Fizzl Buy

Which paid API to buy (free)

x402_pick

Read-only

Free: say what your agent needs and get the paid x402 API that x402 Doctor has seen paying out day after day at a fair price (up to $0.01), with its track record, price and example input (example_in: "query" for URL parameters, "body" for a JSON body), plus alternatives. To buy it in one call, POST https://buy.fizzl.eu/v1/buy {"need": …} ($0.02 over x402): the answer comes back checked, and nothing is charged when no service delivers. Limited to 10 free calls an hour.

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": {
      "need": "<need>"
    },
    "name": "x402_pick"
  }
}

Input Schema

Name	Required	Description	Default
`need`	Yes	What your agent needs, in a sentence, with the details (place, symbol, query)

Output Schema

Name	Required	Description
`buy`	No	How to buy it in one call
`need`	No
`note`	No
`pick`	No	The paid API Fizzl would buy, or null when none fits
`alternatives`	No	The next ones it tries when the pick does not deliver

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedNov 8, 2026

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "additionalProperties": {},
+  "properties": {
+    "alternatives": {
+      "description": "The next ones it tries when the pick does not deliver",
+      "items": {
+        "additionalProperties": {},
+        "properties": {
+          "description": {
+            "type": [
+              "string",
+              "null"
+            ]
+          },
+          "example": {},
+          "example_in": {
+            "description": "\"query\" for URL parameters, \"body\" for a JSON body",
+            "type": [
+              "string",
+              "null"
+            ]
+          },
+          "method": {
+            "type": [
+              "string",
+              "null"
+            ]
+          },
+          "price_usd": {
+            "type": [
+              "number",
+              "null"
+            ]
+          },
+          "record": {
+            "anyOf": [
+              {
+                "additionalProperties": {},
+                "propertyNames": {
+                  "type": "string"
+                },
+                "type": "object"
+              },
+              {
+                "type": "null"
+              }
+            ]
+          },
+          "track_record": {
+            "anyOf": [
+              {
+                "additionalProperties": {},
+                "propertyNames": {
+                  "type": "string"
+                },
+                "type": "object"
+              },
+              {
+                "type": "null"
+              }
+            ]
+          },
+          "url": {
+            "type": "string"
+          }
+        },
+        "type": "object"
+      },
+      "type": "array"
+    },
+    "buy": {
+      "anyOf": [
+        {
+          "additionalProperties": {},
+          "properties": {
+            "method": {
+              "type": "string"
+            },
+            "price_usd": {
+              "type": [
+                "number",
+                "null"
+              ]
+            },
+            "url": {
+              "type": "string"
+            }
+          },
+          "type": "object"
+        },
+        {
+          "type": "null"
+        }
+      ],
+      "description": "How to buy it in one call"
+    },
+    "need": {
+      "type": "string"
+    },
+    "note": {
+      "type": "string"
+    },
+    "pick": {
+      "anyOf": [
+        {
+          "additionalProperties": {},
+          "properties": {
+            "description": {
+              "type": [
+                "string",
+                "null"
+              ]
+            },
+            "example": {},
+            "example_in": {
+              "description": "\"query\" for URL parameters, \"body\" for a JSON body",
+              "type": [
+                "string",
+                "null"
+              ]
+            },
+            "method": {
+              "type": [
+                "string",
+                "null"
+              ]
+            },
+            "price_usd": {
+              "type": [
+                "number",
+                "null"
+              ]
+            },
+            "record": {
+              "anyOf": [
+                {
+                  "additionalProperties": {},
+                  "propertyNames": {
+                    "type": "string"
+                  },
+                  "type": "object"
+                },
+                {
+                  "type": "null"
+                }
+              ]
+            },
+            "track_record": {
+              "anyOf": [
+                {
+                  "additionalProperties": {},
+                  "propertyNames": {
+                    "type": "string"
+                  },
+                  "type": "object"
+                },
+                {
+                  "type": "null"
+                }
+              ]
+            },
+            "url": {
+              "type": "string"
+            }
+          },
+          "type": "object"
+        },
+        {
+          "type": "null"
+        }
+      ],
+      "description": "The paid API Fizzl would buy, or null when none fits"
+    }
+  },
+  "type": "object"
+}

Changed1 schema field changedOct 29, 2026
- removedInput schema / additionalProperties
```
Removed value: -false
```
First observedOct 23, 2026

TDQS

A3.6/5.0

Behavior4/5

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

Annotations only declare readOnlyHint and openWorldHint, so the description adds real value: the call is free, capped at 10 calls/hour, results are limited to services priced up to $0.01 that have a payout track record, alternatives are included, and the buy path charges nothing when no service delivers. The remaining weakness is ambiguity about who performs the embedded purchase POST, which sits oddly next to a read-only tool but is framed as a separate call rather than a behavior of this tool.

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?

The core value proposition is front-loaded, which is good, but the first sentence is a long run-on stacking pricing, payout history, track record and example_in semantics, and the second sentence embeds a full URL plus JSON payload for an out-of-band purchase. That purchase detail is peripheral to invoking this tool and dilutes the definition.

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 one-parameter discovery tool with an output schema and readOnly/openWorld annotations, the description supplies the missing operational context: cost, rate limit, price ceiling, payout vetting, alternatives, and failure charging behavior. The only gap is that it never resolves the relationship between this tool and the x402_buy sibling, leaving the agent to reconcile two buy paths itself.

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 coverage is 100% and the single 'need' parameter already carries its own description, length bounds and guidance to include place/symbol/query. The description adds only a restatement ('say what your agent needs') and instead spends its detail on output-side 'example_in' semantics, so it does not meaningfully deepen parameter understanding beyond the schema. Baseline 3 applies.

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?

The description states a recognizable discovery purpose: given a described need, it returns the paid x402 API plus alternatives, with track record, price and example input. The verb phrasing ('say what your agent needs and get...') is loose, and 'get' could imply purchase rather than recommendation, but the 'plus alternatives' clause and the separate buy path make the read-only recommendation role inferable.

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

Usage Guidelines3/5

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

It sets clear conditions around free usage (10 calls/hour) and describes an alternative paid path ('To buy it in one call, POST https://buy.fizzl.eu/v1/buy ... $0.02'), which is genuine when-to-use guidance. However, it points at a raw HTTP endpoint instead of naming the available sibling tool x402_buy, so an agent must infer that the sibling is the correct way to act on a recommendation rather than following the URL literally.

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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