Read the registry as-of a moment

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

Beyond the readOnlyHint and openWorldHint annotations, the description discloses that the operation is 'never a scan (not credit-metered)', explains tier-based access restrictions (past-minute reads are Pro, returning 403 with an upgrade URL), and notes that results are canonical bytes. This adds meaningful behavioral context beyond what annotations provide.

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?

Although the description is longer than typical, every sentence adds actionable information: the read-only/credit distinction, the connection to run_scan, the deflation warning, and the tier/error details. The TIERS section is clearly separated and front-loaded with the core purpose in the first sentence, so the structure is efficient and purposeful.

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 compensates by listing the exact contents of the returned snapshot (feature values, window aggregates, fired rulebook ids) and describing the canonical-byte format. It also covers the access model (free/present vs. paid/past), error behavior, and its relationship to run_scan, making it complete for an agent to decide when and how to invoke it.

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 100%, so the schema already fully explains both parameters (symbol as lowercase perp, at as RFC3339 datetime floored to grid bucket). The description does not add parameter-specific semantics beyond restating the 'as-of' grid-bucket concept, which is already implied by the schema. Baseline 3 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 uses a specific verb ('Read') and resource ('engine registry AS-OF the grid bucket containing a moment'), and states the exact contents returned (every feature value, window aggregates, fired rulebook ids). It also explicitly distinguishes itself from run_scan, which is the primary sibling, by noting it is a read, never a scan.

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 gives explicit when-to-use guidance: 'Use it to turn one observed moment into candidate clauses, then run_scan to find every other time it looked like that.' It also provides selection discipline for choosing moments and clear tier-based guidance on when to query present vs. past minutes, including the 403 GRANT_REQUIRED behavior on free accounts.

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