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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the description's "Read-only, idempotent" adds nothing beyond structured data. The real contribution is describing the return envelope (user, agent, spaces, projects with role) in the absence of an output schema, plus the caching implication of calling it at session start.

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?

Two sentences, front-loaded with the return contents and ending with the actionable call instruction. The long colon-chained first sentence is dense but every clause (user, agent, spaces, projects+role) conveys real information; only the redundant "Read-only, idempotent" is padding against the annotations.

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 zero-parameter, non-destructive introspection call with no output schema, the description covers what an agent needs: what comes back, that it is safe to call, and when to call it. It does not document exact field names or nesting of the response, which would help an agent parse results without trial.

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 tool takes zero parameters, so there is no parameter semantics to clarify; the baseline of 4 applies. The description does not waste words describing inputs, focusing instead on outputs.

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 (returns the current MCP session identity) and enumerates the returned components: resolved user, agent, spaces, and visible projects with role and space. That enumeration distinguishes it from siblings like skein_project_list or skein_project_members, which cover only part of this surface.

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?

Gives an explicit trigger: "Call once at session start so future tool calls have the user's permission context cached on your side." It also explains how its output feeds a sibling by noting the space short ids are the `space` value for skein_project_create. It stops short of naming when-not-to-use or a direct alternative for the space/project listing it returns.

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