diff --git a/docs/API.md b/docs/API.md
index d5a0bf40..69eb743f 100644
--- a/docs/API.md
+++ b/docs/API.md
@@ -2060,6 +2060,163 @@ print(response.json())
 ```
 
 
+## Classification Rules
+
+The classification rules API lets you manage custom document classification rules.  Rules are evaluated during the `classify` pipeline step to assign a category to each document based on filename patterns, content keywords, and metadata fields.
+
+### Built-in Categories
+
+```bash
+GET /api/classification-rules/categories
+```
+
+Returns the pre-built classification categories.
+
+**Response (200):**
+```json
+{
+  "invoice": "Invoice",
+  "contract": "Contract",
+  "receipt": "Receipt",
+  "letter": "Letter",
+  "report": "Report",
+  "bank_statement": "Bank Statement",
+  "tax_document": "Tax Document",
+  "insurance": "Insurance Document",
+  "payslip": "Payslip",
+  "unknown": "Unknown"
+}
+```
+
+### Rule Types
+
+```bash
+GET /api/classification-rules/rule-types
+```
+
+Returns the supported rule types with descriptions.
+
+**Response (200):**
+```json
+[
+  {
+    "type": "filename_pattern",
+    "label": "Filename Pattern",
+    "description": "Regex pattern matched against the original filename."
+  },
+  {
+    "type": "content_keyword",
+    "label": "Content Keyword",
+    "description": "Pipe-separated keywords matched against the OCR text."
+  },
+  {
+    "type": "metadata_match",
+    "label": "Metadata Match",
+    "description": "field=value pattern matched against existing AI metadata."
+  }
+]
+```
+
+### List Rules
+
+```bash
+GET /api/classification-rules/
+```
+
+List all classification rules visible to the current user (system rules + own rules).
+
+**Response (200):**
+```json
+[
+  {
+    "id": 1,
+    "owner_id": "user@example.com",
+    "name": "German Invoice Filename",
+    "category": "invoice",
+    "rule_type": "filename_pattern",
+    "pattern": "(?i)rechnung",
+    "priority": 10,
+    "case_sensitive": false,
+    "enabled": true
+  }
+]
+```
+
+### Create Rule
+
+```bash
+POST /api/classification-rules/
+```
+
+Create a new custom classification rule.
+
+**Request:**
+```json
+{
+  "name": "German Invoice Filename",
+  "category": "invoice",
+  "rule_type": "filename_pattern",
+  "pattern": "(?i)rechnung",
+  "priority": 10,
+  "case_sensitive": false,
+  "enabled": true
+}
+```
+
+| Field | Type | Required | Description |
+|-------|------|----------|-------------|
+| `name` | string | Yes | Unique rule name (per user) |
+| `category` | string | Yes | Target category (e.g. `invoice`, `contract`, or custom) |
+| `rule_type` | string | Yes | One of: `filename_pattern`, `content_keyword`, `metadata_match` |
+| `pattern` | string | Yes | Regex (filename), pipe-separated keywords (content), or `field=value` (metadata) |
+| `priority` | integer | No | Higher priority rules are evaluated first (default: 0) |
+| `case_sensitive` | boolean | No | Case-sensitive matching (default: false) |
+| `enabled` | boolean | No | Whether the rule is active (default: true) |
+
+**Response (201):**
+```json
+{
+  "id": 1,
+  "owner_id": "user@example.com",
+  "name": "German Invoice Filename",
+  "category": "invoice",
+  "rule_type": "filename_pattern",
+  "pattern": "(?i)rechnung",
+  "priority": 10,
+  "case_sensitive": false,
+  "enabled": true
+}
+```
+
+### Get Rule
+
+```bash
+GET /api/classification-rules/{rule_id}
+```
+
+### Update Rule
+
+```bash
+PUT /api/classification-rules/{rule_id}
+```
+
+**Request (partial update):**
+```json
+{
+  "priority": 20,
+  "enabled": false
+}
+```
+
+### Delete Rule
+
+```bash
+DELETE /api/classification-rules/{rule_id}
+```
+
+**Response:** `204 No Content`
+
+
 ## Further Assistance
 
 For additional help with the API, please contact our support team or refer to the [Development Guide](../CONTRIBUTING.md).
diff --git a/docs/UserGuide.md b/docs/UserGuide.md
index 9906a89c..ad3ebae9 100644
--- a/docs/UserGuide.md
+++ b/docs/UserGuide.md
@@ -573,7 +573,25 @@ Processing pipelines let you define exactly what happens to your documents when
 | `embed_metadata` | Write extracted metadata into the PDF document properties |
 | `compute_embedding` | Compute semantic embeddings for similarity search |
 | `send_to_destinations` | Upload the processed document to all configured storage destinations |
-| `classify` | Classify the document type with AI |
+| `classify` | Classify the document type using rules (filename patterns, content keywords, metadata) |
+
+#### Classify step – rule-based document classification
+
+The `classify` step assigns a category to each document by evaluating **built-in** and **custom** classification rules.  Rules are matched against three signals:
+
+- **Filename patterns** — regex matched against the original filename (e.g. `(?i)invoice` matches filenames containing "invoice").
+- **Content keywords** — pipe-separated keywords matched against the OCR text (e.g. `invoice number|amount due`).
+- **Metadata match** — `field=value` matched against existing AI metadata (e.g. `document_type=Invoice`).
+
+**Pre-built categories** include: Invoice, Contract, Receipt, Letter, Report, Bank Statement, Tax Document, Insurance, and Payslip.  You can also define your own custom categories.
+
+The classification result is stored in the document's `ai_metadata` under the `classification` key with the matched category, confidence score, and list of matched rules.  If no `document_type` was previously set by AI metadata extraction, the classify step will also populate it.
+
+> **Tip:** Manage custom classification rules via **Settings → Classification Rules** or the `/api/classification-rules/` API.  See the [API Documentation](./API.md#classification-rules) for details.
+
+| Option | Type | Default | Description |
+|--------|------|---------|-------------|
+| `use_builtin_rules` | boolean | `true` | Include the pre-built classification rules |
 
 #### OCR step options