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

Adds substantial behavior beyond the annotations: automatic E2E decryption, legacy plaintext passthrough, and the side effect that decoded incoming text is marked read while failed decryption never advances the read pointer. This also explains why readOnlyHint=false despite the tool being a read/list operation, resolving what would otherwise be a confusing annotation.

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?

Four dense sentences, front-loaded with purpose then prerequisites then decryption/read-marking semantics. Nothing is redundant and every sentence carries distinct information.

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?

With no output schema, it still covers the non-obvious parts: decryption, read-pointer advancement, and thread scoping. It does not describe pagination behavior (cursor/limit interplay) or roughly what a returned message looks like, which leaves a small gap for a 4-parameter list tool.

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 75% and already documents contact_id, cursor, and thread_label. The description still adds value by identifying contact_id's provenance (identity_id from mDM_list_contacts) and clarifying thread_label as a filter over the conversation; only `limit` is left undocumented in both places, which is minor given its default.

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 and resource ('List recent direct messages with a specific contact') and implicitly distinguishes itself from mDM_list_contacts, which it names as the source of contact_id. An agent can tell immediately that this reads a conversation rather than enumerating contacts.

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?

It routes the agent to the prerequisite tool ('Pass contact_id as the identity_id from mDM_list_contacts') and states the optional narrowing by thread_label. It gives no explicit when-not-to-use guidance or alternatives for other message-listing paths, but for a read tool in this family the usage context is clear.

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