slackDmChannelCache.set(key, channelId);

}

function isSlackUserRecipient(recipient: SlackRecipient): boolean {

return recipient.kind === "user" || /^U[A-Z0-9]+$/i.test(recipient.id);

}

function resolveDirectUserPostChannelId(params: {

recipient: SlackRecipient;

hasMedia: boolean;

threadTs?: string;

}): string | undefined {

if (!isSlackUserRecipient(params.recipient) || params.hasMedia || params.threadTs) {

return undefined;

}

return params.recipient.id;

}

async function resolveChannelId(

client: WebClient,

recipient: SlackRecipient,

// target string had no explicit prefix (parseSlackTarget defaults bare IDs

// to "channel"). chat.postMessage tolerates user IDs directly, but

// files.uploadV2 → completeUploadExternal validates channel_id against

// ^[CGDZ][A-Z0-9]{8,}$ and rejects U-prefixed IDs. Always resolve user

// IDs via conversations.open to obtain the DM channel ID.

const isUserId = recipient.kind === "user" || /^U[A-Z0-9]+$/i.test(recipient.id);

if (!isUserId) {

// ^[CGDZ][A-Z0-9]{8,}$ and rejects U-prefixed IDs. Resolve user IDs via

// conversations.open only for paths that require the concrete DM channel ID.

if (!isSlackUserRecipient(recipient)) {

return { channelId: recipient.id };

}

const cacheKey = createSlackDmCacheKey({

}): Promise<SlackSendResult> {

const { opts, cfg, account, token, recipient, blocks, trimmedMessage } = params;

const client = opts.client ?? getSlackWriteClient(token);

const { channelId } = await resolveChannelId(client, recipient, {

accountId: account.accountId,

token,

const directUserPostChannelId = resolveDirectUserPostChannelId({

recipient,

hasMedia: Boolean(opts.mediaUrl),

...(opts.threadTs ? { threadTs: opts.threadTs } : {}),

});

const { channelId } = directUserPostChannelId

? { channelId: directUserPostChannelId }

: await resolveChannelId(client, recipient, {

accountId: account.accountId,

token,

});

if (blocks) {

if (opts.mediaUrl) {

throw new Error("Slack send does not support blocks with mediaUrl");