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

The description adds meaningful behavioral context beyond the annotations: it explains that the server stores only the wrapped envelope and never sees the thread key, emphasizing a privacy/security property. It also clarifies that the enveloped key must be unwrapped locally with the agent's private key. Since readOnlyHint and idempotentHint are already present, the description appropriately supplements rather than merely repeats the 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 compact and well-organized: the first sentence says what the tool returns, the second explains the immediate next step, and the third provides a security guarantee. Every sentence earns its place, with no redundant filler.

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-only retrieval tool, the description covers the essential purpose, the returned artifact, the intended local workflow, and a key security property. Gaps are minor: it does not describe the exact shape of the response envelope or explicitly mention what happens if no envelope exists for the caller. But given the simplicity and the strong annotations, the description is largely complete.

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?

The input schema already has 100% coverage with clear descriptions for both agent_key and thread_id. The tool description does not add much parameter-level detail beyond reinforcing that the envelope is specifically for the caller. This matches the baseline of 3, since the schema carries the semantic weight.

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 a specific verb ('Return') and a precise resource ('the thread key envelope that was encrypted for you by an existing participant'). It also distinguishes itself from sibling tools like get_thread by focusing on the key envelope rather than the thread content. This leaves no ambiguity about what the tool does.

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 implies when to use the tool: when you need to recover a thread key from an envelope encrypted for you, so you can decrypt thread content. However, it does not explicitly compare against alternatives like get_thread or request_thread_access, nor does it state preconditions such as having an existing participant who has encrypted the envelope for you. Usage context is present but left to inference.

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