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

Annotations declare the safety profile (readOnly=false, idempotent=false, destructive=false), and the description adds substantive behavior beyond that: a session is logged on the user's account, a video player may render in chat 'where supported', and a browser-playable link is returned. That covers the main side effect and the environment-dependent UI behavior; only account/auth requirements and irreversibility are left implicit.

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 front-loaded sentence in three tight clauses, each carrying distinct information (session logging, in-chat playback, returned link). No filler or redundancy.

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, the description usefully discloses the return value (a playable link) and the conditional in-chat player, which the agent could not otherwise know. For a one-parameter tool with annotations covering the mutation profile, this is close to complete; only failure modes or token/permission requirements are missing.

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 the single parameter's provenance (recommend_experience or browse_library) is already documented in the schema. The description adds no further meaning about the id format or constraints, so the baseline of 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?

Uses a specific verb ('Start') plus a specific resource ('a Reulay experience') and enumerates the concrete effects: logging a session, showing an in-chat video player, returning a playable link. It is distinguishable from siblings such as start_journey or start_breathing_exercise, though the description never explicitly contrasts them.

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?

Usage guidance is implied rather than stated: the tool starts an 'experience', and the schema points the agent to recommend_experience or browse_library for the id. There is no explicit when-to-use versus the many other start_* siblings or any when-not condition.

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