diff --git a/README.md b/README.md
index 83f522f..2f6e5ff 100644
--- a/README.md
+++ b/README.md
@@ -329,6 +329,10 @@ backends:
   gemini: { enabled: false, path: /usr/local/bin/gemini, args: [], timeout_ms: 120000 }
 ```
 
+When `args` is non-empty:
+- use `{prompt}` in an argument to inject the full generated prompt directly into argv.
+- if `{prompt}` is not present, Flynn writes the prompt to stdin.
+
 If multiple external backends are enabled, Flynn selects the first in this order: `codex` -> `claude_code` -> `opencode` -> `gemini`.
 
 You can also route specific named agents to a backend: