OpenCode 1.18.18 fails with session.messages() 400 when using json_schema output format

[Zachary19980701]

Description

When using the VS Code OpenCode extension with OpenCode TUI/server 1.18.18, sending a message fails with the following error:

Failed to send message: OpenCode SDK session.messages() returned no message array (status 400):
{"name":"BadRequest","data":{"message":"Expected OutputFormatJsonSchema, got {"type":"json_schema","schema":{...}}","kind":"Body"}}

The extension appears to send a structured output format similar to:

{
"type": "json_schema",
"schema": {
"type": "object",
"properties": {
"type": {
"type": "string",
"enum": [
"message",
"implementation_plan"
]
},
"text": {
"type": "string"
},
"plan": {
"type": "object"
}
}
}
}

The request then fails when the extension calls session.messages().

Environment
VS Code OpenCode extension: vscode-opencode
Repository: chryzxc/vscode-opencode
OpenCode TUI/server: 1.18.18
OpenCode installation method: npm
OpenCode executable:
/usr/bin/opencode
OS: Ubuntu / Linux
Node.js and npm installed system-wide

OpenCode itself is working from the terminal:

opencode --version

Output:

1.18.18
Error

Full relevant error:

Failed to send message: OpenCode SDK session.messages() returned no message array (status 400):
{"name":"BadRequest","data":{"message":"Expected OutputFormatJsonSchema, got {"type":"json_schema","schema":{"type":"object","properties":{"type":{"type":"string","enum":["message","implementation_plan"],"description":"Response type. MUST be 'implementation_plan' when the user requests a plan..."}}}}","kind":"Body"}}

The Runtime panel also previously showed:

OpenCode TUI 1.18.18
OpenCode SDK -
SDK and TUI versions are incompatible.
Update OpenCode TUI/server.
Expected behavior

The extension should successfully send the prompt and retrieve the response through the OpenCode SDK.

Structured output using:

{
"type": "json_schema",
"schema": {}
}

should either:

be accepted by the OpenCode server version declared compatible by the extension, or
be converted to the output format expected by the installed OpenCode SDK/server.
Actual behavior

The prompt/session workflow fails with HTTP 400 when the extension calls:

session.messages()

The server reports:

Expected OutputFormatJsonSchema

even though the stored/requested value appears to already contain:

{
"type": "json_schema",
"schema": {}
}
Compatibility concern

The extension currently declares support for OpenCode >=1.18.0 <1.19.0, while the packaged/tested SDK/server versions appear to be older 1.18.x versions.

OpenCode 1.18.18 is therefore inside the declared compatibility range, but currently fails with the structured-output/session message path.

Output:

1.18.18
Install/open the vscode-opencode extension.
Start a new OpenCode session.
Send a normal message.
The extension fails with:
OpenCode SDK session.messages() returned no message array (status 400)

[Zachary19980701]

I discovered this was due to opencode updating the structured-output format. I've currently fixed this issue using codex, and the fixed version is available at [vscode-opencode]. I'm not a professional VS Code plugin developer, the code modifications here were entirely done by AI, so I won't be submitting a merge request. This modified repository is only for reference.

[chryzxc]

@Zachary19980701 which version of the VS Code extension and model/provider are you using? Could you confirm whether it’s the latest release, 0.3.8?

[Zachary19980701]

I test at Windows11 work in WSL2(Ubuntu20.04/Ubuntu22.04) and Ubuntu22.04 work in docker(Ubuntu20.04). Extension verison is 0.3.8, downloading from vscode extension market. Model provider is opencode free(chatgpt4o//claude-haiku/deepseek) and KIMI token plan.

[chryzxc]

@Zachary19980701 have you tried setting the thinking to "Auto (let the model decide)" and see if the issue still persist? there are couple of models in opencode go/free that doesn't support structured output

[Zachary19980701]

same error worked in auto think mode