diff --git a/.agents/skills/layerfs-js-sdk/references/methods/fs-write.md b/.agents/skills/layerfs-js-sdk/references/methods/fs-write.md
index 8171f985b..74add024c 100644
--- a/.agents/skills/layerfs-js-sdk/references/methods/fs-write.md
+++ b/.agents/skills/layerfs-js-sdk/references/methods/fs-write.md
@@ -30,6 +30,7 @@ Examples use `layerfs` as connected in [SKILL.md](../../SKILL.md#connect-to-a-pr
 | `NoSuchBranch` | No such Branch | `branch.fork` it, or check the name |
 | `HeadMoved` | Another writer published on the Branch during the call; nothing was published | Read the files you changed again, then decide whether to apply your change |
 | `NoSuchPath` | A path or its parent folder is missing | `fs.list` the parent; `fs.mkdir` first |
+| `NotDirectory` | A file or a symlink is in the way of the path; `message` is that entry | `fs.stat` it; choose another path or remove it first |
 | `TooLarge` | The complete encoded request did not fit one control call; nothing was sent | Inspect the input, then choose a smaller call or a larger atomic Workspace step |
 | `InvalidInput` | A bad argument (message bytes, relative path, …) | Read `message`; fix the call |
 
diff --git a/core/crates/layerfs-api/tests/write_refusals.rs b/core/crates/layerfs-api/tests/write_refusals.rs
index 5b64e0455..ba96142df 100644
--- a/core/crates/layerfs-api/tests/write_refusals.rs
+++ b/core/crates/layerfs-api/tests/write_refusals.rs
@@ -73,3 +73,77 @@ fn a_write_refusal_names_its_cause() {
     assert!(support::items(&commits).is_empty());
     fixture.cleanup();
 }
+
+/// A name below a file or a symlink (which is not followed) is refused
+/// `NotDirectory` at that entry by every write call, as `fs.mkdir` answers.
+/// Such a call keeps no Store call Workspace: more of them than the Store
+/// has call Workspaces leave a valid write working. `fs.mkdir` never took
+/// one, so only the other calls count toward that number.
+#[test]
+fn a_write_through_a_non_directory_is_not_directory() {
+    let fixture = Fixture::new("write-through-non-directory", |source| {
+        fs::create_dir(source.join("dir")).unwrap();
+        fs::write(source.join("dir/inner.txt"), b"inner\n").unwrap();
+        fs::write(source.join("a.txt"), b"a\n").unwrap();
+        std::os::unix::fs::symlink("dir", source.join("dlink")).unwrap();
+        4
+    });
+    fixture.fork("feat");
+    let t = SdkValue::text;
+    let cases = [
+        (
+            "fs.write",
+            vec![("path", t("/a.txt/x")), ("content", t("x"))],
+            "/a.txt",
+        ),
+        (
+            "fs.write",
+            vec![("path", t("/dlink/x")), ("content", t("x"))],
+            "/dlink",
+        ),
+        ("fs.rm", vec![("path", t("/a.txt/x"))], "/a.txt"),
+        ("fs.mkdir", vec![("path", t("/a.txt/x"))], "/a.txt"),
+        (
+            "fs.edit",
+            vec![
+                ("path", t("/dlink/inner.txt")),
+                ("oldText", t("inner")),
+                ("newText", t("x")),
+            ],
+            "/dlink",
+        ),
+        (
+            "fs.cp",
+            vec![("from", t("/a.txt")), ("to", t("/dlink/c.txt"))],
+            "/dlink",
+        ),
+        (
+            "fs.mv",
+            vec![("from", t("/dir/inner.txt")), ("to", t("/a.txt/y"))],
+            "/a.txt",
+        ),
+    ];
+    let calls = layerfs_store_service::OwnerConfig::default().namespaces + 1;
+    let held_a_workspace = cases.iter().filter(|(method, ..)| *method != "fs.mkdir");
+    for (method, fields, at) in cases.iter().chain(held_a_workspace.cycle().take(calls)) {
+        let mut fields = fields.clone();
+        fields.push(("branch", t("feat")));
+        let r = fixture
+            .try_call(method, fields)
+            .expect_err("the call is refused");
+        assert_eq!(
+            (r.code.as_str(), r.message.as_str()),
+            ("NotDirectory", *at),
+            "{method}"
+        );
+    }
+    fixture.call(
+        "fs.write",
+        vec![
+            ("branch", t("feat")),
+            ("path", t("/after.txt")),
+            ("content", t("after")),
+        ],
+    );
+    fixture.cleanup();
+}
diff --git a/core/crates/layerfs-store-service/src/write/mutation.rs b/core/crates/layerfs-store-service/src/write/mutation.rs
index 1b82e714d..bb7e76b44 100644
--- a/core/crates/layerfs-store-service/src/write/mutation.rs
+++ b/core/crates/layerfs-store-service/src/write/mutation.rs
@@ -102,13 +102,22 @@ impl<'a> Mutation<'a> {
             }
         }
     }
+    /// The entry at `path`. A name below an entry that is not a directory (a
+    /// file or a symlink, which is not followed) is `NotDirectory` at that
+    /// entry, as `mkdir` answers, not an engine failure.
     pub fn resolve(&self, path: &str) -> Result<ViewStat> {
         let mut stat = self
             .view
             .stat(self.op.overlay(), self.view.root_serial())
             .map_err(failure)?;
+        let mut at = String::new();
         for name in path.split('/').filter(|s| !s.is_empty()) {
             let name = checked(path, name)?;
+            if stat.kind != layerfs_content::object::inode_leaf::InodeKind::Directory {
+                return Err(args::refuse("NotDirectory", at, SdkValue::Null));
+            }
+            at.push('/');
+            at.push_str(name.as_str());
             stat = self
                 .view
                 .lookup(self.op.overlay(), stat.serial, &name)