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

Adds substantial behavior the single readOnlyHint=false annotation cannot convey: the tool is paid, priced at 1 USDC-cent per call, settled via x402 exact/EVM, and requires a documented unpaid-then-retry handshake with the payment payload in params._meta['x402/payment']. An agent cannot invoke this correctly without that information, and it is disclosed precisely.

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 compact sentences with no filler, and the payment/cost fact is front-loaded so the agent learns the price before anything else. The parenthetical 'x402 exact/EVM' is dense jargon but is genuinely load-bearing for the payment protocol.

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 does the work of describing the return contents (buffers, connections with arrival times, one recommendation) and the payment failure path. It leaves error cases such as an unknown trip_id or insufficient payment uncovered, which is a minor residual gap for a 4-parameter tool.

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%, so the baseline is 3. The description reinforces semantics by framing min_buffer as a 'per-stop transfer buffer' and manual_delay as the fallback when 'the feed has none', but it adds no formats, units, or constraints beyond what the schema already states.

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 ('Re-plan a journey') plus the exact scenario ('a traveller already on a delayed train') and the output shape (per-stop transfer buffer, onward connections with arrival times, one safe recommendation). It is distinguishable from departures/search_stations, which the schema names as the sources of trip_id and stop_id, though it never explicitly contrasts itself with 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?

Gives a clear triggering condition — the traveller is already en route and the train is delayed — and spells out the two-step invocation protocol (call unpaid to get the PaymentRequired object, then retry with the payload). It stops short of naming when to prefer departures instead, but the context is unambiguous.

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