[sashankh]

Purpose

Fixes #52489.

_convert_user_tool_result normalises text and image items inside a tool_result block into OpenAI-shaped content items, but for tool_reference it appended the Anthropic-shaped dict unchanged:

{"type": "tool_reference", "name": ref_name}

That dict becomes the content of a tool-role message and is passed to the model's chat template. Templates that enumerate known content types (Qwen3's among them) fall through to their catch-all raise_exception branch on the unknown tool_reference type, so the request fails for whichever model is being served. Because the offending block stays in the conversation history, every subsequent request in that session fails as well — which is why this surfaces as an agent session going permanently dead rather than a single bad request.

The fix converts the item to a text block, mirroring how text and image are already handled. This is the fix suggested by the reporter in the issue.

Duplicate check. Searched open PRs for 52489 in:body, tool_reference, and anthropic tool_result. The other open Anthropic-frontend PRs — #47877 / #47747 (per-type AnthropicContentBlock field validation), #45653 (search_result blocks), #46162 (streaming error forwarding), #34053 (protocol compliance) — touch different code paths and none change tool_reference handling in _convert_user_tool_result.

One scope note. The test file gains pytestmark = pytest.mark.skip_global_cleanup so this pure-conversion suite can run on a machine with no GPU; without it the run aborts during global cleanup. Five other files under tests/entrypoints/ already set it. Happy to drop it if you would rather the diff stay strictly on the fix.

Test Plan

New case TestToolResultContent::test_tool_result_with_tool_reference. The malformed content shape is the bug, so the test asserts the converted tool message directly rather than going through a template.

.venv/bin/python -m pytest tests/entrypoints/anthropic/test_anthropic_messages_conversion.py

Test Result

• With the new test but serving.py reverted: 1 failed — content came back as [{'type': 'tool_reference', 'name': 'list_files'}].
• With the fix applied: 52 passed.
• ruff check and ruff format --diff clean on both changed files.

No model evaluation is included: this changes neither weights, sampling, nor kernels, and only affects how a request is translated ahead of templating. A before/after quality comparison isn't meaningful here because the affected requests currently fail outright at template rendering.

---

Per the AI-assisted contribution policy: this change was developed with AI assistance (GitHub Copilot), noted in the commit trailer. I reviewed every changed line, ran the tests above, and can speak to the change.

[claude]

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[jagjeet-singh-ambak]

Thanks for picking this up @sashankh — the fix looks correct for the reported path.

One gap worth folding in: this fixes the Anthropic endpoint, but the same crash is
still reachable through the OpenAI /v1/chat/completions path, which doesn't go
through _convert_user_tool_result at all.

tool_reference is a public content-part type on that endpoint too —
CustomChatCompletionContentToolReferenceParam is part of the
ChatCompletionContentPartParam union (vllm/entrypoints/chat_utils.py:330) — so a
client can send it directly in a tool message. _parse_chat_message_content
(chat_utils.py:1849) parses content parts for every role including tool, so it
reaches _parse_chat_message_content_part, which still returns the raw dict:

elif part_type == "tool_reference":
    if wrap_dicts:
        return {"type": "tool_reference", "name": cast(str, content)}

That dict then hits the has_non_text branch at chat_utils.py:1884-1891, which
deliberately preserves structured items instead of flattening them — so the raw
shape reaches the chat template and raises the same
TemplateError: Unexpected item type in content.

Verified against unmodified main:

1) part parser output (wrap_dicts=True): {'type': 'tool_reference', 'name': 'list_files'}
2) has_non_text -> True
   => tool message content stays: [{'type': 'tool_reference', 'name': 'list_files'}]

A one-line change at the same spot closes it, and matches what
vllm/renderers/cohere.py:363-371 already does for this type ("Tool references are
rendered by name; emit as text so the renderer downstream is content-format
agnostic"):

elif part_type == "tool_reference":
    name = cast(str, content)
    placeholder = f"[tool reference: {name}]"
    if wrap_dicts:
        return {"type": "text", "text": placeholder}
    return placeholder

Worth noting nothing in the repo actually consumes the structured
{"type": "tool_reference", ...} shape — git grep tool_reference only hits
anthropic/protocol.py, anthropic/serving.py, chat_utils.py,
renderers/cohere.py and its test. No chat template or example expands it, so the
"passed through as-is for the chat template to expand" comment describes an
expansion that never happens.

Two smaller notes on the current diff:

• The second tool message is still emitted with the same tool_call_id as the
  first (your test asserts len(tool_msg) == 2). Appending the placeholder to
  text_parts instead would collapse it to a single tool message with plain
  string content, which is what every other branch of that function produces and
  what the sibling tests assert.
• Since _convert_user_tool_result reads item.get("tool_name") or item.get("name"),
  it may be worth covering the "name" spelling too — your test only exercises
  "tool_name".

Happy to send the chat_utils.py half as a follow-up PR after this lands if you'd
rather keep this one scoped — whichever you prefer. I had been looking at the same
issue and stopped once I saw your PR; disclosure, I used AI assistance in
investigating this.

[brianosaurus]

Fix looks right, and asserting on converter output rather than template rendering
is the correct level. Three notes from running it against main @ 6914d60b1e.

1. The description's failure mode is incomplete. It says templates fall
through to raise_exception, "Qwen3's among them". Qwen3-0.6B does not raise:

model	symptom
SmolLM2-1.7B-Instruct	raises TypeError
Qwen3-0.6B, Phi-3-mini-4k	silent drop (empty <tool_response>)
Qwen2.5-1.5B, Llama-3.1-8B	raw dict repr leaked into the prompt

Zero of five render it correctly, and only one tells you. So the severity is
worse than "sessions die": on lenient templates you get silent context loss or
prompt pollution instead.

2. The test pins an empty message. assert len(tool_msg) == 2 holds because
tool_text is "" when a tool_result has only a reference, yet
serving.py:438 appends a tool message unconditionally. Message 1 is that empty
<tool_response>. Folding the reference into text_parts would emit one message
and drop it.

3. Two renderings incoming. @jagjeet-singh-ambak's OpenAI-path fix follows
cohere.py:363, which emits the bare name, as does chat_utils.py:1510. This PR
emits [tool reference: {name}]. If both land, the same input renders
differently per endpoint. Worth picking one.

Minor: is the # type: ignore[dict-item] at serving.py:462 still needed now
that the items are text-shaped?

repro (tokenizer only, no GPU)

from types import SimpleNamespace
from transformers import AutoTokenizer
from vllm.entrypoints.anthropic.serving import AnthropicServingMessages

b = SimpleNamespace(tool_use_id="t1",
                    content=[{"type": "tool_reference", "name": "some_mcp_tool"}])
c = []
AnthropicServingMessages._convert_user_tool_result(b, c)
for m in ["Qwen/Qwen3-0.6B", "Qwen/Qwen2.5-1.5B-Instruct",
          "microsoft/Phi-3-mini-4k-instruct",
          "NousResearch/Meta-Llama-3.1-8B-Instruct",
          "HuggingFaceTB/SmolLM2-1.7B-Instruct"]:
    try:
        out = AutoTokenizer.from_pretrained(m).apply_chat_template(
            [{"role": "user", "content": "hi"}, *c],
            tokenize=False, add_generation_prompt=True)
    except Exception as e:
        print(m, "RAISES", type(e).__name__); continue
    print(m, "SILENT DROP" if "some_mcp_tool" not in out
             else "DICT LEAK" if "'type': 'tool_reference'" in out
             else "clean text")

[mergify]

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

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