phone_metrics

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

Beyond the readOnlyHint/openWorldHint annotations, it discloses that no phone connection, Runner launch, or control lease is acquired, that a shared 12 KiB row budget may silently reduce results with truncation explicitly flagged, and that some fields are intentionally null. These are exactly the operational traits an agent cannot infer from annotations.

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 core purpose in the first sentence and groups caveats afterward; nearly every clause carries distinct information (limits, null semantics, no-reconnect rule, excluded counters). It is dense but slightly overloaded with hedging clauses in a single paragraph.

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 no output schema, the description does the necessary work of enumerating what the aggregate contains and how truncation appears in the response. It does not describe the shape of the returned rows (per-run vs. per-section fields), leaving a small gap for a reporting tool.

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 both parameters are already documented, including the limit default of 5, maximum 20, and the shared 12 KiB budget note. The description's mention of 'Default latest 5 open and recent runs, maximum 20' and 'selected instance' largely restates the schema rather than adding new syntax or constraints, so baseline 3 applies.

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 ('Read task performance aggregates') scoped to the caller's X-Phone-Owner and a selected instance, then enumerates the exact contents (HTTP-call counts, batch/flow usage, stale/unknown outcomes, losses, p50/p95 latency). An agent can tell this apart from phone_logs or phone_status without opening the schema, though no sibling is named directly.

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?

Provides explicit exclusions and boundaries: 'Never reconnect solely for metrics' steers away from phone_reconnect, and 'Global cross-owner loss counters are excluded' plus 'model_round_trips remains null without a complete declared trace' clarify when the data will be absent. It gives clear context but does not name a positive alternative tool for adjacent needs (e.g., raw logs).

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