diff --git a/README.md b/README.md
index b1d3812..5c1377d 100644
--- a/README.md
+++ b/README.md
@@ -170,7 +170,7 @@ This won't perform any action over the wire, and just calculates the tokens loca
 
 The system message is used to instruct the model how to behave, see [OpenAI - Instructing Chat Models](https://platform.openai.com/docs/guides/chat/instructing-chat-models).
 
-These can be loaded with `-p`. For convenience any file we place under ~/.config/chatblade/ will be picked up by this command.
+These can be loaded with `-p`. For convenience any file we place under `~/.config/chatblade/` will be picked up by this command.
 
 So for example, given the following file `~/.config/chatblade/etymology`, which contains:
 
@@ -203,7 +203,13 @@ We can now run a command and refer to this prompt with `-p etymology`:
 chatblade -p etymology gregarious
 ```
 
-You can also point -p to a file path directly to load a system message from any arbitrary location
+If you set the environment variable `CHATBLADE_DEFAULT_PROMPT` to the name of a prompt, chatblade will use its value if no other prompt is specified on the command line.
+```bash
+% export CHATBLADE_DEFAULT_PROMPT=default
+% chatblade "Hi there"  # Acts like -p default
+```
+
+You can also point `-p` to a file path directly to load a system message from any arbitrary location
 
 <img src="assets/example5.png">
 
diff --git a/chatblade/cli.py b/chatblade/cli.py
index 14d9806..084156f 100644
--- a/chatblade/cli.py
+++ b/chatblade/cli.py
@@ -57,8 +57,8 @@ def handle_input(query, params):
     messages = None
     if params.session:
         messages = storage.messages_from_cache(params.session)
-    if messages:  # a session specified and it alredy exists
-        if params.prompt_file:
+    if messages:  # a session specified and it already exists
+        if params.prompt_file and not params.prompt_set_by_env:
             printer.warn("refusing to prepend prompt to existing session")
             exit(1)
         if query:  # continue conversation
@@ -143,6 +143,15 @@ def cli():
     migrate_old_cache_file_if_exists()
 
     query, params = parser.parse(sys.argv[1:])
+
+    # If we don't have a prompt on the command line, but the user has
+    # defined a CHATBLADE_DEFAULT_PROMPT environment variable, use the env
+    # variable as the prompt.
+    params.prompt_set_by_env = False
+    if not params.prompt_file and "CHATBLADE_DEFAULT_PROMPT" in os.environ:
+        params.prompt_file = os.environ["CHATBLADE_DEFAULT_PROMPT"]
+        params.prompt_set_by_env = True
+
     if params.session_op:
         ret = do_session_op(params.session, params.session_op, params.rename_to)
         exit(ret)