Python: Fix duplicate system message from instructions - #5051
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Eduard van Valkenburg (eavanvalkenburg) merged 1 commit intoApr 2, 2026
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Add deduplication to `prepend_instructions_to_messages()` to skip instructions that are already present as leading messages with the same role and text. This prevents duplicate system messages when instructions are injected by multiple layers (e.g. Agent + chat client). Fixes microsoft#5049
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Eduard van Valkenburg (eavanvalkenburg)
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… (microsoft#5051) Add deduplication to `prepend_instructions_to_messages()` to skip instructions that are already present as leading messages with the same role and text. This prevents duplicate system messages when instructions are injected by multiple layers (e.g. Agent + chat client). Fixes microsoft#5049
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Summary
Fixes #5049
Root cause:
prepend_instructions_to_messages()in_types.pyblindly prepends instruction messages without checking if identical messages already exist at the head of the list. When instructions are injected by multiple layers (e.g., Agent passesinstructionsin options, and the chat client also prepends them), duplicate system messages are sent to the API.Fix: Added deduplication logic to
prepend_instructions_to_messages()that skips instructions which are already present as leading messages with matching role and text content. This is a centralized fix that benefits all chat clients (OpenAI, Ollama, Anthropic, etc.).Changes:
python/packages/core/agent_framework/_types.py: Added deduplication check before prependingpython/packages/core/tests/core/test_types.py: Added 6 unit tests covering basic prepend, None handling, deduplication (single + list), different instructions, and custom rolespython/packages/openai/tests/openai/test_openai_chat_completion_client.py: Added regression test for the exact scenario in Python: [Bug]: OpenAIChatClient: System message from instructions is duplicated in the messages sent to OpenAI API #5049Test plan
test_types.pytests pass (no regressions)test_prepare_options_with_instructionstests passruff checkandruff formatpass on all changed files