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

Annotations already declare readOnly/idempotent/non-destructive/openWorld=false, so the safety profile is covered. The description adds genuinely new behavioral context beyond those: no API key required, DST-awareness, the Sun 17:00-Fri 17:00 New York FX week, and explicit coverage gaps (holidays, broker hours) plus an informational-only disclaimer.

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?

Front-loaded with the value proposition ('FREE, no key. Forex session clock') and information-dense throughout. It is somewhat packed with parentheticals and a trailing disclaimer, but nearly every clause contributes useful scope or constraint 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?

With no output schema, the description carries the burden of explaining returns, and it does so reasonably: open sessions, active overlaps, next open/close, and display zones. This is enough for an agent to call it correctly, though exact return shape is left implicit.

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 both 'at' and 'tz' are already documented in the schema, making a 3 the baseline. The description reinforces behavior (times shown in UTC and any IANA zone, DST-aware) but adds no syntax or format detail beyond what the schema provides.

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 resource and verb-pattern up front: a Forex session clock listing Sydney, Tokyo, London and New York sessions, open-now status, overlaps, and next open/close. The scope is unambiguous and distinguishable from the generic sibling market_hours by naming the FX-specific session model and week boundary.

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?

It establishes clear usage context ('FREE, no key', defaults to now, 'Informational only, not financial advice') and sets expectations with 'Holidays and broker hours not included.' It does not, however, explicitly compare against the sibling market_hours or state when not to use it.

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