fix: sanitize empty text content blocks in /v1/messages endpoint

[shaeberling]

Summary

• Sanitize empty text content blocks in the /v1/messages endpoint before forwarding to providers
• Fixes 400 errors ("text content blocks must be non-empty") in multi-turn tool-use conversations

Problem

The Anthropic API returns assistant messages with empty text blocks alongside tool_use blocks:

{"type": "text", "text": ""},
{"type": "tool_use", "id": "toolu_xxx", "name": "Bash", "input": {...}}

While the API returns these, it rejects them when sent back in subsequent requests. The /anthropic passthrough handles this transparently, but the /v1/messages native path (used for Anthropic direct and Bedrock) passes messages through without sanitization.

The /v1/chat/completions path already handles this via process_empty_text_blocks() in factory.py, but the /v1/messages path was missing equivalent sanitization. modify_params=True does not help as it targets a different code path.

Fix

Add _sanitize_anthropic_messages() at the entry point of anthropic_messages() in the /v1/messages handler, before any routing. This strips empty/whitespace-only text blocks from message content arrays while preserving non-empty text and other block types (tool_use, tool_result, etc.). Does not leave content arrays empty to avoid a different validation error.

Testing

• 7 unit tests covering: empty text + tool_use, whitespace-only, non-empty preservation, single empty block (no modification), string content, and user messages
• Verified fix on production deployment with Claude Code / Agent SDK multi-turn tool-use conversations routed through /v1/messages to both Anthropic direct and Bedrock Converse

Fixes #22930

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Project	Deployment	Actions	Updated (UTC)
litellm	Ready	Preview, Comment	Mar 5, 2026 11:16pm

[CLAassistant]

All committers have signed the CLA.

[greptile-apps]

Greptile Summary

This PR adds _sanitize_anthropic_messages() to the /v1/messages handler to strip empty and whitespace-only text content blocks from messages before they are forwarded to providers. This fixes 400 errors ("text content blocks must be non-empty") that occur in multi-turn tool-use conversations where the Anthropic API returns—but won't accept back—empty text blocks alongside tool_use blocks.

Key implementation details:

• _sanitize_anthropic_messages() is inserted at the entry point of anthropic_messages(), before any routing or hook execution, ensuring all provider paths (Anthropic direct, Bedrock, etc.) benefit.
• Handles None text values safely via (block.get("text") or "").strip().
• Creates shallow copies of affected message dicts to avoid mutating caller's original message objects.
• When all content blocks in a message are empty text, falls back to DEFAULT_ASSISTANT_CONTINUE_MESSAGE, preventing a different validation error from an empty content array.
• Comprehensive test coverage with 8 unit tests covering empty text + tool_use, whitespace-only text, non-empty preservation, all-empty fallback, string content, user messages, and None text values.

Confidence Score: 5/5

• PR is safe to merge. The fix is targeted, well-tested, and handles all identified edge cases correctly.
• All logic is correct. The _sanitize_anthropic_messages() function properly filters empty/whitespace-only text blocks while preserving non-empty content and other block types (tool_use, tool_result, etc.). Defensive guards against None text values work correctly. When all blocks are empty, the function correctly falls back to DEFAULT_ASSISTANT_CONTINUE_MESSAGE to prevent validation errors. Tests are comprehensive and properly mocked with no real network calls. Both changed files are clean and the implementation is consistent with existing codebase patterns.
• No files require special attention

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["Client sends /v1/messages request\n(multi-turn tool-use conversation)"] --> B["anthropic_messages()"]
    B --> C["_sanitize_anthropic_messages(messages)"]
    C --> D{"For each message:\ncontent is a list?"}
    D -- No --> E["Pass through unchanged"]
    D -- Yes --> F["Filter: remove blocks where\ntype=text AND text is empty/whitespace"]
    F --> G{"Any blocks\nremoved?"}
    G -- No --> E
    G -- Yes --> H{"filtered list\nnon-empty?"}
    H -- Yes --> I["Replace message with shallow copy\n{**message, content: filtered}"]
    H -- No --> J["Replace message with\ncontinuation message\n(DEFAULT_ASSISTANT_CONTINUE_MESSAGE)"]
    I --> K["Return sanitized messages list"]
    J --> K
    E --> K
    K --> L["_execute_pre_request_hooks()"]
    L --> M["Route to provider\n(Anthropic direct / Bedrock / etc.)"]

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_{Last reviewed commit: e8e2a27}