diff --git a/proxy/extensions/upstream-change-detection.mjs b/proxy/extensions/upstream-change-detection.mjs
index 27ac7f65..e6b4675d 100644
--- a/proxy/extensions/upstream-change-detection.mjs
+++ b/proxy/extensions/upstream-change-detection.mjs
@@ -460,6 +460,17 @@ async function _processRequest(body, headers, { dir, map, fs }) {
   };
   map.set(nsKey, updated);
 
+  // messages.count grows on every ordinary turn — a conversation living is
+  // not an upstream structural change, and logging it drowned the alarm
+  // file (97% of all recorded events were count-only diffs, measured
+  // 2026-07-30 over 4661 events). A count-only diff updates the stored
+  // fingerprint silently; any OTHER changed path still alarms, with the
+  // count change riding along in its diff if both moved.
+  const alarmDiff = diff.filter((d) => d.path !== "messages.count");
+  if (alarmDiff.length === 0) {
+    return { event: "noop", nsKey };
+  }
+
   await appendEvent(
     {
       ts,
diff --git a/test/proxy-upstream-change-detection.test.mjs b/test/proxy-upstream-change-detection.test.mjs
index b4a2a218..ae124906 100644
--- a/test/proxy-upstream-change-detection.test.mjs
+++ b/test/proxy-upstream-change-detection.test.mjs
@@ -512,3 +512,62 @@ test("loadBaseline tolerates corrupt file", async () => {
     await rm(dir, { recursive: true, force: true });
   }
 });
+
+// --- 16. Count-only growth is not an upstream change ---
+
+test("16. a messages.count-only diff updates the baseline silently — no structural_change event", async () => {
+  const dir = await newTmp();
+  process.env.CACHE_FIX_UPSTREAM_DETECTION = "1";
+  process.env.CACHE_FIX_UPSTREAM_DIR = dir;
+  try {
+    const ext = await freshExt();
+    await ext.default.onRequest({ body: makeBody() });
+    // Same shape, one more ordinary message — a conversation growing.
+    await ext.default.onRequest({
+      body: makeBody({
+        messages: [
+          { role: "user", content: [{ type: "text", text: "hello" }] },
+          { role: "assistant", content: [{ type: "text", text: "hi" }] },
+          { role: "user", content: [{ type: "text", text: "more" }] },
+        ],
+      }),
+    });
+    const text = await readFile(join(dir, "upstream-changes.jsonl"), "utf8");
+    const events = text.split("\n").filter(Boolean).map((l) => JSON.parse(l).event);
+    assert.deepEqual(events, ["baseline_established"], "growth alone must not alarm");
+  } finally {
+    delete process.env.CACHE_FIX_UPSTREAM_DETECTION;
+    delete process.env.CACHE_FIX_UPSTREAM_DIR;
+    await rm(dir, { recursive: true, force: true });
+  }
+});
+
+test("17. count change RIDING a real structural change still alarms, count in the diff", async () => {
+  const dir = await newTmp();
+  process.env.CACHE_FIX_UPSTREAM_DETECTION = "1";
+  process.env.CACHE_FIX_UPSTREAM_DIR = dir;
+  try {
+    const ext = await freshExt();
+    await ext.default.onRequest({ body: makeBody() });
+    await ext.default.onRequest({
+      body: makeBody({
+        messages: [
+          { role: "user", content: [{ type: "text", text: "hello" }] },
+          { role: "assistant", content: [{ type: "text", text: "hi" }] },
+        ],
+        tools: [
+          { name: "Bash", input_schema: { properties: { command: {} } } },
+        ],
+      }),
+    });
+    const text = await readFile(join(dir, "upstream-changes.jsonl"), "utf8");
+    const parsed = text.split("\n").filter(Boolean).map((l) => JSON.parse(l));
+    const change = parsed.find((e) => e.event === "structural_change");
+    assert.ok(change, "a tools change must still alarm");
+    assert.ok(change.diff.some((d) => d.path === "messages.count"), "the count delta rides in the diff");
+  } finally {
+    delete process.env.CACHE_FIX_UPSTREAM_DETECTION;
+    delete process.env.CACHE_FIX_UPSTREAM_DIR;
+    await rm(dir, { recursive: true, force: true });
+  }
+});