start_session

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

Annotations only declare the generic profile (readOnlyHint=false, openWorldHint=false, destructiveHint=false); the description adds the genuinely non-obvious traits: the isolation guarantee in both directions, and the session's lifetime and termination condition. That is meaningful context the structured fields cannot convey.

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?

Three sentences, front-loaded with the core capability, then the workflow, then the lifetime. The rhetorical pair ('nothing in it reaches the desktop ... nothing of theirs reaches it') earns its space by making the isolation guarantee unambiguous.

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 15-parameter tool with no output schema, the description supplies the mental model, workflow, and lifetime, which is most of what is needed. The one gap is that it never says what starting a session returns (presumably the name/handle that spawn and stop_session take), which an agent must infer.

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% with 15 well-annotated optional properties (each has a concrete description, enum, or bounds), so the schema carries the parameter burden entirely. The description adds no parameter-level syntax or defaults beyond what is already there, making the baseline 3 correct.

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 states a specific verb and resource ('Start a sealed session') and immediately enumerates what that session contains: its own display (X11/Xvfb or Wayland/sway), D-Bus and a11y bus, and a scratch home. That level of specification lets an agent distinguish it from siblings like stop_session or spawn without opening any schema.

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 gives an explicit ordered workflow ('Start one, spawn a program in it, then look with accessible_tree (cheap, text) before screenshot (pixels)') and names the terminating alternative ('lasts until stop_session, or until this server stops'). Both when-to-use and the choice between inspection siblings are covered.

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