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

Annotations already declare readOnly, idempotent, and non-destructive. The description adds significant behavior beyond this: immutability ('Both sides are immutable, so a diff is reproducible indefinitely'), default rule for the baseline, the 'not found' result for the first snapshot, and the explicit disclosure of reverse-chronological behavior with negative elapsed days. It also states auth requirements (tickets:read scope). No contradiction with 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 composed of four purposeful sentences. It starts with the core action, then explains parameter direction and defaults, then covers edge-case disclosure, then immutability and auth. No redundant or filler content; every sentence contributes meaningful information.

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 the tool's moderate complexity (2 params, output schema present, annotations provided), the description covers all essential aspects: purpose, parameter behavior, edge cases, side effects (none, immutability), and required permissions. The output schema presumably documents the return shape, so no further detail is needed. The description is self-sufficient 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% and both parameters already have descriptions in the schema. The tool description reinforces the 'to'/'from' side semantics and explains the defaulting rule for 'against', which adds clarity beyond the schema's individual parameter descriptions. Since the schema already carries most of the meaning, the description's incremental value is moderate.

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 opens with a specific verb and resource: "Compare two of your own portfolio snapshots and return the per-dimension movement between them." This clearly distinguishes it from sibling tools like get_portfolio_snapshot (single snapshot) or list_portfolio_snapshots (listing). The 'your own' scope and the output type (per-dimension movement) are explicit.

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 provides clear context on how to use the tool, including the direction semantics ('snapshot_id is always the to side; against is the from baseline') and the default behavior ('defaulting to your newest snapshot strictly older than it'). It also covers edge cases like first snapshot and reverse comparisons. However, it does not explicitly name alternative tools or state when not to use this tool, so it stops short of a 5.

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