diff --git a/scripts/test.ts b/scripts/test.ts
index 832a537191..39a73c8d5d 100644
--- a/scripts/test.ts
+++ b/scripts/test.ts
@@ -1,5 +1,5 @@
 import { randomUUID } from "node:crypto";
-import { mkdirSync, mkdtempSync, rmSync } from "node:fs";
+import { existsSync, mkdirSync, mkdtempSync, rmSync } from "node:fs";
 import { homedir, tmpdir } from "node:os";
 import { join } from "node:path";
 import { acquireTestRunLock, TEST_RUN_ID_ENV } from "./test-run-lock";
@@ -433,15 +433,67 @@ async function runTestLane(lane: BunTestLane, runId: string, capture = false): P
   }
 }
 
+/**
+ * `gui` is not a workspace of the root package and declares React only in `gui/package.json`, so a
+ * root `bun install` never creates `gui/node_modules`. Twenty-five files under `tests/` import
+ * modules from `gui/src`, which makes those tests fail on a fresh clone or worktree with
+ * `Cannot find package 'react'` — reported as an "Unhandled error between tests" that names no
+ * test, so the cause is not obvious from the output.
+ *
+ * `.github/workflows/ci.yml` already installs them explicitly for exactly this reason; the local
+ * runner had no equivalent. Install on demand rather than fail, because the tests genuinely
+ * require the dependency and `gui/node_modules` is a gitignored build artifact, not source.
+ */
+export function ensureGuiDependencies(io: {
+  cwd?: string;
+  exists?: (path: string) => boolean;
+  install?: (guiDir: string) => { ok: boolean; detail: string };
+  log?: (message: string) => void;
+} = {}): { kind: "present" | "installed" | "absent" | "failed"; detail?: string } {
+  const cwd = io.cwd ?? process.cwd();
+  const exists = io.exists ?? existsSync;
+  const log = io.log ?? (message => console.warn(message));
+  const guiDir = join(cwd, "gui");
+  if (!exists(join(guiDir, "package.json"))) return { kind: "absent" };
+  if (exists(join(guiDir, "node_modules", "react", "package.json"))) return { kind: "present" };
+
+  log("[test] gui dependencies are missing or incomplete; installing them so tests importing gui/src can resolve React.");
+  const install = io.install ?? ((dir: string) => {
+    const result = Bun.spawnSync(["bun", "install", "--frozen-lockfile"], {
+      cwd: dir,
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+    return {
+      ok: result.exitCode === 0,
+      detail: decodeOutput(result.stderr) || decodeOutput(result.stdout),
+    };
+  });
+  const outcome = install(guiDir);
+  if (outcome.ok) return { kind: "installed" };
+  return { kind: "failed", detail: outcome.detail };
+}
+
 if (import.meta.main) {
   const requestedTests = process.argv.slice(2);
-  let changedRun: ReturnType<typeof inspectChangedRun> = null;
-  try {
-    changedRun = inspectChangedRun(requestedTests);
-  } catch (error) {
-    console.error(error instanceof Error ? error.message : String(error));
+  const guiDependencies = ensureGuiDependencies();
+  if (guiDependencies.kind === "failed") {
+    console.error(
+      "[test] could not install gui/node_modules, which tests importing gui/src need to resolve React.\n"
+      + "       Run it manually: cd gui && bun install --frozen-lockfile\n"
+      + (guiDependencies.detail ? `       ${guiDependencies.detail.trim().split("\n").slice(-3).join("\n       ")}` : ""),
+    );
     process.exitCode = 1;
   }
+  let changedRun: ReturnType<typeof inspectChangedRun> = null;
+  if (process.exitCode !== 1) {
+    try {
+      changedRun = inspectChangedRun(requestedTests);
+    } catch (error) {
+      console.error(error instanceof Error ? error.message : String(error));
+      process.exitCode = 1;
+    }
+  }
   if (process.exitCode !== 1) {
     if (changedRun) {
       console.warn(
diff --git a/tests/test-runner.test.ts b/tests/test-runner.test.ts
index 2d5423d628..28e747075b 100644
--- a/tests/test-runner.test.ts
+++ b/tests/test-runner.test.ts
@@ -5,6 +5,7 @@ import { isAbsolute, join } from "node:path";
 import {
   changedSelectionFailure,
   createIsolatedTestEnvironment,
+  ensureGuiDependencies,
   inspectChangedRun,
   resolveBunTestArgs,
   resolveBunTestPlan,
@@ -459,3 +460,87 @@ describe("bun test machine lock", () => {
     }
   });
 });
+
+describe("ensureGuiDependencies", () => {
+  // `gui` is not a workspace, so a root `bun install` leaves gui/node_modules absent and the
+  // twenty-five tests importing gui/src die on `Cannot find package 'react'` — an "Unhandled error
+  // between tests" that names no test. CI already installs them; this closes the local gap.
+  const paths = (present: string[]) => {
+    const normalized = present.map(path => path.replaceAll("\\", "/"));
+    return (path: string) => normalized.some(entry => path.replaceAll("\\", "/").endsWith(entry));
+  };
+
+  test("mocked paths match POSIX and Windows separators", () => {
+    const exists = paths(["gui/package.json"]);
+    expect(exists("/repo/gui/package.json")).toBe(true);
+    expect(exists("C:\\repo\\gui\\package.json")).toBe(true);
+  });
+
+  test("installs when gui/package.json exists but node_modules does not", () => {
+    const installed: string[] = [];
+    const logged: string[] = [];
+    const result = ensureGuiDependencies({
+      cwd: "/repo",
+      exists: paths(["gui/package.json"]),
+      install: dir => { installed.push(dir); return { ok: true, detail: "" }; },
+      log: message => logged.push(message),
+    });
+
+    expect(result).toEqual({ kind: "installed" });
+    expect(installed).toEqual([join("/repo", "gui")]);
+    expect(logged[0]).toContain("gui dependencies are missing or incomplete");
+  });
+
+  test("retries when node_modules exists without the required dependency", () => {
+    let installs = 0;
+    const result = ensureGuiDependencies({
+      cwd: "/repo",
+      exists: paths(["gui/package.json", "gui/node_modules"]),
+      install: () => { installs += 1; return { ok: true, detail: "" }; },
+      log: () => {},
+    });
+
+    expect(result).toEqual({ kind: "installed" });
+    expect(installs).toBe(1);
+  });
+
+  test("does nothing when the required dependency is already there", () => {
+    let installs = 0;
+    const result = ensureGuiDependencies({
+      cwd: "/repo",
+      exists: paths(["gui/package.json", "gui/node_modules/react/package.json"]),
+      install: () => { installs += 1; return { ok: true, detail: "" }; },
+      log: () => {},
+    });
+
+    expect(result).toEqual({ kind: "present" });
+    expect(installs).toBe(0);
+  });
+
+  // A published install tree has no gui/ at all; the runner must not try to install there.
+  test("does nothing when there is no gui package", () => {
+    let installs = 0;
+    const result = ensureGuiDependencies({
+      cwd: "/repo",
+      exists: () => false,
+      install: () => { installs += 1; return { ok: true, detail: "" }; },
+      log: () => {},
+    });
+
+    expect(result).toEqual({ kind: "absent" });
+    expect(installs).toBe(0);
+  });
+
+  // Offline or a lockfile drift has to surface as its own message, not as twenty-five
+  // unexplained React failures once the lanes start.
+  test("reports the failure detail instead of continuing", () => {
+    const result = ensureGuiDependencies({
+      cwd: "/repo",
+      exists: paths(["gui/package.json"]),
+      install: () => ({ ok: false, detail: "lockfile had changes" }),
+      log: () => {},
+    });
+
+    expect(result).toEqual({ kind: "failed", detail: "lockfile had changes" });
+  });
+});