[BugFix] Fix Jamba tool parser deleting all occurrences of the streamed prefix on tool transition

[chuenchen309]

Why is this not duplicating an existing PR?

gh pr list --repo vllm-project/vllm --state open --search "jamba" returns
#46617 (Rust frontend, only touches rust/src/**, unrelated) and #48295.
#48295 ("Stream tool calls that arrive whole in a single delta in
llama3_json, jamba and ernie45 parsers") does touch this file and this same
"starting a new tool" branch, but for a different bug — it adds an early-exit
for the case where a whole message arrives in the first delta. Confirmed via
gh pr diff 48295 that the .replace() call this PR fixes is not present
as an added/removed line anywhere in that diff — only as unchanged
surrounding context. No logical conflict; a straightforward rebase either
way if #48295 merges first.

Bug

When a new tool call starts in the streamed array,
extract_tool_calls_streaming() flushes whatever wasn't yet sent from the
previous tool's arguments:

diff = json.dumps(diff, ensure_ascii=False).replace(
    self.streamed_args_for_tool[self.current_tool_id], ""
)

str.replace() with no count removes every occurrence of the
already-streamed prefix, not just the leading one it's meant to strip. If
that prefix's text recurs elsewhere in the completed arguments — e.g. a
nested object that repeats a top-level key/value, a realistic pattern for
find/replace-style tool arguments — the recurrence is silently deleted from
the flushed diff too, corrupting the JSON sent to the client.

Repro (isolated to this one code path, also the added regression test)

Tool 0's arguments are {"find": "cat", "options": {"find": "cat", "replace": "dog"}}; by the time tool 1 starts, streamed_args_for_tool[0]
is already '{"find": "cat"' (a normal partial-streaming state). Before
this fix, the flushed diff is ', "options": , "replace": "dog"}}' — the
nested "find": "cat" is gone entirely, and the reconstructed JSON fails to
parse (json.decoder.JSONDecodeError: Expecting value). This isn't a
narrow edge case; any tool whose argument schema can legitimately repeat a
key/value pair in a nested structure hits it.

Fix

Pass count=1 to .replace() so only the leading occurrence — the one
that's actually the already-streamed prefix — is removed.

Testing

• Independently reproduced against the real installed vllm package by
  directly driving JambaToolParser.extract_tool_calls_streaming() with
  hand-constructed internal state, isolating this one code path from the
  rest of the streaming pipeline. Confirmed red
  (json.decoder.JSONDecodeError: Expecting value) before the fix, green
  (valid, correctly-nested JSON) after.
• Added test_flush_on_new_tool_replaces_only_leading_occurrence to
  tests/tool_parsers/test_jamba_tool_parser.py. Confirmed red→green:
  reverting only the source change reproduces the exact JSONDecodeError;
  reapplying passes. Existing 8 tests in this file still pass (9 total now).
• pre-commit run ruff-check, ruff-format, mypy-3.12 (--files scoped
  to the 2 changed files): pass.

Model-eval N/A — this only changes streaming-argument diff bookkeeping
logic, not model output/accuracy.

AI-Generated disclosure

Found via an AI-assisted code review pass (Claude Code) over
vllm/tool_parsers/. I independently reproduced the corruption, traced the
root cause, verified the fix, checked it against the overlapping open PR
#48295 line-by-line, and ran the tests above myself before submitting.

[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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