diff --git a/.changeset/codex-store-false.md b/.changeset/codex-store-false.md
new file mode 100644
index 0000000000..846d38a053
--- /dev/null
+++ b/.changeset/codex-store-false.md
@@ -0,0 +1,5 @@
+---
+'@electric-ax/agents': patch
+---
+
+Stop forcing `store: true` on OpenAI Codex reasoning payloads. The ChatGPT-login Codex endpoint is stateless-only and rejects stateful requests with `{"detail":"Store must be set to false"}`, which broke every Codex (`gpt-5.x`) run. The stateful default now applies to the regular OpenAI Responses API only.
diff --git a/packages/agents/src/model-catalog.ts b/packages/agents/src/model-catalog.ts
index e1b74cc5d5..5346e3d05e 100644
--- a/packages/agents/src/model-catalog.ts
+++ b/packages/agents/src/model-catalog.ts
@@ -270,8 +270,12 @@ function withProviderPayloadDefaults(
         // OpenAI Responses reasoning/tool-call continuations replay rs_*
         // reasoning items. With store:false, OpenAI does not persist those
         // items server-side, which can make follow-up requests fail with
-        // "Item with id ... not found". Keep Responses stateful for built-ins.
-        store: true,
+        // "Item with id ... not found". Keep Responses stateful for the
+        // regular API. The ChatGPT-login Codex endpoint is the opposite:
+        // it is stateless-only and rejects any stateful request with
+        // `{"detail":"Store must be set to false"}`, so it must keep the
+        // upstream store:false default.
+        ...(choice.provider === `openai` && { store: true }),
         reasoning: {
           ...existingReasoning,
           effort,
diff --git a/packages/agents/test/model-catalog.test.ts b/packages/agents/test/model-catalog.test.ts
index ec1f3f9a57..6c56fd93ef 100644
--- a/packages/agents/test/model-catalog.test.ts
+++ b/packages/agents/test/model-catalog.test.ts
@@ -191,6 +191,38 @@ describe(`model catalog`, () => {
     expect(deepseekConfig.onPayload).toBeUndefined()
   })
 
+  it(`keeps Codex reasoning model payloads stateless`, async () => {
+    // The ChatGPT-login Codex endpoint rejects stateful requests with
+    // `{"detail":"Store must be set to false"}` — the openai branch's
+    // store:true override must not leak into openai-codex payloads.
+    delete process.env.OPENAI_API_KEY
+    process.env.ELECTRIC_CODEX_ACCESS_TOKEN = `test-codex-token`
+
+    const catalog = await createBuiltinModelCatalog()
+    const config = resolveBuiltinModelConfig(catalog!, {
+      model: `openai-codex:gpt-5.4`,
+    })
+
+    expect(config).toMatchObject({
+      provider: `openai-codex`,
+      model: `gpt-5.4`,
+    })
+    expect(config.onPayload).toBeTypeOf(`function`)
+
+    const payload = config.onPayload!(
+      { store: false, reasoning: { effort: `none` } },
+      {} as any
+    )
+
+    expect(payload).toEqual({
+      store: false,
+      reasoning: { effort: `low` },
+    })
+    expect(
+      config.onPayload!({ reasoning: { effort: `none` } }, {} as any)
+    ).not.toHaveProperty(`store`)
+  })
+
   it(`does not expose providers whose keys are rejected`, async () => {
     vi.stubGlobal(
       `fetch`,