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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds value beyond that: 'Nothing is stored' discloses statelessness (important for a decryption operation), and the x402/per-call payment disclosure plus the sensitive-data routing are operational traits an agent cannot infer from annotations. It does not describe the returned plaintext or failure modes, keeping it at 4.

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?

Two sentences, both load-bearing: the first is the operation, the second carries statelessness, the local alternative, and the payment condition. No filler and the purpose is front-loaded ahead of the caveats.

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?

The tool has nested free-form objects and no output schema, so the description carries the return-value burden and does not state what a successful open yields (plaintext, a file, a receipt). It also does not say where an `envelope` comes from, though the schema points at vault_keygen for the key. Adequate for invocation, thin for understanding results.

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 both parameters are already documented in the schema, including that private_key comes from vault_keygen. The description adds no format, encoding, or provenance detail for `envelope` (a free-form object with additionalProperties), so the baseline 3 applies. It neither compensates for nor worsens the schema.

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: 'Decrypt a Privi Vault envelope.' That separates it cleanly from vault_seal (encrypt), vault_keygen (key generation) and vault_receipt. It stops short of naming those siblings explicitly, but the resource boundary is clear enough for an agent to choose correctly.

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 supplies a real conditional rule: for sensitive data, prefer opening locally with `python -m privi_quantum.vault open`. That is genuine when-not guidance tied to a concrete alternative, plus a cost note pointing at quantum_pricing when payments are enabled. It does not explain when a remote open is actually required (e.g. no local key access), so it is a strong 4 rather than a 5.

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