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

Annotations already establish this as a safe, read-only, idempotent calculation. The description adds substantial behavioral context beyond that: threshold rules, caps, flat fees, zero-rated cases, GST treatment, and what the merchant keeps. It does not fully specify return shape, but with no output schema it gives more transparency than most calculators.

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 description is one dense paragraph but is front-loaded with purpose and then lists the relevant rules efficiently. Every clause carries rule information, though the single-sentence structure is slightly harder to scan than a short bulleted rule list would be.

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 read-only calculator with no output schema, the description supplies the key inputs, thresholds, and a summary of what the result shows ('18% GST on the MDR and what the merchant keeps'). It does not enumerate every possible output field, but it is complete enough for an agent to invoke the tool correctly.

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%, so the baseline is 3, but the description adds business meaning by explaining how category values affect the result: standard 0.4%, essential ₹5, capital 0.02%, and p2p zero. It also ties gst_registered and small_merchant to the 18% GST and ₹1 lakh monthly threshold, adding interpretation beyond the field descriptions.

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 states a specific verb and resource ('Work out the UPI merchant discount rate (MDR) on a payment') and immediately clarifies the legal rule set and effective date. An agent can tell this is a single-payment MDR calculator, not a GST calculator or payment-link tool.

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?

The description makes clear the context is one UPI payment under the 15 October 2026 rules, which implies when to use it. However, it does not explicitly name alternatives such as calculate_gst or state exclusions, so sibling routing is left to inference.

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