Easily debug OpenCode system prompts

log-system-prompts.js

// .opencode/plugins/log-system-prompts.js
// log system prompts to .opencode/system-prompts.jsonl
//
// This makes it easier to directly investigate how OpenCode constructs
// system prompts and what environment data is being provided to the model.
// This does not include tool call schema information.
//
import fs from "fs/promises"
import path from "path"

export const LogSystemPrompts = async ({ worktree, client }) => {
  const outPath = path.join(worktree, ".opencode", "system-prompts.jsonl")

  // We don't get message metadata in `experimental.chat.system.transform`, so we
  // capture the latest system array per session and then log it in `chat.params`.
  /** @type {Map<string, { at: number, system: string[] }>} */
  const lastSystemBySession = new Map()

  async function append(line) {
    try {
      await fs.appendFile(outPath, line + "\n", "utf8")
    } catch (err) {
      // Avoid crashing the whole session if logging fails.
      try {
        const message = err instanceof Error ? err.message : String(err)
        await client.app.log({
          body: {
            service: "log-system-prompts",
            level: "error",
            message: `Failed to append to ${outPath}: ${message}`,
          },
        })
      } catch {
        // ignore
      }
    }
  }

  return {
    "experimental.chat.system.transform": async (input, output) => {
      const sessionID = input?.sessionID ?? ""
      if (!sessionID) return
      const system = Array.isArray(output?.system) ? output.system.map(String) : []
      lastSystemBySession.set(sessionID, { at: Date.now(), system })
    },

    "chat.params": async (input, output) => {
      const sessionID = input?.sessionID ?? ""
      if (!sessionID) return

      const sys = lastSystemBySession.get(sessionID)
      // In case hooks fire in an unexpected order, still log what we can.
      const system = sys?.system ?? []
      const systemAgeMs = sys ? Date.now() - sys.at : null

      const payload = {
        at: new Date().toISOString(),
        outPath,
        sessionID,
        agent: input?.agent?.name ?? input?.agent?.mode ?? null,
        model: {
          providerID: input?.model?.providerID ?? null,
          modelID: input?.model?.id ?? input?.model?.modelID ?? null,
        },
        provider: {
          id: input?.provider?.id ?? null,
          name: input?.provider?.name ?? null,
        },
        message: {
          id: input?.message?.id ?? null,
        },
        system,
        systemAgeMs,
        // OpenAI OAuth (Codex) uses `options.instructions` (not a system message).
        requestInstructions:
          output?.options && typeof output.options.instructions === "string" ? output.options.instructions : null,
      }

      await append(JSON.stringify(payload))
    },
  }
}