fix(anthropic): preserve inline system message position for prefix caching by felix0080 · Pull Request #44602 · vllm-project/vllm

felix0080 · 2026-06-05T02:33:48Z

Problem

PR #44283 merged all inline role: system messages from the messages array into a single leading system message. This changes the conversation prefix, breaking KV-cache hits in multi-turn dialogues.

#44048 (currently open) moves the same merge logic to the protocol layer but retains the same prefix-breaking behavior.

Example of the problem

Input:  [user:A, assistant:B, system:new_rule, user:C]
                ↑ prefix cache can hit here

#44283: [system:(all merged), user:A, assistant:B, user:C]
         ↑ prefix completely different → cache miss

This PR: [system:top-level, user:A, assistant:B, system:new_rule, user:C]
              ↑ prefix unchanged → cache hits preserved

Fix

Remove inline system message extraction from _convert_system_message — only handle top-level system field there
In _convert_messages, handle system messages with a dedicated _extract_system_text helper that:
- Strips x-anthropic-billing-header from inline system messages (previously only done for top-level)
- Only emits a system message if there is real content (avoids empty {"role": "system"} messages that _convert_block could produce)
Add 2 new tests for billing header stripping on inline system messages

Why this approach

Minimal and localized: all system handling is explicit, not spread across _convert_block / _convert_message_content
Prefix structure stays intact for all conversation turns
Billing header stripping is consistent between top-level and inline system messages

Test Plan

(AI assistance was used; I reviewed every changed line.)

python -m pytest tests/entrypoints/anthropic/test_anthropic_messages_conversion.py -v

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.

github-actions · 2026-06-05T02:33:55Z

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felix0080 · 2026-06-05T03:00:12Z

Ready for review — could a maintainer add the ready label to trigger CI? Thanks.

chaunceyjiang · 2026-06-05T06:44:32Z

    ) -> None:
        """Convert Anthropic messages to OpenAI format"""
+
+        def _extract_system_text(msg) -> str | None:


Please convert this method into a class method by adding @classmethod.

@chaunceyjiang ok

@chaunceyjiang @chaunceyjiang Done. I've converted it to a class method

felix0080 · 2026-06-05T07:46:42Z

            openai_messages.append({"role": "system", "content": "".join(system_parts)})

+    @classmethod
+    def _extract_system_text(cls, msg) -> str | None:


@chaunceyjiang Done. I've converted it to a class method

You need to DCO.

@chaunceyjiang Thanks for the reminder. DCO fixed

…ching PR vllm-project#44283 merged all inline system:role messages into a single leading system message, which changes the conversation prefix and breaks KV-cache hits in multi-turn dialogues. This fix keeps inline system messages at their original position: - Remove inline system extraction from _convert_system_message (only top-level system is handled there) - In _convert_messages, handle system messages with a dedicated _extract_system_text helper that strips billing headers and only emits the message if real content exists — avoiding the _convert_block / _convert_message_content path which does not strip billing headers and may omit the "content" key - Add tests for billing header stripping on inline system messages Unlike vllm-project#44048 which moves the same merge logic to the protocol layer, this approach fundamentally avoids the prefix-breaking merge entirely. Co-authored-by: Hermes Agent Signed-off-by: felix0080 <felix0080@users.noreply.github.com>

Per maintainer review feedback. Signed-off-by: felix0080 <felix0080@users.noreply.github.com>

aleksandaryanakiev · 2026-06-05T08:24:08Z

LGTM

chaunceyjiang · 2026-06-05T09:29:39Z

            if msg.role == "system":
+                text = cls._extract_system_text(msg)
+                if text:
+                    openai_messages.append({"role": "system", "content": text})


In fact, after this change, the Qwen3.5/Qwen3.6 series models will no longer be supported.

@chaunceyjiang This change is meant to preserve prefix caching for Anthropic clients like Claude Code that send system messages mid-conversation. The conflict with Qwen's chat template is a template-level limitation — Qwen expects system to appear only at the beginning — and that should be addressed by updating the Qwen template to handle non-leading system messages, not by compromising the conversion layer for all users.

This will impact not only Qwen models - even though many models may allow system messages at any position in the message list it doesn't mean those models were trained on system messages that come after user messages in a conversation. Most are not trained on this kind of data, and expect the system messages (even if more than 1) to come before the user messages.

Are we were of any open weight model specifically trained on system messages that appear later in a conversation? This feels like we're trading KV cache efficiency for worse overall trajectories in these agentic workflows.

chaunceyjiang

LGTM

felix0080 requested review from AndreasKaratzas, DarkLight1337, NickLucche, aarnphm, mgoin and robertgshaw2-redhat as code owners June 5, 2026 02:33

claude Bot reviewed Jun 5, 2026

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mergify Bot added the frontend label Jun 5, 2026

felix0080 force-pushed the fix/anthropic-inline-system-preserve-position branch from 71ef5be to 835f37d Compare June 5, 2026 02:45

This was referenced Jun 5, 2026

[Anthropic] Support system role messages inside messages array #44283

Merged

[Bugfix][Anthropic] Normalize Claude Code system messages #44048

Open

chaunceyjiang reviewed Jun 5, 2026

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felix0080 commented Jun 5, 2026

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felix0080 added 2 commits June 5, 2026 16:00

refactor: convert _extract_system_text to classmethod

4439ea4

Per maintainer review feedback. Signed-off-by: felix0080 <felix0080@users.noreply.github.com>

felix0080 force-pushed the fix/anthropic-inline-system-preserve-position branch from e81f76a to 4439ea4 Compare June 5, 2026 08:00

chaunceyjiang added the verified Run pre-commit for new contributors without triggering other tests label Jun 5, 2026

chaunceyjiang reviewed Jun 5, 2026

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chaunceyjiang approved these changes Jun 5, 2026

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chaunceyjiang added the ready ONLY add when PR is ready to merge/full CI is needed label Jun 5, 2026

Merge branch 'main' into fix/anthropic-inline-system-preserve-position

4e609e8

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felix0080 commented Jun 5, 2026 •

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