fix: handle empty message content in AnthropicCompletion (#4427)

[AtharvaJaiswal005]

Summary

Fixes #4427

AnthropicCompletion crashes with a 400 BadRequestError when messages with empty string content ("") are passed to call(). The Anthropic API requires all non-assistant messages to have non-empty content, but the framework was forwarding empty messages without any validation.

This commonly occurs in multi-agent pipelines where one agent's empty output becomes the next agent's input, causing cascading failures.

Changes

lib/crewai/src/crewai/llms/providers/anthropic/completion.py

• Added empty content filtering in _format_messages_for_anthropic() after message formatting but before the first-message-is-user check
• Messages with empty/whitespace-only string content are filtered out
• Assistant messages are preserved (Anthropic allows empty assistant content)
• List content messages (tool results, multimodal blocks) are preserved
• Raises ValueError with a clear message when all messages have empty content, preventing meaningless API calls
• Logs a warning when some empty messages are filtered from a mixed list

lib/crewai/tests/llms/anthropic/test_anthropic.py

• test_anthropic_filters_empty_user_messages — verifies "" content is filtered
• test_anthropic_filters_whitespace_only_user_messages — verifies " " is filtered
• test_anthropic_preserves_assistant_empty_content — verifies empty assistant messages are kept
• test_anthropic_all_empty_user_messages_raises_error — verifies ValueError when all messages are empty

Behavior

Scenario	Before	After
Mix of valid + empty messages	400 BadRequestError	Empty ones filtered, warning logged, valid ones sent
All messages empty	400 BadRequestError	ValueError raised immediately, no API call wasted
Empty assistant message	Works fine	Still works fine (preserved)

Test Plan

• All 4 new tests pass
• Existing test_anthropic_message_formatting passes (no regression)
• Existing test_anthropic_completion_initialization_parameters passes

---

Note

Low Risk
Small, localized change to message formatting plus tests; primary risk is minor behavior change for callers that previously relied on sending empty user messages.

Overview
Prevents Anthropic 400 BadRequestError failures by filtering out non-assistant messages with empty/whitespace-only content in _format_messages_for_anthropic() before enforcing the “first message is user” constraint.

If filtering removes everything, it now raises a ValueError early; if only some are removed, it logs a warning. Adds focused tests covering empty/whitespace user messages, preserving empty assistant content, and the all-empty error case.

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[AtharvaJaiswal005]

Hi! Just checking in on this PR - is there anything I should update or change? Happy to make adjustments based on any feedback. Would also appreciate if someone could approve the CI workflow to run. Thanks!

[github-actions]

This PR is stale because it has been open for 45 days with no activity.