Trusteed Agentic Commerce

ucp_cancel_checkout

DestructiveIdempotent

Cancel a UCP checkout session. Transitions to 'canceled' status. Cannot cancel already completed checkouts. Use ONLY for a checkout session created with ucp_create_checkout (Universal Commerce Protocol); for the standard flow (create_cart, preview_checkout, complete_checkout) do not call this tool. Checkout guide: read the MCP resource trusteed://guides/checkout.

Input Schema

Name	Required	Description
`reason`	No	Optional cancellation reason
`agent_key`	No	Sandbox credential from `create_sandbox_key`. Use this when your client cannot set an `Authorization: Bearer` header.
`checkout_id`	Yes	UCP checkout session ID (valid UUID)
`idempotency_key`	No	REQUIRED for this write unless sent as the `Idempotency-Key` header or `params._meta["trusteed/idempotency-key"]`. 1-128 chars from [A-Za-z0-9._:/-]. Reuse the same value when retrying the same operation.

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedOct 17, 2026

changedOutput schema / (root)

Previous value: -{
-  "$schema": "http://json-schema.org/draft-07/schema#",
-  "additionalProperties": true,
-  "properties": {
-    "approval_url": {
-      "type": "string"
-    },
-    "created_at": {
-      "type": "string"
-    },
-    "currency": {
-      "type": "string"
-    },
-    "expires_at": {
-      "description": "ISO-8601 UTC instant at which this checkout SESSION stops being usable. It is not a stock reservation and not a price guarantee.",
-      "type": "string"
-    },
-    "expires_in_seconds": {
-      "description": "Whole seconds left on this checkout SESSION at response time (0 when already past). It is not a stock reservation and not a price guarantee.",
-      "minimum": 0,
-      "type": "integer"
-    },
-    "extensions": {
-      "additionalProperties": {},
-      "description": "`warnings[]` with `code`, `message`, `severity`.",
-      "type": "object"
-    },
-    "fulfillment": {
-      "additionalProperties": {},
-      "type": "object"
-    },
-    "id": {
-      "type": "string"
-    },
-    "idempotency_key": {
-      "type": "string"
-    },
-    "intent": {
-      "additionalProperties": {},
-      "type": "object"
-    },
-    "label": {
-      "type": "string"
-    },
-    "line_items": {
-      "items": {
-        "additionalProperties": true,
-        "properties": {
-          "id": {
-            "type": "string"
-          },
-          "price": {
-            "additionalProperties": true,
-            "properties": {
-              "amount": {
-                "type": "number"
-              },
-              "currency": {
-                "type": "string"
-              }
-            },
-            "required": [
-              "amount",
-              "currency"
-            ],
-            "type": "object"
-          },
-          "product_id": {
-            "type": "string"
-          },
-          "quantity": {
-            "type": "number"
-          },
-          "title": {
-            "type": "string"
-          },
-          "variant_id": {
-            "type": "string"
-          },
-          "variant_title": {
-            "type": "string"
-          }
-        },
-        "required": [
-          "id",
-          "product_id",
-          "title",
-          "quantity",
-          "price"
-        ],
-        "type": "object"
-      },
-      "type": "array"
-    },
-    "next_action": {
-      "additionalProperties": {},
-      "description": "Only when a person must approve or pay: `url`, `kind`, `retry`.",
-      "type": "object"
-    },
-    "next_step": {
-      "description": "Next-action code, not a tool name: provide_shipping_destination, refresh_shipping_rates, choose_shipping_option, preview_ucp_checkout, review_not_available_in_ucp.",
-      "type": "string"
-    },
-    "next_step_message": {
-      "type": "string"
-    },
-    "payment_options": {
-      "description": "When present, ask the person which option they want; never choose for them.",
-      "items": {
-        "additionalProperties": {},
-        "type": "object"
-      },
-      "type": "array"
-    },
-    "status": {
-      "enum": [
-        "incomplete",
-        "requires_escalation",
-        "ready_for_complete",
-        "complete_in_progress",
-        "completed",
-        "canceled"
-      ],
-      "type": "string"
-    },
-    "totals": {
-      "items": {
-        "additionalProperties": true,
-        "properties": {
-          "amount": {
-            "type": "number"
-          },
-          "currency": {
-            "type": "string"
-          },
-          "label": {
-            "type": "string"
-          }
-        },
-        "required": [
-          "label",
-          "amount",
-          "currency"
-        ],
-        "type": "object"
-      },
-      "type": "array"
-    },
-    "updated_at": {
-      "type": "string"
-    }
-  },
-  "required": [
-    "id",
-    "status",
-    "currency",
-    "line_items",
-    "totals",
-    "created_at",
-    "updated_at"
-  ],
-  "type": "object"
-}New value: +null

Changed1 schema field changedOct 16, 2026

changedOutput schema / properties / next_step / description

Previous value: -"Next-action code, not a tool name: provide_shipping_destination, refresh_shipping_rates, choose_shipping_option, review_not_available_in_ucp."New value: +"Next-action code, not a tool name: provide_shipping_destination, refresh_shipping_rates, choose_shipping_option, preview_ucp_checkout, review_not_available_in_ucp."

Changed1 schema field changedOct 16, 2026

changedOutput schema / properties / next_step / description

Previous value: -"Next-action code, not a tool name: provide_shipping_destination, refresh_shipping_rates, choose_shipping_option, ask_user_payment_method."New value: +"Next-action code, not a tool name: provide_shipping_destination, refresh_shipping_rates, choose_shipping_option, review_not_available_in_ucp."

Changed5 schema fields changedOct 15, 2026

changedInput schema / properties / idempotency_key / description

Previous value: -"REQUIRED for this write unless sent as the `Idempotency-Key` header or `_meta`. Reuse the same value when retrying the same operation."New value: +"REQUIRED for this write unless sent as the `Idempotency-Key` header or `params._meta[\"trusteed/idempotency-key\"]`. 1-128 chars from [A-Za-z0-9._:/-]. Reuse the same value when retrying the same operation."

changedInput schema / properties / idempotency_key / maxLength
```
Previous value: -255New value: +128
```
removedInput schema / properties / idempotency_key / minLength
```
Removed value: -1
```
addedInput schema / properties / idempotency_key / pattern
```
Added value: +"^[A-Za-z0-9._:/-]{1,128}$"
```

addedOutput schema / properties / next_step / description

Added value: +"Next-action code, not a tool name: provide_shipping_destination, refresh_shipping_rates, choose_shipping_option, ask_user_payment_method."

Changed3 schema fields changedOct 15, 2026

changedInput schema / properties / idempotency_key / description

Previous value: -"Optional idempotency key to prevent duplicate cancellations."New value: +"REQUIRED for this write unless sent as the `Idempotency-Key` header or `_meta`. Reuse the same value when retrying the same operation."

addedInput schema / properties / idempotency_key / maxLength
```
Added value: +255
```
addedInput schema / properties / idempotency_key / minLength
```
Added value: +1
```

Changed17 schema fields changedOct 14, 2026

changedOutput schema / properties / extensions / additionalProperties
```
Previous value: -trueNew value: +{}
```

addedOutput schema / properties / extensions / description

Added value: +"`warnings[]` with `code`, `message`, `severity`."

removedOutput schema / properties / extensions / properties

Removed value: -{
-  "warnings": {
-    "items": {
-      "additionalProperties": true,
-      "properties": {
-        "code": {
-          "type": "string"
-        },
-        "disclosure_url": {
-          "type": "string"
-        },
-        "message": {
-          "type": "string"
-        },
-        "severity": {
-          "enum": [
-            "info",
-            "caution",
-            "critical"
-          ],
-          "type": "string"
-        }
-      },
-      "required": [
-        "code",
-        "message",
-        "severity"
-      ],
-      "type": "object"
-    },
-    "type": "array"
-  }
-}

changedOutput schema / properties / fulfillment / additionalProperties
```
Previous value: -trueNew value: +{}
```

removedOutput schema / properties / fulfillment / properties

Removed value: -{
-  "destination": {
-    "additionalProperties": true,
-    "properties": {
-      "city": {
-        "type": "string"
-      },
-      "country_code": {
-        "type": "string"
-      },
-      "province_code": {
-        "type": "string"
-      },
-      "zip": {
-        "type": "string"
-      }
-    },
-    "required": [
-      "city",
-      "country_code",
-      "zip"
-    ],
-    "type": "object"
-  },
-  "options": {
-    "items": {
-      "additionalProperties": true,
-      "properties": {
-        "authoritative": {
-          "type": "boolean"
-        },
-        "estimated_delivery": {
-          "type": "string"
-        },
-        "id": {
-          "type": "string"
-        },
-        "price": {
-          "additionalProperties": true,
-          "properties": {
-            "amount": {
-              "type": "number"
-            },
-            "currency": {
-              "type": "string"
-            }
-          },
-          "required": [
-            "amount",
-            "currency"
-          ],
-          "type": "object"
-        },
-        "source": {
-          "enum": [
-            "platform",
-            "merchant_policy",
-            "estimated"
-          ],
-          "type": "string"
-        },
-        "title": {
-          "type": "string"
-        }
-      },
-      "required": [
-        "id",
-        "title",
-        "price",
-        "source",
-        "authoritative"
-      ],
-      "type": "object"
-    },
-    "type": "array"
-  },
-  "selected_option_id": {
-    "type": "string"
-  }
-}

changedOutput schema / properties / intent / additionalProperties
```
Previous value: -trueNew value: +{}
```

removedOutput schema / properties / intent / properties

Removed value: -{
-  "action": {
-    "type": "string"
-  },
-  "context": {
-    "type": "string"
-  },
-  "locale": {
-    "type": "string"
-  }
-}

removedOutput schema / properties / intent / required
```
Removed value: -[
-  "action"
-]
```
changedOutput schema / properties / next_action / additionalProperties
```
Previous value: -trueNew value: +{}
```

addedOutput schema / properties / next_action / description

Added value: +"Only when a person must approve or pay: `url`, `kind`, `retry`."

removedOutput schema / properties / next_action / properties

Removed value: -{
-  "actor": {
-    "const": "buyer",
-    "type": "string"
-  },
-  "do": {
-    "enum": [
-      "approve",
-      "pay"
-    ],
-    "type": "string"
-  },
-  "instruction": {
-    "type": "string"
-  },
-  "reuse_arguments": {
-    "items": {
-      "enum": [
-        "checkout_session_id",
-        "checkout_id",
-        "idempotency_key"
-      ],
-      "type": "string"
-    },
-    "type": "array"
-  },
-  "then_call": {
-    "maxLength": 64,
-    "minLength": 1,
-    "type": "string"
-  },
-  "url": {
-    "type": "string"
-  }
-}

removedOutput schema / properties / next_action / required

Removed value: -[
-  "actor",
-  "do",
-  "url",
-  "instruction"
-]

removedOutput schema / properties / next_step / const
```
Removed value: -"ask_user_payment_method"
```

addedOutput schema / properties / payment_options / description

Added value: +"When present, ask the person which option they want; never choose for them."

changedOutput schema / properties / payment_options / items / additionalProperties
```
Previous value: -trueNew value: +{}
```

removedOutput schema / properties / payment_options / items / properties

Removed value: -{
-  "available": {
-    "type": "boolean"
-  },
-  "charges_without_approval": {
-    "const": true,
-    "type": "boolean"
-  },
-  "funding_source": {
-    "anyOf": [
-      {
-        "enum": [
-          "platform_test_card",
-          "platform_test_wallet",
-          "buyer_wallet"
-        ],
-        "type": "string"
-      },
-      {
-        "type": "null"
-      }
-    ]
-  },
-  "id": {
-    "enum": [
-      "demo_card",
-      "demo_wallet",
-      "own_wallet",
-      "merchant_card",
-      "paypal",
-      "kyapay"
-    ],
-    "type": "string"
-  },
-  "label": {
-    "type": "string"
-  },
-  "rail": {
-    "enum": [
-      "stripe",
-      "x402",
-      "kyapay",
-      "paypal"
-    ],
-    "type": "string"
-  },
-  "reason_unavailable": {
-    "enum": [
-      "coming_soon",
-      "not_enabled",
-      "currency_not_supported",
-      "unsupported_on_this_tool"
-    ],
-    "type": "string"
-  }
-}

removedOutput schema / properties / payment_options / items / required

Removed value: -[
-  "id",
-  "label",
-  "rail",
-  "funding_source",
-  "available"
-]

Changed1 schema field changedOct 13, 2026
- addedOutput schema / properties / payment_options / items / properties / charges_without_approval
```
Added value: +{
+  "const": true,
+  "type": "boolean"
+}
```

Changed3 schema fields changedOct 13, 2026

changedOutput schema / properties / payment_options / items / properties / id / enum

Previous value: -[
-  "demo_card",
-  "demo_wallet",
-  "own_wallet",
-  "merchant_card",
-  "kyapay"
-]New value: +[
+  "demo_card",
+  "demo_wallet",
+  "own_wallet",
+  "merchant_card",
+  "paypal",
+  "kyapay"
+]

changedOutput schema / properties / payment_options / items / properties / rail / enum

Previous value: -[
-  "stripe",
-  "x402",
-  "kyapay"
-]New value: +[
+  "stripe",
+  "x402",
+  "kyapay",
+  "paypal"
+]

changedOutput schema / properties / payment_options / items / properties / reason_unavailable / enum

Previous value: -[
-  "coming_soon",
-  "not_enabled",
-  "currency_not_supported"
-]New value: +[
+  "coming_soon",
+  "not_enabled",
+  "currency_not_supported",
+  "unsupported_on_this_tool"
+]

Changed2 schema fields changedOct 10, 2026

addedOutput schema / properties / expires_at

Added value: +{
+  "description": "ISO-8601 UTC instant at which this checkout SESSION stops being usable. It is not a stock reservation and not a price guarantee.",
+  "type": "string"
+}

addedOutput schema / properties / expires_in_seconds

Added value: +{
+  "description": "Whole seconds left on this checkout SESSION at response time (0 when already past). It is not a stock reservation and not a price guarantee.",
+  "minimum": 0,
+  "type": "integer"
+}

Changed1 schema field changedOct 10, 2026

changedOutput schema / properties / payment_options / items / properties / reason_unavailable / enum

Previous value: -[
-  "coming_soon",
-  "not_enabled"
-]New value: +[
+  "coming_soon",
+  "not_enabled",
+  "currency_not_supported"
+]

Changed1 schema field changedOct 5, 2026

changedOutput schema / properties / payment_options / items / properties / id / enum

Previous value: -[
-  "demo_card",
-  "demo_wallet",
-  "own_wallet",
-  "kyapay"
-]New value: +[
+  "demo_card",
+  "demo_wallet",
+  "own_wallet",
+  "merchant_card",
+  "kyapay"
+]

Changed3 schema fields changedOct 1, 2026

addedOutput schema / properties / fulfillment

Added value: +{
+  "additionalProperties": true,
+  "properties": {
+    "destination": {
+      "additionalProperties": true,
+      "properties": {
+        "city": {
+          "type": "string"
+        },
+        "country_code": {
+          "type": "string"
+        },
+        "province_code": {
+          "type": "string"
+        },
+        "zip": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "city",
+        "country_code",
+        "zip"
+      ],
+      "type": "object"
+    },
+    "options": {
+      "items": {
+        "additionalProperties": true,
+        "properties": {
+          "authoritative": {
+            "type": "boolean"
+          },
+          "estimated_delivery": {
+            "type": "string"
+          },
+          "id": {
+            "type": "string"
+          },
+          "price": {
+            "additionalProperties": true,
+            "properties": {
+              "amount": {
+                "type": "number"
+              },
+              "currency": {
+                "type": "string"
+              }
+            },
+            "required": [
+              "amount",
+              "currency"
+            ],
+            "type": "object"
+          },
+          "source": {
+            "enum": [
+              "platform",
+              "merchant_policy",
+              "estimated"
+            ],
+            "type": "string"
+          },
+          "title": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "id",
+          "title",
+          "price",
+          "source",
+          "authoritative"
+        ],
+        "type": "object"
+      },
+      "type": "array"
+    },
+    "selected_option_id": {
+      "type": "string"
+    }
+  },
+  "type": "object"
+}

addedOutput schema / properties / line_items / items / properties / variant_id
```
Added value: +{
+  "type": "string"
+}
```
addedOutput schema / properties / line_items / items / properties / variant_title
```
Added value: +{
+  "type": "string"
+}
```

Changed3 schema fields changedSep 28, 2026

addedOutput schema / properties / next_step

Added value: +{
+  "const": "ask_user_payment_method",
+  "type": "string"
+}

addedOutput schema / properties / next_step_message
```
Added value: +{
+  "type": "string"
+}
```

addedOutput schema / properties / payment_options

Added value: +{
+  "items": {
+    "additionalProperties": true,
+    "properties": {
+      "available": {
+        "type": "boolean"
+      },
+      "funding_source": {
+        "anyOf": [
+          {
+            "enum": [
+              "platform_test_card",
+              "platform_test_wallet",
+              "buyer_wallet"
+            ],
+            "type": "string"
+          },
+          {
+            "type": "null"
+          }
+        ]
+      },
+      "id": {
+        "enum": [
+          "demo_card",
+          "demo_wallet",
+          "own_wallet",
+          "kyapay"
+        ],
+        "type": "string"
+      },
+      "label": {
+        "type": "string"
+      },
+      "rail": {
+        "enum": [
+          "stripe",
+          "x402",
+          "kyapay"
+        ],
+        "type": "string"
+      },
+      "reason_unavailable": {
+        "enum": [
+          "coming_soon",
+          "not_enabled"
+        ],
+        "type": "string"
+      }
+    },
+    "required": [
+      "id",
+      "label",
+      "rail",
+      "funding_source",
+      "available"
+    ],
+    "type": "object"
+  },
+  "type": "array"
+}

Changed2 schema fields changedSep 27, 2026

addedOutput schema / properties / approval_url
```
Added value: +{
+  "type": "string"
+}
```

addedOutput schema / properties / next_action

Added value: +{
+  "additionalProperties": true,
+  "properties": {
+    "actor": {
+      "const": "buyer",
+      "type": "string"
+    },
+    "do": {
+      "enum": [
+        "approve",
+        "pay"
+      ],
+      "type": "string"
+    },
+    "instruction": {
+      "type": "string"
+    },
+    "reuse_arguments": {
+      "items": {
+        "enum": [
+          "checkout_session_id",
+          "checkout_id",
+          "idempotency_key"
+        ],
+        "type": "string"
+      },
+      "type": "array"
+    },
+    "then_call": {
+      "maxLength": 64,
+      "minLength": 1,
+      "type": "string"
+    },
+    "url": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "actor",
+    "do",
+    "url",
+    "instruction"
+  ],
+  "type": "object"
+}

Changed1 schema field changedSep 22, 2026

removedInput schema / properties / payment_mandate

Removed value: -{
-  "anyOf": [
-    {
-      "additionalProperties": {},
-      "type": "object"
-    },
-    {
-      "type": "string"
-    }
-  ],
-  "description": "Payment mandate with `mandate_id`, `max_amount_cents` (integer, minor units), `currency` (3 letters, must match the cart's), `exp` (expiry as an INTEGER of Unix seconds — not an ISO 8601 date string), `sub` (your agent id) and `aud` (the store slug this mandate is for, e.g. the slug in the URL you are calling — not a domain). Use this when your client cannot set an `X-Payment-Mandate` header."
-}

Changed1 schema field changedSep 22, 2026

addedInput schema / properties / agent_key

Added value: +{
+  "description": "Sandbox credential from `create_sandbox_key`. Use this when your client cannot set an `Authorization: Bearer` header.",
+  "type": "string"
+}

Changed1 schema field changedSep 21, 2026

changedInput schema / properties / payment_mandate / description

Previous value: -"Payment mandate with `mandate_id`, `max_amount_cents`, `currency`, `exp`, `sub`, `aud`. Its `currency` must match the cart's. Use this when your client cannot set an `X-Payment-Mandate` header."New value: +"Payment mandate with `mandate_id`, `max_amount_cents` (integer, minor units), `currency` (3 letters, must match the cart's), `exp` (expiry as an INTEGER of Unix seconds — not an ISO 8601 date string), `sub` (your agent id) and `aud` (the store slug this mandate is for, e.g. the slug in the URL you are calling — not a domain). Use this when your client cannot set an `X-Payment-Mandate` header."

Changed1 schema field changedSep 20, 2026

addedInput schema / properties / payment_mandate

Added value: +{
+  "anyOf": [
+    {
+      "additionalProperties": {},
+      "type": "object"
+    },
+    {
+      "type": "string"
+    }
+  ],
+  "description": "Payment mandate with `mandate_id`, `max_amount_cents`, `currency`, `exp`, `sub`, `aud`. Its `currency` must match the cart's. Use this when your client cannot set an `X-Payment-Mandate` header."
+}

Changed8 schema fields changedSep 17, 2026
- changedInput schema / additionalProperties
```
Previous value: -falseNew value: +true
```
- changedOutput schema / additionalProperties
```
Previous value: -falseNew value: +true
```
- changedOutput schema / properties / extensions / additionalProperties
```
Previous value: -falseNew value: +true
```
- changedOutput schema / properties / extensions / properties / warnings / items / additionalProperties
```
Previous value: -falseNew value: +true
```
- changedOutput schema / properties / intent / additionalProperties
```
Previous value: -falseNew value: +true
```
- changedOutput schema / properties / line_items / items / additionalProperties
```
Previous value: -falseNew value: +true
```
- changedOutput schema / properties / line_items / items / properties / price / additionalProperties
```
Previous value: -falseNew value: +true
```
- changedOutput schema / properties / totals / items / additionalProperties
```
Previous value: -falseNew value: +true
```
AddedSep 9, 2026
RemovedSep 1, 2026

Changed4 schema fields changedAug 24, 2026

addedInput schema / $schema

Added value: +"http://json-schema.org/draft-07/schema#"

addedInput schema / additionalProperties
```
Added value: +false
```

addedInput schema / properties

Added value: +{
+  "checkout_id": {
+    "description": "UCP checkout session ID (valid UUID)",
+    "type": "string"
+  },
+  "idempotency_key": {
+    "description": "Optional idempotency key to prevent duplicate cancellations.",
+    "type": "string"
+  },
+  "reason": {
+    "description": "Optional cancellation reason",
+    "type": "string"
+  }
+}

addedInput schema / required
```
Added value: +[
+  "checkout_id"
+]
```

Changed5 schema fields changedJul 10, 2026

removedInput schema / $schema

Removed value: -"http://json-schema.org/draft-07/schema#"

removedInput schema / additionalProperties
```
Removed value: -false
```

removedInput schema / properties

Removed value: -{
-  "checkout_id": {
-    "description": "UCP checkout session ID (valid UUID)",
-    "type": "string"
-  },
-  "idempotency_key": {
-    "description": "Optional idempotency key to prevent duplicate cancellations.",
-    "type": "string"
-  },
-  "reason": {
-    "description": "Optional cancellation reason",
-    "type": "string"
-  }
-}

removedInput schema / required
```
Removed value: -[
-  "checkout_id"
-]
```

changedOutput schema / (root)

Previous value: -{
-  "$schema": "http://json-schema.org/draft-07/schema#",
-  "additionalProperties": false,
-  "properties": {
-    "created_at": {
-      "type": "string"
-    },
-    "currency": {
-      "type": "string"
-    },
-    "extensions": {
-      "additionalProperties": false,
-      "properties": {
-        "warnings": {
-          "items": {
-            "additionalProperties": false,
-            "properties": {
-              "code": {
-                "type": "string"
-              },
-              "disclosure_url": {
-                "type": "string"
-              },
-              "message": {
-                "type": "string"
-              },
-              "severity": {
-                "enum": [
-                  "info",
-                  "caution",
-                  "critical"
-                ],
-                "type": "string"
-              }
-            },
-            "required": [
-              "code",
-              "message",
-              "severity"
-            ],
-            "type": "object"
-          },
-          "type": "array"
-        }
-      },
-      "type": "object"
-    },
-    "id": {
-      "type": "string"
-    },
-    "idempotency_key": {
-      "type": "string"
-    },
-    "intent": {
-      "additionalProperties": false,
-      "properties": {
-        "action": {
-          "type": "string"
-        },
-        "context": {
-          "type": "string"
-        },
-        "locale": {
-          "type": "string"
-        }
-      },
-      "required": [
-        "action"
-      ],
-      "type": "object"
-    },
-    "label": {
-      "type": "string"
-    },
-    "line_items": {
-      "items": {
-        "additionalProperties": false,
-        "properties": {
-          "id": {
-            "type": "string"
-          },
-          "price": {
-            "additionalProperties": false,
-            "properties": {
-              "amount": {
-                "type": "number"
-              },
-              "currency": {
-                "type": "string"
-              }
-            },
-            "required": [
-              "amount",
-              "currency"
-            ],
-            "type": "object"
-          },
-          "product_id": {
-            "type": "string"
-          },
-          "quantity": {
-            "type": "number"
-          },
-          "title": {
-            "type": "string"
-          }
-        },
-        "required": [
-          "id",
-          "product_id",
-          "title",
-          "quantity",
-          "price"
-        ],
-        "type": "object"
-      },
-      "type": "array"
-    },
-    "status": {
-      "enum": [
-        "incomplete",
-        "requires_escalation",
-        "ready_for_complete",
-        "complete_in_progress",
-        "completed",
-        "canceled"
-      ],
-      "type": "string"
-    },
-    "totals": {
-      "items": {
-        "additionalProperties": false,
-        "properties": {
-          "amount": {
-            "type": "number"
-          },
-          "currency": {
-            "type": "string"
-          },
-          "label": {
-            "type": "string"
-          }
-        },
-        "required": [
-          "label",
-          "amount",
-          "currency"
-        ],
-        "type": "object"
-      },
-      "type": "array"
-    },
-    "updated_at": {
-      "type": "string"
-    }
-  },
-  "required": [
-    "id",
-    "status",
-    "currency",
-    "line_items",
-    "totals",
-    "created_at",
-    "updated_at"
-  ],
-  "type": "object"
-}New value: +null

First observedApr 28, 2026

TDQS

A4.4/5.0

Behavior4/5

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

Annotations already declare destructiveHint=true and idempotentHint=true, so the safety profile is covered. The description adds real behavior beyond the annotations: the resulting state ('canceled') and a precondition ('cannot cancel already completed checkouts'). It does not mention authorization or what happens to held carts, keeping it from a 5.

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-loads the action, then the resulting state, then the precondition, then the sibling exclusion, and finally the guide pointer. No redundant phrasing about what the annotations already state; every sentence carries information.

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?

A destruction tool with no output schema. The description covers the resulting status and the sole precondition (already-completed sessions), which is enough to call it correctly. It could clarify whether the cancellation is reversible or what the response contains, but the critical invocation constraints are present.

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 idempotency_key description already covers the retry rule, so the description correctly does not restate it. The one addition 'Use ONLY for a checkout session created with ucp_create_checkout' slightly hints that checkout_id must be a UCP session, but does not add format or semantic detail beyond what the schema already documents. 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 (cancel) and resource (UCP checkout session) and immediately clarifies its scope against the sibling free function ucp_create_checkout and the standard-flow tools (create_cart, preview_checkout, complete_checkout). The agent can distinguish it from those siblings without opening any 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?

Explicitly gives when to use (only for a session created with ucp_create_checkout), when not to use (do not call for the standard create_cart/preview/complete flow), and a prerequisite to consult the guide resource. This is precisely the kind of exclusion-and-alternative routing usage guidance is meant to provide.

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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