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

Annotations already cover readOnly/openWorld/idempotent/non-destructive, and the description adds meaningful context beyond them: data is fetched live from freddiemac.com at call time (network dependence and freshness), it is a weekly average rather than real-time quotes, and it applies only to well-qualified conventional purchase borrowers nationally. It does not discuss latency, caching, or failure behavior when the source is unreachable.

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?

One dense but well-ordered sentence: source and scope first, then the enumerated return fields, then the borrower caveat. With no output schema, the field enumeration is load-bearing rather than redundant, and there is no filler.

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?

Returns are well specified despite the absence of an output schema, and the source, cadence, and borrower scope are all stated. The only gap is coverage boundaries — it is not stated that other products (ARMs, FHA/VA, 30-year jumbo) or non-national geographies are out of scope, which an agent might otherwise assume.

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 tool takes no parameters, so there is nothing for the description to disambiguate; the baseline of 4 applies. The closing scope note correctly sets expectations that this is a national average rather than a quote for a specific borrower or state.

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?

The description names a specific data source (Freddie Mac PMMS), specific resources (30-year and 15-year fixed mortgage rates), and a temporal scope ('latest ... today', 'read at call time') that separates it from get_mortgage_rate_history and get_mortgage_rate_on_date. An agent can tell exactly what this returns without opening a 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?

Usage is only implied: the 'latest ... today' framing and 'next expected release' hint at a current-rates use case, but the description never states when to prefer this over the sibling history or on-date tools, nor any exclusions. The routing must be inferred from the temporal framing alone.

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