diff --git a/packages/agent/src/session-log-writer.test.ts b/packages/agent/src/session-log-writer.test.ts
index f99aec380..b78e7f4e5 100644
--- a/packages/agent/src/session-log-writer.test.ts
+++ b/packages/agent/src/session-log-writer.test.ts
@@ -168,4 +168,91 @@ describe("SessionLogWriter", () => {
       expect(logWriter.isRegistered(sessionId)).toBe(true);
     });
   });
+
+  describe("flush serialization", () => {
+    it("serializes concurrent flush calls so they do not overlap", async () => {
+      const sessionId = "s1";
+      logWriter.register(sessionId, { taskId: "t1", runId: sessionId });
+
+      const callOrder: string[] = [];
+      let resolveFirst!: () => void;
+      const firstBlocked = new Promise<void>((r) => {
+        resolveFirst = r;
+      });
+
+      mockAppendLog
+        .mockImplementationOnce(async () => {
+          callOrder.push("first-start");
+          // Add a new entry while the first flush is in-flight
+          logWriter.appendRawLine(sessionId, JSON.stringify({ method: "b" }));
+          await firstBlocked;
+          callOrder.push("first-end");
+        })
+        .mockImplementationOnce(async () => {
+          callOrder.push("second-start");
+        });
+
+      logWriter.appendRawLine(sessionId, JSON.stringify({ method: "a" }));
+      const flush1 = logWriter.flush(sessionId);
+
+      // Wait for the first flush to be in-flight — "b" is added inside the mock
+      await vi.waitFor(() => expect(callOrder).toContain("first-start"));
+
+      // Queue a second flush for the entry added while first was in-flight
+      const flush2 = logWriter.flush(sessionId);
+
+      // First flush is blocked — second should not have started
+      expect(callOrder).not.toContain("second-start");
+
+      // Unblock first flush
+      resolveFirst?.();
+      await flush1;
+      await flush2;
+
+      // Second started only after first completed
+      expect(callOrder).toEqual(["first-start", "first-end", "second-start"]);
+    });
+
+    it("does not lose entries when flushes are serialized", async () => {
+      const sessionId = "s1";
+      logWriter.register(sessionId, { taskId: "t1", runId: sessionId });
+
+      let resolveFirst!: () => void;
+      const firstBlocked = new Promise<void>((r) => {
+        resolveFirst = r;
+      });
+
+      mockAppendLog
+        .mockImplementationOnce(async () => {
+          // Add a new entry while the first flush is in-flight — simulates
+          // the agent emitting end_turn while a scheduled flush is sending
+          // earlier entries to S3.
+          logWriter.appendRawLine(sessionId, JSON.stringify({ method: "b" }));
+          await firstBlocked;
+        })
+        .mockImplementationOnce(async () => undefined);
+
+      // Batch 1
+      logWriter.appendRawLine(sessionId, JSON.stringify({ method: "a" }));
+      const flush1 = logWriter.flush(sessionId);
+
+      // Wait for first flush to be in-flight (and "b" to be added)
+      await vi.waitFor(() => expect(mockAppendLog).toHaveBeenCalledTimes(1));
+
+      // Queue flush for the entry added while first was in-flight
+      const flush2 = logWriter.flush(sessionId);
+
+      resolveFirst?.();
+      await flush1;
+      await flush2;
+
+      expect(mockAppendLog).toHaveBeenCalledTimes(2);
+      const batch1: StoredNotification[] = mockAppendLog.mock.calls[0][2];
+      const batch2: StoredNotification[] = mockAppendLog.mock.calls[1][2];
+      expect(batch1).toHaveLength(1);
+      expect(batch1[0].notification.method).toBe("a");
+      expect(batch2).toHaveLength(1);
+      expect(batch2[0].notification.method).toBe("b");
+    });
+  });
 });
diff --git a/packages/agent/src/session-log-writer.ts b/packages/agent/src/session-log-writer.ts
index b615752ec..a49890096 100644
--- a/packages/agent/src/session-log-writer.ts
+++ b/packages/agent/src/session-log-writer.ts
@@ -40,6 +40,7 @@ export class SessionLogWriter {
   private lastFlushAttemptTime: Map<string, number> = new Map();
   private retryCounts: Map<string, number> = new Map();
   private sessions: Map<string, SessionState> = new Map();
+  private flushQueues: Map<string, Promise<void>> = new Map();
 
   private logger: Logger;
   private localCachePath?: string;
@@ -155,6 +156,19 @@ export class SessionLogWriter {
   }
 
   async flush(sessionId: string): Promise<void> {
+    // Serialize flushes per session
+    const prev = this.flushQueues.get(sessionId) ?? Promise.resolve();
+    const next = prev.catch(() => {}).then(() => this._doFlush(sessionId));
+    this.flushQueues.set(sessionId, next);
+    next.finally(() => {
+      if (this.flushQueues.get(sessionId) === next) {
+        this.flushQueues.delete(sessionId);
+      }
+    });
+    return next;
+  }
+
+  private async _doFlush(sessionId: string): Promise<void> {
     const session = this.sessions.get(sessionId);
     if (!session) {
       this.logger.warn("flush: no session found", { sessionId });