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 4fa3f049..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);