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

Annotations declare readOnlyHint=true, which the description aligns with by stating it 'Returns' data. The description adds rich context: source methodology (NOAA CPC Nino 3.4), interpretation (50=neutral, higher/lower), return fields, and update frequency. It does not contradict annotations.

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 compact, two sentences, front-loaded with the core output then usage context. Every sentence provides value, with no 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?

Given there is no output schema, the description lists the key returned data points (score, label, episode_strength, trend, last 12 ONI values) and includes update frequency, making it clear what an agent can expect. It does not mention the optional history mode in the main description, but the schema covers that.

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 schema covers the only parameter 'days' with a thorough description (history series, Gold tier requirement). Since schema description coverage is 100%, the description need not repeat it; the baseline of 3 applies as the description adds no extra parameter context.

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 0-100 ENSO climate-regime score with specific components (NOAA state label, episode_strength, trend, ONI values). It uses a specific verb 'Returns' and identifies the resource, distinguishing it from siblings by its specialized ENSO focus.

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 states when to call, e.g., when the user asks about El Nino, La Nina, or ENSO-related decisions. It provides clear use cases but does not mention alternatives or when not to use, so it falls short of full guideline coverage.

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