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

Annotations only cover safety/openness (readOnly, openWorld, idempotent). The description adds behavior annotations cannot express: per-call pricing ($0.03 USDC on Base via x402) and the critical payment handshake — without payment_signature the call returns payment requirements rather than results. That is exactly the non-obvious behavior an agent must know before invoking.

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?

Front-loads purpose, then usage, then cost and payment mechanics, then a sample link — four short sentences with little waste. The pricing/payment sentence is dense but earns its place given it is the difference between getting results and getting a payment challenge.

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 covers the one return-value case an agent must anticipate (payment requirements vs. results) and supplies a sample path. Combined with 100%-covered params and annotations covering safety, nothing needed to call 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?

Schema description coverage is 100%, so every parameter including cpv, countries, and min_days_to_deadline is already documented in the schema. The description only echoes payment_signature indirectly ('unless payment_signature is given') and adds no format or default semantics, so baseline 3 is correct.

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 ('Find open EU public tenders') plus the scope that matters for selection: open, biddable tenders rather than awards. The word 'open' implicitly separates it from siblings like search_tender_awards, but no sibling is named outright, so it falls short of a 5.

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?

'Use when building a bid pipeline or sales leads from public procurement' gives a clear use context that an agent can match against a user request. It offers no exclusions or explicit alternative routing (e.g., grants vs awards vs alerts), so it is clear context without the when-not guidance a 5 would need.

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