Decode JWT headers and payloads, and check expiry, without verifying signatures. Ideal for inspecting token contents locally.
Decode a JWT (header + payload + expiry check) without verifying the signature
| Name | Required | Description | Default |
|---|---|---|---|
| token | Yes | JWT string (three dot-separated base64url parts) |
Changes observed during successful MCP inspections.
v2.0.0Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations provided, the description carries the behavioral burden and does a good job by disclosing the primary limitation ('without verifying the signature') and the included expiry check. It does not mention error behavior or what happens with malformed or expired tokens, but the most important behavioral caveat is explicitly stated.
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?
The description is a single compact sentence that front-loads the verb and resource, then adds the critical caveat. There is no filler or redundant restatement of the parameter schema.
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 one-parameter utility tool, the description is largely sufficient: it identifies the input, the action, the included expiry check, and the key non-behavior. The main gap is the lack of any indication of return shape or error behavior, which would be helpful given there is no output schema.
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 baseline is 3. The description adds meaningful context beyond the schema by indicating what decoding entails (header, payload, expiry check) and aligning with the token format. This raises it above the baseline.
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 a specific verb ('Decode'), a specific resource (JWT), and the exact scope (header + payload + expiry check). It also explicitly carves out what it does not do ('without verifying the signature'), which distinguishes it from any verification-style tool and from all sibling utilities.
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 clearly communicates when to use the tool: for decoding and expiry checking, and explicitly warns that signature verification is not performed, implying it should not be used for security-critical verification. No alternative JWT-specific tool is named, but the guidance is still clear in context.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.