diff --git a/src/gateway/server-methods/agent-job.ts b/src/gateway/server-methods/agent-job.ts
index f08de1f2594d..bb6b2334c517 100644
--- a/src/gateway/server-methods/agent-job.ts
+++ b/src/gateway/server-methods/agent-job.ts
@@ -10,6 +10,7 @@ import { setSafeTimeout } from "../../utils/timer-delay.js";
 import type { AgentWaitTerminalSnapshot } from "./agent-wait-dedupe.js";
 
 const AGENT_RUN_CACHE_TTL_MS = 10 * 60_000;
+const AGENT_RUN_CACHE_MAX_ENTRIES = 5_000;
 /**
  * Embedded runs can emit transient lifecycle `error` events while auth/model
  * failover is still in progress. Give errors a short grace window so a
@@ -63,6 +64,28 @@ function recordAgentRunSnapshot(entry: AgentRunSnapshot) {
     return;
   }
   agentRunCache.set(entry.runId, entry);
+  // Time-based prune only fires on the TTL window; under high run fan-out a
+  // burst can add far more entries than the window reclaims. Cap with a FIFO
+  // drop so the cache cannot grow without bound between prunes.
+  enforceAgentRunCacheMaxEntries();
+}
+
+function enforceAgentRunCacheMaxEntries() {
+  if (agentRunCache.size <= AGENT_RUN_CACHE_MAX_ENTRIES) {
+    return;
+  }
+  const toRemove = agentRunCache.size - AGENT_RUN_CACHE_MAX_ENTRIES;
+  let removed = 0;
+  for (const runId of agentRunCache.keys()) {
+    if (removed >= toRemove) {
+      break;
+    }
+    if ((agentRunWaiterCounts.get(runId) ?? 0) > 0) {
+      continue;
+    }
+    agentRunCache.delete(runId);
+    removed += 1;
+  }
 }
 
 function shouldPreserveTerminalSnapshot(
@@ -494,5 +517,12 @@ export const testing = {
   resetWaiters(): void {
     agentRunWaiterCounts.clear();
   },
+  getAgentRunCacheSize(): number {
+    return agentRunCache.size;
+  },
+  resetAgentRunCache(): void {
+    agentRunCache.clear();
+  },
+  agentRunCacheMaxEntries: AGENT_RUN_CACHE_MAX_ENTRIES,
 };
 export { testing as __testing };
diff --git a/src/gateway/server-methods/server-methods.test.ts b/src/gateway/server-methods/server-methods.test.ts
index f9f51c18ac3c..aa4e22dd7763 100644
--- a/src/gateway/server-methods/server-methods.test.ts
+++ b/src/gateway/server-methods/server-methods.test.ts
@@ -37,7 +37,7 @@ import {
 } from "../chat-display-projection.js";
 import { sanitizeChatSendMessageInput } from "../chat-input-sanitize.js";
 import { ExecApprovalManager } from "../exec-approval-manager.js";
-import { waitForAgentJob } from "./agent-job.js";
+import { __testing as agentJobTesting, waitForAgentJob } from "./agent-job.js";
 import { injectTimestamp, timestampOptsFromConfig } from "./agent-timestamp.js";
 import { normalizeRpcAttachmentsToChatAttachments } from "./attachment-normalize.js";
 import { createExecApprovalHandlers } from "./exec-approval.js";
@@ -592,6 +592,74 @@ describe("waitForAgentJob", () => {
       vi.useRealTimers();
     }
   });
+
+  it("caps agentRunCache at AGENT_RUN_CACHE_MAX_ENTRIES via FIFO drop", () => {
+    agentJobTesting.resetAgentRunCache();
+    const max = agentJobTesting.agentRunCacheMaxEntries;
+    const overflow = 25;
+    const prefix = `cap-${Date.now()}-${Math.random().toString(36).slice(2)}`;
+    for (let i = 0; i < max + overflow; i++) {
+      emitAgentEvent({
+        runId: `${prefix}-${i}`,
+        stream: "lifecycle",
+        data: { phase: "end", startedAt: i, endedAt: i + 1 },
+      });
+    }
+    expect(agentJobTesting.getAgentRunCacheSize()).toBe(max);
+    agentJobTesting.resetAgentRunCache();
+  });
+
+  it("does not evict cached terminal snapshots with active fresh waiters", async () => {
+    agentJobTesting.resetAgentRunCache();
+    const max = agentJobTesting.agentRunCacheMaxEntries;
+    const prefix = `cap-waiter-${Date.now()}-${Math.random().toString(36).slice(2)}`;
+    const waitedRunId = `${prefix}-waited`;
+    emitAgentEvent({
+      runId: waitedRunId,
+      stream: "lifecycle",
+      data: { phase: "end", startedAt: 1_000, endedAt: 1_100 },
+    });
+    const waitPromise = waitForAgentJob({
+      runId: waitedRunId,
+      timeoutMs: 5_000,
+      ignoreCachedSnapshot: true,
+    });
+
+    for (let i = 0; i < max + 25; i++) {
+      emitAgentEvent({
+        runId: `${prefix}-${i}`,
+        stream: "lifecycle",
+        data: { phase: "end", startedAt: i, endedAt: i + 1 },
+      });
+    }
+    const cached = await waitForAgentJob({ runId: waitedRunId, timeoutMs: 0 });
+    expectRecordFields(cached, {
+      status: "ok",
+      startedAt: 1_000,
+      endedAt: 1_100,
+    });
+    expect(agentJobTesting.getAgentRunCacheSize()).toBe(max);
+
+    emitAgentEvent({
+      runId: waitedRunId,
+      stream: "lifecycle",
+      data: { phase: "end", startedAt: 10_000, endedAt: 10_100 },
+    });
+
+    const waited = await waitPromise;
+    expectRecordFields(waited, {
+      status: "ok",
+      startedAt: 10_000,
+      endedAt: 10_100,
+    });
+    emitAgentEvent({
+      runId: `${prefix}-after-waiter`,
+      stream: "lifecycle",
+      data: { phase: "end", startedAt: 20_000, endedAt: 20_100 },
+    });
+    expect(agentJobTesting.getAgentRunCacheSize()).toBe(max);
+    agentJobTesting.resetAgentRunCache();
+  });
 });
 
 describe("augmentChatHistoryWithCanvasBlocks", () => {