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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is fully covered externally. The description adds the coverage scope (EU/EEA MiFIR instruments) but says nothing about auth requirements, rate limits, or behavior on an unknown ISIN, so it adds only modest behavioral context beyond annotations.

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?

The purpose is front-loaded in the opening clause, and the dense enumeration of return fields is justified because no output schema exists. The single run-on sentence is heavy but each clause carries information, with minor structural cost.

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 return-value burden well, enumerating name, CFI, LEI, currency, conditional bond/derivative fields, and venue-level trading data. It omits error/empty-result behavior (invalid or non-EU ISIN), which is the only notable gap for a one-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 coverage is 100% with a single fully documented 'isin' parameter including format and example. The description's 'one ISIN' phrasing reinforces the single-value constraint but adds no syntax or format detail beyond the schema, so the 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 and resource ('Resolve one ISIN via ESMA's FIRDS') and enumerates the returned data domains (name, CFI, LEI, bond/derivative fields, trading venues). The singular 'one ISIN' scope implicitly separates it from the sibling firds_search_by_issuer, so an agent can route without opening either 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?

Provides chaining guidance ('issuer_lei is the join key into the gleif pack and into openfigi'), which tells the agent how to extend results. However, it never states when to prefer this tool over firds_search_by_issuer or what happens for non-EU/non-MiFIR instruments, so the when-to-use decision is only implied.

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