diff --git a/.changeset/fix-strict-mode-double-exchange.md b/.changeset/fix-strict-mode-double-exchange.md
new file mode 100644
index 0000000..0472c9a
--- /dev/null
+++ b/.changeset/fix-strict-mode-double-exchange.md
@@ -0,0 +1,5 @@
+---
+"@onkernel/managed-auth-react": patch
+---
+
+Guard the bootstrap effect in `useManagedAuthSession` against React 18+ Strict Mode's mount → cleanup → mount double-invocation. Without the guard, the second mount re-fires `exchangeHandoffCode` with a now-consumed handoff code and the component lands in the error state ("Failed to start session") even when auth would have worked. Tracked per `(sessionId, handoffCode)` so a genuine prop change still triggers a fresh exchange.
diff --git a/packages/managed-auth-react/src/session/useManagedAuthSession.ts b/packages/managed-auth-react/src/session/useManagedAuthSession.ts
index 515db26..fe45e64 100644
--- a/packages/managed-auth-react/src/session/useManagedAuthSession.ts
+++ b/packages/managed-auth-react/src/session/useManagedAuthSession.ts
@@ -86,6 +86,11 @@ export function useManagedAuthSession(
     success: false,
     error: false,
   });
+  // Tracks the in-flight bootstrap exchange. ``key`` identifies which
+  // (sessionId, handoffCode) pair it belongs to; ``active`` is false
+  // between cleanup and the matching-key remount. See the effect below
+  // for the invariants these fields enforce.
+  const exchangeRef = useRef<{ key: string; active: boolean } | null>(null);
 
   const stopPolling = useCallback(() => {
     if (pollDelayRef.current) {
@@ -167,7 +172,41 @@ export function useManagedAuthSession(
   );
 
   useEffect(() => {
-    let cancelled = false;
+    // Strict-Mode-safe one-shot init. Under React 18+ Strict Mode in dev,
+    // effects run mount → cleanup → mount; the handoff code is one-shot
+    // server-side, so a naive remount refires the exchange against an
+    // already-consumed code. Three invariants make this safe:
+    //
+    //   1. Guard the exchange by ref identity, not a closure-local
+    //      ``cancelled`` flag — a closure flag set by the synthetic
+    //      cleanup would orphan the first mount's in-flight result.
+    //   2. Track an ``active`` flag on the ref so the async can
+    //      distinguish a real unmount (active stays false) from a
+    //      Strict Mode unmount/remount (active flips false → true
+    //      synchronously before the async resolves).
+    //   3. Always return the cleanup, even on the short-circuit path —
+    //      React only keeps the most recent effect's cleanup, so a bare
+    //      ``return`` from the second mount would orphan ``stopPolling``
+    //      and leak the interval at real unmount.
+    const exchangeKey = `${sessionId}::${handoffCode}`;
+    const cleanup = () => {
+      if (exchangeRef.current?.key === exchangeKey) {
+        exchangeRef.current.active = false;
+      }
+      stopPolling();
+    };
+
+    if (exchangeRef.current?.key === exchangeKey) {
+      // Strict Mode remount of the same exchange: cleanup just flipped
+      // active=false; flip it back so the in-flight async can commit.
+      // Return the cleanup so a later *real* unmount still stops polling.
+      exchangeRef.current.active = true;
+      return cleanup;
+    }
+
+    const ref = { key: exchangeKey, active: true };
+    exchangeRef.current = ref;
+
     (async () => {
       try {
         const token = await exchangeHandoffCode(
@@ -175,10 +214,10 @@ export function useManagedAuthSession(
           handoffCode,
           options,
         );
-        if (cancelled) return;
+        if (exchangeRef.current !== ref || !ref.active) return;
         setJwt(token);
         const initial = await retrieveManagedAuth(sessionId, token, options);
-        if (cancelled) return;
+        if (exchangeRef.current !== ref || !ref.active) return;
         setState(initial);
         const derived = deriveUIState(initial);
         if (
@@ -213,7 +252,7 @@ export function useManagedAuthSession(
           setUIState("prime");
         }
       } catch (err) {
-        if (cancelled) return;
+        if (exchangeRef.current !== ref || !ref.active) return;
         const message =
           err instanceof Error ? err.message : "Failed to start session";
         setInitError(message);
@@ -224,10 +263,7 @@ export function useManagedAuthSession(
         }
       }
     })();
-    return () => {
-      cancelled = true;
-      stopPolling();
-    };
+    return cleanup;
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [sessionId, handoffCode]);