diff --git a/agent/context_references.py b/agent/context_references.py
index 1f4e01ec2bfe0..6f428bf87badd 100644
--- a/agent/context_references.py
+++ b/agent/context_references.py
@@ -13,7 +13,7 @@ from typing import Awaitable, Callable
 
 from agent.model_metadata import estimate_tokens_rough
 from hermes_cli._subprocess_compat import IS_WINDOWS, windows_hide_flags
-from hermes_cli.sizefmt import format_bytes as _human_bytes
+from hermes_cli.sizefmt import format_bytes
 
 _QUOTED_REFERENCE_VALUE = r'(?:`[^`\n]+`|"[^"\n]+"|\'[^\'\n]+\')'
 REFERENCE_PATTERN = re.compile(
@@ -559,7 +559,7 @@ def _binary_reference_block(ref: ContextReference, path: Path) -> str:
     mime, _ = mimetypes.guess_type(path.name)
     mime = mime or "application/octet-stream"
     try:
-        size = _human_bytes(path.stat().st_size)
+        size = format_bytes(path.stat().st_size)
     except OSError:
         size = "unknown size"
     return (
diff --git a/agent/curator_backup.py b/agent/curator_backup.py
index 4d17f3743c0a3..d7ea51755cfa4 100644
--- a/agent/curator_backup.py
+++ b/agent/curator_backup.py
@@ -50,10 +50,7 @@ from typing import Any, Dict, List, Optional, Set, Tuple
 
 from hermes_constants import get_hermes_home
 from agent.skill_utils import is_excluded_skill_path
-
-# Shared byte formatter; public name ``format_size`` is part of this module's
-# established surface, so alias rather than churn the callers.
-from hermes_cli.sizefmt import format_bytes as format_size
+from hermes_cli.sizefmt import format_bytes
 
 logger = logging.getLogger(__name__)
 
@@ -749,6 +746,6 @@ def summarize_backups() -> str:
             f"{r.get('id','?'):<24}  "
             f"{(r.get('reason','?') or '?')[:40]:<40}  "
             f"{r.get('skill_files', 0):>6}  "
-            f"{format_size(int(r.get('archive_bytes', 0))):>8}"
+            f"{format_bytes(int(r.get('archive_bytes', 0))):>8}"
         )
     return "\n".join(lines)
diff --git a/hermes_cli/checkpoints.py b/hermes_cli/checkpoints.py
index cecd22df2d7e0..0d7c866b40028 100644
--- a/hermes_cli/checkpoints.py
+++ b/hermes_cli/checkpoints.py
@@ -27,14 +27,7 @@ from datetime import datetime
 from pathlib import Path
 from typing import Any, Optional
 
-from hermes_cli.sizefmt import format_bytes
-
-
-def _fmt_bytes(n: Optional[int]) -> str:
-    # Delegates to the shared formatter; ``or 0`` preserves this module's
-    # historical None/0 -> "0 B" display (the shared helper renders None
-    # as "?", which is wrong for a size total that is genuinely zero).
-    return format_bytes(n or 0)
+from hermes_cli.sizefmt import format_bytes as _fmt_bytes
 
 
 def _fmt_ts(ts: Any) -> str:
diff --git a/hermes_cli/doctor.py b/hermes_cli/doctor.py
index 202942b476d4f..88ce3ecba73bf 100644
--- a/hermes_cli/doctor.py
+++ b/hermes_cli/doctor.py
@@ -320,8 +320,8 @@ def check_info(text: str):
 STATE_DB_SIZE_WARN_BYTES = 1 * 1024 * 1024 * 1024   # 1 GiB logical size
 
 
-# Shared byte formatter (private alias: this module's rendering helpers and
-# tests refer to ``_human_bytes``).
+# Shared byte formatter, aliased to the name this module's three rendering
+# call sites already use.
 from hermes_cli.sizefmt import format_bytes as _human_bytes
 
 
diff --git a/hermes_cli/sizefmt.py b/hermes_cli/sizefmt.py
index 99cf9e8c83634..f0680f4229d85 100644
--- a/hermes_cli/sizefmt.py
+++ b/hermes_cli/sizefmt.py
@@ -1,11 +1,8 @@
 """Small shared size-formatting helpers for CLI/agent output.
 
-Public home for the human-readable byte formatter that previously existed
-as five near-identical private copies (``hermes_cli/backup.py``,
-``hermes_cli/checkpoints.py``, ``hermes_cli/doctor.py``,
-``agent/context_references.py``, ``agent/curator_backup.py``). Sibling of
-``hermes_cli.timefmt``, and kept dependency-free for the same reason:
-lightweight consumers must not drag in the whole CLI surface.
+Sibling of ``hermes_cli.timefmt`` (same extraction rationale: a tiny
+purpose-named module lightweight consumers can import without dragging in
+the CLI surface). Replaces six near-identical private byte formatters.
 
 Two in-repo formatters intentionally do NOT delegate here:
 
@@ -19,20 +16,21 @@ Two in-repo formatters intentionally do NOT delegate here:
 from __future__ import annotations
 
 
-def format_bytes(n, *, fallback: str = "?") -> str:
+def format_bytes(n) -> str:
     """1234567 -> '1.2 MB' (B/KB/MB/GB/TB; integer bytes, one decimal above).
 
-    Accepts anything ``float()`` accepts; returns *fallback* for None or
-    unparseable input so display call sites never raise.
+    Accepts anything ``float()`` accepts; returns ``"?"`` for None or
+    unparseable input so display call sites never raise (contract inherited
+    from doctor's original copy — its stats dict tolerates None fields).
     """
     try:
         size = float(n)
     except (TypeError, ValueError):
-        return fallback
+        return "?"
     if size < 1024:
         return f"{int(size)} B"
-    for unit in ("KB", "MB", "GB", "TB"):
+    for unit in ("KB", "MB", "GB"):
         size /= 1024.0
-        if size < 1024 or unit == "TB":
+        if size < 1024:
             return f"{size:.1f} {unit}"
-    return f"{size:.1f} TB"  # unreachable; keeps type-checkers satisfied
+    return f"{size / 1024.0:.1f} TB"
diff --git a/hermes_cli/update_cmd.py b/hermes_cli/update_cmd.py
index c99d56a81e18b..6d6c139633e09 100644
--- a/hermes_cli/update_cmd.py
+++ b/hermes_cli/update_cmd.py
@@ -2692,12 +2692,9 @@ def _run_pre_update_backup(args) -> Optional[str]:
         size_bytes = 0
 
     # Human-readable size
-    size_str = f"{size_bytes} B"
-    for unit in ("KB", "MB", "GB"):
-        if size_bytes < 1024:
-            break
-        size_bytes /= 1024
-        size_str = f"{size_bytes:.1f} {unit}"
+    from hermes_cli.sizefmt import format_bytes
+
+    size_str = format_bytes(size_bytes)
 
     # Render path using display_hermes_home so the user sees ~/.hermes/...
     try:
diff --git a/tests/hermes_cli/test_sizefmt.py b/tests/hermes_cli/test_sizefmt.py
index f2ad56a927b21..17fe0d0e7f0b9 100644
--- a/tests/hermes_cli/test_sizefmt.py
+++ b/tests/hermes_cli/test_sizefmt.py
@@ -1,10 +1,10 @@
 """Tests for the shared byte formatter (hermes_cli.sizefmt).
 
-Consolidates five near-identical private formatters (backup, checkpoints,
-doctor, context_references, curator_backup). The contract below locks the
-shared behavior, including the two deliberate changes vs the old copies:
-a real TB tier (doctor/context_references/curator_backup previously
-rendered 1 TiB as '1024.0 GB') and non-raising fallback for None/garbage.
+Consolidates six near-identical formatters (backup, checkpoints, doctor,
+update_cmd, context_references, curator_backup). The contract below locks
+the shared behavior, including the one deliberate change vs the old
+copies: a real TB tier (doctor/context_references/curator_backup
+previously rendered 1 TiB as '1024.0 GB').
 """
 
 import pytest
@@ -40,24 +40,23 @@ def test_format_bytes_tb_tier_not_gb_overflow():
 
 
 def test_format_bytes_never_raises():
+    """Doctor's stats dict tolerates None in every field; the formatter must
+    render (not raise) for None/garbage."""
     assert format_bytes(None) == "?"
     assert format_bytes("garbage") == "?"
     assert format_bytes("2048") == "2.0 KB"  # numeric strings accepted
-    assert format_bytes(None, fallback="unknown") == "unknown"
 
 
-def test_delegating_aliases_share_the_implementation():
-    """The five migrated call sites must all resolve to the shared helper
-    (checkpoints wraps it to preserve its None -> '0 B' display)."""
-    from agent.context_references import _human_bytes as ctx
-    from agent.curator_backup import format_size as curator
+def test_migrated_sites_render_through_the_shared_helper():
+    """Behavior contract for the aliased call sites: the module-local names
+    must render byte-identically to the shared helper (how they delegate is
+    an implementation detail — only the rendering is pinned)."""
     from hermes_cli.backup import _format_size as backup
     from hermes_cli.checkpoints import _fmt_bytes as checkpoints
     from hermes_cli.doctor import _human_bytes as doctor
 
-    assert backup is format_bytes
-    assert ctx is format_bytes
-    assert curator is format_bytes
-    assert doctor is format_bytes
-    assert checkpoints(None) == "0 B"
-    assert checkpoints(2048) == format_bytes(2048)
+    for n in (0, 512, 2048, 1234567, 1024**3, 1024**4):
+        expected = format_bytes(n)
+        assert backup(n) == expected
+        assert checkpoints(n) == expected
+        assert doctor(n) == expected