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

Annotations already declare readOnlyHint=false, idempotentHint=true, destructiveHint=false. The description adds that it's an upsert (so it overwrites/creates), that empty string clears instructions, and that it's account-wide applying to all models. It also discloses the read-only key restriction. This adds meaningful behavioral context beyond the annotations, though it doesn't detail side effects like whether existing instructions are replaced atomically.

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?

Three sentences, each with a distinct purpose: what it does, scope, and routing guidance. No fluff. The key facts (upsert, max length, account-level) are front-loaded.

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 single-parameter tool with full schema coverage and annotations covering idempotency and safety, the description is nearly complete. It covers scope, alternatives, and auth restriction. The only minor gap is not describing the return value/confirmation, but there's no output schema and the operation is simple enough that this is a small omission.

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 the 'content' parameter fully, including maxLength and the fact that empty string clears. The description repeats the max 3000 chars and Markdown/text but doesn't add much beyond the schema. Baseline 3 is appropriate since the schema does the heavy lifting.

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 clearly states the verb 'Upsert' and the resource 'account-wide custom instructions for the authenticated user', with a max length constraint. It distinguishes itself from sibling tools like layerz_set_finance_md by explicitly saying to prefer editing FINANCE.md for model-specific conventions. This is a specific, unambiguous purpose.

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 explicitly says when to use this tool vs alternatives: 'Prefer editing FINANCE.md for model-specific conventions; use this for cross-model preferences.' It also notes it's not available for read-only API keys, which is a clear usage constraint. This is excellent guidance.

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