diff --git a/server/src/__tests__/ai-connection-binding-mode.test.ts b/server/src/__tests__/ai-connection-binding-mode.test.ts
new file mode 100644
index 00000000000..e3b4a6d0ea6
--- /dev/null
+++ b/server/src/__tests__/ai-connection-binding-mode.test.ts
@@ -0,0 +1,34 @@
+import { describe, expect, it, vi } from "vitest";
+import type { Db } from "@paperclipai/db";
+import type { AiConnectionBinding } from "@paperclipai/shared";
+import { readAiConnectionConfigurationFailure } from "../services/ai-connection-configuration-failure.js";
+import { aiConnectionService } from "../services/ai-connections.js";
+
+describe("AI connection binding mode guard", () => {
+  it("rejects an unknown binding mode before it reads any connection", async () => {
+    // Any database read fails the test: the guard must run first.
+    const select = vi.fn(() => {
+      throw new Error("unexpected database read");
+    });
+    const service = aiConnectionService({ select } as unknown as Db);
+    const binding = {
+      provider: "openai",
+      method: "api_key",
+      mode: "connection",
+      connectionId: "11111111-1111-4111-8111-111111111111",
+      grantId: "22222222-2222-4222-8222-222222222222",
+    } as unknown as AiConnectionBinding;
+
+    const error = await service.select({
+      companyId: "33333333-3333-4333-8333-333333333333",
+      userId: "user-1",
+      agentId: "44444444-4444-4444-8444-444444444444",
+      adapterType: "codex_local",
+      binding,
+    }).catch((caught: unknown) => caught);
+
+    expect(error).toMatchObject({ status: 422 });
+    expect(readAiConnectionConfigurationFailure(error)).toBe("ai_connection_incompatible");
+    expect(select).not.toHaveBeenCalled();
+  });
+});
diff --git a/server/src/services/ai-connections.ts b/server/src/services/ai-connections.ts
index 3622fc2c1c8..a4081a4b4d7 100644
--- a/server/src/services/ai-connections.ts
+++ b/server/src/services/ai-connections.ts
@@ -281,6 +281,10 @@ export function aiConnectionService(db: Db) {
     binding: AiConnectionBinding;
   }) {
     const { companyId, userId, agentId, binding } = input;
+    // The grant-kind checks below cover only these three modes. Callers parse
+    // the binding first, but an unknown mode must never skip those checks.
+    if (binding.mode !== "responsible_user" && binding.mode !== "shared" && binding.mode !== "delegated")
+      throw aiConnectionConfigurationFailure("ai_connection_incompatible", "Select a supported AI connection mode");
     if (
       !isAiConnectionCompatible(
         binding,