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

With no annotations provided, the description carries the full burden of behavioral disclosure, and it does so well. It explicitly warns that the server stores exactly the bytes supplied and cannot enforce client encryption, discloses that writes overwrite existing content, and exposes free-tier slot and size limits. This is unusually transparent for a mutation tool.

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 short sentences deliver the core action, the overwrite consequence, the client-side encryption requirement, the base64 encoding requirement, and resource limits. Every sentence carries distinct information and nothing is wasted.

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 write tool with no output schema, the description covers the essential calling requirements: what to store, how to encode it, what security caveat applies, and what quota limits exist. It omits explicit return-value behavior, but the core invocation is well-specified and the schema supplies the remaining parameter and auth-header guidance.

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 slug, data_base64, and vigil_key. The description still adds meaningful semantic context by emphasizing that data_base64 must contain already-encrypted bytes, that the blob is opaque to the server, and that the free tier imposes a 64KB per-slot ceiling, which an agent needs to know before invoking.

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 the specific action 'Store an opaque blob under a slug' with the resource being a journal entry addressed by slug. The parenthetical '(overwrites)' adds a key behavioral differentiator, and the use of 'opaque blob' clearly distinguishes this write-oriented tool from siblings like vigil_journal_get, vigil_journal_delete, and vigil_journal_list.

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 gives clear operational guidance: encrypt client-side, send exactly the supplied bytes, and base64-encode the data. It does not explicitly name alternative tools or state when not to use this one, but the context is sufficiently clear that an agent can infer the write-intent versus sibling read/delete/list operations.

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