fix(guard): derive cross-repository target base ref

Cross-repository mutation gating assumed the tracking base ref was
prgs/master, and parity reporting independently assumed origin/master.
A namespace bound to any other repository -- for example remote MDCPS on
integration branch dev -- could not prove base equivalence, so every
gated mutation failed closed with no reachable remedy. The two modules
also disagreed with each other, so at most one could be right for any
given repository.

Derive the target instead of assuming it. canonical_repository_root
already discovered the correct remote while resolving repository
identity and then discarded its name; it now returns that name with its
exact configured case preserved, and resolve_target_base_ref() builds
refs/remotes/<remote>/<branch> from it. The integration branch comes
from refs/remotes/<remote>/HEAD -- git's own record of the remote's
default branch -- so no new configuration field is required. Only when
a remote publishes no such default does it fall back to exactly one
present integration-branch candidate.

Resolution fails closed with a machine-checkable reason_code when
identity is unprovable, when distinct remotes claim different
repositories, when several candidate branches exist with no recorded
default, or when no candidate exists. It never invents a branch, writes
a ref, or falls back to another repository's base.

Both the mutation guard and the parity report now consume that one
resolved target, so they cannot disagree again. Root-checkout
contamination names the ref it actually compared rather than a literal
prgs/master the target repository may not have.

Fixes an observable defect in this repository: refs/remotes/origin/master
survives as an orphan ref from a removed remote, so parity reported the
target stale against a dead commit while reporting its identity as
underivable.

PRGS behaviour is unchanged -- prgs/master still resolves via the
recorded remote HEAD to the same SHA, and an explicit remote_refs
override keeps the historical probe path verbatim.

Tests: 24 new hermetic regression tests covering PRGS prgs/master,
MDCPS/dev, no origin remote, exact remote-name case, equal/behind/
divergent targets, missing remote or ref, ambiguous remote and branch
resolution, gate/report agreement, and every affected production
caller. Full suite 6191 passed / 28 failed, byte-identical failure set
to the baseline at 108cbfa (zero introduced failures).

Refs #983

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

anti_stomp_preflight.py

@@ -355,6 +355,10 @@ def assess_anti_stomp_preflight(
     root_head_sha: str | None = None,
     root_porcelain: str | None = None,
     remote_master_sha: str | None = None,
+    # #983: the tracking integration ref the SHA above came from, so root-checkout
+    # contamination names the ref actually compared instead of a hardcoded
+    # 'prgs/master'. None preserves the previous generic wording.
+    remote_master_ref: str | None = None,
     check_root_checkout: bool = True,
     check_worktree: bool = True,
     create_issue_bootstrap_assessment: dict[str, Any] | None = None,
@@ -553,6 +557,7 @@ def assess_anti_stomp_preflight(
             remote_master_sha=remote_master_sha,
             resolved_role=req_role or role,
             actual_role=role,
+            remote_master_ref=remote_master_ref,
         )
         checks["root_checkout"] = {
             "block": bool(root_assessment.get("block")),

canonical_repository_root.py

@@ -37,6 +37,23 @@ CANONICAL_ROOT_ENV = "GITEA_CANONICAL_REPOSITORY_ROOT"
 # Candidate git remote names probed when deriving repository identity.
 _IDENTITY_REMOTE_CANDIDATES = ("prgs", "origin", "dadeschools", "mdcps")
 
+# Fallback integration-branch names, probed only when the remote publishes no
+# ``refs/remotes/<remote>/HEAD`` symbolic ref (#983). Mirrors the stable base
+# branches recognised elsewhere in the workflow (``stacked_pr_support``). The
+# fallback is deliberately *not* ordered-first-wins: when more than one of these
+# refs exists and git records no default, the integration branch is genuinely
+# ambiguous and resolution fails closed instead of guessing.
+INTEGRATION_BRANCH_CANDIDATES: tuple[str, ...] = ("master", "main", "dev")
+
+# Reason codes for base-ref derivation outcomes (#983), so callers and tests can
+# assert the refusal cause instead of string-matching prose.
+BASE_REF_SOURCE_REMOTE_HEAD = "remote_head_symref"
+BASE_REF_SOURCE_UNIQUE_CANDIDATE = "unique_integration_branch_ref"
+DENY_NO_IDENTITY_REMOTE = "no_identity_remote"
+DENY_AMBIGUOUS_REMOTE = "ambiguous_identity_remote"
+DENY_AMBIGUOUS_BASE_BRANCH = "ambiguous_integration_branch"
+DENY_NO_BASE_BRANCH = "no_integration_branch_ref"
+
 # Repository-authority modes (#973 B10). Exactly two values are supported.
 # ``mode`` selects how repository authority is established, so an unrecognised
 # value must never be normalised onto one of these: aliasing a trusted mode is
@@ -120,17 +137,15 @@ def resolve_repo_toplevel(path: str) -> str | None:
     return os.path.realpath(top) if top else None
 
 
-def repository_identity_slug(path: str, *, remote: str | None = None) -> str | None:
-    """``owner/repository`` derived from a git remote configured at *path*.
+def _identity_remote_candidates(path: str, remote: str | None) -> list[str]:
+    """Ordered remote names to probe for identity at *path*.
 
-    Tries the caller-named remote first, then a small set of known remote names,
-    then whatever remote the repository actually has. Returns None when no remote
-    URL is parseable (identity cannot be proven).
+    Names are used verbatim — never case-folded. Git config subsection names are
+    case-sensitive, so a repository whose remote is ``MDCPS`` is reached only by
+    the exact string ``MDCPS``; the lowercase entry in
+    :data:`_IDENTITY_REMOTE_CANDIDATES` simply does not resolve, and the exact
+    name arrives from ``git remote`` below (#983).
     """
-    text = (path or "").strip()
-    if not text:
-        return None
-
     ordered: list[str] = []
     for name in (remote, *_IDENTITY_REMOTE_CANDIDATES):
         clean = (name or "").strip()
@@ -139,7 +154,7 @@ def repository_identity_slug(path: str, *, remote: str | None = None) -> str | N
 
     try:
         listed = subprocess.run(
-            ["git", "-C", text, "remote"],
+            ["git", "-C", path, "remote"],
             capture_output=True,
             text=True,
             check=True,
@@ -149,11 +164,20 @@ def repository_identity_slug(path: str, *, remote: str | None = None) -> str | N
     for name in listed:
         if name and name not in ordered:
             ordered.append(name)
+    return ordered
 
-    for name in ordered:
+
+def _configured_remote_identities(path: str, remote: str | None) -> list[tuple[str, str]]:
+    """``(remote_name, owner/repository)`` for every probe name that resolves.
+
+    Remote names are returned exactly as configured so downstream tracking refs
+    (``refs/remotes/<remote>/<branch>``) address the real ref (#983).
+    """
+    found: list[tuple[str, str]] = []
+    for name in _identity_remote_candidates(path, remote):
         try:
             url = subprocess.run(
-                ["git", "-C", text, "remote", "get-url", name],
+                ["git", "-C", path, "remote", "get-url", name],
                 capture_output=True,
                 text=True,
                 check=True,
@@ -162,8 +186,213 @@ def repository_identity_slug(path: str, *, remote: str | None = None) -> str | N
             continue
         parsed = remote_repo_guard.parse_org_repo_from_remote_url(url)
         if parsed:
-            return f"{parsed[0]}/{parsed[1]}"
-    return None
+            found.append((name, f"{parsed[0]}/{parsed[1]}"))
+    return found
+
+
+def resolve_identity_remote(
+    path: str, *, remote: str | None = None
+) -> tuple[str | None, str | None]:
+    """``(remote_name, owner/repository)`` for the remote that proves identity.
+
+    The remote *name* is the piece historically thrown away by
+    :func:`repository_identity_slug`, even though resolving the slug already
+    required discovering it. Cross-repository base-ref derivation needs that
+    name to build ``refs/remotes/<remote>/<branch>``, so it is now returned
+    rather than discarded (#983). Returns ``(None, None)`` when no remote URL is
+    parseable (identity cannot be proven).
+    """
+    text = (path or "").strip()
+    if not text:
+        return None, None
+    for name, slug in _configured_remote_identities(text, remote):
+        return name, slug
+    return None, None
+
+
+def repository_identity_slug(path: str, *, remote: str | None = None) -> str | None:
+    """``owner/repository`` derived from a git remote configured at *path*.
+
+    Tries the caller-named remote first, then a small set of known remote names,
+    then whatever remote the repository actually has. Returns None when no remote
+    URL is parseable (identity cannot be proven).
+    """
+    return resolve_identity_remote(path, remote=remote)[1]
+
+
+def _base_ref_result(
+    *,
+    proven: bool,
+    reasons: list[str],
+    remote: str | None = None,
+    branch: str | None = None,
+    repository_slug: str | None = None,
+    source: str | None = None,
+    reason_code: str | None = None,
+) -> dict:
+    """Build the base-ref derivation payload.
+
+    ``tracking_refs`` is the ordered probe tuple downstream guards hand to
+    ``git rev-parse``: the ``<remote>/<branch>`` shorthand first, then the fully
+    qualified ``refs/remotes/<remote>/<branch>``. It is empty whenever the
+    derivation is not ``proven``, so an unresolved target can never be probed
+    against some other repository's ref.
+    """
+    tracking_ref = f"refs/remotes/{remote}/{branch}" if proven and remote and branch else None
+    tracking_refs: tuple[str, ...] = (
+        (f"{remote}/{branch}", tracking_ref) if tracking_ref else ()
+    )
+    return {
+        "proven": proven,
+        "block": not proven,
+        "remote": remote,
+        "branch": branch,
+        "repository_slug": repository_slug,
+        "tracking_ref": tracking_ref,
+        "tracking_refs": tracking_refs,
+        "source": source,
+        "reason_code": reason_code,
+        "reasons": list(reasons),
+    }
+
+
+def _ref_exists(path: str, ref: str) -> bool:
+    """Whether *ref* resolves in the checkout at *path*."""
+    try:
+        res = subprocess.run(
+            ["git", "-C", path, "rev-parse", "--verify", "--quiet", ref],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+    except Exception:
+        return False
+    return res.returncode == 0 and bool((res.stdout or "").strip())
+
+
+def resolve_target_base_ref(path: str, *, remote: str | None = None) -> dict:
+    """Derive the authoritative integration base ref for the checkout at *path*.
+
+    This is the single resolved target shared by cross-repository mutation
+    gating and parity reporting, so the two can never disagree about which
+    commit a checkout is supposed to match (#983).
+
+    Resolution order:
+
+    1. The identity remote — the remote that already proves repository identity
+       via :func:`resolve_identity_remote`, with its **exact configured case**
+       preserved (``MDCPS`` stays ``MDCPS``).
+    2. The integration branch, from ``refs/remotes/<remote>/HEAD`` — git's own
+       record of that remote's default branch, written by ``clone`` and
+       ``remote set-head``. This is authoritative repository state, which is why
+       no new configuration field is required.
+    3. Only if the remote publishes no such symbolic ref, exactly one of
+       :data:`INTEGRATION_BRANCH_CANDIDATES` present as a remote-tracking ref.
+
+    Fails closed — ``proven`` False, empty ``tracking_refs``, and a
+    ``reason_code`` — when identity is unprovable, when distinct remotes claim
+    different repositories, when several candidate branches exist with no
+    recorded default, or when no candidate exists at all. Nothing here invents a
+    branch, writes a ref, or falls back to another repository's base.
+    """
+    text = (path or "").strip()
+    if not text:
+        return _base_ref_result(
+            proven=False,
+            reasons=["no repository path supplied for base-ref derivation (fail closed)"],
+            reason_code=DENY_NO_IDENTITY_REMOTE,
+        )
+
+    identities = _configured_remote_identities(text, remote)
+    if not identities:
+        return _base_ref_result(
+            proven=False,
+            reasons=[
+                f"no git remote at '{text}' yields a parseable repository identity, so "
+                "the integration base ref cannot be derived (fail closed)"
+            ],
+            reason_code=DENY_NO_IDENTITY_REMOTE,
+        )
+
+    # Ambiguity only matters when the caller named no remote: if distinct remotes
+    # describe different repositories there is no single authoritative target,
+    # and picking the first would silently gate against the wrong repository.
+    if not (remote or "").strip():
+        distinct = {slug for _, slug in identities}
+        if len(distinct) > 1:
+            listed = ", ".join(f"{name} -> {slug}" for name, slug in identities)
+            return _base_ref_result(
+                proven=False,
+                reasons=[
+                    f"ambiguous repository identity at '{text}': remotes resolve to "
+                    f"different repositories ({listed}); no single authoritative "
+                    "integration base ref can be derived (fail closed)"
+                ],
+                reason_code=DENY_AMBIGUOUS_REMOTE,
+            )
+
+    remote_name, slug = identities[0]
+
+    symref = None
+    try:
+        res = subprocess.run(
+            ["git", "-C", text, "symbolic-ref", "--quiet", f"refs/remotes/{remote_name}/HEAD"],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+        if res.returncode == 0:
+            symref = (res.stdout or "").strip() or None
+    except Exception:
+        symref = None
+
+    prefix = f"refs/remotes/{remote_name}/"
+    if symref and symref.startswith(prefix):
+        branch = symref[len(prefix):].strip()
+        if branch and branch != "HEAD":
+            return _base_ref_result(
+                proven=True,
+                reasons=[],
+                remote=remote_name,
+                branch=branch,
+                repository_slug=slug,
+                source=BASE_REF_SOURCE_REMOTE_HEAD,
+            )
+
+    present = [
+        candidate
+        for candidate in INTEGRATION_BRANCH_CANDIDATES
+        if _ref_exists(text, f"{prefix}{candidate}")
+    ]
+    if len(present) == 1:
+        return _base_ref_result(
+            proven=True,
+            reasons=[],
+            remote=remote_name,
+            branch=present[0],
+            repository_slug=slug,
+            source=BASE_REF_SOURCE_UNIQUE_CANDIDATE,
+        )
+    if len(present) > 1:
+        return _base_ref_result(
+            proven=False,
+            reasons=[
+                f"remote '{remote_name}' publishes no '{prefix}HEAD' default and "
+                f"several integration branches exist ({', '.join(present)}); the "
+                "integration base ref is ambiguous (fail closed)"
+            ],
+            reason_code=DENY_AMBIGUOUS_BASE_BRANCH,
+        )
+    return _base_ref_result(
+        proven=False,
+        reasons=[
+            f"remote '{remote_name}' publishes no '{prefix}HEAD' default and none of "
+            f"{'/'.join(INTEGRATION_BRANCH_CANDIDATES)} exists as a remote-tracking "
+            f"ref under '{prefix}'; the integration base ref cannot be derived "
+            "(fail closed; no fetch is performed here)"
+        ],
+        reason_code=DENY_NO_BASE_BRANCH,
+    )
 
 
 def assess_canonical_repository_root(

gitea_mcp_server.py

@@ -1052,9 +1052,16 @@ def _create_issue_bootstrap_assessment(
         git_state = issue_lock_worktree.read_worktree_git_state(workspace)
         remote_master_sha_error: str | None = None
         try:
-            remote_master_sha = root_checkout_guard.resolve_remote_master_sha(
+            # #983: consume the resolved-target state so an *underivable* base ref
+            # reaches the assessor as named missing evidence rather than a bare
+            # None, which the bootstrap would otherwise report only as "live
+            # master tip is unknown" with no cause.
+            _base_state = root_checkout_guard.resolve_remote_master_ref_state(
                 ctx["canonical_repo_root"]
             )
+            remote_master_sha = _base_state["sha"]
+            if not remote_master_sha and _base_state.get("reasons"):
+                remote_master_sha_error = "; ".join(_base_state["reasons"])
         except Exception as exc:
             remote_master_sha = None
             remote_master_sha_error = (
@@ -1082,9 +1089,14 @@ def _create_issue_bootstrap_assessment(
     # closed instead of proceeding without base-equivalence proof.
     remote_master_sha_error: str | None = None
     try:
-        remote_master_sha = root_checkout_guard.resolve_remote_master_sha(
+        # #983: same resolved-target consumption as the author bootstrap above —
+        # a target whose base ref cannot be derived must say why.
+        _base_state = root_checkout_guard.resolve_remote_master_ref_state(
             ctx["canonical_repo_root"]
         )
+        remote_master_sha = _base_state["sha"]
+        if not remote_master_sha and _base_state.get("reasons"):
+            remote_master_sha_error = "; ".join(_base_state["reasons"])
     except Exception as exc:
         remote_master_sha = None
         remote_master_sha_error = f"{type(exc).__name__}: {exc}".strip() or "resolver failed"
@@ -1303,7 +1315,12 @@ def _run_anti_stomp_preflight(
     workspace = ctx["workspace_path"]
     canonical_root = ctx["canonical_repo_root"]
     git_state = issue_lock_worktree.read_worktree_git_state(canonical_root)
-    remote_master_sha = root_checkout_guard.resolve_remote_master_sha(canonical_root)
+    # #983: derive the target base ref for this repository and carry the ref
+    # itself alongside the SHA, so the anti-stomp root-checkout check reports the
+    # ref it actually compared.
+    _root_base_state = root_checkout_guard.resolve_remote_master_ref_state(canonical_root)
+    remote_master_sha = _root_base_state["sha"]
+    remote_master_ref = _root_base_state.get("ref")
 
     # Repo/org facts (best-effort; explicit org/repo when provided).
     resolved_org = org
@@ -1432,6 +1449,7 @@ def _run_anti_stomp_preflight(
         root_head_sha=git_state.get("head_sha"),
         root_porcelain=git_state.get("porcelain_status") or "",
         remote_master_sha=remote_master_sha,
+        remote_master_ref=remote_master_ref,
         startup_head=startup_head,
         current_code_head=current_code_head,
         lease_required=lease_required,
@@ -1987,7 +2005,11 @@ def _enforce_root_checkout_guard(worktree_path: str | None = None) -> None:
     canonical_root = ctx["canonical_repo_root"]
     workspace = ctx["workspace_path"]
     git_state = issue_lock_worktree.read_worktree_git_state(canonical_root)
-    remote_master_sha = root_checkout_guard.resolve_remote_master_sha(canonical_root)
+    # #983: consume the resolved target so the contamination message names the
+    # ref that was actually compared (refs/remotes/<remote>/<branch>) instead of
+    # a hardcoded 'prgs/master' the target repository may not have.
+    base_state = root_checkout_guard.resolve_remote_master_ref_state(canonical_root)
+    remote_master_sha = base_state["sha"]
     assessment = root_checkout_guard.assess_root_checkout_guard(
         workspace_path=workspace,
         canonical_repo_root=canonical_root,
@@ -1997,6 +2019,7 @@ def _enforce_root_checkout_guard(worktree_path: str | None = None) -> None:
         remote_master_sha=remote_master_sha,
         resolved_role=_preflight_resolved_role,
         actual_role=_actual_profile_role(),
+        remote_master_ref=base_state.get("ref"),
     )
     if assessment["block"]:
         raise RuntimeError(root_checkout_guard.format_root_checkout_guard_error(assessment))

master_parity_gate.py

@@ -378,6 +378,11 @@ def format_parity(assessment: dict) -> str:
 # feeds the mutation gate and never changes startup_head/current_head.
 # ---------------------------------------------------------------------------
 
+# Legacy hardcoded target tracking ref. Retained for callers that still pass an
+# explicit ref, but no longer the default: assuming a remote named ``origin``
+# read an unrelated (often orphaned) remote-tracking ref in any checkout whose
+# remote is named something else, and reported the target stale against a commit
+# from a remote that may no longer even be configured (#983).
 DEFAULT_TARGET_TRACKING_REF = "refs/remotes/origin/master"
 
 
@@ -408,7 +413,7 @@ def assess_target_repository_parity(
     *,
     canonical_root: str | None,
     source: str | None,
-    tracking_ref: str = DEFAULT_TARGET_TRACKING_REF,
+    tracking_ref: str | None = None,
 ) -> dict:
     """Assess the configured cross-repository target checkout.
 
@@ -421,6 +426,11 @@ def assess_target_repository_parity(
     An unconfigured namespace is ``configured=False`` and never ``stale`` — the
     single-repository default has no second dimension to be stale about. A
     configured root that cannot be read is ``determinable=False`` with reasons.
+
+    *tracking_ref* defaults to None, meaning **derive the target from the
+    repository itself** through the same resolver the mutation guard uses, so
+    gating and reporting can never disagree about which ref is authoritative
+    (#983). Passing an explicit ref preserves the previous behaviour verbatim.
     """
     result = {
         "configured": bool(canonical_root),
@@ -429,6 +439,9 @@ def assess_target_repository_parity(
         "repository_slug": None,
         "checkout_head": None,
         "tracking_ref": tracking_ref,
+        "base_remote": None,
+        "base_branch": None,
+        "tracking_ref_source": "explicit_tracking_ref" if tracking_ref else None,
         "remote_tracking_head": None,
         "determinable": False,
         "stale": False,
@@ -463,19 +476,35 @@ def assess_target_repository_parity(
     result["checkout_head"] = head
     result["determinable"] = True
 
-    remote_url = _git_capture(canonical_root, "remote", "get-url", "origin")
-    if remote_url:
-        # Local import keeps this module dependency-light for its startup role.
-        import remote_repo_guard
+    # Local import keeps this module dependency-light for its startup role.
+    import canonical_repository_root as _crr
 
-        parsed = remote_repo_guard.parse_org_repo_from_remote_url(remote_url)
-        if parsed:
-            result["repository_slug"] = f"{parsed[0]}/{parsed[1]}"
-    if not result["repository_slug"]:
+    # #983: identity comes from whichever remote actually proves it, not from a
+    # remote assumed to be named 'origin'. In a checkout whose only remote is
+    # 'prgs', the old lookup failed outright and reported the identity as
+    # underivable while a leftover refs/remotes/origin/master still resolved.
+    #
+    # Identity is resolved independently of the base ref: a target that has never
+    # been fetched still has a provable repository identity, and reporting it as
+    # unidentifiable would lose real information over an unrelated missing ref.
+    identity_remote, slug = _crr.resolve_identity_remote(canonical_root)
+    result["repository_slug"] = slug
+    if not slug:
         result["reasons"].append(
             "target repository identity could not be derived from its git remote"
         )
 
+    if not tracking_ref:
+        base = _crr.resolve_target_base_ref(canonical_root, remote=identity_remote)
+        if not base.get("proven"):
+            result["reasons"].extend(base.get("reasons") or [])
+            return result
+        tracking_ref = base["tracking_ref"]
+        result["tracking_ref"] = tracking_ref
+        result["tracking_ref_source"] = base.get("source")
+        result["base_remote"] = base.get("remote")
+        result["base_branch"] = base.get("branch")
+
     tracking_head = _git_capture(canonical_root, "rev-parse", tracking_ref)
     if not tracking_head:
         result["reasons"].append(

root_checkout_guard.py

@@ -1,9 +1,16 @@
 """Root checkout guard (#475).
 
-The project root checkout is the stable control checkout on master/prgs/master.
-Author/reviewer/merge flows must fail closed when the control checkout is
-contaminated (wrong branch, detached HEAD, dirty, or HEAD behind/ahead of
-prgs/master). Isolated ``branches/...`` worktrees remain allowed.
+The project root checkout is the stable control checkout on its integration
+branch. Author/reviewer/merge flows must fail closed when the control checkout
+is contaminated (wrong branch, detached HEAD, dirty, or HEAD behind/ahead of the
+tracking integration ref). Isolated ``branches/...`` worktrees remain allowed.
+
+The tracking ref is *derived per repository* (#983) rather than assumed to be
+``prgs/master``: a namespace bound to another repository — say remote ``MDCPS``
+on branch ``dev`` — is gated against ``refs/remotes/MDCPS/dev``. Derivation is
+delegated to :mod:`canonical_repository_root`, the authoritative
+repository/context resolver, so gating and parity reporting share one resolved
+target instead of maintaining two disagreeing hardcoded defaults.
 """
 
 from __future__ import annotations
@@ -11,6 +18,7 @@ from __future__ import annotations
 import os
 import subprocess
 
+import canonical_repository_root
 from author_mutation_worktree import is_path_under_branches
 from reviewer_worktree import parse_dirty_tracked_files
 
@@ -20,19 +28,57 @@ REMEDIATION = (
 )
 
 BASE_BRANCHES = frozenset({"master", "main", "dev"})
+
+# Legacy PRGS-specific probe order. Retained only for callers that pass an
+# explicit ``remote_refs`` override; it is no longer the silent default, because
+# inheriting it in a non-PRGS checkout compared that checkout against a ref it
+# can never have (#983).
 REMOTE_MASTER_REFS = ("prgs/master", "refs/remotes/prgs/master")
 
 
+def _derive_probe_refs(root: str, remote: str | None) -> dict:
+    """Derive the ordered tracking refs to probe for *root*.
+
+    Returns the derivation payload from :mod:`canonical_repository_root` plus a
+    ``refs`` tuple, which is empty when the target is not provable.
+    """
+    derived = canonical_repository_root.resolve_target_base_ref(root, remote=remote)
+    return {
+        "refs": tuple(derived.get("tracking_refs") or ()),
+        "remote": derived.get("remote"),
+        "branch": derived.get("branch"),
+        "source": derived.get("source"),
+        "proven": bool(derived.get("proven")),
+        "reason_code": derived.get("reason_code"),
+        "reasons": list(derived.get("reasons") or []),
+    }
+
+
 def resolve_remote_master_sha(
     canonical_repo_root: str,
     *,
     remote_refs: tuple[str, ...] | None = None,
+    remote: str | None = None,
 ) -> str | None:
-    """Return the commit SHA for the tracking master ref when available."""
+    """Return the commit SHA for the tracking integration ref when available.
+
+    This remains the single place that turns a ref into a SHA. Returns None when
+    the target cannot be derived or resolved — exactly what this function already
+    returned when ``rev-parse`` failed. Callers that must fail closed on missing
+    evidence (the #749/#757 bootstrap path) surface that None as *missing
+    evidence*, never as "no constraint".
+    """
     root = (canonical_repo_root or "").strip()
     if not root:
         return None
-    for ref in remote_refs or REMOTE_MASTER_REFS:
+    if remote_refs:
+        probe: tuple[str, ...] = tuple(remote_refs)
+    else:
+        derived = _derive_probe_refs(root, remote)
+        if not derived["proven"]:
+            return None
+        probe = derived["refs"]
+    for ref in probe:
         res = subprocess.run(
             ["git", "-C", root, "rev-parse", "--verify", ref],
             capture_output=True,
@@ -46,6 +92,80 @@ def resolve_remote_master_sha(
     return None
 
 
+def resolve_remote_master_ref_state(
+    canonical_repo_root: str,
+    *,
+    remote_refs: tuple[str, ...] | None = None,
+    remote: str | None = None,
+) -> dict:
+    """Resolve the tracking integration ref together with its commit SHA.
+
+    Returns ``sha``, the ``ref`` it came from, the derived ``remote`` /
+    ``branch``, a machine-checkable ``reason_code``, and ``reasons``. ``sha`` is
+    None whenever the target cannot be resolved — never a fallback to some other
+    repository's commit.
+
+    The SHA itself is obtained through :func:`resolve_remote_master_sha` rather
+    than by re-probing here, so one public function stays authoritative for
+    ref-to-SHA resolution. An explicit *remote_refs* keeps the historical
+    behaviour exactly: those refs are probed in order and no derivation happens.
+    """
+    root = (canonical_repo_root or "").strip()
+    state: dict = {
+        "sha": None,
+        "ref": None,
+        "remote": None,
+        "branch": None,
+        "source": None,
+        "reason_code": None,
+        "reasons": [],
+    }
+    if not root:
+        state["reasons"].append("no canonical repository root supplied (fail closed)")
+        return state
+
+    if remote_refs:
+        probe: tuple[str, ...] = tuple(remote_refs)
+        state["source"] = "explicit_remote_refs"
+    else:
+        derived = _derive_probe_refs(root, remote)
+        state["remote"] = derived["remote"]
+        state["branch"] = derived["branch"]
+        state["source"] = derived["source"]
+        if not derived["proven"]:
+            state["reason_code"] = derived["reason_code"]
+            state["reasons"] = derived["reasons"]
+            return state
+        probe = derived["refs"]
+
+    sha = resolve_remote_master_sha(root, remote_refs=probe, remote=remote)
+    if not sha:
+        state["reasons"].append(
+            "tracking integration ref "
+            f"{' / '.join(probe) if probe else '(none derived)'} does not resolve in "
+            f"'{root}' (fail closed)"
+        )
+        return state
+
+    state["sha"] = sha
+    # Name the ref that actually carries this commit. When the SHA comes from a
+    # test double no probe will match, so fall back to the first derived ref,
+    # which is the one the guard is conceptually comparing against.
+    for ref in probe:
+        res = subprocess.run(
+            ["git", "-C", root, "rev-parse", "--verify", ref],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+        if res.returncode == 0 and (res.stdout or "").strip() == sha:
+            state["ref"] = ref
+            break
+    else:
+        state["ref"] = probe[0] if probe else None
+    return state
+
+
 resolve_tracking_master_sha = resolve_remote_master_sha
 
 
@@ -59,8 +179,9 @@ def assess_root_checkout_guard(
     remote_master_sha: str | None,
     resolved_role: str | None = None,
     actual_role: str | None = None,
+    remote_master_ref: str | None = None,
 ) -> dict:
-    """Fail closed when the control checkout is not clean master/prgs/master.
+    """Fail closed when the control checkout is not clean on its integration ref.
 
     ``resolved_role`` is the preflight-resolved *task* role and ``actual_role``
     is the *active profile* role (#540). The reconciler exemption honours either
@@ -102,9 +223,13 @@ def assess_root_checkout_guard(
         )
 
     if remote_master_sha and head_sha and head_sha != remote_master_sha:
+        # #983: name the ref that was actually compared. Reporting a literal
+        # 'prgs/master' in a checkout gated against refs/remotes/MDCPS/dev sends
+        # the operator to inspect a ref that repository does not have.
+        ref_label = (remote_master_ref or "").strip() or "the tracking integration ref"
         reasons.append(
-            "control checkout HEAD does not match prgs/master "
-            f"(HEAD {head_sha[:12]}, prgs/master {remote_master_sha[:12]})"
+            f"control checkout HEAD does not match {ref_label} "
+            f"(HEAD {head_sha[:12]}, {ref_label} {remote_master_sha[:12]})"
         )
 
     proven = not reasons

tests/test_issue_983_cross_repo_base_ref.py

@@ -0,0 +1,482 @@
+"""Regression tests for Issue #983: derived target base ref for cross-repository checkouts.
+
+The mutation guard previously assumed ``prgs/master`` and the parity report
+assumed ``origin/master``. Any repository using neither — for example remote
+``MDCPS`` on integration branch ``dev`` — could not prove base equivalence, so
+every gated mutation failed closed with no reachable remedy.
+
+These tests build hermetic git repositories on disk (no network, no fetch) and
+assert the derived target end to end: identity remote, integration branch,
+tracking ref, fail-closed refusals, and agreement between the mutation guard and
+the parity report.
+"""
+
+from __future__ import annotations
+
+import inspect
+import os
+import subprocess
+import tempfile
+import unittest
+
+import anti_stomp_preflight
+import canonical_repository_root as crr
+import master_parity_gate
+import root_checkout_guard
+
+
+def _git(root: str, *args: str) -> str:
+    res = subprocess.run(
+        ["git", "-C", root, *args],
+        capture_output=True,
+        text=True,
+        check=True,
+    )
+    return (res.stdout or "").strip()
+
+
+def _make_repo(root: str, *, remote: str | None, url: str | None) -> str:
+    """Initialise a repository with one commit and an optional named remote."""
+    os.makedirs(root, exist_ok=True)
+    _git(root, "init", "--quiet")
+    _git(root, "config", "user.email", "[email protected]")
+    _git(root, "config", "user.name", "Issue983 Test")
+    _git(root, "config", "commit.gpgsign", "false")
+    with open(os.path.join(root, "seed.txt"), "w", encoding="utf-8") as fh:
+        fh.write("seed\n")
+    _git(root, "add", "seed.txt")
+    _git(root, "commit", "--quiet", "-m", "seed")
+    if remote and url:
+        _git(root, "remote", "add", remote, url)
+    return _git(root, "rev-parse", "HEAD")
+
+
+def _set_remote_branch(root: str, remote: str, branch: str, sha: str) -> None:
+    """Create refs/remotes/<remote>/<branch> without contacting a network."""
+    _git(root, "update-ref", f"refs/remotes/{remote}/{branch}", sha)
+
+
+def _set_remote_head(root: str, remote: str, branch: str) -> None:
+    _git(
+        root,
+        "symbolic-ref",
+        f"refs/remotes/{remote}/HEAD",
+        f"refs/remotes/{remote}/{branch}",
+    )
+
+
+def _advance(root: str, message: str) -> str:
+    with open(os.path.join(root, "seed.txt"), "a", encoding="utf-8") as fh:
+        fh.write(message + "\n")
+    _git(root, "add", "seed.txt")
+    _git(root, "commit", "--quiet", "-m", message)
+    return _git(root, "rev-parse", "HEAD")
+
+
+class _RepoCase(unittest.TestCase):
+    def setUp(self) -> None:
+        self._tmp = tempfile.TemporaryDirectory()
+        self.addCleanup(self._tmp.cleanup)
+        self.root = os.path.join(self._tmp.name, "repo")
+
+
+class TestPrgsBehaviourPreserved(_RepoCase):
+    """AC1: existing PRGS behaviour using prgs/master is unchanged."""
+
+    def test_prgs_master_resolves_unchanged(self):
+        head = _make_repo(
+            self.root,
+            remote="prgs",
+            url="https://gitea.prgs.cc/Scaled-Tech-Consulting/Gitea-Tools.git",
+        )
+        _set_remote_branch(self.root, "prgs", "master", head)
+        _set_remote_head(self.root, "prgs", "master")
+
+        got = crr.resolve_target_base_ref(self.root)
+        self.assertTrue(got["proven"], got["reasons"])
+        self.assertEqual(got["remote"], "prgs")
+        self.assertEqual(got["branch"], "master")
+        self.assertEqual(got["tracking_ref"], "refs/remotes/prgs/master")
+        self.assertEqual(got["repository_slug"], "Scaled-Tech-Consulting/Gitea-Tools")
+        self.assertEqual(root_checkout_guard.resolve_remote_master_sha(self.root), head)
+
+    def test_legacy_explicit_remote_refs_path_is_untouched(self):
+        """An explicit remote_refs override still short-circuits derivation."""
+        head = _make_repo(
+            self.root,
+            remote="prgs",
+            url="https://gitea.prgs.cc/Scaled-Tech-Consulting/Gitea-Tools.git",
+        )
+        _set_remote_branch(self.root, "prgs", "master", head)
+
+        state = root_checkout_guard.resolve_remote_master_ref_state(
+            self.root, remote_refs=root_checkout_guard.REMOTE_MASTER_REFS
+        )
+        self.assertEqual(state["sha"], head)
+        self.assertEqual(state["source"], "explicit_remote_refs")
+
+
+class TestCrossRepositoryTarget(_RepoCase):
+    """AC2/AC3/AC4: MDCPS/dev, no origin remote, exact remote-name case."""
+
+    def _mdcps(self) -> str:
+        head = _make_repo(
+            self.root,
+            remote="MDCPS",
+            url="https://gitea.example.net/MDCPS/WeeklyBriefings-Meta.git",
+        )
+        _set_remote_branch(self.root, "MDCPS", "dev", head)
+        _set_remote_head(self.root, "MDCPS", "dev")
+        return head
+
+    def test_mdcps_dev_resolves(self):
+        head = self._mdcps()
+        got = crr.resolve_target_base_ref(self.root)
+        self.assertTrue(got["proven"], got["reasons"])
+        self.assertEqual(got["remote"], "MDCPS")
+        self.assertEqual(got["branch"], "dev")
+        self.assertEqual(got["tracking_ref"], "refs/remotes/MDCPS/dev")
+        self.assertEqual(got["repository_slug"], "MDCPS/WeeklyBriefings-Meta")
+        self.assertEqual(root_checkout_guard.resolve_remote_master_sha(self.root), head)
+
+    def test_no_remote_named_origin(self):
+        self._mdcps()
+        self.assertEqual(_git(self.root, "remote"), "MDCPS")
+        got = crr.resolve_target_base_ref(self.root)
+        self.assertTrue(got["proven"], got["reasons"])
+        self.assertNotIn("origin", got["tracking_ref"])
+
+    def test_remote_name_case_is_preserved_exactly(self):
+        self._mdcps()
+        got = crr.resolve_target_base_ref(self.root)
+        self.assertEqual(got["remote"], "MDCPS")
+        self.assertNotEqual(got["remote"], "mdcps")
+        # The tracking ref must address the real ref, which is case-sensitive.
+        self.assertEqual(got["tracking_ref"], "refs/remotes/MDCPS/dev")
+        self.assertTrue(
+            _git(self.root, "rev-parse", "--verify", got["tracking_ref"]),
+            "case-preserved tracking ref must resolve",
+        )
+
+    def test_lowercase_candidate_never_supplies_the_remote_name(self):
+        """Guards against silently case-folding MDCPS to the candidate 'mdcps'.
+
+        ``_IDENTITY_REMOTE_CANDIDATES`` contains a lowercase ``mdcps`` entry and
+        is probed *before* the repository's own remote listing. Git remote names
+        live in case-sensitive config subsections on every platform, so the
+        lowercase probe cannot resolve and the exact-case name must arrive from
+        ``git remote``. Asserted through config rather than ref lookup because a
+        case-insensitive filesystem (macOS) resolves loose refs either way, which
+        would make a ref-based assertion test the filesystem instead of the code.
+        """
+        self._mdcps()
+        res = subprocess.run(
+            ["git", "-C", self.root, "remote", "get-url", "mdcps"],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+        self.assertNotEqual(res.returncode, 0, "git remote names are case-sensitive")
+
+        # Even when the caller *hints* the wrong case, the resolved name is exact.
+        got = crr.resolve_target_base_ref(self.root, remote="mdcps")
+        self.assertTrue(got["proven"], got["reasons"])
+        self.assertEqual(got["remote"], "MDCPS")
+        self.assertEqual(got["tracking_ref"], "refs/remotes/MDCPS/dev")
+
+        name, slug = crr.resolve_identity_remote(self.root)
+        self.assertEqual(name, "MDCPS")
+        self.assertEqual(slug, "MDCPS/WeeklyBriefings-Meta")
+
+
+class TestTargetStaleness(_RepoCase):
+    """AC5/AC6: local equal to, behind, or divergent from the resolved tip."""
+
+    def _repo_with_tip(self) -> tuple[str, str]:
+        head = _make_repo(
+            self.root,
+            remote="MDCPS",
+            url="https://gitea.example.net/MDCPS/WeeklyBriefings-Meta.git",
+        )
+        _set_remote_branch(self.root, "MDCPS", "dev", head)
+        _set_remote_head(self.root, "MDCPS", "dev")
+        return head, self.root
+
+    def test_local_equal_to_resolved_tip_is_not_stale(self):
+        self._repo_with_tip()
+        got = master_parity_gate.assess_target_repository_parity(
+            canonical_root=self.root, source="test"
+        )
+        self.assertTrue(got["determinable"])
+        self.assertFalse(got["stale"])
+        self.assertEqual(got["tracking_ref"], "refs/remotes/MDCPS/dev")
+        self.assertEqual(got["base_remote"], "MDCPS")
+        self.assertEqual(got["base_branch"], "dev")
+
+    def test_local_behind_resolved_tip_is_stale(self):
+        head, _ = self._repo_with_tip()
+        advanced = _advance(self.root, "remote moved on")
+        _set_remote_branch(self.root, "MDCPS", "dev", advanced)
+        _git(self.root, "reset", "--hard", "--quiet", head)
+
+        got = master_parity_gate.assess_target_repository_parity(
+            canonical_root=self.root, source="test"
+        )
+        self.assertTrue(got["determinable"])
+        self.assertTrue(got["stale"])
+        self.assertEqual(got["checkout_head"], head)
+        self.assertEqual(got["remote_tracking_head"], advanced)
+
+    def test_local_divergent_from_resolved_tip_is_stale(self):
+        head, _ = self._repo_with_tip()
+        remote_side = _advance(self.root, "remote side")
+        _set_remote_branch(self.root, "MDCPS", "dev", remote_side)
+        _git(self.root, "reset", "--hard", "--quiet", head)
+        local_side = _advance(self.root, "local side")
+
+        got = master_parity_gate.assess_target_repository_parity(
+            canonical_root=self.root, source="test"
+        )
+        self.assertTrue(got["stale"])
+        self.assertEqual(got["checkout_head"], local_side)
+        self.assertNotEqual(local_side, remote_side)
+
+
+class TestFailClosed(_RepoCase):
+    """AC7/AC8: missing remote/ref and ambiguous resolution never guess."""
+
+    def test_missing_remote_fails_closed(self):
+        _make_repo(self.root, remote=None, url=None)
+        got = crr.resolve_target_base_ref(self.root)
+        self.assertFalse(got["proven"])
+        self.assertEqual(got["reason_code"], crr.DENY_NO_IDENTITY_REMOTE)
+        self.assertEqual(got["tracking_refs"], ())
+        self.assertIsNone(root_checkout_guard.resolve_remote_master_sha(self.root))
+
+    def test_missing_tracking_ref_fails_closed(self):
+        _make_repo(
+            self.root,
+            remote="MDCPS",
+            url="https://gitea.example.net/MDCPS/WeeklyBriefings-Meta.git",
+        )
+        # Remote configured, but nothing has ever been fetched.
+        got = crr.resolve_target_base_ref(self.root)
+        self.assertFalse(got["proven"])
+        self.assertEqual(got["reason_code"], crr.DENY_NO_BASE_BRANCH)
+        self.assertIsNone(root_checkout_guard.resolve_remote_master_sha(self.root))
+
+    def test_ambiguous_integration_branch_fails_closed(self):
+        head = _make_repo(
+            self.root,
+            remote="MDCPS",
+            url="https://gitea.example.net/MDCPS/WeeklyBriefings-Meta.git",
+        )
+        # Two candidate integration branches and no recorded remote default.
+        _set_remote_branch(self.root, "MDCPS", "dev", head)
+        _set_remote_branch(self.root, "MDCPS", "main", head)
+
+        got = crr.resolve_target_base_ref(self.root)
+        self.assertFalse(got["proven"])
+        self.assertEqual(got["reason_code"], crr.DENY_AMBIGUOUS_BASE_BRANCH)
+        self.assertIsNone(root_checkout_guard.resolve_remote_master_sha(self.root))
+
+    def test_recorded_remote_head_resolves_otherwise_ambiguous_branches(self):
+        """Ambiguity is refused only when git records no default."""
+        head = _make_repo(
+            self.root,
+            remote="MDCPS",
+            url="https://gitea.example.net/MDCPS/WeeklyBriefings-Meta.git",
+        )
+        _set_remote_branch(self.root, "MDCPS", "dev", head)
+        _set_remote_branch(self.root, "MDCPS", "main", head)
+        _set_remote_head(self.root, "MDCPS", "dev")
+
+        got = crr.resolve_target_base_ref(self.root)
+        self.assertTrue(got["proven"], got["reasons"])
+        self.assertEqual(got["branch"], "dev")
+        self.assertEqual(got["source"], crr.BASE_REF_SOURCE_REMOTE_HEAD)
+
+    def test_ambiguous_identity_remote_fails_closed(self):
+        head = _make_repo(
+            self.root,
+            remote="MDCPS",
+            url="https://gitea.example.net/MDCPS/WeeklyBriefings-Meta.git",
+        )
+        _git(
+            self.root,
+            "remote",
+            "add",
+            "prgs",
+            "https://gitea.prgs.cc/Scaled-Tech-Consulting/Gitea-Tools.git",
+        )
+        _set_remote_branch(self.root, "MDCPS", "dev", head)
+        _set_remote_branch(self.root, "prgs", "master", head)
+
+        got = crr.resolve_target_base_ref(self.root)
+        self.assertFalse(got["proven"])
+        self.assertEqual(got["reason_code"], crr.DENY_AMBIGUOUS_REMOTE)
+        self.assertEqual(got["tracking_refs"], ())
+
+    def test_named_remote_disambiguates(self):
+        """An explicitly named remote is authoritative and not ambiguous."""
+        head = _make_repo(
+            self.root,
+            remote="MDCPS",
+            url="https://gitea.example.net/MDCPS/WeeklyBriefings-Meta.git",
+        )
+        _git(
+            self.root,
+            "remote",
+            "add",
+            "prgs",
+            "https://gitea.prgs.cc/Scaled-Tech-Consulting/Gitea-Tools.git",
+        )
+        _set_remote_branch(self.root, "MDCPS", "dev", head)
+        _set_remote_branch(self.root, "prgs", "master", head)
+
+        got = crr.resolve_target_base_ref(self.root, remote="MDCPS")
+        self.assertTrue(got["proven"], got["reasons"])
+        self.assertEqual(got["remote"], "MDCPS")
+        self.assertEqual(got["branch"], "dev")
+
+    def test_orphan_tracking_ref_from_removed_remote_is_ignored(self):
+        """The live Gitea-Tools symptom: refs/remotes/origin/* outlives its remote."""
+        head = _make_repo(
+            self.root,
+            remote="prgs",
+            url="https://gitea.prgs.cc/Scaled-Tech-Consulting/Gitea-Tools.git",
+        )
+        _set_remote_branch(self.root, "prgs", "master", head)
+        _set_remote_head(self.root, "prgs", "master")
+        # An abandoned ref left behind by a remote that no longer exists.
+        _set_remote_branch(self.root, "origin", "master", head)
+        _advance(self.root, "orphan must not be consulted")
+
+        got = master_parity_gate.assess_target_repository_parity(
+            canonical_root=self.root, source="test"
+        )
+        self.assertEqual(got["tracking_ref"], "refs/remotes/prgs/master")
+        self.assertEqual(got["repository_slug"], "Scaled-Tech-Consulting/Gitea-Tools")
+        self.assertNotIn(
+            "target repository identity could not be derived from its git remote",
+            got["reasons"],
+        )
+
+
+class TestGatingAndReportingAgree(_RepoCase):
+    """AC9: mutation gating and parity reporting resolve the same target."""
+
+    def test_same_resolved_target_for_gate_and_report(self):
+        head = _make_repo(
+            self.root,
+            remote="MDCPS",
+            url="https://gitea.example.net/MDCPS/WeeklyBriefings-Meta.git",
+        )
+        _set_remote_branch(self.root, "MDCPS", "dev", head)
+        _set_remote_head(self.root, "MDCPS", "dev")
+
+        gate = root_checkout_guard.resolve_remote_master_ref_state(self.root)
+        report = master_parity_gate.assess_target_repository_parity(
+            canonical_root=self.root, source="test"
+        )
+
+        self.assertEqual(gate["remote"], report["base_remote"])
+        self.assertEqual(gate["branch"], report["base_branch"])
+        self.assertEqual(gate["sha"], report["remote_tracking_head"])
+        self.assertIn(
+            gate["ref"],
+            (report["tracking_ref"], f"{gate['remote']}/{gate['branch']}"),
+        )
+
+    def test_both_sides_refuse_the_same_unresolvable_target(self):
+        _make_repo(self.root, remote=None, url=None)
+
+        self.assertIsNone(root_checkout_guard.resolve_remote_master_sha(self.root))
+        report = master_parity_gate.assess_target_repository_parity(
+            canonical_root=self.root, source="test"
+        )
+        self.assertIsNone(report["remote_tracking_head"])
+        self.assertFalse(report["stale"])
+        self.assertTrue(report["reasons"])
+
+
+class TestProductionCallers(unittest.TestCase):
+    """AC10: every affected production caller supplies/consumes the resolved target."""
+
+    def test_guard_reports_the_ref_it_actually_compared(self):
+        assessment = root_checkout_guard.assess_root_checkout_guard(
+            workspace_path="/tmp/nonexistent-workspace-983",
+            canonical_repo_root="/tmp/nonexistent-root-983",
+            current_branch="dev",
+            head_sha="a" * 40,
+            porcelain_status="",
+            remote_master_sha="b" * 40,
+            remote_master_ref="refs/remotes/MDCPS/dev",
+        )
+        self.assertTrue(assessment["block"])
+        joined = " ".join(assessment["reasons"])
+        self.assertIn("refs/remotes/MDCPS/dev", joined)
+        self.assertNotIn("prgs/master", joined)
+
+    def test_guard_message_without_a_ref_stays_generic(self):
+        assessment = root_checkout_guard.assess_root_checkout_guard(
+            workspace_path="/tmp/nonexistent-workspace-983",
+            canonical_repo_root="/tmp/nonexistent-root-983",
+            current_branch="master",
+            head_sha="a" * 40,
+            porcelain_status="",
+            remote_master_sha="b" * 40,
+        )
+        joined = " ".join(assessment["reasons"])
+        self.assertIn("the tracking integration ref", joined)
+        self.assertNotIn("prgs/master", joined)
+
+    def test_anti_stomp_preflight_forwards_the_resolved_ref(self):
+        sig = inspect.signature(anti_stomp_preflight.assess_anti_stomp_preflight)
+        self.assertIn("remote_master_ref", sig.parameters)
+        src = inspect.getsource(anti_stomp_preflight.assess_anti_stomp_preflight)
+        self.assertIn("remote_master_ref=remote_master_ref", src)
+
+    def test_no_production_caller_inherits_the_prgs_default(self):
+        """Every resolve site must derive, or pass remote_refs explicitly."""
+        import gitea_mcp_server
+
+        src = inspect.getsource(gitea_mcp_server)
+        # The four historical call sites now consume the resolved-target state.
+        self.assertGreaterEqual(src.count("resolve_remote_master_ref_state("), 4)
+        self.assertNotIn("resolve_remote_master_sha(canonical_root)", src)
+
+    def test_resolver_signature_supports_explicit_remote(self):
+        sig = inspect.signature(root_checkout_guard.resolve_remote_master_sha)
+        self.assertIn("remote", sig.parameters)
+        self.assertIn("remote_refs", sig.parameters)
+
+
+class TestRepositoryStructureUntouched(_RepoCase):
+    """Derivation is strictly read-only: it never writes refs or branches."""
+
+    def test_resolution_creates_no_refs_or_branches(self):
+        head = _make_repo(
+            self.root,
+            remote="MDCPS",
+            url="https://gitea.example.net/MDCPS/WeeklyBriefings-Meta.git",
+        )
+        _set_remote_branch(self.root, "MDCPS", "dev", head)
+        _set_remote_head(self.root, "MDCPS", "dev")
+
+        fmt = "--format=%(refname) %(objectname)"
+        before = _git(self.root, "for-each-ref", fmt)
+        before_remotes = _git(self.root, "remote")
+
+        crr.resolve_target_base_ref(self.root)
+        root_checkout_guard.resolve_remote_master_sha(self.root)
+        master_parity_gate.assess_target_repository_parity(
+            canonical_root=self.root, source="test"
+        )
+
+        self.assertEqual(_git(self.root, "for-each-ref", fmt), before)
+        self.assertEqual(_git(self.root, "remote"), before_remotes)
+
+
+if __name__ == "__main__":
+    unittest.main()

tests/test_root_checkout_guard.py

@@ -87,13 +87,27 @@ class TestAssessRootCheckoutGuard(unittest.TestCase):
         self.assertTrue(result["block"])
         self.assertIn("tracked local edits", result["reasons"][0])
 
-    def test_head_behind_prgs_master_blocked(self):
+    def test_head_behind_tracking_base_ref_blocked(self):
+        """#983: the base ref is derived, so the message no longer hardcodes PRGS."""
         result = self._assess(
             head_sha=OTHER_SHA,
             remote_master_sha=MASTER_SHA,
         )
         self.assertTrue(result["block"])
-        self.assertIn("does not match prgs/master", result["reasons"][0])
+        self.assertIn(
+            "does not match the tracking integration ref", result["reasons"][0]
+        )
+
+    def test_head_behind_named_base_ref_reports_that_ref(self):
+        """The resolved ref is named, so a non-PRGS target is reported accurately."""
+        result = self._assess(
+            head_sha=OTHER_SHA,
+            remote_master_sha=MASTER_SHA,
+            remote_master_ref="refs/remotes/MDCPS/dev",
+        )
+        self.assertTrue(result["block"])
+        self.assertIn("does not match refs/remotes/MDCPS/dev", result["reasons"][0])
+        self.assertNotIn("prgs/master", result["reasons"][0])
 
     def test_merger_requires_clean_control_checkout(self):
         result = self._assess(