[#19227][fix] Complete zero-argument tool calls in Qwen3-Coder streaming

[Yigtwxx]

Description

Fixes #19227.

A tool call whose function takes no parameters is written as a <function=...> block with
no <parameter> children. In streaming, Qwen3CoderToolParser.parse_streaming_increment
emits the name and then, on </tool_call>, only appends the closing brace when some
argument text was already streamed. Nothing was, so the call completes with
arguments == "" while detect_and_parse returns {} for the same text:

from tensorrt_llm.serve.tool_parser.qwen3_coder_parser import Qwen3CoderToolParser

text = "<tool_call>\n<function=get_time>\n</function>\n</tool_call>"

[(c.name, c.parameters) for c in Qwen3CoderToolParser().parse_streaming_increment(text, tools).calls]
# before: [('get_time', '')]
# after:  [('get_time', ''), (None, '{}')]
[(c.name, c.parameters) for c in Qwen3CoderToolParser().detect_and_parse(text, tools).calls]
# [('get_time', '{}')]  (unchanged)

The fragments _parse_and_stream_parameters emits are {"k": v and , "k": v — never
the outer closing brace — so the completion step now appends "}" whenever something was
streamed and "{}" when nothing was. That replaces the previous brace count, which the
second commit removes: a string value containing } (<parameter=location>a}b</parameter>)
made the counts equal, so the outer brace was never sent and the streamed arguments were
not valid JSON, while detect_and_parse returned {"location": "a}b"}. Both shapes are the
same invariant — the streamed arguments end as the complete JSON object that
detect_and_parse produces. This is the contract the package already follows:
Glm47ToolParser._finalize_tool_call sends "{}" when no parameter was streamed, and
BaseToolParser.parse_streaming_increment got the same treatment in #17575. Calls with
ordinary values take exactly the path they took before. There is no API change and no
change to the non-streaming path.

tool_parser_factory.py maps nemotron_h_omni to this parser as well, so both are
covered.

Test Coverage

tests/unittest/llmapi/apps/test_tool_parsers.py, already registered as cpu_only in
tests/integration/test_lists/test-db/l0_cpu.yml, class TestQwen3CoderToolParser:

• test_streaming_zero_arg_tool[one_delta|chunked] — streams a zero-argument call whole
  and split at every tag, and asserts the name is emitted once, the concatenated argument
  deltas are exactly {}, and that this equals what detect_and_parse returns. Mirrors
  TestGlm47ToolParser::test_streaming_zero_arg_tool.
• test_streaming_closes_arguments_with_brace_in_value — streams a call whose string value
  contains } in three deltas and asserts the concatenated arguments parse to
  {"location": "a}b"} and equal the detect_and_parse result.

All three fail on main and pass here. Runtime on this box, measured over 2000
repetitions each: 18.9 / 30.6 / 42.1 us. The rest of the class (17 cases) and the file
are unchanged: 350 passed locally versus 347 before, with an identical set of 50
pre-existing failures that need the full openai_protocol/postprocess_handlers import
chain, which this machine cannot load.

PR Checklist

• Commit is signed off (DCO)
• PR title follows [#issue][type] description
• Pre-commit hooks run on the changed files
• Tests added, verified red on main and green here
• Single concern

@zhaoyangwang-nvidia this is the same zero-argument case you reviewed in #17575, in the
one parser that still dead-ends on it. Would you mind taking a look and triggering the
pipeline when you have a moment?

Dev Engineer Review

• Streaming completion now emits {} for zero-argument tool calls.
• Parameterized calls retain existing behavior, including arguments that contain }.
• Non-streaming parsing and public APIs remain unchanged.
• The shared parser change also applies to nemotron_h_omni.
• No material implementation risks are evident from the supplied changes.

QA Engineer Review

• Modified tests/unittest/llmapi/apps/test_tool_parsers.py.
• Added coverage for zero-argument calls in one and multiple streaming deltas.
• Added coverage for } inside string arguments and parity with one-shot parsing.
• Integration test-list updates are not applicable.
• Coverage verdict: sufficient.

Per-File QA Perspective

• tensorrt_llm/serve/tool_parser/qwen3_coder_parser.py: Verify that zero-argument calls emit {} and parameterized calls emit the correct closing fragment without brace-counting errors. Confirm behavior for both qwen3_coder and nemotron_h_omni.
• tests/unittest/llmapi/apps/test_tool_parsers.py: Covers zero-argument streaming, split deltas, embedded } characters, and one-shot parsing parity. This unit test is not listed in the integration test lists.

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Walkthrough

The Qwen3 Coder streaming parser now emits {} for zero-argument calls and appends } to nonempty streamed arguments. Regression tests cover separate increments and string values containing }.

Changes

Qwen3 Coder streaming completion

Layer / File(s)	Summary
Streaming JSON finalization and regression coverage tensorrt_llm/serve/tool_parser/qwen3_coder_parser.py, tests/unittest/llmapi/apps/test_tool_parsers.py	The parser emits {} when no arguments are streamed. For nonempty arguments, it emits and stores a closing } without counting braces. Tests cover single- and multi-increment zero-argument calls and parameter values containing }.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: brnguyen2

Merge Risk: ⚪ Minimal · up to c3b94

The streaming completion change has coverage for zero-argument and parameterized tool calls, with no actionable current-head risk identified.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes satisfy issue #19227. The streaming completion path emits {} for zero-argument calls. It appends the closing } for non-empty argument fragments without relying on brace counts, so } …
Out of Scope Changes check	✅ Passed	The changes are limited to Qwen3-Coder streaming completion logic and regression tests. The tests directly support issue #19227, including the affected parser behavior. No unrelated API, non-streaming…
Docstring Coverage	✅ Passed	Docstring coverage is 80.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 5 functions across 2 files.
Title check	✅ Passed	The title clearly identifies the issue number, fix type, and primary change: completing zero-argument Qwen3-Coder streaming tool calls.
Description check	✅ Passed	The description includes the issue, root cause, implementation details, non-goals, affected mappings, test coverage, results, and checklist status. It satisfies the required template sections.

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Inline comments:
In `@tensorrt_llm/serve/tool_parser/qwen3_coder_parser.py`:
- Line 160: Update _parse_and_stream_parameters to stop inferring JSON
completion by counting braces; when a tool call closes, append the outer closing
brace unconditionally for nonempty streamed fragments, while preserving {} for
empty arguments. Add a chunked regression test with a string value containing }
and verify concatenated streaming arguments match detect_and_parse.

In `@tests/unittest/llmapi/apps/test_tool_parsers.py`:
- Line 1249: Update test_streaming_zero_arg_tool to split the complete
zero-argument tool call across separate parse_streaming_increment calls, collect
results from both increments, and assert their concatenated parameters equal {}
and match detect_and_parse.

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