Build custom tools and plugins using the OpenCode SDK packages. Use when creating custom tools, implementing tool schemas, building plugins, managing OpenCode sessions, using opencode-ai/sdk or opencode-ai/plugin, creating a tool.ts file, writing an execute handler, using tool.schema, ToolContext, createOpencode, plugin hooks, custom auth, automating OpenCode from a script, building a CI tool, or integrating external APIs. Keywords: Zod schema, execute handler, event streaming, session management, abort signal, tool discovery, multiple tools per file, tool factory.