feat(auth): add Gemini OAuth credentials converter script

Add a Typescript script that converts Gemini OAuth credentials from ~/.gemini/oauth_creds.json to OpenCode's auth.json format. Supports environment variable overrides for custom paths and handles various expiry date formats (seconds, milliseconds, ISO strings).

Author: tmeckel

convert-gemini.auth.ts

@@ -0,0 +1,92 @@
+#!/usr/bin/env bun
+import { chmod, mkdir, readFile, writeFile } from "node:fs/promises"
+import os from "node:os"
+import path from "node:path"
+
+type Gemini = {
+  access_token?: string
+  refresh_token?: string
+  expiry_date?: number | string
+}
+
+type OAuth = {
+  type: "oauth"
+  refresh: string
+  access: string
+  expires: number
+}
+
+type Auth = Record<string, OAuth | Record<string, unknown>>
+
+function fail(msg: string): never {
+  console.error(`[convert-gemini.auth] ${msg}`)
+  process.exit(1)
+}
+
+function home() {
+  return process.env.HOME || os.homedir()
+}
+
+function data() {
+  return process.env.XDG_DATA_HOME || path.join(home(), ".local", "share")
+}
+
+function src() {
+  return process.env.GEMINI_OAUTH_CREDS_PATH || path.join(home(), ".gemini", "oauth_creds.json")
+}
+
+function dst() {
+  return process.env.OPENCODE_AUTH_PATH || path.join(data(), "opencode", "auth.json")
+}
+
+function ms(value: unknown) {
+  if (typeof value === "number" && Number.isFinite(value)) {
+    return value < 1e12 ? value * 1000 : value
+  }
+
+  if (typeof value === "string" && value.trim()) {
+    const num = Number(value)
+    if (Number.isFinite(num)) return num < 1e12 ? num * 1000 : num
+    const date = Date.parse(value)
+    if (!Number.isNaN(date)) return date
+  }
+
+  return Date.now() + 55 * 60 * 1000
+}
+
+async function json(file: string) {
+  return JSON.parse(await readFile(file, "utf8")) as Record<string, unknown>
+}
+
+async function existing(file: string): Promise<Auth> {
+  try {
+    return (await json(file)) as Auth
+  } catch {
+    return {}
+  }
+}
+
+async function main() {
+  const source = src()
+  const target = dst()
+  const creds = (await json(source)) as Gemini
+
+  if (!creds.refresh_token) fail(`missing refresh_token in ${source}`)
+  if (!creds.access_token) fail(`missing access_token in ${source}`)
+
+  const auth = await existing(target)
+  auth.google = {
+    type: "oauth",
+    refresh: creds.refresh_token,
+    access: creds.access_token,
+    expires: ms(creds.expiry_date),
+  }
+
+  await mkdir(path.dirname(target), { recursive: true })
+  await writeFile(target, `${JSON.stringify(auth, null, 2)}\n`, { mode: 0o600 })
+  await chmod(target, 0o600)
+}
+
+await main().catch((err) => {
+  fail(err instanceof Error ? err.message : String(err))
+})