config-audit/scanners/output-style-scanner.mjs

/**
 * OST Scanner — Output-style validation (v5.6 C)
 *
 * Output styles are live (the standalone `/output-style` command was removed in
 * v2.1.91; styles are now managed via `/config`). They are the most surprising
 * steering surface because they rewrite the system prompt:
 *
 *   CA-OST-001  A custom (user/project) output style that does NOT set
 *               `keep-coding-instructions: true` → when active, Claude Code
 *               REMOVES its built-in software-engineering instructions (how to
 *               scope changes, write comments, verify work) and keeps only the
 *               style's text. `keep-coding-instructions` defaults to false, so
 *               this is the headline footgun. Severity medium.
 *
 *   CA-OST-002  A PLUGIN output style with `force-for-plugin: true` → Claude
 *               Code auto-applies it whenever the plugin is enabled, OVERRIDING
 *               the user's selected `outputStyle`. If several enabled plugins set
 *               it, the first loaded wins. Severity low (awareness). Note:
 *               `force-for-plugin` is plugin-styles-only per the docs, so this
 *               keys on `source === 'plugin'` — a user/project style cannot
 *               trigger the override (it would simply be ignored).
 *
 *   CA-OST-003  A settings `outputStyle` value that matches no built-in and no
 *               discovered custom style → dead config: Claude Code falls back to
 *               the default style, so the configured behavior is silently not
 *               applied. Severity medium.
 *
 * Every claim traces to a CONFIRMED row of docs/v5.5-steering-model-plan.md
 * (V9/V10/V11/V12), verified against code.claude.com/docs/en/output-styles and
 * .../plugins-reference. The scanner is fixture-gated: with no output styles and
 * no `outputStyle` setting it emits nothing (keeps the SC-5 snapshot byte-stable).
 *
 * Zero external dependencies.
 */

import { readFile } from 'node:fs/promises';
import { finding, scannerResult } from './lib/output.mjs';
import { SEVERITY } from './lib/severity.mjs';
import { readActiveConfig } from './lib/active-config-reader.mjs';
import { parseFrontmatter, parseJson } from './lib/yaml-parser.mjs';

const SCANNER = 'OST';

// Built-in output styles, verified against code.claude.com/docs/en/output-styles.
// Compared case-insensitively so OST-003 never false-flags a valid built-in.
const BUILTIN_STYLES = ['default', 'explanatory', 'learning', 'proactive'];

/**
 * Read + parse the frontmatter of each enumerated output style once.
 * @param {Array<object>} styles - readActiveConfig().outputStyles entries
 */
async function withFrontmatter(styles) {
  const out = [];
  for (const s of styles) {
    let frontmatter = null;
    try {
      frontmatter = parseFrontmatter(await readFile(s.path, 'utf-8')).frontmatter;
    } catch { /* unreadable → treat as no frontmatter */ }
    out.push({ ...s, frontmatter });
  }
  return out;
}

/**
 * Resolve the effective `outputStyle` setting from the cascade (user → project →
 * local; later scope wins). Returns null when unset everywhere.
 * @param {object} activeConfig
 * @returns {Promise<{value:string, scope:string, path:string} | null>}
 */
async function resolveOutputStyleSetting(activeConfig) {
  const cascade = (activeConfig.settings && activeConfig.settings.cascade) || [];
  let resolved = null;
  for (const entry of cascade) {
    if (!entry.exists || !entry.path) continue;
    let json = null;
    try { json = parseJson(await readFile(entry.path, 'utf-8')); } catch { continue; }
    if (json && typeof json.outputStyle === 'string' && json.outputStyle.trim()) {
      resolved = { value: json.outputStyle.trim(), scope: entry.scope, path: entry.path };
    }
  }
  return resolved;
}

/**
 * Main scanner entry point.
 * @param {string} targetPath - repo root to scan
 * @param {object} _discovery - unused (OST reads the active config cascade itself)
 */
export async function scan(targetPath, _discovery) {
  const start = Date.now();
  const findings = [];

  const activeConfig = await readActiveConfig(targetPath);
  const styles = await withFrontmatter(activeConfig.outputStyles || []);

  // CA-OST-001 — user/project custom style missing keep-coding-instructions:true.
  for (const s of styles) {
    if (s.source !== 'project' && s.source !== 'user') continue;
    const kci = s.frontmatter ? s.frontmatter.keep_coding_instructions : undefined;
    if (kci === true) continue;
    findings.push(finding({
      scanner: SCANNER,
      code: 'strips-coding-instructions',
      severity: SEVERITY.medium,
      title: 'Custom output style removes built-in coding instructions',
      description:
        `The ${s.source} output style "${s.name}" does not set ` +
        '`keep-coding-instructions: true`. While this style is active, Claude Code ' +
        'drops its built-in software-engineering instructions — how to scope changes, ' +
        'write comments, and verify work — and keeps only this style\'s text. The ' +
        'frontmatter flag defaults to false, so the strip is easy to miss.',
      file: s.path,
      evidence:
        `output_style="${s.name}"; source=${s.source}; ` +
        `keep-coding-instructions=${kci === undefined ? 'unset (default false)' : String(kci)}`,
      recommendation:
        'To keep Claude Code\'s software-engineering behavior while applying this style, ' +
        'add `keep-coding-instructions: true` to the frontmatter. If the strip is ' +
        'intentional (a non-coding persona), no change is needed.',
      category: 'output-styles',
    }));
  }

  // CA-OST-002 — plugin output style with force-for-plugin:true (overrides user choice).
  for (const s of styles) {
    if (s.source !== 'plugin') continue;
    const ffp = s.frontmatter ? s.frontmatter.force_for_plugin : undefined;
    if (ffp !== true) continue;
    findings.push(finding({
      scanner: SCANNER,
      code: 'plugin-forces-style',
      severity: SEVERITY.low,
      title: 'Plugin output style overrides your selected output style',
      description:
        `The plugin "${s.pluginName}" ships an output style "${s.name}" with ` +
        '`force-for-plugin: true`, so Claude Code applies it automatically whenever the ' +
        'plugin is enabled — overriding whatever `outputStyle` you selected. When more ' +
        'than one enabled plugin does this, the first one loaded wins.',
      file: s.path,
      evidence: `output_style="${s.name}"; source=plugin:${s.pluginName}; force-for-plugin=true`,
      recommendation:
        'If you did not expect this style, disable the plugin or remove ' +
        '`force-for-plugin: true` from its output style. This is awareness only — the ' +
        'plugin is behaving as designed.',
      category: 'output-styles',
    }));
  }

  // CA-OST-003 — settings outputStyle resolving to a non-existent style (dead config).
  const resolved = await resolveOutputStyleSetting(activeConfig);
  if (resolved) {
    const known = new Set([
      ...BUILTIN_STYLES,
      ...styles.map(s => String(s.name).toLowerCase()),
    ]);
    if (!known.has(resolved.value.toLowerCase())) {
      const customNames = styles.map(s => s.name);
      findings.push(finding({
        scanner: SCANNER,
        code: 'style-not-found',
        severity: SEVERITY.medium,
        title: 'Configured output style does not exist',
        description:
          `Your ${resolved.scope} settings set \`outputStyle: "${resolved.value}"\`, but no ` +
          'built-in or discovered custom style has that name. Claude Code falls back to the ' +
          'default style, so the output behavior you configured is silently never applied.',
        file: resolved.path,
        evidence:
          `outputStyle="${resolved.value}"; scope=${resolved.scope}; ` +
          `builtins=[${BUILTIN_STYLES.join(', ')}]; ` +
          `known_custom=[${customNames.join(', ')}]`,
        recommendation:
          'Fix the value to match an existing style (built-ins: Default, Explanatory, ' +
          'Learning, Proactive), create the missing style under `.claude/output-styles/`, ' +
          'or remove the `outputStyle` setting.',
        category: 'output-styles',
      }));
    }
  }

  return scannerResult(SCANNER, 'ok', findings, styles.length, Date.now() - start);
}