fix(anthropic): handle empty message content gracefully

[Jason Meng (JasonCZMeng)]

Summary

Replace empty string content in human and system messages with a single space ( ) before sending to the Anthropic API. This prevents a BadRequestError (400) when HumanMessage or SystemMessage has content="".

Fixes #35081

Motivation

Anthropic's API rejects messages with empty content: "all messages must have non-empty content except for the optional final assistant message". This is a problem in multi-agent pipelines where one agent's empty output becomes the next agent's user input. The same approach was adopted by AutoGen.

Changes

libs/partners/anthropic/langchain_anthropic/chat_models.py

• In _format_messages(), empty string content ("") in human and system messages is replaced with " " before calling the API
• Handles string content, list content blocks, and dict text blocks
• Also fixes a minor inefficiency in tool_result content formatting for plain string content

libs/partners/anthropic/tests/unit_tests/test_chat_models.py

• Added test__format_messages_with_empty_system_and_human_content() covering the fix

[codspeed]

Merging this PR will not alter performance

✅ 3 untouched benchmarks
⏩ 33 skipped benchmarks¹

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_{Comparing JasonCZMeng:fix/issue-35081 (6215543) with master (29134dc)}

Footnotes

1. 33 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Jason Meng (JasonCZMeng)]

Friendly ping — CI is green and no performance impact per CodSpeed. This is a minimal fix (replaces empty content with " " in ChatAnthropic, matching AutoGen's approach) for issue #35081 where empty message content causes crashes. Would love a review when you get a chance! 🙏

[Kevin Kakolla (torontodeveloper)]

i think this can be written for better readability this
block_text = " " if block=="" else block
obj = {"type":"text", "text:block_text}
system.append(obj)

though this codes looks like a lot, but's more readable in my 2 cents. I let u decide than compacting the code

[Kevin Kakolla (torontodeveloper)]

.

[Jason Meng (JasonCZMeng)]

Closing this to keep my PR queue focused — no reviewer activity in ~40 days. The change is still mergeable; happy to reopen if a maintainer wants to pick it up, or I'll revisit if the underlying issue resurfaces. Thanks!