diff --git a/src/server/responses/core.ts b/src/server/responses/core.ts
index c6ce07c05a..760e9c0d8a 100644
--- a/src/server/responses/core.ts
+++ b/src/server/responses/core.ts
@@ -1080,6 +1080,19 @@ async function applyFinalRouteRequestNormalization(args: {
     }
   }
 
+  // Generic Responses clients (e.g. AI-SDK apps) omit `store`, but the canonical
+  // forward Codex backend rejects a native request without an explicit store:false.
+  // Default it only there — every other Responses upstream (key-auth providers and
+  // custom forward gateways) intentionally keeps the omitted-store server-side
+  // default for previous_response_id reuse — and never override an explicit value.
+  if (
+    isCanonicalOpenAiForwardProvider(route.provider)
+    && parsed._rawBody && typeof parsed._rawBody === "object"
+    && (parsed._rawBody as Record<string, unknown>).store === undefined
+  ) {
+    (parsed._rawBody as Record<string, unknown>).store = false;
+  }
+
   // Final selected model before virtual wire-model rewriting (Pro aliases).
   const finalSelectedModelId = route.modelId;
 
diff --git a/tests/responses-inbound-store-default.test.ts b/tests/responses-inbound-store-default.test.ts
new file mode 100644
index 0000000000..85038187d1
--- /dev/null
+++ b/tests/responses-inbound-store-default.test.ts
@@ -0,0 +1,122 @@
+/**
+ * Generic Responses-API clients (AI-SDK apps such as ZCode) omit `store`, but the
+ * canonical forward Codex backend rejects a native request without an explicit
+ * store:false ("Store must be set to false"). The default is applied only after
+ * routing settles and only on the canonical Codex forward backend
+ * (isCanonicalOpenAiForwardProvider: adapter + forward auth + the canonical base
+ * URL). Every other Responses upstream — key-auth providers and custom forward
+ * gateways alike — intentionally keeps the omitted-store server-side default so a
+ * later turn can continue through an unexpanded previous_response_id. These tests
+ * pin the scoped default (forward positive cases), the key-auth and custom-gateway
+ * negatives, and explicit-value survival on both sides.
+ *
+ * End-to-end cases assert the captured upstream request body — the externally
+ * observable payload. Pattern mirrors tests/responses-compaction-routing.test.ts
+ * and tests/github-copilot-wire-defaults.test.ts.
+ */
+import { afterEach, describe, expect, test } from "bun:test";
+import { CODEX_FORWARD_BASE_URL } from "../src/providers/openai-tiers";
+import { handleResponses } from "../src/server/responses";
+import type { OcxConfig, OcxProviderConfig } from "../src/types";
+
+function providerConfig(overrides: Partial<OcxProviderConfig> = {}): OcxConfig {
+  return {
+    defaultProvider: "gw",
+    providers: {
+      gw: {
+        adapter: "openai-responses",
+        baseUrl: CODEX_FORWARD_BASE_URL,
+        authMode: "key",
+        apiKey: "test-key",
+        ...overrides,
+      },
+    },
+  } as unknown as OcxConfig;
+}
+
+describe("/v1/responses defaults store:false only for the canonical forward Codex backend", () => {
+  const originalFetch = globalThis.fetch;
+  afterEach(() => { globalThis.fetch = originalFetch; });
+
+  function captureUpstream(): { urls: string[]; bodies: string[] } {
+    const urls: string[] = [];
+    const bodies: string[] = [];
+    globalThis.fetch = (async (input: RequestInfo | URL, init?: RequestInit) => {
+      urls.push(String(input));
+      const body =
+        input instanceof Request ? await input.clone().text()
+        : typeof init?.body === "string" ? init.body
+        : "";
+      bodies.push(body);
+      return new Response("data: [DONE]\n\n", {
+        status: 200,
+        headers: { "content-type": "text/event-stream" },
+      });
+    }) as typeof fetch;
+    return { urls, bodies };
+  }
+
+  async function drive(
+    config: OcxConfig,
+    store: unknown,
+  ): Promise<{ url: string; body: Record<string, unknown> | null }> {
+    const { urls, bodies } = captureUpstream();
+    await handleResponses(
+      new Request("http://localhost/v1/responses", {
+        method: "POST",
+        headers: { "content-type": "application/json" },
+        body: JSON.stringify({
+          model: "gw/some-model",
+          input: [{ type: "message", role: "user", content: [{ type: "input_text", text: "ping" }] }],
+          stream: true,
+          ...(store === undefined ? {} : { store }),
+        }),
+      }),
+      config,
+      { model: "", provider: "" },
+    );
+    let parsed: Record<string, unknown> | null = null;
+    try { parsed = bodies[0] ? (JSON.parse(bodies[0]) as Record<string, unknown>) : null; } catch { parsed = null; }
+    return { url: urls[0] ?? "", body: parsed };
+  }
+
+  test("forward route: omitted store reaches the upstream Responses request as false", async () => {
+    const { url, body } = await drive(providerConfig({ authMode: "forward" }), undefined);
+    expect(url).toContain("/responses");
+    expect(body).not.toBeNull();
+    expect((body as Record<string, unknown>).store).toBe(false);
+  });
+
+  test("key-auth route: omitted store is NOT injected (stateful upstream keeps its default)", async () => {
+    const { body } = await drive(providerConfig(), undefined);
+    expect(body).not.toBeNull();
+    expect(!("store" in (body as Record<string, unknown>))).toBe(true);
+  });
+
+  test("custom forward gateway: omitted store is NOT injected (non-canonical base URL keeps its default)", async () => {
+    const { body } = await drive(
+      providerConfig({ authMode: "forward", baseUrl: "https://gateway.example/v1" }),
+      undefined,
+    );
+    expect(body).not.toBeNull();
+    expect(!("store" in (body as Record<string, unknown>))).toBe(true);
+  });
+
+  test("forward route: explicit store:true is preserved", async () => {
+    const { body } = await drive(providerConfig({ authMode: "forward" }), true);
+    expect(body).not.toBeNull();
+    expect((body as Record<string, unknown>).store).toBe(true);
+  });
+
+  test("forward route: explicit store:false is preserved", async () => {
+    const { body } = await drive(providerConfig({ authMode: "forward" }), false);
+    expect(body).not.toBeNull();
+    expect((body as Record<string, unknown>).store).toBe(false);
+  });
+
+  test("key-auth route: explicit store:false is preserved", async () => {
+    const { body } = await drive(providerConfig(), false);
+    expect(body).not.toBeNull();
+    expect((body as Record<string, unknown>).store).toBe(false);
+  });
+});