[Fix] Gemma4 tool parser: support multiple tool calls in single delta and refactor streaming extraction

[Alex-ai-future]

Summary

Fixes #42696 — Gemma4 tool parser fails in streaming mode when multiple tool calls arrive in a single delta.

Root Cause

The old streaming parser used a tag-counting state machine that assumed only one active tool call at a time. When MTP/speculative decoding produced multiple <tool_call|> delimiters in one delta, it would:

1. Lose subsequent tool calls — only the first one was emitted
2. Emit incomplete DeltaToolCall — follow-up chunks lacked id, type, function.name, causing @ai-sdk (OpenCode) validation failures

Solution

Replace the tag-counting state machine with Hermes-style accumulated-text scan-and-diff:

1. regex.findall() enumerates all complete tool calls in current_text
2. Per-index state tracking (prev_tool_call_arr[i]) instead of global current_tool_id
3. Each DeltaToolCall emission includes id, type, and function.name on first chunk
4. Content between tool calls is properly extracted and emitted

Code changes

File	+	-
vllm/tool_parsers/gemma4_tool_parser.py	~130	~200
tests/tool_parsers/test_gemma4_tool_parser.py	~110	~5

Key refactorings:

• Split _extract_streaming into 7 focused functions
• Removed ~150 lines of dead code (_handle_tool_call_middle, _handle_tool_call_end, etc.)
• Removed unused buffered_delta_text and _buffer_delta_text()

Tests

.venv/bin/python -m pytest tests/tool_parsers/test_gemma4_tool_parser.py -v
# 57 passed

New test cases:

• test_streaming_multiple_tool_calls_in_single_delta — 2 calls in one delta
• test_streaming_four_tool_calls_with_interleaved_text_in_single_delta — 4 calls + text
• test_streaming_text_between_tool_calls_in_single_delta — content extraction between calls
• test_streaming_multiple_tool_calls_sequential — cross-chunk multi-call
• test_streaming_filename_suffix_preserved_across_chunks — file extension split
• test_streaming_string_prefix_preserved_across_chunks — string prefix split

Why this is not duplicating an existing PR

• [Bugfix] Rewrite Gemma4 streaming tool parser #42237 (whym:codex/gemma4-hermes-style-parser) has the same high-level approach (Hermes-style scan-and-diff). Our PR differs in:
  • Better code structure: 7 focused functions vs fewer large functions
  • More comprehensive tests: 4-call + interleaved text + content extraction (57 vs ~55)
  • No external dependency: Doesn't introduce partial_tag_overlap utility
  • Both PRs address the same root cause; maintainers can choose either approach

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[gemini-code-assist]

Code Review

This pull request refactors the Gemma4 streaming tool parser to support multiple tool calls within a single delta by adopting a full-match enumeration approach, accompanied by new regression tests. Review feedback identifies several critical issues: the prev_end_count variable is now unused, the use of a global current_tool_name_sent flag fails to correctly track state for individual tool calls in a multi-call sequence, and the content extraction logic is prone to data loss for text appearing before or between tool call tags.

[gemini-code-assist]

The variable prev_end_count is calculated but never used in the new implementation of _extract_streaming. It should be removed to avoid confusion and unnecessary computation.

[gemini-code-assist]

The use of the global self.current_tool_name_sent flag is incorrect for multi-tool parsing. If a previous tool call in the same request has already finished, this flag will be True. When a subsequent tool call starts, line 622 may evaluate to True even if the current tool's name hasn't been sent yet (e.g., if func_name is still None at line 605). This could lead to streaming arguments for the wrong tool or before the tool identity is established. You should use the per-tool state stored in self.prev_tool_call_arr[partial_idx] to track if the name has been sent for the specific tool call being processed.

                if "name" in self.prev_tool_call_arr[partial_idx] and args_part:

[gemini-code-assist]

The content extraction logic in Step 3 is incomplete and will lead to data loss in several scenarios:

1. Preamble Loss: Any text appearing before the first tool call in the same delta (e.g., "Preamble <|tool_call>...") is ignored because curr_start_count > 0 and curr_start_count != curr_end_count.
2. Inter-call Content Loss: Text between multiple tool calls (e.g., "<tool_call|> Inter <|tool_call>") is lost because the parser assumes content only exists when no tool calls are active.
3. Delayed Emission: Content after a tool call is only emitted once that specific tool call is fully completed, which may delay the display of text to the user.

You should refactor this to identify all text segments outside of <|tool_call>...<tool_call|> blocks and emit the difference relative to what was previously sent. A more robust approach would be to calculate the total content in current_text (by removing all tool call blocks) and diffing it against the total content in previous_text.