feat: add per-PR reviewer leases for parallel review (Closes #407)

Structured PR-thread leases with acquire/heartbeat MCP tools and
mutation-time enforcement so reviewers cannot race the same PR queue.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

gitea_mcp_server.py

@@ -543,6 +543,7 @@ import already_landed_reconcile  # noqa: E402
 import author_mutation_worktree  # noqa: E402
 import issue_claim_heartbeat  # noqa: E402
 import issue_work_duplicate_gate  # noqa: E402
+import reviewer_pr_lease  # noqa: E402
 import merged_cleanup_reconcile  # noqa: E402
 import reconciler_profile  # noqa: E402
 import reconciliation_workflow  # noqa: E402
@@ -2016,6 +2017,7 @@ def init_review_decision_lock(remote: str | None, task: str | None):
         (os.environ.get(SESSION_PROFILE_LOCK_ENV) or "").strip()
         or profile_name
     )
+    reviewer_pr_lease.clear_session_lease()
     _save_review_decision_lock({
         "task": task,
         "remote": remote,
@@ -2458,6 +2460,20 @@ def _evaluate_pr_review_submission(
             result["permission_report"] = elig["permission_report"]
         return result
 
+    if live:
+        reasons.extend(_reviewer_pr_lease_gate(
+            pr_number=pr_number,
+            remote=remote,
+            host=host,
+            org=org,
+            repo=repo,
+            mutation=action,
+            live_head_sha=result.get("head_sha"),
+            pinned_head_sha=expected_head_sha,
+        ))
+        if reasons:
+            return result
+
     auth_user = result["authenticated_user"]
     pr_author = result["pr_author"]
     if action == "approve" and auth_user and pr_author and auth_user == pr_author:
@@ -3257,6 +3273,19 @@ def gitea_merge_pr(
             result["permission_report"] = elig["permission_report"]
         return result
 
+    reasons.extend(_reviewer_pr_lease_gate(
+        pr_number=pr_number,
+        remote=remote,
+        host=host,
+        org=org,
+        repo=repo,
+        mutation="merge",
+        live_head_sha=result.get("head_sha"),
+        pinned_head_sha=expected_head_sha,
+    ))
+    if reasons:
+        return result
+
     # Gate 4 — head SHA must match if the caller pinned a reviewed SHA.
     actual_sha = result["head_sha"]
     if expected_head_sha and actual_sha and expected_head_sha != actual_sha:
@@ -4556,6 +4585,261 @@ def _namespace_mutation_block(mutation_task: str, **extra_fields) -> dict | None
     return blocked
 
 
+def _fetch_pr_comments(
+    pr_number: int,
+    *,
+    remote: str,
+    host: str | None,
+    org: str | None,
+    repo: str | None,
+) -> list[dict]:
+    h, o, r = _resolve(remote, host, org, repo)
+    auth = _auth(h)
+    api = f"{repo_api_url(h, o, r)}/issues/{pr_number}/comments"
+    return api_request("GET", api, auth) or []
+
+
+def _reviewer_pr_lease_gate(
+    *,
+    pr_number: int,
+    remote: str,
+    host: str | None,
+    org: str | None,
+    repo: str | None,
+    mutation: str,
+    live_head_sha: str | None,
+    pinned_head_sha: str | None,
+) -> list[str]:
+    """Return block reasons when the session lacks an owned PR reviewer lease."""
+    session = reviewer_pr_lease.get_session_lease()
+    session_id = (session or {}).get("session_id")
+    identity = _authenticated_username(remote) or ""
+    try:
+        comments = _fetch_pr_comments(
+            pr_number, remote=remote, host=host, org=org, repo=repo)
+    except Exception as exc:
+        return [f"cannot fetch PR comments for lease gate: {_redact(str(exc))}"]
+    assessment = reviewer_pr_lease.assess_mutation_lease_gate(
+        pr_number=pr_number,
+        comments=comments,
+        reviewer_identity=identity,
+        session_id=session_id,
+        mutation=mutation,
+        live_head_sha=live_head_sha,
+        pinned_head_sha=pinned_head_sha,
+    )
+    return list(assessment.get("reasons") or []) if assessment.get("block") else []
+
+
+@mcp.tool()
+def gitea_acquire_reviewer_pr_lease(
+    pr_number: int,
+    worktree: str,
+    candidate_head: str | None = None,
+    target_branch: str = "master",
+    target_branch_sha: str | None = None,
+    issue_number: int | None = None,
+    session_id: str | None = None,
+    remote: str = "dadeschools",
+    host: str | None = None,
+    org: str | None = None,
+    repo: str | None = None,
+) -> dict:
+    """Acquire a per-PR reviewer lease before review/merge mutations (#407)."""
+    read_block = _profile_operation_gate("gitea.read")
+    if read_block:
+        return {
+            "success": False,
+            "acquired": False,
+            "reasons": read_block,
+            "permission_report": _permission_block_report("gitea.read"),
+        }
+    comment_block = _profile_operation_gate("gitea.pr.comment")
+    if comment_block:
+        return {
+            "success": False,
+            "acquired": False,
+            "reasons": comment_block,
+            "permission_report": _permission_block_report("gitea.pr.comment"),
+        }
+
+    verify_preflight_purity(remote)
+    h, o, r = _resolve(remote, host, org, repo)
+    auth = _auth(h)
+    profile = get_profile()
+    identity = _authenticated_username(remote) or profile.get("username") or ""
+    sid = (session_id or "").strip() or reviewer_pr_lease.new_session_id()
+    repo_label = f"{o}/{r}"
+
+    comments = _fetch_pr_comments(
+        pr_number, remote=remote, host=host, org=org, repo=repo)
+    assessment = reviewer_pr_lease.assess_acquire_lease(
+        comments,
+        pr_number=pr_number,
+        reviewer_identity=identity,
+        profile=profile.get("profile_name") or "unknown",
+        session_id=sid,
+        repo=repo_label,
+        issue_number=issue_number,
+        worktree=worktree,
+        candidate_head=candidate_head,
+        target_branch=target_branch,
+        target_branch_sha=target_branch_sha,
+    )
+    if not assessment.get("acquire_allowed"):
+        return {
+            "success": False,
+            "acquired": False,
+            "reasons": assessment.get("reasons") or [],
+            "existing_lease": assessment.get("existing_lease"),
+        }
+
+    body = assessment["lease_body"]
+    comment_url = f"{repo_api_url(h, o, r)}/issues/{pr_number}/comments"
+    with _audited(
+        "comment_pr",
+        host=h,
+        remote=remote,
+        org=o,
+        repo=r,
+        pr_number=pr_number,
+        request_metadata={"source": "acquire_reviewer_pr_lease"},
+    ):
+        posted = api_request("POST", comment_url, auth, {"body": body})
+
+    session_lease = reviewer_pr_lease.record_session_lease({
+        "pr_number": pr_number,
+        "issue_number": issue_number,
+        "session_id": sid,
+        "reviewer_identity": identity,
+        "profile": profile.get("profile_name"),
+        "worktree": worktree,
+        "phase": "claimed",
+        "candidate_head": candidate_head,
+        "target_branch": target_branch,
+        "target_branch_sha": target_branch_sha,
+        "repo": repo_label,
+        "comment_id": posted.get("id"),
+    })
+    return {
+        "success": True,
+        "acquired": True,
+        "pr_number": pr_number,
+        "session_id": sid,
+        "comment_id": posted.get("id"),
+        "session_lease": session_lease,
+        "reasons": [],
+    }
+
+
+@mcp.tool()
+def gitea_heartbeat_reviewer_pr_lease(
+    pr_number: int,
+    phase: str,
+    worktree: str | None = None,
+    candidate_head: str | None = None,
+    target_branch_sha: str | None = None,
+    remote: str = "dadeschools",
+    host: str | None = None,
+    org: str | None = None,
+    repo: str | None = None,
+) -> dict:
+    """Post a reviewer lease heartbeat / phase update on the PR thread (#407)."""
+    comment_block = _profile_operation_gate("gitea.pr.comment")
+    if comment_block:
+        return {
+            "success": False,
+            "posted": False,
+            "reasons": comment_block,
+            "permission_report": _permission_block_report("gitea.pr.comment"),
+        }
+    session = reviewer_pr_lease.get_session_lease()
+    if not session or session.get("pr_number") != pr_number:
+        return {
+            "success": False,
+            "posted": False,
+            "reasons": [
+                f"no in-session lease for PR #{pr_number}; acquire first "
+                "(fail closed)"
+            ],
+        }
+
+    verify_preflight_purity(remote)
+    h, o, r = _resolve(remote, host, org, repo)
+    auth = _auth(h)
+    body = reviewer_pr_lease.format_lease_body(
+        repo=f"{o}/{r}",
+        pr_number=pr_number,
+        issue_number=session.get("issue_number"),
+        reviewer_identity=session.get("reviewer_identity") or "",
+        profile=session.get("profile") or "unknown",
+        session_id=session.get("session_id") or reviewer_pr_lease.new_session_id(),
+        worktree=worktree or session.get("worktree") or "",
+        phase=phase,
+        candidate_head=candidate_head or session.get("candidate_head"),
+        target_branch=session.get("target_branch") or "master",
+        target_branch_sha=target_branch_sha or session.get("target_branch_sha"),
+    )
+    comment_url = f"{repo_api_url(h, o, r)}/issues/{pr_number}/comments"
+    with _audited(
+        "comment_pr",
+        host=h,
+        remote=remote,
+        org=o,
+        repo=r,
+        pr_number=pr_number,
+        request_metadata={"source": "heartbeat_reviewer_pr_lease", "phase": phase},
+    ):
+        posted = api_request("POST", comment_url, auth, {"body": body})
+
+    updated = reviewer_pr_lease.record_session_lease({
+        **session,
+        "phase": phase,
+        "worktree": worktree or session.get("worktree"),
+        "candidate_head": candidate_head or session.get("candidate_head"),
+        "target_branch_sha": target_branch_sha or session.get("target_branch_sha"),
+        "last_comment_id": posted.get("id"),
+    })
+    return {
+        "success": True,
+        "posted": True,
+        "pr_number": pr_number,
+        "phase": phase,
+        "comment_id": posted.get("id"),
+        "session_lease": updated,
+        "reasons": [],
+    }
+
+
+@mcp.tool()
+def gitea_assess_reviewer_pr_lease(
+    pr_number: int,
+    remote: str = "dadeschools",
+    host: str | None = None,
+    org: str | None = None,
+    repo: str | None = None,
+) -> dict:
+    """Read-only: assess active reviewer lease state for a PR (#407)."""
+    read_block = _profile_operation_gate("gitea.read")
+    if read_block:
+        return {
+            "success": False,
+            "reasons": read_block,
+            "permission_report": _permission_block_report("gitea.read"),
+        }
+    comments = _fetch_pr_comments(
+        pr_number, remote=remote, host=host, org=org, repo=repo)
+    active = reviewer_pr_lease.find_active_reviewer_lease(
+        comments, pr_number=pr_number)
+    return {
+        "success": True,
+        "pr_number": pr_number,
+        "active_lease": active,
+        "session_lease": reviewer_pr_lease.get_session_lease(),
+        "reasons": [],
+    }
+
+
 @mcp.tool()
 def gitea_list_issue_comments(
     issue_number: int,

reviewer_pr_lease.py

@@ -0,0 +1,382 @@
+"""Per-PR reviewer leases for safe parallel review sessions (#407)."""
+
+from __future__ import annotations
+
+import os
+import re
+import uuid
+from datetime import datetime, timedelta, timezone
+from typing import Any
+
+MARKER = "<!-- mcp-review-lease:v1 -->"
+
+_FIELD_RE = re.compile(
+    r"^\s*([a-z_]+)\s*:\s*(.+?)\s*$",
+    re.IGNORECASE | re.MULTILINE,
+)
+_FULL_SHA = re.compile(r"^[0-9a-f]{40}$", re.IGNORECASE)
+
+_TERMINAL_PHASES = frozenset({"done", "released", "blocked"})
+_ACTIVE_PHASES = frozenset({
+    "claimed",
+    "validating",
+    "approved",
+    "request-changes",
+    "merging",
+})
+
+DEFAULT_LEASE_TTL_MINUTES = 120
+STALE_WARNING_MINUTES = 30
+RECLAIMABLE_MINUTES = 60
+
+_SESSION_LEASE: dict[str, Any] | None = None
+
+
+def _parse_timestamp(value: str | None) -> datetime | None:
+    if not value:
+        return None
+    text = value.strip()
+    if text.endswith("Z"):
+        text = text[:-1] + "+00:00"
+    try:
+        parsed = datetime.fromisoformat(text)
+    except ValueError:
+        return None
+    if parsed.tzinfo is None:
+        return parsed.replace(tzinfo=timezone.utc)
+    return parsed.astimezone(timezone.utc)
+
+
+def _normalize_sha(value: str | None) -> str | None:
+    text = (value or "").strip().lower()
+    return text if text and _FULL_SHA.match(text) else None
+
+
+def _parse_pr_ref(value: str | None) -> int | None:
+    digits = re.sub(r"[^\d]", "", value or "")
+    return int(digits) if digits.isdigit() else None
+
+
+def new_session_id() -> str:
+    return f"{os.getpid()}-{uuid.uuid4().hex[:12]}"
+
+
+def format_lease_body(
+    *,
+    repo: str,
+    pr_number: int,
+    issue_number: int | None,
+    reviewer_identity: str,
+    profile: str,
+    session_id: str,
+    worktree: str,
+    phase: str,
+    candidate_head: str | None,
+    target_branch: str,
+    target_branch_sha: str | None,
+    last_activity: datetime | None = None,
+    expires_at: datetime | None = None,
+    blocker: str = "none",
+) -> str:
+    now = last_activity or datetime.now(timezone.utc)
+    expires = expires_at or (now + timedelta(minutes=DEFAULT_LEASE_TTL_MINUTES))
+    last_text = now.astimezone(timezone.utc).replace(microsecond=0).isoformat().replace(
+        "+00:00", "Z"
+    )
+    expires_text = expires.astimezone(timezone.utc).replace(microsecond=0).isoformat().replace(
+        "+00:00", "Z"
+    )
+    issue_text = f"#{issue_number}" if issue_number else "none"
+    lines = [
+        MARKER,
+        f"repo: {repo}",
+        f"pr: #{pr_number}",
+        f"issue: {issue_text}",
+        f"reviewer_identity: {reviewer_identity}",
+        f"profile: {profile}",
+        f"session_id: {session_id}",
+        f"worktree: {worktree}",
+        f"phase: {phase}",
+        f"candidate_head: {candidate_head or 'none'}",
+        f"target_branch: {target_branch}",
+        f"target_branch_sha: {target_branch_sha or 'none'}",
+        f"last_activity: {last_text}",
+        f"expires_at: {expires_text}",
+        f"blocker: {blocker}",
+    ]
+    return "\n".join(lines)
+
+
+def parse_lease_comment(body: str) -> dict[str, Any] | None:
+    text = body or ""
+    if MARKER not in text:
+        return None
+    fields: dict[str, str] = {}
+    for match in _FIELD_RE.finditer(text):
+        fields[match.group(1).strip().lower()] = match.group(2).strip()
+    if not fields:
+        return None
+    return {
+        "repo": fields.get("repo"),
+        "pr_number": _parse_pr_ref(fields.get("pr")),
+        "issue_number": _parse_pr_ref(fields.get("issue")),
+        "reviewer_identity": fields.get("reviewer_identity"),
+        "profile": fields.get("profile"),
+        "session_id": fields.get("session_id"),
+        "worktree": fields.get("worktree"),
+        "phase": (fields.get("phase") or "").strip().lower() or None,
+        "candidate_head": _normalize_sha(fields.get("candidate_head")),
+        "target_branch": fields.get("target_branch"),
+        "target_branch_sha": _normalize_sha(fields.get("target_branch_sha")),
+        "last_activity": fields.get("last_activity"),
+        "expires_at": fields.get("expires_at"),
+        "blocker": fields.get("blocker"),
+        "raw_fields": fields,
+    }
+
+
+def _lease_entries(comments: list[dict], *, pr_number: int) -> list[dict]:
+    entries: list[dict] = []
+    for comment in comments or []:
+        parsed = parse_lease_comment(comment.get("body") or "")
+        if not parsed:
+            continue
+        if parsed.get("pr_number") not in (None, pr_number):
+            continue
+        entries.append({
+            **parsed,
+            "comment_id": comment.get("id"),
+            "author": (comment.get("user") or {}).get("login") or comment.get("author"),
+            "created_at": comment.get("created_at"),
+            "updated_at": comment.get("updated_at"),
+        })
+    return entries
+
+
+def _lease_expired(lease: dict, *, now: datetime) -> bool:
+    expires_at = _parse_timestamp(lease.get("expires_at"))
+    return bool(expires_at and expires_at <= now)
+
+
+def _minutes_since_activity(lease: dict, *, now: datetime) -> float | None:
+    last = _parse_timestamp(lease.get("last_activity"))
+    if not last:
+        return None
+    return (now - last).total_seconds() / 60.0
+
+
+def classify_lease_freshness(lease: dict, *, now: datetime | None = None) -> str:
+    """Return active, stale_warning, reclaimable, expired, or terminal."""
+    now = now or datetime.now(timezone.utc)
+    phase = (lease.get("phase") or "").strip().lower()
+    if phase in _TERMINAL_PHASES:
+        return "terminal"
+    if _lease_expired(lease, now=now):
+        return "expired"
+    minutes = _minutes_since_activity(lease, now=now)
+    if minutes is None:
+        return "active"
+    if minutes >= RECLAIMABLE_MINUTES:
+        return "reclaimable"
+    if minutes >= STALE_WARNING_MINUTES:
+        return "stale_warning"
+    return "active"
+
+
+def find_active_reviewer_lease(
+    comments: list[dict],
+    *,
+    pr_number: int,
+    now: datetime | None = None,
+) -> dict[str, Any] | None:
+    """Newest non-terminal, unexpired lease for *pr_number*."""
+    now = now or datetime.now(timezone.utc)
+    for lease in reversed(_lease_entries(comments, pr_number=pr_number)):
+        phase = (lease.get("phase") or "").strip().lower()
+        if phase in _TERMINAL_PHASES:
+            continue
+        if _lease_expired(lease, now=now):
+            continue
+        if phase in _ACTIVE_PHASES or phase:
+            lease = dict(lease)
+            lease["freshness"] = classify_lease_freshness(lease, now=now)
+            return lease
+    return None
+
+
+def assess_acquire_lease(
+    comments: list[dict],
+    *,
+    pr_number: int,
+    reviewer_identity: str,
+    profile: str,
+    session_id: str,
+    repo: str,
+    issue_number: int | None,
+    worktree: str,
+    candidate_head: str | None,
+    target_branch: str,
+    target_branch_sha: str | None,
+    now: datetime | None = None,
+) -> dict[str, Any]:
+    """Fail closed when another session holds an active lease."""
+    now = now or datetime.now(timezone.utc)
+    reasons: list[str] = []
+    existing = find_active_reviewer_lease(comments, pr_number=pr_number, now=now)
+    if existing:
+        owner_session = (existing.get("session_id") or "").strip()
+        freshness = existing.get("freshness") or classify_lease_freshness(existing, now=now)
+        if owner_session and owner_session != session_id and freshness in {
+            "active", "stale_warning"
+        }:
+            reasons.append(
+                f"PR #{pr_number} already has active reviewer lease "
+                f"(session_id={owner_session}, phase={existing.get('phase')})"
+            )
+        elif owner_session and owner_session != session_id and freshness == "reclaimable":
+            reasons.append(
+                f"PR #{pr_number} lease is reclaimable but still held by "
+                f"session_id={owner_session}; explicit reclaim not implemented "
+                "(fail closed)"
+            )
+
+    if not (reviewer_identity or "").strip():
+        reasons.append("reviewer identity required for lease acquisition")
+    if not (session_id or "").strip():
+        reasons.append("session_id required for lease acquisition")
+    if not (worktree or "").strip():
+        reasons.append("worktree path required for lease acquisition")
+
+    allowed = not reasons
+    body = None
+    if allowed:
+        body = format_lease_body(
+            repo=repo,
+            pr_number=pr_number,
+            issue_number=issue_number,
+            reviewer_identity=reviewer_identity,
+            profile=profile,
+            session_id=session_id,
+            worktree=worktree,
+            phase="claimed",
+            candidate_head=candidate_head,
+            target_branch=target_branch,
+            target_branch_sha=target_branch_sha,
+            last_activity=now,
+        )
+    return {
+        "acquire_allowed": allowed,
+        "reasons": reasons,
+        "existing_lease": existing,
+        "lease_body": body,
+        "session_id": session_id,
+    }
+
+
+def record_session_lease(lease: dict[str, Any]) -> dict[str, Any]:
+    global _SESSION_LEASE
+    _SESSION_LEASE = dict(lease)
+    return dict(_SESSION_LEASE)
+
+
+def clear_session_lease() -> None:
+    global _SESSION_LEASE
+    _SESSION_LEASE = None
+
+
+def get_session_lease() -> dict[str, Any] | None:
+    return dict(_SESSION_LEASE) if _SESSION_LEASE else None
+
+
+def assess_mutation_lease_gate(
+    *,
+    pr_number: int,
+    comments: list[dict],
+    reviewer_identity: str,
+    session_id: str | None,
+    mutation: str,
+    live_head_sha: str | None,
+    pinned_head_sha: str | None,
+    now: datetime | None = None,
+) -> dict[str, Any]:
+    """Reviewer mutations require an owned, current PR lease."""
+    now = now or datetime.now(timezone.utc)
+    reasons: list[str] = []
+    session = get_session_lease()
+    active = find_active_reviewer_lease(comments, pr_number=pr_number, now=now)
+
+    if not session:
+        reasons.append(
+            f"no in-session reviewer lease recorded; acquire via "
+            f"gitea_acquire_reviewer_pr_lease before {mutation}"
+        )
+    elif session.get("pr_number") != pr_number:
+        reasons.append(
+            f"session lease is for PR #{session.get('pr_number')}, not #{pr_number}"
+        )
+    elif (session.get("session_id") or "") != (session_id or session.get("session_id")):
+        reasons.append("session lease session_id mismatch (fail closed)")
+
+    if active:
+        owner = (active.get("session_id") or "").strip()
+        if owner and session_id and owner != session_id:
+            reasons.append(
+                f"active PR lease owned by session_id={owner}; current session "
+                f"cannot {mutation}"
+            )
+        pinned = _normalize_sha(pinned_head_sha)
+        live = _normalize_sha(live_head_sha)
+        lease_head = active.get("candidate_head")
+        if pinned and live and pinned != live:
+            reasons.append(
+                "PR head changed during lease; stop and re-validate before "
+                f"reviewer {mutation}"
+            )
+        if lease_head and live and lease_head != live:
+            reasons.append(
+                "live PR head differs from lease candidate_head; refresh lease "
+                f"before {mutation}"
+            )
+        freshness = active.get("freshness") or classify_lease_freshness(active, now=now)
+        if freshness in {"expired", "reclaimable"}:
+            reasons.append(f"reviewer lease freshness is '{freshness}' (fail closed)")
+    else:
+        reasons.append(f"no active reviewer lease found on PR #{pr_number}")
+
+    allowed = not reasons
+    return {
+        "mutation_allowed": allowed,
+        "block": not allowed,
+        "reasons": reasons,
+        "active_lease": active,
+        "session_lease": session,
+    }
+
+
+def assess_lease_inventory(
+    comments_by_pr: dict[int, list[dict]],
+    *,
+    now: datetime | None = None,
+) -> dict[str, Any]:
+    """Summarize lease states across PR comment threads."""
+    now = now or datetime.now(timezone.utc)
+    active: list[dict] = []
+    stale: list[dict] = []
+    reclaimable: list[dict] = []
+    for pr_number, comments in (comments_by_pr or {}).items():
+        lease = find_active_reviewer_lease(comments, pr_number=pr_number, now=now)
+        if not lease:
+            continue
+        freshness = lease.get("freshness") or classify_lease_freshness(lease, now=now)
+        entry = {"pr_number": pr_number, "session_id": lease.get("session_id"), "freshness": freshness}
+        if freshness == "stale_warning":
+            stale.append(entry)
+        elif freshness == "reclaimable":
+            reclaimable.append(entry)
+        else:
+            active.append(entry)
+    return {
+        "active_review_leases": active,
+        "stale_review_leases": stale,
+        "reclaimable_review_leases": reclaimable,
+    }

skills/llm-project-workflow/workflows/review-merge-pr.md

@@ -732,6 +732,26 @@ The final report must identify:
 * whether same-PR merge continuation was allowed
 * whether the run stopped as required
 
+## 26B. Per-PR reviewer lease (#407)
+
+Parallel reviewer sessions are allowed only when each session holds a distinct,
+live PR lease.
+
+Before validation or review mutation on a selected PR:
+
+1. Call `gitea_acquire_reviewer_pr_lease` with worktree path, candidate head SHA,
+   and target branch SHA.
+2. Post heartbeats via `gitea_heartbeat_reviewer_pr_lease` before validation,
+   after validation, before review mutation, and before merge.
+3. Do not approve, request changes, or merge unless the in-session lease
+   matches the selected PR.
+
+If PR head or target branch advances during the lease, stop and refresh
+inventory before continuing.
+
+Final reports must include lease session id, acquisition proof, heartbeat
+status, and release/blocked status.
+
 ## 27. Merge rules
 
 Before merge, rerun fresh live checks:

tests/test_reviewer_pr_lease.py

@@ -0,0 +1,193 @@
+"""Tests for per-PR reviewer leases (#407)."""
+
+import sys
+import unittest
+from datetime import datetime, timedelta, timezone
+from unittest.mock import patch
+
+sys.path.insert(0, str(__import__("pathlib").Path(__file__).resolve().parent.parent))
+
+import reviewer_pr_lease as leases
+
+
+def _lease_comment(
+    pr_number: int,
+    session_id: str,
+    *,
+    phase: str = "claimed",
+    minutes_ago: int = 0,
+    candidate_head: str = "a" * 40,
+) -> dict:
+    now = datetime.now(timezone.utc) - timedelta(minutes=minutes_ago)
+    body = leases.format_lease_body(
+        repo="Scaled-Tech-Consulting/Gitea-Tools",
+        pr_number=pr_number,
+        issue_number=295,
+        reviewer_identity="rev1",
+        profile="prgs-reviewer",
+        session_id=session_id,
+        worktree="branches/review-pr382",
+        phase=phase,
+        candidate_head=candidate_head,
+        target_branch="master",
+        target_branch_sha="b" * 40,
+        last_activity=now,
+    )
+    return {"id": 1, "body": body, "user": {"login": "rev1"}}
+
+
+class TestReviewerLeaseAcquire(unittest.TestCase):
+    def setUp(self):
+        leases.clear_session_lease()
+
+    def test_two_reviewers_cannot_lease_same_pr(self):
+        comments = [_lease_comment(382, "session-a")]
+        result = leases.assess_acquire_lease(
+            comments,
+            pr_number=382,
+            reviewer_identity="rev2",
+            profile="prgs-reviewer",
+            session_id="session-b",
+            repo="Scaled-Tech-Consulting/Gitea-Tools",
+            issue_number=295,
+            worktree="branches/review-pr382-b",
+            candidate_head="c" * 40,
+            target_branch="master",
+            target_branch_sha="d" * 40,
+        )
+        self.assertFalse(result["acquire_allowed"])
+        self.assertTrue(any("already has active" in r for r in result["reasons"]))
+
+    def test_two_reviewers_can_lease_different_prs(self):
+        comments = [_lease_comment(382, "session-a")]
+        result = leases.assess_acquire_lease(
+            comments,
+            pr_number=383,
+            reviewer_identity="rev2",
+            profile="prgs-reviewer",
+            session_id="session-b",
+            repo="Scaled-Tech-Consulting/Gitea-Tools",
+            issue_number=296,
+            worktree="branches/review-pr383",
+            candidate_head="c" * 40,
+            target_branch="master",
+            target_branch_sha="d" * 40,
+        )
+        self.assertTrue(result["acquire_allowed"])
+        self.assertIsNotNone(result["lease_body"])
+
+
+class TestReviewerLeaseFreshness(unittest.TestCase):
+    def test_stale_warning_after_30_minutes(self):
+        lease = leases.parse_lease_comment(
+            _lease_comment(382, "session-a", minutes_ago=35)["body"]
+        )
+        self.assertEqual(
+            leases.classify_lease_freshness(lease),
+            "stale_warning",
+        )
+
+    def test_reclaimable_after_60_minutes(self):
+        lease = leases.parse_lease_comment(
+            _lease_comment(382, "session-a", minutes_ago=65)["body"]
+        )
+        self.assertEqual(
+            leases.classify_lease_freshness(lease),
+            "reclaimable",
+        )
+
+
+class TestReviewerLeaseMutationGate(unittest.TestCase):
+    def setUp(self):
+        leases.clear_session_lease()
+
+    def test_reviewer_without_lease_cannot_mutate(self):
+        head = "f" * 40
+        comments = [_lease_comment(382, "other-session", candidate_head=head)]
+        result = leases.assess_mutation_lease_gate(
+            pr_number=382,
+            comments=comments,
+            reviewer_identity="rev1",
+            session_id="my-session",
+            mutation="approve",
+            live_head_sha=head,
+            pinned_head_sha=head,
+        )
+        self.assertTrue(result["block"])
+
+    def test_owned_lease_allows_mutation(self):
+        head = "f" * 40
+        comments = [_lease_comment(382, "my-session", candidate_head=head)]
+        leases.record_session_lease({
+            "pr_number": 382,
+            "session_id": "my-session",
+            "candidate_head": head,
+            "target_branch": "master",
+        })
+        result = leases.assess_mutation_lease_gate(
+            pr_number=382,
+            comments=comments,
+            reviewer_identity="rev1",
+            session_id="my-session",
+            mutation="approve",
+            live_head_sha=head,
+            pinned_head_sha=head,
+        )
+        self.assertFalse(result["block"])
+
+    def test_head_change_invalidates_lease(self):
+        reviewed = "f" * 40
+        live = "e" * 40
+        comments = [_lease_comment(382, "my-session", candidate_head=reviewed)]
+        leases.record_session_lease({
+            "pr_number": 382,
+            "session_id": "my-session",
+            "candidate_head": reviewed,
+        })
+        result = leases.assess_mutation_lease_gate(
+            pr_number=382,
+            comments=comments,
+            reviewer_identity="rev1",
+            session_id="my-session",
+            mutation="merge",
+            live_head_sha=live,
+            pinned_head_sha=reviewed,
+        )
+        self.assertTrue(result["block"])
+        self.assertTrue(any("head" in r.lower() for r in result["reasons"]))
+
+
+class TestReviewerLeaseMcpGate(unittest.TestCase):
+    def setUp(self):
+        leases.clear_session_lease()
+        patch("mcp_server.verify_preflight_purity").start()
+        patch("gitea_audit.audit_enabled", return_value=False).start()
+        mcp_server = __import__("mcp_server")
+        mcp_server._IDENTITY_CACHE.clear()
+        mcp_server.init_review_decision_lock("prgs", "review_pr")
+        mcp_server.record_preflight_check("whoami")
+        mcp_server.record_preflight_check("capability", "reviewer")
+
+    def tearDown(self):
+        patch.stopall()
+        leases.clear_session_lease()
+
+    def test_reviewer_pr_lease_gate_helper_blocks_without_session(self):
+        import mcp_server
+        head = "a" * 40
+        with patch("mcp_server._fetch_pr_comments", return_value=[]):
+            reasons = mcp_server._reviewer_pr_lease_gate(
+                pr_number=382,
+                remote="prgs",
+                host=None,
+                org=None,
+                repo=None,
+                mutation="approve",
+                live_head_sha=head,
+                pinned_head_sha=head,
+            )
+        self.assertTrue(any("lease" in r.lower() for r in reasons))
+
+
+if __name__ == "__main__":
+    unittest.main()