diff --git a/index.ts b/index.ts
index e705bbb6..d7f5af75 100644
--- a/index.ts
+++ b/index.ts
@@ -314,8 +314,9 @@ export const OpenAIOAuthPlugin: Plugin = async ({ client }: PluginInput) => {
 	let refreshGuardian: RefreshGuardian | null = null;
 	let refreshGuardianConfigKey: string | null = null;
 	let refreshGuardianCleanupRegistered = false;
-	let sessionAffinityStore: SessionAffinityStore | null =
-		new SessionAffinityStore();
+let sessionAffinityStore: SessionAffinityStore | null =
+	new SessionAffinityStore();
+let sessionAffinityWriteVersion = 0;
 	let sessionAffinityConfigKey: string | null = null;
 	const entitlementCache = new EntitlementCache();
 	const preemptiveQuotaScheduler = new PreemptiveQuotaScheduler();
@@ -862,6 +863,8 @@ export const OpenAIOAuthPlugin: Plugin = async ({ client }: PluginInput) => {
 										.trim() || undefined;
 								const sessionAffinityKey =
 									threadIdCandidate ?? promptCacheKey ?? null;
+								const sessionAffinityVersion =
+									(sessionAffinityWriteVersion += 1);
 								const effectivePromptCacheKey =
 									(sessionAffinityKey ?? promptCacheKey ?? "")
 										.toString()
@@ -2273,15 +2276,19 @@ export const OpenAIOAuthPlugin: Plugin = async ({ client }: PluginInput) => {
 												isStreaming,
 												{
 													onResponseId: (responseId) => {
-														if (!responseContinuationEnabled) return;
-														sessionAffinityStore?.remember(
-															sessionAffinityKey,
-															successAccountForResponse.index,
-														);
-														sessionAffinityStore?.updateLastResponseId(
-															sessionAffinityKey,
-															responseId,
-														);
+												if (!responseContinuationEnabled) return;
+												sessionAffinityStore?.rememberWithVersion(
+													sessionAffinityKey,
+													successAccountForResponse.index,
+													Date.now(),
+													sessionAffinityVersion,
+												);
+												sessionAffinityStore?.updateLastResponseId(
+													sessionAffinityKey,
+													responseId,
+													Date.now(),
+													sessionAffinityVersion,
+												);
 														storedResponseIdForSuccess = true;
 													},
 													streamStallTimeoutMs,
diff --git a/lib/session-affinity.ts b/lib/session-affinity.ts
index 1ce27e30..91efda33 100644
--- a/lib/session-affinity.ts
+++ b/lib/session-affinity.ts
@@ -12,6 +12,7 @@ interface SessionAffinityEntry {
 	expiresAt: number;
 	lastResponseId?: string;
 	updatedAt: number;
+	writeVersion: number;
 }
 
 const DEFAULT_TTL_MS = 20 * 60 * 1000;
@@ -62,17 +63,34 @@ export class SessionAffinityStore {
 	}
 
 	remember(sessionKey: string | null | undefined, accountIndex: number, now = Date.now()): void {
+		this.rememberWithVersion(sessionKey, accountIndex, now, now);
+	}
+
+	rememberWithVersion(
+		sessionKey: string | null | undefined,
+		accountIndex: number,
+		now = Date.now(),
+		writeVersion = now,
+	): void {
 		const key = normalizeSessionKey(sessionKey);
 		if (!key) return;
 		if (!Number.isFinite(accountIndex) || accountIndex < 0) return;
 
 		const existingEntry = this.entries.get(key);
+		if (
+			existingEntry &&
+			existingEntry.expiresAt > now &&
+			existingEntry.writeVersion > writeVersion
+		) {
+			return;
+		}
 
 		this.setEntry(key, {
 			accountIndex,
 			expiresAt: now + this.ttlMs,
 			lastResponseId: existingEntry?.lastResponseId,
 			updatedAt: now,
+			writeVersion,
 		});
 	}
 
@@ -110,6 +128,7 @@ export class SessionAffinityStore {
 		sessionKey: string | null | undefined,
 		responseId: string | null | undefined,
 		now = Date.now(),
+		writeVersion = now,
 	): void {
 		const key = normalizeSessionKey(sessionKey);
 		const normalizedResponseId = typeof responseId === "string" ? responseId.trim() : "";
@@ -121,12 +140,16 @@ export class SessionAffinityStore {
 			this.entries.delete(key);
 			return;
 		}
+		if (entry.writeVersion > writeVersion) {
+			return;
+		}
 
 		this.setEntry(key, {
 			...entry,
 			expiresAt: now + this.ttlMs,
 			lastResponseId: normalizedResponseId,
 			updatedAt: now,
+			writeVersion,
 		});
 	}
 
diff --git a/test/index.test.ts b/test/index.test.ts
index b5aed127..97d3a468 100644
--- a/test/index.test.ts
+++ b/test/index.test.ts
@@ -1642,7 +1642,7 @@ describe("OpenAIOAuthPlugin fetch handler", () => {
 		expect(secondBody?.previous_response_id).toBe("resp_standalone_789");
 	});
 
-	it("keeps account and previous_response_id aligned across overlapping same-session streams", async () => {
+	it("keeps the newest account and previous_response_id across overlapping same-session streams", async () => {
 		const { AccountManager } = await import("../lib/accounts.js");
 		const configModule = await import("../lib/config.js");
 		const fetchHelpers = await import("../lib/request/fetch-helpers.js");
@@ -1748,9 +1748,9 @@ describe("OpenAIOAuthPlugin fetch handler", () => {
 			previous_response_id?: string;
 		};
 		const thirdHeaders = new Headers(thirdInit.headers);
-		expect(thirdBody.previous_response_id).toBe("resp_first_123");
-		expect(thirdHeaders.get("x-test-account-id")).toBe("acc-1");
-		expect(thirdHeaders.get("x-test-access-token")).toBe("access-alpha");
+		expect(thirdBody.previous_response_id).toBe("resp_second_456");
+		expect(thirdHeaders.get("x-test-account-id")).toBe("acc-2");
+		expect(thirdHeaders.get("x-test-access-token")).toBe("access-beta");
 	});
 	it("compacts fast-session input before sending the upstream request when compaction succeeds", async () => {
 		const fetchHelpers = await import("../lib/request/fetch-helpers.js");
diff --git a/test/session-affinity.test.ts b/test/session-affinity.test.ts
index 1bf1ae73..dd304823 100644
--- a/test/session-affinity.test.ts
+++ b/test/session-affinity.test.ts
@@ -143,4 +143,18 @@ describe("SessionAffinityStore", () => {
 		expect(store.getLastResponseId("session-a", 3_500)).toBe("resp_123");
 		expect(store.getPreferredAccountIndex("session-a", 3_500)).toBe(2);
 	});
+
+	it("ignores stale response-id writes from older overlapping requests", () => {
+		const store = new SessionAffinityStore({ ttlMs: 10_000, maxEntries: 4 });
+		store.rememberWithVersion("session-a", 1, 1_000, 1);
+		store.updateLastResponseId("session-a", "resp_first", 2_000, 1);
+		store.rememberWithVersion("session-a", 2, 3_000, 2);
+		store.updateLastResponseId("session-a", "resp_second", 4_000, 2);
+
+		store.rememberWithVersion("session-a", 1, 5_000, 1);
+		store.updateLastResponseId("session-a", "resp_stale", 5_000, 1);
+
+		expect(store.getPreferredAccountIndex("session-a", 5_500)).toBe(2);
+		expect(store.getLastResponseId("session-a", 5_500)).toBe("resp_second");
+	});
 });