brew upgrade --fetch-only versus --quiet for low-noise update alerts

Short answer: neither flag alone does what you want. brew outdated is the right alerting primitive, and --fetch-only is a safe staging step — but neither one can promise an upgrade is free of dependency problems. --quiet is the wrong tool for monitoring because it performs the actual upgrade rather than just reporting on it.

What is confirmed vs. what is likely

Confirmed behavior (well-established in Homebrew):

• brew outdated compares installed versions against available versions and makes no system changes. It is the canonical "is anything upgradable?" check.
• brew upgrade --fetch-only downloads the new packages into Homebrew's cache without installing or linking them.
• brew upgrade --quiet still runs the complete upgrade; it only suppresses progress output on stdout. Errors still go to stderr and failures still produce a non-zero exit code.

Likely, but not guaranteed: a conflict that only appears at install or link time (for example, a keg that cannot be linked because of leftover files, or a broken dependency in a third-party tap) will not be surfaced by --fetch-only. Fetching validates that the download works, not that the install will succeed.

Your three questions, directly

• Does --fetch-only + brew outdated surface dependency conflicts? No, not reliably. brew outdated is a version comparison only, and --fetch-only stops before the resolution, build, and link phases where most conflicts appear. Treat a successful fetch as "the download is staged," not "the upgrade is safe."
• Can --quiet be trusted to expose important warnings? Only partially. Critical failures still fail loudly via exit code and stderr. But non-fatal caveats and warnings that would explain why an upgrade is brittle can be suppressed or easy to miss, and by the time you see them the system has already been modified. That is the opposite of an alert-first workflow.
• Which yields fewer false positives? brew outdated. It fires only when a newer version actually exists, so every alert is real. The trade-off is that "upgradable" is not the same as "will upgrade cleanly" — and no read-only Homebrew command fully closes that gap.

The honest limitation

There is no Homebrew flag that dry-runs a full upgrade including dependency resolution and linking. If "actionable without hidden dependency problems" is a hard requirement, the practical ceiling is: alert on brew outdated, optionally pre-stage with --fetch-only, and accept that the final upgrade is the first moment conflicts can truly surface. Capturing stderr from the real upgrade (quiet or not) is how you catch those.

Assumption: standard Homebrew on macOS or a supported Linux setup, recent Homebrew 4.x. Behavior with third-party taps carrying custom install logic can differ, and I have not verified flag behavior against a specific pinned Homebrew release.