diff --git a/crates/socket-patch-cli/CLI_CONTRACT.md b/crates/socket-patch-cli/CLI_CONTRACT.md
index f74b21f3..9a319d24 100644
--- a/crates/socket-patch-cli/CLI_CONTRACT.md
+++ b/crates/socket-patch-cli/CLI_CONTRACT.md
@@ -753,6 +753,7 @@ These exist for staged rollouts and the launcher wrappers. They are **internal**
 | `SOCKET_UPDATE_STATE_DIR` | Overrides the per-user dir holding `update-check.json` + `update.lock` (tests point it into a tempdir). |
 | `SOCKET_UPDATE_TIMEOUT_MS` | Caps the update fetches' connect/metadata/download budgets (defaults 10 s / 30 s / 300 s; the notice's fetch defaults to 2 s). Doubles as the slow-network escape hatch. |
 | `SOCKET_UPDATE_NOTIFIER_FORCE` | Test hook: bypasses the update notice's stderr-TTY guard — and nothing else (opt-out, offline, `--silent`, `--json`, CI all still win). |
+| `SOCKET_UPDATE_GRACE_MS` | Test hook: overrides the notice's post-command join grace (default 500 ms — how long the run waits for the background check before abandoning it and exiting). Lets the e2e suite await the loopback fetch to completion so its observable effect is deterministic; production keeps the tight 500 ms ceiling. |
 
 ### Deprecated env vars
 
diff --git a/crates/socket-patch-cli/src/update_notifier.rs b/crates/socket-patch-cli/src/update_notifier.rs
index f6be28c7..1ecc22b0 100644
--- a/crates/socket-patch-cli/src/update_notifier.rs
+++ b/crates/socket-patch-cli/src/update_notifier.rs
@@ -264,6 +264,38 @@ fn format_notice(
     )
 }
 
+const DEFAULT_GRACE_MS: u64 = 500;
+
+/// How long `finish` waits for the background fetch before abandoning it.
+///
+/// 500 ms in production — deliberately tight so a real command never stalls
+/// on the notifier. The catch: `main` calls `std::process::exit` the instant
+/// `finish` returns, which kills a still-running detached fetch before its
+/// state write (or even its outbound request) can land. On a fast host the
+/// loopback fetch finishes in milliseconds and comfortably beats the ceiling;
+/// on a slow one (a loaded Windows CI runner, fsync latency, a cold TLS/HTTP
+/// client) the fetch can miss the 500 ms window, the task is killed, and its
+/// `latestSeen` write / `expect`-counted request simply never happens — an
+/// e2e that asserts on that observable effect then fails intermittently.
+///
+/// `SOCKET_UPDATE_GRACE_MS` lets the e2e suite lift the ceiling so the fetch
+/// is awaited to completion instead of raced. Same shape as the
+/// `SOCKET_UPDATE_TIMEOUT_MS` fetch-budget hook; the default is preserved, so
+/// production behavior is byte-identical.
+fn grace_budget() -> Duration {
+    grace_budget_from(std::env::var("SOCKET_UPDATE_GRACE_MS").ok().as_deref())
+}
+
+/// Pure core of [`grace_budget`]: parse the raw env value, falling back to
+/// the production default on absence, emptiness, or garbage.
+fn grace_budget_from(raw: Option<&str>) -> Duration {
+    let ms = raw
+        .filter(|v| !v.is_empty())
+        .and_then(|v| v.parse::<u64>().ok())
+        .unwrap_or(DEFAULT_GRACE_MS);
+    Duration::from_millis(ms)
+}
+
 /// Join the background fetch within the grace budget and print the notice
 /// if one is warranted. Runs after all command output; never touches
 /// stdout or the exit code.
@@ -273,7 +305,7 @@ pub async fn finish(notifier: Option<Notifier>) {
     };
     let fetched = match notifier.task {
         Some(handle) => {
-            match tokio::time::timeout(Duration::from_millis(500), handle).await {
+            match tokio::time::timeout(grace_budget(), handle).await {
                 Ok(Ok(result)) => result,
                 // Timed out (the task keeps running until process exit —
                 // its own state write may still land) or panicked; either
@@ -407,6 +439,18 @@ mod tests {
         }
     }
 
+    #[test]
+    fn grace_budget_defaults_preserved_and_override_honored() {
+        // Absence, emptiness, and garbage all keep the tight production
+        // ceiling — the override never silently changes shipped behavior.
+        assert_eq!(grace_budget_from(None), Duration::from_millis(500));
+        assert_eq!(grace_budget_from(Some("")), Duration::from_millis(500));
+        assert_eq!(grace_budget_from(Some("not-a-number")), Duration::from_millis(500));
+        // A valid value lifts the ceiling (the e2e suite's escape hatch).
+        assert_eq!(grace_budget_from(Some("30000")), Duration::from_millis(30_000));
+        assert_eq!(grace_budget_from(Some("0")), Duration::from_millis(0));
+    }
+
     #[test]
     fn notice_names_versions_hint_and_optout() {
         let current = semver::Version::new(3, 3, 0);
diff --git a/crates/socket-patch-cli/tests/update_notifier_e2e.rs b/crates/socket-patch-cli/tests/update_notifier_e2e.rs
index 8e887de5..09cbfdd0 100644
--- a/crates/socket-patch-cli/tests/update_notifier_e2e.rs
+++ b/crates/socket-patch-cli/tests/update_notifier_e2e.rs
@@ -87,6 +87,29 @@ fn eligible_kit(base_url: &str) -> Vec<(&str, &str)> {
     ]
 }
 
+/// [`eligible_kit`] plus a lifted join grace, for every row that asserts on
+/// the background check's *observable effect* — a rewritten `latestSeen`, a
+/// notice on stderr, or a request an `expect_resolves` mock counts.
+///
+/// The notifier's fetch runs in a detached task, and the child calls
+/// `std::process::exit` the instant the notifier's `finish` returns. Under
+/// the production 500 ms grace that is a genuine race: on a fast host the
+/// loopback fetch lands its state write / request in a few ms and wins, but
+/// on a slow one (a loaded Windows CI runner) it can miss the window, get
+/// killed at exit, and never write `latestSeen` or reach the mock at all —
+/// the "reads the STALE version" / "expect(1) unmet" Windows flake. Lifting
+/// the ceiling via `SOCKET_UPDATE_GRACE_MS` makes `finish` await the fetch to
+/// completion instead of racing it, so the effect is deterministic. It does
+/// NOT weaken any assertion: the fetch still completes in milliseconds, only
+/// the artificial cutoff is gone (`grace_budget_bounds_command_latency`
+/// keeps the real 500 ms ceiling, since testing that cutoff is its whole
+/// point).
+fn eligible_kit_await_fetch(base_url: &str) -> Vec<(&str, &str)> {
+    let mut kit = eligible_kit(base_url);
+    kit.push(("SOCKET_UPDATE_GRACE_MS", "30000"));
+    kit
+}
+
 /// The notifier must never mutate the install or the project dir, on any
 /// path — every row re-proves it.
 fn assert_install_pristine(install: &StagedInstall) {
@@ -110,7 +133,7 @@ async fn first_eligible_run_checks_and_notices() {
         .await;
 
     let (code, stdout, stderr) =
-        run_installed(&install, &["apply"], &eligible_kit(&release.base_url));
+        run_installed(&install, &["apply"], &eligible_kit_await_fetch(&release.base_url));
     assert_eq!(code, 0, "stdout:\n{stdout}\nstderr:\n{stderr}");
     assert!(
         stderr.contains("Update available") && stderr.contains("9.9.9"),
@@ -174,7 +197,7 @@ async fn stale_state_rechecks() {
     write_state(&install.state_dir, STALE, Some(CURRENT), None);
 
     let (code, stdout, stderr) =
-        run_installed(&install, &["apply"], &eligible_kit(&release.base_url));
+        run_installed(&install, &["apply"], &eligible_kit_await_fetch(&release.base_url));
     assert_eq!(code, 0, "stdout:\n{stdout}\nstderr:\n{stderr}");
 
     let state = read_state(&install.state_dir);
@@ -202,7 +225,8 @@ async fn up_to_date_prints_nothing() {
         .await;
     write_state(&install.state_dir, STALE, Some(CURRENT), None);
 
-    let (code, _, stderr) = run_installed(&install, &["apply"], &eligible_kit(&release.base_url));
+    let (code, _, stderr) =
+        run_installed(&install, &["apply"], &eligible_kit_await_fetch(&release.base_url));
     assert_eq!(code, 0);
     assert!(
         !stderr.contains("Update available"),
@@ -265,7 +289,7 @@ async fn corrupt_state_recovers() {
     .unwrap();
 
     let (code, stdout, stderr) =
-        run_installed(&install, &["apply"], &eligible_kit(&release.base_url));
+        run_installed(&install, &["apply"], &eligible_kit_await_fetch(&release.base_url));
     assert_eq!(code, 0, "stdout:\n{stdout}\nstderr:\n{stderr}");
     assert!(
         !stderr.contains("panicked"),
@@ -293,7 +317,7 @@ async fn future_timestamp_tolerated() {
     write_state(&install.state_dir, -48 * HOUR, Some("9.9.9"), None);
 
     let (code, stdout, stderr) =
-        run_installed(&install, &["apply"], &eligible_kit(&release.base_url));
+        run_installed(&install, &["apply"], &eligible_kit_await_fetch(&release.base_url));
     assert_eq!(code, 0, "stdout:\n{stdout}\nstderr:\n{stderr}");
     assert_install_pristine(&install);
 }
@@ -636,6 +660,10 @@ mod pty {
             ("CI", ""),
             ("GITHUB_ACTIONS", ""),
             ("SOCKET_UPDATE_BASE_URL", &release.base_url),
+            // Await the background fetch rather than race the child's exit —
+            // the notice only appears once the check lands (see
+            // `eligible_kit_await_fetch`).
+            ("SOCKET_UPDATE_GRACE_MS", "30000"),
         ];
         let (code, output) = run_in_pty(
             &install.bin,