nix-compact

Full build logs on disk. Small, useful output for humans and coding agents.

nix-compact source-patches Nix's native CLI to suppress routine nix build output, report progress every 15 seconds, and surface bounded diagnostic excerpts. It prints the full-log location and supplies a reader for targeted investigation. It is not a shell alias or a subprocess wrapper.

[nix] Output suppressed; progress every 15s. Full log: /home/user/.local/state/nix/build-logs/session.jsonl
[nix] L# = log line; D# = diagnostic. Read: nix build-log '/home/user/.local/state/nix/build-logs/session.jsonl' --diagnostics
[nix] 15s: 3 built, 1 running, 8 copied; phase: checkPhase
[nix] D1 — example-app — level-json/error
L842 [example-app] [rendered] Connection refused
[nix] SUCCESS in 19s | exit 0 | 1 diagnostic candidates; 0 omitted; 0 repeats
[nix] Log: /home/user/.local/state/nix/build-logs/session.jsonl

An application saying error does not fail the build. Only Nix's actual execution result determines its exit status. Detection affects presentation only.

Try it

nix run github:ghuntley/nix-compact -- build .#example-app

Or add the executable to a temporary shell:

nix shell github:ghuntley/nix-compact
nix build .#example-app

Inspect the archive advertised by that invocation:

nix build-log /absolute/path/session.jsonl --diagnostics --limit 3
nix build-log /absolute/path/session.jsonl --diagnostic D1 --context 40
nix build-log /absolute/path/session.jsonl --lines 800:900

L# identifies a received payload line; D# identifies a diagnostic. Read results default to 100 lines / 8 KiB and include continuation commands when shortened.

Consume as a flake

inputs.nix-compact.url = "github:ghuntley/nix-compact";

Commit your flake.lock to pin the source revision. The standalone flake pins its own package inputs; following a consumer's unrelated Nixpkgs is unnecessary.

NixOS

imports = [ inputs.nix-compact.nixosModules.default ];
programs.nix-compact.enable = true;

The module defaults nix.package to the project's tested package. Its package input is independent of the consuming host's Nixpkgs version. Enable it explicitly and rebuild the system normally. An explicit nix.package override takes priority.

devenv

In devenv.yaml:

inputs:
  nix-compact:
    url: github:ghuntley/nix-compact

In devenv.nix:

{ inputs, ... }: {
  imports = [ inputs.nix-compact.devenvModules.default ];
}

Commit devenv.lock to pin the dependency. Alternatively add inputs.nix-compact.packages.x86_64-linux.default to your shell's packages.

Public outputs

Supported: Nix 2.34.8, x86_64-linux. Unsupported versions/platforms fail explicitly. The native suite is exercised against both stable NixOS and development package inputs. Other platforms are not advertised as supported.

Bounds and compatibility

• At most one 160-byte progress line every 15 seconds.
• Up to 20 preceding and 20 following lines per error; 4 KiB per excerpt.
• At most 12 KiB automatic diagnostic details per invocation. Low-confidence word matches get at most 4 KiB of that total, reserving capacity for clearer errors.
• A separate 2 KiB final-result allowance.
• Structured Elixir LoggerJSON, Rust tracing and Cargo diagnostics are recognized before heuristic text matching.
• All received diagnostic/builder payloads are saved before console filtering.
• Private archives, exact line references, repeated-diagnostic counting, bounded context, configurable completed-log retention and visible capture-failure fallback.
• --json and --print-out-paths preserve machine-readable stdout.

Use nix build -L …, an explicit --log-format, or NIX_COMPACT_LOG=0 to select upstream output and bypass compact capture. The patch does not alter daemon/store protocols or build scheduling. Capture starts after command parsing and cannot recover messages disabled by their producer or historical substituted-build logs.

See MANUAL.md for storage, retention, archive format, limits and replay.

Develop and test

This repository was initialized with devenv. Both entry points share the pinned toolchain definition:

devenv shell
# or
nix develop

nix flake check --no-build --option allow-import-from-derivation false
nix build --no-link .#default .#checks.x86_64-linux.core \
  .#checks.x86_64-linux.native .#checks.x86_64-linux.native-development \
  .#checks.x86_64-linux.configuration

Hegel's native C ABI tests the actual C++: 1,000 generated core cases and 50 generated real-build cases per native variant, plus deterministic/runtime fixtures. Coverage includes arbitrary bytes, structured severity, interleaving, output budgets, pagination, retention, Unix daemon and remote protocol capture, TTY, quiet builds, interruptions, stdout compatibility and unchanged exit status. The complete package also retains upstream Nix test gates.

A synthetic 10,000-line build in the original implementation retained every builder line while reducing stderr from 359,114 to 609 bytes (over 99.8%). This is a console-byte measurement for that fixture, not a universal tokenizer guarantee.

License

LGPL-2.1-or-later. See LICENSE and NOTICE.md for licensing, upstream attribution and extraction provenance. Architecture and maintenance boundaries are documented in DESIGN.md.