diff --git a/src/web/routes/status-telemetry-routes.ts b/src/web/routes/status-telemetry-routes.ts
index a568ac74..a93cb4e1 100644
--- a/src/web/routes/status-telemetry-routes.ts
+++ b/src/web/routes/status-telemetry-routes.ts
@@ -51,12 +51,17 @@ export function registerStatusTelemetryRoutes(app: FastifyInstance, ctx: Session
 
     const payload = data as RawStatuslinePayload | undefined;
 
+    // Only a CLI that has a statusline exporter at all speaks for its own model and
+    // prompt-cache state: both ride the SAME statusline payload the exporter POSTs, so
+    // both are gated on capabilities.statusLineTelemetry (the gate that also decides
+    // whether the exporter is injected at all, in tmux-manager._resolveStatusLine).
+    const session = ctx.sessions.get(sessionId);
+    const reportsStatusLine = !!session && !!getCli(session.mode)?.capabilities.statusLineTelemetry;
+
     // The model the CLI says it is running, for the session headers (displayModel). It
     // rides every render, so it follows an in-session `/model`; the session dedupes and
-    // broadcasts only a change. Only a CLI that has a statusline exporter at all may
-    // report one here.
-    const session = ctx.sessions.get(sessionId);
-    if (session && getCli(session.mode)?.capabilities.statusLineTelemetry) {
+    // broadcasts only a change.
+    if (session && reportsStatusLine) {
       session.noteReportedModel('statusline', payload?.model?.display_name);
     }
 
@@ -68,7 +73,7 @@ export function registerStatusTelemetryRoutes(app: FastifyInstance, ctx: Session
     // it backgrounds this POST (hooks-config.ts), so two sub-second loopback reports can
     // arrive out of order; a stale one then briefly overwrites newer state until the next
     // render corrects it. Accepted for a display-only readout rather than ordered here.
-    const promptCache = parsePromptCache(payload);
+    const promptCache = reportsStatusLine ? parsePromptCache(payload) : null;
     if (promptCache && session?.setPromptCache(promptCache)) {
       ctx.broadcast(SessionPromptCache, { sessionId, ...session.promptCache });
       // Persisted on change, like a model change, so a crash restores the latest report.
diff --git a/src/web/schemas.ts b/src/web/schemas.ts
index f40a2a16..6a23a304 100644
--- a/src/web/schemas.ts
+++ b/src/web/schemas.ts
@@ -707,7 +707,16 @@ export const StatusTelemetrySchema = z.object({
           miss_recache_tokens: z.number().nullish(),
           last_miss_at: z.number().nullish(),
           recache_tokens_if_cold: z.number().nullish(),
-          last_miss_cause: z.object({ causes: z.array(z.unknown()).max(20).nullish() }).nullish(),
+          // Keep the first 20 causes instead of 400-ing the whole post (which also drops the
+          // footer, the plan chip and the model report it carries) when Claude Code reports more. The
+          // preprocess slices BEFORE the array validates, so an over-long array costs one bounded
+          // slice, never per-element validation across every entry (parsePromptCache then keeps
+          // the string ones among those 20).
+          last_miss_cause: z
+            .object({
+              causes: z.preprocess((v) => (Array.isArray(v) ? v.slice(0, 20) : v), z.array(z.unknown()).nullish()),
+            })
+            .nullish(),
         })
         .nullish(),
     })
diff --git a/test/routes/status-telemetry-routes.test.ts b/test/routes/status-telemetry-routes.test.ts
index 0741b17f..faa96ebd 100644
--- a/test/routes/status-telemetry-routes.test.ts
+++ b/test/routes/status-telemetry-routes.test.ts
@@ -154,6 +154,20 @@ describe('POST /api/status-telemetry', () => {
     expect(events.filter((e) => e === SessionPromptCache)).toHaveLength(1);
   });
 
+  it('keeps the first 20 miss causes instead of refusing a post that carries more', async () => {
+    const causes = Array.from({ length: 25 }, (_, i) => `cause_${i}`);
+    const res = await post({
+      sessionId: SID,
+      data: { ...REAL, prompt_cache: { warm: false, ttl: '1h', last_miss_cause: { causes } } },
+    });
+    // Accepted, not 400: an over-long causes array must not blank the whole statusline.
+    expect(res.statusCode).toBe(200);
+    const session = h.ctx.sessions.get(SID) as unknown as { promptCache: { lastMissCauses?: string[] } | null };
+    expect(session.promptCache?.lastMissCauses).toHaveLength(20);
+    expect(session.promptCache?.lastMissCauses?.[0]).toBe('cause_0');
+    expect(session.promptCache?.lastMissCauses?.[19]).toBe('cause_19');
+  });
+
   it('rejects a malformed body (missing sessionId) with 400', async () => {
     const res = await post({ data: REAL });
     expect(res.statusCode).toBe(400);
@@ -212,6 +226,20 @@ describe('POST /api/status-telemetry: the session model (displayModel)', () => {
     expect(session.toState().displayModel).toBeUndefined();
   });
 
+  it('stores no prompt-cache from a CLI with no statusline exporter, but still broadcasts plan usage', async () => {
+    const session = realSession('codex', 'codex-cache');
+    const res = await post({
+      sessionId: 'codex-cache',
+      data: { ...REAL, prompt_cache: { warm: true, ttl: '1h', expires_at: Math.floor(Date.now() / 1000) + 3600 } },
+    });
+    expect(res.statusCode).toBe(200);
+    // Prompt cache is gated like the model report; the rest of the post still lands.
+    expect(session.toState().promptCache).toBeUndefined();
+    const events = h.ctx.broadcast.mock.calls.map(([name]) => name);
+    expect(events).not.toContain(SessionPromptCache);
+    expect(events).toContain(SessionStatusTelemetry);
+  });
+
   it('broadcasts the STORED prompt-cache object, so a compaction stamp reaches the live readout', async () => {
     const session = realSession('claude', 'claude-compact');
     const expiresAtS = Math.floor(Date.now() / 1000) + 3600;