diff --git a/src/tests/web-bridge-package-root.test.ts b/src/tests/web-bridge-package-root.test.ts
new file mode 100644
index 000000000..f919ce873
--- /dev/null
+++ b/src/tests/web-bridge-package-root.test.ts
@@ -0,0 +1,70 @@
+/**
+ * Regression tests for the default package root fallback in bridge-service.
+ *
+ * Issue: gsd-build/gsd-2#1881
+ * The standalone Next.js bundle bakes import.meta.url at build time with the
+ * CI runner's absolute path.  On Windows, fileURLToPath() rejects the Unix
+ * file:// URL at module load time, 500-ing all API routes.
+ *
+ * The fix makes the fallback lazy and catch-guarded so the module loads safely
+ * on any OS regardless of what import.meta.url resolved to at build time.
+ */
+
+import test from "node:test";
+import assert from "node:assert/strict";
+import { resolve } from "node:path";
+
+const bridge = await import("../web/bridge-service.ts");
+
+test("resolveBridgeRuntimeConfig uses GSD_WEB_PACKAGE_ROOT when set", () => {
+  const env = {
+    GSD_WEB_PACKAGE_ROOT: "/custom/package/root",
+    GSD_WEB_PROJECT_CWD: "/some/project",
+  } as unknown as NodeJS.ProcessEnv;
+
+  const config = bridge.resolveBridgeRuntimeConfig(env);
+  assert.equal(config.packageRoot, "/custom/package/root");
+});
+
+test("resolveBridgeRuntimeConfig falls back to lazy default when GSD_WEB_PACKAGE_ROOT is absent", () => {
+  // Reset the memoized value so we exercise the lazy computation path.
+  bridge.resetDefaultPackageRootForTests();
+
+  const env = {
+    GSD_WEB_PROJECT_CWD: "/some/project",
+  } as unknown as NodeJS.ProcessEnv;
+
+  // Should not throw — the lazy getter catches cross-platform failures.
+  const config = bridge.resolveBridgeRuntimeConfig(env);
+  assert.equal(typeof config.packageRoot, "string");
+  assert.ok(config.packageRoot.length > 0, "packageRoot must be a non-empty string");
+});
+
+test("lazy default package root is an absolute path", () => {
+  bridge.resetDefaultPackageRootForTests();
+
+  const env = {
+    GSD_WEB_PROJECT_CWD: "/some/project",
+  } as unknown as NodeJS.ProcessEnv;
+
+  const config = bridge.resolveBridgeRuntimeConfig(env);
+  // resolve() returns the same path if already absolute.
+  assert.equal(config.packageRoot, resolve(config.packageRoot));
+});
+
+test("lazy default package root is memoized across calls", () => {
+  bridge.resetDefaultPackageRootForTests();
+
+  const env = {} as unknown as NodeJS.ProcessEnv;
+
+  const first = bridge.resolveBridgeRuntimeConfig(env).packageRoot;
+  const second = bridge.resolveBridgeRuntimeConfig(env).packageRoot;
+  assert.equal(first, second, "memoized value should be stable across calls");
+});
+
+test("module loads without throwing (regression: eager fileURLToPath crash)", () => {
+  // The fact that we can import bridge-service at the top of this file without
+  // an unhandled exception is itself the primary regression gate.  This test
+  // makes that contract explicit.
+  assert.ok(typeof bridge.resolveBridgeRuntimeConfig === "function");
+});
diff --git a/src/web/bridge-service.ts b/src/web/bridge-service.ts
index ebac2e8b1..796873fc7 100644
--- a/src/web/bridge-service.ts
+++ b/src/web/bridge-service.ts
@@ -39,7 +39,30 @@ import {
 } from "./auto-dashboard-service.ts";
 import { resolveGsdCliEntry } from "./cli-entry.ts";
 
-const DEFAULT_PACKAGE_ROOT = resolve(dirname(fileURLToPath(import.meta.url)), "../..");
+// Lazily computed fallback — import.meta.url is baked in at build time by
+// webpack, so when the standalone bundle built on Linux CI runs on Windows the
+// literal file:// URL contains a Unix path that fileURLToPath() rejects.
+// Deferring the computation means it only fires when GSD_WEB_PACKAGE_ROOT is
+// absent, and if it does fire we handle the cross-platform failure gracefully.
+let _defaultPackageRoot: string | undefined;
+function getDefaultPackageRoot(): string {
+  if (_defaultPackageRoot !== undefined) return _defaultPackageRoot;
+  try {
+    _defaultPackageRoot = resolve(dirname(fileURLToPath(import.meta.url)), "../..");
+  } catch {
+    // Standalone bundle running on a different OS than the builder — the
+    // baked-in import.meta.url is not a valid local file URL.  Fall back to
+    // cwd which is the best available approximation; callers that need the
+    // real package root should set GSD_WEB_PACKAGE_ROOT.
+    _defaultPackageRoot = process.cwd();
+  }
+  return _defaultPackageRoot;
+}
+
+/** @internal — test-only: reset the memoized default package root */
+export function resetDefaultPackageRootForTests(): void {
+  _defaultPackageRoot = undefined;
+}
 const RESPONSE_TIMEOUT_MS = 30_000;
 const START_TIMEOUT_MS = 150_000;
 const MAX_STDERR_BUFFER = 8_000;
@@ -1058,7 +1081,7 @@ async function fallbackWorkspaceIndex(basePath: string): Promise<GSDWorkspaceInd
 export function resolveBridgeRuntimeConfig(env: NodeJS.ProcessEnv = getBridgeDeps().env ?? process.env, projectCwdOverride?: string): BridgeRuntimeConfig {
   const projectCwd = projectCwdOverride || env.GSD_WEB_PROJECT_CWD || process.cwd();
   const projectSessionsDir = env.GSD_WEB_PROJECT_SESSIONS_DIR || getProjectSessionsDir(projectCwd);
-  const packageRoot = env.GSD_WEB_PACKAGE_ROOT || DEFAULT_PACKAGE_ROOT;
+  const packageRoot = env.GSD_WEB_PACKAGE_ROOT || getDefaultPackageRoot();
   return { projectCwd, projectSessionsDir, packageRoot };
 }