Bug report: Anthropic provider broken after auto-update to 2026.4.26

[atomic obsidian]

After OpenClaw auto-updated to version 2026.4.26 today, Anthropic models (Haiku and Sonnet) fail at session startup with:
messages: at least one message is required
Error from gateway.err.log:
embedded run failover decision: decision=fallback_model reason=format from=anthropic/claude-haiku-4-5-20251001 rawError=400 {"type":"error","error":{"type":"invalid_request_error","message":"messages: at least one message is required"}}
Was running Haiku as primary for weeks with no issues. Broke with this update. Had to revert to Grok as primary. Please advise on fix or rollback.

[spark charm]

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[twilit tangle]

Yep — this looks like a real 2026.4.26 regression, not bad Anthropic config.

I found a post-2026.4.26 fix on main: “keep bare reset transcript prompt non-empty”, changing empty session/reset transcript prompts to [OpenClaw session new]. That matches Anthropic rejecting startup with messages: at least one message is required. (github.com)

Fastest safe rollback:

openclaw update --tag 2026.4.25
openclaw doctor
openclaw gateway restart
openclaw --version

If the auto-updater keeps pulling 2026.4.26 back in, temporarily set OPENCLAW_NO_AUTO_UPDATE=1 in the Gateway environment; the docs call that out for downgrade/incident recovery. (github.com)

If you’re comfortable running main instead of stable, this should also pick up the fix: