diff --git a/internal/cloud/workbench-projection-writer.test.ts b/internal/cloud/workbench-projection-writer.test.ts
index 8bf74f22..f6d0aade 100644
--- a/internal/cloud/workbench-projection-writer.test.ts
+++ b/internal/cloud/workbench-projection-writer.test.ts
@@ -90,6 +90,74 @@ test("workbench projection writer commits terminal owner evidence as sealed dura
   assert.equal(facts.checkpoints[0].timing.finishedAt, "2026-06-20T11:00:00.000Z");
 });
 
+test("workbench projection writer does not rewrite prior-turn messages from merged owner session", async () => {
+  const factWrites = [];
+  const runtimeStore = {
+    async writeWorkbenchFacts(params, requestMeta) {
+      factWrites.push({ params, requestMeta });
+      return { written: true, facts: params.facts };
+    }
+  };
+  const accessController = {
+    async recordAgentSessionOwner(input) {
+      return {
+        id: input.sessionId,
+        projectId: input.projectId,
+        ownerUserId: input.ownerUserId,
+        conversationId: input.conversationId,
+        threadId: input.threadId,
+        lastTraceId: input.traceId,
+        status: input.status,
+        updatedAt: "2026-06-20T11:10:00.000Z",
+        session: {
+          sessionStatus: "completed",
+          lastTraceId: input.traceId,
+          messages: [
+            { messageId: "msg_writer_r1_user", role: "user", text: "round1 prompt", status: "sent", turnId: "trc_writer_r1", traceId: "trc_writer_r1" },
+            { messageId: "msg_writer_r1_agent", role: "agent", text: "WRONG_ROUND2_TEXT", status: "completed", turnId: "trc_writer_r1", traceId: "trc_writer_r1" },
+            ...input.session.messages
+          ]
+        }
+      };
+    }
+  };
+
+  await writeWorkbenchProjectionSession({
+    accessController,
+    runtimeStore,
+    traceId: "trc_writer_r2",
+    ownerUserId: "usr_writer",
+    ownerRole: "user",
+    sessionId: "ses_writer_multi_turn",
+    projectId: "prj_writer",
+    conversationId: "cnv_writer_multi_turn",
+    threadId: "thread-writer-multi-turn",
+    status: "completed",
+    payload: {
+      traceId: "trc_writer_r2",
+      status: "completed",
+      finalResponse: { text: "ROUND2_DONE", status: "completed", traceId: "trc_writer_r2" },
+      agentRun: { runId: "run_writer_r2", commandId: "cmd_writer_r2", status: "completed", terminalStatus: "completed", lastSeq: 9 },
+      updatedAt: "2026-06-20T11:10:00.000Z"
+    },
+    session: {
+      sessionStatus: "completed",
+      messages: [
+        { messageId: "msg_writer_r2_user", role: "user", text: "round2 prompt", status: "sent", turnId: "trc_writer_r2", traceId: "trc_writer_r2" },
+        { messageId: "msg_writer_r2_agent", role: "agent", text: "ROUND2_DONE", status: "completed", turnId: "trc_writer_r2", traceId: "trc_writer_r2" }
+      ]
+    }
+  });
+
+  assert.equal(factWrites.length, 1);
+  const facts = factWrites[0].params.facts;
+  assert.deepEqual(facts.messages.map((message) => message.messageId), ["msg_writer_r2_user", "msg_writer_r2_agent"]);
+  assert.equal(facts.messages.some((message) => message.traceId === "trc_writer_r1"), false);
+  assert.equal(facts.messages.find((message) => message.messageId === "msg_writer_r2_agent").text, "ROUND2_DONE");
+  assert.equal(facts.turns[0].traceId, "trc_writer_r2");
+  assert.equal(facts.turns[0].finalResponse.text, "ROUND2_DONE");
+});
+
 test("workbench projection writer seals canceled AgentRun turns with canonical cancel final response", async () => {
   const factWrites = [];
   const runtimeStore = {
diff --git a/internal/cloud/workbench-projection-writer.ts b/internal/cloud/workbench-projection-writer.ts
index f4ddfdd2..0e8e4b6e 100644
--- a/internal/cloud/workbench-projection-writer.ts
+++ b/internal/cloud/workbench-projection-writer.ts
@@ -35,7 +35,7 @@ export async function writeWorkbenchProjectionSession({ accessController, runtim
       conversationId: ownerRecord?.conversationId ?? conversationId,
       threadId: ownerRecord?.threadId ?? threadId,
       status: ownerRecord?.status ?? status,
-      session: ownerRecord?.session ?? session,
+      session,
       payload,
       params
     });