVER_PORT: "49161",
       BB_HOST_DAEMON_PORT: "49162",
     },
-    projects: [
-      {
-        extends: true,
-        test: {
-          name: "@bb/server",
-          include: ["src/**/*.test.ts", "test/**/*.test.ts"],
-          exclude: ["dist/**", "node_modules/**", ...isolationTests],
-          isolate: false,
-        },
-      },
-      {
-        extends: true,
-        test: {
-          name: "@bb/server:isolated",
-          include: isolationTests,
-        },
-      },
-    ],
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "@bb/server",
+      include: ["src/**/*.test.ts", "test/**/*.test.ts"],
+    }),
   },
 });
diff --git a/apps/web/vitest.config.ts b/apps/web/vitest.config.ts
index d66ff13049..82261e9da9 100644
--- a/apps/web/vitest.config.ts
+++ b/apps/web/vitest.config.ts
@@ -1,13 +1,16 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
-  // vite.config.ts injects this from the target deployment's APP_URL; tests
-  // load modules without that config, so they get an obviously-not-real origin.
   define: { __SITE_ORIGIN__: JSON.stringify("https://web.test") },
   test: {
     silent: "passed-only",
-    name: "@bb/web",
-    include: ["src/**/*.test.ts"],
-    exclude: ["dist/**", "node_modules/**"],
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "@bb/web",
+      include: ["src/**/*.test.ts"],
+    }),
   },
 });
diff --git a/packages/agent-runtime/src/pi/bridge/__tests__/sdk-session.test.ts b/packages/agent-runtime/src/pi/bridge/__tests__/sdk-session.test.ts
index 78c3094f4c..82f685ded7 100644
--- a/packages/agent-runtime/src/pi/bridge/__tests__/sdk-session.test.ts
+++ b/packages/agent-runtime/src/pi/bridge/__tests__/sdk-session.test.ts
@@ -1003,37 +1003,66 @@ describe("PiSdkSession", () => {
     );
   });
 
+  /**
+   * Each transient auth miss waits a real 250ms before the retry; drive the
+   * clock instead of sleeping through eight of them per test.
+   */
+  async function settleThroughTransientAuthRetries<T>(
+    pending: Promise<T>,
+    retries: number,
+  ): Promise<T> {
+    for (let i = 0; i < retries; i += 1) {
+      await vi.advanceTimersByTimeAsync(250);
+    }
+    return pending;
+  }
+
   it("allows eight transient Pi auth storage misses before succeeding", async () => {
-    const authError = new Error("No API key found for anthropic.");
-    rejectPromptWithTransientAuthError(8, authError);
-    mockPrompt.mockResolvedValueOnce(undefined);
-    const onDone = vi.fn();
-    const session = new PiSdkSession({ cwd: "/tmp/project" }, vi.fn(), onDone);
+    vi.useFakeTimers();
+    try {
+      const authError = new Error("No API key found for anthropic.");
+      rejectPromptWithTransientAuthError(8, authError);
+      mockPrompt.mockResolvedValueOnce(undefined);
+      const onDone = vi.fn();
+      const session = new PiSdkSession({ cwd: "/tmp/project" }, vi.fn(), onDone);
 
-    await session.start();
-    await session.prompt("retry after auth storage miss").settled;
+      await session.start();
+      await settleThroughTransientAuthRetries(
+        session.prompt("retry after auth storage miss").settled,
+        8,
+      );
 
-    expect(mockPrompt).toHaveBeenCalledTimes(9);
-    expect(onDone).not.toHaveBeenCalled();
+      expect(mockPrompt).toHaveBeenCalledTimes(9);
+      expect(onDone).not.toHaveBeenCalled();
+    } finally {
+      vi.useRealTimers();
+    }
   });
 
   it("fails after nine transient Pi auth storage misses", async () => {
-    const authError = new Error("No API key found for anthropic.");
-    rejectPromptWithTransientAuthError(9, authError);
-    const onDone = vi.fn();
-    const session = new PiSdkSession({ cwd: "/tmp/project" }, vi.fn(), onDone);
+    vi.useFakeTimers();
+    try {
+      const authError = new Error("No API key found for anthropic.");
+      rejectPromptWithTransientAuthError(9, authError);
+      const onDone = vi.fn();
+      const session = new PiSdkSession({ cwd: "/tmp/project" }, vi.fn(), onDone);
 
-    await session.start();
-    const dispatch = session.prompt("fail after retry budget");
-    void dispatch.consumed.catch(() => undefined);
-    await expect(dispatch.settled).resolves.toEqual({ error: authError });
-    await expect(dispatch.consumed).rejects.toThrow(
-      "No API key found for anthropic.",
-    );
+      await session.start();
+      const dispatch = session.prompt("fail after retry budget");
+      void dispatch.consumed.catch(() => undefined);
+      await expect(
+        settleThroughTransientAuthRetries(dispatch.settled, 8),
+      ).resolves.toEqual({ error: authError });
+      await expect(dispatch.consumed).rejects.toThrow(
+        "No API key found for anthropic.",
+      );
 
-    expect(mockPrompt).toHaveBeenCalledTimes(9);
-    expect(onDone).toHaveBeenCalledTimes(1);
-    expect(onDone).toHaveBeenCalledWith(authError);
+      expect(mockPrompt).toHaveBeenCalledTimes(9);
+      expect(onDone).toHaveBeenCalledTimes(1);
+      expect(onDone).toHaveBeenCalledWith(authError);
+    } finally {
+      vi.useRealTimers();
+    }
   });
 
   it("stays processing across retryable agent-end events", async () => {
diff --git a/packages/agent-runtime/vitest.config.ts b/packages/agent-runtime/vitest.config.ts
index 0215da0789..5b50089d6d 100644
--- a/packages/agent-runtime/vitest.config.ts
+++ b/packages/agent-runtime/vitest.config.ts
@@ -1,11 +1,8 @@
 import {
   defineWorkspaceTestConfig,
-  findIsolationRequiringTests,
+  sharedWorkerProjects,
 } from "../../vitest.shared.js";
 
-const exclude = ["dist/**", "node_modules/**", "src/integration*.test.ts"];
-const isolationTests = findIsolationRequiringTests(__dirname, ["src"]);
-
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
@@ -15,24 +12,11 @@ export default defineWorkspaceTestConfig({
     // Both projects below extend this root, so the cap applies to each.
     testTimeout: 15_000,
     hookTimeout: 15_000,
-    projects: [
-      {
-        extends: true,
-        test: {
-          name: "@bb/agent-runtime",
-          include: ["src/**/*.test.ts"],
-          exclude: [...exclude, ...isolationTests],
-          isolate: false,
-        },
-      },
-      {
-        extends: true,
-        test: {
-          name: "@bb/agent-runtime:isolated",
-          include: isolationTests,
-          exclude,
-        },
-      },
-    ],
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "@bb/agent-runtime",
+      include: ["src/**/*.test.ts"],
+      exclude: ["dist/**", "node_modules/**", "src/integration*.test.ts"],
+    }),
   },
 });
diff --git a/packages/client-core/vitest.config.ts b/packages/client-core/vitest.config.ts
index d5e14c37c3..4bd16a812f 100644
--- a/packages/client-core/vitest.config.ts
+++ b/packages/client-core/vitest.config.ts
@@ -1,11 +1,16 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
-    name: "@bb/client-core",
     environment: "node",
-    include: ["test/**/*.test.ts"],
-    exclude: ["dist/**", "node_modules/**"],
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "@bb/client-core",
+      include: ["test/**/*.test.ts"],
+    }),
   },
 });
diff --git a/packages/db/src/connection.ts b/packages/db/src/connection.ts
index a4d83d2eb6..4c3a6b0c32 100644
--- a/packages/db/src/connection.ts
+++ b/packages/db/src/connection.ts
@@ -161,11 +161,17 @@ function instrumentSqliteClient(
   });
 }
 
+/**
+ * Opens the database at `source`. A path (or `:memory:`) opens a file or a
+ * fresh in-memory database; a `Buffer` opens an in-memory copy of a
+ * serialized database image (`db.$client.serialize()`), which is how the test
+ * harnesses clone a migrated template instead of replaying every migration.
+ */
 export function createConnection(
-  dbPath: string = "bb.db",
+  source: string | Buffer = "bb.db",
   options: CreateConnectionOptions = {},
 ) {
-  const sqlite = new Database(dbPath);
+  const sqlite = new Database(source);
 
   // Reclaim freed pages incrementally (via PRAGMA incremental_vacuum in the
   // periodic maintenance sweep) instead of relying on a full-file VACUUM. This
diff --git a/packages/db/test/data/app-settings.test.ts b/packages/db/test/data/app-settings.test.ts
index c3c4e223b2..e586c668e2 100644
--- a/packages/db/test/data/app-settings.test.ts
+++ b/packages/db/test/data/app-settings.test.ts
@@ -1,21 +1,19 @@
 import { afterEach, beforeEach, describe, expect, it } from "vitest";
 import { defaultAppSettings } from "@bb/domain";
 import {
-  createConnection,
   getAppKeybindingOverrides,
   getAppSettings,
-  migrate,
   setAppKeybindingOverrides,
   setAppSettings,
   type DbConnection,
 } from "../../src/index.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 describe("app settings data", () => {
   let db: DbConnection;
 
   beforeEach(() => {
-    db = createConnection(":memory:");
-    migrate(db);
+    db = createMigratedConnection();
   });
 
   afterEach(() => {
diff --git a/packages/db/test/data/environment-lifecycle.test.ts b/packages/db/test/data/environment-lifecycle.test.ts
index 37bea65160..12817b15df 100644
--- a/packages/db/test/data/environment-lifecycle.test.ts
+++ b/packages/db/test/data/environment-lifecycle.test.ts
@@ -1,8 +1,6 @@
 import { describe, expect, it, vi } from "vitest";
 import { eq } from "drizzle-orm";
-import { createConnection } from "../../src/connection.js";
 import type { DbTransaction } from "../../src/connection.js";
-import { migrate } from "../../src/migrate.js";
 import { noopNotifier } from "../../src/notifier.js";
 import type { DbNotifier } from "../../src/notifier.js";
 import { environments, threads } from "../../src/schema.js";
@@ -26,10 +24,10 @@ import {
 import { createProject } from "../../src/data/projects.js";
 import { upsertHost } from "../../src/data/hosts.js";
 import { withWriteAfterFirstRead } from "../helpers/interleave.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 function setup() {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   const host = upsertHost(db, noopNotifier, {
     name: "test-host",
     type: "persistent",
diff --git a/packages/db/test/data/environments.test.ts b/packages/db/test/data/environments.test.ts
index 1370d09e52..e4db3bb7e9 100644
--- a/packages/db/test/data/environments.test.ts
+++ b/packages/db/test/data/environments.test.ts
@@ -1,6 +1,4 @@
 import { describe, expect, it, vi } from "vitest";
-import { createConnection } from "../../src/connection.js";
-import { migrate } from "../../src/migrate.js";
 import { noopNotifier } from "../../src/notifier.js";
 import type { DbNotifier } from "../../src/notifier.js";
 import {
@@ -12,10 +10,10 @@ import {
 } from "../../src/data/environments.js";
 import { createProject } from "../../src/data/projects.js";
 import { upsertHost } from "../../src/data/hosts.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 function setup() {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   const host = upsertHost(db, noopNotifier, {
     name: "test-host",
     type: "persistent",
diff --git a/packages/db/test/data/events.test.ts b/packages/db/test/data/events.test.ts
index 3586a97e95..1465b3418a 100644
--- a/packages/db/test/data/events.test.ts
+++ b/packages/db/test/data/events.test.ts
@@ -8,8 +8,6 @@ import {
   turnScope,
   type PromptInput,
 } from "@bb/domain";
-import { createConnection } from "../../src/connection.js";
-import { migrate } from "../../src/migrate.js";
 import { noopNotifier } from "../../src/notifier.js";
 import type { DbNotifier } from "../../src/notifier.js";
 import {
@@ -63,10 +61,10 @@ import { createEnvironment } from "../../src/data/environments.js";
 import { createProject } from "../../src/data/projects.js";
 import { createThread } from "../../src/data/threads.js";
 import { upsertHost } from "../../src/data/hosts.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 function setup() {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   const host = upsertHost(db, noopNotifier, {
     name: "test-host",
     type: "persistent",
@@ -4402,8 +4400,7 @@ describe("events", () => {
   });
 
   it("lists the latest lifecycle row per open backgroundTask item on a host", () => {
-    const db = createConnection(":memory:");
-    migrate(db);
+    const db = createMigratedConnection();
     const host = upsertHost(db, noopNotifier, {
       name: "task-host",
       type: "persistent",
diff --git a/packages/db/test/data/hosts.test.ts b/packages/db/test/data/hosts.test.ts
index 49b9ac4c07..36ba3fa47b 100644
--- a/packages/db/test/data/hosts.test.ts
+++ b/packages/db/test/data/hosts.test.ts
@@ -1,6 +1,4 @@
 import { describe, expect, it, vi } from "vitest";
-import { createConnection } from "../../src/connection.js";
-import { migrate } from "../../src/migrate.js";
 import { noopNotifier } from "../../src/notifier.js";
 import {
   deleteHost,
@@ -13,10 +11,10 @@ import {
   updateHost,
   upsertHost,
 } from "../../src/data/hosts.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 function setup() {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   return { db };
 }
 
diff --git a/packages/db/test/data/maintenance.test.ts b/packages/db/test/data/maintenance.test.ts
index 6965006502..3d7fc5b878 100644
--- a/packages/db/test/data/maintenance.test.ts
+++ b/packages/db/test/data/maintenance.test.ts
@@ -23,6 +23,7 @@ import {
 import { upsertHost } from "../../src/data/hosts.js";
 import { createProject } from "../../src/data/projects.js";
 import { createThread, markThreadDeleted } from "../../src/data/threads.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 const TEST_INCREMENTAL_VACUUM_MAX_PAGES = 128;
 
@@ -49,8 +50,7 @@ const TEST_DEFERRED_LEGACY_TABLE_NAMES = [
 ];
 
 function setup() {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   const host = upsertHost(db, noopNotifier, {
     name: "maintenance-host",
     type: "persistent",
diff --git a/packages/db/test/data/pending-interactions.test.ts b/packages/db/test/data/pending-interactions.test.ts
index a5372826f8..2577050750 100644
--- a/packages/db/test/data/pending-interactions.test.ts
+++ b/packages/db/test/data/pending-interactions.test.ts
@@ -1,6 +1,4 @@
 import { describe, expect, it } from "vitest";
-import { createConnection } from "../../src/connection.js";
-import { migrate } from "../../src/migrate.js";
 import { noopNotifier } from "../../src/notifier.js";
 import { createEnvironment } from "../../src/data/environments.js";
 import { upsertHost } from "../../src/data/hosts.js";
@@ -15,10 +13,10 @@ import {
   setPendingInteractionResolved,
 } from "../../src/data/pending-interactions.js";
 import { createThread } from "../../src/data/threads.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 function setup() {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   const host = upsertHost(db, noopNotifier, {
     name: "test-host",
     type: "persistent",
diff --git a/packages/db/test/data/plugin-persistence.test.ts b/packages/db/test/data/plugin-persistence.test.ts
index 44fc3441da..37bcdfe63e 100644
--- a/packages/db/test/data/plugin-persistence.test.ts
+++ b/packages/db/test/data/plugin-persistence.test.ts
@@ -1,24 +1,22 @@
 import { afterEach, beforeEach, describe, expect, it } from "vitest";
 import {
-  createConnection,
   createPluginArtifact,
   deletePluginArtifact,
   deleteInstalledPlugin,
   getInstalledPluginRegistration,
   getInstalledPlugin,
   listPluginArtifacts,
-  migrate,
   upsertInstalledPlugin,
   type DbConnection,
 } from "../../src/index.js";
 import type { UpsertInstalledPluginInput } from "../../src/data/plugins.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 describe("normalized plugin persistence", () => {
   let db: DbConnection;
 
   beforeEach(() => {
-    db = createConnection(":memory:");
-    migrate(db);
+    db = createMigratedConnection();
   });
 
   afterEach(() => db.$client.close());
diff --git a/packages/db/test/data/project-execution-defaults.test.ts b/packages/db/test/data/project-execution-defaults.test.ts
index 5491647217..851cfcfce4 100644
--- a/packages/db/test/data/project-execution-defaults.test.ts
+++ b/packages/db/test/data/project-execution-defaults.test.ts
@@ -1,6 +1,4 @@
 import { describe, expect, it } from "vitest";
-import { createConnection } from "../../src/connection.js";
-import { migrate } from "../../src/migrate.js";
 import { noopNotifier } from "../../src/notifier.js";
 import { deleteProject, createProject } from "../../src/data/projects.js";
 import {
@@ -8,10 +6,10 @@ import {
   upsertProjectExecutionDefaults,
 } from "../../src/data/project-execution-defaults.js";
 import { upsertHost } from "../../src/data/hosts.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 function setup() {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   const host = upsertHost(db, noopNotifier, {
     name: "defaults-host",
     type: "persistent",
diff --git a/packages/db/test/data/project-sources.test.ts b/packages/db/test/data/project-sources.test.ts
index fd4cf66802..7436c7d11d 100644
--- a/packages/db/test/data/project-sources.test.ts
+++ b/packages/db/test/data/project-sources.test.ts
@@ -1,7 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { createConnection } from "../../src/connection.js";
 import { createProjectSourceId } from "../../src/ids.js";
-import { migrate } from "../../src/migrate.js";
 import { noopNotifier } from "../../src/notifier.js";
 import { projectSources } from "../../src/schema.js";
 import {
@@ -17,10 +15,10 @@ import {
 } from "../../src/data/project-sources.js";
 import { createProject } from "../../src/data/projects.js";
 import { upsertHost } from "../../src/data/hosts.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 function setup() {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   const host = upsertHost(db, noopNotifier, {
     name: "test-host",
     type: "persistent",
diff --git a/packages/db/test/data/projects.test.ts b/packages/db/test/data/projects.test.ts
index bc2cf8c9c2..3c6f370adc 100644
--- a/packages/db/test/data/projects.test.ts
+++ b/packages/db/test/data/projects.test.ts
@@ -1,7 +1,5 @@
 import { describe, expect, it } from "vitest";
 import { PERSONAL_PROJECT_ID } from "@bb/domain";
-import { createConnection } from "../../src/connection.js";
-import { migrate } from "../../src/migrate.js";
 import { noopNotifier } from "../../src/notifier.js";
 import {
   createProject,
@@ -15,10 +13,10 @@ import {
   setProjectGitRemoteUrlIfMissing,
 } from "../../src/data/projects.js";
 import { upsertHost } from "../../src/data/hosts.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 function setup() {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   const host = upsertHost(db, noopNotifier, {
     name: "projects-host",
     type: "persistent",
diff --git a/packages/db/test/data/queued-thread-messages.test.ts b/packages/db/test/data/queued-thread-messages.test.ts
index 28c0418f1d..bbc64d5810 100644
--- a/packages/db/test/data/queued-thread-messages.test.ts
+++ b/packages/db/test/data/queued-thread-messages.test.ts
@@ -1,7 +1,5 @@
 import { describe, expect, it, vi } from "vitest";
 import type { PromptInput } from "@bb/domain";
-import { createConnection } from "../../src/connection.js";
-import { migrate } from "../../src/migrate.js";
 import { noopNotifier } from "../../src/notifier.js";
 import {
   claimNextQueuedThreadMessageGroup,
@@ -21,6 +19,7 @@ import {
 import { createProject } from "../../src/data/projects.js";
 import { createThread } from "../../src/data/threads.js";
 import { upsertHost } from "../../src/data/hosts.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 function textInput(text: string): PromptInput[] {
   return [{ type: "text", text, mentions: [] }];
@@ -30,8 +29,7 @@ const defaultInput = textInput("hello");
 const altInput = textInput("world");
 
 function setup() {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   const host = upsertHost(db, noopNotifier, {
     name: "test-host",
     type: "persistent",
diff --git a/packages/db/test/data/sessions.test.ts b/packages/db/test/data/sessions.test.ts
index 3f70362906..ff1641fd64 100644
--- a/packages/db/test/data/sessions.test.ts
+++ b/packages/db/test/data/sessions.test.ts
@@ -1,7 +1,5 @@
 import { describe, expect, it } from "vitest";
 import { eq } from "drizzle-orm";
-import { createConnection } from "../../src/connection.js";
-import { migrate } from "../../src/migrate.js";
 import { noopNotifier } from "../../src/notifier.js";
 import {
   closeSession,
@@ -13,10 +11,10 @@ import {
 } from "../../src/data/sessions.js";
 import { getHost, upsertHost } from "../../src/data/hosts.js";
 import { hostDaemonSessions } from "../../src/schema.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 function setup() {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   const host = upsertHost(db, noopNotifier, {
     name: "test-host",
     type: "persistent",
diff --git a/packages/db/test/data/sweeps.test.ts b/packages/db/test/data/sweeps.test.ts
index 251e11bd8a..081af51eb0 100644
--- a/packages/db/test/data/sweeps.test.ts
+++ b/packages/db/test/data/sweeps.test.ts
@@ -1,9 +1,7 @@
 import { describe, expect, it, vi } from "vitest";
 import { eq } from "drizzle-orm";
-import { createConnection } from "../../src/connection.js";
 import type { DbConnection } from "../../src/connection.js";
 import { createEventId } from "../../src/ids.js";
-import { migrate } from "../../src/migrate.js";
 import { noopNotifier } from "../../src/notifier.js";
 import type { DbNotifier } from "../../src/notifier.js";
 import {
@@ -31,10 +29,10 @@ import {
   events,
   hostDaemonSessions,
 } from "../../src/schema.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 function setup() {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   const host = upsertHost(db, noopNotifier, {
     name: "test-host",
     type: "persistent",
diff --git a/packages/db/test/data/terminal-sessions.test.ts b/packages/db/test/data/terminal-sessions.test.ts
index eff92cb201..634dee472d 100644
--- a/packages/db/test/data/terminal-sessions.test.ts
+++ b/packages/db/test/data/terminal-sessions.test.ts
@@ -1,6 +1,5 @@
 import { describe, expect, it } from "vitest";
 import { createConnection } from "../../src/connection.js";
-import { migrate } from "../../src/migrate.js";
 import { noopNotifier } from "../../src/notifier.js";
 import {
   createTerminalSession,
@@ -14,6 +13,7 @@ import { upsertHost } from "../../src/data/hosts.js";
 import { createProject } from "../../src/data/projects.js";
 import { openSession } from "../../src/data/sessions.js";
 import { createThread } from "../../src/data/threads.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 type TestDb = ReturnType<typeof createConnection>;
 type TestHost = ReturnType<typeof upsertHost>;
@@ -188,8 +188,7 @@ function openTestSession(db: TestDb, hostId: string): TestSession {
 }
 
 function setup(): TerminalSessionFixture {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   const host = upsertHost(db, noopNotifier, {
     name: "test-host",
     type: "persistent",
diff --git a/packages/db/test/data/thread-lifecycle.test.ts b/packages/db/test/data/thread-lifecycle.test.ts
index 1e6068cd3e..adefa12edf 100644
--- a/packages/db/test/data/thread-lifecycle.test.ts
+++ b/packages/db/test/data/thread-lifecycle.test.ts
@@ -1,8 +1,6 @@
 import { describe, expect, it, vi } from "vitest";
 import { eq } from "drizzle-orm";
-import { createConnection } from "../../src/connection.js";
 import type { DbTransaction } from "../../src/connection.js";
-import { migrate } from "../../src/migrate.js";
 import { noopNotifier } from "../../src/notifier.js";
 import { threads } from "../../src/schema.js";
 import {
@@ -17,10 +15,10 @@ import {
 import { createProject } from "../../src/data/projects.js";
 import { upsertHost } from "../../src/data/hosts.js";
 import { withWriteAfterFirstRead } from "../helpers/interleave.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 function setup() {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   const host = upsertHost(db, noopNotifier, {
     name: "test-host",
     type: "persistent",
diff --git a/packages/db/test/data/thread-search.test.ts b/packages/db/test/data/thread-search.test.ts
index 74691e6f59..7c35a92183 100644
--- a/packages/db/test/data/thread-search.test.ts
+++ b/packages/db/test/data/thread-search.test.ts
@@ -8,7 +8,6 @@ import {
   type PromptInput,
 } from "@bb/domain";
 import { createConnection } from "../../src/connection.js";
-import { migrate } from "../../src/migrate.js";
 import { noopNotifier } from "../../src/notifier.js";
 import {
   appendStoredThreadEvent,
@@ -24,6 +23,7 @@ import {
   updateThread,
   upsertThreadSearchSegments,
 } from "../../src/data/threads.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 interface SetupResult {
   db: ReturnType<typeof createConnection>;
@@ -31,8 +31,7 @@ interface SetupResult {
 }
 
 function setup(): SetupResult {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   const host = upsertHost(db, noopNotifier, {
     name: "test-host",
     type: "persistent",
diff --git a/packages/db/test/data/threads.test.ts b/packages/db/test/data/threads.test.ts
index 6e48cc226c..69349b53fd 100644
--- a/packages/db/test/data/threads.test.ts
+++ b/packages/db/test/data/threads.test.ts
@@ -1,7 +1,6 @@
 import { describe, expect, it, vi } from "vitest";
 import { isRawThreadId } from "@bb/domain";
 import { createConnection } from "../../src/connection.js";
-import { migrate } from "../../src/migrate.js";
 import { noopNotifier } from "../../src/notifier.js";
 import type { DbNotifier } from "../../src/notifier.js";
 import {
@@ -45,10 +44,10 @@ import {
 } from "../../src/data/projects.js";
 import { upsertHost } from "../../src/data/hosts.js";
 import { createEnvironment } from "../../src/data/environments.js";
+import { createMigratedConnection } from "../helpers/migrated-connection.js";
 
 function setup() {
-  const db = createConnection(":memory:");
-  migrate(db);
+  const db = createMigratedConnection();
   const host = upsertHost(db, noopNotifier, {
     name: "test-host",
     type: "persistent",
diff --git a/packages/db/test/helpers/migrated-connection.ts b/packages/db/test/helpers/migrated-connection.ts
new file mode 100644
index 0000000000..6c8136d2df
--- /dev/null
+++ b/packages/db/test/helpers/migrated-connection.ts
@@ -0,0 +1,22 @@
+import { createConnection, migrate } from "../../src/index.js";
+import type { DbConnection } from "../../src/index.js";
+
+let migratedTemplate: Buffer | null = null;
+
+/**
+ * A fresh in-memory database with every migration applied, exactly as
+ * `createConnection(":memory:")` followed by `migrate(db)` leaves it. The
+ * first call migrates for real and keeps the serialized image; every later
+ * call opens an independent copy of that image. Replaying the 100+
+ * migrations costs ~57ms, which the data suites paid once per test.
+ *
+ * Suites that exercise `migrate` itself keep calling it directly.
+ */
+export function createMigratedConnection(): DbConnection {
+  if (migratedTemplate === null) {
+    const db = createConnection(":memory:");
+    migrate(db);
+    migratedTemplate = db.$client.serialize();
+  }
+  return createConnection(migratedTemplate);
+}
diff --git a/packages/db/vitest.config.ts b/packages/db/vitest.config.ts
index 941f66bbdf..0a98abf966 100644
--- a/packages/db/vitest.config.ts
+++ b/packages/db/vitest.config.ts
@@ -1,10 +1,15 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
-    name: "@bb/db",
-    include: ["test/**/*.test.ts"],
-    exclude: ["dist/**", "node_modules/**"],
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "@bb/db",
+      include: ["test/**/*.test.ts"],
+    }),
   },
 });
diff --git a/packages/domain/vitest.config.ts b/packages/domain/vitest.config.ts
index 58e36e1927..c876b0fbad 100644
--- a/packages/domain/vitest.config.ts
+++ b/packages/domain/vitest.config.ts
@@ -1,10 +1,15 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
-    name: "@bb/domain",
-    include: ["test/**/*.test.ts"],
-    exclude: ["dist/**", "node_modules/**"],
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "@bb/domain",
+      include: ["test/**/*.test.ts"],
+    }),
   },
 });
diff --git a/packages/host-workspace/vitest.config.ts b/packages/host-workspace/vitest.config.ts
index b910540e27..7bf9defcc5 100644
--- a/packages/host-workspace/vitest.config.ts
+++ b/packages/host-workspace/vitest.config.ts
@@ -1,15 +1,16 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
-    name: "@bb/host-workspace",
-    include: ["test/**/*.test.ts"],
-    exclude: ["dist/**", "node_modules/**"],
-    // Several tests drive real git subprocesses (concurrent reset/checkout,
-    // stash) that run fast in isolation but can exceed the 5s default under
-    // full-suite CPU contention. Match the 15s used by other subprocess-heavy
-    // packages (@bb/host-daemon, @bb/app, @bb/logger).
     testTimeout: 15_000,
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "@bb/host-workspace",
+      include: ["test/**/*.test.ts"],
+    }),
   },
 });
diff --git a/packages/plugin-build/vitest.config.ts b/packages/plugin-build/vitest.config.ts
index fc6fc9075b..5c7dfed678 100644
--- a/packages/plugin-build/vitest.config.ts
+++ b/packages/plugin-build/vitest.config.ts
@@ -1,10 +1,15 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
-    name: "@bb/plugin-build",
-    include: ["src/**/*.test.ts"],
-    exclude: ["dist/**", "node_modules/**"],
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "@bb/plugin-build",
+      include: ["src/**/*.test.ts"],
+    }),
   },
 });
diff --git a/packages/plugin-sdk/vitest.config.ts b/packages/plugin-sdk/vitest.config.ts
index 68738e02fc..f7e0361dfd 100644
--- a/packages/plugin-sdk/vitest.config.ts
+++ b/packages/plugin-sdk/vitest.config.ts
@@ -1,15 +1,20 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
-    name: "@get-bb/plugin-sdk",
-    include: [
-      "src/**/*.test.ts",
-      "src/**/*.test.tsx",
-      // Build/release scripts are plain .mjs and live outside src.
-      "scripts/**/*.test.mjs",
-    ],
-    exclude: ["dist/**", "node_modules/**"],
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "@get-bb/plugin-sdk",
+      include: [
+        "src/**/*.test.ts",
+        "src/**/*.test.tsx",
+        // Build/release scripts are plain .mjs and live outside src.
+        "scripts/**/*.test.mjs",
+      ],
+    }),
   },
 });
diff --git a/packages/provider-bridge-protocol/vitest.config.ts b/packages/provider-bridge-protocol/vitest.config.ts
index 462991f13c..c5034860e2 100644
--- a/packages/provider-bridge-protocol/vitest.config.ts
+++ b/packages/provider-bridge-protocol/vitest.config.ts
@@ -1,10 +1,15 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
-    name: "@bb/provider-bridge-protocol",
-    include: ["src/**/*.test.ts", "test/**/*.test.ts"],
-    exclude: ["dist/**", "node_modules/**"],
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "@bb/provider-bridge-protocol",
+      include: ["src/**/*.test.ts", "test/**/*.test.ts"],
+    }),
   },
 });
diff --git a/packages/provider-parity/vitest.config.ts b/packages/provider-parity/vitest.config.ts
index b7b8d67278..84fbf0b3df 100644
--- a/packages/provider-parity/vitest.config.ts
+++ b/packages/provider-parity/vitest.config.ts
@@ -6,5 +6,10 @@ export default defineWorkspaceTestConfig({
     name: "@bb/provider-parity",
     include: ["src/**/*.test.ts"],
     exclude: ["dist/**", "node_modules/**"],
+    // Every replay cell is its own bridge child that mostly waits on
+    // settle/drain pacing, so the suite is wall-clock bound by how many are
+    // in flight. Pinned to 4 CPUs, 43 cells took 37s at vitest's default of
+    // 5, 21s at 10, and 15s at 16, all green.
+    maxConcurrency: 16,
   },
 });
diff --git a/packages/scripts/vitest.config.ts b/packages/scripts/vitest.config.ts
index 999c160037..4abea6a543 100644
--- a/packages/scripts/vitest.config.ts
+++ b/packages/scripts/vitest.config.ts
@@ -1,10 +1,15 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
-    name: "@bb/scripts",
-    include: ["test/**/*.test.ts", "test/**/*.test.mjs"],
-    exclude: ["dist/**", "node_modules/**"],
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "@bb/scripts",
+      include: ["test/**/*.test.ts", "test/**/*.test.mjs"],
+    }),
   },
 });
diff --git a/packages/templates/test/plugin-scaffold-external.test.ts b/packages/templates/test/plugin-scaffold-external.test.ts
index 96648df6da..15d016fbc2 100644
--- a/packages/templates/test/plugin-scaffold-external.test.ts
+++ b/packages/templates/test/plugin-scaffold-external.test.ts
@@ -268,6 +268,11 @@ async function installPackedSdk(
       "--legacy-peer-deps",
       "--no-package-lock",
       "--no-save",
+      // Registry round trips the install does not need: the audit and the
+      // funding banner, and a metadata refresh for packages the cache holds.
+      "--no-audit",
+      "--no-fund",
+      "--prefer-offline",
       tarball,
     ],
     { cwd: targetDir },
@@ -327,10 +332,33 @@ describe("external plugin scaffold types", () => {
   let workDir: string;
   let packRoot: string;
   let tarball: string;
+  let installedNodeModules: string;
+
+  /**
+   * Point a scaffold's `node_modules` at the one real install made in
+   * `beforeAll`: the packed SDK, its registry dependencies, and the linked
+   * workspace copies. Each of the installs this file used to run cost
+   * ~13s cold, and every scaffold here resolves the same packages.
+   */
+  async function useInstalledNodeModules(targetDir: string): Promise<void> {
+    await symlink(installedNodeModules, join(targetDir, "node_modules"), "dir");
+  }
 
   beforeAll(async () => {
     packRoot = await mkdtemp(join(tmpdir(), "bb-external-pack-"));
     tarball = await packPluginSdk(join(packRoot, "pack"));
+    // The app scaffold declares the superset of the backend scaffold's
+    // dependencies, so its install serves both.
+    const templateDir = join(packRoot, "template");
+    await scaffoldPlugin({
+      targetDir: templateDir,
+      packageName: "bb-plugin-external-template",
+      bbVersion: "0.9.0",
+      app: true,
+    });
+    await installPackedSdk(templateDir, tarball);
+    await linkExternalDependencies(templateDir);
+    installedNodeModules = join(templateDir, "node_modules");
   }, 180_000);
 
   afterAll(async () => {
@@ -357,8 +385,7 @@ describe("external plugin scaffold types", () => {
     await writeFile(join(targetDir, "app.tsx"), REPRESENTATIVE_APP);
     // The scaffold's own pin, satisfied by the packed artifact.
     expect(await scaffoldSdkPin(targetDir)).toBe(PLUGIN_SDK_VERSION);
-    await installPackedSdk(targetDir, tarball);
-    await linkExternalDependencies(targetDir);
+    await useInstalledNodeModules(targetDir);
 
     const tsconfig = JSON.parse(
       await readFile(join(targetDir, "tsconfig.json"), "utf8"),
@@ -416,8 +443,7 @@ describe("external plugin scaffold types", () => {
       packageName: "bb-plugin-external-backend",
       bbVersion: "0.9.0",
     });
-    await installPackedSdk(backendDir, tarball);
-    await linkExternalDependencies(backendDir);
+    await useInstalledNodeModules(backendDir);
     await writeFile(join(backendDir, "server.test.ts"), BACKEND_TEST);
     await includeTestsInTypecheck(backendDir);
 
@@ -490,8 +516,6 @@ describe("external plugin scaffold types", () => {
     // No mapping to fall back on: the testing declarations import the package
     // root, which has to resolve through the install alone.
     expect(backendTsconfig.compilerOptions.paths).toBeUndefined();
-    await runTypecheck(backendDir);
-    await runVitest(backendDir);
 
     const frontendDir = join(workDir, "bb-plugin-external-frontend");
     await scaffoldPlugin({
@@ -500,19 +524,18 @@ describe("external plugin scaffold types", () => {
       bbVersion: "0.9.0",
       app: true,
     });
-    const frontendSdk = join(
-      frontendDir,
-      "node_modules",
-      "@get-bb",
-      "plugin-sdk",
-    );
-    await mkdir(dirname(frontendSdk), { recursive: true });
-    await symlink(installedSdk, frontendSdk, "dir");
-    await linkExternalDependencies(frontendDir);
+    await useInstalledNodeModules(frontendDir);
     await writeFile(join(frontendDir, "app.test.tsx"), FRONTEND_TEST);
     await writeFile(join(frontendDir, "vitest.config.ts"), VITEST_CONFIG);
     await includeTestsInTypecheck(frontendDir);
-    await runTypecheck(frontendDir);
-    await runVitest(frontendDir);
+
+    // The two scaffolds are independent; their typechecks (~3.5s each) and
+    // test runs overlap.
+    await Promise.all(
+      [backendDir, frontendDir].map(async (dir) => {
+        await runTypecheck(dir);
+        await runVitest(dir);
+      }),
+    );
   }, 300_000);
 });
diff --git a/packages/thread-view/vitest.config.ts b/packages/thread-view/vitest.config.ts
index 38107c1b38..4e1d04326d 100644
--- a/packages/thread-view/vitest.config.ts
+++ b/packages/thread-view/vitest.config.ts
@@ -1,10 +1,15 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
-    name: "@bb/thread-view",
-    include: ["test/**/*.test.ts"],
-    exclude: ["dist/**", "node_modules/**"],
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "@bb/thread-view",
+      include: ["test/**/*.test.ts"],
+    }),
   },
 });
diff --git a/plugins/github/vitest.config.ts b/plugins/github/vitest.config.ts
index b0119f903d..05b11f76a7 100644
--- a/plugins/github/vitest.config.ts
+++ b/plugins/github/vitest.config.ts
@@ -1,5 +1,8 @@
 import path from "node:path";
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   resolve: {
@@ -9,8 +12,12 @@ export default defineWorkspaceTestConfig({
   },
   test: {
     silent: "passed-only",
-    name: "bb-plugin-github",
-    include: ["**/*.test.{ts,tsx}"],
-    exclude: ["node_modules/**"],
+    projects: sharedWorkerProjects({
+      pkgDir: import.meta.dirname,
+      aliases: { "@": path.resolve(import.meta.dirname, ".") },
+      name: "bb-plugin-github",
+      include: ["**/*.test.{ts,tsx}"],
+      exclude: ["node_modules/**"],
+    }),
   },
 });
diff --git a/plugins/provider-acp/vitest.config.ts b/plugins/provider-acp/vitest.config.ts
index 59659ca205..bf0d34c3dc 100644
--- a/plugins/provider-acp/vitest.config.ts
+++ b/plugins/provider-acp/vitest.config.ts
@@ -1,10 +1,15 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
-    name: "bb-plugin-provider-acp",
-    include: ["src/**/*.test.ts"],
-    exclude: ["node_modules/**", "dist/**"],
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "bb-plugin-provider-acp",
+      include: ["src/**/*.test.ts"],
+    }),
   },
 });
diff --git a/plugins/provider-claude-code/vitest.config.ts b/plugins/provider-claude-code/vitest.config.ts
index e5e9131388..198e21631e 100644
--- a/plugins/provider-claude-code/vitest.config.ts
+++ b/plugins/provider-claude-code/vitest.config.ts
@@ -1,10 +1,15 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
-    name: "bb-plugin-provider-claude-code",
-    include: ["src/**/*.test.ts"],
-    exclude: ["node_modules/**", "dist/**"],
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "bb-plugin-provider-claude-code",
+      include: ["src/**/*.test.ts"],
+    }),
   },
 });
diff --git a/plugins/provider-codex/vitest.config.ts b/plugins/provider-codex/vitest.config.ts
index 2b010b254e..8e7f74ed7c 100644
--- a/plugins/provider-codex/vitest.config.ts
+++ b/plugins/provider-codex/vitest.config.ts
@@ -1,10 +1,15 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   test: {
     silent: "passed-only",
-    name: "bb-plugin-provider-codex",
-    include: ["src/**/*.test.ts"],
-    exclude: ["node_modules/**", "dist/**"],
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "bb-plugin-provider-codex",
+      include: ["src/**/*.test.ts"],
+    }),
   },
 });
diff --git a/plugins/tasks/vitest.config.ts b/plugins/tasks/vitest.config.ts
index d718182340..88cdfceba3 100644
--- a/plugins/tasks/vitest.config.ts
+++ b/plugins/tasks/vitest.config.ts
@@ -1,4 +1,7 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
   resolve: {
@@ -10,13 +13,10 @@ export default defineWorkspaceTestConfig({
   },
   test: {
     silent: "passed-only",
-    name: "bb-plugin-tasks",
     // CI runners are slow enough that testing-library's default 1s findBy*/
     // waitFor timeout flakes on the heavier UI suites (pager, manage, rail).
     testTimeout: 20_000,
     setupFiles: ["./vitest.setup.ts"],
-    include: ["**/*.test.{ts,tsx}"],
-    exclude: ["node_modules/**"],
     server: {
       deps: {
         // Inlined so the tippy.js alias above applies to its import; left
@@ -25,5 +25,11 @@ export default defineWorkspaceTestConfig({
         inline: ["@tiptap/extension-bubble-menu"],
       },
     },
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "bb-plugin-tasks",
+      include: ["**/*.test.{ts,tsx}"],
+      exclude: ["node_modules/**"],
+    }),
   },
 });
diff --git a/plugins/workflows/vitest.config.ts b/plugins/workflows/vitest.config.ts
index 631695e1f4..4cf4306dae 100644
--- a/plugins/workflows/vitest.config.ts
+++ b/plugins/workflows/vitest.config.ts
@@ -1,5 +1,16 @@
-import { defineWorkspaceTestConfig } from "../../vitest.shared.js";
+import {
+  defineWorkspaceTestConfig,
+  sharedWorkerProjects,
+} from "../../vitest.shared.js";
 
 export default defineWorkspaceTestConfig({
-  test: { environment: "node", testTimeout: 15_000 },
+  test: {
+    environment: "node",
+    testTimeout: 15_000,
+    projects: sharedWorkerProjects({
+      pkgDir: __dirname,
+      name: "bb-plugin-workflows",
+      include: ["src/**/*.test.{ts,tsx}"],
+    }),
+  },
 });
diff --git a/tests/integration/vitest.config.ts b/tests/integration/vitest.config.ts
index aa8e1dfdb0..1b42554649 100644
--- a/tests/integration/vitest.config.ts
+++ b/tests/integration/vitest.config.ts
@@ -12,7 +12,7 @@ export default defineWorkspaceTestConfig({
     // so we can safely parallelize across files for a large runtime win.
     fileParallelism: true,
     // No file here mocks modules or stubs globals/env (vitest.shared.ts's
-    // findIsolationRequiringTests would flag it), so workers can reuse their
+    // partitionTestFiles would flag it), so workers can reuse their
     // context across files instead of re-importing the server graph per file.
     isolate: false,
     globalSetup: ["./global-setup.ts"],
diff --git a/tests/qa/vitest.config.ts b/tests/qa/vitest.config.ts
index c6f15c8074..6a238fea4a 100644
--- a/tests/qa/vitest.config.ts
+++ b/tests/qa/vitest.config.ts
@@ -4,5 +4,8 @@ export default defineWorkspaceTestConfig({
   test: {
     include: ["test/**/*.test.ts"],
     name: "@bb/qa",
+    // These suites spawn real child processes; the 5s default tipped over
+    // when the whole workspace ran at once.
+    testTimeout: 15_000,
   },
 });
diff --git a/vitest.config.ts b/vitest.config.ts
index cef78a464e..d2f0d1a751 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -1,6 +1,7 @@
 import { existsSync, readdirSync } from "node:fs";
 import path from "node:path";
 import { defineConfig } from "vitest/config";
+import { SharedWorkerSequencer } from "./vitest.shared.js";
 
 const workspaceRoots = [
   "apps",
@@ -26,6 +27,7 @@ function discoverVitestProjects(): string[] {
 export default defineConfig({
   test: {
     silent: "passed-only",
+    sequence: { sequencer: SharedWorkerSequencer },
     // Keep the default workspace test entrypoint aligned with every package/app
     // that defines its own Vitest config so new suites are not silently skipped.
     projects: discoverVitestProjects(),
diff --git a/vitest.shared.ts b/vitest.shared.ts
index 6539329a7a..4475482a78 100644
--- a/vitest.shared.ts
+++ b/vitest.shared.ts
@@ -1,48 +1,246 @@
-import { readdirSync, readFileSync } from "node:fs";
+import { readdirSync, readFileSync, statSync } from "node:fs";
+import { createRequire } from "node:module";
 import path from "node:path";
 import { mergeConfig, type ViteUserConfig } from "vitest/config";
+import { BaseSequencer, type TestSpecification } from "vitest/node";
+
+const GLOBAL_OBJECT = String.raw`(?:window|globalThis|global|document|navigator|[A-Z][\w$]*\.prototype)`;
+/** A global object, optionally through a TypeScript cast: `(window as X)`. */
+const GLOBAL_TARGET = String.raw`(?:${GLOBAL_OBJECT}|\(\s*${GLOBAL_OBJECT}\s+as\b[^)]*\))`;
 
 /**
- * Wraps a package's Vitest config so workspace imports (`@bb/*`) resolve to
- * package sources instead of built `dist/` output.
- *
- * Every workspace package's export map carries a `source` condition pointing
- * at `src/` — the same condition used by `node --conditions=source` in dev,
- * esbuild bundling (`scripts/build-utils.mjs`), and tsc (`customConditions`
- * in `packages/tsconfig/typecheck-overrides.json`). Vitest resolves test
- * imports through Vite's server environment, which only honors conditions
- * under `ssr.resolve`, so a plain `resolve.conditions` entry has no effect on
- * tests. Only `source` is listed here: Vitest contributes its own default
- * conditions through a config plugin, and Vite concatenates these arrays
- * with them during config merge.
+ * Syntax that mutates worker-global state: the vitest module registry and
+ * stubs, `process.env`, the working directory, and properties of the global
+ * objects (`window`, `document`, `navigator`, `globalThis`, prototypes),
+ * including through a cast such as `(window as X).bbDesktop = ...`. A
+ * file that contains any of it keeps the default isolated worker, whether or
+ * not it restores what it changed: in a shared worker (`isolate: false`) a
+ * missed restore bleeds into the next file, and mocks fail to apply when the
+ * target module is already loaded.
  */
+const ISOLATION_REQUIRING_API = new RegExp(
+  [
+    String.raw`\bvi\.(mock|doMock|unmock|doUnmock|resetModules|stubGlobal|stubEnv)\(`,
+    String.raw`\bprocess\.chdir\(`,
+    String.raw`\bprocess\.env(\.[A-Za-z_$][\w$]*|\[[^\]]+\])\s*=[^=]`,
+    String.raw`\bdelete\s+process\.env\b`,
+    String.raw`\b${GLOBAL_TARGET}\.[A-Za-z_$][\w$.]*\s*=[^=]`,
+    String.raw`\bdelete\s+${GLOBAL_TARGET}(?![\w$])`,
+    String.raw`\b(?:Object\.(?:defineProperty|defineProperties|assign)|Reflect\.(?:set|defineProperty|deleteProperty))\(\s*${GLOBAL_TARGET}(?![\w$])`,
+  ].join("|"),
+);
+
+/**
+ * Import specifiers in a module: `import x from "..."`, `export ... from
+ * "..."`, `import "..."`, `import("...")`, and `require("...")`. Only
+ * relative and aliased specifiers are followed; the rest resolve to `null`.
+ */
+const IMPORT_SPECIFIER =
+  /\b(?:import|export)\b[^'"]*?\bfrom\s*["']([^"']+)["']|\bimport\s*\(?\s*["']([^"']+)["']|\brequire\s*\(\s*["']([^"']+)["']\s*\)/g;
+
+const SOURCE_EXTENSIONS = [".ts", ".tsx", ".mts", ".cts", ".mjs", ".cjs", ".js", ".jsx"];
+
 /**
- * Vitest APIs that mutate worker-global state (the module registry, globals,
- * or `process.env`). Files calling any of these need their own isolated
- * worker; running them with `isolate: false` makes mocks bleed across files
- * or silently fail to apply when the target module is already loaded.
+ * The scan follows a test's local imports only into test-support modules
+ * (harnesses, helpers, fixtures, mocks): a `vi.mock` in a shared harness
+ * makes every test that imports it need a fresh module registry. Production
+ * sources are not followed — a module that sets `process.env` when the app
+ * boots does so once per worker, exactly as it does once per process in
+ * production, and following the whole source graph would flag nearly every
+ * test.
  */
-const ISOLATION_REQUIRING_API =
-  /\bvi\.(mock|doMock|unmock|doUnmock|resetModules|stubGlobal|stubEnv)\(/;
+const TEST_SUPPORT_DIRS = new Set([
+  "test",
+  "tests",
+  "__tests__",
+  "__mocks__",
+  "__fixtures__",
+  "fixtures",
+  "testing",
+  "test-utils",
+]);
+const TEST_SUPPORT_FILE =
+  /(^|[.-])(test|tests|mock|mocks|harness|fixture|fixtures|helpers?)(\.|-|$)/;
 
-const TEST_FILE = /\.test\.tsx?$/;
+function isTestSupportModule(relativePath: string): boolean {
+  const segments = relativePath.split(path.sep);
+  const baseName = (segments.pop() ?? "").replace(/\.[cm]?[jt]sx?$/, "");
+  return (
+    segments.some((segment) => TEST_SUPPORT_DIRS.has(segment)) ||
+    TEST_SUPPORT_FILE.test(baseName)
+  );
+}
+
+/**
+ * Every file name shape vitest's default `include` treats as a test. The
+ * partition below must see every file a package's `include` globs select;
+ * a file matched by `include` but not by this pattern would run in both
+ * halves.
+ */
+const TEST_FILE = /\.(test|spec)\.[cm]?[jt]sx?$/;
 const SKIP_DIRS = new Set(["node_modules", "dist"]);
 
+/** The per-file environment docblock vitest honors. */
+const ENVIRONMENT_DOCBLOCK = /@(?:vitest|jest)-environment\s+([\w-]+)/;
+
 /**
- * Finds test files under `roots` (relative to `pkgDir`) that use
- * worker-global vitest APIs and therefore must keep the default isolated
- * worker. Everything else can run in a shared worker context
+ * Environments whose files never share a worker. A DOM test mutates the
+ * shared `document` in ways no source scan can enumerate — portals left in
+ * `body`, focus, listeners on `window`, module caches keyed by the document
+ * (media-query lists, drawer stacks) — and running the app's jsdom files
+ * through one worker failed a different file on every ordering.
+ */
+const ISOLATED_ENVIRONMENTS = new Set(["jsdom", "happy-dom"]);
+
+export interface PartitionOptions {
+  /**
+   * Import aliases to follow when the scan walks a test file's local
+   * imports, as alias prefix to directory (absolute or package-relative),
+   * e.g. `{ "@": "src" }`.
+   */
+  aliases?: Record<string, string>;
+  /**
+   * The root config's `test.environment`, which files without a docblock
+   * use. Defaults to `node`. A DOM default isolates every such file.
+   */
+  defaultEnvironment?: string;
+}
+
+export interface SharedTestFileGroup {
+  /**
+   * The environment the files' docblocks select, or `null` for the files
+   * that use the project default.
+   */
+  environment: string | null;
+  files: string[];
+}
+
+export interface TestFilePartition {
+  /**
+   * Files safe to run in a shared worker context, grouped by environment.
+   * The default-environment group comes first; the rest sort by name.
+   */
+  shared: SharedTestFileGroup[];
+  /**
+   * Files that need their own isolated worker: they mutate worker-global
+   * state, or they run in a DOM environment.
+   */
+  isolated: string[];
+}
+
+interface IsolationScan {
+  pkgDir: string;
+  aliases: Record<string, string>;
+  memo: Map<string, boolean>;
+  visiting: Set<string>;
+}
+
+function isFile(filePath: string): boolean {
+  try {
+    return statSync(filePath).isFile();
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * Resolves a relative or aliased import to a test-support module inside the
+ * package, mapping the `.js` specifiers TypeScript sources use back to
+ * `.ts`/`.tsx`. Production sources and anything outside the package resolve
+ * to `null`.
+ */
+function resolveLocalImport(
+  fromFile: string,
+  specifier: string,
+  scan: IsolationScan,
+): string | null {
+  let base: string | null = null;
+  if (specifier.startsWith("./") || specifier.startsWith("../")) {
+    base = path.resolve(path.dirname(fromFile), specifier);
+  } else {
+    for (const [alias, target] of Object.entries(scan.aliases)) {
+      if (specifier === alias || specifier.startsWith(`${alias}/`)) {
+        base = path.resolve(scan.pkgDir, target, specifier.slice(alias.length + 1));
+        break;
+      }
+    }
+  }
+  if (base === null) return null;
+  const withoutJs = base.replace(/\.[cm]?jsx?$/, "");
+  const candidates = [
+    base,
+    ...SOURCE_EXTENSIONS.map((extension) => withoutJs + extension),
+    ...SOURCE_EXTENSIONS.map((extension) => path.join(base, `index${extension}`)),
+  ];
+  for (const candidate of candidates) {
+    const relative = path.relative(scan.pkgDir, candidate);
+    if (relative.startsWith("..") || relative.split(path.sep).includes("node_modules")) {
+      continue;
+    }
+    if (!isFile(candidate)) continue;
+    return isTestSupportModule(relative) ? candidate : null;
+  }
+  return null;
+}
+
+/**
+ * Whether `file` — or any local module it imports, transitively — uses an
+ * isolation-requiring API. A test helper that calls `vi.mock` makes every
+ * test that imports it depend on a fresh module registry.
+ */
+function requiresIsolation(file: string, scan: IsolationScan): boolean {
+  const memo = scan.memo.get(file);
+  if (memo !== undefined) return memo;
+  if (scan.visiting.has(file)) return false;
+  scan.visiting.add(file);
+  const source = readFileSync(file, "utf8");
+  let result = ISOLATION_REQUIRING_API.test(source);
+  if (!result) {
+    for (const match of source.matchAll(IMPORT_SPECIFIER)) {
+      const specifier = match[1] ?? match[2] ?? match[3];
+      if (specifier === undefined) continue;
+      const target = resolveLocalImport(file, specifier, scan);
+      if (target !== null && requiresIsolation(target, scan)) {
+        result = true;
+        break;
+      }
+    }
+  }
+  scan.visiting.delete(file);
+  scan.memo.set(file, result);
+  return result;
+}
+
+/**
+ * Walks `roots` (relative to `pkgDir`) and splits the test files found there
+ * by whether they need an isolated worker: files that, directly or through
+ * a local test helper, use worker-global APIs, and files that run in a DOM
+ * environment. Everything else can run in a shared worker context
  * (`isolate: false`), which skips re-importing the module graph for every
- * file — by far the dominant cost of the big suites in CI.
+ * file — by far the dominant cost of the big suites.
  *
- * Returns package-relative posix paths, usable directly as vitest
- * `include`/`exclude` entries.
+ * Shared files are further grouped by the environment their docblock names.
+ * Vitest only hands a finished worker the next queued file when that file
+ * has the same project and environment, so a queue that mixes `node` and
+ * `jsdom` files churns workers instead of reusing them.
+ *
+ * Every path is package-relative posix, usable directly as a vitest
+ * `include`/`exclude` entry.
  */
-export function findIsolationRequiringTests(
+export function partitionTestFiles(
   pkgDir: string,
   roots: string[],
-): string[] {
-  const matches: string[] = [];
+  options: PartitionOptions = {},
+): TestFilePartition {
+  const defaultEnvironment = options.defaultEnvironment ?? "node";
+  const scan: IsolationScan = {
+    pkgDir,
+    aliases: options.aliases ?? {},
+    memo: new Map(),
+    visiting: new Set(),
+  };
+  const sharedByEnvironment = new Map<string | null, Set<string>>();
+  const isolated = new Set<string>();
   const walk = (dir: string) => {
     let entries;
     try {
@@ -54,27 +252,230 @@ export function findIsolationRequiringTests(
       const fullPath = path.join(dir, entry.name);
       if (entry.isDirectory()) {
         if (!SKIP_DIRS.has(entry.name)) walk(fullPath);
-      } else if (
-        TEST_FILE.test(entry.name) &&
-        ISOLATION_REQUIRING_API.test(readFileSync(fullPath, "utf8"))
-      ) {
-        matches.push(path.relative(pkgDir, fullPath).split(path.sep).join("/"));
+      } else if (TEST_FILE.test(entry.name)) {
+        const relative = path
+          .relative(pkgDir, fullPath)
+          .split(path.sep)
+          .join("/");
+        const source = readFileSync(fullPath, "utf8");
+        const environment = ENVIRONMENT_DOCBLOCK.exec(source)?.[1] ?? null;
+        if (
+          ISOLATED_ENVIRONMENTS.has(environment ?? defaultEnvironment) ||
+          requiresIsolation(fullPath, scan)
+        ) {
+          isolated.add(relative);
+        } else {
+          let group = sharedByEnvironment.get(environment);
+          if (!group) {
+            group = new Set();
+            sharedByEnvironment.set(environment, group);
+          }
+          group.add(relative);
+        }
       }
     }
   };
-  for (const root of roots) walk(path.join(pkgDir, root));
-  return matches.sort();
+  for (const root of new Set(roots)) walk(path.join(pkgDir, root));
+  const shared = [...sharedByEnvironment]
+    .map(([environment, files]) => ({ environment, files: [...files].sort() }))
+    .sort((a, b) => {
+      if (a.environment === null) return -1;
+      if (b.environment === null) return 1;
+      return a.environment.localeCompare(b.environment);
+    });
+  return { shared, isolated: [...isolated].sort() };
+}
+
+type TestProjects = NonNullable<NonNullable<ViteUserConfig["test"]>["projects"]>;
+
+export interface SharedWorkerProjectsArgs {
+  /** The package directory, normally the config file's `__dirname`. */
+  pkgDir: string;
+  /**
+   * The project name. Shared files that select another environment report
+   * as `${name}:${environment}`; the isolated files as `${name}:isolated`.
+   */
+  name: string;
+  /** Package-relative globs that select the package's test files. */
+  include: string[];
+  /** Globs excluded from every project. Defaults to `dist/**` and `node_modules/**`. */
+  exclude?: string[];
+  /** Import aliases the isolation scan follows; see {@link PartitionOptions}. */
+  aliases?: Record<string, string>;
+  /** The root config's `test.environment`; see {@link PartitionOptions}. */
+  defaultEnvironment?: string;
+}
+
+/**
+ * Splits a package's tests into projects that extend the package's root
+ * config: one shared-worker project (`isolate: false`) per non-DOM
+ * environment for the files {@link partitionTestFiles} finds safe, and an
+ * isolated project for the rest. Every project keeps the package's own
+ * `include` globs and excludes the other projects' files, so the split never
+ * changes which files run — only where.
+ *
+ * {@link SharedWorkerSequencer} (installed by {@link defineWorkspaceTestConfig})
+ * orders the run queue so the files of one shared project sit together.
+ * Vitest hands a finished worker the next queued file only when that file
+ * has the same project and environment, so this ordering is what turns
+ * `isolate: false` into actual worker reuse. Re-importing the module graph
+ * for every file is the dominant cost of most suites here (80–90% of total
+ * CPU for the large ones).
+ */
+export function sharedWorkerProjects(
+  args: SharedWorkerProjectsArgs,
+): TestProjects {
+  const exclude = args.exclude ?? ["dist/**", "node_modules/**"];
+  const options: PartitionOptions = {};
+  if (args.aliases !== undefined) options.aliases = args.aliases;
+  if (args.defaultEnvironment !== undefined) {
+    options.defaultEnvironment = args.defaultEnvironment;
+  }
+  const partition = partitionTestFiles(
+    args.pkgDir,
+    args.include.map(globRoot),
+    options,
+  );
+  const allFiles = [
+    ...partition.shared.flatMap((group) => group.files),
+    ...partition.isolated,
+  ];
+  if (allFiles.length === 0) {
+    return [{ extends: true, test: { name: args.name, include: args.include, exclude } }];
+  }
+  const otherFiles = (own: readonly string[]) => {
+    const ownSet = new Set(own);
+    return allFiles.filter((file) => !ownSet.has(file));
+  };
+  const projects: TestProjects = partition.shared.map((group) => ({
+    extends: true,
+    test: {
+      name:
+        group.environment === null
+          ? args.name
+          : `${args.name}:${group.environment}`,
+      include: args.include,
+      exclude: [...exclude, ...otherFiles(group.files)],
+      isolate: false,
+    },
+  }));
+  if (partition.isolated.length > 0) {
+    projects.push({
+      extends: true,
+      test: {
+        name: `${args.name}:isolated`,
+        include: args.include,
+        exclude: [...exclude, ...otherFiles(partition.isolated)],
+      },
+    });
+  }
+  return projects;
+}
+
+/**
+ * Orders the run queue for worker reuse: isolated files first, then the
+ * shared-worker projects one after another, keeping vitest's own
+ * slowest-first order inside each block.
+ *
+ * Vitest reuses a finished shared worker only when the file at the head of
+ * the queue belongs to the same project and environment; otherwise it
+ * terminates the worker and starts a new one. Interleaved projects therefore
+ * churn workers, and running the projects as separate phases
+ * (`sequence.groupOrder`) leaves every worker idle at each phase's tail.
+ * One queue with contiguous blocks avoids both. Isolated files go first
+ * because each one pays a full module-graph import, so they are the longest
+ * tasks and scheduling them early keeps the end of the run short.
+ */
+export class SharedWorkerSequencer extends BaseSequencer {
+  override async sort(
+    files: TestSpecification[],
+  ): Promise<TestSpecification[]> {
+    const sorted = await super.sort(files);
+    const rank = (spec: TestSpecification) =>
+      spec.project.config.isolate ? 0 : 1;
+    return sorted
+      .map((spec, index) => ({ spec, index }))
+      .sort(
+        (a, b) =>
+          rank(a.spec) - rank(b.spec) ||
+          a.spec.project.name.localeCompare(b.spec.project.name) ||
+          a.index - b.index,
+      )
+      .map(({ spec }) => spec);
+  }
 }
 
+/**
+ * The literal directory prefix of a glob: `src/** /*.test.ts` is rooted at
+ * `src`, and `** /*.test.ts` at the package itself.
+ */
+function globRoot(glob: string): string {
+  const segments = glob.split("/");
+  const literal: string[] = [];
+  for (const segment of segments) {
+    if (/[*?{}[\]]/.test(segment)) break;
+    literal.push(segment);
+  }
+  // A fully literal glob names one file; its root is the containing directory.
+  if (literal.length === segments.length) literal.pop();
+  return literal.length > 0 ? literal.join("/") : ".";
+}
+
+/**
+ * `@hugeicons/core-free-icons` resolves to a barrel that re-exports 5,122
+ * one-icon modules, which Node loads in ~0.4–0.7s — paid by every isolated
+ * worker, since the shared-ui icon registry imports it. The package also
+ * ships the same exports as one self-contained minified bundle (its
+ * `production` entry), which loads in ~45ms. Tests alias the bare specifier
+ * to that bundle; deep imports are untouched. Resolved from the package
+ * under test, and skipped when that package cannot see the dependency.
+ */
+function hugeiconsBundleAlias(): { find: RegExp; replacement: string }[] {
+  try {
+    const require = createRequire(path.join(process.cwd(), "package.json"));
+    const packageJson = require.resolve("@hugeicons/core-free-icons/package.json");
+    return [
+      {
+        find: /^@hugeicons\/core-free-icons$/,
+        replacement: path.join(
+          path.dirname(packageJson),
+          "dist",
+          "esm",
+          "index.min.js",
+        ),
+      },
+    ];
+  } catch {
+    return [];
+  }
+}
+
+/**
+ * Wraps a package's Vitest config so workspace imports (`@bb/*`) resolve to
+ * package sources instead of built `dist/` output, installs
+ * {@link SharedWorkerSequencer}, and applies {@link hugeiconsBundleAlias}.
+ *
+ * Every workspace package's export map carries a `source` condition pointing
+ * at `src/` — the same condition used by `node --conditions=source` in dev,
+ * esbuild bundling (`scripts/build-utils.mjs`), and tsc (`customConditions`
+ * in `packages/tsconfig/typecheck-overrides.json`). Vitest resolves test
+ * imports through Vite's server environment, which only honors conditions
+ * under `ssr.resolve`, so a plain `resolve.conditions` entry has no effect on
+ * tests. Only `source` is listed here: Vitest contributes its own default
+ * conditions through a config plugin, and Vite concatenates these arrays
+ * with them during config merge.
+ */
 export function defineWorkspaceTestConfig(
   config: ViteUserConfig,
 ): ViteUserConfig {
   return mergeConfig(
     {
       resolve: {
+        alias: hugeiconsBundleAlias(),
         conditions: ["source"],
       },
       test: {
+        sequence: { sequencer: SharedWorkerSequencer },
         coverage: {
           provider: "v8",
           include: ["**/*.{ts,tsx,js,jsx,mjs,cjs}"],