This Slack MCP server has solid naming conventions and complete JSON schemas for all 9 tools, but suffers from insufficient description depth and missing output schema documentation. All tools start with action verbs (add_, edit_, get_, post_, reply_) following the verb_noun pattern. Parameter schemas are well-formed with type definitions and descriptions. However, tool descriptions average ~60 characters, below the 194-char baseline for A+ tools, lacking context about when to use each tool versus alternatives. Output schemas are completely undocumented, LLMs cannot plan downstream calls or extract fields with confidence. Error handling is not visible in the provided code; the client.ts shows rate limiting and sanitization but no recovery guidance in responses. Input validation occurs (reaction sanitization, limit capping) but error messages are not documented. The server correctly injects secrets via environment variables (SLACK_BOT_TOKEN) rather than tool parameters, following security best practices.

• Document output schema for every tool. For example, add_reaction should document: {ok: boolean, error?: string}. get_channel_history should document message array structure with fields: {type, text, user, ts, thread_ts?, replies_count?}. This enables LLMs to extract IDs for chaining.
• Expand all tool descriptions to 100-200 characters. Pattern: 'Does X. Returns Y. Use when Z. Requires permission A.' E.g., add_reaction: 'Add an emoji reaction to a Slack message. Returns success/error. Use to indicate agreement or acknowledgment. Requires message timestamp in format UNIX.milliseconds.'
• Add recovery guidance for write operations. E.g., edit_message: '...If the message is not found, the original message may have been deleted. No undo is available; the edit is permanent.'
• Document Slack API error categories in tool descriptions. State that 'not_found' errors are user-actionable (message was deleted); 'permission_denied' errors require token scope changes; 'rate_limited' errors warrant a retry with backoff.
• Standardize verb pairing: rename 'get_users' to 'list_users' for consistency with 'list_channels'. Both list resources; both should use the same verb.
• Add parameter validation details to descriptions. E.g., reaction: 'Emoji name without colons (e.g., "thumbsup", not ":thumbsup:"). Invalid names are automatically sanitized; however, some names may not exist in your workspace.'
• Document pagination semantics. E.g., list_channels: 'Returns up to 'limit' channels (max 200). If cursor is present in response, more results exist; pass it as the next request's cursor parameter.'