fix(harmony): run render_for_completion in thread pool to unblock event loop

[ztimothy96]

Purpose

Resolves #38266. When a request contains a very long repetitive sequence, the synchronous Harmony tokenizer call (render_for_completion) could block the asyncio event loop for minutes, making the server unresponsive to all
other requests.

Fix: wrap render_for_completion with make_async() (run_in_executor) and propagate async/await through the entire call chain (_make_request_with_harmony, HarmonyContext.render_for_completion,
StreamingHarmonyContext.render_for_completion) so the event loop is free to serve concurrent requests while tokenization runs in a thread. The same fix is applied to OpenAIServingRender._make_request_with_harmony in the Chat
Completions path.

Checked for existing PRs addressing #38266 — none found as of March 26, 2026.

Test Plan

Add a unit test, test_render_for_completion_async.py, to ensure all callers of the function are async and that the function itself releases the event loop.

Also run existing tests that call _make_request_with_harmony, since those call sites were updated to add await.

.venv/bin/python -m pytest tests/entrypoints/openai/parser/test_render_for_completion_async.py -v
.venv/bin/python -m pytest tests/entrypoints/openai/responses/test_serving_responses.py -v

Chat Completion test (requires model artifacts)

.venv/bin/python -m pytest tests/entrypoints/openai/chat_completion/test_serving_chat.py -v

Test Result

This fix was developed with AI assistance (Claude). I have reviewed every changed line and personally ran the first two test commands above.

Note: test_serving_chat.py::TestGPTOSSChat requires the Harmony vocab file (not available in a standard dev environment) and was not run locally. The await fixes to that file are mechanical call-site corrections that follow directly from the async signature change.

.venv/bin/python -m pytest tests/entrypoints/openai/parser/test_render_for_completion_async.py -v
====================================== test session starts ======================================
platform darwin -- Python 3.12.13, pytest-9.0.2, pluggy-1.6.0 -- /Users/tzhou/Documents/GitHub/vllm/.venv/bin/python
cachedir: .pytest_cache
rootdir: /Users/tzhou/Documents/GitHub/vllm
configfile: pyproject.toml
plugins: asyncio-1.3.0, anyio-4.13.0
asyncio: mode=Mode.STRICT, debug=False, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collected 9 items                                                                               

tests/entrypoints/openai/parser/test_render_for_completion_async.py::TestMakeAsyncReleasesEventLoop::test_blocking_call_in_executor_allows_other_coroutines PASSED [ 11%]
tests/entrypoints/openai/parser/test_render_for_completion_async.py::TestMakeAsyncReleasesEventLoop::test_make_async_returns_awaitable PASSED [ 22%]
tests/entrypoints/openai/parser/test_render_for_completion_async.py::TestRenderForCompletionAsyncIsAwaitable::test_render_for_completion_async_exported PASSED [ 33%]
tests/entrypoints/openai/parser/test_render_for_completion_async.py::TestRenderForCompletionAsyncIsAwaitable::test_harmony_context_render_for_completion_is_coroutine PASSED [ 44%]
tests/entrypoints/openai/parser/test_render_for_completion_async.py::TestRenderForCompletionAsyncIsAwaitable::test_streaming_harmony_context_render_for_completion_is_coroutine PASSED [ 55%]
tests/entrypoints/openai/parser/test_render_for_completion_async.py::TestRenderForCompletionAsyncIsAwaitable::test_make_request_with_harmony_is_coroutine_in_render_serving PASSED [ 66%]
tests/entrypoints/openai/parser/test_render_for_completion_async.py::TestRenderForCompletionAsyncIsAwaitable::test_make_request_with_harmony_is_coroutine_in_responses_serving PASSED [ 77%]
tests/entrypoints/openai/parser/test_render_for_completion_async.py::TestGetEncodingThreadSafety::test_get_encoding_initialized_exactly_once_under_concurrency PASSED [ 88%]
tests/entrypoints/openai/parser/test_render_for_completion_async.py::TestRenderForCompletionAsyncResult::test_async_wrapper_preserves_return_value PASSED [100%]

.venv/bin/python -m pytest tests/entrypoints/openai/responses/test_serving_responses.py -v
====================================== test session starts ======================================
platform darwin -- Python 3.12.13, pytest-9.0.2, pluggy-1.6.0 -- /Users/tzhou/Documents/GitHub/vllm/.venv/bin/python
cachedir: .pytest_cache
rootdir: /Users/tzhou/Documents/GitHub/vllm
configfile: pyproject.toml
plugins: asyncio-1.3.0, anyio-4.13.0
asyncio: mode=Mode.STRICT, debug=False, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collected 16 items                                                                              

tests/entrypoints/openai/responses/test_serving_responses.py::test_extract_tool_types PASSED [  6%]
tests/entrypoints/openai/responses/test_serving_responses.py::TestInitializeToolSessions::test_initialize_tool_sessions PASSED [ 12%]
tests/entrypoints/openai/responses/test_serving_responses.py::TestInitializeToolSessions::test_validate_create_responses_input PASSED [ 18%]
tests/entrypoints/openai/responses/test_serving_responses.py::TestValidateGeneratorInput::test_validate_generator_input PASSED [ 25%]
tests/entrypoints/openai/responses/test_serving_responses.py::test_reasoning_tokens_counted_for_text_reasoning_model PASSED [ 31%]
tests/entrypoints/openai/responses/test_serving_responses.py::TestExtractAllowedToolsFromMcpRequests::test_extract_allowed_tools_basic_formats PASSED [ 37%]
tests/entrypoints/openai/responses/test_serving_responses.py::TestExtractAllowedToolsFromMcpRequests::test_extract_allowed_tools_star_normalization PASSED [ 43%]
tests/entrypoints/openai/responses/test_serving_responses.py::TestExtractAllowedToolsFromMcpRequests::test_extract_allowed_tools_filters_non_mcp PASSED [ 50%]
tests/entrypoints/openai/responses/test_serving_responses.py::TestHarmonyPreambleStreaming::test_preamble_delta_emits_text_events PASSED [ 56%]
tests/entrypoints/openai/responses/test_serving_responses.py::TestHarmonyPreambleStreaming::test_preamble_delta_second_token_no_added PASSED [ 62%]
tests/entrypoints/openai/responses/test_serving_responses.py::TestHarmonyPreambleStreaming::test_commentary_with_function_recipient_not_preamble PASSED [ 68%]
tests/entrypoints/openai/responses/test_serving_responses.py::TestHarmonyPreambleStreaming::test_preamble_done_emits_text_done_events PASSED [ 75%]
tests/entrypoints/openai/responses/test_serving_responses.py::TestHarmonyPreambleStreaming::test_commentary_with_recipient_no_preamble_done PASSED [ 81%]
tests/entrypoints/openai/responses/test_serving_responses.py::TestStreamingReasoningToContentTransition::test_mixed_delta_reasoning_and_content_emits_reasoning_delta PASSED [ 87%]
tests/entrypoints/openai/responses/test_serving_responses.py::TestStreamingReasoningToContentTransition::test_transition_without_mixed_delta_no_extra_reasoning_event PASSED [ 93%]
tests/entrypoints/openai/responses/test_serving_responses.py::TestStreamingReasoningToContentTransition::test_reasoning_only_stream_no_content PASSED [100%]

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
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• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[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 addresses a performance bottleneck where the synchronous render_for_completion tokenizer call could block the event loop for extended periods. By wrapping this call with make_async, the rendering now runs in a thread pool, and the associated methods in HarmonyContext, OpenAIServingRender, and OpenAIServingResponses have been updated to be asynchronous. New unit tests were added to ensure the event loop remains responsive during tokenization. A critical issue was identified regarding a race condition in the global encoding singleton's lazy initialization, which now requires thread-safe locking because the tokenization logic is executed concurrently across multiple threads.

[DarkLight1337]

Can you double check whether this is still needed after #34789?

[Gnoale]

It will not have any effect if you don't provide make_async with another ThreadPoolExecutor, so the function is executed inside a dedicated thread

from concurrent.futures import ThreadPoolExecutor

render_for_completion_async = make_async(render_for_completion, ThreadPoolExecutor(max_workers=4))

[Gnoale]

Maybe that's not needed because the underlying code should already be thread safe ? tokenizer: Arc::new(encoding_ext.load()?)

https://doc.rust-lang.org/std/sync/struct.Arc.html#thread-safety