diff --git a/scripts/gen_ac_traceability.py b/scripts/gen_ac_traceability.py
new file mode 100644
index 0000000..1f3f19c
--- /dev/null
+++ b/scripts/gen_ac_traceability.py
@@ -0,0 +1,164 @@
+"""Generate .planning/AC-TRACEABILITY.md from pytest collection.
+
+Bidirectional AC<->test traceability per Phase 2 / AC-06.
+
+Forward orphan  : declared AC with no test         -> visible + (with --check) exit 1
+Backward orphan : test references unknown AC ID    -> visible + (with --check) exit 1
+Deferred-hw AC  : `validation_method: deferred-hardware` in the AC entry
+                  -> excluded from orphan check, rendered as "DEFERRED (hardware)"
+
+Usage:
+    python scripts/gen_ac_traceability.py             # write matrix, exit 0
+    python scripts/gen_ac_traceability.py --check     # CI: exit 1 on drift
+"""
+from __future__ import annotations
+
+import argparse
+import json
+import logging
+import re
+import subprocess
+import sys
+import tempfile
+from pathlib import Path
+
+logging.basicConfig(level=logging.INFO, format="%(message)s")
+log = logging.getLogger("gen_ac_traceability")
+
+ROOT = Path(__file__).resolve().parent.parent
+AC_DOC = ROOT / "_docs" / "00_problem" / "acceptance_criteria.md"
+OUT_MD = ROOT / ".planning" / "AC-TRACEABILITY.md"
+
+_AC_HEADER_RE = re.compile(r"^\s*-\s*\*\*(AC-(?:\d+\.\d+[a-z]?|NEW-\d+))\*\*")
+_DEFERRED_RE = re.compile(r"deferred-hardware", re.IGNORECASE)
+
+
+def collect_acs_from_doc() -> dict[str, dict]:
+    """Parse _docs/00_problem/acceptance_criteria.md.
+
+    Returns: {ac_id: {"deferred": bool}}.
+    An AC is `deferred` if the literal token `deferred-hardware` (case-insensitive)
+    appears anywhere in the block between this AC's header and the next AC's header
+    (or EOF).
+    """
+    if not AC_DOC.exists():
+        log.error("AC doc not found: %s", AC_DOC)
+        sys.exit(2)
+
+    acs: dict[str, dict] = {}
+    current_id: str | None = None
+    for line in AC_DOC.read_text(encoding="utf-8").splitlines():
+        m = _AC_HEADER_RE.match(line)
+        if m:
+            current_id = m.group(1)
+            acs[current_id] = {"deferred": False}
+            continue
+        if current_id is not None and _DEFERRED_RE.search(line):
+            acs[current_id]["deferred"] = True
+    return acs
+
+
+def collect_acs_from_tests() -> dict[str, list[str]]:
+    """Run `pytest --collect-only --ac-dump=<tmp>` and parse the JSON.
+
+    Returns: {ac_id: [test_nodeid, ...]}.
+    """
+    with tempfile.NamedTemporaryFile(suffix=".json", delete=False) as tmp:
+        tmp_path = Path(tmp.name)
+    try:
+        result = subprocess.run(
+            ["pytest", "--collect-only", "-q", f"--ac-dump={tmp_path}", "tests/"],
+            cwd=ROOT,
+            check=False,             # collection may report 0 errors but exit non-zero on warnings -- handle below
+            capture_output=True,
+            text=True,
+        )
+        # Collection errors are a fatal traceability problem; surface stderr.
+        if result.returncode != 0 and not tmp_path.exists():
+            log.error("pytest --collect-only failed:\n%s", result.stderr or result.stdout)
+            sys.exit(2)
+        if not tmp_path.exists() or tmp_path.stat().st_size == 0:
+            return {}
+        return json.loads(tmp_path.read_text(encoding="utf-8"))
+    finally:
+        tmp_path.unlink(missing_ok=True)
+
+
+def render_md(doc_acs: dict[str, dict], test_map: dict[str, list[str]]) -> str:
+    """Render the AC-TRACEABILITY.md content."""
+    declared = sorted(doc_acs)
+    tested = set(test_map)
+    declared_set = set(declared)
+
+    lines: list[str] = [
+        "# AC Traceability Matrix",
+        "",
+        "> Auto-generated by `scripts/gen_ac_traceability.py`. Do not edit by hand.",
+        "> Run `python scripts/gen_ac_traceability.py` to regenerate after AC doc or test edits.",
+        "",
+        f"**ACs declared in acceptance_criteria.md:** {len(declared)}",
+        f"**ACs covered by at least one test:** {len(declared_set & tested)}",
+        f"**ACs deferred to hardware:** {sum(1 for a in declared if doc_acs[a]['deferred'])}",
+        "",
+        "## AC -> Test mapping",
+        "",
+        "| AC ID | Test count | Tests | Status |",
+        "|-------|-----------|-------|--------|",
+    ]
+    for ac_id in declared:
+        tests = test_map.get(ac_id, [])
+        if doc_acs[ac_id]["deferred"]:
+            status = "DEFERRED (hardware)"
+        elif tests:
+            status = "OK"
+        else:
+            status = "**ORPHAN -- no test**"
+        tests_str = "<br>".join(f"`{t}`" for t in tests) if tests else "_none_"
+        lines.append(f"| {ac_id} | {len(tests)} | {tests_str} | {status} |")
+
+    unknown = sorted(tested - declared_set)
+    if unknown:
+        lines += ["", "## Tests reference unknown AC IDs (backward orphans)", ""]
+        for ac_id in unknown:
+            for nid in test_map[ac_id]:
+                lines.append(f"- `{ac_id}` <- `{nid}`")
+
+    return "\n".join(lines) + "\n"
+
+
+def main() -> int:
+    p = argparse.ArgumentParser(description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter)
+    p.add_argument(
+        "--check", "--strict", action="store_true",
+        help="Exit 1 if any non-deferred AC has 0 tests, or any test references an AC ID not in the doc.",
+    )
+    args = p.parse_args()
+
+    doc_acs = collect_acs_from_doc()
+    test_map = collect_acs_from_tests()
+    out = render_md(doc_acs, test_map)
+    OUT_MD.parent.mkdir(parents=True, exist_ok=True)
+    OUT_MD.write_text(out, encoding="utf-8")
+    log.info("wrote %s (%d AC, %d tagged, %d deferred)",
+             OUT_MD.relative_to(ROOT),
+             len(doc_acs),
+             sum(1 for a in doc_acs if test_map.get(a)),
+             sum(1 for a in doc_acs if doc_acs[a]["deferred"]))
+
+    if args.check:
+        bad_orphans = sorted(
+            a for a, meta in doc_acs.items() if not meta["deferred"] and not test_map.get(a)
+        )
+        unknown = sorted(set(test_map) - set(doc_acs))
+        if bad_orphans or unknown:
+            log.error("AC traceability drift detected:")
+            for a in bad_orphans:
+                log.error("  ORPHAN AC (no test):       %s", a)
+            for a in unknown:
+                log.error("  UNKNOWN AC ID in test:     %s", a)
+            return 1
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())