Why a node exists

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

Annotations already establish readOnlyHint=true and destructiveHint=false, so safety is covered. The description adds real behavioral detail beyond the annotations: results are truncated to eight open siblings and eight open children, every blocking item is retained, and full lifts the caps. That truncation behavior is valuable for interpreting results.

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 sentences, front-loaded with the purpose and the composition of the answer, with the cap behavior last where it belongs. The line 'This is the question the whole tool exists to answer' is partly rhetorical and does not encode new facts, keeping it just short of a 5.

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 carries the burden of describing the return, and it does: the goal-to-node chain, parallel-open items, descendants, blockers, and the eight-item caps. For a read-only query tool this is nearly sufficient; only explicit sibling differentiation is missing.

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 the schema already documents id, full, and project, and the description largely restates the eight-item cap that the schema's full description already mentions. It adds the 'every blocking one kept' rule, a minor increment, so the baseline 3 for schema-driven parameters is appropriate.

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 states a specific capability with concrete scope: the chain from goal to node, parallel-open items, descendants, and blockers. It reads as a distinct 'explain a node's rationale' tool. It does not explicitly name a sibling (brief, open, find) it differs from, so it stops short of a 5.

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?

It asserts 'This is the question the whole tool exists to answer,' implying this is the canonical rationale query, and it enables deeper retrieval via full. But there is no explicit when-to-use/when-not, no routing against vivac_brief or vivac_open, and no stated prerequisites, so usage is only implied.

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