Slack error channel_not_found: Value passed for channel was invalid

Slack could not resolve the channel value you sent to any conversation this token can see. The documented description is deliberately narrow: "Value passed for channel was invalid." It is the single most-searched Slack API error, and in the overwhelming majority of cases the value is a channel name where a channel ID belongs. Slack conversation IDs look like C0123ABCDEF (public/private channels), D... (DMs), or G... (legacy groups/MPIMs); #general or general is a display label, accepted by some endpoints for convenience but not a stable identifier — renaming the channel breaks every integration that used the name.

The second major branch is visibility rather than format. A syntactically perfect ID still returns channel_not_found when the token cannot see the conversation: a private channel the bot has never been invited to, a DM that belongs to a different app's token, a conversation in another workspace of an Enterprise Grid org, or an ID copied from a different (staging) workspace. Slack does not distinguish "does not exist" from "exists but is invisible to you" — an information-leak-avoidance choice that means this error covers both.

The third branch is plumbing: the value never made it into the request. An undefined interpolated into JSON, an empty string from an unset environment variable, or a trailing newline from a config file all fail resolution. Since the HTTP status is 200, only body inspection reveals any of this.

Note the sibling for DMs: to message a user, you should not post to a bare user ID's DM without first calling conversations.open with that user ID and posting to the channel.id it returns; skipping that step on some paths surfaces as channel_not_found. In the SDKs it arrives as a plain platform error — ErrorCode.PlatformError with error.data.error === "channel_not_found" in @slack/web-api, SlackApiError with e.response["error"] in slack_sdk — and is never retried automatically, because it is deterministic.

POST /api/chat.postMessage
{"channel": "#general", "text": "hello"}

HTTP 200
{"ok": false, "error": "channel_not_found"}

Store conversation IDs, never names, the moment you learn them (from events, conversations.list, or interactive payloads), and validate configured destinations with conversations.info at setup time so a bad ID fails during onboarding rather than at 3 a.m. Keep staging and production configuration strictly separated — cross-workspace IDs are this error's favorite disguise. The name-vs-ID trap is cause number one in Slack bot not responding.