diff --git a/hermes_cli/config_defaults.py b/hermes_cli/config_defaults.py
index f86c3c6da1..3776bf8f21 100644
--- a/hermes_cli/config_defaults.py
+++ b/hermes_cli/config_defaults.py
@@ -1458,6 +1458,10 @@ DEFAULT_CONFIG = {
         # curator ledger` / `rollback <entry-id>`. Never a gate — failures can't block.
         # See #79686.
         "ledger": True,
+        # Size cap for that ledger: once the file grows past this, the next append rewrites it
+        # through the unchanged-file dedup and, if still over, drops the oldest entries (0 = keep
+        # the ledger append-only forever, the previous behaviour).
+        "ledger_max_bytes": 5 * 1024 * 1024,
     },
 
     # Curator — background maintenance of AGENT-CREATED skills (never hub-installed): marks
diff --git a/tests/tools/test_skill_ledger.py b/tests/tools/test_skill_ledger.py
index 57d444bcf9..89371ab6dd 100644
--- a/tests/tools/test_skill_ledger.py
+++ b/tests/tools/test_skill_ledger.py
@@ -600,3 +600,107 @@ def test_backup_fill_ignores_tar_path_traversal(ledger_env):
     )
     # Malicious members are not.
     assert not any(p.endswith("evil.md") or p.endswith("outside.md") for p in paths)
+import json
+from pathlib import Path
+
+import pytest
+
+
+def _append_padded(skill_ledger, action: str, pad: str, n: int = 1) -> None:
+    """Append *n* entries padded with evidence text so the ledger file grows fast."""
+    for _ in range(n):
+        skill_ledger.append_entry(action, "my-skill", before=[], after=[], evidence={"pad": pad})
+
+
+def test_auto_compact_triggers_at_threshold(ledger_env, monkeypatch):
+    """Crossing skills.ledger_max_bytes rewrites the ledger through the delta
+    dedup: identical before/after manifests shrink to nothing while ids and
+    entry order survive."""
+    from tools import skill_ledger
+
+    import hermes_cli.config as _cfg
+
+    monkeypatch.setattr(_cfg, "load_config", lambda *a, **k: {
+        "skills": {"ledger_max_bytes": 8192}})
+
+    for i in range(3):  # pre-delta-style entries: identical fat manifests on both sides
+        fat = [{"path": f"my-skill/f{i}j{j}.md", "sha256": "a" * 64} for j in range(40)]
+        skill_ledger.append_entry("patch", "my-skill", before=fat, after=list(fat))
+
+    # the maintenance sweep fired mid-append: the file stays under the cap
+    assert skill_ledger.ledger_path().stat().st_size <= 8192
+    rows = skill_ledger.list_entries()
+    assert len(rows) == 3, "dedup alone must reach the cap — nothing trimmed"
+    assert all(r["before"] == [] and r["after"] == [] for r in rows), (
+        "identical manifests must be dropped by compaction"
+    )
+
+
+def test_trim_oldest_when_still_over_cap(ledger_env, monkeypatch):
+    """When compaction alone cannot reach the cap (every entry genuinely
+    differs), the oldest entries are dropped until it fits — newest entries
+    survive, malformed lines are kept verbatim."""
+    from tools import skill_ledger
+
+    import hermes_cli.config as _cfg
+
+    monkeypatch.setattr(_cfg, "load_config", lambda *a, **k: {
+        "skills": {"ledger_max_bytes": 8192}})
+
+    newest_id = None
+    for i in range(5):
+        before = [{"path": f"my-skill/old{i}.md", "sha256": f"{i}" * 64}]
+        after = [{"path": f"my-skill/new{i}.md", "sha256": f"{i + 1}" * 64}]
+        newest_id = skill_ledger.append_entry(
+            "edit", "my-skill", before=before, after=after,
+            evidence={"pad": "y" * 2048})
+    with open(skill_ledger.ledger_path(), "a", encoding="utf-8") as fh:
+        fh.write("{not json at all\n")
+
+    skill_ledger._maintain_size()
+
+    rows = skill_ledger.list_entries()
+    assert len(rows) < 5, "oldest entries must be trimmed when compaction is not enough"
+    assert rows[0]["id"] == newest_id, "the newest entry always survives"
+    # malformed lines are never parsed away — they stay in the file verbatim
+    raw = skill_ledger.ledger_path().read_text(encoding="utf-8")
+    assert "{not json at all" in raw
+    assert skill_ledger.ledger_path().stat().st_size <= 8192
+
+
+def test_disabled_threshold_never_touches_the_file(ledger_env, monkeypatch):
+    """ledger_max_bytes: 0 keeps the append-only contract: no
+    rewrite ever happens, however large the file is."""
+    from tools import skill_ledger
+
+    import hermes_cli.config as _cfg
+
+    monkeypatch.setattr(_cfg, "load_config", lambda *a, **k: {
+        "skills": {"ledger": True, "ledger_max_bytes": 0}})
+
+    _append_padded(skill_ledger, "patch", "z" * 4096, n=4)
+    raw = skill_ledger.ledger_path().read_text(encoding="utf-8")
+
+    skill_ledger._maintain_size()
+
+    assert skill_ledger.ledger_path().read_text(encoding="utf-8") == raw
+
+
+def test_maintenance_failure_never_blocks_append(ledger_env, monkeypatch):
+    """Telemetry contract: a broken maintenance sweep logs and leaves the
+    appended entries on disk."""
+    from tools import skill_ledger
+
+    import hermes_cli.config as _cfg
+
+    monkeypatch.setattr(_cfg, "load_config", lambda *a, **k: {
+        "skills": {"ledger_max_bytes": 8192}})
+
+    def _boom(*a, **k):
+        raise OSError("disk full")
+
+    monkeypatch.setattr(skill_ledger, "compact_ledger", _boom)
+    # appends past the threshold hit the broken sweep and must survive it
+    _append_padded(skill_ledger, "patch", "w" * 4096, n=6)
+
+    assert len(skill_ledger.list_entries()) == 6
diff --git a/tools/skill_ledger.py b/tools/skill_ledger.py
index 5e3831980e..0a5f2aa98e 100644
--- a/tools/skill_ledger.py
+++ b/tools/skill_ledger.py
@@ -96,6 +96,21 @@ def ledger_enabled() -> bool:
         return True
 
 
+def _max_ledger_bytes() -> int:
+    """Config ``skills.ledger_max_bytes`` (default 5 MB, 0 disables): above it the
+    next append triggers the maintenance sweep instead of growing the file forever."""
+    try:
+        from hermes_cli.config import cfg_get, load_config
+        return int(
+            cfg_get(
+                load_config(), "skills", "ledger_max_bytes", default=5 * 1024 * 1024
+            )
+        )
+    except Exception as e:  # pragma: no cover — best-effort config read
+        logger.debug("skill_ledger: config read failed (%s); defaulting to 5 MB", e)
+        return 5 * 1024 * 1024
+
+
 def _rel_posix(path: Path | str, root: Path) -> Optional[str]:
     """POSIX path of ``path`` relative to ``root`` (both normalized), or None when outside."""
     try:
@@ -290,6 +305,7 @@ def append_entry(
         path.parent.mkdir(parents=True, exist_ok=True)
         with open(path, "a", encoding="utf-8") as fh:
             fh.write(json.dumps(entry, ensure_ascii=False) + "\n")
+        _maintain_size()
         return entry["id"]
     except Exception as e:
         logger.warning("skill_ledger: failed to append entry (%s) — mutation unaffected", e)
@@ -309,6 +325,57 @@ def _read_ledger(what: str, *, quiet_missing: bool = False) -> Optional[bytes]:
         return None
 
 
+def _maintain_size() -> None:
+    """Keep the ledger bounded: once it crosses ``skills.ledger_max_bytes`` run the
+    delta-dedup rewrite, and if genuinely-divergent entries still exceed the cap,
+    drop the oldest ones until it fits. Best-effort telemetry, never a gate: any
+    failure is logged and the just-appended entry stays on disk."""
+    max_bytes = _max_ledger_bytes()
+    if max_bytes <= 0:
+        return
+    try:
+        if ledger_path().stat().st_size <= max_bytes:
+            return
+        _, _, size_after = compact_ledger()
+        if size_after > max_bytes:
+            _trim_oldest(max_bytes)
+        gc_blobs()
+    except Exception as e:
+        logger.warning(
+            "skill_ledger: maintenance sweep failed (%s) — ledger left as-is", e
+        )
+
+
+def _trim_oldest(max_bytes: int) -> None:
+    """Rewrite the ledger without its oldest parsed entries until it is at most
+    *max_bytes*; the newest entry always survives, malformed lines are kept verbatim."""
+    path = ledger_path()
+    raw = _read_ledger("trim skipped")
+    if raw is None:
+        return
+    lines = raw.decode("utf-8").splitlines()
+    newest_json = None
+    for line in reversed(lines):
+        if line.strip():
+            newest_json = line
+            break
+    kept: List[str] = []
+    size = 2  # trailing newline + rounding slack
+    for line in reversed(lines):
+        addition = len(line.encode("utf-8")) + 1
+        if kept and size + addition > max_bytes:
+            break
+        kept.append(line)
+        size += addition
+    kept.reverse()
+    data = ("\n".join(kept) + "\n").encode("utf-8") if kept else b""
+    if newest_json is not None and newest_json not in kept:
+        data = (newest_json + "\n").encode("utf-8")
+    tmp = path.with_name(path.name + ".trim.tmp")
+    tmp.write_bytes(data)
+    os.replace(tmp, path)
+
+
 def compact_ledger() -> Tuple[int, int, int]:
     """Rewrite the ledger with every entry's unchanged paths dropped (see ``_delta``); ids, order and
     rollback semantics are preserved. Returns ``(entries, bytes_before, bytes_after)``. Atomic: the
diff --git a/website/docs/user-guide/features/curator.md b/website/docs/user-guide/features/curator.md
index ad668cf2b8..303dc69184 100644
--- a/website/docs/user-guide/features/curator.md
+++ b/website/docs/user-guide/features/curator.md
@@ -175,6 +175,13 @@ skills:
   ledger: false
 ```
 
+The file is also size-bounded: once it grows past `skills.ledger_max_bytes` (default 5 MB), the next mutation first rewrites it through the unchanged-file dedup (exactly what `hermes curator ledger --compact` does) and, if genuinely-divergent entries still exceed the cap, drops the oldest ones — the newest entry and any malformed lines always survive, and the sweep frees blobs nothing references anymore. Set it to `0` to keep the ledger append-only forever.
+
+```yaml
+skills:
+  ledger_max_bytes: 5242880   # 0 = never auto-maintain
+```
+
 ## Archive TTL purge
 
 Archived skills are kept forever by default. If you want `~/.hermes/skills/.archive/` bounded, set a TTL and purge explicitly — purging never runs automatically, and every purged skill is captured into the ledger (with blobs) first, so even a purge leaves an auditable, recoverable trail: