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

Annotations already declare readOnlyHint and openWorldHint, so the safety profile is covered. The description adds genuine behavioral context beyond that: the lookup 'runs independently' and is not a prerequisite for quote retrieval, which tells the agent the call ordering is unconstrained. Minor gap: no note on what the related instruments look like, though the output schema covers that.

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 tight sentences, zero waste, with the core action front-loaded and the sibling-routing caveat immediately after. Every 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?

For a single-parameter related-instruments lookup with an output schema present, the definition covers the action, the parameter source, and the sibling relationship. Nothing needed to invoke it correctly is missing.

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?

With one parameter at 100% schema description coverage, the schema already documents 'quote' (symbol, e.g. AAPL:NASDAQ). The description says 'quote key' but adds no syntax, format, or lookup-by-name-vs-key nuance beyond the schema, so baseline 3 applies.

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: 'Look up instruments related to a Google Finance quote key.' It distinguishes itself from google_finance_quote by naming that sibling and clarifying the relationship, so an agent can tell the two apart 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?

Explicit routing guidance: 'call google_finance_quote only when you also need the seed instrument's quote' and 'not as a prerequisite for quote retrieval.' This states both when to use this tool and when the alternative is warranted, with the exclusion spelled out.

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