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

Annotations already declare readOnlyHint/destructiveHint=false, and the description adds genuinely new behavioural context: content is delivered in the chat as a multi-part sequence (Part 1 of N) and ends with an in-app check-in, which tells the agent the call is the beginning of a session rather than a one-shot result. It still doesn't say how subsequent parts advance or whether complete_journey must be called at the end.

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 short sentences with zero filler; the delivery model is front-loaded and the prerequisite follows. Every clause carries 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?

For a one-parameter tool with full schema coverage, annotations and no output schema, the description covers delivery format, multi-part structure, and terminal check-in well. The remaining gap is the lifecycle: it never mentions the sibling complete_journey or how to progress through remaining parts, which matters given the "Part 1 of N" framing.

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% and the single field is already documented as "Journey id from list_journeys"; the description's sourcing hint largely duplicates that. No format, constraint, or alternate-source meaning is added beyond the schema, so the baseline 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?

"Open a Reulay journey in the chat, page by page (Part 1 of N)" gives a concrete verb and resource plus the delivery mechanic, which is far more than a restatement of the name. It does not explicitly distinguish itself from the many other start_* siblings (start_experience, start_reflection, start_breathing_exercise), so an agent must infer that a "journey" is a distinct content type.

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?

"Use the id from list_journeys" establishes a clear prerequisite workflow and points at the right sibling for sourcing the parameter. However it gives no guidance on when to pick this over start_experience or browse_library, and no exclusions or preconditions, so choice among the start_* family remains implicit.

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