Three Mac CI failures, three root causes:
1. MLX CI 'Studio prebuilt llama.cpp install + GGUF inference' hit
GitHub API 403 resolving the b9049 release tag because anonymous
API calls share the runner-IP rate-limit bucket. Pass GH_TOKEN /
GITHUB_TOKEN so install_llama_prebuilt.py uses the workflow's
authenticated 5000/hr quota.
2. Mac Studio UI CI's click_nav('New Chat', ...) failed with
'nav not found' because macOS Chromium's accessible-name resolver
doesn't always pick up the tooltip-derived name on the icon-only
collapsed sidebar. Add a fallback locator cascade: ARIA name first,
then has-text on button / a / [data-sidebar=menu-button], and
scroll into view before clicking.
3. Mac Studio GGUF Tool calling hit 'finish_reason=length' on
Qwen3.5-2B IQ3_XXS because Metal output drifts vs Linux CPU and
120 max_tokens isn't enough for the model to produce a tool_call.
Bump to 600 and accept finish_reason=length as long as tool_calls
are present.
4. Mac Studio GGUF JSON/images failed json.loads on empty content
because the IQ3_XXS gemma-4 json_object grammar produced
whitespace-only output. Bump max_tokens 200 -> 600, log the raw
content, treat empty/non-JSON output from the constrained grammar
as a model-quality WARN (not a hard fail), and add a second
unconstrained call that must mention 'paris' to prove the
inference path itself is healthy.
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