diff --git a/src/gateway/server-methods/chat.send-deleted-agent.test.ts b/src/gateway/server-methods/chat.send-deleted-agent.test.ts
new file mode 100644
index 00000000000..21e51466f27
--- /dev/null
+++ b/src/gateway/server-methods/chat.send-deleted-agent.test.ts
@@ -0,0 +1,52 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { ErrorCodes } from "../protocol/index.js";
+import type { RespondFn } from "./types.js";
+
+const loadSessionEntryMock = vi.fn();
+const resolveDeletedAgentIdFromSessionKeyMock = vi.fn();
+
+vi.mock("../session-utils.js", async () => {
+  const actual = await vi.importActual<typeof import("../session-utils.js")>("../session-utils.js");
+  return {
+    ...actual,
+    loadSessionEntry: (...args: unknown[]) => loadSessionEntryMock(...args),
+    resolveDeletedAgentIdFromSessionKey: (...args: unknown[]) =>
+      resolveDeletedAgentIdFromSessionKeyMock(...args),
+  };
+});
+
+import { chatHandlers } from "./chat.js";
+
+describe("chat.send deleted-agent guard", () => {
+  beforeEach(() => {
+    loadSessionEntryMock.mockReset();
+    resolveDeletedAgentIdFromSessionKeyMock.mockReset();
+  });
+
+  it("rejects keys belonging to a deleted agent", async () => {
+    const orphanKey = "agent:deleted-agent:main";
+    loadSessionEntryMock.mockReturnValue({
+      cfg: {},
+      canonicalKey: orphanKey,
+      storePath: "/tmp/sessions.json",
+      entry: { sessionId: "sess-orphan" },
+    });
+    resolveDeletedAgentIdFromSessionKeyMock.mockReturnValue("deleted-agent");
+
+    const respond = vi.fn() as unknown as RespondFn;
+
+    await chatHandlers["chat.send"]({
+      req: { id: "req-1" } as never,
+      params: { sessionKey: orphanKey, message: "hi", idempotencyKey: "run-1" },
+      respond,
+      context: {} as never,
+      client: null,
+      isWebchatConnect: () => false,
+    });
+
+    expect(respond).toHaveBeenCalledWith(false, undefined, {
+      code: ErrorCodes.INVALID_REQUEST,
+      message: 'Agent "deleted-agent" no longer exists in configuration',
+    });
+  });
+});
diff --git a/src/gateway/server-methods/chat.ts b/src/gateway/server-methods/chat.ts
index 3c351398a2a..dc4e6a2319d 100644
--- a/src/gateway/server-methods/chat.ts
+++ b/src/gateway/server-methods/chat.ts
@@ -74,6 +74,7 @@ import {
   capArrayByJsonBytes,
   loadSessionEntry,
   resolveGatewayModelSupportsImages,
+  resolveDeletedAgentIdFromSessionKey,
   readSessionMessages,
   resolveSessionModelRef,
 } from "../session-utils.js";
@@ -1846,6 +1847,18 @@ export const chatHandlers: GatewayRequestHandlers = {
     }
     const rawSessionKey = p.sessionKey;
     const { cfg, entry, canonicalKey: sessionKey } = loadSessionEntry(rawSessionKey);
+    const deletedAgentId = resolveDeletedAgentIdFromSessionKey(cfg, sessionKey);
+    if (deletedAgentId !== null) {
+      respond(
+        false,
+        undefined,
+        errorShape(
+          ErrorCodes.INVALID_REQUEST,
+          `Agent "${deletedAgentId}" no longer exists in configuration`,
+        ),
+      );
+      return;
+    }
     const agentId = resolveSessionAgentId({
       sessionKey,
       config: cfg,