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

Annotations already declare readOnlyHint=true, destructiveHint=false, and openWorldHint=false, so safety is covered. With no output schema, the description compensates by naming the returned fields (totalUnits, totalCost, avgEntry) and disclosing the computational method, which is genuinely useful behavioral context.

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-loads the core computation, then the method note, trigger, and return fields. The parenthetical comparing harmonic vs arithmetic means is somewhat long but earns its place by preempting a plausible sibling confusion.

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 usefully enumerates return fields, and the method note plus trigger cover the call context. For a simple 2-param, read-only computational tool the definition is essentially complete.

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% with only 2 parameters, so the schema already documents pair format (BASE/QUOTE) and fill structure (price, units). The description's 'size-weighted' phrasing adds marginal semantic clarity about units but nothing 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 a specific verb+resource ('size-weighted average entry price across multiple forex fills') and explicitly differentiates from the sibling primitive.average_entry by explaining why the arithmetic mean is used rather than the harmonic mean. An agent can distinguish 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?

Provides a clear trigger ('Use when user asks "what's my average entry after these fills?"') and contrasts the method against the crypto average_entry tool. It does not state an explicit exclusion or when to prefer the sibling instead, but the routing context is clear.

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