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google-vertex-anthropic: claude-sonnet-5 fails on every message in v1.18.26 ("Extra inputs are not permitted" / thinking.block_binding) — works in v1.18.20 #46777
Description
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This issue might be a duplicate of existing issues. Please check:
- [BUG] thinking.adaptive.block_binding.prefix_mismatch_behavior: Extra inputs are not permitted #46729: Same root cause —
thinking.block_binding.prefix_mismatch_behaviorfield introduced in v1.18.26 is rejected by cloud provider backends (Bedrock in that case, Vertex here) as an unrecognized field.
Reacted by Marcus Burghardt- [BUG] thinking.adaptive.block_binding.prefix_mismatch_behavior: Extra inputs are not permitted #46729: Same root cause —
- added a commit that references this issue
on Sep 2, 2026 Same error for Claude 4. I cannot use claude models because VertexAI rejects thinking.block_binding.prefix_mismatch_behavior.
This issue might be a duplicate of existing issues. Please check:
- [BUG] Claude Opus 5 on Bedrock rejects thinking block binding in 1.18.26 #46729: Same root cause —
thinking.block_binding.prefix_mismatch_behaviorfield introduced in v1.18.26 is rejected by cloud provider backends (Bedrock in that case, Vertex here) as an unrecognized field.
Is it? From what I see the issue seems to be that Claude Fable requires a new value, VertexAI rejects this unknown value. Also, if this field is needed for Claude Fable, why are submitting this field for Claude 4 models? Also Bedrock and VertexAI may handle this differently. One might support this field for all models, the other might only support it for Fable?
I would argue that these changes broke Claude 4 models which appears to be a bug on itself.
- [BUG] Claude Opus 5 on Bedrock rejects thinking block binding in 1.18.26 #46729: Same root cause —
Deleted my Bug Report as duplicate as it is propably the same underlying issue.
Issue is NOT fixed with Opencode v1.18.27 using Opencode Zen and Fable 5.1That's a separate issue actually @FG-RVT
that's a bug on our inference. We will address it tho.
Description
Every message sent to Claude Sonnet 5 (and the Haiku 4.5 title-generation call) via the
google-vertex-anthropicprovider fails immediately with:This happens on the very first message of a brand-new session, not just after long conversations, so it looks unconditional for these models on this provider.
The same model works fine through the desktop app, including when configured to use Vertex too, so the model/backend itself isn't broken — something in the request opencode builds is different.
Likely cause: the 1.18.26 release note "Claude 5 sessions now tolerate stale thinking blocks instead of failing after prompt or tool changes" appears to add a
thinking.block_binding.prefix_mismatch_behaviorfield to requests for the newest Claude generation (Sonnet 5, Opus 4.7/4.8/5, Fable 5). Google Vertex rejects that field as unrecognized.Downgrading to v1.18.20 fixes it completely — same config, same model, same provider, nothing else changed. A colleague on 1.18.20 has no issue with the exact same setup.
Plugins
superpowers,code-simplifier(unrelated — same result with both disabled)OpenCode version
Broken on 1.18.26 (haven't bisected 1.18.21–1.18.25). Confirmed working on 1.18.20.
Steps to reproduce
google-vertex-anthropicprovider.claude-sonnet-5@defaultas the model.claude-haiku-4-5and the main response onclaude-sonnet-5).Screenshot and/or share link
No response
Operating System
macOS 26.5 (arm64)
Terminal
Warp