fix(vertexai): allow system messages at any position in ChatAnthropicVertex#1610
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Leonid Kuligin (lkuligin) merged 1 commit intolangchain-ai:mainfrom Feb 24, 2026
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…Vertex The _format_messages_anthropic() function required system messages to be at index 0, raising ValueError otherwise. This prevented valid patterns like [HumanMessage, SystemMessage, ...] that are supported by the Anthropic API and by langchain-anthropic. This change removes the position check and instead validates that non-consecutive system messages are not present (raising a clear error). Consecutive system messages are merged. The _merge_messages() function is extended to merge consecutive SystemMessages the same way it already merged consecutive HumanMessages. Fixes langchain-ai#1022
Leonid Kuligin (lkuligin)
approved these changes
Feb 24, 2026
Leonid Kuligin (lkuligin)
merged commit 7453adb
into
langchain-ai:main
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Problem
The
_format_messages_anthropic()function required system messages to be at index 0, raisingValueError("System message must be at beginning of message list.")otherwise. This prevented valid patterns like[HumanMessage(...), SystemMessage(...), ...]that are supported by the Anthropic API and bylangchain-anthropic.Error from issue #1022:
Solution
if i != 0) in_format_messages_anthropic()ValueError)_merge_messages()to merge consecutiveSystemMessages the same way it already merged consecutiveHumanMessagesmodel_copy(update=...)instead of mutating message content in-placeTests
ValueErrorChanges
libs/vertexai/langchain_google_vertexai/_anthropic_utils.pylibs/vertexai/tests/unit_tests/test_anthropic_utils.pyFixes #1022