Applies label changes to a conversation and links or unlinks a contact. System labels move the conversation: adding `spam` files it (and its received messages) as spam, adding `archive` files it away without deleting it, and adding `inbox` — or removing `spam`, `blocked`, or `archive` — returns it to the inbox; unread counts recompute to match. An archived conversation returns to the inbox by itself when a new message arrives. To block a sender going forward, add a receive rule instead. Omitted fields are left unchanged. Label changes to apply. Labels in `add` are applied and labels in `remove` are taken off; other labels are left untouched. Adding a label that is already present, or removing one that is not, has no effect. System labels express state changes: on a conversation, adding `spam` files it as spam, adding `archive` files it away without deleting it, adding `inbox` (or removing `spam` or `archive`) returns it to the inbox, and removing `unread` marks all retained received messages as read in one call; on a message, adding or removing `unread` flips read state, and adding or removing `trash` moves it to or out of the trash. Changes that contradict this model are rejected: adding more than one placement label in one request, adding `blocked` (blocking a sender is a receive-rule decision), removing `inbox` without adding a destination, adding `trash` or `unread` to a conversation (removing `unread` is the mark-all-read shortcut; `trash` uses the DELETE verb), placement labels on a message (move its conversation instead), and `unread` on a sent message. Custom labels are 1-64 characters with no commas, control characters, or leading or trailing whitespace. System label names and a small reserved set (`all`, `archived`, `deleted`, `draft`, `drafts`, `flagged`, `important`, `junk`, `muted`, `none`, `outbox`, `pinned`, `read`, `scheduled`, `snoozed`, `starred`) cannot be used as custom labels, in any casing. A conversation or message carries at most 20 labels, system labels included. Labels to apply. Labels to take off. Contact to link this conversation to, or null to unlink the current contact. Thread ID. Mailbox this conversation belongs to. Channel this conversation lives on. Always `email`. Contact linked to this conversation, or null when none is linked. Subject of the conversation, taken from its first message. Null when that message had no subject. Addresses that appear on the retained messages in this conversation, including the mailbox's own address. Number of retained messages in this conversation, both directions. Number of retained received messages that are still unread. Spam and blocked mail is not counted. When the most recent retained message in this conversation was received or sent. Direction of the most recent message — `inbound` for a received message, `outbound` for a sent one. Possible values: `inbound`, `outbound` Labels on this conversation. Exactly one system placement label is always present — `inbox`, `archive` (filed away, done for now), `spam` (the opening message failed sender authentication), or `blocked` (rejected by the mailbox's receive policy or rules) — set by the message that started the conversation. Move a conversation by updating its labels: add `spam` to file it as spam, add `archive` to clean it out of the inbox, and add `inbox` — or remove `spam`, `blocked`, or `archive` — to bring it back. An archived conversation returns to the inbox by itself when a new message arrives. Custom labels share the same list; a conversation carries at most 20. When the thread was created. When the thread last changed. Matched search fragments, keyed by the field that matched. Returned only by thread search; omitted when listing threads. Matched fragments from the conversation's subject. Matched fragments from a message's body text.