diff --git a/packages/jupyter-ai-magics/jupyter_ai_magics/magics.py b/packages/jupyter-ai-magics/jupyter_ai_magics/magics.py
index ebfdf7b4a..8acb40c6d 100644
--- a/packages/jupyter-ai-magics/jupyter_ai_magics/magics.py
+++ b/packages/jupyter-ai-magics/jupyter_ai_magics/magics.py
@@ -105,17 +105,61 @@ def _repr_mimebundle_(self, include=None, exclude=None):
 MULTIENV_REQUIRES = "Requires environment variables:"
 
 
-class FormatDict(dict):
-    """Subclass of dict to be passed to str#format(). Suppresses KeyError and
-    leaves replacement field unchanged if replacement field is not associated
-    with a value."""
-
-    def __missing__(self, key):
-        return key.join("{}")
-
-
-class EnvironmentError(BaseException):
-    pass
+class PromptStr(str):
+    """
+    A string subclass that processes its content to support a custom
+    placeholder delimiter. Custom placeholders are marked with "@{...}".
+
+    When format() or format_map() is called, the instance is first processed:
+      - Custom placeholders (e.g. "@{var}") are converted into standard
+        placeholders ("{var}") for interpolation.
+      - All other literal curly braces are doubled (e.g. "{" becomes "{{")
+        so that they are preserved literally.
+
+    If any custom placeholder contains additional curly braces (i.e. nested
+    braces), a ValueError is raised.
+    """
+
+    def __init__(self, text):
+        self._template = self._process_template(text)
+
+    @staticmethod
+    def _process_template(template: str) -> str:
+        """
+        Process the input template so that:
+          - Any custom placeholder of the form "@{...}" is converted into
+            a normal placeholder "{...}".
+          - All other literal curly braces are doubled so that they remain
+            unchanged during formatting.
+
+        Assumes that the custom placeholder does not contain nested braces.
+        If nested or extra curly braces are found within a custom placeholder,
+        a ValueError is raised.
+        """
+        # Pattern to match custom placeholders: "@{...}" where ... has no braces.
+        pattern = r"@{([^{}]+)}"
+        tokens = []
+
+        def token_replacer(match):
+            inner = match.group(1)
+            assert ("{" not in inner) and ("}" not in inner)
+            tokens.append(inner)
+            return f"<<<{len(tokens)-1}>>>"
+
+        template_with_tokens = re.sub(pattern, token_replacer, template)
+        if "@{" in template_with_tokens:
+            raise ValueError("Curly braces are not allowed inside custom placeholders.")
+
+        escaped = template_with_tokens.replace("{", "{{").replace("}", "}}")
+        for i, token in enumerate(tokens):
+            escaped = escaped.replace(f"<<<{i}>>>", f"{{{token}}}")
+        return escaped
+
+    def format(self, *args, **kwargs):
+        return self._template.format(*args, **kwargs)
+
+    def format_map(self, mapping):
+        return self._template.format_map(mapping)
 
 
 class CellMagicError(BaseException):
@@ -587,10 +631,6 @@ def run_ai_cell(self, args: CellArgs, prompt: str):
         # Apply a prompt template.
         prompt = provider.get_prompt_template(args.format).format(prompt=prompt)
 
-        # interpolate user namespace into prompt
-        ip = self.shell
-        prompt = prompt.format_map(FormatDict(ip.user_ns))
-
         context = self.transcript[-2 * self.max_history :] if self.max_history else []
         if provider.is_chat_provider:
             result = provider.generate([[*context, HumanMessage(content=prompt)]])
@@ -676,10 +716,10 @@ def ai(self, line, cell=None):
                 subcommands."""
             )
 
-        prompt = cell.strip()
+        prompt = PromptStr(cell.strip())
 
         # interpolate user namespace into prompt
         ip = self.shell
-        prompt = prompt.format_map(FormatDict(ip.user_ns))
+        prompt = prompt.format_map(ip.user_ns)
 
         return self.run_ai_cell(args, prompt)