# C14 — Headless core, two faces

**Guarantee.** The engine is a UI-agnostic library. Two faces sit on it: a
**programmatic/CLI** face (the primary surface for the LLM and automation —
author a node, run a sim/sweep, emit structured metrics) and a **visual** face
for human exploration. Visualization is only a downstream consumer node
([C8](c08-node-contract.md)) on the streams.

**Forbids.** Any UI/pixel knowledge inside the engine.

**Why.** The LLM drives programmatically, the human visually; a headless core
serves both. The visual face is the **playground** ([C22](c22-playground-traces.md)) —
a **web frontend served from disk-persisted traces** (#101, ratified 2026-06):
the engine writes recorded traces to disk and a browser charts them. It is
staged after the runnable substrate but is core to aura's identity, not
optional. Reading a serialized trace file is a stricter form of "no UI knowledge
in the engine" than an in-process native UI reaching zero-copy into the live SoA
columns — the pivot to the disk-trace frontend keeps this contract's core intact
and arguably tightens it.

## Current state

**The CLI conventions.** The programmatic/CLI face is a `clap` derive parser
(`aura-cli`, `crates/aura-cli/src/main.rs`) that meets GNU / clig.dev
conventions: one declarative source yields scoped `aura <sub> --help`,
`--version`/`-V`, per-flag Options sections, and GNU `--flag=value` / `--` /
long-option abbreviation. `clap` is admitted under the
[C16](c16-engine-project-split.md) per-case dependency policy — a
research-side, dev-loop/compile tax confined to `aura-cli`, a leaf binary the
frozen deploy artifact ([invariant 8](c13-hot-reload-frozen-deploy.md)) cannot
pick up. Usage lines follow the clap house style (`Usage: aura <verb> …`);
refusal diagnostics stay unprefixed, since a diagnostic is not a usage line.

**The stderr class markers (#278/#313).** Diagnostics on a *continuing* run
carry a stable class marker the exit code cannot supply: `aura: note: <text>`
for a benign, expected diagnostic — a gate-emptied cell, a skipped tap, an
all-zero-trade walk-forward — where the run stays valid and the exit code is
unaffected (a null result is a valid research result, #198); and
`aura: warning: <text>` for a recorded fault or suspect condition the run
survives — a failed cell feeding exit 3, a failed scaffold git step, a stale
hot-reload dylib. Error lines that accompany a non-zero exit — and plain
info lines such as the run-record summary — keep the bare `aura: ` prefix;
for errors, the exit-code partition below is the machine contract.
The markers are grep-stable: `grep '^aura: warning: '` isolates faults,
`grep '^aura: note: '` benign notices. The grammar's single source is
`aura-cli/src/diag.rs`; the few runner-side literals repeat it verbatim.

**The exit-code partition** is a durable part of the automation contract: a
caller branches on the failure class without parsing stderr.

- **0 — success.**
- **2 — usage error:** a command-line fault (clap parse errors + aura's
  post-parse argument-structure validations, *including* the content of an
  argv-named blueprint file, which is itself an argument).
- **1 — runtime failure:** a well-formed command whose needed environment /
  recorded state is missing, or bad piped stdin data.
- **3 — campaign completed with failed cells:** a well-formed campaign that ran
  to completion but with at least one failed cell (#272; `exit_on_campaign_result`
  in `crates/aura-cli/src/main.rs`, threaded from the run registry —
  [C18](c18-registry.md)).

**Dual grammar.** The four dual-grammar subcommands (run/sweep/walkforward/mc)
keep both grammars under one token via an optional `[blueprint]` positional plus
a post-parse `is_file()` dispatch; the execution layer is unchanged (arg-plumbing
via thin `*_from` adapters). The machine-first help surface (JSON/manifest help,
stdin op-scripts) is deferred to the #157 / [C21](c21-world.md) track, distinct
from this human/GNU-convention compliance.

**Two artifact classes, two redundancy budgets** (#249, ratified 2026-07-13).
The data layer the programmatic face emits is a public interface read raw (by
humans and LLMs), and its records split into two classes with opposite
redundancy budgets. **Generated outcome records** (manifests, metrics, family
reports) have a single writer at run time — redundancy there cannot drift and is
deliberately spent on direct readability. **Authored intent artifacts**
(blueprints, op-scripts, campaign documents) are author-maintained — every
redundancy is a drift site and stays out. Consequence in the run manifest
(`RunManifest`, `crates/aura-engine/src/report.rs`): the untouched bound
`defaults` are stamped one-directionally beside `params` (which keeps its "what
varied" reproduce semantics, disjoint by construction from `defaults`, "what was
held"), so a raw reader of a fully-bound run no longer sees a misleading
`"params": []`; the blueprint itself carries no such duplication.

**The programmatic face is an executor** (#295, ratified 2026-07-20). The
member-run recipe — the definition of what a standard aura backtest is — is
library content (`aura-runner` / `aura-backtest` / `aura-measurement`,
[C28](c28-stratification.md) assembly position), not CLI content: the binary is
argv → document → executor → presentation, and a downstream World program
reaches the same recipe with no binary involved. See
[C25](c25-role-model.md)'s control-surface amendment for the projection rule.

## See also

- [C8](c08-node-contract.md) — visualization is a downstream sink/consumer node
- [C16](c16-engine-project-split.md) — the per-case dependency policy under
  which `clap` is admitted
- [C18](c18-registry.md) — the run registry that yields the exit-3
  completed-with-failures class
- [C21](c21-world.md) — the #157 machine-first help track (deferred)
- [C22](c22-playground-traces.md) — the visual face / playground
- [C25](c25-role-model.md) — the control-surface amendment (executor projection)
- [C28](c28-stratification.md) — the assembly position of the member-run recipe

> History: [c14-headless-two-faces.history.md](c14-headless-two-faces.history.md)