diff --git a/tools/hygiene/audit-tick-shard-relative-paths.ts b/tools/hygiene/audit-tick-shard-relative-paths.ts
new file mode 100644
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+++ b/tools/hygiene/audit-tick-shard-relative-paths.ts
@@ -0,0 +1,310 @@
+#!/usr/bin/env bun
+// audit-tick-shard-relative-paths.ts — detect broken relative-path
+// links in `docs/hygiene-history/ticks/**/HHMMZ.md` tick shards.
+//
+// Tick shards live at docs/hygiene-history/ticks/YYYY/MM/DD/HHMMZ.md
+// (5 directories below `docs/`). Common relative-link bug class —
+// the agent counts directory levels wrong:
+//
+//   ../../../../.claude/rules/X.md   (4 levels)  → lands at docs/hygiene-history/ticks/2026/  — WRONG
+//   ../../../../../.claude/rules/X.md  (5 levels)  → lands at docs/                              — WRONG
+//   ../../../../../../.claude/rules/X.md (6 levels) → lands at repo root                          — CORRECT
+//
+// The bug class shipped twice in the same session (PR #3676 / PR #3679,
+// both with 5-level paths that didn't reach .claude/rules/ at repo root).
+// Copilot caught both via review threads. This audit mechanizes the catch
+// so future shards fail CI before merge, not after.
+//
+// Scope:
+//
+//   - Walk docs/hygiene-history/ticks/**/*.md
+//   - For each markdown link `[text](path)`, if path is relative (./...
+//     or ../...) and not a URL/anchor/code-block, resolve from the shard's
+//     directory and check the result is an existing file or directory
+//     under the repo root.
+//
+// Out of scope:
+//
+//   - Absolute URLs (http://, https://, mailto:, etc.)
+//   - In-page anchors (#section)
+//   - Bare paths without parens (markdown text mentions, not links)
+//   - Image embeds (![alt](path)) — same resolution rules apply, but
+//     skipping for MVP; can extend later
+//
+// Usage:
+//
+//   bun tools/hygiene/audit-tick-shard-relative-paths.ts                # detect-only
+//   bun tools/hygiene/audit-tick-shard-relative-paths.ts --enforce      # exit 1 on findings (CI gate)
+//   bun tools/hygiene/audit-tick-shard-relative-paths.ts --files <p...> # scan specific files
+//   bun tools/hygiene/audit-tick-shard-relative-paths.ts --json         # JSON output
+//
+// Exit codes:
+//
+//   0   no findings, OR detect-only mode (default)
+//   1   findings present AND --enforce flag set
+//   64  argument error
+//
+// Composes with: audit-section-33-migration-xrefs.ts (sibling template),
+// audit-memory-references.ts (relative-link resolution pattern), the
+// blocked-green-ci-investigate-threads.md rule (why this lint exists).
+
+import { existsSync, readdirSync, readFileSync, statSync } from "node:fs";
+import { join, relative, resolve, dirname, sep as PATH_SEP } from "node:path";
+import { spawnSync } from "node:child_process";
+
+// repo root via git rev-parse — same safe pattern as check-tick-history-shard-schema.ts
+
+function repoRoot(): string {
+  if (process.env["REPO_ROOT"]) return process.env["REPO_ROOT"]!;
+  // eslint-disable-next-line sonarjs/no-os-command-from-path -- git invoked as explicit args array; no shell, no user input.
+  const r = spawnSync("git", ["rev-parse", "--show-toplevel"], { encoding: "utf8" });
+  return r.status === 0 ? r.stdout.trim() : process.cwd();
+}
+
+const ROOT = resolve(repoRoot());
+const SHARD_DIR = join(ROOT, "docs/hygiene-history/ticks");
+
+// arg parsing
+
+interface Args {
+  enforce: boolean;
+  json: boolean;
+  files: readonly string[] | null;
+}
+
+function parseArgs(argv: readonly string[]): Args {
+  let enforce = false;
+  let json = false;
+  let files: string[] | null = null;
+  for (let i = 0; i < argv.length; i++) {
+    const a = argv[i]!;
+    if (a === "--enforce") enforce = true;
+    else if (a === "--json") json = true;
+    else if (a === "--files") {
+      files = [];
+      while (i + 1 < argv.length && !argv[i + 1]!.startsWith("--")) {
+        files.push(argv[++i]!);
+      }
+    } else {
+      process.stderr.write(`unknown argument: ${a}\n`);
+      process.exit(64);
+    }
+  }
+  return { enforce, json, files };
+}
+
+// shard discovery
+
+function walkShards(dir: string): string[] {
+  const out: string[] = [];
+  if (!existsSync(dir)) return out;
+  for (const entry of readdirSync(dir, { withFileTypes: true })) {
+    const p = join(dir, entry.name);
+    if (entry.isDirectory()) out.push(...walkShards(p));
+    else if (entry.isFile() && entry.name.endsWith(".md")) out.push(p);
+  }
+  return out;
+}
+
+// link extraction
+
+interface LinkRef {
+  readonly file: string;
+  readonly line: number;
+  readonly target: string;
+}
+
+// Matches markdown links `[text](target)`. Captures the target.
+// Excludes images (preceded by `!`) and reference-style links.
+const MD_LINK_RE = /(?<!!)\[(?:[^\]]*)\]\(([^)\s]+)(?:\s+"[^"]*")?\)/g;
+
+// URI-scheme prefixes that mark a target as absolute (non-relative).
+// Any `<scheme>:` prefix is treated as absolute except for ambiguous
+// cases (e.g. a Windows-style drive letter `C:` would never appear
+// inside a tick shard, so the generic `<scheme>:` rule is safe).
+const URI_SCHEME_RE = /^[A-Za-z][A-Za-z0-9+.-]*:/;
+
+function isRelativeTarget(target: string): boolean {
+  if (URI_SCHEME_RE.test(target)) return false; // http(s)://, mailto:, ftp:, file:, tel:, data:, etc.
+  if (target.startsWith("//")) return false;
+  if (target.startsWith("#")) return false;
+  if (target.startsWith("/")) return false;
+  return true;
+}
+
+// Skip targets that are clearly placeholders or shape examples within prose
+// rather than real navigation links. Tick shards sometimes contain inline
+// `[label](path-shape)` constructs to illustrate a pattern; those should
+// not be checked for resolution. Filters:
+//
+//   - contains `...` (ellipsis placeholder, e.g. `docs/research/...amara-...md`)
+//   - contains `(` or `)` (malformed or fragmentary link syntax)
+//   - pure identifier-only (no `/` and no `.`)
+function isPlaceholderTarget(target: string): boolean {
+  if (target.includes("...")) return true;
+  if (target.includes("(") || target.includes(")")) return true;
+  if (!target.includes("/") && !target.includes(".")) return true;
+  return false;
+}
+
+function stripAnchor(target: string): string {
+  const hash = target.indexOf("#");
+  return hash === -1 ? target : target.substring(0, hash);
+}
+
+function buildCodeFenceFlags(lines: readonly string[]): boolean[] {
+  const flags = new Array<boolean>(lines.length).fill(false);
+  let inFence = false;
+  for (let i = 0; i < lines.length; i++) {
+    const trimmed = lines[i]!.trim();
+    if (trimmed.startsWith("```")) {
+      inFence = !inFence;
+      flags[i] = inFence;
+    } else {
+      flags[i] = inFence;
+    }
+  }
+  return flags;
+}
+
+function extractLinks(file: string): LinkRef[] {
+  const text = readFileSync(file, "utf8");
+  const lines = text.split("\n");
+  const fenceFlags = buildCodeFenceFlags(lines);
+  const out: LinkRef[] = [];
+  for (let i = 0; i < lines.length; i++) {
+    if (fenceFlags[i]) continue;
+    const line = lines[i]!;
+    MD_LINK_RE.lastIndex = 0;
+    let m: RegExpExecArray | null;
+    while ((m = MD_LINK_RE.exec(line)) !== null) {
+      const target = m[1]!;
+      if (isRelativeTarget(target) && !isPlaceholderTarget(target)) {
+        out.push({ file, line: i + 1, target });
+      }
+    }
+  }
+  return out;
+}
+
+// resolution + verdict
+
+interface Finding {
+  readonly file: string;
+  readonly line: number;
+  readonly target: string;
+  readonly resolved: string;
+  readonly reason: "missing" | "escapes-repo";
+}
+
+function repoRelative(p: string): string {
+  return relative(ROOT, p).replaceAll("\\", "/");
+}
+
+function checkLink(link: LinkRef): Finding | null {
+  const base = dirname(link.file);
+  const stripped = stripAnchor(link.target);
+  const abs = resolve(base, stripped);
+  const rel = repoRelative(abs);
+
+  // Use platform-correct path separator (PATH_SEP from node:path), so the
+  // repo-boundary check works on Windows where resolve() returns paths with
+  // backslash separators. ROOT itself comes from `git rev-parse --show-toplevel`
+  // which returns forward-slash paths on all platforms; resolve() normalises
+  // separators to the platform's native form, so we compare with PATH_SEP.
+  if (!abs.startsWith(ROOT + PATH_SEP) && abs !== ROOT) {
+    return {
+      file: repoRelative(link.file),
+      line: link.line,
+      target: link.target,
+      resolved: abs,
+      reason: "escapes-repo",
+    };
+  }
+
+  if (!existsSync(abs)) {
+    return {
+      file: repoRelative(link.file),
+      line: link.line,
+      target: link.target,
+      resolved: rel,
+      reason: "missing",
+    };
+  }
+
+  return null;
+}
+
+// main
+
+export function main(argv: readonly string[]): 0 | 1 | 64 {
+  const args = parseArgs(argv);
+
+  // Validate --files inputs before reading: a missing OR non-file path
+  // should produce a structured exit (64), not an uncaught readFileSync
+  // exception that crashes the run partway through. A directory passing
+  // existsSync would later throw EISDIR inside extractLinks; reject
+  // upfront with a clear message.
+  if (args.files) {
+    const bad: { path: string; reason: string }[] = [];
+    for (const f of args.files) {
+      const abs = resolve(f);
+      if (!existsSync(abs)) {
+        bad.push({ path: f, reason: "not found" });
+        continue;
+      }
+      try {
+        if (!statSync(abs).isFile()) {
+          bad.push({ path: f, reason: "not a regular file" });
+        }
+      } catch (e) {
+        bad.push({ path: f, reason: `stat failed (${(e as Error).message})` });
+      }
+    }
+    if (bad.length > 0) {
+      for (const b of bad) process.stderr.write(`input ${b.reason}: ${b.path}\n`);
+      return 64;
+    }
+  }
+
+  const shards = args.files
+    ? args.files.map((f) => resolve(f))
+    : walkShards(SHARD_DIR);
+
+  if (shards.length === 0) {
+    if (args.files) {
+      process.stderr.write("no input files\n");
+      return 64;
+    }
+    process.stderr.write(`no tick shards found under ${SHARD_DIR}\n`);
+    return 0;
+  }
+
+  const findings: Finding[] = [];
+  for (const shard of shards) {
+    for (const link of extractLinks(shard)) {
+      const f = checkLink(link);
+      if (f) findings.push(f);
+    }
+  }
+
+  if (args.json) {
+    process.stdout.write(JSON.stringify({ shardsScanned: shards.length, findings }, null, 2) + "\n");
+  } else if (findings.length === 0) {
+    process.stdout.write(`ok: scanned ${shards.length} tick shards; 0 broken relative-path links\n`);
+  } else {
+    process.stdout.write(`scanned ${shards.length} tick shards; ${findings.length} broken relative-path links:\n\n`);
+    for (const f of findings) {
+      process.stdout.write(`  ${f.file}:${f.line}  ${f.reason}\n`);
+      process.stdout.write(`    target:   ${f.target}\n`);
+      process.stdout.write(`    resolved: ${f.resolved}\n\n`);
+    }
+  }
+
+  if (args.enforce && findings.length > 0) return 1;
+  return 0;
+}
+
+if (import.meta.main) {
+  process.exit(main(process.argv.slice(2)));
+}