Sentinel

One JSON-RPC call with no provider account (paid, x402)

rpc_call

PAID: one JSON-RPC call on base, base-sepolia, ethereum, solana for $0.0020. You name the chain and the method, NEVER an endpoint URL: we pick the node and absorb its IP rate limit, which is the whole product. No signup, no API key, no provider account, and no key of yours is held here. Chain reads, fee queries and raw-transaction broadcast only. You get the node's own JSON-RPC object back with YOUR id echoed; an upstream error object ("execution reverted") is a paid answer, because it is what you paid to learn. Every refusal (unknown chain, method off the allowlist, batch array, oversized params) is free and before the paywall. A repeated payment buys another call -- there is nothing to extend. Call it once without payment to receive the price as an x402 PaymentRequired (Base USDC, and Solana USDC where offered); call it again with a signed payment in _meta['x402/payment'] to complete it. The receipt comes back in _meta['x402/payment-response']. No account, no API key. Complaints, questions, quote requests, or a service you wish we had: POST /feedback on this host -- free, no account. Every report is read and analysed daily; there is no individual reply.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "chain": "<chain>",
      "method": "<method>"
    },
    "name": "rpc_call"
  }
}

Input Schema

Name	Required	Description
`id`	No	Your own call id, echoed back: a string of at most 64 chars or a safe integer. Omitted means we supply one.
`chain`	Yes	Which chain to ask: one of base, base-sepolia, ethereum, solana. You never name a URL -- we pick the endpoint, e.g. base.
`method`	Yes	The JSON-RPC method, e.g. eth_blockNumber. Reads, fee queries and raw-transaction broadcast only: 19 EVM methods, 14 Solana. Anything else is refused free.
`params`	No	The method's params, as the node takes them; at most 8192 serialized bytes. Omit for a method that takes none.

Schema Changelog

Changes observed during successful MCP inspections.

AddedNov 4, 2026

TDQS

A4.5/5.0

Behavior5/5

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

Goes well beyond the supplied annotations: it discloses the paywall ordering (refusals free and before the paywall), that an upstream error object such as 'execution reverted' is a paid answer, that repeated payments buy new calls rather than extending anything, where the receipt lands (_meta['x402/payment-response']), and that the provider absorbs node IP rate limits. The readOnlyHint=false annotation is consistent with the description's allowance for raw-transaction broadcast.

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

Conciseness3/5

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

Front-loading is good — price, chains and the no-URL constraint come first — but 'no signup, no API key, no provider account' is repeated twice and the closing feedback paragraph is tangential to invoking the tool. Roughly a fifth of the text could be cut without losing call-relevant 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 paid, two-phase protocol tool with no output schema, the description supplies everything an agent needs: the payment negotiation sequence, the _meta keys for payment and receipt, error-vs-refusal semantics, and the absence of any account requirement. Nothing material about calling it correctly is left unstated.

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

Parameters3/5

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

Schema description coverage is 100%, so chain/method/params/id semantics are already documented in structured form, and the description largely restates them ('name the chain and the method'). It adds only marginal context, such as the allowlist size (19 EVM, 14 Solana) and the id echo behavior, so baseline 3 applies.

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?

States a specific verb and resource ('one JSON-RPC call on base, base-sepolia, ethereum, solana for $0.0020') and immediately defines scope: reads, fee queries and raw-transaction broadcast only. It also distinguishes itself from endpoint-provisioning siblings by saying 'You name the chain and the method, NEVER an endpoint URL'.

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

Usage Guidelines5/5

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

Explicitly prescribes the two-step invocation: call once without payment to receive the x402 PaymentRequired, then call again with a signed payment in _meta['x402/payment']. It also names what is refused (unknown chain, off-allowlist method, batch array, oversized params) and that those refusals are free and pre-paywall, plus a fallback route (POST /feedback) for anything out of scope.

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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