Problem

When scripting with slck, operations like adding a reaction that already exists return errors from the Slack API (e.g. already_reacted). These aren't real failures — the desired state is already achieved. This causes unnecessary error noise in automation.

slck react :eyes: failed: Error: add reaction :eyes:: slack API error: already_reacted

Semantically, these operations are like PUT — "ensure this state exists." If the state already exists, that's success, not failure.

Operations to make idempotent

Excluded: messages delete + message_not_found — ambiguous (wrong timestamp vs already deleted).

Implementation plan

1. Add IsSlackError helper — internal/client/errors.go

Add a reusable helper to standardize error code checking (replaces ad-hoc strings.Contains usage):

func IsSlackError(err error, code string) bool {
    if err == nil {
        return false
    }
    return strings.Contains(err.Error(), code)
}

Also add missing error hints for already_reacted, no_reaction, and already_in_channel to the errorHints map (useful if these errors surface in other contexts).

2. Update command files

Handle the idempotent error before calling WrapError, print a descriptive message, and return nil:

• internal/cmd/messages/react.go — check already_reacted after c.AddReaction()
• internal/cmd/messages/unreact.go — check no_reaction after c.RemoveReaction()
• internal/cmd/channels/archive.go — check already_archived after c.ArchiveChannel()
• internal/cmd/channels/unarchive.go — check not_archived after c.UnarchiveChannel(). Also refactor existing not_in_channel check to use IsSlackError
• internal/cmd/channels/invite.go — check already_in_channel after c.InviteToChannel() (only when len(userIDs) == 1). Also add missing WrapError call for non-idempotent errors

3. Add tests

• internal/cmd/messages/messages_test.go — TestRunReact_AlreadyReacted, TestRunUnreact_NoReaction
• internal/cmd/channels/channels_test.go — TestRunArchive_AlreadyArchived, TestRunUnarchive_NotArchived, TestRunInvite_AlreadyInChannel
• internal/client/errors_test.go — TestIsSlackError + new hint entries

4. Update integration-tests.md

Update expected outputs for the idempotent operations.

Edge case: channels invite with multiple users

Slack's conversations.invite API is all-or-nothing. If you invite 2 users and one is already in the channel, Slack fails the entire batch with already_in_channel — the second user does NOT get invited.

Decision: Only treat as idempotent when len(userIDs) == 1. Multi-user invites still surface the error so callers know not everyone was added.

Verification

make test      # All tests pass
make lint      # No lint errors
make build     # Builds cleanly