diff --git a/src/slack/monitor/message-handler/prepare.test.ts b/src/slack/monitor/message-handler/prepare.test.ts
index 0db6a635303..3c16609a1a2 100644
--- a/src/slack/monitor/message-handler/prepare.test.ts
+++ b/src/slack/monitor/message-handler/prepare.test.ts
@@ -516,7 +516,7 @@ describe("slack prepareSlackMessage inbound contract", () => {
     expect(replies).toHaveBeenCalledTimes(2);
   });
 
-  it("keeps loading thread history when thread session already exists in store", async () => {
+  it("skips loading thread history when thread session already exists in store (bloat fix)", async () => {
     const { storePath } = makeTmpStorePath();
     const cfg = {
       session: { store: storePath },
@@ -533,24 +533,15 @@ describe("slack prepareSlackMessage inbound contract", () => {
       baseSessionKey: route.sessionKey,
       threadId: "200.000",
     });
+    // Simulate existing session - thread history should NOT be fetched (bloat fix)
     fs.writeFileSync(
       storePath,
       JSON.stringify({ [threadKeys.sessionKey]: { updatedAt: Date.now() } }, null, 2),
     );
 
-    const replies = vi
-      .fn()
-      .mockResolvedValueOnce({
-        messages: [{ text: "starter", user: "U2", ts: "200.000" }],
-      })
-      .mockResolvedValueOnce({
-        messages: [
-          { text: "starter", user: "U2", ts: "200.000" },
-          { text: "assistant follow-up", bot_id: "B1", ts: "200.500" },
-          { text: "user follow-up", user: "U1", ts: "200.800" },
-          { text: "current message", user: "U1", ts: "201.000" },
-        ],
-      });
+    const replies = vi.fn().mockResolvedValueOnce({
+      messages: [{ text: "starter", user: "U2", ts: "200.000" }],
+    });
     const slackCtx = createThreadSlackCtx({ cfg, replies });
     slackCtx.resolveUserName = async () => ({ name: "Alice" });
     slackCtx.resolveChannelName = async () => ({ name: "general", type: "channel" });
@@ -563,10 +554,12 @@ describe("slack prepareSlackMessage inbound contract", () => {
 
     expect(prepared).toBeTruthy();
     expect(prepared!.ctxPayload.IsFirstThreadTurn).toBeUndefined();
-    expect(prepared!.ctxPayload.ThreadHistoryBody).toContain("assistant follow-up");
-    expect(prepared!.ctxPayload.ThreadHistoryBody).toContain("user follow-up");
-    expect(prepared!.ctxPayload.ThreadHistoryBody).not.toContain("current message");
-    expect(replies).toHaveBeenCalledTimes(2);
+    // Thread history should NOT be fetched for existing sessions (bloat fix)
+    expect(prepared!.ctxPayload.ThreadHistoryBody).toBeUndefined();
+    // Thread starter should also be skipped for existing sessions
+    expect(prepared!.ctxPayload.ThreadStarterBody).toBeUndefined();
+    // Replies API should only be called once (for thread starter lookup, not history)
+    expect(replies).toHaveBeenCalledTimes(1);
   });
 
   it("includes thread_ts and parent_user_id metadata in thread replies", async () => {
diff --git a/src/slack/monitor/message-handler/prepare.ts b/src/slack/monitor/message-handler/prepare.ts
index 247dac92edc..4bf192858e1 100644
--- a/src/slack/monitor/message-handler/prepare.ts
+++ b/src/slack/monitor/message-handler/prepare.ts
@@ -594,7 +594,8 @@ export async function prepareSlackMessage(params: {
       storePath,
       sessionKey, // Thread-specific session key
     });
-    if (threadInitialHistoryLimit > 0) {
+    // Only fetch thread history for NEW sessions (existing sessions already have this context in their transcript)
+    if (threadInitialHistoryLimit > 0 && !threadSessionPreviousTimestamp) {
       const threadHistory = await resolveSlackThreadHistory({
         channelId: message.channel,
         threadTs,
@@ -684,7 +685,8 @@ export async function prepareSlackMessage(params: {
     // Preserve thread context for routed tool notifications.
     MessageThreadId: threadContext.messageThreadId,
     ParentSessionKey: threadKeys.parentSessionKey,
-    ThreadStarterBody: threadStarterBody,
+    // Only include thread starter body for NEW sessions (existing sessions already have it in their transcript)
+    ThreadStarterBody: !threadSessionPreviousTimestamp ? threadStarterBody : undefined,
     ThreadHistoryBody: threadHistoryBody,
     IsFirstThreadTurn:
       isThreadReply && threadTs && !threadSessionPreviousTimestamp ? true : undefined,