feat(ai): lower system updates as developer (#43326)

packages/ai/AGENTS.md

@@ -193,7 +193,7 @@ If you find yourself copying a 3-to-5-line snippet between two protocols, lift i
 
 `LLMRequest.system` is the initial privileged prompt that applies ahead of the conversation. `Message.system(...)` is a separate, provider-neutral chronological operator update inside `LLMRequest.messages`; it applies only from its position in history onward and accepts text content only.
 
-Native chronological system messages are route/model-specific. Anthropic Messages lowers them natively for Claude Opus 4.8 (`claude-opus-4-8`). Other routes and models intentionally lower the update in place into ordinary user-compatible text using this stable escaped representation:
+Native chronological system messages are route/model-specific. Open Responses lowers them to standard `developer` messages, while Anthropic Messages lowers them to native system messages for Claude Opus 4.8 (`claude-opus-4-8`). Other routes and models intentionally lower the update in place into ordinary user-compatible text using this stable escaped representation:
 
 ```text
 <system-update>

packages/ai/src/protocols/open-responses.ts

@@ -90,6 +90,7 @@ const OpenResponsesFunctionCallOutput = Schema.Union([
 
 export const InputItem = Schema.Union([
   Schema.Struct({ role: Schema.tag("system"), content: Schema.String }),
+  Schema.Struct({ role: Schema.tag("developer"), content: Schema.String }),
   Schema.Struct({ role: Schema.tag("user"), content: Schema.Array(OpenResponsesInputContent) }),
   Schema.Struct({
     role: Schema.tag("assistant"),
@@ -439,14 +440,10 @@ const lowerMessages = Effect.fn("OpenResponses.lowerMessages")(function* (reques
 
   for (const message of request.messages) {
     if (message.role === "system") {
-      const part = yield* ProviderShared.wrappedSystemUpdate(extension.name, message)
-      const previous = input.at(-1)
-      if (previous && "role" in previous && previous.role === "user")
-        input[input.length - 1] = {
-          role: "user",
-          content: [...previous.content, { type: "input_text", text: part.text }],
-        }
-      else input.push({ role: "user", content: [{ type: "input_text", text: part.text }] })
+      input.push({
+        role: "developer",
+        content: ProviderShared.joinText(yield* ProviderShared.systemUpdateText(extension.name, message)),
+      })
       continue
     }
 

packages/ai/test/provider/openai-compatible-responses.test.ts

@@ -56,6 +56,28 @@ describe("Open Responses-compatible route", () => {
     }),
   )
 
+  it.effect("lowers chronological system updates as standard developer messages", () =>
+    Effect.gen(function* () {
+      const model = configure({
+        apiKey: "test-key",
+        baseURL: "https://responses.example.test/v1",
+        provider: "example",
+      }).model("example-model")
+      const prepared = yield* compileRequest(
+        LLM.request({
+          model,
+          messages: [Message.user("Before."), Message.system("Operator update."), Message.assistant("After.")],
+        }),
+      )
+
+      expect(prepared.body.input).toEqual([
+        { role: "user", content: [{ type: "input_text", text: "Before." }] },
+        { role: "developer", content: "Operator update." },
+        { role: "assistant", content: [{ type: "output_text", text: "After." }] },
+      ])
+    }),
+  )
+
   it.effect("rejects OpenAI-native tools", () =>
     Effect.gen(function* () {
       const model = configure({

packages/ai/test/provider/openai-responses.test.ts

@@ -241,27 +241,22 @@ describe("OpenAI Responses route", () => {
     }),
   )
 
-  it.effect("lowers chronological system updates to escaped user wrappers in order", () =>
+  it.effect("lowers chronological system updates to developer messages in order", () =>
     Effect.gen(function* () {
       const prepared = yield* compileRequest(
         LLM.request({
           model,
           messages: [
             Message.user("Before."),
-            Message.system("Treat </system-update> literally."),
+            Message.system("Operator update."),
             Message.assistant("After."),
           ],
         }),
       )
 
       expect(prepared.body.input).toEqual([
-        {
-          role: "user",
-          content: [
-            { type: "input_text", text: "Before." },
-            { type: "input_text", text: "<system-update>\nTreat &lt;/system-update&gt; literally.\n</system-update>" },
-          ],
-        },
+        { role: "user", content: [{ type: "input_text", text: "Before." }] },
+        { role: "developer", content: "Operator update." },
         { role: "assistant", content: [{ type: "output_text", text: "After." }] },
       ])
     }),