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

Annotations already declare readOnlyHint and openWorldHint, and the description is fully consistent with a pure computation. It adds real behavioral disclosure beyond that: the 12.2 constant when TDS exceeds 1,000 ppm, and the table-lookup rule that a between-rows reading takes the next greater row. It stops short of stating output precision or rounding.

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?

Front-loaded with the purpose, then formula, then the lookup and balance-pH rules — every sentence carries computational meaning. It is dense and slightly run-on, packing two formulas and a lookup rule into a compact block, but nothing is filler.

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 compensates by naming the two return values (the SI and the pH for SI = 0), and it explains the factor-table derivation, which is materially useful. It does not state rounding/precision or the valid input ranges for each reading, a minor gap for a calculation tool with no output schema.

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 baseline is 3, but the description earns above baseline by explaining the two non-obvious parameters: the `method` choice (table vs interpolation, with the 'next greater row' tie-break spelled out) and the TDS threshold that switches the constant from 12.1 to 12.2. Those are semantic rules the schema cannot express.

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 computational verb and named index (Langelier Saturation Index) plus the exact inputs it consumes, so an agent can distinguish it from the neighboring single-chemistry calculators (calc_pool_alkalinity, calc_pool_calcium_hardness, calc_pool_chlorine). The formula is given inline, removing any ambiguity about what is produced.

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?

Usage is implied by the chemistry context (assessing overall water balance from four readings) but there is no explicit statement of when to reach for this versus the individual pool-chemistry siblings, nor any prerequisites. An agent can infer the fit, but must reason about it.

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