AgentIndex — measured reliability for agent services

search_agents

Search the AgentIndex for AI agents, MCP servers, A2A agents, and x402-gated APIs by capability keyword. Returns ranked results with type, description, URL, and tags. FREE for the first 50 calls a day per caller, then $0.005 in USDC on base via x402. Over the allowance, call without payment to receive the exact payment requirements, then call again with payment to be served.

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": {
      "capability": "<capability>"
    },
    "name": "search_agents"
  }
}

Input Schema

Name	Required	Description
`type`	No	Filter by resource type: application/mcp-server-card+json, application/ai-skill, application/openapi+json, etc.
`limit`	No	Max results (default 10, max 50)
`payment`	No	Optional. An x402 X-PAYMENT header value, needed only past the free daily allowance.
`capability`	Yes	Capability or keyword to search (e.g. trading, data retrieval, mcp)

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedOct 1, 2026

addedInput schema / properties / payment

Added value: +{
+  "description": "Optional. An x402 X-PAYMENT header value, needed only past the free daily allowance.",
+  "type": "string"
+}

First observedJul 28, 2026

TDQS

A4.2/5.0

Behavior5/5

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

With no annotations, the description carries the full burden and does so well: it discloses the free quota (50 calls/day per caller), the exact price and rail past it ($0.005 USDC on base via x402), and the recovery handshake for the over-allowance case. Cost, rate limit, and auth/payment requirements are all surfaced before the agent calls.

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

Conciseness4/5

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

Three sentences, front-loaded with purpose then return shape then pricing/payment flow; no filler. The final sentence is dense with a nested call-again instruction, which costs a little readability but earns its place given the x402 complexity.

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

Completeness4/5

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

For a search tool with no output schema and no annotations, the description covers return contents (type, description, URL, tags), cost model, and payment mechanics. It omits pagination behavior and what the `type` filter values map to in practice, minor gaps given the schema documents parameters.

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?

Schema coverage is 100%, so the baseline is 3, but the description goes beyond it by explaining the operational semantics of the `payment` parameter – that omitting it yields payment requirements and re-calling with it completes the transaction. The `capability` and `type`/`limit` params are left to the schema, which documents them adequately.

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

Purpose4/5

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

States a specific verb and resource ('Search the AgentIndex for AI agents, MCP servers, A2A agents, and x402-gated APIs by capability keyword'), which is concrete and unambiguous. However, it never differentiates itself from siblings like find_tool or cheapest_that_works, which an agent scanning the toolset must distinguish on its own.

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?

Gives explicit conditional guidance for the paid path: 'Over the allowance, call without payment to receive the exact payment requirements, then call again with payment to be served' – a genuinely actionable two-step flow. It stops short of naming when to prefer this over find_tool or the other search-adjacent siblings, so no true alternatives/exclusions are given.

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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