diff --git a/.agents/skills/lib_py/herdr_config.py b/.agents/skills/lib_py/herdr_config.py
new file mode 100644
index 0000000..69f1500
--- /dev/null
+++ b/.agents/skills/lib_py/herdr_config.py
@@ -0,0 +1,196 @@
+"""Issue #45: the config file MAM's isolated herdr servers start with.
+
+herdr restores a named session's saved layout when its server starts and, by
+default, relaunches each saved agent with herdr's own resume command - without
+the permission flags MAM adds, and even for agents MAM deleted while the server
+was down. `[session] resume_agents_on_restore = false` turns that off. The
+shim's headless bootstrap (lib.sh) hands the server `HERDR_CONFIG_PATH` pointing
+at the file built here: the user's own herdr config with only that key forced
+to false. The user's file is never modified.
+
+  * Source: the file herdr itself would read - HERDR_CONFIG_PATH, else
+    $XDG_CONFIG_HOME/herdr/config.toml, else ~/.config/herdr/config.toml. A
+    HERDR_CONFIG_PATH naming a file generated here (every MAM pane inherits it
+    from the server) is followed back to the source recorded in that file.
+  * Output: `mam-server-config.toml` next to the source, so relative paths in
+    it (herdr resolves them against the config file's directory) keep their
+    meaning; `<.mam>/herdr-server-config.toml` only when that directory is not
+    writable.
+  * The merge edits text (stdlib has no TOML writer) and is then re-parsed: the
+    key must read false and everything else must equal the source. A merge that
+    fails the check falls back to a file holding only the key - herdr would
+    drop a broken file whole and restore again, silently.
+
+CLI: `python3 -m lib_py.herdr_config server-config [--fallback-dir DIR]` writes
+the file and prints its path; warnings go to stderr; exit 1 when no file could
+be written.
+"""
+from __future__ import annotations
+
+import argparse
+import os
+import re
+import sys
+import tempfile
+import tomllib
+from typing import List, Mapping, Optional, Tuple
+
+MARKER = "# generated by multi-agent-mux (Issue #45) - do not edit"
+SOURCE_PREFIX = "# source: "
+OUTPUT_NAME = "mam-server-config.toml"
+FALLBACK_NAME = "herdr-server-config.toml"
+KEY = "resume_agents_on_restore"
+MINIMAL = f"[session]\n{KEY} = false\n"
+
+_SESSION_HEADER = re.compile(r"^\s*\[\s*session\s*\]\s*(#.*)?$")
+_ANY_HEADER = re.compile(r"^\s*\[")
+_KEY_LINE = re.compile(rf"^\s*{KEY}\s*=")
+
+
+def _generated_source(path: str) -> Optional[str]:
+    """The source recorded in a file generated here; '' when it is one without
+    a source line; None when it is not a generated file."""
+    try:
+        with open(path, encoding="utf-8") as f:
+            if f.readline().rstrip("\n") != MARKER:
+                return None
+            line = f.readline().rstrip("\n")
+    except OSError:
+        return None
+    return line[len(SOURCE_PREFIX):] if line.startswith(SOURCE_PREFIX) else ""
+
+
+def user_config_path(env: Mapping[str, str]) -> str:
+    """The config file herdr reads under `env`, seen through generated files."""
+    explicit = env.get("HERDR_CONFIG_PATH", "")
+    if explicit:
+        source = _generated_source(explicit)
+        if source is None:
+            return explicit
+        if source:
+            return source
+    xdg = env.get("XDG_CONFIG_HOME", "")
+    if xdg:
+        # herdr takes a relative XDG_CONFIG_HOME against its cwd, too.
+        return os.path.join(os.path.abspath(xdg), "herdr", "config.toml")
+    return os.path.join(env.get("HOME", os.path.expanduser("~")), ".config", "herdr", "config.toml")
+
+
+def _without_key(d: dict) -> dict:
+    d = dict(d)
+    session = dict(d.get("session") or {})
+    session.pop(KEY, None)
+    if session:
+        d["session"] = session
+    else:
+        d.pop("session", None)
+    return d
+
+
+def _merge_text(user_text: str) -> str:
+    lines = user_text.splitlines()
+    header = next((i for i, l in enumerate(lines) if _SESSION_HEADER.match(l)), None)
+    if header is None:
+        sep = "" if user_text.endswith("\n") else "\n"
+        return f"{user_text}{sep}\n{MINIMAL}"
+    end = next((i for i in range(header + 1, len(lines)) if _ANY_HEADER.match(lines[i])), len(lines))
+    key_line = next((i for i in range(header + 1, end) if _KEY_LINE.match(lines[i])), None)
+    if key_line is None:
+        lines.insert(header + 1, f"{KEY} = false")
+    else:
+        lines[key_line] = f"{KEY} = false"
+    return "\n".join(lines) + "\n"
+
+
+def server_config_text(user_text: Optional[str]) -> Tuple[str, List[str]]:
+    """(the TOML body for the server, warnings). The body always parses and
+    always has session.resume_agents_on_restore = false."""
+    if not user_text or not user_text.strip():
+        return MINIMAL, []
+    try:
+        user = tomllib.loads(user_text)
+    except tomllib.TOMLDecodeError as e:
+        return MINIMAL, [f"herdr config does not parse ({e}); the MAM server uses only {KEY} = false"]
+    if (user.get("session") or {}).get(KEY) is False:
+        return user_text, []
+    merged = _merge_text(user_text)
+    try:
+        result = tomllib.loads(merged)
+        ok = result["session"][KEY] is False and _without_key(result) == _without_key(user)
+    except (tomllib.TOMLDecodeError, KeyError, TypeError):
+        ok = False
+    if not ok:
+        return MINIMAL, [f"could not set [session] {KEY} in the herdr config (dotted key or inline table?); "
+                         f"the MAM server uses only {KEY} = false, without the rest of your settings"]
+    return merged, []
+
+
+def _write_atomic(path: str, text: str, mode: int) -> None:
+    d = os.path.dirname(path)
+    os.makedirs(d, exist_ok=True)
+    fd, tmp = tempfile.mkstemp(dir=d, prefix=".mam-server-config.", suffix=".tmp")
+    try:
+        with os.fdopen(fd, "w", encoding="utf-8") as f:
+            f.write(text)
+        os.chmod(tmp, mode)
+        os.replace(tmp, path)
+    except BaseException:
+        try:
+            os.unlink(tmp)
+        except OSError:
+            pass
+        raise
+
+
+def write_server_config(env: Mapping[str, str], fallback_dir: Optional[str] = None) -> Tuple[Optional[str], List[str]]:
+    """Write the server config; (its path or None, warnings)."""
+    source = user_config_path(env)
+    warnings: List[str] = []
+    user_text: Optional[str] = None
+    mode = 0o644
+    try:
+        with open(source, encoding="utf-8") as f:
+            user_text = f.read()
+        mode = os.stat(source).st_mode & 0o777
+    except FileNotFoundError:
+        pass
+    except (OSError, UnicodeDecodeError) as e:
+        # herdr cannot use it either, so it runs on defaults: same as no file.
+        warnings.append(f"cannot read herdr config {source}: {e}")
+    body, merge_warnings = server_config_text(user_text)
+    warnings += merge_warnings
+    text = f"{MARKER}\n{SOURCE_PREFIX}{source}\n{body}"
+    target = os.path.join(os.path.dirname(source), OUTPUT_NAME)
+    try:
+        _write_atomic(target, text, mode)
+        return target, warnings
+    except OSError as e:
+        warnings.append(f"cannot write {target}: {e}")
+    if fallback_dir:
+        target = os.path.join(fallback_dir, FALLBACK_NAME)
+        try:
+            _write_atomic(target, text, mode)
+            warnings.append(f"wrote {target} instead; relative paths in the herdr config may not resolve")
+            return target, warnings
+        except OSError as e:
+            warnings.append(f"cannot write {target}: {e}")
+    return None, warnings
+
+
+def main(argv=None) -> int:
+    p = argparse.ArgumentParser(description="herdr config for MAM's isolated servers (Issue #45)")
+    sub = p.add_subparsers(dest="cmd", required=True)
+    sc = sub.add_parser("server-config", help="write the server config and print its path")
+    sc.add_argument("--fallback-dir", default=None, help="where to write when the config dir is not writable")
+    args = p.parse_args(argv)
+    path, warnings = write_server_config(os.environ, args.fallback_dir)
+    for w in warnings:
+        print(f"WARNING: {w}", file=sys.stderr)
+    if not path:
+        return 1
+    print(path)
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/tests/test_issue45_herdr_config.py b/tests/test_issue45_herdr_config.py
new file mode 100644
index 0000000..add63e4
--- /dev/null
+++ b/tests/test_issue45_herdr_config.py
@@ -0,0 +1,192 @@
+"""Issue #45 (A): the herdr config MAM's isolated servers start with.
+
+lib_py/herdr_config.py builds the user's herdr config with only
+[session] resume_agents_on_restore forced to false. herdr drops a config that
+does not parse and runs on defaults - restore on - so every result here is
+re-parsed, and fed to the real `herdr config check` when herdr is installed.
+That restore really is off on a live server is test_issue45_restore.py.
+"""
+from __future__ import annotations
+
+import os
+import shutil
+import stat
+import subprocess
+import sys
+import tomllib
+from pathlib import Path
+
+import pytest
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(REPO_ROOT / ".agents" / "skills"))
+
+from lib_py import herdr_config as hc  # noqa: E402
+
+HERDR = shutil.which("herdr")
+
+# (name, user config text or None, expect the user's other settings kept, expect a warning)
+CASES = [
+    ("missing", None, True, False),
+    ("empty", "", True, False),
+    ("broken", "[session]\n[session]\n", False, True),
+    ("no_session", "[ui]\nconfirm_close = false\n", True, False),
+    ("no_session_no_newline", "[ui]\nconfirm_close = false", True, False),
+    ("session_without_key", "[session]\n# a comment\n\n[ui]\nconfirm_close = false\n", True, False),
+    ("key_true", "[ui]\nconfirm_close = false\n[session]\nresume_agents_on_restore = true  # mine\n", True, False),
+    ("key_false", "[session]\nresume_agents_on_restore=false\n", True, False),
+    ("sub_table_only", "[session.extra]\na = 1\n", True, False),
+    ("dotted_key", "session.resume_agents_on_restore = true\n", False, True),
+    ("inline_table", "session = { resume_agents_on_restore = true }\n", False, True),
+    ("header_inside_string", 'banner = """\n[session]\n"""\n', False, True),
+]
+
+
+def _without_key(d):
+    return hc._without_key(d)
+
+
+@pytest.mark.parametrize("name,user,kept,warned", CASES, ids=[c[0] for c in CASES])
+def test_merge_always_parses_with_restore_off(name, user, kept, warned):
+    text, warnings = hc.server_config_text(user)
+    parsed = tomllib.loads(text)
+    assert parsed["session"]["resume_agents_on_restore"] is False
+    assert bool(warnings) == warned, warnings
+    if kept and user:
+        assert _without_key(parsed) == _without_key(tomllib.loads(user))
+    if not kept:
+        assert text == hc.MINIMAL
+
+
+def test_existing_false_is_left_byte_for_byte():
+    user = "[session]\nresume_agents_on_restore=false # keep\n"
+    assert hc.server_config_text(user) == (user, [])
+
+
+def test_a_bad_value_type_falls_back_to_minimal():
+    # herdr rejects a non-bool here as a parse error of the whole file.
+    text, warnings = hc.server_config_text('[session]\nresume_agents_on_restore = "no"\n')
+    assert tomllib.loads(text)["session"]["resume_agents_on_restore"] is False
+    # tomllib reads the string fine, so the merge replaces the line instead.
+    assert warnings == []
+
+
+def _env(tmp_path, **kw):
+    env = {"HOME": str(tmp_path / "home")}
+    env.update(kw)
+    return env
+
+
+def test_source_is_home_config_by_default(tmp_path):
+    assert hc.user_config_path(_env(tmp_path)) == str(tmp_path / "home" / ".config" / "herdr" / "config.toml")
+
+
+def test_source_follows_xdg_config_home(tmp_path, monkeypatch):
+    assert hc.user_config_path(_env(tmp_path, XDG_CONFIG_HOME=str(tmp_path / "x"))) == \
+        str(tmp_path / "x" / "herdr" / "config.toml")
+    monkeypatch.chdir(tmp_path)  # herdr takes a relative one against its cwd
+    assert hc.user_config_path(_env(tmp_path, XDG_CONFIG_HOME="rel")) == \
+        str(tmp_path / "rel" / "herdr" / "config.toml")
+
+
+def test_source_is_herdr_config_path_unless_generated(tmp_path):
+    user = tmp_path / "custom.toml"
+    user.write_text("[ui]\nconfirm_close = false\n")
+    env = _env(tmp_path, HERDR_CONFIG_PATH=str(user), XDG_CONFIG_HOME=str(tmp_path / "x"))
+    assert hc.user_config_path(env) == str(user)
+
+    # A MAM pane inherits HERDR_CONFIG_PATH=<generated>; a server bootstrapped
+    # from there must still build from the user's file, not from the copy.
+    generated, _ = hc.write_server_config(env)
+    assert generated == str(tmp_path / hc.OUTPUT_NAME)
+    assert hc.user_config_path(dict(env, HERDR_CONFIG_PATH=generated)) == str(user)
+
+    # A generated file without a source line falls through to XDG / HOME.
+    bare = tmp_path / "bare.toml"
+    bare.write_text(hc.MARKER + "\n" + hc.MINIMAL)
+    assert hc.user_config_path(dict(env, HERDR_CONFIG_PATH=str(bare))) == \
+        str(tmp_path / "x" / "herdr" / "config.toml")
+
+
+def test_written_next_to_source_with_its_mode(tmp_path):
+    cfg_dir = tmp_path / "home" / ".config" / "herdr"
+    cfg_dir.mkdir(parents=True)
+    user = cfg_dir / "config.toml"
+    user.write_text('[ui.sound]\npath = "sounds/done.wav"\n')
+    user.chmod(0o600)
+    path, warnings = hc.write_server_config(_env(tmp_path))
+    assert path == str(cfg_dir / hc.OUTPUT_NAME) and warnings == []
+    text = Path(path).read_text()
+    assert text.splitlines()[:2] == [hc.MARKER, hc.SOURCE_PREFIX + str(user)]
+    parsed = tomllib.loads(text)
+    # same directory, so the relative sound path resolves to the same file
+    assert parsed["ui"]["sound"]["path"] == "sounds/done.wav"
+    assert parsed["session"]["resume_agents_on_restore"] is False
+    assert stat.S_IMODE(os.stat(path).st_mode) == 0o600
+    assert user.read_text() == '[ui.sound]\npath = "sounds/done.wav"\n'  # untouched
+    assert [p.name for p in cfg_dir.iterdir() if p.name.endswith(".tmp")] == []
+
+
+def test_written_without_any_user_config(tmp_path):
+    path, warnings = hc.write_server_config(_env(tmp_path))
+    assert path == str(tmp_path / "home" / ".config" / "herdr" / hc.OUTPUT_NAME) and warnings == []
+    assert stat.S_IMODE(os.stat(path).st_mode) == 0o644
+    assert tomllib.loads(Path(path).read_text()) == {"session": {"resume_agents_on_restore": False}}
+
+
+def test_rewritten_on_every_call(tmp_path):
+    cfg_dir = tmp_path / "home" / ".config" / "herdr"
+    cfg_dir.mkdir(parents=True)
+    (cfg_dir / "config.toml").write_text("[ui]\nconfirm_close = true\n")
+    path, _ = hc.write_server_config(_env(tmp_path))
+    (cfg_dir / "config.toml").write_text("[ui]\nconfirm_close = false\n")
+    hc.write_server_config(_env(tmp_path))
+    assert tomllib.loads(Path(path).read_text())["ui"]["confirm_close"] is False
+
+
+@pytest.mark.skipif(os.geteuid() == 0, reason="root ignores directory permissions")
+def test_unwritable_config_dir_falls_back_to_mam_dir(tmp_path):
+    cfg_dir = tmp_path / "home" / ".config" / "herdr"
+    cfg_dir.mkdir(parents=True)
+    (cfg_dir / "config.toml").write_text("[ui]\nconfirm_close = false\n")
+    cfg_dir.chmod(0o555)
+    try:
+        mam = tmp_path / ".mam"
+        mam.mkdir()
+        path, warnings = hc.write_server_config(_env(tmp_path), str(mam))
+        assert path == str(mam / hc.FALLBACK_NAME)
+        assert any("relative paths" in w for w in warnings), warnings
+        assert tomllib.loads(Path(path).read_text())["ui"]["confirm_close"] is False
+        assert hc.write_server_config(_env(tmp_path))[0] is None  # no fallback given
+    finally:
+        cfg_dir.chmod(0o755)
+
+
+def test_cli_prints_the_path(tmp_path):
+    env = dict(os.environ, HOME=str(tmp_path), PYTHONPATH=str(REPO_ROOT / ".agents" / "skills"))
+    env.pop("HERDR_CONFIG_PATH", None)
+    env.pop("XDG_CONFIG_HOME", None)
+    cfg_dir = tmp_path / ".config" / "herdr"
+    cfg_dir.mkdir(parents=True)
+    (cfg_dir / "config.toml").write_text("[session]\n[session]\n")
+    res = subprocess.run([sys.executable, "-m", "lib_py.herdr_config", "server-config"],
+                         capture_output=True, text=True, env=env)
+    assert res.returncode == 0, res.stderr
+    assert res.stdout == str(cfg_dir / hc.OUTPUT_NAME) + "\n"
+    assert "WARNING: herdr config does not parse" in res.stderr
+
+
+@pytest.mark.skipif(not HERDR, reason="herdr is not installed")
+@pytest.mark.parametrize("name,user,kept,warned", CASES, ids=[c[0] for c in CASES])
+def test_real_herdr_accepts_every_result(tmp_path, name, user, kept, warned):
+    text, _ = hc.server_config_text(user)
+    cfg = tmp_path / "server.toml"
+    cfg.write_text(hc.MARKER + "\n" + text)
+    env = {k: v for k, v in os.environ.items() if not k.startswith("HERDR_")}
+    env.update(HOME=str(tmp_path), HERDR_CONFIG_PATH=str(cfg))
+    res = subprocess.run([HERDR, "config", "check"], capture_output=True, text=True, env=env)
+    out = res.stdout + res.stderr
+    assert "parse error" not in out, out
+    # the fixtures use only keys herdr knows, apart from the made-up ones below
+    issues = [l for l in out.splitlines()[1:] if l.strip()]
+    assert all(l.startswith("unknown config key") and ("extra" in l or "banner" in l) for l in issues), out