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

Beyond the annotations (idempotentHint=true, readOnlyHint=false), the description discloses rich behavior: Apollo auto-creates contacts, duplicate adds return already_present, activateSequence is best-effort, Instantly handoffs sync reply/bounce/unsubscribe state, the operation is free, and it enumerates possible statuses including specific error codes like INTEGRATION_NOT_CONNECTED and INTEGRATION_KEY_INSUFFICIENT_SCOPE. This is far more than the structured annotations provide.

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 but every sentence adds value: scope, provider differences, idempotency, sequencing activation, state-sync side effects, billing, return statuses, and lookup tool references. The core purpose is front-loaded, and the supporting detail is compactly organized. Nothing feels like filler or repetition of the schema.

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 tool with no output schema and 8 parameters across two providers, the description is exceptionally complete: it explains the return status vocabulary, provider-specific requirements, error scenarios, idempotency behavior, and side effects. An agent has enough context to invoke the tool correctly and interpret the result.

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?

Although the schema already covers 88% of parameters, the description adds significant provider-specific meaning: emailAccountId is required for Apollo and unused by Instantly, activateSequence is Apollo-only and best-effort, skipIfInWorkspace is Instantly-only, and campaignId comes from campaignstack_list_external_sequences. These semantics materially help an agent choose and fill parameters correctly.

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: 'Add one lead to an external provider's sequence', and immediately distinguishes it from sibling tools by naming the provider-parameterized nature (Apollo sequences, Instantly campaigns) and the external scope. This makes it clearly separable from similar tools like campaignstack_add_lead_to_external_list and campaignstack_add_leads_to_list.

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 context is clear: this is for adding a single lead to an external sequence, with provider-specific behaviors spelled out. It also directs the agent to campaignstack_list_external_sequences for sequence IDs and campaignstack_list_external_email_accounts for Apollo mailbox IDs. However, it does not explicitly state when to prefer this tool over the closely related sibling add_lead_to_external_list, so it stops short of a full 5.

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