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

Annotations already cover safety (readOnlyHint=true, destructiveHint=false, openWorldHint=false), so the bar is lower. The description adds genuinely non-obvious behavioral context: the 100-pointer cap, the missing-vs-null preservation semantics, the free-during-validation pricing model, and that no key/credits/wallet are required. It does not explain what happens when a pointer matches nothing beyond the missing/null note, but that is a minor residual 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?

Roughly three short sentences, front-loaded with the core operation before the pricing/auth caveats. It is slightly choppy and the 'No key, credits or wallet' fragment partly restates 'FREE during validation', but there is no real padding.

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?

An output schema exists, so return-value shape need not be described. For a read-only extraction tool with nested input (data + paths) and legacy optional fields, the description covers the cap, authorization/pricing model, and a subtle semantic guarantee, leaving only the no-match behavior as an unaddressed edge case.

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 67% and the description compensates by clarifying the pointer format (RFC 6901), the maximum number of paths (100, matching schema maxItems), and the null-vs-missing distinction that governs how an absent match is reported. The two legacy fields (requestId, maxPriceMicroUsd) are already documented in the schema as ignored in free mode.

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?

Specific verb ('Select') plus resource ('JSON values') plus mechanism ('RFC 6901 pointer strings'), and it names the count limit (100). The clause 'preserving missing versus null' distinguishes it behaviorally from data_json_canonical, data_json_diff, and data_json_patch, which are the nearest siblings in the data_json family.

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 states a cost/auth condition ('FREE during validation. No key, credits or wallet') and that existing limits apply, which is useful context, but it gives no guidance on when to choose this over data_json_canonical/diff/patch or what kinds of inputs are inappropriate. No when-not or alternative routing is present.

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