Slack MCP has well-structured tool definitions with complete input schemas and clear descriptions. All 7 tools are explicitly registered with proper JSON Schema. Naming follows verb_noun convention (list, post, get, add). However, output schemas are entirely undocumented, LLMs cannot predict response structure or plan downstream calls. Parameter descriptions are adequate but lack constraint details (e.g., timestamp format is explained in prose but not formalized). Error handling is absent, no guidance on retryability, user-fixable errors, or recovery paths. Missing tool annotations (readOnlyHint, destructiveHint) despite clear risk classifications. No batch operations despite agents likely calling post_message/add_reaction in loops.

• Document output schemas for all 7 tools. Example: slack_post_message returns {ok: boolean, ts: string, channel: string, message: {type, user, text, ts}}. Include all fields agents need for chaining (ts for thread operations, channel for history lookups).
• Add destructiveHint and readOnlyHint annotations to tool definitions. Mark slack_post_message, slack_reply_to_thread, slack_add_reaction with destructiveHint=true. Mark read-only tools with readOnlyHint=true.
• Implement error recovery guidance. Wrap Slack API responses; map error codes to actionable messages: 'channel_not_found → try slack_list_channels to find the correct channel', 'not_in_channel → request channel access', 'rate_limited → retry after 30 seconds'.
• Formalize timestamp constraints. Add 'pattern': '^\d{10}\.\d{6}$' to thread_ts and timestamp parameters. Document in description: 'Unix timestamp with microseconds (e.g., 1234567890.123456)'.
• Add batch_post_messages(channel_id, messages: [{text, thread_ts?}]) and batch_add_reactions(channel_id, reactions: [{timestamp, reaction}]) to reduce sequential calls and improve agent efficiency.