Autoza UK

Calculate UK car tax (VED)

calculate_car_tax

Read-only

Calculate annual UK vehicle tax (VED / road tax) for a car. Handles all three UK systems: cars first registered before 1 March 2001 (taxed on engine size, so engine_size_cc is required), 1 March 2001 to 31 March 2017 (CO2 bands £20–£790, so co2 is required), and from 1 April 2017 (flat standard rate). Also works out the Expensive Car Supplement: over £40,000 list price when new for petrol, diesel and alternative fuel; over £50,000 for a zero-emission car first registered on or after 1 April 2025; and none for a zero-emission car first registered before 1 April 2025, whatever it listed for. The boundaries are dates, so registration_date gives an exact answer where the year alone is ambiguous. If an input the car's tax system needs is missing, the tool returns an error naming it rather than a figure. Use for questions about UK car tax, road tax or VED.

Input Schema

TableJSON Schema

Name	Required	Description
`co2`	No	CO2 emissions in g/km, from the V5C. Required for cars first registered 1 March 2001 – 31 March 2017. For cars registered from 1 April 2017 it only sets the one-off first-year rate. Ignored for pre-2001 cars.
`fuel`	Yes	Fuel type. Use "alternative" for hybrid/LPG, "diesel-rde2" only for diesels meeting RDE2.
`engine_size_cc`	No	Engine size in cc. Required for cars first registered before 1 March 2001; not used otherwise.
`registration_date`	No	Full date of FIRST registration as YYYY-MM-DD, from the V5C. Optional but preferred: every UK VED boundary is a date (1 March 2001, 1 April 2017, 1 April 2025), so the year alone cannot tell a zero-emission car registered 1 March 2025 (no Expensive Car Supplement) from one registered 1 April 2025 (supplement). Without it, an ambiguous case is answered with a caveat in notes rather than a figure.
`registration_year`	Yes	Year the car was FIRST registered (not the year it was bought). This decides which tax system applies.
`list_price_when_new`	No	Original list price in GBP when the car was new. Needed only to work out the Expensive Car Supplement — based on the price when NEW, not what you pay for it used.

Output Schema

TableJSON Schema

Name	Required	Description
`_note`	Yes	Where the figures come from: Autoza, prices in GBP, distances in miles.
`basis`	Yes
`notes`	Yes
`system`	Yes	Which of the three UK systems applies.
`co2_band`	No
`tax_year`	Yes	The tax year the rates apply to, for example 2026/27.
`annual_tax_gbp`	Yes	Annual vehicle tax in GBP. Null only when the answer depends on a date the caller did not give; annual_tax_outcomes then lists each case.
`official_source`	Yes	The GOV.UK page the rates come from.
`rates_verified_on`	Yes	When the rates were last checked against GOV.UK.
`annual_tax_outcomes`	No	One entry per possible answer. Present only when annual_tax_gbp is null.
`first_year_rate_gbp`	No
`annual_tax_depends_on`	No	What is needed to give one figure. Present only when annual_tax_gbp is null.
`historic_class_eligible`	No
`expensive_car_supplement_gbp`	Yes	Zero when no Expensive Car Supplement applies.

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedOct 6, 2026

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "properties": {
+    "_note": {
+      "description": "Where the figures come from: Autoza, prices in GBP, distances in miles.",
+      "type": "string"
+    },
+    "annual_tax_depends_on": {
+      "description": "What is needed to give one figure. Present only when annual_tax_gbp is null.",
+      "type": "string"
+    },
+    "annual_tax_gbp": {
+      "description": "Annual vehicle tax in GBP. Null only when the answer depends on a date the caller did not give; annual_tax_outcomes then lists each case.",
+      "type": [
+        "number",
+        "null"
+      ]
+    },
+    "annual_tax_outcomes": {
+      "description": "One entry per possible answer. Present only when annual_tax_gbp is null.",
+      "items": {
+        "properties": {
+          "annual_tax_gbp": {
+            "type": "number"
+          },
+          "if": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "if",
+          "annual_tax_gbp"
+        ],
+        "type": "object"
+      },
+      "type": "array"
+    },
+    "basis": {
+      "type": "string"
+    },
+    "co2_band": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "expensive_car_supplement_gbp": {
+      "description": "Zero when no Expensive Car Supplement applies.",
+      "type": "number"
+    },
+    "first_year_rate_gbp": {
+      "type": [
+        "number",
+        "null"
+      ]
+    },
+    "historic_class_eligible": {
+      "type": "boolean"
+    },
+    "notes": {
+      "items": {
+        "type": "string"
+      },
+      "type": "array"
+    },
+    "official_source": {
+      "description": "The GOV.UK page the rates come from.",
+      "type": "string"
+    },
+    "rates_verified_on": {
+      "description": "When the rates were last checked against GOV.UK.",
+      "type": "string"
+    },
+    "system": {
+      "description": "Which of the three UK systems applies.",
+      "type": "string"
+    },
+    "tax_year": {
+      "description": "The tax year the rates apply to, for example 2026/27.",
+      "type": "string"
+    }
+  },
+  "required": [
+    "annual_tax_gbp",
+    "tax_year",
+    "system",
+    "basis",
+    "expensive_car_supplement_gbp",
+    "notes",
+    "rates_verified_on",
+    "official_source",
+    "_note"
+  ],
+  "type": "object"
+}

Changed4 schema fields changedSep 23, 2026

changedInput schema / properties / co2 / description

Previous value: -"CO2 emissions in g/km. Required for cars registered March 2001–March 2017; ignored for pre-2001 cars."New value: +"CO2 emissions in g/km, from the V5C. Required for cars first registered 1 March 2001 – 31 March 2017. For cars registered from 1 April 2017 it only sets the one-off first-year rate. Ignored for pre-2001 cars."

changedInput schema / properties / engine_size_cc / description

Previous value: -"Engine size in cc. Only used for cars registered before March 2001."New value: +"Engine size in cc. Required for cars first registered before 1 March 2001; not used otherwise."

changedInput schema / properties / registration_date / description

Previous value: -"Full date of FIRST registration as YYYY-MM-DD, from the V5C. Optional but strongly preferred: every UK VED boundary is a date (1 March 2001, 1 April 2017, 1 April 2025), so the year alone cannot tell a zero-emission car registered 1 March 2025 (no Expensive Car Supplement) from one registered 1 April 2025 (supplement). Without it, an ambiguous case is answered with a caveat in notes rather than a figure."New value: +"Full date of FIRST registration as YYYY-MM-DD, from the V5C. Optional but preferred: every UK VED boundary is a date (1 March 2001, 1 April 2017, 1 April 2025), so the year alone cannot tell a zero-emission car registered 1 March 2025 (no Expensive Car Supplement) from one registered 1 April 2025 (supplement). Without it, an ambiguous case is answered with a caveat in notes rather than a figure."

changedInput schema / properties / registration_year / description

Previous value: -"Year the car was FIRST registered (not the year it was bought). This decides which tax system applies and is the single most important input."New value: +"Year the car was FIRST registered (not the year it was bought). This decides which tax system applies."

Changed1 schema field changedSep 10, 2026

addedInput schema / properties / registration_date

Added value: +{
+  "description": "Full date of FIRST registration as YYYY-MM-DD, from the V5C. Optional but strongly preferred: every UK VED boundary is a date (1 March 2001, 1 April 2017, 1 April 2025), so the year alone cannot tell a zero-emission car registered 1 March 2025 (no Expensive Car Supplement) from one registered 1 April 2025 (supplement). Without it, an ambiguous case is answered with a caveat in notes rather than a figure.",
+  "type": "string"
+}

First observedSep 1, 2026

TDQS

A4.6/5.0

Behavior5/5

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

Annotations confirm a safe read-only, closed-world operation, and the description adds substantial behaviour beyond that: it returns a named error rather than a figure when a required input is missing, and it caveats ambiguous year-only cases via notes instead of guessing. The Expensive Car Supplement thresholds and the fuel/registration-date interactions are genuine behavioural disclosure.

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?

Front-loaded with the core action and the three regimes, then the supplement exceptions and the error/ambiguous-case behaviour. Dense and purposeful, though the multi-clause supplement sentence is heavy enough to slow scanning slightly.

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?

An output schema exists, so return values need no explanation, and the description covers the remaining burden: regime selection, conditional inputs, error behaviour and ambiguity handling. Nothing an agent needs to invoke this correctly is missing.

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?

Coverage is 100% so the schema already documents each field, but the description adds cross-parameter semantics the schema states only per-field: which inputs each tax regime requires, that registration_date resolves date boundaries the year alone cannot, and that list_price matters only for the supplement. This conditional-requirement framing is real added value over the flat field list.

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 and resource ('Calculate annual UK vehicle tax (VED / road tax) for a car') and immediately scopes it to the three UK registration regimes. It is trivially distinguishable from siblings like check_mot_due_date or check_clean_air_zone.

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?

Explicitly says 'Use for questions about UK car tax, road tax or VED' and lays out which regime applies to which registration period, which effectively tells the agent when a given input set is appropriate. It does not name a sibling alternative, but the domain is narrow enough that this costs little.

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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