ViaFrei

find_cheapest_fuel

Read-onlyIdempotent

Returns the cheapest stations for one fuel grade near a place or coordinate: price per litre, brand, address, distance, open state. Use when the user asks where to fill up or what fuel costs. Do NOT use for charging an electric car (call find_charging_station), price history, or traffic (call check_autobahn_traffic). Radius ≤ 25 km, at most 10 stations. For a brand, a name, open now or at a named time, or nearest-first, call find_fuel_station. Each line names the age of its price and opening-hours claim. Prices are consumer information only; the attribution line and MTS-K note must be shown.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`lat`	No	Latitude in WGS 84, e.g. 48.137. Use with lon when the caller already holds coordinates; otherwise use place.
`lon`	No	Longitude in WGS 84, e.g. 11.576. Use with lat; otherwise use place.
`fuel`	No	Fuel grade: "e5" (Super E5), "e10" (Super E10, the standard German petrol) or "diesel". Default "e10". Pass the grade the person named — a diesel driver is not helped by a petrol price.	e10
`limit`	No	How many stations to return, cheapest first (1–10, default 5). The provider's terms cap it at 10.
`place`	No	Where to look, as free text: a city ("München", "Munich"), a district or Kreis ("Kreis Fulda"), a Bundesland, a station or stop ("Hamburg Hbf"), a motorway junction ("AK Neufahrn"), or a street address with a house number ("Bahnhofstraße 12, 36037 Fulda"). A motorway number alone ("A7") resolves to the road, which has no single point — name a town or junction on it instead. Use this instead of coordinates whenever the person named a place. An address needs its town or postcode — a street and a number alone exist in many towns. Give either place OR lat+lon, never both.
`language`	No	Set this on every call to the language the person is writing in: "en" if they wrote English, "de" if they wrote German. Do not leave it out: it defaults to German ("de"), which is only the fallback when the language is genuinely unclear, so an English question sent without it is answered in German — a wrong answer. Place names, station names and road numbers are never translated in either language; in English the German term is kept in parentheses so the person recognises it on signs and in local apps.	de
`radius_km`	No	Search radius around the place in kilometres (1–25, default 5). The provider's terms cap it at 25 km — a larger circle is a dataset request, not a consumer question.

Schema Changelog

Changes observed during successful MCP inspections.

First observedOct 15, 2026

TDQS

A4.7/5.0

Behavior5/5

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

Annotations already declare readOnly/idempotent/openWorld/non-destructive, and the description adds context beyond them: each returned line discloses the age of its price and opening-hours claim, the provider's terms cap radius and result count, and the attribution line and MTS-K note are mandatory outputs. Nothing here contradicts the safety annotations.

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?

Front-loaded with the core return description, then when-to-use, when-not, parameter routing, and output/legal constraints in that order. Despite covering a 7-parameter tool with no output schema, every clause carries distinct information and there is no filler.

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?

With no output schema, the description compensates by enumerating the returned fields, the price-age and hours-age disclosures, and the required attribution/MTS-K output. Combined with the schema's 100% parameter coverage and the safety annotations, an agent has everything needed to call and interpret this tool.

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% and the schema text already explains lat/lon vs place, the fuel enum intent, language defaulting, and the radius/limit caps. The description mostly restates the radius and limit bounds rather than adding new parameter meaning, so the 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+resource with a clear scope: cheapest stations for one fuel grade near a place/coordinate, plus exactly what each result carries (price per litre, brand, address, distance, open state). It explicitly distinguishes itself from find_fuel_station (brand, name, open-now, nearest-first) and from find_charging_station and check_autobahn_traffic, so an agent can route correctly 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.

Usage Guidelines5/5

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

Gives explicit when-to-use ("where to fill up or what fuel costs"), explicit when-not (electric charging, price history, traffic) with the alternative tool named for each, and a routing rule to find_fuel_station for brand/name/open-now/named-time/nearest-first. Constraint envelopes (radius <= 25 km, at most 10 stations) are stated up front.

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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