Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare this as a safe read operation (readOnlyHint=true, destructiveHint=false), and the description adds meaningful behavioral context beyond that: it reads from the local IndexedDB cache, not directly from Slack servers, and may only contain the last few hundred messages for active channels. This informs the agent about potential incompleteness of data. No contradictions with annotations.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is three sentences, front-loaded with the core function, and every sentence adds value: what it reads, the cache limitation, and the prerequisite for the channel_id. There is no wasted wording.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a simple read tool with full schema coverage, safety annotations, and an output schema, the description is sufficiently complete. It covers the key limitation (local cache) and the source of the required parameter, leaving no significant gaps for an agent to invoke it correctly.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%—both parameters (channel_id and limit) are described in the schema itself. The description repeats the channel_id source instruction ('must come from slack_list_channels') but does not add new semantic meaning beyond what the schema already provides, so it meets the baseline but does not exceed it.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the tool's function with a specific verb and resource: 'Reads recent messages from a Slack channel or DM.' It distinguishes itself from siblings like slack_search_messages and send_message by specifying the read-only nature and the data source (local IndexedDB cache).

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description provides clear context for when to use the tool: to access recent cached messages, with the limitation that only synced messages are available. It also instructs that channel_id must come from slack_list_channels, serving as a prerequisite. However, it does not explicitly name alternative tools like slack_search_messages for searching older messages, so it lacks explicit when-not-to-use guidance.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.