Claude Code returns HTTP 400 while OpenCode CLI works

[AdaviDigital]

Description

Bug Report: Claude Code returns HTTP 400 while OpenCode CLI works

Summary

Claude Code cannot communicate with OpenCode Zen API. Every prompt in Claude Code returns:

API Error: 400 Error from provider (Console): Upstream request failed

However, the OpenCode CLI works correctly using the same account and API key.

Environment

• OS: Windows 11
• Claude Code: 2.1.212
• OpenCode: 1.18.3
• Plugin: @opencode-ai/plugin 1.18.3
• Provider: OpenCode Zen API

Expected behavior

Claude Code should send prompts successfully through the OpenCode Zen API, just like the OpenCode CLI.

Actual behavior

Every prompt entered in Claude Code returns:

API Error: 400 Error from provider (Console): Upstream request failed

Steps to reproduce

1. Install Claude Code 2.1.212.
2. Install OpenCode 1.18.3.
3. Authenticate with OpenCode Zen API.
4. Launch Claude Code.
5. Enter a simple prompt such as:

hello

6. Claude Code immediately returns:

API Error: 400 Error from provider (Console): Upstream request failed

Diagnostics performed

OpenCode CLI works

opencode run "hello"

Output:

> build · deepseek-v4-flash-free

Hello. How can I help you?

Claude Doctor

• Installation successful
• No installation issues found

OpenCode

• Authentication successful
• Models load correctly
• opencode models works
• opencode run works

Configuration

opencode debug config returns a valid configuration.

Authentication is stored in:

C:\Users\USER\.local\share\opencode\auth.json

Configuration folder:

C:\Users\USER\.config\opencode

Additional observations

The issue only occurs in the Claude Code interactive interface.

The OpenCode CLI works correctly using the same provider and API key.

This suggests a compatibility issue between Claude Code 2.1.212 and the OpenCode Zen gateway (or the Anthropic-compatible request handling).

I can provide additional logs or test builds if needed.

Plugins

No response

OpenCode version

No response

Steps to reproduce

No response

Screenshot and/or share link

No response

Operating System

No response

Terminal

No response

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Zen returns 400 Upstream request failed when thinking is enabled for latest Claude models (fable-5, sonnet-5, opus-4-8); older models (opus-4-6, haiku-4-5) work fine #35124: Same 400 Upstream request failed error from the Zen/Console provider — may share the same underlying Zen API issue
• opencode-go (Console Go) provider returns 400/401/500 for subscribed models #37056: Also reports 400 Error from provider (Console): Upstream request failed through the Zen gateway

If your issue is distinct (e.g., the Claude Code client sends requests in a format incompatible with the Zen API, rather than a model-specific or request-size issue), please clarify what differs so maintainers can track it separately.

[AdaviDigital]

Thanks for the pointers.

I believe my issue is different from #35124 and #37056 because I have narrowed it down to the Claude Code integration.

Here are the results of my testing:

OpenCode CLI works correctly.
opencode run "hello" succeeds using the same OpenCode Zen API key.
Authentication is successful.
opencode models lists all models successfully.
The same provider and API key fail only when requests originate from Claude Code.

Environment:

Claude Code 2.1.212
OpenCode 1.18.3
@opencode-ai/plugin 1.18.3
Windows 11

Reproduction:

opencode run "hello"

returns:

Hello. How can I help you?

But starting Claude Code and entering:

hello

consistently returns:

API Error: 400 Error from provider (Console): Upstream request failed

Since the CLI succeeds with the same credentials and provider, this appears to be specific to the request format or protocol used by the Claude Code integration rather than a general Zen API or authentication issue.

If there are additional debug logs that would help isolate the incompatibility, I'm happy to provide them.