diff --git a/packages/chat/src/store.ts b/packages/chat/src/store.ts
index 32279e628..71c8abc68 100644
--- a/packages/chat/src/store.ts
+++ b/packages/chat/src/store.ts
@@ -10,7 +10,7 @@
 // Routing against the interface (rather than a raw drizzle handle) keeps the
 // route layer testable with a plain in-memory fake, with no database and no
 // drizzle SQL-condition internals involved.
-import { and, eq, inArray } from "drizzle-orm";
+import { and, eq, inArray, sql } from "drizzle-orm";
 import type { PostgresJsDatabase } from "drizzle-orm/postgres-js";
 
 import { participantsOf } from "./workbench-settings";
@@ -311,8 +311,8 @@ export function createDrizzleChatStore<TSchema extends Record<string, unknown>>(
             workbenchReadState.principalId,
           ],
           set: {
-            lastSeenCreatedAt: input.lastSeenCreatedAt,
-            lastSeenId: input.lastSeenId,
+            lastSeenCreatedAt: sql`CASE WHEN excluded.last_seen_created_at >= ${workbenchReadState.lastSeenCreatedAt} THEN excluded.last_seen_created_at ELSE ${workbenchReadState.lastSeenCreatedAt} END`,
+            lastSeenId: sql`CASE WHEN excluded.last_seen_created_at >= ${workbenchReadState.lastSeenCreatedAt} THEN excluded.last_seen_id ELSE ${workbenchReadState.lastSeenId} END`,
           },
         })
         .returning();
@@ -459,11 +459,20 @@ export function createInMemoryChatStore(): ChatStore {
     },
 
     async putReadState(input) {
-      const row: ReadStateRow = { ...input };
-      readStateByKey.set(
-        readStateKey(input.tenantId, input.workbenchId, input.principalId),
-        row,
+      const key = readStateKey(
+        input.tenantId,
+        input.workbenchId,
+        input.principalId,
       );
+      const existing = readStateByKey.get(key);
+      if (
+        existing !== undefined &&
+        existing.lastSeenCreatedAt > input.lastSeenCreatedAt
+      ) {
+        return existing;
+      }
+      const row: ReadStateRow = { ...input };
+      readStateByKey.set(key, row);
       return row;
     },
 
diff --git a/packages/chat/test/read-state.drizzle.test.ts b/packages/chat/test/read-state.drizzle.test.ts
new file mode 100644
index 000000000..a8c844b93
--- /dev/null
+++ b/packages/chat/test/read-state.drizzle.test.ts
@@ -0,0 +1,162 @@
+// DB-gated: skipped when no DATABASE_URL is reachable (a fresh
+// checkout still runs the unit gates), mirroring `migrations.test.ts`.
+// Runs against its own scratch database.
+//
+// `store.test.ts` proves `putReadState`'s monotonicity guard against
+// the in-memory store. This exercises the real `createDrizzleChatStore`
+// path, where the guard is a conditional `ON CONFLICT DO UPDATE ... SET`
+// rather than an in-process comparison — proving the SQL itself never
+// regresses a reader's cursor.
+import { afterAll, beforeAll, describe, expect, test } from "bun:test";
+import { drizzle } from "drizzle-orm/postgres-js";
+import postgres from "postgres";
+
+import { e2eDatabaseUrl } from "../../../scripts/e2e/harness";
+import { applyChatMigrations } from "../src/migrations";
+import { createDrizzleChatStore } from "../src/store";
+
+function scratchUrlFor(e2eUrl: string): string {
+  const url = new URL(e2eUrl);
+  const database = url.pathname.replace(/^\//, "");
+  url.pathname = `/${database}_chat_read_state_drizzle_test`;
+  return url.toString();
+}
+
+const databaseUrl = e2eDatabaseUrl();
+const describeIfDb = databaseUrl === undefined ? describe.skip : describe;
+
+const TENANT = "tnt_1";
+const WORKBENCH = "run_workbench1";
+const PRINCIPAL = "prn_alice";
+
+describeIfDb("createDrizzleChatStore: putReadState monotonicity", () => {
+  const scratchUrl = scratchUrlFor(
+    databaseUrl ?? "postgres://localhost:5432/unused",
+  );
+  const scratchTarget = new URL(scratchUrl);
+  const scratchDatabase = scratchTarget.pathname.replace(/^\//, "");
+
+  beforeAll(async () => {
+    const maintenanceUrl = new URL(scratchUrl);
+    maintenanceUrl.pathname = "/postgres";
+    const maintenance = postgres(maintenanceUrl.toString(), {
+      max: 1,
+      onnotice: () => undefined,
+    });
+    try {
+      await maintenance.unsafe(`DROP DATABASE IF EXISTS "${scratchDatabase}"`);
+      await maintenance.unsafe(`CREATE DATABASE "${scratchDatabase}"`);
+    } finally {
+      await maintenance.end();
+    }
+    await applyChatMigrations(scratchUrl);
+  });
+
+  afterAll(async () => {
+    const maintenanceUrl = new URL(scratchUrl);
+    maintenanceUrl.pathname = "/postgres";
+    const maintenance = postgres(maintenanceUrl.toString(), {
+      max: 1,
+      onnotice: () => undefined,
+    });
+    try {
+      await maintenance.unsafe(`DROP DATABASE IF EXISTS "${scratchDatabase}"`);
+    } finally {
+      await maintenance.end();
+    }
+  });
+
+  test("a stale write landing after a newer one never moves the cursor backward", async () => {
+    const sql = postgres(scratchUrl, { max: 5, onnotice: () => undefined });
+    try {
+      const store = createDrizzleChatStore(drizzle(sql));
+
+      await store.putReadState({
+        tenantId: TENANT,
+        workbenchId: WORKBENCH,
+        principalId: PRINCIPAL,
+        lastSeenCreatedAt: new Date("2026-01-02T00:00:00.000Z"),
+        lastSeenId: "mail_2",
+      });
+
+      const result = await store.putReadState({
+        tenantId: TENANT,
+        workbenchId: WORKBENCH,
+        principalId: PRINCIPAL,
+        lastSeenCreatedAt: new Date("2026-01-01T00:00:00.000Z"),
+        lastSeenId: "mail_1",
+      });
+
+      expect(result.lastSeenId).toBe("mail_2");
+      expect(result.lastSeenCreatedAt).toEqual(
+        new Date("2026-01-02T00:00:00.000Z"),
+      );
+
+      const stored = await store.getReadState(TENANT, WORKBENCH, PRINCIPAL);
+      expect(stored?.lastSeenId).toBe("mail_2");
+    } finally {
+      await sql.end();
+    }
+  });
+
+  test("a newer write still moves the cursor forward", async () => {
+    const sql = postgres(scratchUrl, { max: 5, onnotice: () => undefined });
+    try {
+      const store = createDrizzleChatStore(drizzle(sql));
+
+      await store.putReadState({
+        tenantId: TENANT,
+        workbenchId: WORKBENCH,
+        principalId: "prn_bob",
+        lastSeenCreatedAt: new Date("2026-01-01T00:00:00.000Z"),
+        lastSeenId: "mail_1",
+      });
+
+      const result = await store.putReadState({
+        tenantId: TENANT,
+        workbenchId: WORKBENCH,
+        principalId: "prn_bob",
+        lastSeenCreatedAt: new Date("2026-01-03T00:00:00.000Z"),
+        lastSeenId: "mail_3",
+      });
+
+      expect(result.lastSeenId).toBe("mail_3");
+
+      const stored = await store.getReadState(TENANT, WORKBENCH, "prn_bob");
+      expect(stored?.lastSeenId).toBe("mail_3");
+    } finally {
+      await sql.end();
+    }
+  });
+
+  test("a same-millisecond forward move to a different message still lands", async () => {
+    const sql = postgres(scratchUrl, { max: 5, onnotice: () => undefined });
+    try {
+      const store = createDrizzleChatStore(drizzle(sql));
+      const sameCreatedAt = new Date("2026-01-04T00:00:00.001Z");
+
+      await store.putReadState({
+        tenantId: TENANT,
+        workbenchId: WORKBENCH,
+        principalId: "prn_carol",
+        lastSeenCreatedAt: sameCreatedAt,
+        lastSeenId: "mail_4",
+      });
+
+      const result = await store.putReadState({
+        tenantId: TENANT,
+        workbenchId: WORKBENCH,
+        principalId: "prn_carol",
+        lastSeenCreatedAt: sameCreatedAt,
+        lastSeenId: "mail_5",
+      });
+
+      expect(result.lastSeenId).toBe("mail_5");
+
+      const stored = await store.getReadState(TENANT, WORKBENCH, "prn_carol");
+      expect(stored?.lastSeenId).toBe("mail_5");
+    } finally {
+      await sql.end();
+    }
+  });
+});
diff --git a/packages/chat/test/store.test.ts b/packages/chat/test/store.test.ts
index 06bc0a0d8..f7cbab837 100644
--- a/packages/chat/test/store.test.ts
+++ b/packages/chat/test/store.test.ts
@@ -122,3 +122,55 @@ test("putReadState upserts a per-principal cursor without disturbing other princ
   const bob = await store.getReadState("tnt_1", "chn_1", "prn_bob");
   expect(bob).toBeUndefined();
 });
+
+test("putReadState never moves the cursor backward when a stale write lands after a newer one", async () => {
+  const store = createInMemoryChatStore();
+  await store.putReadState({
+    tenantId: "tnt_1",
+    workbenchId: "chn_1",
+    principalId: "prn_alice",
+    lastSeenCreatedAt: new Date("2026-01-02T00:00:00.000Z"),
+    lastSeenId: "mail_2",
+  });
+
+  const result = await store.putReadState({
+    tenantId: "tnt_1",
+    workbenchId: "chn_1",
+    principalId: "prn_alice",
+    lastSeenCreatedAt: new Date("2026-01-01T00:00:00.000Z"),
+    lastSeenId: "mail_1",
+  });
+
+  expect(result.lastSeenId).toBe("mail_2");
+  expect(result.lastSeenCreatedAt).toEqual(
+    new Date("2026-01-02T00:00:00.000Z"),
+  );
+
+  const alice = await store.getReadState("tnt_1", "chn_1", "prn_alice");
+  expect(alice?.lastSeenId).toBe("mail_2");
+});
+
+test("putReadState still lands a same-millisecond forward move to a different message", async () => {
+  const store = createInMemoryChatStore();
+  const sameCreatedAt = new Date("2026-01-02T00:00:00.001Z");
+  await store.putReadState({
+    tenantId: "tnt_1",
+    workbenchId: "chn_1",
+    principalId: "prn_alice",
+    lastSeenCreatedAt: sameCreatedAt,
+    lastSeenId: "mail_2",
+  });
+
+  const result = await store.putReadState({
+    tenantId: "tnt_1",
+    workbenchId: "chn_1",
+    principalId: "prn_alice",
+    lastSeenCreatedAt: sameCreatedAt,
+    lastSeenId: "mail_3",
+  });
+
+  expect(result.lastSeenId).toBe("mail_3");
+
+  const alice = await store.getReadState("tnt_1", "chn_1", "prn_alice");
+  expect(alice?.lastSeenId).toBe("mail_3");
+});