diff --git a/test/helpers/skill-census.ts b/test/helpers/skill-census.ts
new file mode 100644
index 000000000..378201041
--- /dev/null
+++ b/test/helpers/skill-census.ts
@@ -0,0 +1,96 @@
+/**
+ * Shared skill census — the ONE place that counts skills (C11).
+ *
+ * Three consumers (hermetic seeding, context-bill ground truth, catalog-budget
+ * test) each need a DIFFERENT count of "the skills", and hand-rolled walks
+ * encode the wrong one somewhere. The counts diverge because of two facts:
+ *
+ *   1. `connect-chrome/` is a directory SYMLINK to `open-gstack-browser/`.
+ *      `Dirent.isDirectory()` is false for it (scripts/discover-skills.ts
+ *      skips it) while setup's trailing-slash shell glob follows it.
+ *   2. The root `SKILL.md` is a router, registered by `setup` under the
+ *      alias name `_gstack-command`, not as an authored skill.
+ *
+ * So: use `physicalSkillFiles` when you mean "every SKILL.md a filesystem
+ * walk can reach" (context-bill's walker), `authoredSkills` when you mean
+ * "distinct skills a human maintains" (catalog budget), and
+ * `registryEntries` when you mean "what a host discovers after ./setup"
+ * (hermetic seeding must mirror this exactly).
+ */
+
+import * as fs from 'fs';
+import * as path from 'path';
+
+const SKIP = new Set(['node_modules', '.git', 'dist']);
+
+export interface SkillCensus {
+  /** Every reachable `<dir>/SKILL.md` (symlinked dirs INCLUDED) plus the
+   * root `SKILL.md` router. Relative paths from root. */
+  physicalSkillFiles: string[];
+  /** Distinct authored skills: symlink-deduped by realpath, root router
+   * EXCLUDED. Each entry is the canonical directory name. */
+  authoredSkills: string[];
+  /** What `setup` registers into `~/.claude/skills/` with prefixing off:
+   * one entry per unique frontmatter `name:` (falling back to dir name),
+   * plus the `_gstack-command` root alias when the root router exists.
+   * Symlinked dirs collapse here because they share a frontmatter name. */
+  registryEntries: string[];
+}
+
+/** First `name:` from SKILL.md frontmatter, mirroring `setup`'s
+ * `grep -m1 '^name:'` (whitespace stripped). Empty string if absent. */
+export function frontmatterName(skillMdPath: string): string {
+  let body: string;
+  try {
+    body = fs.readFileSync(skillMdPath, 'utf-8');
+  } catch {
+    return '';
+  }
+  const m = body.match(/^name:\s*(.+)$/m);
+  return m ? m[1].replace(/\s+/g, '') : '';
+}
+
+export function skillCensus(root: string): SkillCensus {
+  const physicalSkillFiles: string[] = [];
+  const authoredByRealpath = new Map<string, string>();
+  const registrySet = new Set<string>();
+
+  if (fs.existsSync(path.join(root, 'SKILL.md'))) {
+    physicalSkillFiles.push('SKILL.md');
+    registrySet.add('_gstack-command');
+  }
+
+  for (const entry of fs.readdirSync(root, { withFileTypes: true })) {
+    if (entry.name.startsWith('.') || SKIP.has(entry.name)) continue;
+    const dirPath = path.join(root, entry.name);
+    // Follow directory symlinks like setup's shell glob does; Dirent alone
+    // reports symlinks as non-directories.
+    let isDir = entry.isDirectory();
+    if (!isDir && entry.isSymbolicLink()) {
+      try {
+        isDir = fs.statSync(dirPath).isDirectory();
+      } catch {
+        continue; // dangling symlink
+      }
+    }
+    if (!isDir) continue;
+
+    const skillMd = path.join(dirPath, 'SKILL.md');
+    if (!fs.existsSync(skillMd)) continue;
+
+    physicalSkillFiles.push(`${entry.name}/SKILL.md`);
+
+    const real = fs.realpathSync(dirPath);
+    if (!authoredByRealpath.has(real)) {
+      authoredByRealpath.set(real, path.basename(real));
+    }
+
+    registrySet.add(frontmatterName(skillMd) || entry.name);
+  }
+
+  return {
+    physicalSkillFiles: physicalSkillFiles.sort(),
+    authoredSkills: [...authoredByRealpath.values()].sort(),
+    registryEntries: [...registrySet].sort(),
+  };
+}
diff --git a/test/skill-census.test.ts b/test/skill-census.test.ts
new file mode 100644
index 000000000..b9bb21578
--- /dev/null
+++ b/test/skill-census.test.ts
@@ -0,0 +1,69 @@
+/**
+ * Pins the three-count contract of test/helpers/skill-census.ts (C11).
+ *
+ * No hardcoded totals here — the catalog-budget test owns the ratchet.
+ * This file pins the STRUCTURAL relationships that make the three counts
+ * mean different things, using the live repo as the fixture.
+ */
+
+import { describe, expect, it } from 'bun:test';
+import * as fs from 'fs';
+import * as path from 'path';
+import { frontmatterName, skillCensus } from './helpers/skill-census';
+
+const ROOT = path.resolve(new URL(import.meta.url).pathname, '..', '..');
+const census = skillCensus(ROOT);
+
+describe('skillCensus', () => {
+  it('physicalSkillFiles includes the root router and the symlinked dir', () => {
+    expect(census.physicalSkillFiles).toContain('SKILL.md');
+    expect(census.physicalSkillFiles).toContain('connect-chrome/SKILL.md');
+    expect(census.physicalSkillFiles).toContain('open-gstack-browser/SKILL.md');
+  });
+
+  it('authoredSkills dedupes the connect-chrome symlink and excludes the root router', () => {
+    expect(census.authoredSkills).toContain('open-gstack-browser');
+    expect(census.authoredSkills).not.toContain('connect-chrome');
+    // Root router is not an authored skill; its dir entry would be '' anyway.
+    for (const name of census.authoredSkills) expect(name.length).toBeGreaterThan(0);
+  });
+
+  it('registryEntries carries the root alias and collapses shared frontmatter names', () => {
+    expect(census.registryEntries).toContain('_gstack-command');
+    expect(
+      census.registryEntries.filter((n) => n === 'open-gstack-browser'),
+    ).toHaveLength(1);
+  });
+
+  it('count relationships hold: physical = authored + root + symlink dups', () => {
+    const symlinkDups = census.physicalSkillFiles.length - 1 - census.authoredSkills.length;
+    expect(symlinkDups).toBeGreaterThanOrEqual(1); // connect-chrome today
+    // Registry = unique frontmatter names + root alias. It can only collapse
+    // entries relative to physical, never invent them.
+    expect(census.registryEntries.length).toBeLessThanOrEqual(census.physicalSkillFiles.length);
+    expect(census.registryEntries.length).toBeGreaterThan(census.authoredSkills.length - 1);
+  });
+
+  it('frontmatterName mirrors setup: first ^name: line, whitespace stripped', () => {
+    const qa = frontmatterName(path.join(ROOT, 'qa', 'SKILL.md'));
+    expect(qa).toBe('qa');
+    const alias = frontmatterName(path.join(ROOT, 'connect-chrome', 'SKILL.md'));
+    expect(alias).toBe('open-gstack-browser');
+    expect(frontmatterName(path.join(ROOT, 'no-such-dir', 'SKILL.md'))).toBe('');
+  });
+
+  it('every registry entry a host would see resolves back to a physical SKILL.md', () => {
+    const names = new Set(
+      census.physicalSkillFiles
+        .filter((p) => p !== 'SKILL.md')
+        .map((p) => frontmatterName(path.join(ROOT, p)) || path.dirname(p)),
+    );
+    for (const entry of census.registryEntries) {
+      if (entry === '_gstack-command') {
+        expect(fs.existsSync(path.join(ROOT, 'SKILL.md'))).toBe(true);
+      } else {
+        expect(names.has(entry)).toBe(true);
+      }
+    }
+  });
+});