fix(sdk): wrap thrown error bodies in Error

SDK throwOnError paths now convert structured response bodies into real Error instances while preserving the original body and status in cause.

packages/sdk/js/src/error-interceptor.ts

@@ -0,0 +1,51 @@
+/**
+ * Wrap whatever the generated client decoded from a non-2xx error body
+ * into a real `Error` so downstream formatters (TUI, plugins) get a
+ * useful `.message` instead of `[object Object]` or blank. The original
+ * parsed body and status live under `.cause` for callers that need
+ * structured fields.
+ *
+ * Only fires when the caller used `{ throwOnError: true }`. Callers that
+ * read `result.error` directly (the result-tuple path) get the parsed
+ * body unchanged so existing field-level reads (`.error.name`,
+ * `JSON.stringify(error)`, etc.) are byte-for-byte identical to before.
+ */
+export function wrapClientError(
+  error: unknown,
+  response: Response | undefined,
+  request: Request | undefined,
+  opts: { throwOnError?: boolean } | undefined,
+): unknown {
+  if (!opts?.throwOnError) return error
+  if (error instanceof Error) return error
+
+  // NamedError-shaped responses (the common case for opencode 4xx) come
+  // through as POJOs — extract a useful message first, then wrap.
+  if (typeof error === "object" && error !== null && Object.keys(error).length > 0) {
+    const obj = error as { data?: { message?: unknown }; message?: unknown; name?: unknown }
+    const message =
+      (typeof obj.data?.message === "string" && obj.data.message) ||
+      (typeof obj.message === "string" && obj.message) ||
+      (typeof obj.name === "string" && obj.name) ||
+      describe(request, response)
+    return new Error(message, { cause: { body: error, status: response?.status } })
+  }
+
+  if (typeof error === "string" && error.length > 0) {
+    return new Error(error, { cause: { body: error, status: response?.status } })
+  }
+
+  // Empty body / network failure / undefined / null / empty object.
+  const reason = response ? "(empty response body)" : "network error (no response)"
+  return new Error(`opencode server ${describe(request, response)}: ${reason}`, {
+    cause: { body: error, status: response?.status },
+  })
+}
+
+function describe(request: Request | undefined, response: Response | undefined) {
+  const method = request?.method ?? "?"
+  const url = request?.url ?? "?"
+  const status = response?.status
+  const statusText = response?.statusText
+  return `${method} ${url}${status ? " → " + status : ""}${statusText ? " " + statusText : ""}`
+}