# AGENTS.md

## Aim

nxdns: a self-hosted DNS sinkhole for a household LAN, written in Zig 0.16.0.
Portfolio-grade public repo. PLAN.md is the source of truth for scope and design;
specs/ holds per-milestone contracts; specs/research/ holds verified stdlib facts.

## Values

We intentionally architect this code to be robust, maintainable, pragmatic —
good craftsmanship and good engineering. We explicitly avoid tech debt, code
smells, bad architecture decisions, and brittle implementations.

What that means in practice:

- This is a greenfield project. Breaking changes are allowed. Never keep a bad
  interface for compatibility; fix it at the root.
- No versioning of scope. A feature is in scope (build it completely) or out of
  scope (do not build it). No "v2 later", no stubs left behind.
- Fix root causes, not symptoms. Do not iterate on workarounds.
- Scope is small on purpose: household scale, two targets, few dependencies.
  Do not add generality nobody asked for.
- Dependencies are liabilities: stdlib first; vendored + pinned C deps
  (sqlite3, mbedTLS) only where the stdlib has nothing.
- Verify stdlib claims against ../zig at tag 0.16.0 — pre-0.16 knowledge is
  stale (std.Io migration). See specs/research/zig-0.16-api-notes.md.
- Pure core: dns/, filter/, local/, cache/ take bytes and return bytes — no Io,
  no sockets, no clocks hidden inside.
- Every failure mode must be visible: no silent drops, no unbounded logs, no
  swallowed errors. Counters + health surfaces over log spam.
- Tests are runnable acceptance criteria, not decoration. Required CI stays
  deterministic — no network-dependent tests in blocking jobs.
- Comments state constraints the code cannot show. No narration, no
  commented-out code.
- Git: GPG-signed commits (`git commit -S`), simple lowercase messages, no
  generated-by footers.

## Reading `zig build test` output

A fully passing `zig build test` still prints a line like `failed command:
.../test --cache-dir=... --seed=... --listen=-`, and still exits 0. That line
is a known upstream zig 0.16.0 labelling defect. It does not mean a test
failed, and no test binary crashed.

The build runner sets a step's `result_failed_command` on every spawn
(`std/Build/Step/Run.zig:1540`) and never clears it on success. It then prints
a step's diagnostics whenever the step wrote anything to stderr, explicitly "no
matter the result" (`compiler/build_runner.zig:1381`), and that printer emits
the `failed command: ` label unconditionally when the field is set
(`compiler/build_runner.zig:1515`). Our suite writes to stderr on every run,
because the tests that cover the warning paths log through the real sink. A
minimal reproducer with no mbedTLS and no C — one passing test whose body is a
`std.debug.print` — prints the same label and reports "3/3 steps succeeded;
1/1 tests passed"; deleting the print removes the label. No upstream issue
matched a search, so the reference is the 0.16.0 source lines above.

Any *other* failure text is real. Trust the summary line: `zig build test`
exiting non-zero, a `N failed` count, or a panic backtrace all mean a genuine
failure. Do not filter, wrap, or suppress the runner's output to hide the
label — that would hide real failures with it.

One trap: running a cached test binary by hand with `--listen=-` aborts with
`internal test runner failure: EndOfStream`. That is not a teardown bug; the
IPC runner is talking to a closed stdin because no build runner is on the other
end. Run the binary with no arguments to get the plain stdio report.