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

readOnlyHint=true already establishes the safe read profile, so the description needn't restate it. Beyond that it adds little behavior: no mention of missing-id errors, response size, or rate limits; the "allFields" tip duplicates the schema rather than disclosing new behavior.

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?

Two short, front-loaded sentences; the core action leads and the endpoint mapping trails. The endpoint line is mildly redundant for invocation purposes but costs almost nothing.

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 two-parameter, annotation-covered read tool with no output schema, this covers the essentials: what it returns conceptually (contact plus custom fields when requested) and how to widen the field set. Only the absence of any return-shape or error hint keeps it from 5.

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 both `id` and `fields` are already documented in the input schema. The description's fields="allFields" note merely restates the schema's own enum-like guidance, so it earns the baseline rather than more.

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 ("Fetch one contact by id") plus the underlying REST endpoint, which lets an agent separate it from list/search siblings such as fub_search_people and fub_list_* tools. It never explicitly names a sibling, so it stops 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?

Usage is only implied: "by id" signals this is the lookup for when you already know the identifier, versus fub_search_people for discovery. There is no explicit when-to-use/when-not statement or named alternative, which is the minimum viable level.

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