Hopi

JWT decoder

hopi_jwt_decoder

Read-onlyIdempotent

Decode a JSON Web Token's header and payload from base64url. The signature is never verified, so a decoded claim proves nothing on its own. Reports the algorithm, the exp, iat and nbf time claims, and whether the token is expired. A leading Bearer prefix and whitespace are handled. Source: https://hopi.co.uk/jwt-decoder/

Input Schema

TableJSON Schema

Name	Required	Description	Default
`token`	Yes	The JWT to decode, optionally with a leading 'Bearer ' prefix

Output Schema

TableJSON Schema

Name	Required	Description
`exp`	No
`iat`	No
`nbf`	No
`type`	No
`header`	No
`status`	No
`payload`	No
`summary`	Yes
`algorithm`	No
`source_url`	Yes	The tool's page on hopi.co.uk (cite or open to check the figure)
`statusNote`	No

Schema Changelog

Changes observed during successful MCP inspections.

Changed3 schema fields changedSep 20, 2026
- addedOutput schema / properties / exp / type
```
Added value: +[
+  "number",
+  "null"
+]
```
- addedOutput schema / properties / nbf / type
```
Added value: +[
+  "number",
+  "null"
+]
```
- addedOutput schema / required
```
Added value: +[
+  "source_url",
+  "summary"
+]
```
First observedSep 20, 2026

TDQS

A4.7/5.0

Behavior5/5

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

Annotations already mark the tool as read-only and idempotent, but the description adds substantial behavioral details: the signature is never verified, it reports algorithm, exp, iat, nbf, and expired status, and it tolerates a leading Bearer prefix and whitespace. These disclosures go well beyond the annotations and do not contradict them.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is three focused sentences with no filler. The primary action and the critical non-verification caveat are front-loaded, followed by output details and input edge-case handling. The trailing source URL is unobtrusive and does not dilute the key information.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a single-parameter tool with a full output schema, the description covers everything an agent needs: what the tool does, what it reports, its limitations, and how it handles input variations. There are no significant gaps in understanding how to invoke or interpret the result.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The schema covers 100% of parameter semantics by describing the token with an optional Bearer prefix, so the baseline is 3. The description adds value by clarifying that whitespace is also handled and by framing the input as a JWT header/payload to be decoded. This small but meaningful enrichment justifies a 4.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific verb and resource: 'Decode a JSON Web Token's header and payload from base64url.' It also distinguishes itself from generic utilities by focusing on JWT structure and explicitly noting it does not verify signatures, which separates it from any sibling that might decode base64 or hashes.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description gives clear context for when to use the tool—when you need to inspect a JWT's header, payload, or expiry status. It also provides a when-not-to-use caveat: the signature is never verified, so decoded claims prove nothing on their own. No sibling tool covers JWT decoding, so naming an explicit alternative is not applicable.

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

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