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

Beyond the annotations (readOnlyHint=true, openWorldHint=true), the description adds valuable behavioral context: the tool may return a specific error early in the trading day, and the agent should treat that as 'not ready' rather than a failure. This is practical operational knowledge that annotations do not convey.

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 two sentences with no unnecessary words. It front-loads the core function and return values, then adds the caveat about early-day behavior. Every word 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?

Given the tool has no parameters, no output schema, and the annotations declare read-only safety, the description sufficiently covers what the tool returns (1-day and YTD returns) and an important edge case. It is complete for a simple, parameterless read-only 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?

The tool has zero parameters, so baseline is 4. The description does not need to explain parameter semantics beyond what the schema (empty) already provides. It focuses on output semantics, which 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 clearly states the tool's function: it retrieves current performance (1-day and YTD returns) for major index ETFs like SPY, QQQ, and IWM. It also distinguishes itself by noting the use case of benchmarking an individual stock against the market, which differentiates it from sibling tools that focus on quotes, sectors, or signals.

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 provides explicit guidance on when to use the tool: 'Use to frame an individual stock against the market.' It also communicates an important temporal condition (early trading day behavior), which helps the agent decide when to call it. It does not explicitly list alternatives, but the use case is clear enough.

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