Look up an HSN/SAC code

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

Annotations already declare readOnly, idempotent and closed-world; the description adds genuinely non-obvious behavior beyond them — the nearest-valid-parent fallback for unknown codes and the fact that it works offline. It does not say what happens for malformed input (e.g. a 1-digit or non-numeric code), which is the one behavioral gap.

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 short sentences, zero filler, and the primary outcome is front-loaded before the fallback case and the alternative-tool pointer. Nothing redundant with the title.

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 correctly carries the return-value burden by naming description, chapter, heading and sub-codes, and it defines the miss case. Only the invalid/malformed-input path and any partial-match vs exact-match distinction are left unstated.

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% and the single param already documents the 2-8 digit format with examples, so the description adds no syntax detail. Its added meaning is about results (fallback to parent code), not about the parameter itself, 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?

"Check whether an HSN or SAC code exists in the GST portal's official list" gives a specific verb plus resource, and the enumerated payload (description, chapter, heading, sub-codes) makes it distinguishable from search_hsn_sac and find_gst_rate without opening any 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?

Explicitly routes rate queries away: "For the GST rate, use find_gst_rate." It does not mention search_hsn_sac, which is the obvious alternative when the agent only has a partial or keyword description rather than a code, so the sibling routing is incomplete.

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