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

Annotations already declare the write, non-destructive, open-world profile, so the bar is lower. The description adds the meaningful provenance rule (source must be an https page stating the fact, otherwise mark synthetic), but it largely restates the `source` and `synthetic` field descriptions and says nothing about duplicates, one-fact-per-call limits, or failure modes.

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?

It is a single sentence with the action front-loaded, but the construction "with the https page that states it, or synthetic: \"yes\" when made up for the plan" is grammatically tangled and takes a second read to parse.

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?

For a simple one-fact append with a schema that covers most fields and an adequate safety annotation profile, the description supplies the key decision (real source URL vs synthetic). It is a bit thin on the builder-token/space-ownership precondition, but nothing critical to calling it correctly is absent.

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 80%, so the schema itself documents `fact`, `token`, `source`, and `synthetic`. The description reinforces the source-vs-synthetic choice but adds no syntax, format, or constraint detail beyond that; 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 and resource: "Add one fact to a Space you built." That is concrete enough to distinguish it from write-free siblings like fathom_find or fathom_forget, but it never contrasts with fathom_remember or fathom_space, which an agent could easily confuse with it.

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 conditional it carries (use it with an https source page, or mark synthetic when the fact is invented for the plan). There is no explicit when-not-to-use rule and no named alternative such as fathom_remember, so the routing decision is left partly to inference.

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