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

Annotations already declare idempotentHint=true, destructiveHint=false, and openWorldHint=false, so the safety profile is covered; the description adds substantive behavior beyond that: dedup-by-email upsert semantics, the return payload ('the id of every lead written'), and how customVariables become merge tags. It doesn't discuss permissions, rate limits, or partial-failure behavior for a 1,000-item batch, which keeps it from a 5.

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?

Four front-loaded sentences, each carrying distinct information (capacity, upsert rule, return value, custom attributes) with no filler. The closing customVariables sentence mildly overlaps with the schema, so not a perfect 5.

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 compensates by stating the return value (lead ids for subsequent fetch/update). Batch limits, dedup behavior, optional campaign attachment, and custom attribute handling are all present, leaving no material gap for correct invocation.

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 the schema already documents email, the named fields, customVariables, and campaignId. The description restates the customVariables merge-tag concept and explains campaignId's purpose, but adds no syntax or format detail beyond the schema, so the baseline 3 applies.

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 ('Creates or updates ... leads') with scope ('up to 1,000 in one request') and an optional side effect ('adding them to a campaign via campaignId'). This cleanly separates it from single-record siblings like update_lead and read-only siblings like list_leads/get_lead.

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?

Explains the upsert rule clearly ('Existing leads (matched by email) are updated instead of duplicated'), which tells the agent when this tool is the right choice for existing contacts, and notes campaignId is optional. It stops short of explicitly naming alternatives (update_lead for one record, list_leads to discover leads), so it's clear context without explicit routing.

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