Strict JSON Schema validation fail on backends like SGLang (Missing required field in empty tool parameters) #8184

Description

Description

When using opencode with backends that perform strict JSON Schema validation (such as SGLang or other Pydantic-based LLM servers), tool calls for built-in tools with empty parameters (e.g., todoread) fail during the initial request.
The server returns a 400 Bad Request or a validation error similar to:
Tool function has invalid 'parameters' schema: None is not of type 'array' on schema['required']

Root Cause Analysis

The issue stems from how zod schemas are converted to JSON Schema for the LLM provider.
In packages/opencode/src/tool/todo.ts, the todoread tool defines its parameters as an empty object:

export const TodoReadTool = Tool.define("todoread", {
  description: "Use this tool to read your todo list",
  parameters: z.object({}), // This results in a schema without a 'required' field
  // ...
})

In packages/opencode/src/session/prompt.ts, the schema is generated using z.toJSONSchema(item.parameters). For an empty object, it produces:

"parameters": {
  "type": "object",
  "properties": {}
}

Strict validators like SGLang's internal pydantic models fail because they expect "required": [] to be explicitly present if the schema is of type object.

Suggested Fix

The most robust place to fix this is in the provider transformation layer to ensure all object-based schemas satisfy strict requirements.
File: packages/opencode/src/provider/transform.ts

export function schema(model: Provider.Model, schema: any) {
  // ... existing logic ...
  // Ensure 'required' is always an array for object types to satisfy strict backends
  if (schema.type === 'object' && !schema.required) {
    schema.required = [];
  }
  return schema;
}

Environment

• Opencode Version: 1.1.14
• LLM Backend: SGLang (v1/chat/completions)
• Platform: macOS / Linux

Plugins

No response

OpenCode version

1.1.14

Steps to reproduce

1. Configure opencode to use an SGLang backend (or any backend using strict JSON Schema validation).
2. Start a session where built-in tools are enabled.
3. Observe the v1/chat/completions request failing because the backend rejects the todoread tool definition.

Screenshot and/or share link

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Operating System

No response

Terminal

No response