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openclaw - 💡(How to fix) Fix [Bug] 4.25 regression: Bedrock Claude 4.6 "assistant message prefill" error on first message (Feishu channel) [1 participants]

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openclaw/openclaw#72865•Fetched 2026-04-28 06:31:11
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After upgrading from OpenClaw 2026.4.24 to 2026.4.25, all new sessions on the Feishu channel fail on the first user message with:

Validation error: The model returned the following errors: This model does not support assistant message prefill. The conversation must end with a user message.

Rolling back to 2026.4.24 resolves the issue immediately.

Error Message

Validation error: The model returned the following errors: This model does not support assistant message prefill. The conversation must end with a user message.

  1. Send any message → error
  • larksuite/openclaw-lark#285 (similar prefill error after session compaction)

Root Cause

Root Cause Hypothesis

Claude 4.6 dropped support for assistant message prefill (Anthropic breaking change since Feb 2026). OpenClaw 4.25 introduced a "shorter startup greeting" change (Crestodian/TUI) that appears to insert an assistant message into the conversation context before the first user message, causing the Bedrock Converse API to reject the request.

Fix Action

Workaround

Rollback to OpenClaw 2026.4.24: pnpm install -g [email protected]

RAW_BUFFERClick to expand / collapse

Bug type

Regression (worked before, now fails)

Summary

After upgrading from OpenClaw 2026.4.24 to 2026.4.25, all new sessions on the Feishu channel fail on the first user message with:

Validation error: The model returned the following errors: This model does not support assistant message prefill. The conversation must end with a user message.

Rolling back to 2026.4.24 resolves the issue immediately.

Environment

  • OpenClaw: 2026.4.25 (aa36ee6)
  • Working version: 2026.4.24
  • Provider: Amazon Bedrock (bedrock-converse-stream)
  • Model: global.anthropic.claude-opus-4-6-v1 (Claude Opus 4.6)
  • Channel: Feishu (@larksuite/[email protected])
  • Config: thinkingDefault: "high", api: "bedrock-converse-stream"
  • OS: Linux (Ubuntu)

Steps to Reproduce

  1. Install OpenClaw 2026.4.25
  2. Configure Bedrock with Claude Opus 4.6 as primary model
  3. Connect Feishu channel ([email protected])
  4. Start a new session (e.g. via /new)
  5. Send any message → error

Expected Behavior

First message in a new session should work without errors, as it does in 2026.4.24.

Root Cause Hypothesis

Claude 4.6 dropped support for assistant message prefill (Anthropic breaking change since Feb 2026). OpenClaw 4.25 introduced a "shorter startup greeting" change (Crestodian/TUI) that appears to insert an assistant message into the conversation context before the first user message, causing the Bedrock Converse API to reject the request.

Related

Workaround

Rollback to OpenClaw 2026.4.24: pnpm install -g [email protected]

extent analysis

TL;DR

The most likely fix is to rollback to OpenClaw version 2026.4.24 due to a compatibility issue with Claude Opus 4.6's dropped support for assistant message prefill.

Guidance

  • Verify that the issue is indeed caused by the introduction of the "shorter startup greeting" in OpenClaw 2026.4.25, which may be inserting an assistant message before the first user message.
  • Check the Bedrock Converse API documentation to see if there are any configuration options or workarounds for handling assistant message prefill with Claude Opus 4.6.
  • Consider reaching out to the OpenClaw or Bedrock support teams to report the issue and request a fix or guidance on a potential workaround.
  • Review the related issues, such as larksuite/openclaw-lark#285, to see if there are any additional insights or potential solutions.

Example

No code snippet is provided as the issue seems to be related to a version compatibility problem rather than a code-specific error.

Notes

The provided workaround of rolling back to OpenClaw 2026.4.24 is a temporary solution and may not be suitable for all users, especially if they rely on features introduced in version 2026.4.25.

Recommendation

Apply the workaround by rolling back to OpenClaw version 2026.4.24, as it is a known stable version that does not exhibit this issue, and wait for a potential fix in a future version of OpenClaw that addresses the compatibility issue with Claude Opus 4.6.

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openclaw - 💡(How to fix) Fix [Bug] 4.25 regression: Bedrock Claude 4.6 "assistant message prefill" error on first message (Feishu channel) [1 participants]