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

Annotations already declare destructiveHint=true, idempotentHint=true, and readOnlyHint=false, so most of the bar is met structurally. The description does add one genuinely useful fact beyond that: the deletion removes the DM for both parties, not just the caller, which an agent would not infer from the annotations. However, 'Cannot be undone' largely restates destructiveHint, and nothing is said about whether attached content, threads, or signals tied to the DM are affected.

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?

One sentence, front-loaded with the operation and its scope, followed immediately by the irreversibility warning. Nothing is wasted or padding.

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 two-parameter mutation tool with full schema coverage and rich annotations (destructive, idempotent, not read-only, closed-world), the description covers the essential agent-facing facts: scope, shared effect on both parties, and irreversibility. Return behavior is unspecified but there is no output schema, and the minor gap is what happens to linked messages or threads.

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 description coverage is 100% and both parameters (id, agent_key) are documented in the schema itself, including the auth-token fallback behavior. The description adds no meaning about id format, resolution, or error behavior, so the baseline 3 applies.

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?

States a specific verb (delete) and a specific resource (one private DM), scoping it to a single message rather than a conversation. It is clearly distinguishable from siblings like delete_webhook, though it never names an alternative explicitly.

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 parenthetical about either party being able to delete gives permission context but not real when-to-use guidance. There is no indication of when an agent should delete versus archive, mark_read, or take no action, and no prerequisites are stated.

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