1820.ch – Telefonbuch Schweiz

Reverse phone lookup

reverse_phone_lookup

Read-onlyIdempotent

Looks up the public directory entry (or entries) associated with a Swiss phone number. Accepts national ("079 123 45 67"), E.164 ("+41791234567"), or compact notation. Returns an empty result list when the number has no findable public listing.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`lang`	No	Response language: de, fr, it, or en. Defaults to de.
`phone_number`	Yes	The phone number to look up, in any common Swiss notation (national, E.164, or compact).

Output Schema

TableJSON Schema

Name	Required	Description	Default
`source`	Yes
`results`	Yes
`retrieved_at`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

Changed2 schema fields changedSep 27, 2026

addedInput schema / properties / lang

Added value: +{
+  "description": "Response language: de, fr, it, or en. Defaults to de.",
+  "enum": [
+    "de",
+    "fr",
+    "it",
+    "en"
+  ],
+  "type": "string"
+}

addedInput schema / properties / phone_number / maxLength
```
Added value: +40
```

First observedSep 27, 2026

TDQS

A4.1/5.0

Behavior4/5

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

Annotations already cover readOnlyHint and idempotentHint, so safety is off the table. The description adds genuine behavioral value by disclosing the empty-result-list outcome when no public listing exists — an edge case not derivable from annotations and only implied by the output schema.

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?

Two tightly written sentences, no filler, with the core operation and accepted formats front-loaded before the edge-case note.

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 annotations covering the safety profile, a full-coverage input schema, and an output schema, the description only needed to add the non-obvious empty-result behavior — which it does. Nothing an agent needs to invoke it correctly is missing.

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 both parameters are already documented in the schema. The description restates the accepted phone notations (duplicating the phone_number description) and adds nothing about the lang enum. Baseline 3 applies when the schema does the heavy lifting.

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 (looks up) and resource (public directory entry) scoped to Swiss phone numbers, with the accepted input notations named. An agent can distinguish this from get_directory_entry or search_swiss_directory without opening the schemas.

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?

Usage is implied by the name and by the 'reverse lookup' framing, but the description never states when to prefer this over search_swiss_directory or get_directory_entry, nor any prerequisites. Adequate but no explicit routing.

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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