* Studio: strip orphan tool_call XML from streamed visible content
The speculative-buffer state machine in
`studio/backend/core/inference/llama_cpp.py` can slice a tool_call XML
block between the silent DRAINING path and the user-visible
content_accum, depending on when in the model's emission the BUFFERING
-> STREAMING -> DRAINING transitions fire. Three leak shapes were
observed in a 2026-05-22 sweep of 900 Qwen3.5 / Qwen3.6 GGUF runs:
Pre-fix XML leak rate: 20/900 (2.22%), concentrated 6.7% on the
larger Q8 / MTP configs:
Qwen3.6-35B-A3B Q8_0 4/60 (6.7%)
Qwen3.6-35B-A3B-MTP Q4 4/60 (6.7%)
Qwen3.5-35B-A3B Q8_0 3/60 (5.0%)
Qwen3.6-27B Q8_0 3/60 (5.0%)
The existing `_TOOL_XML_RE` only matched well-formed
`<tool_call>...</tool_call>` and `<function=...></function>` pairs, so
unterminated openings (close was DRAINED) and orphan closes (opening
was DRAINED) survived the strip and reached the user.
Fix relaxes the regex to also strip:
1. Orphan opening up to end-of-string: `(?:</tool_call>|\Z)`
2. Orphan closing tag: bare `</tool_call>` / `</function>`
Verified on the full sweep: 20/900 -> 0/900 (100% of detected leaks
eliminated). 16 unit tests in `test_tool_xml_strip.py` pin all three
leak shapes plus the well-formed cases, plus parametrised checks on
the 5 actual real-world leak samples from the sweep data.
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* Studio: strip tail-only </parameter> orphan + tighten regex
The 2026-05-22 gdpval sweep surfaced a 4th XML-leak shape not caught
by the earlier regex: a bare `</parameter>\n\n` at end-of-buffer (7
of 192 trials, all Qwen3.5-27B + a few Qwen3.6-27B). The model emits
the full `<tool_call><function=...><parameter=...>...content...
</parameter></function></tool_call>` envelope, the speculative buffer
DRAINS the opening tags as intended, but EOS (max_tokens cutoff)
truncates the outer `</function></tool_call>` close, leaving just
`</parameter>` as the visible tail.
We strip this ONLY when end-anchored (`\s*\Z`) so legitimate
mid-text uses (user code samples, documentation discussing the
Qwen tool-call XML shape) survive. Verified on the 192-trial
gdpval corpus: before=7, after=0.
While at it, fold the five top-level alternations into three by
sharing tag-name and prefix subgroups:
<tool_call>... + <function=\w+>... + --> <(?:tool_call|function=\w+)>...
</tool_call> | </function> --> </(?:tool_call|function)>
Semantically identical (verified by replay over the 192-trial
corpus + adversarial inputs, 0 diffs) and 1.34x faster on real
workloads. Backtracking-safety pinned by two new perf guards
(256KB '<' spam, 1000x orphan opens).
Tests: 16 -> 28 (6 new functional + 4 well-formed-vs-orphan +
2 perf guards).
* Tighten comments in XML-strip regex and tests
Code says what it does; comments were repeating it. Strip the verbose
explanations down to the WHY-only bits (engine quirk, tail-anchor
rationale, real-world source of each test sample). No code changes.
inference.py: 21 -> 12 lines around _TOOL_XML_RE
test_tool_xml_strip.py: 343 -> 259 lines (-84)
Tests: 28/28 still pass.
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