diff --git a/reme/components/tag_index/base_tag_index.py b/reme/components/tag_index/base_tag_index.py
index d1e33ec9..c7f2f5ec 100644
--- a/reme/components/tag_index/base_tag_index.py
+++ b/reme/components/tag_index/base_tag_index.py
@@ -24,9 +24,13 @@ class BaseTagIndex(BaseComponent):
 
     @tag_key.setter
     def tag_key(self, value: object) -> None:
+        self._tag_key = self._validate_tag_key(value)
+
+    def _validate_tag_key(self, value: object) -> str:
+        """Validate and normalize the configured frontmatter tag field."""
         if not isinstance(value, str) or not value.strip():
             raise ValueError("tag_key must be a non-empty string")
-        self._tag_key = value.strip()
+        return value.strip()
 
     def set_healthy(self, healthy: bool) -> None:
         """Mark whether lookups can safely use the current derived state."""
diff --git a/reme/components/tag_index/local_tag_index.py b/reme/components/tag_index/local_tag_index.py
index 387565a4..f1e3eb0f 100644
--- a/reme/components/tag_index/local_tag_index.py
+++ b/reme/components/tag_index/local_tag_index.py
@@ -19,11 +19,6 @@ class LocalTagIndex(BaseTagIndex):
         max_tag_length: int = 64,
         **kwargs,
     ):
-        if not isinstance(tag_key, str) or not tag_key.strip():
-            raise ValueError("tag_key must be a non-empty string")
-        tag_key = tag_key.strip()
-        if tag_key in FileFrontMatter.model_fields:
-            raise ValueError(f"tag_key must not be a reserved frontmatter key: {tag_key!r}")
         super().__init__(tag_key=tag_key, **kwargs)
         self.max_tags_per_file = self._positive_int("max_tags_per_file", max_tags_per_file)
         self.max_tag_length = self._positive_int("max_tag_length", max_tag_length)
@@ -31,6 +26,12 @@ class LocalTagIndex(BaseTagIndex):
         self.tag_to_paths: dict[str, set[str]] = {}
         self._maintenance_lock = asyncio.Lock()
 
+    def _validate_tag_key(self, value: object) -> str:
+        tag_key = super()._validate_tag_key(value)
+        if tag_key in FileFrontMatter.model_fields:
+            raise ValueError(f"tag_key must not be a reserved frontmatter key: {tag_key!r}")
+        return tag_key
+
     @property
     def n_files(self) -> int:
         return len(self.path_to_tags)
diff --git a/reme/config/default.yaml b/reme/config/default.yaml
index 53ca0f4e..c6ea233f 100644
--- a/reme/config/default.yaml
+++ b/reme/config/default.yaml
@@ -552,7 +552,7 @@ jobs:
 
   frontmatter_update:
     backend: base
-    description: "Merge key-values into a file's frontmatter."
+    description: "Merge key-value pairs into a Markdown file's frontmatter while preserving its body and unrelated fields."
     parameters:
       type: object
       properties:
diff --git a/reme/steps/evolve/auto_tag.py b/reme/steps/evolve/auto_tag.py
index d05c1e64..63c5c730 100644
--- a/reme/steps/evolve/auto_tag.py
+++ b/reme/steps/evolve/auto_tag.py
@@ -78,7 +78,10 @@ class AutoTagStep(BaseStep):
             self.prompt_format("user_message", path=path, tags_key=tag_key),
             system_prompt=self.prompt_format("system_prompt", tags_key=tag_key),
             job_tools=self.tools,
-            injected_job_kwargs={"_allowed_paths": [path]},
+            injected_job_kwargs={
+                "_allowed_paths": [path],
+                "_allowed_frontmatter_keys": [tag_key],
+            },
         )
 
         target = Path(self.file_store.workspace_path or ".") / path
@@ -90,6 +93,7 @@ class AutoTagStep(BaseStep):
                 path=path,
                 metadata={tag_key: normalized},
                 _allowed_paths=[path],
+                _allowed_frontmatter_keys=[tag_key],
             )
             if not response.success:
                 raise RuntimeError(str(response.answer))
diff --git a/reme/steps/file_io/frontmatter_update.py b/reme/steps/file_io/frontmatter_update.py
index cc4b8f2d..e8801c13 100644
--- a/reme/steps/file_io/frontmatter_update.py
+++ b/reme/steps/file_io/frontmatter_update.py
@@ -30,6 +30,10 @@ class FrontmatterUpdateStep(BaseStep):
     injected by the server into the RuntimeContext;
     without it, restricting read/edit/write alone would still leave
     frontmatter of arbitrary workspace Markdown files mutable.
+
+    ``_allowed_frontmatter_keys`` optionally limits updates to an injected
+    list of top-level keys.  Omitting it (or setting it to ``None``) preserves
+    the unrestricted historical behavior.
     """
 
     async def execute(self):
@@ -39,6 +43,18 @@ class FrontmatterUpdateStep(BaseStep):
         metadata = self.context.get("metadata") or {}
         assert isinstance(metadata, dict), "metadata must be a dict"
 
+        allowed_keys = self.context.get("_allowed_frontmatter_keys")
+        key_error: str | None = None
+        if allowed_keys is not None:
+            if not isinstance(allowed_keys, list) or any(not isinstance(key, str) for key in allowed_keys):
+                key_error = "_allowed_frontmatter_keys must be a list of strings"
+            else:
+                allowed = set(allowed_keys)
+                denied = [key for key in metadata if not isinstance(key, str) or key not in allowed]
+                if denied:
+                    names = ", ".join(sorted(repr(key) for key in denied))
+                    key_error = f"frontmatter key(s) not allowed: {names}"
+
         workspace_dir = Path(self.file_store.workspace_path or ".").resolve()
         target, err = resolve_path(workspace_dir, path)
         if err or target is None:
@@ -57,6 +73,8 @@ class FrontmatterUpdateStep(BaseStep):
                         payload = {"path": path, "error": "not markdown"}
                     elif not metadata:
                         payload = {"path": path, "error": "no fields to update"}
+                    elif key_error:
+                        payload = {"path": path, "error": key_error}
                     else:
                         post = frontmatter.loads(target.read_text(encoding="utf-8"))
                         post.metadata.update(metadata)
diff --git a/tests/unit/test_auto_tag.py b/tests/unit/test_auto_tag.py
index 0c689433..95037205 100644
--- a/tests/unit/test_auto_tag.py
+++ b/tests/unit/test_auto_tag.py
@@ -95,8 +95,14 @@ async def test_auto_tag_filters_paths_and_continues_after_one_file_fails(tmp_pat
 
     assert response.success is False
     assert [call[1]["injected_job_kwargs"] for call in wrapper.calls] == [
-        {"_allowed_paths": ["daily/2026-09-09/failed.md"]},
-        {"_allowed_paths": ["daily/2026-09-09/first.md"]},
+        {
+            "_allowed_paths": ["daily/2026-09-09/failed.md"],
+            "_allowed_frontmatter_keys": ["tags"],
+        },
+        {
+            "_allowed_paths": ["daily/2026-09-09/first.md"],
+            "_allowed_frontmatter_keys": ["tags"],
+        },
     ]
     assert all(
         call[1]["job_tools"] == ["read", "list_tags", "frontmatter_read", "frontmatter_update"]
@@ -131,6 +137,7 @@ async def test_auto_tag_uses_configured_key_and_normalizes_agent_output(tmp_path
 
     async def update_frontmatter(name, /, **kwargs):
         assert name == "frontmatter_update"
+        assert kwargs["_allowed_frontmatter_keys"] == ["keywords"]
         post = frontmatter.loads(note.read_text(encoding="utf-8"))
         post.metadata.update(kwargs["metadata"])
         note.write_text(frontmatter.dumps(post), encoding="utf-8")
diff --git a/tests/unit/test_frontmatter_steps.py b/tests/unit/test_frontmatter_steps.py
index 1ea9a6ef..51a49f34 100644
--- a/tests/unit/test_frontmatter_steps.py
+++ b/tests/unit/test_frontmatter_steps.py
@@ -14,6 +14,7 @@ import os
 import tempfile
 from pathlib import Path
 
+import frontmatter
 import pytest
 
 from reme.components.file_store import LocalFileStore
@@ -89,6 +90,58 @@ async def test_update_no_suffix_autoappends_md():
         await store.close()
 
 
+@pytest.mark.asyncio
+async def test_update_honors_injected_frontmatter_key_scope():
+    """Injected key scope rejects the entire update when any key is outside it."""
+    with tempfile.TemporaryDirectory() as tmp, temp_chdir(tmp):
+        note = _seed(Path(tmp), NOTE, BODY)
+        store = await _make_store()
+
+        before = note.read_bytes()
+        resp = await _run(
+            FrontmatterUpdateStep,
+            store,
+            path=NOTE,
+            metadata={"tags": ["new"], "name": "renamed"},
+            _allowed_frontmatter_keys=["tags"],
+        )
+        assert resp.success is False
+        assert resp.metadata["error"] == "frontmatter key(s) not allowed: 'name'"
+        assert note.read_bytes() == before
+
+        resp = await _run(
+            FrontmatterUpdateStep,
+            store,
+            path=NOTE,
+            metadata={"tags": ["new"]},
+            _allowed_frontmatter_keys=["tags"],
+        )
+        assert resp.success is True
+        assert frontmatter.loads(note.read_text(encoding="utf-8")).metadata == {"name": "n", "tags": ["new"]}
+        await store.close()
+
+
+@pytest.mark.asyncio
+async def test_update_empty_frontmatter_key_scope_denies_all_updates():
+    """An empty injected list is restrictive, unlike an omitted constraint."""
+    with tempfile.TemporaryDirectory() as tmp, temp_chdir(tmp):
+        note = _seed(Path(tmp), NOTE, BODY)
+        store = await _make_store()
+        before = note.read_bytes()
+
+        resp = await _run(
+            FrontmatterUpdateStep,
+            store,
+            path=NOTE,
+            metadata={"tags": ["new"]},
+            _allowed_frontmatter_keys=[],
+        )
+
+        assert resp.success is False
+        assert note.read_bytes() == before
+        await store.close()
+
+
 @pytest.mark.asyncio
 async def test_delete_no_suffix_autoappends_md():
     """frontmatter_delete on a suffix-less path resolves to the ``.md`` file."""
diff --git a/tests/unit/test_tag_index.py b/tests/unit/test_tag_index.py
index 64bd1513..6e1ccc41 100644
--- a/tests/unit/test_tag_index.py
+++ b/tests/unit/test_tag_index.py
@@ -279,6 +279,17 @@ def test_tag_index_rejects_invalid_runtime_frontmatter_key() -> None:
     assert index.tag_key == "tags"
 
 
+@pytest.mark.parametrize("tag_key", ["name", "description"])
+def test_tag_index_rejects_reserved_runtime_frontmatter_key(tag_key: str) -> None:
+    """Runtime updates preserve the same reserved-field invariant as construction."""
+    index = LocalTagIndex()
+
+    with pytest.raises(ValueError, match="tag_key must not be a reserved frontmatter key"):
+        index.tag_key = tag_key
+
+    assert index.tag_key == "tags"
+
+
 def test_file_store_updates_tag_index_from_file_nodes(monkeypatch, tmp_path: Path) -> None:
     """Keep daily and digest tags aligned through file-store mutations."""