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

Annotations already declare readOnlyHint=true, so safety is covered by structured data. The description still adds genuine behavioral context by clarifying that results are scoped to the authenticated caller's direct access, which matters for interpreting the returned IDs. It says nothing about pagination or result ordering, so it is not exhaustive.

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?

Two short sentences, no filler, and the primary guidance ('use this tool first') is front-loaded in the second sentence before the reason. Every sentence earns its place.

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 zero-parameter discovery tool with an output schema already covering the return shape, the description supplies everything needed: what it returns, its auth scope, and its role as the entry point before other customer-ID-dependent tools. Nothing required for correct invocation 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?

The tool takes zero parameters, which is the baseline-4 case; there is no parameter behavior left to explain. The description correctly implies the call is fully parameterless and derived from the caller's auth context.

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 names a specific verb and resource (returns IDs of accessible customers) and adds the scoping constraint that access is limited to the authenticating user. It does not explicitly name or contrast the siblings (metadata_get_resource_metadata, search_search), so it is clear but not fully differentiated.

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 exactly when to reach for this tool: 'Use this tool first to discover available customer IDs if the user hasn't provided one,' plus the rationale that most other tools need a valid customer ID. The sequencing and trigger condition are unambiguous.

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

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

The annotation readOnlyHint=true already covers the safety profile, and the description complements it by disclosing the response contents (fields, metrics, segments) and the naming convention for metrics/segments. It also adds an operational trait: 'responses ... should be cached, as they don't change frequently'. No contradiction with 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?

The description is front-loaded with the core purpose and adds concise, actionable guidance. There is minor redundancy: the first sentence says 'including compatible metrics and segments' and the third sentence repeats that metrics and segments can be selected. Still, it is well-structured and every section adds value.

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 metadata-lookup tool with an output schema and readOnlyHint annotation, the description covers everything an agent needs: what the tool returns, naming conventions, mandatory usage before querying, and caching guidance. No significant gaps remain for correct invocation.

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?

The input schema already describes both parameters comprehensively (100% coverage), so the description need not repeat them. It adds a small amount of extra context with examples like 'campaign', 'ad_group', but this does not significantly exceed what the schema provides. Baseline 3 is appropriate.

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 uses a specific verb ('Retrieves') and names a clear resource: the selectable, filterable, and sortable fields for a Google Ads resource, including metrics and segments. This clearly distinguishes the tool from the siblings: search_search executes queries, while this tool provides metadata for building them.

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 states when to use the tool: before constructing a query for the `search` tool, and it warns 'Do not guess fields, you MUST use this tool to discover them'. This gives the agent an unambiguous directive and contextual placement relative to the sibling search tool.

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

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

The only annotation is readOnlyHint=true, which already tells the agent this is a safe read operation. The description adds no additional behavioral context—nothing about pagination, limits, error handling, or response format. It fails to enrich the annotation-provided information.

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 only one sentence and is technically concise, but it is under-specified to the point of being nearly useless. It does not front-load critical information or structure the content in a way that helps an agent understand the tool's scope. The brevity is a deficiency, not a virtue.

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?

This is a 7-parameter tool with 3 required parameters, yet the description provides no context about what the tool actually accomplishes beyond a generic 'search'. It lacks any explanation of the query syntax, the relationship between fields and resource, or expected output. Given the complexity, the description is grossly incomplete.

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%, meaning all parameters have descriptions in the schema. The description adds no extra meaning beyond what the schema already provides, so the baseline of 3 applies. It does not compensate with any additional clarifications about parameter relationships or formatting.

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 states a verb ('Fetches data') and a resource ('Google Ads API') but is extremely vague about what kind of data or which resources are searchable. It does not differentiate from sibling tools like metadata_get_resource_metadata or customers_list_accessible_customers, making it unclear when this tool is the right choice.

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?

There is no guidance on when to use this tool versus the siblings. No mention of alternatives, exclusions, or prerequisites. An agent has to infer that this is a general search tool, but the description gives no contextual hints about when it should be preferred.

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