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

Annotations provide readOnlyHint=true and idempotentHint=true, covering safety. The description adds value by disclosing that decoding malformed input returns a recoverable error rather than silent best-effort (important behavioral detail), and explains how url uses encodeURIComponent/decodeURIComponent. It doesn't contradict annotations; readOnly hint is consistent with a transform tool. Missing specifics like output format or edge cases (empty string, whitespace), but the error disclosure is valuable.

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 with no fluff: the first states the full capability, the second explains the two operations and specific encodings, the third discloses error behavior. All sentences earn their place, and the most important info (what it does, how to invoke) is front-loaded.

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?

Given the tool's simplicity (3 parameters, simple enum values), the description is complete enough. It covers the operation semantics, encoding specifics, and error handling. It does not mention that output is likely a string, but an output schema exists, so return values are covered. Minor gap: doesn't explicitly state that encode assumes raw UTF-8 text input, but the description says 'raw UTF-8 text' for encode, so it's covered. Overall complete.

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%: each parameter (value, encoding, operation) has a clear description and encoding has enum with meanings. The description reinforces parameter semantics and adds context for base64url vs url and error handling, but doesn't add much beyond the schema. Baseline 3 is appropriate because schema does the heavy lifting.

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 clearly states the tool encodes/decodes values across four specific encodings, in either direction, and details the exact operation semantics ('encode' vs 'decode'). It distinguishes it from siblings like toolkit_hash_value (hash vs encode) and toolkit_generate_id (generation vs transformation). Purpose is specific with verb, resource, and scope.

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 explains when to use encodings: base64url for URL-safe contexts, url for percent-encoding, and specifies encodeURIComponent/decodeURIComponent semantics. It doesn't explicitly exclude cases where this tool should NOT be used or mention alternatives like hash for one-way transforms, but it gives sufficient context for an agent to decide based on the operation. Slight gap in not contrasting with hash_value or other transforms, but clear enough.

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