Write a message or a thread to a file

A forty-message thread is perhaps two hundred thousand characters of somebody else's writing. Pasted into the conversation it is paid for once on the way in and again in every turn that follows, it pushes out whatever the user actually asked about, and it is harder to read than a file: no scrolling back, no searching, nothing to keep. This skill is where that thread goes instead. It writes one message or one conversation to disk and returns a path, and the path is what belongs in the conversation.

The first failure mode is reading the thread in order to decide whether to export it. By the time you know it is long you have already paid for it. The decision is made from what you already hold — the totalChars from a read that hit its cap, the user's own "this is too long" — and not from the body. A search row will not settle it: beyond the ids it carries a subject, a snippet, labels and an attachment count, and nothing at all about how many messages the thread holds. When the length itself is the deciding fact, gmail_thread_timeline reports the thread's messageCount while reading every body at a single character, so the count arrives and the thread does not. The second is exporting and then pasting the file back, which is the same cost with an extra step. The file exists so it can be read in pieces, and only the lines that answer the question come into the conversation.