diff --git a/.github/workflows/lint.yml b/.github/workflows/lint.yml
index bae4299b1b..633ea9ca66 100644
--- a/.github/workflows/lint.yml
+++ b/.github/workflows/lint.yml
@@ -184,3 +184,12 @@ jobs:
       # reverting one commit, so in-tree code must never depend on them.
       - name: Forbid in-tree use of plugin-compat pointers
         run: python scripts/check_compat_pointers.py
+
+      # Advisory: dropped public names / methods / test defs vs the PR base, printed into the log.
+      # A refactor that silently removes a public symbol breaks plugins that import it; the Sep 2026
+      # decomposition opened with 1,703 such drops that reviewers had to find by hand.
+      - name: Public-surface diff vs base (advisory)
+        if: github.event_name == 'pull_request'
+        run: |
+          git fetch --no-tags --depth=1 origin "${{ github.base_ref }}"
+          python scripts/ci/check_public_surface.py --base "origin/${{ github.base_ref }}" --head HEAD
diff --git a/scripts/ci/check_public_surface.py b/scripts/ci/check_public_surface.py
new file mode 100644
index 0000000000..f878e9dbd4
--- /dev/null
+++ b/scripts/ci/check_public_surface.py
@@ -0,0 +1,159 @@
+#!/usr/bin/env python3
+"""Public-surface diff against a base ref: dropped public names and dropped test functions.
+
+The Sep 2026 whole-codebase refactor (PR #102117) opened with 1,703 public top-level names dropped
+across 341 modules, 1,000 public methods in 166, and 130 ``def test_`` deleted in 54 files.
+Reviewers found ~30 of the names by hand; the rest surfaced as post-merge fixes (10 commits
+restoring symbols/re-exports, 6 restoring tests, plus a qwen OAuth break that went past import
+smoke because the caller used ``module.attr``). Every one of those was catchable in seconds with
+this script; nothing ran it because it did not exist.
+
+Rules (deliberately narrow, no allowlist file to maintain):
+- A PUBLIC top-level name (function, class, module-level assignment, re-exported import) or a
+  public/dunder method of a module present on BOTH sides may be moved and re-exported or
+  deprecated, never silently removed. A deleted MODULE is not a drop here (that is a visible
+  decision; ``check_doc_paths`` territory).
+- A ``tests/**`` file present on both sides must not end with fewer ``def test_`` than it started
+  with. Deleting a whole test file is, again, a visible decision and not flagged.
+
+Advisory by default (prints the report, exit 0). ``--strict`` exits 1 on any finding so a
+refactor brief or a CI lane can make it a hard gate. Runs in ~30 s over the whole tree.
+
+Usage:
+    python scripts/ci/check_public_surface.py [--base origin/main] [--head HEAD] [--strict] [--json out.json]
+"""
+from __future__ import annotations
+
+import argparse
+import ast
+import json
+import re
+import subprocess
+import sys
+
+SRC_DIRS = ("agent", "gateway", "hermes_cli", "tools", "tui_gateway", "cron", "acp_adapter", "plugins")
+_TEST_DEF_RE = re.compile(r"^\s*(?:async\s+)?def test_", re.M)
+
+
+def _git(*args: str) -> str:
+    return subprocess.run(["git", *args], capture_output=True, text=True, encoding="utf-8", errors="replace").stdout
+
+
+def _changed_py(base: str, head: str) -> list[str]:
+    out = _git("diff", "--name-only", "--diff-filter=M", base, head)
+    return [f for f in out.split() if f.endswith(".py")]
+
+
+def _show(rev: str, path: str) -> str | None:
+    r = subprocess.run(["git", "show", f"{rev}:{path}"], capture_output=True, text=True, encoding="utf-8", errors="replace")
+    return r.stdout if r.returncode == 0 else None
+
+
+def public_toplevel(src: str) -> set[str] | None:
+    """Public top-level names: defs, classes, assigned names, and imported/re-exported names."""
+    try:
+        tree = ast.parse(src)
+    except SyntaxError:
+        return None
+    names: set[str] = set()
+    for node in tree.body:
+        if isinstance(node, (ast.FunctionDef, ast.AsyncFunctionDef, ast.ClassDef)):
+            names.add(node.name)
+        elif isinstance(node, ast.Assign):
+            for target in node.targets:
+                names.update(x.id for x in ast.walk(target) if isinstance(x, ast.Name))
+        elif isinstance(node, ast.AnnAssign) and isinstance(node.target, ast.Name):
+            names.add(node.target.id)
+        elif isinstance(node, (ast.Import, ast.ImportFrom)):
+            names.update((a.asname or a.name).split(".")[0] for a in node.names)
+    return {n for n in names if not n.startswith("_")}
+
+
+def public_methods(src: str) -> set[str]:
+    """``Class.method`` for public and dunder methods of top-level classes."""
+    try:
+        tree = ast.parse(src)
+    except SyntaxError:
+        return set()
+    out: set[str] = set()
+    for node in tree.body:
+        if isinstance(node, ast.ClassDef):
+            for m in node.body:
+                if isinstance(m, (ast.FunctionDef, ast.AsyncFunctionDef)) and (
+                    not m.name.startswith("_") or m.name.startswith("__")
+                ):
+                    out.add(f"{node.name}.{m.name}")
+    return out
+
+
+def _is_source(path: str) -> bool:
+    return "/" not in path or path.split("/", 1)[0] in SRC_DIRS
+
+
+def diff_surface(base: str, head: str) -> dict:
+    dropped_names: dict[str, list[str]] = {}
+    dropped_methods: dict[str, list[str]] = {}
+    test_drops: dict[str, tuple[int, int]] = {}
+    for path in _changed_py(base, head):
+        before, after = _show(base, path), _show(head, path)
+        if before is None or after is None:
+            continue
+        if path.startswith("tests/"):
+            nb, na = len(_TEST_DEF_RE.findall(before)), len(_TEST_DEF_RE.findall(after))
+            if na < nb:
+                test_drops[path] = (nb, na)
+            continue
+        if not _is_source(path) or path.startswith("tests"):
+            continue
+        pb, pa = public_toplevel(before), public_toplevel(after)
+        if pb is not None and pa is not None and (gone := sorted(pb - pa)):
+            dropped_names[path] = gone
+        if gone_m := sorted(public_methods(before) - public_methods(after)):
+            dropped_methods[path] = gone_m
+    return {"dropped_names": dropped_names, "dropped_methods": dropped_methods, "test_drops": test_drops}
+
+
+def render(report: dict) -> str:
+    lines: list[str] = []
+    n_names = sum(len(v) for v in report["dropped_names"].values())
+    n_meth = sum(len(v) for v in report["dropped_methods"].values())
+    n_tests = sum(b - a for b, a in report["test_drops"].values())
+    lines.append(
+        f"public-surface: {n_names} public top-level name(s) dropped in {len(report['dropped_names'])} module(s); "
+        f"{n_meth} public/dunder method(s) dropped in {len(report['dropped_methods'])}; "
+        f"{n_tests} test def(s) dropped in {len(report['test_drops'])} file(s)"
+    )
+    for path, names in sorted(report["dropped_names"].items()):
+        lines.append(f"  {path}: -{', '.join(names[:12])}{' …' if len(names) > 12 else ''}")
+    for path, names in sorted(report["dropped_methods"].items()):
+        lines.append(f"  {path}: -{', '.join(names[:8])}{' …' if len(names) > 8 else ''}")
+    for path, (b, a) in sorted(report["test_drops"].items()):
+        lines.append(f"  {path}: {b} -> {a} test defs")
+    if n_names or n_meth or n_tests:
+        lines.append(
+            "  A public name may be moved and re-exported or deprecated, never silently removed (in-tree refs say "
+            "nothing about plugins); a test file must not lose test defs unless the commit names the private symbol "
+            "they pinned."
+        )
+    return "\n".join(lines)
+
+
+def main(argv: list[str] | None = None) -> int:
+    ap = argparse.ArgumentParser(description=(__doc__ or "").split("\n\n")[0])
+    ap.add_argument("--base", default="origin/main")
+    ap.add_argument("--head", default="HEAD")
+    ap.add_argument("--strict", action="store_true", help="exit 1 on any finding")
+    ap.add_argument("--json", dest="json_out", help="also write the report as JSON")
+    args = ap.parse_args(argv)
+    base = _git("merge-base", args.base, args.head).strip() or args.base
+    report = diff_surface(base, args.head)
+    print(render(report))
+    if args.json_out:
+        with open(args.json_out, "w", encoding="utf-8") as fh:
+            json.dump(report, fh, indent=1)
+    findings = any(report[k] for k in ("dropped_names", "dropped_methods", "test_drops"))
+    return 1 if (args.strict and findings) else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/tests/scripts/test_check_public_surface.py b/tests/scripts/test_check_public_surface.py
new file mode 100644
index 0000000000..e0e5f038f2
--- /dev/null
+++ b/tests/scripts/test_check_public_surface.py
@@ -0,0 +1,72 @@
+"""scripts/ci/check_public_surface.py detects silently dropped public names, methods and test defs.
+
+Replayed against the Sep 2026 refactor PR at open it reports 1,703 public names dropped in 341 modules
+(the figure reviewers had to find ~30 of by hand); here the contract is pinned on a throwaway repo.
+"""
+import importlib.util
+import subprocess
+import textwrap
+from pathlib import Path
+
+SCRIPT = Path(__file__).resolve().parents[2] / "scripts" / "ci" / "check_public_surface.py"
+
+
+def _load():
+    spec = importlib.util.spec_from_file_location("check_public_surface", SCRIPT)
+    mod = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(mod)
+    return mod
+
+
+def _git(repo, *args):
+    subprocess.run(["git", "-C", str(repo), *args], check=True, capture_output=True,
+                   env={"GIT_AUTHOR_NAME": "t", "GIT_AUTHOR_EMAIL": "t@t", "GIT_COMMITTER_NAME": "t",
+                        "GIT_COMMITTER_EMAIL": "t@t", "PATH": "/usr/bin:/bin:/usr/local/bin"})
+
+
+def test_dropped_public_names_methods_and_test_defs_are_reported_and_moves_are_not(tmp_path, monkeypatch):
+    repo = tmp_path
+    (repo / "agent").mkdir(); (repo / "tests").mkdir(); (repo / "docs").mkdir()
+    (repo / "agent" / "mod.py").write_text(textwrap.dedent('''
+        import os
+        from json import loads as parse
+
+        LIMIT = 3
+        def keep(): ...
+        def gone(): ...
+        def _private(): ...
+        class K:
+            def pub(self): ...
+            def __len__(self): return 0
+            def _hidden(self): ...
+    '''), encoding="utf-8")
+    (repo / "agent" / "moved.py").write_text("def relocated(): ...\n", encoding="utf-8")
+    (repo / "tests" / "test_x.py").write_text("def test_a(): ...\ndef test_b(): ...\nasync def test_c(): ...\n", encoding="utf-8")
+    (repo / "docs" / "notes.py").write_text("def doc_helper(): ...\n", encoding="utf-8")
+    _git(repo, "init", "-q", "-b", "main"); _git(repo, "add", "."); _git(repo, "commit", "-qm", "base")
+
+    # HEAD: drop `gone`, `parse`, `K.pub`, `K.__len__`; drop `_private`/`K._hidden` (private: fine);
+    # move `relocated` into mod with a re-export left behind (fine); lose one test def; change a
+    # non-source module (ignored).
+    (repo / "agent" / "mod.py").write_text(textwrap.dedent('''
+        import os
+        from agent.moved import relocated  # re-export
+
+        LIMIT = 3
+        def keep(): ...
+        class K: ...
+    '''), encoding="utf-8")
+    (repo / "agent" / "moved.py").write_text("def relocated(): ...\ndef relocated2(): ...\n", encoding="utf-8")
+    (repo / "tests" / "test_x.py").write_text("def test_a(): ...\nasync def test_c(): ...\n", encoding="utf-8")
+    (repo / "docs" / "notes.py").write_text("def other(): ...\n", encoding="utf-8")
+    _git(repo, "add", "."); _git(repo, "commit", "-qm", "head")
+
+    monkeypatch.chdir(repo)
+    mod = _load()
+    report = mod.diff_surface("HEAD~1", "HEAD")
+    assert report["dropped_names"] == {"agent/mod.py": ["gone", "parse"]}
+    assert report["dropped_methods"] == {"agent/mod.py": ["K.__len__", "K.pub"]}
+    assert report["test_drops"] == {"tests/test_x.py": (3, 2)}
+    assert mod.main(["--base", "HEAD~1", "--head", "HEAD"]) == 0          # advisory
+    assert mod.main(["--base", "HEAD~1", "--head", "HEAD", "--strict"]) == 1
+    assert mod.main(["--base", "HEAD", "--head", "HEAD", "--strict"]) == 0  # nothing dropped