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

Annotations already declare readOnly/idempotent/non-destructive, and the description goes well beyond them: it discloses the exact return shape (CSV column list), the JSON-candidate fallback for ambiguous input, and the critical behavioral difference that headline indices (ASPI, S&P SL20) populate OHLC while industry sub-indices populate turnover/volume/trades and leave high/low empty.

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, with the core purpose front-loaded before the return-format and index-type caveats. The CSV column enumeration is long but earns its place since no output schema exists; nothing is redundant.

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 is responsible for describing returns and it does so thoroughly, including the column set, the JSON candidate edge case, and the headline-vs-sub-index field divergence. An agent has everything needed to call it and interpret the result.

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% and the schema already documents 'from', 'to', and 'index' including format and ambiguity behavior. The description adds no parameter syntax or format detail beyond what the schema provides, so the baseline of 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: 'Get historical data for a CSE market/sector index.' The word 'historical' distinguishes it from sibling tools like get_indices (current listings) and screen_indices, so an agent can route to it without opening schemas.

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 explains the ambiguity-recovery flow (ask the user to pick a candidate and call again), which is genuine usage guidance. However, it never states when to prefer this over get_indices, screen_indices, or get_ohlc_data, so the alternative-selection guidance is only implied.

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