OpenRouter Users Report Invalid Signature Bug in Sonnet 4.5 Thinking Blocks

Users of OpenRouter are reporting issues with Claude Sonnet 4.5 extended thinking mode, where an "Invalid signature in thinking block" error breaks multi-turn conversations.

The Issue

The first message with reasoning mode works correctly, but subsequent messages fail with the error: "Invalid request: messages.1.content.0: Invalid signature in thinking block."

This appears to be related to how Anthropic changed their message format for thinking blocks, with OpenRouter not yet updated to handle the new format.

Workaround

For developers with backend access, Claude suggests redacting previous thinking blocks before sending messages back to the API:

for msg in messages:
    if msg["role"] == "assistant":
        for block in msg["content"]:
            if block["type"] == "thinking":
                block["thinking"] = "redacted"
                block["signature"] = "redacted"

Current Status

This is a known issue affecting the OpenRouter frontend directly. Users relying on extended thinking through OpenRouter will need to wait for a platform update or use the Anthropic API directly as an alternative.

The issue affects all providers through OpenRouter, not just specific ones.