fix: keep OpenAI reasoning responses stateful

.changeset/openai-reasoning-store-true.md

@@ -0,0 +1,5 @@
+---
+'@electric-ax/agents': patch
+---
+
+Force `store: true` for built-in OpenAI reasoning model payloads so reasoning/tool-call continuations can replay `rs_*` reasoning items without follow-up requests failing due to missing persisted items.

packages/agents/src/model-catalog.ts

@@ -267,6 +267,11 @@ function withProviderPayloadDefaults(
 
       return {
         ...body,
+        // 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,
         reasoning: {
           ...existingReasoning,
           effort,

packages/agents/test/model-catalog.test.ts

@@ -126,6 +126,7 @@ describe(`model catalog`, () => {
     )
 
     expect(payload).toEqual({
+      store: true,
       reasoning: { effort: `minimal` },
     })
   })
@@ -145,10 +146,51 @@ describe(`model catalog`, () => {
     )
 
     expect(payload).toEqual({
+      store: true,
       reasoning: { effort: `high` },
     })
   })
 
+  it(`forces store true only for OpenAI reasoning model payloads`, async () => {
+    const openAiCatalog = await createBuiltinModelCatalog()
+    const openAiConfig = resolveBuiltinModelConfig(openAiCatalog!, {
+      model: `openai:gpt-5`,
+    })
+
+    expect(
+      openAiConfig.onPayload!(
+        { store: false, reasoning: { effort: `none` } },
+        {} as any
+      )
+    ).toEqual({
+      store: true,
+      reasoning: { effort: `minimal` },
+    })
+
+    delete process.env.OPENAI_API_KEY
+    process.env.DEEPSEEK_API_KEY = `test-deepseek-key`
+    vi.stubGlobal(
+      `fetch`,
+      vi.fn(async (url: string) => {
+        if (String(url).includes(`deepseek.com`)) {
+          return {
+            ok: true,
+            status: 200,
+            json: async () => ({ data: [{ id: `deepseek-v4-flash` }] }),
+          }
+        }
+        return { ok: false, status: 401, json: async () => ({}) }
+      })
+    )
+
+    const deepseekCatalog = await createBuiltinModelCatalog()
+    const deepseekConfig = resolveBuiltinModelConfig(deepseekCatalog!, {
+      model: `deepseek:deepseek-v4-flash`,
+    })
+
+    expect(deepseekConfig.onPayload).toBeUndefined()
+  })
+
   it(`does not expose providers whose keys are rejected`, async () => {
     vi.stubGlobal(
       `fetch`,