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

Annotations already declare idempotentHint=true and destructiveHint=false; the description reinforces this usefully by explaining the email-matching upsert rule and disclosing that every written lead's id is returned, which matters because no output schema exists. It stops short of disclosing partial-failure behavior, validation errors, or rate limits for a 1,000-item write, which is the remaining gap for a mutation tool.

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 short sentences, front-loaded with the operation and scope, then upsert behavior, then return value, then the escape hatch for extra fields. No filler, no repetition of the tool name, and each sentence adds a distinct fact.

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 only two top-level parameters, full schema coverage, and no output schema, the description covers everything an agent needs: what it writes, how duplicates are handled, the batch limit, the optional campaign, and what comes back (lead ids). Nothing essential is missing 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 coverage is 100%, so the baseline is 3, but the description adds real meaning beyond the schema: it reveals that email is the match key determining create-vs-update, which the schema does not state, and that campaignId optionally attaches the batch to a campaign. The customVariables/merge-tag explanation largely duplicates the schema text rather than extending it.

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 a specific verb and resource ('Creates or updates up to 1,000 leads in one request') and adds the distinguishing scope: bulk upsert keyed on email, with an optional campaign association. That immediately separates it from single-lead or read-only siblings such as list_leads or delete_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?

The description gives clear usage context by stating the batch ceiling (1,000), that campaignId is optional, and that existing leads are updated instead of duplicated, so an agent knows when bulk-upsert semantics apply. It does not, however, explicitly say when to prefer a sibling such as add_lead_finder_prospects or how to handle invalid or duplicate rows within the request.

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