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

Adds real context beyond the readOnly/idempotent annotations: unknown IDs are flagged not_found rather than silently replaced, and the derived fields (cheapest plan, lowest EUR/GB, longest validity) are disclosed. The 'no reservation or purchase' line is slightly redundant with readOnlyHint=true, keeping it just short of 5.

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 verb and input constraint, then the return shape, then the usage boundary. No filler; each sentence carries distinct 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?

No output schema exists, so the description must describe returns — it does, including the summary fields and the not_found behavior. For a 2-parameter read-only tool this is fully sufficient to call it correctly.

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 schema already documents the 2–10 range, provenance from search_plans, and not_found handling, so the description mostly restates it. It adds no format or edge-case detail about plan_ids or lang beyond the schema.

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+resource ('Compare 2–10 existing eSIM plan IDs') and scopes the input provenance to search_plans or get_plan, which cleanly separates it from both siblings. An agent can identify the operation without opening the 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?

Explicitly says when to use it ('an explicit shortlist') and when not to ('not as the initial destination search'), naming the upstream alternatives. The 'makes no reservation or purchase' clause further excludes it from the purchase path.

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