diff --git a/chat.el b/chat.el
index 6813b64..977e8b6 100644
--- a/chat.el
+++ b/chat.el
@@ -418,7 +418,8 @@ fn path_within_root(path: String, root: String) -> Bool {
         return false
     }
     if str_starts_with(path, "/") {
-        return str_starts_with(path, root)
+        let root_normalized: String = root + "/"
+        return str_starts_with(path, root_normalized)
     }
     return true
 }
@@ -509,12 +510,17 @@ fn dispatch_tool(tool_name: String, tool_input: String) -> String {
         let path: String = json_get(tool_input, "path")
         let old_text: String = json_get(tool_input, "old_text")
         let new_text: String = json_get(tool_input, "new_text")
-        let content: String = fs_read(path)
+        let root: String = agent_workspace_root()
+        if !path_within_root(path, root) {
+            return json_safe("denied: path is outside the agent workspace root")
+        }
+        let resolved: String = resolve_in_root(path, root)
+        let content: String = fs_read(resolved)
         if str_eq(content, "") {
             return json_safe("{\"error\":\"file not found\"}")
         }
         let updated: String = str_replace(content, old_text, new_text)
-        fs_write(path, updated)
+        fs_write(resolved, updated)
         return json_safe("{\"ok\":true}")
     }
     if str_eq(tool_name, "remember") {
@@ -634,13 +640,13 @@ fn handle_chat_agentic(body: String) -> String {
     // Workspace scope (#23): the desktop UI sends the user-chosen Agent Workspace root
     // on every agentic request. Persist it to state so agent_workspace_root() — and the
     // path/command tool guards that read it — confine this turn's file/command tools to
-    // that subtree. The UI is the source of truth per request: empty means unscoped (the
-    // backward-compatible default), and it also lets agent_workspace_root() fall through
-    // to the NEURON_AGENT_ROOT env when no root is sent. FLAGGED FOR REVIEW: setting
-    // state from the body each turn (vs. only-when-nonempty) so clearing the folder in
-    // the UI un-scopes — confirm this is the intended ownership model.
+    // that subtree. Only set when non-empty: an empty/absent field means the client sent
+    // no root (or cleared the field), and we must not overwrite a server-configured root
+    // from NEURON_AGENT_ROOT with an empty string, which would silently un-scope the agent.
     let ws_root: String = json_get(body, "agent_workspace_root")
-    state_set("agent_workspace_root", ws_root)
+    if !str_eq(ws_root, "") {
+        state_set("agent_workspace_root", ws_root)
+    }
 
     let req_model: String = json_get(body, "model")
     let model: String = if str_eq(req_model, "") { chat_default_model() } else { req_model }