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

Beyond the readOnlyHint/openWorldHint annotations, the description adds the commercial contract that an agent must know before calling: $0.002 USDC per successful call via x402 on Base, Solana or Arc, with failed calls not charged. It also discloses the data limitation that only trains currently reporting appear, which distinguishes this from a schedule and prevents misinterpretation of sparse results.

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 what the tool returns, followed by the coverage caveat and pricing. Each element — field list, 'not a schedule', per-call cost, no-charge-on-failure — is information the agent needs and nothing is padded.

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?

An output schema exists, so return values did not need spelling out, yet the description still names the key fields. Pricing, payment rails, failure policy and the 'currently reporting only' caveat cover the remaining gaps for a simple read-only, single-optional-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?

The single optional 'limit' parameter is fully documented in the schema (1–200, default 50), and the description adds nothing about it. With schema description coverage at 100%, the schema carries this dimension and a baseline 3 is appropriate.

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 ('Live SEPTA regional rail train positions from the public TrainView feed') and enumerates the returned fields, so the agent knows exactly what data comes back. It scopes itself by agency ('SEPTA regional rail') and excludes schedules, which implicitly separates it from schedule-style siblings, but it never names related live-transit tools like travel_amtrak_train_tracker or travel_mbta_realtime.

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?

'Covers SEPTA regional rail trains currently reporting; not a schedule' gives a clear usage boundary and an explicit exclusion. What's missing is a positive routing statement — nothing tells the agent which sibling to use when it wants a timetable, other agencies, or other modes.

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