Python README missing system message customization documentation

[Halcyonhal9]

The .NET README documents system message customization (append, customize, replace modes) under "System Message Customization", but the Python README has no equivalent section. The Python SDK supports all three modes plus transform callbacks (see SystemMessageAppendConfig, SystemMessageReplaceConfig, SystemMessageCustomizeConfig in session.py).

[github-actions]

Thanks for reporting this! Here's a summary of the documentation gap:

Documentation Gap Summary

What's missing: The Python README (python/README.md) lacks a "System Message Customization" section, even though the Python SDK fully supports all three modes.

Affected file: python/README.md

Reference: The .NET README (dotnet/README.md) has a well-documented "System Message Customization" section covering append, customize, and replace modes — the Python docs should be brought to parity.

What should be added

A new section in python/README.md documenting system message customization, covering:

1. Append mode — using SystemMessageAppendConfig to append content to the default system message
2. Replace mode — using SystemMessageReplaceConfig to fully replace the system message
3. Customize mode — using SystemMessageCustomizeConfig for structured customization
4. Transform callbacks — Python-specific feature for applying a callable transform to the system message

Each mode should include a brief description and a Python code example, similar to how the .NET README presents the equivalent C# examples.

Generated by Documentation Handler for issue #1065 · ● 148.7K · ◷

[patniko]

Closing as fixed after verification against the current public SDK sources and bundled behavior.

Public evidence

• #1066 — docs: add system message customization section to Python README (merged 2026-06-24)
• commit 481ccac32a80 — docs: add system message customization section to Python README (docs: add system message customization section to Python README #1066)
• python/README.md — System Message Customization

The reported behavior/request is implemented at the current SDK baseline, so this issue is being closed as completed.

Public SDK baseline: github/copilot-sdk d5c9d06d8c41.