diff --git a/packages/vscode/e2e/rstest/suite/helpers.ts b/packages/vscode/e2e/rstest/suite/helpers.ts
index 899f05f7..67b322fb 100644
--- a/packages/vscode/e2e/rstest/suite/helpers.ts
+++ b/packages/vscode/e2e/rstest/suite/helpers.ts
@@ -3,6 +3,7 @@
 // the `Rstest` instance directly, while here the shell republishes the stack's
 // exports through `RstackExtensionExports` (see `src/types.ts`).
 import assert from 'node:assert';
+import fs from 'node:fs/promises';
 import path from 'node:path';
 import vscode from 'vscode';
 import type { RstackExtensionExports } from '../../../src/types';
@@ -62,6 +63,59 @@ export function currentRstestExports(): RstestExports {
   return stackExports as unknown as RstestExports;
 }
 
+/** How long one identical-content rewrite waits for the event to come back. */
+const WATCHER_ARM_PROBE_WAIT_MS = 500;
+/** Overall budget for proving the watcher armed, rewriting between probes. */
+const WATCHER_ARM_TIMEOUT_MS = 15_000;
+
+/**
+ * Proves the detection file watcher delivers events for `file` before the
+ * suite mutates watched fixture files. `fs.rename`/`write` issued right after
+ * the watchers changed — a folder added or `configFileGlobPattern` updated —
+ * race the watcher's arming: on Linux the backend has been observed taking
+ * hundreds of milliseconds to arm and dropping the whole event burst written
+ * in that window, which is the "detection never re-ran" ecosystem-CI failure.
+ * The FileSystemWatcher API has no ready signal, so the suite instead rewrites
+ * `file` with its identical content and waits for a watcher event through
+ * `getDetectionWatcherEventCount()`: if the write is not observed, the
+ * upcoming mutations would not be either. The rewrite never changes
+ * detection's output, so this only costs a no-op detection pass.
+ *
+ * `file` must be a file the currently-configured watch patterns match (e.g. a
+ * live `rstest.config.*` after the fixture creates it), or the probe never
+ * completes.
+ */
+export async function waitForWatcherArmed(file: string): Promise<void> {
+  const api =
+    vscode.extensions.getExtension<RstackExtensionExports>(
+      EXTENSION_ID,
+    )?.exports;
+  assert.ok(api, `extension ${EXTENSION_ID} should be activated`);
+  const eventCount = () => api.getDetectionWatcherEventCount();
+  const content = await fs.readFile(file, 'utf8');
+  let attempts = 0;
+  const deadline = Date.now() + WATCHER_ARM_TIMEOUT_MS;
+  while (Date.now() < deadline) {
+    attempts += 1;
+    const baseline = eventCount();
+    await fs.writeFile(file, content);
+    const probeDeadline = Math.min(
+      Date.now() + WATCHER_ARM_PROBE_WAIT_MS,
+      deadline,
+    );
+    while (Date.now() < probeDeadline) {
+      await delay(25);
+      if (eventCount() !== baseline) {
+        return;
+      }
+    }
+  }
+  throw new Error(
+    `the file watcher never delivered an event within ${WATCHER_ARM_TIMEOUT_MS} ms: rewrote ${file} ` +
+      `${attempts} times with identical content`,
+  );
+}
+
 export async function delay(ms: number) {
   await new Promise((resolve) => setTimeout(resolve, ms));
 }
diff --git a/packages/vscode/e2e/rstest/suite/workspace.test.ts b/packages/vscode/e2e/rstest/suite/workspace.test.ts
index c1b211bd..30b87100 100644
--- a/packages/vscode/e2e/rstest/suite/workspace.test.ts
+++ b/packages/vscode/e2e/rstest/suite/workspace.test.ts
@@ -28,6 +28,7 @@ import {
   getTestItemByLabels,
   toLabelTree,
   waitFor,
+  waitForWatcherArmed,
 } from './helpers';
 
 // The unwrapped single-folder tree of workspace-1 (the standard layout: one
@@ -122,6 +123,12 @@ suite('Workspace discover suite', () => {
     // (detection scopes per folder): with no matching config the folder is undetected and
     // dropped entirely, and workspace-1 — the only detected folder left —
     // is shown unwrapped again.
+    // The renames below must be seen by the freshly installed workspace-2
+    // watcher (nothing else re-runs detection): on Linux the backend arms
+    // late, so probe it first (see `waitForWatcherArmed`).
+    await waitForWatcherArmed(
+      path.resolve(fixturesRoot, 'workspace-2/project-1/rstest.config.ts'),
+    );
     await fs.rename(
       path.resolve(fixturesRoot, 'workspace-2/project-1/rstest.config.ts'),
       path.resolve(fixturesRoot, 'workspace-2/project-1/foo.config.ts'),
@@ -167,6 +174,13 @@ suite('Workspace discover suite', () => {
     // add config file.
     // Adapted like the step above: workspace-2 is still the only detected
     // folder, so its two projects are shown unwrapped.
+    // The rename's create event under the new glob is the only signal: probe
+    // the watcher reinstalled by `config.update` above before renaming
+    // (`project-2` still holds a `bar.config.ts`, which the `foo.config.*`
+    // glob does not watch, so the probe targets `project-1`).
+    await waitForWatcherArmed(
+      path.resolve(fixturesRoot, 'workspace-2/project-1/foo.config.ts'),
+    );
     await fs.rename(
       path.resolve(fixturesRoot, 'workspace-2/folder/project-2/bar.config.ts'),
       path.resolve(fixturesRoot, 'workspace-2/folder/project-2/foo.config.ts'),
diff --git a/packages/vscode/src/detection.ts b/packages/vscode/src/detection.ts
index 088d167e..ee263d44 100644
--- a/packages/vscode/src/detection.ts
+++ b/packages/vscode/src/detection.ts
@@ -277,6 +277,10 @@ export class DetectionService implements vscode.Disposable {
   // rebuild itself does not need the event, it already has the fresh snapshot.
   #notifyUnchanged = false;
   #watchers: vscode.Disposable[] = [];
+  /** E2E only: monotonic count of watcher events received — the arm probe in
+   * `e2e/rstest/suite/helpers.ts` compares against its own baseline, so the
+   * counter is never reset. */
+  #watcherEventCount = 0;
   #debounce: ReturnType<typeof setTimeout> | undefined;
   #running: Promise<DetectionSnapshot> | undefined;
   #rerun = false;
@@ -305,6 +309,11 @@ export class DetectionService implements vscode.Disposable {
     return this.#snapshot;
   }
 
+  /** E2E only — see `#watcherEventCount`. */
+  get watcherEventCount(): number {
+    return this.#watcherEventCount;
+  }
+
   /** Installs the watchers and runs the first detection pass. */
   async initialize(): Promise<DetectionSnapshot> {
     this.installWatchers();
@@ -399,6 +408,7 @@ export class DetectionService implements vscode.Disposable {
     // A lockfile event is a dependency change wherever it is watched: a
     // monorepo opened at a subdirectory installs at an ancestor.
     const onEvent = (uri: vscode.Uri) => {
+      this.#watcherEventCount += 1;
       if (isLockfile(uri)) {
         this.#notifyUnchanged = true;
       }
diff --git a/packages/vscode/src/extension.ts b/packages/vscode/src/extension.ts
index eac8eb74..70be0341 100644
--- a/packages/vscode/src/extension.ts
+++ b/packages/vscode/src/extension.ts
@@ -538,6 +538,7 @@ class ExtensionShell {
         }
         this.syncDependencyPoll();
       },
+      getDetectionWatcherEventCount: () => this.#detection.watcherEventCount,
     };
   }
 
diff --git a/packages/vscode/src/types.ts b/packages/vscode/src/types.ts
index e2e82772..bbaa0c25 100644
--- a/packages/vscode/src/types.ts
+++ b/packages/vscode/src/types.ts
@@ -210,6 +210,8 @@ export interface RstackExtensionExports {
   whenStackActive(stack: StackId): Promise<Record<string, unknown>>;
   /** E2E only: shorten the shell's dependency-recovery polling interval. */
   setDependencyPollIntervalForTest(intervalMs: number): void;
+  /** E2E only: see `watcherEventCount` (`src/detection.ts`). */
+  getDetectionWatcherEventCount(): number;
 }
 
 export type StackControllerFactory = () => StackController;