diff --git a/open-sse/services/accountFallback.ts b/open-sse/services/accountFallback.ts
index dbc087252d..12271a22cf 100644
--- a/open-sse/services/accountFallback.ts
+++ b/open-sse/services/accountFallback.ts
@@ -474,8 +474,14 @@ export function lockModelIfPerModelQuota(
 export function shouldMarkAccountExhaustedFrom429(
   provider: string | null | undefined,
   model: string | null | undefined = null,
-  connectionPassthroughModels?: boolean
+  connectionPassthroughModels?: boolean,
+  failureKind?: FailureKind
 ): boolean {
+  // A plain 429 means transient rate limiting / high traffic for many OAuth providers.
+  // Only connection-poison the quota cache when the upstream body explicitly says
+  // the long-window quota is exhausted; otherwise fallback should try another account
+  // without making this one look quota-depleted for 5 minutes.
+  if (failureKind === "rate_limit" || failureKind === "transient") return false;
   return (
     shouldPreserveQuotaSignalsFor429(provider) &&
     !hasPerModelQuota(provider, model, connectionPassthroughModels)
diff --git a/src/sse/handlers/chat.ts b/src/sse/handlers/chat.ts
index e816960249..9a9bb63a90 100644
--- a/src/sse/handlers/chat.ts
+++ b/src/sse/handlers/chat.ts
@@ -1064,15 +1064,18 @@ async function handleSingleModelChat(
         dailyQuotaExhausted = true;
       }
 
-      // 7. Mark account as quota-exhausted on 429 response (non-daily-quota errors)
-      // For providers that route quota/cooldown at model scope, a 429 on one model
-      // does not mean the whole connection is exhausted.
-      // Daily quota errors are handled above; only process regular rate_limit here
+      // 7. Mark account as quota-exhausted only for explicit long-window quota signals.
+      // A plain 429/high-traffic response should trigger fallback/cooldown, not poison
+      // quotaCache as exhausted for 5 minutes while usage quota may still be available.
       if (!dailyQuotaExhausted) {
         const passthroughModels = credentials.providerSpecificData?.passthroughModels;
+        const failureKind =
+          result.status === 429
+            ? classify429FromError({ status: result.status, message: errorStr })
+            : undefined;
         if (
           result.status === 429 &&
-          shouldMarkAccountExhaustedFrom429(provider, model, passthroughModels)
+          shouldMarkAccountExhaustedFrom429(provider, model, passthroughModels, failureKind)
         ) {
           markAccountExhaustedFrom429(credentials.connectionId, provider);
         }
diff --git a/tests/unit/account-fallback-service.test.ts b/tests/unit/account-fallback-service.test.ts
index d3dac846d1..dc153c37d0 100644
--- a/tests/unit/account-fallback-service.test.ts
+++ b/tests/unit/account-fallback-service.test.ts
@@ -295,6 +295,21 @@ test("shouldMarkAccountExhaustedFrom429 skips connection poisoning for compatibl
   assert.equal(shouldMarkAccountExhaustedFrom429("claude", "claude-sonnet-4-6"), true);
 });
 
+test("shouldMarkAccountExhaustedFrom429 does not poison quota cache for transient 429s", () => {
+  assert.equal(
+    shouldMarkAccountExhaustedFrom429("kiro", "claude-opus-4.7", undefined, "rate_limit"),
+    false
+  );
+  assert.equal(
+    shouldMarkAccountExhaustedFrom429("kiro", "claude-opus-4.7", undefined, "transient"),
+    false
+  );
+  assert.equal(
+    shouldMarkAccountExhaustedFrom429("kiro", "claude-opus-4.7", undefined, "quota_exhausted"),
+    true
+  );
+});
+
 test("hasPerModelQuota returns true for GitHub Copilot provider (#1624)", () => {
   assert.equal(hasPerModelQuota("github"), true);
   assert.equal(hasPerModelQuota("github", "gpt-5.1-codex-max"), true);
diff --git a/tests/unit/classify429.test.ts b/tests/unit/classify429.test.ts
index 75d1a4fbd3..a57b04e134 100644
--- a/tests/unit/classify429.test.ts
+++ b/tests/unit/classify429.test.ts
@@ -66,6 +66,13 @@ test("classify429: 429 without quota keyword returns 'rate_limit'", () => {
     }),
     "rate_limit"
   );
+  assert.equal(
+    classify429({
+      status: 429,
+      body: "I am experiencing high traffic, please try again shortly.",
+    }),
+    "rate_limit"
+  );
 });
 
 test("looksLikeQuotaExhausted: detects all known keyword variants", () => {