diff --git a/tools/diff-events/diff_events.py b/tools/diff-events/diff_events.py
new file mode 100644
index 0000000..ecc2c0b
--- /dev/null
+++ b/tools/diff-events/diff_events.py
@@ -0,0 +1,434 @@
+#!/usr/bin/env python3
+"""Phase A event-log diff tool.
+
+Reads two schema-v1 JSONL event logs (one per engine) and reports the
+first behavioral divergence per guest-thread. Aligns streams by
+`tid_event_idx`. Field-comparison rules come straight from
+`audit-runs/phase-a-diff-harness/schema-v1.md` — keep both in sync.
+
+Usage:
+    diff_events.py --canary canary.jsonl --ours ours.jsonl [--out report.md]
+    diff_events.py --canary a.jsonl --ours b.jsonl --validate-identical
+    diff_events.py --canary a.jsonl --ours b.jsonl --tid-map 6=1,7=2
+"""
+
+import argparse
+import json
+import sys
+from pathlib import Path
+
+SCHEMA_VERSION = 1
+
+# Fields the diff tool skips (engine-local or host-clock).
+SKIP_TOP_FIELDS = {"engine", "host_ns", "guest_cycle", "deterministic"}
+# Within a payload: skipped fields by kind (in addition to the global set).
+SKIP_PAYLOAD_FIELDS_BY_KIND = {
+    # raw_handle_id is engine-local; the diff key is handle_semantic_id.
+    "handle.create": {"raw_handle_id"},
+    "handle.destroy": {"raw_handle_id"},
+    # wait_duration_cycles is non-deterministic (host scheduling).
+    "wait.end": {"wait_duration_cycles"},
+}
+
+# Allocator-returning kernel exports whose `kernel.return.payload.return_value`
+# is a host-allocator-dependent guest VA. Canary and ours legitimately route
+# allocations to different heap regions (e.g. canary `MmAllocatePhysicalMemoryEx`
+# returns `0xBC220000` from `vC0000000` while ours returns `0x40105000` from
+# its single user-heap region — see AUDIT-043 "ε host-allocator address-space
+# divergence" and Phase B `report.md` ε-class). Comparing raw VAs would always
+# diverge at the first allocator call.
+#
+# Canonicalization: per `(tid, export_name)` we assign a stable ordinal
+# (0, 1, 2, …) to each successive `kernel.return.return_value`, replacing
+# both sides' value with the sentinel string `<ALLOC_<NAME>_<ORDINAL>>`
+# before payload comparison. As long as both engines call the same
+# allocator the same number of times in the same order on a given thread,
+# the comparison treats them as equivalent.
+#
+# Limitations (documented):
+# * If one engine calls an allocator more times than the other, ordinals
+#   drift and subsequent allocator returns appear as divergences. That's
+#   the correct outcome — ordinal-count mismatch IS a behavioral
+#   divergence.
+# * `payload.status` is left untouched: it's a copy of the raw VA in
+#   hex-string form, useful in diff context.
+# * Other payload fields that happen to embed an allocator VA (e.g. a
+#   future `args_resolved.base_address` in a free-call) are NOT
+#   canonicalized — out of scope for this divergence. Extend the set
+#   below as new divergence classes surface.
+ALLOCATOR_RETURN_FNS = frozenset(
+    [
+        "MmAllocatePhysicalMemoryEx",
+        "MmAllocatePhysicalMemory",
+        "NtAllocateVirtualMemory",
+        "RtlAllocateHeap",
+        "MmCreateKernelStack",
+    ]
+)
+
+
+def canonicalize_allocator_returns(events_by_tid: dict) -> None:
+    """In-place: rewrite `payload.return_value` for every kernel.return whose
+    `payload.name` is in ALLOCATOR_RETURN_FNS, replacing the raw VA with
+    `<ALLOC_<NAME>_<ORDINAL>>`. Ordinals are per (tid, name) and assigned
+    in event order.
+
+    Called on each engine's stream independently; because ordinals are
+    assigned deterministically by per-tid call order, equivalent streams
+    produce equivalent sentinels."""
+    for tid, evs in events_by_tid.items():
+        # name -> next ordinal to assign on this tid
+        counters: dict[str, int] = {}
+        for ev in evs:
+            if ev.get("kind") != "kernel.return":
+                continue
+            payload = ev.get("payload") or {}
+            name = payload.get("name")
+            if name not in ALLOCATOR_RETURN_FNS:
+                continue
+            ordinal = counters.get(name, 0)
+            counters[name] = ordinal + 1
+            sentinel = f"<ALLOC_{name}_{ordinal}>"
+            payload["return_value"] = sentinel
+            # `payload.status` mirrors `return_value` as a hex string for
+            # allocator entries (xboxkrnl trampoline doesn't distinguish
+            # NTSTATUS from pointer-typed returns). Canonicalize together
+            # so they stay in lockstep.
+            if "status" in payload:
+                payload["status"] = sentinel
+
+
+def load_events(path: Path) -> dict:
+    """Return {tid: [event, ...]} keyed by tid, ordered by tid_event_idx.
+
+    Validates the schema header (first line must be schema_version=1).
+    """
+    events_by_tid: dict[int, list[dict]] = {}
+    with path.open("r", encoding="utf-8") as f:
+        first = f.readline()
+        if not first:
+            raise SystemExit(f"{path}: empty file")
+        hdr = json.loads(first)
+        if hdr.get("kind") != "schema_version":
+            raise SystemExit(
+                f"{path}: first event is not schema_version (got {hdr.get('kind')!r})"
+            )
+        if hdr.get("schema_version") != SCHEMA_VERSION:
+            raise SystemExit(
+                f"{path}: schema_version mismatch (expected {SCHEMA_VERSION}, got {hdr.get('schema_version')!r})"
+            )
+        for lineno, line in enumerate(f, start=2):
+            line = line.rstrip("\n")
+            if not line:
+                continue
+            try:
+                ev = json.loads(line)
+            except json.JSONDecodeError as e:
+                raise SystemExit(f"{path}:{lineno}: invalid JSON ({e})")
+            tid = ev.get("tid")
+            if tid is None:
+                raise SystemExit(f"{path}:{lineno}: missing tid")
+            events_by_tid.setdefault(tid, []).append(ev)
+    # Ensure each per-tid list is already monotonic by tid_event_idx.
+    for tid, evs in events_by_tid.items():
+        for i, ev in enumerate(evs):
+            if ev.get("tid_event_idx") != i:
+                # Note: the schema permits one engine to emit fewer events; we
+                # only validate the in-file ordering is strictly monotonic.
+                if i > 0 and ev["tid_event_idx"] <= evs[i - 1]["tid_event_idx"]:
+                    raise SystemExit(
+                        f"{path}: tid={tid} events out of order at index {i}"
+                    )
+    return events_by_tid
+
+
+def auto_tid_map(canary_evs: dict, ours_evs: dict) -> dict[int, int]:
+    """Naive tid mapping: pair canary tids with ours tids by the first
+    kernel.call name in each stream. Documented limitation in README."""
+    def first_call_name(evs: list[dict]) -> str | None:
+        for ev in evs:
+            if ev.get("kind") == "kernel.call":
+                return ev["payload"].get("name")
+        return None
+
+    canary_by_first = {}
+    for tid, evs in canary_evs.items():
+        name = first_call_name(evs)
+        if name is not None:
+            canary_by_first.setdefault(name, []).append(tid)
+
+    ours_by_first = {}
+    for tid, evs in ours_evs.items():
+        name = first_call_name(evs)
+        if name is not None:
+            ours_by_first.setdefault(name, []).append(tid)
+
+    mapping: dict[int, int] = {}
+    for name, c_tids in canary_by_first.items():
+        o_tids = ours_by_first.get(name, [])
+        for c, o in zip(sorted(c_tids), sorted(o_tids)):
+            mapping[c] = o
+    return mapping
+
+
+def parse_tid_map_arg(s: str) -> dict[int, int]:
+    """Parse `--tid-map 6=1,7=2` into {6: 1, 7: 2}."""
+    out: dict[int, int] = {}
+    for token in s.split(","):
+        token = token.strip()
+        if not token:
+            continue
+        if "=" not in token:
+            raise SystemExit(f"--tid-map: bad token {token!r} (expected canary=ours)")
+        a, b = token.split("=", 1)
+        out[int(a.strip(), 0)] = int(b.strip(), 0)
+    return out
+
+
+def compare_payload(kind: str, p_canary: dict, p_ours: dict) -> str | None:
+    """Compare two payloads. Returns None if equivalent, else a short
+    human-readable description of the first differing field."""
+    skip = SKIP_PAYLOAD_FIELDS_BY_KIND.get(kind, set())
+    # Compare the union of keys excluding skipped ones, in canary's key order
+    # first (stable), then any ours-only fields.
+    keys_seen: set[str] = set()
+    for k in p_canary.keys():
+        if k in skip:
+            continue
+        keys_seen.add(k)
+        vc = p_canary.get(k)
+        vo = p_ours.get(k)
+        if vc != vo:
+            return f"payload.{k}: canary={vc!r} ours={vo!r}"
+    for k in p_ours.keys():
+        if k in skip or k in keys_seen:
+            continue
+        if p_ours[k] is not None:
+            return f"payload.{k}: canary=<missing> ours={p_ours[k]!r}"
+    return None
+
+
+def compare_event(ev_canary: dict, ev_ours: dict) -> str | None:
+    """Compare two events. Returns None if equivalent, else a short description."""
+    # Top-level comparison: kind must match.
+    if ev_canary.get("kind") != ev_ours.get("kind"):
+        return f"kind: canary={ev_canary.get('kind')!r} ours={ev_ours.get('kind')!r}"
+    # tid_event_idx must match (it's our diff key).
+    if ev_canary.get("tid_event_idx") != ev_ours.get("tid_event_idx"):
+        return (
+            f"tid_event_idx: canary={ev_canary.get('tid_event_idx')!r} "
+            f"ours={ev_ours.get('tid_event_idx')!r}"
+        )
+    # Payload comparison.
+    pc = ev_canary.get("payload", {})
+    po = ev_ours.get("payload", {})
+    diff = compare_payload(ev_canary["kind"], pc, po)
+    if diff:
+        return diff
+    return None
+
+
+def render_event(ev: dict) -> str:
+    """One-line summary of an event for the diff report."""
+    kind = ev.get("kind", "?")
+    idx = ev.get("tid_event_idx", "?")
+    payload = ev.get("payload", {})
+    if kind in ("kernel.call", "kernel.return", "import.call"):
+        name = payload.get("name") or payload.get("ord")
+        return f"[{idx}] {kind} {name}"
+    if kind in ("handle.create", "handle.destroy"):
+        sid = payload.get("handle_semantic_id", "?")
+        return f"[{idx}] {kind} sid={sid}"
+    if kind in ("thread.create", "thread.exit"):
+        return f"[{idx}] {kind} {payload}"
+    if kind in ("wait.begin", "wait.end"):
+        return f"[{idx}] {kind} {payload}"
+    return f"[{idx}] {kind} {payload}"
+
+
+def diff_one_tid(
+    canary_evs: list[dict], ours_evs: list[dict], canary_tid: int, ours_tid: int
+) -> dict:
+    """Walk one mapped pair. Stop at the first divergence."""
+    matched = 0
+    n = min(len(canary_evs), len(ours_evs))
+    pre_context: list[tuple[dict, dict]] = []
+    diverged_at: int | None = None
+    diff_descr: str | None = None
+    for i in range(n):
+        ec = canary_evs[i]
+        eo = ours_evs[i]
+        d = compare_event(ec, eo)
+        if d is None:
+            matched += 1
+            pre_context.append((ec, eo))
+            if len(pre_context) > 5:
+                pre_context.pop(0)
+            continue
+        diverged_at = i
+        diff_descr = d
+        break
+    return {
+        "canary_tid": canary_tid,
+        "ours_tid": ours_tid,
+        "matched": matched,
+        "canary_total": len(canary_evs),
+        "ours_total": len(ours_evs),
+        "diverged_at": diverged_at,
+        "diff_descr": diff_descr,
+        "pre_context": pre_context,
+        "post_canary": canary_evs[diverged_at] if diverged_at is not None else None,
+        "post_ours": ours_evs[diverged_at] if diverged_at is not None else None,
+        "next_canary": (
+            canary_evs[diverged_at + 1]
+            if diverged_at is not None and diverged_at + 1 < len(canary_evs)
+            else None
+        ),
+        "next_ours": (
+            ours_evs[diverged_at + 1]
+            if diverged_at is not None and diverged_at + 1 < len(ours_evs)
+            else None
+        ),
+    }
+
+
+def render_report(per_tid_results: list[dict]) -> str:
+    out: list[str] = []
+    out.append("# Phase A diff report")
+    out.append("")
+    out.append("**This report is the output of Phase A's diff harness. Divergences")
+    out.append("shown here are INPUT for Phase B (first-divergence localization),")
+    out.append("not findings of Phase A.** Phase A's job is to make the harness")
+    out.append("itself correct, not to analyze what it surfaces.")
+    out.append("")
+    out.append("## Summary")
+    out.append("")
+    out.append("| canary_tid | ours_tid | matched | canary_total | ours_total | first_divergence_at |")
+    out.append("|---|---|---|---|---|---|")
+    for r in per_tid_results:
+        div = r["diverged_at"] if r["diverged_at"] is not None else "—"
+        out.append(
+            f"| {r['canary_tid']} | {r['ours_tid']} | {r['matched']} | "
+            f"{r['canary_total']} | {r['ours_total']} | {div} |"
+        )
+    out.append("")
+    for r in per_tid_results:
+        out.append(f"## canary_tid={r['canary_tid']} → ours_tid={r['ours_tid']}")
+        out.append("")
+        if r["diverged_at"] is None:
+            out.append(
+                f"No divergence within the {r['matched']} compared events "
+                f"(canary has {r['canary_total']}, ours has {r['ours_total']})."
+            )
+            out.append("")
+            continue
+        out.append(f"First divergence at `tid_event_idx={r['diverged_at']}`: {r['diff_descr']}")
+        out.append("")
+        out.append("**Pre-context (last 5 matching events):**")
+        out.append("```")
+        for ec, eo in r["pre_context"]:
+            out.append(f"  canary: {render_event(ec)}")
+            out.append(f"  ours:   {render_event(eo)}")
+        out.append("```")
+        out.append("")
+        out.append("**Divergent event:**")
+        out.append("```")
+        out.append(f"  canary: {render_event(r['post_canary'])}")
+        out.append(f"  ours:   {render_event(r['post_ours'])}")
+        out.append("```")
+        out.append("")
+        out.append("**Next event after the divergence (if any):**")
+        out.append("```")
+        if r["next_canary"]:
+            out.append(f"  canary: {render_event(r['next_canary'])}")
+        else:
+            out.append("  canary: <end of stream>")
+        if r["next_ours"]:
+            out.append(f"  ours:   {render_event(r['next_ours'])}")
+        else:
+            out.append("  ours:   <end of stream>")
+        out.append("```")
+        out.append("")
+        out.append("**Raw events (JSON):**")
+        out.append("```json")
+        out.append(json.dumps(r["post_canary"], sort_keys=True))
+        out.append(json.dumps(r["post_ours"], sort_keys=True))
+        out.append("```")
+        out.append("")
+    return "\n".join(out)
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser(description="Phase A event-log diff tool")
+    ap.add_argument("--canary", required=True, type=Path)
+    ap.add_argument("--ours", required=True, type=Path)
+    ap.add_argument("--out", type=Path, help="Write markdown report here (else stdout)")
+    ap.add_argument(
+        "--tid-map",
+        type=str,
+        help="Manual tid mapping like '6=1,7=2'. Overrides auto-mapping.",
+    )
+    ap.add_argument(
+        "--validate-identical",
+        action="store_true",
+        help="Exit non-zero if any mapped tid pair has any divergence. "
+        "Used by gate-4 negative-test and by self-diff smoke tests.",
+    )
+    ap.add_argument(
+        "--no-canonicalize-allocators",
+        action="store_true",
+        help="Disable per-tid ordinal canonicalization of allocator return "
+        "values (default: enabled). See ALLOCATOR_RETURN_FNS for the "
+        "covered set. Disabling reproduces the raw-VA comparison.",
+    )
+    args = ap.parse_args()
+
+    canary_evs = load_events(args.canary)
+    ours_evs = load_events(args.ours)
+
+    if not args.no_canonicalize_allocators:
+        canonicalize_allocator_returns(canary_evs)
+        canonicalize_allocator_returns(ours_evs)
+
+    if args.tid_map:
+        tid_map = parse_tid_map_arg(args.tid_map)
+    else:
+        tid_map = auto_tid_map(canary_evs, ours_evs)
+
+    if not tid_map:
+        sys.stderr.write(
+            "no tid mapping (auto-mapping found no shared first-kernel-call). "
+            "Pass --tid-map manually.\n"
+        )
+        return 2
+
+    per_tid: list[dict] = []
+    for c_tid, o_tid in sorted(tid_map.items()):
+        if c_tid not in canary_evs:
+            sys.stderr.write(f"warn: canary tid {c_tid} not in stream; skipping\n")
+            continue
+        if o_tid not in ours_evs:
+            sys.stderr.write(f"warn: ours tid {o_tid} not in stream; skipping\n")
+            continue
+        per_tid.append(diff_one_tid(canary_evs[c_tid], ours_evs[o_tid], c_tid, o_tid))
+
+    report = render_report(per_tid)
+    if args.out:
+        args.out.write_text(report, encoding="utf-8")
+        sys.stderr.write(f"diff report written to {args.out}\n")
+    else:
+        sys.stdout.write(report)
+
+    if args.validate_identical:
+        for r in per_tid:
+            if r["diverged_at"] is not None:
+                sys.stderr.write(
+                    f"validate-identical: divergence in canary_tid={r['canary_tid']} "
+                    f"at tid_event_idx={r['diverged_at']} ({r['diff_descr']})\n"
+                )
+                return 1
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())