diff --git a/src/transcription/services/prompts.py b/src/transcription/services/prompts.py
index b0b18be..31c7ed9 100644
--- a/src/transcription/services/prompts.py
+++ b/src/transcription/services/prompts.py
@@ -104,8 +104,11 @@ class PromptStore:
 
     def _resolve_existing_prompt(self, name: str) -> Path:
         normalized_name = name.strip()
+        # Path().name is platform-dependent: POSIX treats "\" as an ordinary filename
+        # character, so reject both separators explicitly to match config.PromptFilename.
         if (
             not normalized_name
+            or any(separator in normalized_name for separator in ("/", "\\"))
             or Path(normalized_name).name != normalized_name
             or Path(normalized_name).suffix.lower() != PROMPT_EXTENSION
         ):