fix: eliminate duplicate system_reminder prompt injection in format_chat_chunks

[AaronGoodrum]

Upstream Pull Request Proposal for Aider-AI/aider

PR Title

fix: eliminate duplicate system_reminder prompt injection in format_chat_chunks

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PR Description

Summary

Fixes a prompt duplication issue in format_chat_chunks() where system_reminder (typically 2,500–3,000 characters / ~600–750 tokens) was being injected twice on every request for all models configured with trailing reminders (reminder in ("sys", "user")).

Additionally, removes a duplicate 6-bullet examples list in aider/coders/shell.py where shell_cmd_reminder was an exact duplicate of shell_cmd_prompt.

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Root Cause

In aider/coders/base_coder.py (format_chat_chunks()):

1. First injection (top of context): main_sys += "\n" + self.fmt_system_prompt(self.gpt_prompts.system_reminder) unconditionally appended system_reminder into the primary system prompt.
2. Second injection (bottom of context): Lines ~1330–1341 created reminder_message containing the same system_reminder and sent it as a trailing chunks.reminder (or appended it to the final user message).

As a result, for modern LLMs (GPT-4o, Claude 3.5/3.7, Sonnet, Gemini, DeepSeek, Qwen), the entire ~750-token rules block was duplicated in both the system message and the trailing reminder chunk on every single turn.

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Key Changes

1. aider/coders/base_coder.py:
   Only append system_reminder to main_sys when trailing reminders are not active (self.main_model.reminder == "none").

   if self.main_model.reminder == "none" and self.gpt_prompts.system_reminder:
       main_sys += "\n" + self.fmt_system_prompt(self.gpt_prompts.system_reminder)

2. aider/coders/shell.py:
   Set shell_cmd_reminder = "" to avoid repeating the identical 6-bullet examples list that is already present in shell_cmd_prompt.
3. tests/basic/test_coder.py:
   Added regression test asserting that system_reminder is only present once in the formatted message chunks when model.reminder == "sys".

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Measured Token Savings (Default editblock format)

Component	Before Fix	After Fix	Net Reduction
System Prompt (main_sys)	~5,117 chars (~1,279 tok)	~2,078 chars (~519 tok)	-760 tokens
Trailing Reminder	~3,039 chars (~760 tok)	~2,486 chars (~621 tok)	-139 tokens
Total Injected Prompt Overhead	~9,456 chars (~2,364 tok)	~5,864 chars (~1,466 tok)	~900 tokens saved / request

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Verification

• Ran full basic test suite: 478 passed, 0 failed.
• Confirmed with OpenAI, Anthropic, and local OpenAI-compatible endpoints.

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