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

With no annotations, the description carries the full burden of behavioral disclosure. It reveals that close ranks exist only before 2026-07-31, daily data starts 2026-07-21, intraday data starts 2026-08-19, and data is never backfilled. It also states the key requirement. It does not mention error behavior or response format, but the availability quirks are well covered.

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 two sentences, but the first is dense with parentheticals and multiple clauses, making it slightly harder to parse. Still, every sentence earns its place: the first covers the core functionality and data quirks, the second covers access requirements and sibling alternatives. It could be restructured for readability but is not excessive.

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 no output schema, the description does a good job explaining what daily rows contain (rank, % move, breadth per sector) and the intraday mode. It also includes availability dates, backfill status, and key constraints. Gaps include the intraday row structure and behavior for dates outside the available range (empty vs. error), but these are secondary 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?

Schema coverage is 100%, so the baseline is 3. The description does add that intraday=true returns '5-minute intraday rows', but the schema already says 'true = 5-minute rows instead of the daily close', so this is redundant. The date parameter's meaning is not expanded beyond the schema's pattern and description.

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 returns a daily close record for one date (with rank, % move, and breadth per sector) or 5-minute intraday rows when intraday=true. It distinguishes itself from siblings by noting that free keys cover get_sectors and get_roster, implying this is the paid history tool.

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 says a paid Sector API key is required and free keys cover the sibling tools, which tells an agent when not to use this tool. It also warns that data is never backfilled and gives start dates, clarifying when historical data is available. It does not directly state 'use this instead of X', but the context is sufficient.

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