OpenCode plugin to provide custom request instructions

request-instructions.js

// .opencode/plugins/request-instructions.js
// replaces requestInstructions (model-specific coding instructions) with 
// the contents of .opencode/REQUEST_INSTRUCTIONS.md
//
// When using OpenCode for non-coding tasks (such as creative writing or 
// knowledge management), this reduces coding-related context poisoning 
// when paired with a general purpose prompt.
//
import fs from "fs/promises"
import path from "path"

export const RequestInstructions = async ({ worktree, client }) => {
  const filePath = path.join(worktree, ".opencode", "REQUEST_INSTRUCTIONS.md")

  /** @type {{ mtimeMs: number, text: string } | null} */
  let cache = null

  async function load() {
    try {
      const stat = await fs.stat(filePath)
      if (cache && cache.mtimeMs === stat.mtimeMs) return cache.text

      const text = await fs.readFile(filePath, "utf8")
      cache = { mtimeMs: stat.mtimeMs, text }
      return text
    } catch (err) {
      // If the file doesn't exist, treat it as an intentionally empty override.
      if (err && typeof err === "object" && "code" in err && err.code === "ENOENT") return ""

      try {
        const message = err instanceof Error ? err.message : String(err)
        await client.app.log({
          body: {
            service: "request-instructions",
            level: "error",
            message: `Failed to read ${filePath}: ${message}`,
          },
        })
      } catch {
        // ignore
      }
      return null
    }
  }

  return {
    "chat.params": async (_input, output) => {
      // Only override when OpenCode is already using request-level instructions
      // (e.g. OpenAI OAuth/Codex sessions). Avoid injecting an unsupported field
      // into providers that don't accept it.
      if (!output?.options) return
      if (typeof output.options.instructions !== "string") return

      const text = await load()
      if (typeof text !== "string") return
      output.options.instructions = text
    },
  }
}