Decode JWT header and payload claims locally without verifying the signature. The response always marks verification as false.
| Name | Required | Description | Default |
|---|---|---|---|
| token | Yes | JWT compact serialization, up to 20 KB. | |
| l402_proof | No | L402 payment proof "<macaroon>:<preimage>". Omit on the first call; an unpaid call returns a 0 sat Lightning invoice and the macaroon. Pay the invoice, then repeat the call with this argument set. |
Changes observed during successful MCP inspections.
Input schema / properties / l402_proof / descriptionPrevious value: -"L402 payment proof \"<macaroon>:<preimage>\". Omit on the first call; an unpaid call returns a 5 sat Lightning invoice and the macaroon. Pay the invoice, then repeat the call with this argument set."New value: +"L402 payment proof \"<macaroon>:<preimage>\". Omit on the first call; an unpaid call returns a 0 sat Lightning invoice and the macaroon. Pay the invoice, then repeat the call with this argument set."Input schema / properties / l402_proofAdded value: +{
+ "description": "L402 payment proof \"<macaroon>:<preimage>\". Omit on the first call; an unpaid call returns a 5 sat Lightning invoice and the macaroon. Pay the invoice, then repeat the call with this argument set.",
+ "type": "string"
+}Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations declare the safety profile (readOnly, non-destructive, open-world), so the description's job is to add operational context. It does: processing happens 'locally' and the response 'always marks verification as false', which is a meaningful caveat an agent could otherwise misread as a validation result. It stays silent on malformed-token behavior and on the paid L402 retry flow, but that flow is documented in the schema.
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 tight sentences, with the non-verification caveat front-loaded in the first sentence and the output guarantee in the second. Nothing is redundant or padded.
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 supplies the single most important return-value fact (verification always false) and the local-processing guarantee, which is enough to call the tool correctly. It could go slightly further by noting behavior on malformed tokens or that the decoded claims are returned as an object, but the critical risk is covered.
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%, including the 20 KB token limit and the full L402 payment-proof workflow, so the schema carries parameter meaning. The description adds nothing about either parameter; 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 states a specific verb and resource ('Decode JWT header and payload claims') and adds the essential scope qualifier 'without verifying the signature', which distinguishes it from any verifying or signature-checking alternative. An agent knows exactly what it gets without opening the schema.
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 'without verifying the signature' phrase implies when this tool is appropriate (inspection only) versus when signature validation is needed, but it never names an alternative or states an explicit exclusion. Usage is inferable rather than prescribed.
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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