Would this index help?

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

The description goes far beyond the readOnly/idempotent/destructive annotations by explaining the hypopg mechanism: index registered in backend memory only, gone when the call returns. It also discloses the failure mode if hypopg is missing and clarifies that results are planner estimates, not measured times. This aligns with annotations with no contradiction.

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 dense yet well-structured, front-loading purpose and safety, then covering inputs, outputs, interpretation, and prerequisites. Every sentence earns its place; there is no filler or repetition.

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?

Given no output schema, the description fully explains what is returned: plan and cost with and without, plus whether the planner used the index. It also covers prerequisite, absence of side effects, and interpretation nuance, making it complete for a complex 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?

The schema already covers 100% of parameters with descriptions, so baseline is 3. The description adds value by stating 'the index definition is assembled here,' clarifying that users supply table and columns rather than a full DDL statement, which goes beyond the schema's individual parameter 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 opens with 'Answers whether an index would change the plan for a given query — WITHOUT creating it,' a specific verb and resource. It explicitly distinguishes itself from sibling tools like analyze_indexes and explain_query by emphasizing 'the planner sees it, storage never does,' making the hypothetical nature unmistakable.

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?

It gives clear usage guidance: provide query, table, and columns; the index is assembled automatically; no DDL can be sent. It also warns that these are estimates, not proof. However, it does not explicitly name alternative tools or state when not to use this tool, so it falls short of a 5.

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