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

Beyond the readOnlyHint annotation, the description discloses real behavioral traits: it explicitly reports negative forward variance and missing tenor brackets, and it documents a deterministic 503 historical_source_unhealthy (daily_series_disabled) until the daily-series release is enabled. That failure-mode disclosure is exactly the kind of context annotations cannot carry. It stops short of describing pagination or granularity limits.

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?

Three dense sentences, front-loaded with what the tool returns, then edge-case behavior, then availability. No filler or restatement of the title.

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 must convey return content, and it does: the specific tenors, derived metrics, edge-case signaling, and a hard availability gate. A trace of what breadth of history or how the percentile is computed is missing, but for a read-only series tool this is close to sufficient.

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 100%, so symbol/start/end/apiKey semantics are already fully documented, including the 366-day cap, the ET-before-today rule, and the OAuth-vs-/mcp apiKey requirement. The description repeats the SPY/QQQ scope but adds no syntax or format detail beyond the schema. 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 the specific resource and its payload precisely: daily SPY/QQQ constant-tenor 30/60/90-day IV, slope, forward variance and prior-only slope percentile. This distinguishes it from get_term_structure (current tenor) and get_historical_volatility. It lacks an explicit verb, but the data it returns is unambiguous.

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 when-to-use guidance relative to siblings like get_term_structure or get_historical_volatility; the 'not a calendar trade recommendation' line is a disclaimer, not a selection rule. The only actionable guidance is the release-pending failure mode, which tells the agent when the tool will fail rather than when to prefer it.

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