Told, never guessed

magus does not guess where it runs to change what it does. It does not read CI, GITHUB_ACTIONS or GITLAB_CI, probe for a container, or work out which agent host is calling, and then pick a default, a result, or which provider or feature is on. When a caller needs different behavior, the caller asks for it: a flag, a charm, a key in magus.yaml, or a variable the workflow sets on purpose. The same command does the same work on a laptop, on a runner and inside an agent's shell.

My words, when this was written down: "We should never be doing environment sniffing, ever. Thinking that we're smarter than the developer, or trying to be too clever, is incredibly dangerous."

The danger is the same one No silent defaults names, arriving from the other side. A guess is a setting nobody wrote. It cannot be read in the file the reader is already looking at, it cannot be turned off without first learning it exists, and it is wrong exactly where the environment differs from the one its author pictured: a self-hosted runner that looks hosted, a laptop with CI=true exported by a test harness, an agent host that happens to share a field name with another. A command that passed locally and failed in CI because magus noticed it was in CI is the worst kind of red, because the difference is invisible in both places.

Two things are not guessing. The first is adapting to the terminal in front of the person: whether a stream is a terminal, what kind and how big, whether it is reached over ssh, and which window a once-only notice belongs to. That decides how magus talks to them (color, links, hover, which prompt it can show), never what work it does. The second is reading the input a caller sent: a flag, an argument, the event a hook was handed. NO_COLOR belongs with them: a person set it to say what they want.

An agent host is a caller like any other, so it says who it is. The hook configuration magus describe harness prints names the host explicitly, and a hook invoked without that name is refused rather than defaulted to one host.

Enforced by

• TestNoEnvironmentSniffing in conventions_test.go fails the build when code reads a CI, hosting or agent-host variable outside a short list of inputs read after the caller chose them, and the failure says to take a flag instead.

Good

The workflow says what it wants, in the file a reader is already looking at:

- run: magus affected ci --plan -o json

Bad

if runningInCI() {
    opts.NoCache = true
}

runningInCI guesses from the environment. Nobody wrote this setting, nobody can see it from the workflow, and a laptop whose test harness exports the variable it reads gets it too.

See also

• No silent defaults
• Provider I/O is Buzz