Add --system-prompt-extras option to append file content to system prompt by mykolarudenko · Pull Request #4818 · Aider-AI/aider

mykolarudenko · 2026-02-10T02:59:28Z

Summary

Add a new --system-prompt-extras CLI option that reads a file and appends its content to the system prompt before each LLM request.

Motivation

Many LLMs treat system prompts and user messages differently. To ensure the model strictly follows instructions, they should be added to the system prompt rather than user messages. This option allows users to inject custom instructions into the system prompt via an external file.

Closes #4817

Changes

aider/args.py: Added --system-prompt-extras argument in the "Other settings" group, accepting a file path
aider/coders/base_coder.py:
- Added system_prompt_extras_file parameter to __init__
- Modified fmt_system_prompt() to read the extras file and append its content to the system prompt on each call
aider/main.py: Pass args.system_prompt_extras to Coder.create()

Behavior

The file is read dynamically before each LLM request (not cached at startup), so changes to the file take effect immediately
If the file is not found or unreadable, a warning is displayed but aider continues to work normally
Compatible with CLI, YAML config (system-prompt-extras), and environment variables (AIDER_SYSTEM_PROMPT_EXTRAS)

Usage Example

# Create an extras file with custom system-level instructions
cat > extras.txt << 'EOF'
When a user makes a remark or comment, append it to ./aider_lore/user_comments.md.
If the user points out a mistake or you discover you made one, record the details
in ./aider_lore/my_mistakes.md so you do not repeat it and the user does not have
to flag the same issues multiple times.
EOF

# Run aider with the new option
aider --system-prompt-extras extras.txt

…ompt - Add new CLI argument --system-prompt-extras in 'Other settings' group - Read extras file dynamically before each LLM request in fmt_system_prompt() - Fail gracefully with warning if file is not found or unreadable - Support for CLI, YAML config, and environment variables Closes Aider-AI#4817

Copilot

Pull request overview

Adds support for injecting additional instructions into the system prompt by specifying a file whose contents get appended before LLM requests.

Changes:

Added --system-prompt-extras CLI/config option to accept a file path.
Plumbed the option through main into coder creation.
Appended the file’s content during system prompt formatting.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
aider/args.py	Adds the new `--system-prompt-extras` argument.
aider/main.py	Passes `args.system_prompt_extras` into `Coder.create()`.
aider/coders/base_coder.py	Stores `system_prompt_extras_file` and appends file content during prompt formatting.

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Copilot · 2026-02-10T03:03:46Z

aider/coders/base_coder.py

+        # Add extra system prompt content from file if specified
+        if self.system_prompt_extras_file:
+            extras_content = self.io.read_text(self.system_prompt_extras_file)
+            if extras_content:
+                prompt = prompt + "\n\n" + extras_content.strip()
+


Appending extras inside fmt_system_prompt() causes the extras file to be read and appended for every call (main system prompt, each example message, system reminders, etc.). This can duplicate the extras multiple times per request and even inject it into non-system example messages. Consider reading the extras file once per request (eg when assembling main_sys / system messages in format_chat_chunks()) and appending it only to the actual system prompt content.

Copilot · 2026-02-10T03:03:47Z

aider/coders/base_coder.py

+        if self.system_prompt_extras_file:
+            extras_content = self.io.read_text(self.system_prompt_extras_file)
+            if extras_content:
+                prompt = prompt + "\n\n" + extras_content.strip()


extras_content.strip() changes the file’s content (removes leading/trailing whitespace/newlines). Since the option is described as appending the file content, it’s safer to preserve the text exactly (or at most rstrip a single trailing newline) so users can control formatting precisely.

Suggested change

prompt = prompt + "\n\n" + extras_content.strip()

# Preserve extras content formatting; at most remove a single trailing newline

if extras_content.endswith("\n"):

extras_content = extras_content[:-1]

prompt = prompt + "\n\n" + extras_content

Copilot · 2026-02-10T03:03:47Z

aider/coders/base_coder.py

+        if self.system_prompt_extras_file:
+            extras_content = self.io.read_text(self.system_prompt_extras_file)
+            if extras_content:
+                prompt = prompt + "\n\n" + extras_content.strip()


InputOutput.read_text() reports failures via tool_error; with the current placement this can emit an error message repeatedly on every prompt formatting when the file is missing/unreadable. To match the intended “warning but continue” behavior (and avoid spamming/inflating error counters), read silently and emit a tool_warning once (or throttle/reuse the last error).

Copilot · 2026-02-10T03:03:47Z

aider/main.py

            auto_copy_context=args.copy_paste,
            auto_accept_architect=args.auto_accept_architect,
            add_gitignore_files=args.add_gitignore_files,
+            system_prompt_extras_file=args.system_prompt_extras,


This new CLI/config option introduces new behavior in prompt construction; please add/extend tests to cover that the extras file content is appended to the system prompt and that missing/unreadable files only produce a warning without breaking requests.

Read the extras file once per request and append only to the main system prompt, avoiding duplication across example messages and system reminders.

Only remove a single trailing newline instead of strip() to let users control formatting precisely.

- test_system_prompt_extras_file: verify extras content is appended to system prompt - test_system_prompt_extras_file_missing: verify graceful handling of missing file - test_system_prompt_extras_file_reread_each_request: verify file is re-read dynamically

CLAassistant · 2026-02-10T03:24:45Z

All committers have signed the CLA.

Copilot AI review requested due to automatic review settings February 10, 2026 02:59

Copilot started reviewing on behalf of mykolarudenko February 10, 2026 02:59 View session

Copilot AI reviewed Feb 10, 2026

View reviewed changes

mykolarudenko added 3 commits February 10, 2026 03:10

Move extras reading from fmt_system_prompt to format_chat_chunks

73bb653

Read the extras file once per request and append only to the main system prompt, avoiding duplication across example messages and system reminders.

Preserve extras file formatting instead of stripping whitespace

acf54c2

Only remove a single trailing newline instead of strip() to let users control formatting precisely.

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Add --system-prompt-extras option to append file content to system prompt#4818

Add --system-prompt-extras option to append file content to system prompt#4818
mykolarudenko wants to merge 4 commits intoAider-AI:mainfrom
mykolarudenko:feature/system-prompt-extras

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-                prompt = prompt + "\n\n" + extras_content.strip()
+                # Preserve extras content formatting; at most remove a single trailing newline
+                if extras_content.endswith("\n"):
+                    extras_content = extras_content[:-1]
+                prompt = prompt + "\n\n" + extras_content

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mykolarudenko commented Feb 10, 2026

Summary

Motivation

Changes

Behavior

Usage Example

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CLAassistant commented Feb 10, 2026 •

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