Crypto Bot Audit + Market Data (x402 paid)

One transaction by block and index

chain_tx_at_index

Read-onlyIdempotent

eth_getTransactionByBlockNumberAndIndex — pick the Nth transaction of a block without transferring the whole block, which is how you sample a block instead of downloading it. Costs $0.001 USDC per call. Chains: base, ethereum, arbitrum, bsc, polygon, optimism, avalanche. Byte-identical to GET /chain/tx-at-index?chain=base&block=latest&index=0.

Input Schema

TableJSON Schema

Name	Required	Description
`block`	No	block number or latest\|safe\|finalized\|earliest\|pending (default "latest")
`chain`	Yes	chain key: base, ethereum, arbitrum, bsc, polygon, optimism, avalanche [required]
`index`	Yes	transaction index inside the block (decimal) [required]

Output Schema

TableJSON Schema

Name	Required	Description	Default
`ts`	No
`note`	No
`chain`	No
`found`	No
`index`	No
`stale`	No
`caveat`	No
`reason`	No
`source`	No
`chainId`	No
`chainLabel`	No
`transaction`	No

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedSep 25, 2026

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "additionalProperties": false,
+  "properties": {
+    "caveat": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "chain": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "chainId": {
+      "anyOf": [
+        {
+          "type": "integer"
+        },
+        {
+          "type": "null"
+        }
+      ]
+    },
+    "chainLabel": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "found": {
+      "type": "boolean"
+    },
+    "index": {
+      "type": "integer"
+    },
+    "note": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "reason": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "source": {
+      "type": "string"
+    },
+    "stale": {
+      "type": [
+        "boolean",
+        "null"
+      ]
+    },
+    "transaction": {
+      "additionalProperties": {},
+      "type": "object"
+    },
+    "ts": {
+      "type": "string"
+    }
+  },
+  "type": "object"
+}

AddedSep 24, 2026

TDQS

A4.3/5.0

Behavior4/5

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

Annotations already declare the tool read-only and idempotent, and the description adds meaningful behavioral context: the per-call cost, the list of supported chains, and byte-identical equivalence to a REST endpoint. This goes beyond what annotations alone provide without contradicting 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?

Three sentences deliver purpose, use case, cost, supported chains, and REST equivalence with no filler. The core behavioral guidance is front-loaded before secondary details.

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 simple lookup tool with a full input schema, an output schema, and safety annotations, the description covers the essential operational details: what it does, when to use it, what it costs, and which chains it supports. Nothing critical is missing for an agent to select and invoke it correctly.

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 the schema already documents all three parameters. The description only reinforces the parameter shapes through an example URL, adding no substantial semantic detail beyond the schema.

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 clearly states the tool picks the Nth transaction of a block without transferring the whole block, making its purpose specific and distinct from block-level or transaction-by-hash siblings. The RPC method name and explicit block sampling framing reinforce the exact resource and operation.

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 explains that this is the way to sample a block instead of downloading it, which gives clear context for when to use it. It does not explicitly name sibling alternatives or exclusion conditions, but the sampling-vs-downloading contrast is sufficient guidance for the common use case.

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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