System prompts guide how the generation agent works. The repository includes separate examples for replacing its instructions and appending project rules:

# Replace the resolved system instructions.
$ llmc++ \
    -fllm-system-prompt=examples/markdown/13_prompt.md \
    examples/13_system_prompt.cpp
 
# Keep the built-in instructions and append project rules.
$ llmc++ \
    -fllm-append-prompt=examples/markdown/14_rules.md \
    examples/14_append_prompt.cpp

Both files must be UTF-8. Paths resolve from the compiler's working directory. Their contents affect cache identity, so edits can invalidate a previous implementation.

Function modifiers provide the same controls for individual targets. A function-level replacement discards driver-appended instructions; its own additions follow the replacement regardless of modifier order. Reference context attaches supporting data separately from these instructions.