diff --git a/docs/specification/additional-fields.md b/docs/specification/additional-fields.md
new file mode 100644
index 00000000..dd94db5a
--- /dev/null
+++ b/docs/specification/additional-fields.md
@@ -0,0 +1,179 @@
+<!--
+   Copyright 2026 UCP Authors
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+-->
+
+# Additional Fields Extension
+
+## Overview
+
+The additional fields extension enables businesses to request checkout inputs
+that are not covered by the core schema, and to bind those inputs to cart,
+buyer, or fulfillment data via JSONPath.
+
+**Key features**
+
+- Declare required and optional additional fields with typed UI controls
+- Map each field to cart, buyer, or fulfillment data using JSONPath
+- Receive input in a consistent structure keyed by field `key`
+- Surface validation via `messages[]` with JSONPath to the field value
+
+**Dependencies**
+
+- Checkout capability
+
+## Discovery
+
+Businesses advertise additional fields support in their profile:
+
+```json
+{
+  "ucp": {
+    "version": "2026-01-11",
+    "capabilities": {
+      "dev.ucp.shopping.additional_fields": [
+        {
+          "version": "2026-01-11",
+          "extends": "dev.ucp.shopping.checkout",
+          "spec": "https://ucp.dev/specification/additional-fields",
+          "schema": "https://ucp.dev/schemas/shopping/additional_fields.json"
+        }
+      ]
+    }
+  }
+}
+```
+
+## Schema
+
+When this extension is active, checkout gains an `additional_fields` object with
+field definitions and buyer-provided values.
+
+### Additional Fields
+
+{{ extension_schema_fields('additional_fields.json#/$defs/additional_fields', 'additional-fields') }}
+
+### Field Definition
+
+{{ extension_schema_fields('additional_fields.json#/$defs/additional_field', 'additional-fields') }}
+
+### Validation Hints
+
+{{ extension_schema_fields('additional_fields.json#/$defs/validation_hints', 'additional-fields') }}
+
+## Input types and values
+
+- `single_line_text` and `multi_line_text`: string values. Platforms **SHOULD**
+  honor `min_length`, `max_length`, and `pattern` hints when rendering input.
+- `checkbox`: boolean values.
+- `date`: RFC 3339 full-date string (e.g., `2026-02-14`).
+- `date_time`: RFC 3339 date-time string with offset (e.g.,
+  `2026-02-14T15:04:05Z`).
+- `default_value` must match the field's input type.
+
+## Validation
+
+- Businesses validate submitted values and return issues in `messages[]` using
+  `type: "error"` with `severity: "recoverable"` and `path` set to the field
+  value (e.g., `$.additional_fields.values.delivery_date`).
+- No new message type is introduced; `type: "error"` + `severity` communicates
+  that the issue is blocking until resolved.
+- Suggested error codes: `additional_field_required`, `additional_field_invalid`
+  (type or format mismatch), `additional_field_not_supported` (key unknown).
+- The `validation_hints` object is a **hint** for platforms to improve UX;
+  businesses remain the source of truth and enforce rules via `messages[]`.
+
+## Operations
+
+- **Create/Update Checkout:** Platforms send buyer input in
+  `additional_fields.values`. Submitting the object replaces previously sent
+  values. Send an empty object to clear all additional fields.
+- **Responses:** Businesses return `additional_fields.fields` to describe what
+  is supported, and may echo `additional_fields.values` received from the
+  platform.
+- **Mapping:** `path` is a JSONPath from the checkout root to the data the field
+  influences (e.g., buyer attributes, line items, fulfillment options). It
+  allows platforms to render the field near its context while keeping the core
+  schema unchanged.
+
+## Example
+
+```json
+{
+  "additional_fields": {
+    "fields": [
+      {
+        "key": "po_number",
+        "label": "PO number",
+        "description": "Optional purchase order for your records",
+        "type": "single_line_text",
+        "path": "$.buyer",
+        "validation_hints": {
+          "max_length": 24
+        }
+      },
+      {
+        "key": "gift_wrap",
+        "label": "Gift wrap",
+        "type": "checkbox",
+        "default_value": false,
+        "path": "$.line_items[?(@.id=='shirt')]"
+      },
+      {
+        "key": "delivery_date",
+        "label": "Delivery date",
+        "type": "date",
+        "required": true,
+        "path": "$.fulfillment.methods[0].groups[0]",
+        "validation_hints": {
+          "min_date": "2026-02-01"
+        }
+      },
+      {
+        "key": "gift_message",
+        "label": "Gift message",
+        "type": "multi_line_text",
+        "path": "$.line_items[?(@.id=='gift_box')]"
+      },
+      {
+        "key": "agree_to_terms",
+        "label": "Agree to terms",
+        "type": "checkbox",
+        "required": true,
+        "path": "$"
+      }
+    ],
+    "values": {
+      "po_number": "PO-45822",
+      "gift_wrap": true,
+      "delivery_date": "2026-02-05",
+      "gift_message": "Happy birthday, Alex!",
+      "agree_to_terms": true
+    }
+  },
+  "messages": [
+    {
+      "type": "error",
+      "code": "additional_field_required",
+      "severity": "recoverable",
+      "path": "$.additional_fields.values.delivery_date",
+      "content": "Select an eligible delivery date."
+    }
+  ]
+}
+```
+
+Use cases include buyer birthdays, order notes, line-item personalization
+(e.g., monogramming), delivery dates, gift wrap and gift messages, PO numbers,
+tax IDs, and terms acceptance.
diff --git a/mkdocs.yml b/mkdocs.yml
index 6686c99c..dd4883e7 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -38,6 +38,7 @@ nav:
       - EP Binding: specification/embedded-checkout.md
       - AP2 Mandates Extension: specification/ap2-mandates.md
       - Buyer Consent Extension: specification/buyer-consent.md
+      - Additional Fields Extension: specification/additional-fields.md
       - Discounts Extension: specification/discount.md
       - Fulfillment Extension: specification/fulfillment.md
     - Cart Capability:
diff --git a/source/schemas/shopping/additional_fields.json b/source/schemas/shopping/additional_fields.json
new file mode 100644
index 00000000..ee840e88
--- /dev/null
+++ b/source/schemas/shopping/additional_fields.json
@@ -0,0 +1,172 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://ucp.dev/schemas/shopping/additional_fields.json",
+  "name": "dev.ucp.shopping.additional_fields",
+  "title": "Additional Fields Extension",
+  "description": "Extends Checkout with merchant-defined additional fields mapped to cart, buyer, or fulfillment data via JSONPath.",
+  "$defs": {
+    "input_type": {
+      "type": "string",
+      "description": "UI control type for the additional field.",
+      "enum": [
+        "single_line_text",
+        "multi_line_text",
+        "checkbox",
+        "date",
+        "date_time"
+      ]
+    },
+    "validation_hints": {
+      "type": "object",
+      "description": "Optional validation hints for platforms. Enforcement is communicated via messages.",
+      "properties": {
+        "pattern": {
+          "type": "string",
+          "description": "Regular expression hint for text inputs."
+        },
+        "min_length": {
+          "type": "integer",
+          "minimum": 0,
+          "description": "Minimum characters for text inputs."
+        },
+        "max_length": {
+          "type": "integer",
+          "minimum": 1,
+          "description": "Maximum characters for text inputs."
+        },
+        "min_date": {
+          "type": "string",
+          "format": "date",
+          "description": "Earliest allowed date for date inputs (RFC 3339 full-date)."
+        },
+        "max_date": {
+          "type": "string",
+          "format": "date",
+          "description": "Latest allowed date for date inputs (RFC 3339 full-date)."
+        },
+        "min_date_time": {
+          "type": "string",
+          "format": "date-time",
+          "description": "Earliest allowed timestamp for date_time inputs (RFC 3339 date-time)."
+        },
+        "max_date_time": {
+          "type": "string",
+          "format": "date-time",
+          "description": "Latest allowed timestamp for date_time inputs (RFC 3339 date-time)."
+        }
+      }
+    },
+    "additional_field": {
+      "type": "object",
+      "description": "Definition for a merchant-requested input.",
+      "required": [
+        "key",
+        "label",
+        "type",
+        "path"
+      ],
+      "properties": {
+        "key": {
+          "type": "string",
+          "pattern": "^[A-Za-z0-9_-]+$",
+          "description": "Stable identifier for the field. Used in values object keys and in messages.path (e.g., $.additional_fields.values.po_number)."
+        },
+        "label": {
+          "type": "string",
+          "description": "Buyer-facing label for the field."
+        },
+        "description": {
+          "type": "string",
+          "description": "Additional context or instructions for the buyer."
+        },
+        "type": {
+          "$ref": "#/$defs/input_type"
+        },
+        "required": {
+          "type": "boolean",
+          "default": false,
+          "description": "If true, checkout remains incomplete until a value is provided."
+        },
+        "default_value": {
+          "description": "Default value used when the platform does not provide buyer input.",
+          "oneOf": [
+            {
+              "type": "string"
+            },
+            {
+              "type": "boolean"
+            }
+          ]
+        },
+        "path": {
+          "type": "string",
+          "description": "RFC 9535 JSONPath to the target checkout data (e.g., buyer, cart totals, fulfillment option, or specific line items)."
+        },
+        "validation_hints": {
+          "$ref": "#/$defs/validation_hints"
+        }
+      }
+    },
+    "values": {
+      "type": "object",
+      "description": "Buyer-provided values keyed by additional field key.",
+      "propertyNames": {
+        "pattern": "^[A-Za-z0-9_-]+$"
+      },
+      "additionalProperties": {
+        "oneOf": [
+          {
+            "type": "string"
+          },
+          {
+            "type": "boolean"
+          }
+        ]
+      }
+    },
+    "additional_fields": {
+      "type": "object",
+      "description": "Definitions provided by the business and values supplied by the platform.",
+      "properties": {
+        "fields": {
+          "type": "array",
+          "items": {
+            "$ref": "#/$defs/additional_field"
+          },
+          "description": "Available fields and how they map into checkout data.",
+          "ucp_request": "omit"
+        },
+        "values": {
+          "$ref": "#/$defs/values",
+          "ucp_request": {
+            "create": "optional",
+            "update": "optional",
+            "complete": "omit"
+          }
+        }
+      }
+    },
+    "dev.ucp.shopping.checkout": {
+      "title": "Checkout with Additional Fields",
+      "description": "Checkout extended with merchant-defined additional fields.",
+      "allOf": [
+        {
+          "$ref": "checkout.json"
+        },
+        {
+          "type": "object",
+          "properties": {
+            "additional_fields": {
+              "$ref": "#/$defs/additional_fields",
+              "ucp_request": {
+                "create": "optional",
+                "update": "optional",
+                "complete": "omit"
+              }
+            }
+          }
+        }
+      ]
+    }
+  }
+}