[Bugfix][Frontend] Validate reused prompt token ids; render messages for media and echo

[shijie-lyu]

Purpose

Bug fixes for the existing kv_transfer_params["prompt_token_ids"] reuse path from #48145. A P/D decode instance uses it to skip rendering and tokenizing messages again, and routers such as Ray's kv-aware router set it too. The PR adds no new request field, CLI flag or other API surface. Earlier revisions added a public prompt_token_ids field to /v1/chat/completions; after the discussion with @DarkLight1337 that field is gone.

Related to #55770. Mitigates #58445: the frontend now type-checks the ids, which removes that trigger. The engine-core bug itself is still open (EngineCoreProc.process_input_sockets decodes the ADD frame outside the try); #58481 handles it.

Changes:

• vllm/renderers/online_renderer.py: _reused_prompt_token_ids, the only reader of the alias, returns a 400 (parameter="kv_transfer_params.prompt_token_ids") unless the ids are a non-empty list of non-negative ints. The type check is exact, so True and 1.0 are rejected. With echo set it returns None without checking the ids and logs this at debug level, and messages is rendered. The check runs at render time, inside _with_kv_transfer_rejection_cleanup, so a rejected do_remote_prefill request notifies the KV connector. Valid ids are still used as sent; the HF and Harmony call sites are unchanged.
• vllm/entrypoints/openai/chat_completion/protocol.py: a mode="before" validator on ChatCompletionRequest removes the key when a content part is not text (a non-text or non-string type, or a media key on any part), so messages is rendered instead, and logs this at debug level. It reads the raw body because validation can drop the media keys of some parts and can turn content into a one-shot iterator. BatchChatCompletionRequest rejects the alias.
• vllm/entrypoints/chat_utils.py: the text part types become a shared TEXT_PART_TYPES constant, used by the content-part parser and by the validator above.
• docs/features/disagg_prefill.md: two sentences on the above.

Why media and echo fall back to rendering instead of failing: the ids stand in for the rendered prompt, but they carry no multimodal data or hashes and no prompt text. Reusing them drops the media (and their prefix-cache blocks cannot be told apart from those of a request with the same tokens and different media), and echo has nothing to return. A 400 from body validation would be raised before _with_kv_transfer_rejection_cleanup runs, so the prefill KV of a decode request would stay pinned until the connector lease expires. Rendering messages is always correct. It is what the decode instance does without the alias, so multimodal P/D and routers that always set the alias keep working, at the cost of one tokenization.

Behaviour:

Request (alias on /v1/chat/completions unless noted)	Before (main)	After
valid ids, text-only messages (text, thinking, refusal parts or string content)	ids used as sent, key removed from kv_transfer_params, cache_salt kept	unchanged
non-text content: typed media, a part without type, a media key on a type: text part, an unknown or non-string type	ids used, media silently dropped	ids ignored and messages rendered, as on a decode request without the alias
echo=true (HF and Harmony)	ids used, nothing echoed	ids ignored without being checked, messages rendered
ids []	ignored, messages rendered	400 (kv_transfer_params.prompt_token_ids)
ids [-1]	rejected later by the engine input processor (an error event inside the 200 stream with stream=true)	400 at render time, before the stream opens
ids [1.5], [true], [1.0], ["1"], "abc", 5	reach the engine untyped: a TypeError, or a decode failure that stops the engine-core input thread (#58445)	400 (kv_transfer_params.prompt_token_ids) on chat (HF and Harmony), non-Harmony /v1/responses, /v1/messages and /tokenize
malformed ids on a do_remote_prefill request with stream=true	fails (or hangs, #58445) after the stream opens; KV connector not notified	400 inside _with_kv_transfer_rejection_cleanup; connector notified
/v1/chat/completions/batch with the alias	one id list served every conversation	400 (kv_transfer_params.prompt_token_ids)
/v1/messages with an image block	media silently dropped	ids ignored, messages rendered (the Anthropic adapter builds a ChatCompletionRequest)
top-level prompt_token_ids on chat	ignored as an extra field	unchanged

Known limits, all unchanged by this PR:

• Code that sets the alias on a request object that is already built (for example an in-process router) skips the media scan, so media is still dropped there. The id check and the echo fallback still apply.
• The Harmony /v1/responses path does not read the alias. Media in the input of non-Harmony /v1/responses is not scanned.
• Cohere's catch-all handlers turn these 400s into 500s.
• With stream=true, ids that only the engine rejects (out of vocabulary, or longer than the context when truncate_prompt_tokens is set, since it is not applied to the ids) fail after the stream opens, so the KV connector is not notified.

Test Plan

export VLLM_WORKER_MULTIPROC_METHOD=spawn
cd tests
pytest -v entrypoints/openai/chat_completion/test_serving_chat.py -k kv_transfer_prompt_token_ids
pytest -v entrypoints/openai/chat_completion/test_batched_chat_completions.py -k kv_transfer
pytest -v entrypoints/openai/chat_completion/test_chat_completion.py -k kv_transfer_prompt_token_ids
# Every ChatCompletionRequest now runs the new validator.
pytest -q entrypoints/openai/chat_completion/test_serving_chat.py \
  entrypoints/openai/chat_completion/test_batched_chat_completions.py \
  entrypoints/openai/chat_completion/test_chat_echo.py \
  entrypoints/anthropic/test_anthropic_messages_conversion.py \
  entrypoints/cohere/test_serving_conversion.py entrypoints/scale_out/render/test_render.py
cd ..
pre-commit run ruff-check ruff-format typos check-spdx-header markdownlint-cli2 --files \
  vllm/entrypoints/chat_utils.py \
  vllm/entrypoints/openai/chat_completion/protocol.py vllm/renderers/online_renderer.py \
  tests/entrypoints/openai/chat_completion/test_serving_chat.py \
  tests/entrypoints/openai/chat_completion/test_batched_chat_completions.py docs/features/disagg_prefill.md
pre-commit run --hook-stage manual mypy-3.10 --files vllm/entrypoints/chat_utils.py \
  vllm/entrypoints/openai/chat_completion/protocol.py vllm/renderers/online_renderer.py

New tests:

• malformed ids on HF chat ([], [-1], [1.5], [1.0], ["1"], [True], "abc", 5) and on Harmony ([-1], [1.5]) return a 400
• a render-time 400 on a do_remote_prefill request notifies the KV connector, and the payload no longer has prompt_token_ids
• with echo, valid and malformed ids are ignored and messages is rendered
• non-text parts (typed, without type, media key on a type: text part, unknown type, non-string type) drop the ids
• text, thinking and refusal parts and string content keep the ids
• the batch endpoint rejects the alias

The existing #48145 tests run unchanged: Harmony reuse, and the e2e round-trip and streaming tests, which cover HF reuse as sent.

Test Result

Hardware: 1x NVIDIA L4 24 GB (EC2 g6.2xlarge), Python 3.12.14, torch 2.13.0+cu130, pytest 9.1.1. PR tree = this diff (5 files, +222/-1) applied to main 5c3b61e. Base = 5c3b61e. The later review commit (shared TEXT_PART_TYPES, two debug logs) changes no behaviour and was checked with ruff only; CI covers it.

Tests on the PR tree

Command	Result
test_serving_chat.py -k kv_transfer_prompt_token_ids	20 passed
test_batched_chat_completions.py -k kv_transfer	1 passed
test_serving_chat.py + test_batched_chat_completions.py (full)	71 passed, 4 skipped (the tests' own skips)
test_chat_completion.py + test_chat_echo.py + test_chat_error.py	32 passed, including the #48145 test_kv_transfer_prompt_token_ids_round_trip and _streaming
anthropic/test_anthropic_messages_conversion.py, cohere/, scale_out/render/	316 passed
test_request_input_bounds.py	32 passed, 7 errors. The same 7 error on main: a fixture Mock model_config fails at vllm/multimodal/cache/factories.py:32. Unrelated to this PR.

I also ran the new test files against main's source. 19 of the 21 selected tests fail there, all on the behaviour under test (DID NOT RAISE, messages not rendered with echo, ids kept with non-text content). The 2 that pass on main are the existing #48145 Harmony reuse test and allows_text_only_parts. Both pin behaviour this PR does not change.

Lint: pre-commit ruff-check, ruff-format, typos, check-spdx-header and markdownlint-cli2 on the 5 files, and mypy-3.10 (manual stage) on protocol.py and online_renderer.py, all pass.

Live server, main vs PR (same vllm serve command on both trees, temperature 0)

Qwen/Qwen2.5-1.5B-Instruct, 96 requests per tree, no unmet expectation on either tree:

• Valid ids with text-only messages: same result on both trees. Other messages plus the ids return the output for the ids, so the ids are used as sent. This holds for streams, tool calls, and text/thinking/refusal parts. cache_salt is kept: a fresh salt gives 0 prefix hits. Lengths are unchanged: 4088 ids return 200, 4200 ids return the context-length 400, and truncate_prompt_tokens is not applied to the ids.
• Malformed ids ([], [-1], [1.5], [1.0], ["1"], [true], "abc", 5), non-stream and stream=true, also with do_remote_prefill: the PR returns 400 param=kv_transfer_params.prompt_token_ids as JSON, before any stream opens. On main:
  • [] returns 200.
  • [-1] returns the engine's 400, or with stream=true a 200 stream carrying an error event.
  • [1.5], [1.0], ["1"] and "abc" return 500.
  • 5 returns 400 "no len()".
• [Bug]: EngineCore input socket thread dies on an undecodable request and the core stays alive but stops accepting requests #58445: on main, [true] stopped the engine-core input thread twice (msgspec.ValidationError: Expected int, got bool in process_input_sockets). Chat then hung for 60 s while /health returned 200, and the server had to be restarted. On the PR, the probe request after every malformed request succeeded, and no server log has an engine-core exception.
• echo=true with valid, other-prompt or malformed ids: the PR output is identical to plain echo. Main echoes nothing, or answers the prompt the ids encode.
• Non-text content plus ids (typed image, part without type, image behind type: text + uuid, unknown type, non-string type): the PR returns the same status and body as the same request without the ids. Main returns 200 with the media dropped.
• Batch plus ids: the PR returns 400. Main answered both conversations from one id list.
• Adapters: /v1/responses ([1.5], "abc", [-1], []), /v1/messages ([1.5], []) and /tokenize ([1.5]) return 400. /v1/messages with an image block plus ids gives the same result as without ids. Cohere returns 500 carrying the 400 message (known limit).
• Unrelated difference: a plain batch request without ids differed between the two runs in the wording of the second choice. Fresh-server reruns on both trees gave byte-identical output under 3 matched prefix-cache states, and that output depends on cache state.

Qwen/Qwen3-VL-2B-Instruct, 17 requests per tree. The test image is a red square with the word "CAT", and the plain request answers "red ... cat" with 229 prompt tokens:

• On main, the image plus forwarded ids answers "blue ... Hello" or "blue ... SUN", so the image was dropped. The ids came from /tokenize of the same request or of the text-only request.
• On the PR, all 7 image-plus-ids variants give exactly the plain answer with 229 prompt tokens: typed, typeless, behind type: text + uuid, stream, /v1/messages image block, and [1.5].
• Text-only reuse still works on the VLM.

Reasoning, Qwen/Qwen3-0.6B with --reasoning-parser qwen3 (PR tree only), 13/13:

• Reuse gives identical reasoning and content, stream and non-stream, with thinking on and off.
• Malformed ids return 400.

Harmony, openai/gpt-oss-20b (PR tree only), 19/19:

• /tokenize ids equal the Harmony-rendered ids (79 tokens), and reuse matches the plain output.
• [], [-1], [1.5], [true], "abc", 5 and stream [1.5] return 400.
• With echo, the ids are ignored. For a user-final message, Harmony's echo prepends nothing, so this run shows the ids were not used rather than echoed text.

P/D: NixlConnector (nixl 1.4.1, nixl-cu13), prefill and decode instances of Qwen2.5-1.5B-Instruct on the one L4, main vs PR. 89/89 checks pass.

• Decode with the forwarded ids: it pulls the whole prompt KV over NIXL (402/402 external prefix hits, one transfer of 11,927,552 bytes) and uses the ids as sent. It returns the plain single-instance output, identical on main and PR, stream and non-stream.
• [1.5] on the decode request: the PR returns 400, also with stream=true. The decode instance aborts the request, and the prefill instance frees the blocks in the same second. On main, non-stream returns 500 and stream=true returns 200 with an error event. In the stream=true case the connector is not notified, and the prefill blocks are held until the 30 s lease expires.
• echo plus ids: the PR echoes the message, the same as without ids. Main echoes nothing.
• Media key on a type: text part plus ids: the PR ignores the ids.
• Typed image on the text-only model: the PR returns the same 400 as without the ids, and the connector is notified. Main uses the ids, dropping the image.

Not tested:

• The known limits: alias set on an already-built request; Harmony /v1/responses; media in /v1/responses input; engine-only rejections under stream=true
• Every malformed value on every endpoint: only HF chat got all 8
• Multimodal and Harmony P/D
• The Harmony and reasoning suites on main

Written with Claude Code; the design, review and testing are mine, and the commit carries a Co-authored-by: Claude trailer per the AI-assistance guidance in docs/contributing/README.md.

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Documentation preview: https://vllm--55771.org.readthedocs.build/en/55771/

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[chaunceyjiang]

Thanks for your work. Have you looked into the /inference/v1/generate endpoint?

This endpoint was specifically designed for this kind of use case. You may want to take a closer look at docs/serving/online_serving/README.md for more details.

[shijie-lyu]

Thanks for your work. Have you looked into the /inference/v1/generate endpoint?

This endpoint was specifically designed for this kind of use case. You may want to take a closer look at docs/serving/online_serving/README.md for more details.

Thanks, yes. I looked at the token-in/token-out path and the render/derender split before opening this, and it is the right design when a CPU frontend owns the whole OpenAI surface. Our case is narrower: a thin router sits in front of each engine and only needs to skip the second tokenization; the engine should still produce its own chat response.

Going through /inference/v1/generate for that means enabling scale-out on every serving container, carrying the chat request and prompt tokens to /derender, and for streaming one extra HTTP round trip per chunk with client-held stream_state, or reimplementing tool and reasoning parsing in the router. The chat endpoint already does all of this when the ids arrive via kv_transfer_params["prompt_token_ids"] (#48145); this PR makes that channel a typed, documented field and closes its validation gaps rather than adding a new mechanism.

[shijie-lyu]

@DarkLight1337 I agree with @shijie-lyu here. The two cover different sides of the problem. output_mode="text" changes what generate sends back. It helps callers that already use the GenerateRequest format. This PR changes what chat accepts for routers that tokenize a /v1/chat/completions body. Even the derender level coming in Phases 2 and 3 (of #56851) wouldn't cover this case. The caller would still need a /render built GenerateRequest, --enable-scale-out on the engines and a wrap of the generate response into chat.completion.

Alternatives item 4 in the #56851 says the same thing from the other side. It's under "alternatives considered" rather than "out of scope". I'll update it to link this PR as the input side counterpart so the split is clear.

Thanks @hickeyma, and thanks for the correction: it's listed under alternatives considered, not out of scope. Linking the two as the output and input sides makes sense. After talking with @DarkLight1337, I shrank this PR to bug fixes for the existing kv_transfer_params["prompt_token_ids"] path from #48145, with no new public field, so it doesn't add to the chat API. The description has the details and test results.

[DarkLight1337]

LGTM but better for @NickLucche to confirm

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This pull request has merge conflicts that must be resolved before it can be
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✅ @shijie-lyu, CI is now available for this PR.

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[NickLucche]

Thanks for the fix @shijie-lyu , I think the enginecore crash one is close to some of the ~cve fix we get :)
Left a couple of minor comments

[shijie-lyu]

/ci run

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✅ Triggered Buildkite CI #91558 for commit d9091c2f6dc2.

[shijie-lyu]

/ci retry

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✅ Queued 2 failed job(s) for retry in Buildkite CI #91558.

[shijie-lyu]

@NickLucche I've addressed your comments in a52df23, and CI is green now. If it looks good to you, could you approve and merge it? Thanks!

[hickeyma]

LGTM, thanks @shijie-lyu.

Not blocking but /v1/responses hits the same path without this check, so image input plus forwarded prompt_token_ids still drops the image there. That was broken before this PR too. Could be a quick follow up?

[shijie-lyu]

LGTM, thanks @shijie-lyu.

Not blocking but /v1/responses hits the same path without this check, so image input plus forwarded prompt_token_ids still drops the image there. That was broken before this PR too. Could be a quick follow up?

Thanks @hickeyma, Agreed, it's in the known limits: media in the non-Harmony /v1/responses input isn't scanned yet. I'll open a follow-up PR for it.