Small local models drift off the tool-call format, especially with a large
tool schema in play. Two failures followed from that, both of which ended
the turn looking like the model had simply stopped mid-sentence.
Salvage handled well-formed Qwen/Hermes XML but gave up entirely on a call
cut off partway through. It now takes the function name plus every
parameter that actually closed. A trailing unterminated `<parameter=…>` is
dropped rather than guessed, so a half-written path or file body can never
reach a tool; if that leaves a required argument missing, the call fails
validation and the model gets a corrective tool error, which is the agent
loop working.
Worse, a call that never parsed at all was reported as `status: completed`
— indistinguishable from the model deciding it was done, leaving the user
with narration trailing off into stray closing tags. It now ends on the
error path, so the GUI offers Retry. That is the right affordance here: the
drift is probabilistic, not deterministic, so the same model usually
succeeds on a second attempt.
Detection ignores fenced and inline code, so a model *explaining* tool-call
syntax is still a real answer, and requires that tools were offered at all.
Reported against qwen3.5-9b on LM Studio, 2026-07-26.
Co-Authored-By: Claude <noreply@anthropic.com>
