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

Annotations already declare readOnlyHint=true, destructiveHint=false, and openWorldHint=true, so the safety profile is covered. The description adds genuine behavioral context beyond that: the Realtime API enforces a small fixed schema and many run_report fields (e.g. pagePath, sessionSource) are invalid here, which tells the agent what will error. It doesn't mention rate limits or quotas, so it falls short of 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?

Front-loads the purpose before the field lists, and the long enumeration is necessary given zero schema coverage. It is dense but every clause earns its place; the only minor cost is the parenthetical that interrupts the opening sentence.

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 read-only report tool with no output schema, nothing an agent needs to invoke it correctly is missing, including the routing rule to run_report. The annotation and output-schema picture means no further return-value explanation is required.

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 0% and the dimension/metric arrays are bare strings with no enums, so the description carries the full burden. It compensates well by enumerating the complete valid set of dimensions (including the `customUser:<parameter_name>` pattern) and metrics, which is exactly what the schema lacks. It says nothing about `limit` or `property`, leaving those two params unexplained.

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 ('Run a GA4 realtime report') and immediately defines the scope ('active users right now, by dimension'). It explicitly distinguishes itself from the similar-sounding sibling run_report, so an agent can pick the right tool without opening either 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?

Gives an explicit when and when-not: use this for realtime/active-user reporting, and 'use run_report instead for anything outside this list.' The condition selecting the alternative is spelled out rather than inferred, which is exactly the routing guidance the sibling set needs.

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