[Feature] Tolerate malformed Harmony streams.

[sanjaradylov]

Purpose

Problem

Currently, vllm.parser.harmony.HarmonyParser.process_chunk aborts Harmony parsing upon encountering stray tokens. Since both HarmonyParser.parse and HarmonyParser.parse_delta delegate to process_chunk, the model can still fail on malformed control-token sequences such as:

• role tokens twice: ... <|start|><|start|>assistant ... => ❌ Unknown role: <|start|>assistant ...;
• unexpected tokens breaking the template: ... <|end|> 364 <|start|> => ❌ Unexpected token ...;
• empty channels: ... <|channel|><|message|> ... => ❌ channel marker present but no ....

P.S. This was originally experienced by the Tenstorrent fork while running AIME25 against gpt-oss-120b.

Solution

Gracefully handle a malformed Harmony chunk by preserving parsed segments up to a stray token, breaking the iteration, and logging a warning.

This allows HarmonyParser.parse to return partial reasoning/content from completed messages, and causes HarmonyParser.parse_delta to stop yielding later chunks after the first parse error.

Before:

>>> from transformers import AutoTokenizer
>>> from vllm.entrypoints.openai.parser.harmony_utils import get_encoding
>>> from vllm.parser.harmony import HarmonyParser
>>> parser = HarmonyParser(AutoTokenizer.from_pretrained("openai/gpt-oss-20b"))
>>> malformed_stream = (
...     "<|channel|>analysis"
...     "<|message|>Reasoning here.<|end|>"
...     # Below 'assistant' appears 2 times => stray token.
...     "<|start|>assistantassistant<|channel|>final"
...     "<|message|>Final answer.<|end|>")
>>> token_ids = list(get_encoding().encode(malformed_stream, allowed_special="all"))
>>> result = parser.process_chunk(token_ids)
Traceback (most recent call last)
...
HarmonyError: Unknown role: assistantassistant

After:

>>> result = parser.process_chunk(token_ids)
WARNING ... Harmony parser error at token ID 200008, returning partial parse: HarmonyError('Unknown role: assistantassistant')
>>> [
...     (segment.completed_message.channel, segment.completed_message.content[0].text)
...     for segment in result.segments
...     if segment.completed_message is not None
... ]
[('analysis', 'Reasoning here.')]

Alternative Solutions We Could Consider

1. Continue after the error

Instead of breaking on the first HarmonyError, skip the offending token and keep iterating to recover additional content after an isolated malformed token.

2. Preprocess malformed tokens

Instead of handling malformed output only at parse time, normalize obviously corrupt control-token patterns before feeding them into the Harmony parser, e.g., collapsing duplicated headers, dropping stray tokens between <|end|> and the next valid header, or skipping empty channel markers.

3. Re-synchronize and continue

A middle ground would be to stop on HarmonyError, scan ahead for the next plausible Harmony message boundary, and resume parsing from there.

For example, given:

"<|channel|>analysis<|message|>Reasoning here.<|end|>"
"<|start|>assistantassistant<|channel|>final"
"<|message|>Corrupted answer.<|end|>"
"<|start|>assistant<|channel|>final"
"<|message|>Recovered answer.<|end|>"

a re-synchronizing parser could drop the malformed assistantassistant header, search forward to the next valid <|start|>assistant<|channel|>...<|message|> boundary, and continue from "Recovered answer.".

Test Plan

Run the Harmony parser suite test_harmony.py to verify parsing of both valid and malformed streams across non-streaming parsing, streaming parsing, and low-level chunk parsing.

Test Result

Passed.

---

P.S. After resolving merge conflicts with main, this PR composes with the
existing Harmony process_eos() recovery already on main.

The resulting behavior is:

• mid-stream HarmonyError in process_chunk: preserve completed segments, log a warning, and stop parsing;
• non-terminal process_eos() failure in flush: preserve main's existing raw-output recovery for the buffered unfinished message.

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[gemini-code-assist]

Code Review

This pull request introduces error handling in the parse_output_into_messages function to catch HarmonyError exceptions, allowing the parser to return a partial result instead of failing when encountering stray or corrupt tokens. Corresponding unit tests were added to validate both successful and malformed input scenarios. The reviewer suggested clarifying the log message to specify "token ID" for better debugging and identified a logic error in the test expectations for malformed streams where processing should terminate early.

[mergify]

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

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

[sanjaradylov]

I see that the current layout changes Harmony parsing logic and makes the proposed implementations outdated. But if the core issues from the PR are still not addressed, we still might want to incorporate the corresponding changes. [EDIT]: Refactoring is implemented.

Additionally, instead of merely breaking a cycle, we might want to introduce more refined stopping criteria and/or preprocess malformed tokens.

[mergify]

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

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

[mergify]

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

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