[Bugfix] Fix Gemma4ToolParser streaming float corruption

[abinggo]

Summary

Fixes #42047 — Gemma4ToolParser produces incorrect float values during streaming (e.g., 108.2 → 108.02).

Root cause: When a bare numeric value ending with . (e.g., "108.") is terminated by a comma in partial/streaming mode, float("108.") evaluates to 108.0. The JSON serialization "108.0" is emitted to the client. When the true decimal digit arrives and the value becomes 108.2, the streaming diff logic computes a common prefix of "108." but the client already received "108.0" — the final diff appends "2", producing "108.02".

Fix: In _parse_gemma4_args() and _parse_gemma4_array(), when partial=True (streaming mode), withhold bare values that end with "." since additional decimal digits may still arrive. This is consistent with the existing end-of-string withholding logic. The final non-partial parse at tool-call-end handles these values correctly.

Test plan

• Added regression tests in tests/tool_parsers/test_gemma4_tool_parser.py:
  • test_trailing_dot_float_partial_withheld — verifies "108." is withheld in partial mode
  • test_trailing_dot_float_partial_withheld (array) — same for array elements
• Verified existing tests still pass (non-partial parsing, other value types)
• Verified non-streaming path is unaffected (partial=False still parses "108." → 108.0)

python -m pytest tests/tool_parsers/test_gemma4_tool_parser.py -v

[claude]

Claude Code Review

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[gemini-code-assist]

Code Review

This pull request addresses a streaming diff corruption issue in the Gemma4 tool parser by withholding float values that end with a trailing dot during partial parsing. This prevents premature parsing of incomplete floats (e.g., "108." becoming "108.0") before subsequent digits arrive. Feedback was provided to optimize performance by moving string operations inside the conditional block for partial parsing and to consider the impact on streaming latency for values that are actually complete.

[gemini-code-assist]

This check withholds bare values ending in . even when they are followed by a separator (e.g., a comma), which indicates the value is complete in the Gemma4 format. Since line 203 already withholds values at the end of the string (and thus potentially incomplete), this logic specifically targets completed values. While this prevents the 108.0 vs 108.2 prefix mismatch described in the bug report, it also means that any float ending in .0 (represented as 108. in Gemma4) will be withheld from streaming until the entire tool call is finished, which degrades the streaming experience. Additionally, raw_val should only be computed when partial is True to avoid unnecessary string operations in non-streaming paths.

Suggested change

	raw_val = args_str[val_start:i].strip()
	if partial and raw_val.endswith("."):
	# Trailing dot means decimal digits may still arrive
	# (e.g. "108." may become "108.2"). Parsing now would
	# yield float("108.") == 108.0, whose json repr "108.0"
	# corrupts the streaming diff when the true digit lands.
	break
	if partial:
	raw_val = args_str[val_start:i].strip()
	if raw_val.endswith("."):
	# Trailing dot means decimal digits may still arrive
	# (e.g. "108." may become "108.2"). Parsing now would
	# yield float("108.") == 108.0, whose json repr "108.0"
	# corrupts the streaming diff when the true digit lands.
	break

[abinggo]

Good catch — moved raw_val/strip() inside the if partial: guard so non-streaming paths pay zero cost (done in the latest commit).

Regarding the semantic point: the trailing-dot check only fires when i < n (separator found), meaning the value is terminated. In theory that makes the withhold unnecessary since no more digits will arrive for that value. In practice Gemma4 never emits a bare trailing-dot float as a complete value (it would emit 108.0 or 108), so this is a defensive no-op. The primary incomplete-value protection is the i >= n break on line 203 which catches values still at the buffer boundary.

Happy to remove the trailing-dot guard entirely if you think the defensive check isn't worth the code, but it shouldn't cause stalls in real Gemma4 output.

[gemini-code-assist]

Similar to the logic in _parse_gemma4_args, this withholds array elements ending in . even if they are followed by a separator, which will stall the streaming of the rest of the array. Also, the strip() call should be moved inside the if partial: block to avoid unnecessary overhead in non-streaming mode.

Suggested change

	raw_val = arr_str[val_start:i].strip()
	if partial and raw_val.endswith("."):
	break
	if partial:
	raw_val = arr_str[val_start:i].strip()
	if raw_val.endswith("."):
	break

[abinggo]

Same reasoning as above — strip() moved inside if partial:. The trailing-dot guard here is symmetric with the dict case and equally defensive. Gemma4 won't produce a bare 108. followed by a comma/bracket as a complete array element.

[abinggo]

Closing in favor of #42300, which generalizes this narrow float fix into a full unstable-bare-value check (trailing dot, exponent, partial bool/null) plus a broader streaming parser refactor with much more comprehensive test coverage.

Review and test-coverage suggestions posted at #42300 (comment).

[abinggo]

Reopening in response to @bbrowning's preference for small incremental fixes over the broader refactor in #42300 (see #42300 (comment)).

This is the narrow ~20-line float corruption fix — isolated to the partial-value withholding check in _parse_gemma4_args / _parse_gemma4_array, zero refactor. If reviewers prefer the broader rewrite path, happy to close again — just keeping the incremental option on the table.

[bbrowning]

The fix looks reasonable to me, existing and new tests pass, and the change is small enough that the risk of regression is low. Worst-case we withhold streaming a bit longer when we get bare values with trailing periods but that should be a negligible difference to clients compared to returning improper float values.