[Responses API] Fold developer-role input messages into instructions

[kdcyberdude]

Summary

OpenAI's Responses API allows `role: "developer"` items in the `input` array. Clients such as Codex send developer messages for harness / policy text. vLLM's non-harmony path previously forwarded those items into the chat template, which only accepts standard roles and returned `Unexpected message role`.

This change merges developer message text into the top-level `instructions` field (appending after any existing instructions with `\n\n`) and removes those items from `input`, so downstream chat construction only sees `system` / `user` / `assistant` / tool roles.

Motivation / references

• Developer role is Incompatible Locally Deployed Model Interface openai/codex#9392 (developer role vs local providers)
• [Bug]: unsupport local vllm +qwen3.5-27b, because the role 'developer' openclaw/openclaw#28711 (similar `developer` role issue with vLLM)

Tests

• `tests/entrypoints/openai/responses/test_protocol.py`: validate folding + `construct_input_messages` produces a leading `system` message from merged instructions.

Implementation notes

• Text is extracted from `input_text` and `output_text` content parts (same shape Codex sends for developer messages).
• Handles `type` omitted or `type: "message"` with `role: "developer"`.

Made with Cursor

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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[gemini-code-assist]

Code Review

This pull request introduces logic to handle role='developer' messages in the Responses API by folding them into the top-level instructions field. This change ensures compatibility with chat templates that typically only support system, user, and assistant roles. The implementation includes a new helper function _text_chunks_from_responses_message_content to extract text from message content and updates the input_item_parsing logic to merge these developer messages into the instructions. Unit tests were added to verify that developer messages are correctly extracted and appended to existing instructions. I have no feedback to provide as there were no review comments to assess.

[chaunceyjiang]

LGTM.

you need to DCO.

[chaunceyjiang]

LGTM.

you need to DCO.

[cjackal]

This kind of internal message folding can change the request API spec I think. For example, most chat template asserts that the system message must be the first and unique within a message array, but with this PR any number of system messages are accepted and the position within the message array is not checked. Which can raise a once-blocked now-accepted chat histories problem in live services.

I wonder if a less destructive alternative of identifying the developer role with system role can be considered instead. In practice developer role is the correct role for instruction which deprecates system role (OpenAI codex does not send system role message as well).

[chaunceyjiang]

Thanks @cjackal for pointing that out. I’ll test this case locally.

[chaunceyjiang]

I don't think we should make modifications in the protocol.py.

The changes should only be made in construct_input_messages().

diff --git a/vllm/entrypoints/openai/responses/utils.py b/vllm/entrypoints/openai/responses/utils.py
index ad94c8d8d..89fc16f94 100644
--- a/vllm/entrypoints/openai/responses/utils.py
+++ b/vllm/entrypoints/openai/responses/utils.py
@@ -118,7 +118,63 @@ def construct_input_messages(
     else:
         input_messages = construct_chat_messages_with_tool_call(request_input)
         messages.extend(input_messages)
-    return messages
+
+    return _normalize_messages_for_chat_template(messages)
+
+
+def _extract_text_from_content(
+    content: str | list[dict[str, str]] | None,
+) -> list[str]:
+    """Extract plain text from a message content field.
+
+    Handles both string content and OpenAI-style list of content parts.
+    Returns None if there is no extractable text.
+    """
+    if content is None:
+        return []
+    if isinstance(content, str):
+        return [content] if content else []
+    if not isinstance(content, list):
+        return []
+    chunks: list[str] = []
+    for part in content:
+        if not isinstance(part, dict):
+            continue
+        if part.get("type") in ("input_text", "output_text"):
+            text = part.get("text")
+            if isinstance(text, str) and text:
+                chunks.append(text)
+    return chunks
+def _normalize_messages_for_chat_template(
+    messages: list[ChatCompletionMessageParam],
+) -> list[ChatCompletionMessageParam]:
+    """Normalize messages before passing them to the chat template.
+
+    1. Maps the "developer" role to "system" because most HF chat templates
+       do not recognize "developer".
+    2. Merges all system messages into a single message so that chat
+       templates (which typically expect at most one) do not receive multiples.
+    """
+    system_contents: list[str] = []
+    other_messages: list[ChatCompletionMessageParam] = []
+
+    for msg in messages:
+        if isinstance(msg, dict) and msg.get("role") in ("developer", "system"):
+            texts = _extract_text_from_content(msg.get("content"))
+            if texts:
+                system_contents.extend(texts)
+            continue
+        other_messages.append(msg)
+
+    if system_contents:
+        merged_system: ChatCompletionMessageParam = {
+            "role": "system",
+            "content": "\n\n".join(system_contents),
+        }
+        return [merged_system, *other_messages]
+    return other_messages

[lazyn1997]

Could you please let me know when this improvement will be merged into the official release? I'm currently using vllm 0.21.0, and this issue still persists.

[chaunceyjiang]

#41828 (review)

Hi @kdcyberdude, I noticed that you’ve been inactive for quite a while, and several issues are currently blocked by this PR. Based on my review, I submitted a new PR along with tests, and I’ve also added you as a collaborator.

see #43590

[tammypi]

When will this patch be merged into the main branch?

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @kdcyberdude.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[sfeng33]

Closing as completed in #43590