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

Annotations only declare the mutation posture (readOnlyHint=false, non-destructive, non-idempotent), which tells an agent nothing about the actual flow. The description supplies the critical non-obvious behavior: the call returns a terms_url the user must open exactly once, the account does not exist until they do, and a failure surfaces as fallback=true plus Google's own sign-up page. That is precisely the context annotations cannot carry.

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?

Front-loaded with the purpose, then the return contract, then a walk-through of the follow-on tools, with zero filler sentences. The trailing 'Tools revision: d80f65c0.' line is the one element that does not earn its place for an agent selecting or invoking the tool.

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?

An output schema exists, so return values need not be enumerated, yet the description still highlights the two return fields that actually change agent behavior (terms_url and fallback). For a two-parameter creation tool that kicks off a human-in-the-loop handshake, nothing an agent needs in order to call it correctly is missing.

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 0%, so the description carries the burden. It does well on region_code, defining it as a CLDR business country code with concrete examples ('IN', 'US'), which the schema's bare 'Region Code' title does not convey. display_name is left unexplained, though its meaning is largely self-evident from the name.

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 first sentence names a specific verb and resource ('Start a new Google Analytics account') and constrains scope ('for a login that has none'), which cleanly separates it from ga4_create_property and ga4_list_account_summaries. An agent can identify this as the account-creation entry point without opening any schema.

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?

It states the precondition (a login with no account), names the alternative for the opposite case implicitly, and spells out the follow-on sequence: accept the terms link, then ga4_list_account_summaries confirms it, then ga4_create_property. The refusal path ('If Google refuses... give the user that link') is an explicit branch instruction rather than something to infer.

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