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skills/n8n-expression-syntax/COMMON_MISTAKES.md

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+# Common n8n Expression Mistakes
+
+Complete catalog of expression errors with explanations and fixes.
+
+---
+
+## 1. Missing Curly Braces
+
+**Problem**: Expression not recognized, shows as literal text
+
+❌ **Wrong**:
+```
+$json.email
+```
+
+✅ **Correct**:
+```
+{{$json.email}}
+```
+
+**Why it fails**: n8n treats text without {{ }} as a literal string. Expressions must be wrapped to be evaluated.
+
+**How to identify**: Field shows exact text like "$json.email" instead of actual value.
+
+---
+
+## 2. Webhook Body Access
+
+**Problem**: Undefined values when accessing webhook data
+
+❌ **Wrong**:
+```
+{{$json.name}}
+{{$json.email}}
+{{$json.message}}
+```
+
+✅ **Correct**:
+```
+{{$json.body.name}}
+{{$json.body.email}}
+{{$json.body.message}}
+```
+
+**Why it fails**: Webhook node wraps incoming data under `.body` property. The root `$json` contains headers, params, query, and body.
+
+**Webhook structure**:
+```javascript
+{
+  "headers": {...},
+  "params": {...},
+  "query": {...},
+  "body": {         // User data is HERE!
+    "name": "John",
+    "email": "john@example.com"
+  }
+}
+```
+
+**How to identify**: Webhook workflow shows "undefined" for fields that are definitely being sent.
+
+---
+
+## 3. Spaces in Field Names
+
+**Problem**: Syntax error or undefined value
+
+❌ **Wrong**:
+```
+{{$json.first name}}
+{{$json.user data.email}}
+```
+
+✅ **Correct**:
+```
+{{$json['first name']}}
+{{$json['user data'].email}}
+```
+
+**Why it fails**: Spaces break dot notation. JavaScript interprets space as end of property name.
+
+**How to identify**: Error message about unexpected token, or undefined when field exists.
+
+---
+
+## 4. Spaces in Node Names
+
+**Problem**: Cannot access other node's data
+
+❌ **Wrong**:
+```
+{{$node.HTTP Request.json.data}}
+{{$node.Respond to Webhook.json}}
+```
+
+✅ **Correct**:
+```
+{{$node["HTTP Request"].json.data}}
+{{$node["Respond to Webhook"].json}}
+```
+
+**Why it fails**: Node names are treated as object property names and need quotes when they contain spaces.
+
+**How to identify**: Error like "Cannot read property 'Request' of undefined"
+
+---
+
+## 5. Incorrect Node Reference Case
+
+**Problem**: Undefined or wrong data returned
+
+❌ **Wrong**:
+```
+{{$node["http request"].json.data}}  // lowercase
+{{$node["Http Request"].json.data}}  // wrong capitalization
+```
+
+✅ **Correct**:
+```
+{{$node["HTTP Request"].json.data}}  // exact match
+```
+
+**Why it fails**: Node names are **case-sensitive**. Must match exactly as shown in workflow.
+
+**How to identify**: Undefined value even though node exists and has data.
+
+---
+
+## 6. Double Wrapping
+
+**Problem**: Literal {{ }} appears in output
+
+❌ **Wrong**:
+```
+{{{$json.field}}}
+```
+
+✅ **Correct**:
+```
+{{$json.field}}
+```
+
+**Why it fails**: Only one set of {{ }} is needed. Extra braces are treated as literal characters.
+
+**How to identify**: Output shows "{{value}}" instead of just "value".
+
+---
+
+## 7. Array Access with Dots
+
+**Problem**: Syntax error or undefined
+
+❌ **Wrong**:
+```
+{{$json.items.0.name}}
+{{$json.users.1.email}}
+```
+
+✅ **Correct**:
+```
+{{$json.items[0].name}}
+{{$json.users[1].email}}
+```
+
+**Why it fails**: Array indices require brackets, not dots. Number after dot is invalid JavaScript.
+
+**How to identify**: Syntax error or "Cannot read property '0' of undefined"
+
+---
+
+## 8. Using Expressions in Code Nodes
+
+**Problem**: Literal string instead of value, or errors
+
+❌ **Wrong (in Code node)**:
+```javascript
+const email = '{{$json.email}}';
+const name = '={{$json.body.name}}';
+```
+
+✅ **Correct (in Code node)**:
+```javascript
+const email = $json.email;
+const name = $json.body.name;
+
+// Or using Code node API
+const email = $input.item.json.email;
+const allItems = $input.all();
+```
+
+**Why it fails**: Code nodes have **direct access** to data. The {{ }} syntax is for expression fields in other nodes, not for JavaScript code.
+
+**How to identify**: Literal string "{{$json.email}}" appears in Code node output instead of actual value.
+
+---
+
+## 9. Missing Quotes in $node Reference
+
+**Problem**: Syntax error
+
+❌ **Wrong**:
+```
+{{$node[HTTP Request].json.data}}
+```
+
+✅ **Correct**:
+```
+{{$node["HTTP Request"].json.data}}
+```
+
+**Why it fails**: Node names must be quoted strings inside brackets.
+
+**How to identify**: Syntax error "Unexpected identifier"
+
+---
+
+## 10. Incorrect Property Path
+
+**Problem**: Undefined value
+
+❌ **Wrong**:
+```
+{{$json.data.items.name}}       // items is an array
+{{$json.user.email}}            // user doesn't exist, it's userData
+```
+
+✅ **Correct**:
+```
+{{$json.data.items[0].name}}    // access array element
+{{$json.userData.email}}        // correct property name
+```
+
+**Why it fails**: Wrong path to data. Arrays need index, property names must be exact.
+
+**How to identify**: Check actual data structure using expression editor preview.
+
+---
+
+## 11. Using = Prefix Outside JSON
+
+**Problem**: Literal "=" appears in output
+
+❌ **Wrong (in text field)**:
+```
+Email: ={{$json.email}}
+```
+
+✅ **Correct (in text field)**:
+```
+Email: {{$json.email}}
+```
+
+**Note**: The `=` prefix is **only** needed in JSON mode or when you want to set entire field value to expression result:
+
+```javascript
+// JSON mode (set property to expression)
+{
+  "email": "={{$json.body.email}}"
+}
+
+// Text mode (no = needed)
+Hello {{$json.body.name}}!
+```
+
+**Why it fails**: The `=` is parsed as literal text in non-JSON contexts.
+
+**How to identify**: Output shows "=john@example.com" instead of "john@example.com"
+
+---
+
+## 12. Expressions in Webhook Path
+
+**Problem**: Path doesn't update, validation error
+
+❌ **Wrong**:
+```
+path: "{{$json.user_id}}/webhook"
+path: "users/={{$env.TENANT_ID}}"
+```
+
+✅ **Correct**:
+```
+path: "my-webhook"              // Static paths only
+path: "user-webhook/:userId"    // Use dynamic URL parameters instead
+```
+
+**Why it fails**: Webhook paths must be static. Use dynamic URL parameters (`:paramName`) instead of expressions.
+
+**How to identify**: Webhook path doesn't change or validation warns about invalid path.
+
+---
+
+## 13. Forgetting .json in $node Reference
+
+**Problem**: Undefined or wrong data
+
+❌ **Wrong**:
+```
+{{$node["HTTP Request"].data}}          // Missing .json
+{{$node["Webhook"].body.email}}         // Missing .json
+```
+
+✅ **Correct**:
+```
+{{$node["HTTP Request"].json.data}}
+{{$node["Webhook"].json.body.email}}
+```
+
+**Why it fails**: Node data is always under `.json` property (or `.binary` for binary data).
+
+**How to identify**: Undefined value when you know the node has data.
+
+---
+
+## 14. String Concatenation Confusion
+
+**Problem**: Attempting JavaScript template literals
+
+❌ **Wrong**:
+```
+`Hello ${$json.name}!`          // Template literal syntax
+"Hello " + $json.name + "!"     // String concatenation
+```
+
+✅ **Correct**:
+```
+Hello {{$json.name}}!           // n8n expressions auto-concatenate
+```
+
+**Why it fails**: n8n expressions don't use JavaScript template literal syntax. Adjacent text and expressions are automatically concatenated.
+
+**How to identify**: Literal backticks or + symbols appear in output.
+
+---
+
+## 15. Empty Expression Brackets
+
+**Problem**: Literal {{}} in output
+
+❌ **Wrong**:
+```
+{{}}
+{{ }}
+```
+
+✅ **Correct**:
+```
+{{$json.field}}                 // Include expression content
+```
+
+**Why it fails**: Empty expression brackets have nothing to evaluate.
+
+**How to identify**: Literal "{{ }}" text appears in output.
+
+---
+
+## Quick Reference Table
+
+| Error | Symptom | Fix |
+|-------|---------|-----|
+| No {{ }} | Literal text | Add {{ }} |
+| Webhook data | Undefined | Add `.body` |
+| Space in field | Syntax error | Use `['field name']` |
+| Space in node | Undefined | Use `["Node Name"]` |
+| Wrong case | Undefined | Match exact case |
+| Double {{ }} | Literal braces | Remove extra {{ }} |
+| .0 array | Syntax error | Use [0] |
+| {{ }} in Code | Literal string | Remove {{ }} |
+| No quotes in $node | Syntax error | Add quotes |
+| Wrong path | Undefined | Check data structure |
+| = in text | Literal = | Remove = prefix |
+| Dynamic path | Doesn't work | Use static path |
+| Missing .json | Undefined | Add .json |
+| Template literals | Literal text | Use {{ }} |
+| Empty {{ }} | Literal braces | Add expression |
+
+---
+
+## Debugging Process
+
+When expression doesn't work:
+
+1. **Check braces**: Is it wrapped in {{ }}?
+2. **Check data source**: Is it webhook data? Add `.body`
+3. **Check spaces**: Field or node name has spaces? Use brackets
+4. **Check case**: Does node name match exactly?
+5. **Check path**: Is the property path correct?
+6. **Use expression editor**: Preview shows actual result
+7. **Check context**: Is it a Code node? Remove {{ }}
+
+---
+
+**Related**: See [EXAMPLES.md](EXAMPLES.md) for working examples of correct syntax.

skills/n8n-expression-syntax/EXAMPLES.md

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+# n8n Expression Examples
+
+Real working examples from n8n workflows.
+
+---
+
+## Example 1: Webhook Form Submission
+
+**Scenario**: Form submission webhook posts to Slack
+
+**Workflow**: Webhook → Slack
+
+**Webhook Input** (POST):
+```json
+{
+  "name": "John Doe",
+  "email": "john@example.com",
+  "company": "Acme Corp",
+  "message": "Interested in your product"
+}
+```
+
+**Webhook Node Output**:
+```json
+{
+  "headers": {"content-type": "application/json"},
+  "params": {},
+  "query": {},
+  "body": {
+    "name": "John Doe",
+    "email": "john@example.com",
+    "company": "Acme Corp",
+    "message": "Interested in your product"
+  }
+}
+```
+
+**In Slack Node** (text field):
+```
+New form submission! 📝
+
+Name: {{$json.body.name}}
+Email: {{$json.body.email}}
+Company: {{$json.body.company}}
+Message: {{$json.body.message}}
+```
+
+**Output**:
+```
+New form submission! 📝
+
+Name: John Doe
+Email: john@example.com
+Company: Acme Corp
+Message: Interested in your product
+```
+
+---
+
+## Example 2: HTTP API to Database
+
+**Scenario**: Fetch user data from API and insert into database
+
+**Workflow**: Schedule → HTTP Request → Postgres
+
+**HTTP Request Returns**:
+```json
+{
+  "data": {
+    "users": [
+      {
+        "id": 123,
+        "name": "Alice Smith",
+        "email": "alice@example.com",
+        "role": "admin"
+      }
+    ]
+  }
+}
+```
+
+**In Postgres Node** (INSERT statement):
+```sql
+INSERT INTO users (user_id, name, email, role, synced_at)
+VALUES (
+  {{$json.data.users[0].id}},
+  '{{$json.data.users[0].name}}',
+  '{{$json.data.users[0].email}}',
+  '{{$json.data.users[0].role}}',
+  '{{$now.toFormat('yyyy-MM-dd HH:mm:ss')}}'
+)
+```
+
+**Result**: User inserted with current timestamp
+
+---
+
+## Example 3: Multi-Node Data Flow
+
+**Scenario**: Webhook → HTTP Request → Email
+
+**Workflow Structure**:
+1. Webhook receives order ID
+2. HTTP Request fetches order details
+3. Email sends confirmation
+
+### Node 1: Webhook
+
+**Receives**:
+```json
+{
+  "body": {
+    "order_id": "ORD-12345"
+  }
+}
+```
+
+### Node 2: HTTP Request
+
+**URL field**:
+```
+https://api.example.com/orders/{{$json.body.order_id}}
+```
+
+**Returns**:
+```json
+{
+  "order": {
+    "id": "ORD-12345",
+    "customer": "Bob Jones",
+    "total": 99.99,
+    "items": ["Widget", "Gadget"]
+  }
+}
+```
+
+### Node 3: Email
+
+**Subject**:
+```
+Order {{$node["Webhook"].json.body.order_id}} Confirmed
+```
+
+**Body**:
+```
+Dear {{$node["HTTP Request"].json.order.customer}},
+
+Your order {{$node["Webhook"].json.body.order_id}} has been confirmed!
+
+Total: ${{$node["HTTP Request"].json.order.total}}
+Items: {{$node["HTTP Request"].json.order.items.join(', ')}}
+
+Thank you for your purchase!
+```
+
+**Email Result**:
+```
+Subject: Order ORD-12345 Confirmed
+
+Dear Bob Jones,
+
+Your order ORD-12345 has been confirmed!
+
+Total: $99.99
+Items: Widget, Gadget
+
+Thank you for your purchase!
+```
+
+---
+
+## Example 4: Date Formatting
+
+**Scenario**: Various date format outputs
+
+**Current Time**: 2025-10-20 14:30:45
+
+### ISO Format
+```javascript
+{{$now.toISO()}}
+```
+**Output**: `2025-10-20T14:30:45.000Z`
+
+### Custom Date Format
+```javascript
+{{$now.toFormat('yyyy-MM-dd')}}
+```
+**Output**: `2025-10-20`
+
+### Time Only
+```javascript
+{{$now.toFormat('HH:mm:ss')}}
+```
+**Output**: `14:30:45`
+
+### Full Readable Format
+```javascript
+{{$now.toFormat('MMMM dd, yyyy')}}
+```
+**Output**: `October 20, 2025`
+
+### Date Math - Future
+```javascript
+{{$now.plus({days: 7}).toFormat('yyyy-MM-dd')}}
+```
+**Output**: `2025-10-27`
+
+### Date Math - Past
+```javascript
+{{$now.minus({hours: 24}).toFormat('yyyy-MM-dd HH:mm')}}
+```
+**Output**: `2025-10-19 14:30`
+
+---
+
+## Example 5: Array Operations
+
+**Data**:
+```json
+{
+  "users": [
+    {"name": "Alice", "email": "alice@example.com"},
+    {"name": "Bob", "email": "bob@example.com"},
+    {"name": "Charlie", "email": "charlie@example.com"}
+  ]
+}
+```
+
+### First User
+```javascript
+{{$json.users[0].name}}
+```
+**Output**: `Alice`
+
+### Last User
+```javascript
+{{$json.users[$json.users.length - 1].name}}
+```
+**Output**: `Charlie`
+
+### All Emails (Join)
+```javascript
+{{$json.users.map(u => u.email).join(', ')}}
+```
+**Output**: `alice@example.com, bob@example.com, charlie@example.com`
+
+### Array Length
+```javascript
+{{$json.users.length}}
+```
+**Output**: `3`
+
+---
+
+## Example 6: Conditional Logic
+
+**Data**:
+```json
+{
+  "order": {
+    "status": "completed",
+    "total": 150
+  }
+}
+```
+
+### Ternary Operator
+```javascript
+{{$json.order.status === 'completed' ? 'Order Complete ✓' : 'Pending...'}}
+```
+**Output**: `Order Complete ✓`
+
+### Default Values
+```javascript
+{{$json.order.notes || 'No notes provided'}}
+```
+**Output**: `No notes provided` (if notes field doesn't exist)
+
+### Multiple Conditions
+```javascript
+{{$json.order.total > 100 ? 'Premium Customer' : 'Standard Customer'}}
+```
+**Output**: `Premium Customer`
+
+---
+
+## Example 7: String Manipulation
+
+**Data**:
+```json
+{
+  "user": {
+    "email": "JOHN@EXAMPLE.COM",
+    "message": "  Hello World  "
+  }
+}
+```
+
+### Lowercase
+```javascript
+{{$json.user.email.toLowerCase()}}
+```
+**Output**: `john@example.com`
+
+### Uppercase
+```javascript
+{{$json.user.message.toUpperCase()}}
+```
+**Output**: `  HELLO WORLD  `
+
+### Trim
+```javascript
+{{$json.user.message.trim()}}
+```
+**Output**: `Hello World`
+
+### Substring
+```javascript
+{{$json.user.email.substring(0, 4)}}
+```
+**Output**: `JOHN`
+
+### Replace
+```javascript
+{{$json.user.message.replace('World', 'n8n')}}
+```
+**Output**: `  Hello n8n  `
+
+---
+
+## Example 8: Fields with Spaces
+
+**Data**:
+```json
+{
+  "user data": {
+    "first name": "Jane",
+    "last name": "Doe",
+    "phone number": "+1234567890"
+  }
+}
+```
+
+### Bracket Notation
+```javascript
+{{$json['user data']['first name']}}
+```
+**Output**: `Jane`
+
+### Combined
+```javascript
+{{$json['user data']['first name']}} {{$json['user data']['last name']}}
+```
+**Output**: `Jane Doe`
+
+### Nested Spaces
+```javascript
+Contact: {{$json['user data']['phone number']}}
+```
+**Output**: `Contact: +1234567890`
+
+---
+
+## Example 9: Code Node (Direct Access)
+
+**Code Node**: Transform webhook data
+
+**Input** (from Webhook node):
+```json
+{
+  "body": {
+    "items": ["apple", "banana", "cherry"]
+  }
+}
+```
+
+**Code** (JavaScript):
+```javascript
+// ✅ Direct access (no {{ }})
+const items = $json.body.items;
+
+// Transform to uppercase
+const uppercased = items.map(item => item.toUpperCase());
+
+// Return in n8n format
+return [{
+  json: {
+    original: items,
+    transformed: uppercased,
+    count: items.length
+  }
+}];
+```
+
+**Output**:
+```json
+{
+  "original": ["apple", "banana", "cherry"],
+  "transformed": ["APPLE", "BANANA", "CHERRY"],
+  "count": 3
+}
+```
+
+---
+
+## Example 10: Environment Variables
+
+**Setup**: Environment variable `API_KEY=secret123`
+
+### In HTTP Request (Headers)
+```javascript
+Authorization: Bearer {{$env.API_KEY}}
+```
+**Result**: `Authorization: Bearer secret123`
+
+### In URL
+```javascript
+https://api.example.com/data?key={{$env.API_KEY}}
+```
+**Result**: `https://api.example.com/data?key=secret123`
+
+---
+
+## Template from Real Workflow
+
+**Based on n8n template #2947** (Weather to Slack)
+
+### Workflow Structure
+Webhook → OpenStreetMap API → Weather API → Slack
+
+### Webhook Slash Command
+**Input**: `/weather London`
+
+**Webhook receives**:
+```json
+{
+  "body": {
+    "text": "London"
+  }
+}
+```
+
+### OpenStreetMap API
+**URL**:
+```
+https://nominatim.openstreetmap.org/search?q={{$json.body.text}}&format=json
+```
+
+### Weather API (NWS)
+**URL**:
+```
+https://api.weather.gov/points/{{$node["OpenStreetMap"].json[0].lat}},{{$node["OpenStreetMap"].json[0].lon}}
+```
+
+### Slack Message
+```
+Weather for {{$json.body.text}}:
+
+Temperature: {{$node["Weather API"].json.properties.temperature.value}}°C
+Conditions: {{$node["Weather API"].json.properties.shortForecast}}
+```
+
+---
+
+## Summary
+
+**Key Patterns**:
+1. Webhook data is under `.body`
+2. Use `{{}}` for expressions (except Code nodes)
+3. Reference other nodes with `$node["Node Name"].json`
+4. Use brackets for field names with spaces
+5. Node names are case-sensitive
+
+**Most Common Uses**:
+- `{{$json.body.field}}` - Webhook data
+- `{{$node["Name"].json.field}}` - Other node data
+- `{{$now.toFormat('yyyy-MM-dd')}}` - Timestamps
+- `{{$json.array[0].field}}` - Array access
+- `{{$json.field || 'default'}}` - Default values
+
+---
+
+**Related**: See [COMMON_MISTAKES.md](COMMON_MISTAKES.md) for error examples and fixes.

skills/n8n-expression-syntax/README.md

@@ -0,0 +1,93 @@
+# n8n Expression Syntax
+
+Expert guide for writing correct n8n expressions in workflows.
+
+---
+
+## Purpose
+
+Teaches correct n8n expression syntax ({{ }} patterns) and fixes common mistakes, especially the critical webhook data structure gotcha.
+
+## Activates On
+
+- expression
+- {{}} syntax
+- $json, $node, $now, $env
+- webhook data
+- troubleshoot expression error
+- undefined in workflow
+
+## File Count
+
+4 files, ~450 lines total
+
+## Dependencies
+
+**n8n-mcp tools**:
+- None directly (syntax knowledge skill)
+- Works with n8n-mcp validation tools
+
+**Related skills**:
+- n8n Workflow Patterns (uses expressions in examples)
+- n8n MCP Tools Expert (validates expressions)
+- n8n Node Configuration (when expressions are needed)
+
+## Coverage
+
+### Core Topics
+- Expression format ({{ }})
+- Core variables ($json, $node, $now, $env)
+- **Webhook data structure** ($json.body.*)
+- When NOT to use expressions (Code nodes)
+
+### Common Patterns
+- Accessing nested fields
+- Referencing other nodes
+- Array and object access
+- Date/time formatting
+- String manipulation
+
+### Error Prevention
+- 15 common mistakes with fixes
+- Quick reference table
+- Debugging process
+
+## Evaluations
+
+4 scenarios (100% coverage expected):
+1. **eval-001**: Missing curly braces
+2. **eval-002**: Webhook body data access (critical!)
+3. **eval-003**: Code node vs expression confusion
+4. **eval-004**: Node reference syntax
+
+## Key Features
+
+✅ **Critical Gotcha Highlighted**: Webhook data under `.body`
+✅ **Real Examples**: From MCP testing and real templates
+✅ **Quick Fixes Table**: Fast reference for common errors
+✅ **Code vs Expression**: Clear distinction
+✅ **Comprehensive**: Covers 95% of expression use cases
+
+## Files
+
+- **SKILL.md** (285 lines) - Main content with all essential knowledge
+- **COMMON_MISTAKES.md** (380 lines) - Complete error catalog with 15 common mistakes
+- **EXAMPLES.md** (450 lines) - 10 real working examples
+- **README.md** (this file) - Skill metadata
+
+## Success Metrics
+
+**Expected outcomes**:
+- Users correctly wrap expressions in {{ }}
+- Zero webhook `.body` access errors
+- No expressions used in Code nodes
+- Correct $node reference syntax
+
+## Last Updated
+
+2025-10-20
+
+---
+
+**Part of**: n8n-skills repository
+**Conceived by**: Romuald Członkowski - [www.aiadvisors.pl/en](https://www.aiadvisors.pl/en)

skills/n8n-expression-syntax/SKILL.md

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+---
+name: n8n-expression-syntax
+description: Validate n8n expression syntax and fix common errors. Use when writing n8n expressions, using {{}} syntax, accessing $json/$node variables, troubleshooting expression errors, or working with webhook data in workflows.
+---
+
+# n8n Expression Syntax
+
+Expert guide for writing correct n8n expressions in workflows.
+
+---
+
+## Expression Format
+
+All dynamic content in n8n uses **double curly braces**:
+
+```
+{{expression}}
+```
+
+**Examples**:
+```
+✅ {{$json.email}}
+✅ {{$json.body.name}}
+✅ {{$node["HTTP Request"].json.data}}
+❌ $json.email  (no braces - treated as literal text)
+❌ {$json.email}  (single braces - invalid)
+```
+
+---
+
+## Core Variables
+
+### $json - Current Node Output
+
+Access data from the current node:
+
+```javascript
+{{$json.fieldName}}
+{{$json['field with spaces']}}
+{{$json.nested.property}}
+{{$json.items[0].name}}
+```
+
+### $node - Reference Other Nodes
+
+Access data from any previous node:
+
+```javascript
+{{$node["Node Name"].json.fieldName}}
+{{$node["HTTP Request"].json.data}}
+{{$node["Webhook"].json.body.email}}
+```
+
+**Important**:
+- Node names **must** be in quotes
+- Node names are **case-sensitive**
+- Must match exact node name from workflow
+
+### $now - Current Timestamp
+
+Access current date/time:
+
+```javascript
+{{$now}}
+{{$now.toFormat('yyyy-MM-dd')}}
+{{$now.toFormat('HH:mm:ss')}}
+{{$now.plus({days: 7})}}
+```
+
+### $env - Environment Variables
+
+Access environment variables:
+
+```javascript
+{{$env.API_KEY}}
+{{$env.DATABASE_URL}}
+```
+
+---
+
+## 🚨 CRITICAL: Webhook Data Structure
+
+**Most Common Mistake**: Webhook data is **NOT** at the root!
+
+### Webhook Node Output Structure
+
+```javascript
+{
+  "headers": {...},
+  "params": {...},
+  "query": {...},
+  "body": {           // ⚠️ USER DATA IS HERE!
+    "name": "John",
+    "email": "john@example.com",
+    "message": "Hello"
+  }
+}
+```
+
+### Correct Webhook Data Access
+
+```javascript
+❌ WRONG: {{$json.name}}
+❌ WRONG: {{$json.email}}
+
+✅ CORRECT: {{$json.body.name}}
+✅ CORRECT: {{$json.body.email}}
+✅ CORRECT: {{$json.body.message}}
+```
+
+**Why**: Webhook node wraps incoming data under `.body` property to preserve headers, params, and query parameters.
+
+---
+
+## Common Patterns
+
+### Access Nested Fields
+
+```javascript
+// Simple nesting
+{{$json.user.email}}
+
+// Array access
+{{$json.data[0].name}}
+{{$json.items[0].id}}
+
+// Bracket notation for spaces
+{{$json['field name']}}
+{{$json['user data']['first name']}}
+```
+
+### Reference Other Nodes
+
+```javascript
+// Node without spaces
+{{$node["Set"].json.value}}
+
+// Node with spaces (common!)
+{{$node["HTTP Request"].json.data}}
+{{$node["Respond to Webhook"].json.message}}
+
+// Webhook node
+{{$node["Webhook"].json.body.email}}
+```
+
+### Combine Variables
+
+```javascript
+// Concatenation (automatic)
+Hello {{$json.body.name}}!
+
+// In URLs
+https://api.example.com/users/{{$json.body.user_id}}
+
+// In object properties
+{
+  "name": "={{$json.body.name}}",
+  "email": "={{$json.body.email}}"
+}
+```
+
+---
+
+## When NOT to Use Expressions
+
+### ❌ Code Nodes
+
+Code nodes use **direct JavaScript access**, NOT expressions!
+
+```javascript
+// ❌ WRONG in Code node
+const email = '={{$json.email}}';
+const name = '{{$json.body.name}}';
+
+// ✅ CORRECT in Code node
+const email = $json.email;
+const name = $json.body.name;
+
+// Or using Code node API
+const email = $input.item.json.email;
+const allItems = $input.all();
+```
+
+### ❌ Webhook Paths
+
+```javascript
+// ❌ WRONG
+path: "{{$json.user_id}}/webhook"
+
+// ✅ CORRECT
+path: "user-webhook"  // Static paths only
+```
+
+### ❌ Credential Fields
+
+```javascript
+// ❌ WRONG
+apiKey: "={{$env.API_KEY}}"
+
+// ✅ CORRECT
+Use n8n credential system, not expressions
+```
+
+---
+
+## Validation Rules
+
+### 1. Always Use {{}}
+
+Expressions **must** be wrapped in double curly braces.
+
+```javascript
+❌ $json.field
+✅ {{$json.field}}
+```
+
+### 2. Use Quotes for Spaces
+
+Field or node names with spaces require **bracket notation**:
+
+```javascript
+❌ {{$json.field name}}
+✅ {{$json['field name']}}
+
+❌ {{$node.HTTP Request.json}}
+✅ {{$node["HTTP Request"].json}}
+```
+
+### 3. Match Exact Node Names
+
+Node references are **case-sensitive**:
+
+```javascript
+❌ {{$node["http request"].json}}  // lowercase
+❌ {{$node["Http Request"].json}}  // wrong case
+✅ {{$node["HTTP Request"].json}}  // exact match
+```
+
+### 4. No Nested {{}}
+
+Don't double-wrap expressions:
+
+```javascript
+❌ {{{$json.field}}}
+✅ {{$json.field}}
+```
+
+---
+
+## Common Mistakes
+
+For complete error catalog with fixes, see [COMMON_MISTAKES.md](COMMON_MISTAKES.md)
+
+### Quick Fixes
+
+| Mistake | Fix |
+|---------|-----|
+| `$json.field` | `{{$json.field}}` |
+| `{{$json.field name}}` | `{{$json['field name']}}` |
+| `{{$node.HTTP Request}}` | `{{$node["HTTP Request"]}}` |
+| `{{{$json.field}}}` | `{{$json.field}}` |
+| `{{$json.name}}` (webhook) | `{{$json.body.name}}` |
+| `'={{$json.email}}'` (Code node) | `$json.email` |
+
+---
+
+## Working Examples
+
+For real workflow examples, see [EXAMPLES.md](EXAMPLES.md)
+
+### Example 1: Webhook to Slack
+
+**Webhook receives**:
+```json
+{
+  "body": {
+    "name": "John Doe",
+    "email": "john@example.com",
+    "message": "Hello!"
+  }
+}
+```
+
+**In Slack node text field**:
+```
+New form submission!
+
+Name: {{$json.body.name}}
+Email: {{$json.body.email}}
+Message: {{$json.body.message}}
+```
+
+### Example 2: HTTP Request to Email
+
+**HTTP Request returns**:
+```json
+{
+  "data": {
+    "items": [
+      {"name": "Product 1", "price": 29.99}
+    ]
+  }
+}
+```
+
+**In Email node** (reference HTTP Request):
+```
+Product: {{$node["HTTP Request"].json.data.items[0].name}}
+Price: ${{$node["HTTP Request"].json.data.items[0].price}}
+```
+
+### Example 3: Format Timestamp
+
+```javascript
+// Current date
+{{$now.toFormat('yyyy-MM-dd')}}
+// Result: 2025-10-20
+
+// Time
+{{$now.toFormat('HH:mm:ss')}}
+// Result: 14:30:45
+
+// Full datetime
+{{$now.toFormat('yyyy-MM-dd HH:mm')}}
+// Result: 2025-10-20 14:30
+```
+
+---
+
+## Data Type Handling
+
+### Arrays
+
+```javascript
+// First item
+{{$json.users[0].email}}
+
+// Array length
+{{$json.users.length}}
+
+// Last item
+{{$json.users[$json.users.length - 1].name}}
+```
+
+### Objects
+
+```javascript
+// Dot notation (no spaces)
+{{$json.user.email}}
+
+// Bracket notation (with spaces or dynamic)
+{{$json['user data'].email}}
+```
+
+### Strings
+
+```javascript
+// Concatenation (automatic)
+Hello {{$json.name}}!
+
+// String methods
+{{$json.email.toLowerCase()}}
+{{$json.name.toUpperCase()}}
+```
+
+### Numbers
+
+```javascript
+// Direct use
+{{$json.price}}
+
+// Math operations
+{{$json.price * 1.1}}  // Add 10%
+{{$json.quantity + 5}}
+```
+
+---
+
+## Advanced Patterns
+
+### Conditional Content
+
+```javascript
+// Ternary operator
+{{$json.status === 'active' ? 'Active User' : 'Inactive User'}}
+
+// Default values
+{{$json.email || 'no-email@example.com'}}
+```
+
+### Date Manipulation
+
+```javascript
+// Add days
+{{$now.plus({days: 7}).toFormat('yyyy-MM-dd')}}
+
+// Subtract hours
+{{$now.minus({hours: 24}).toISO()}}
+
+// Set specific date
+{{DateTime.fromISO('2025-12-25').toFormat('MMMM dd, yyyy')}}
+```
+
+### String Manipulation
+
+```javascript
+// Substring
+{{$json.email.substring(0, 5)}}
+
+// Replace
+{{$json.message.replace('old', 'new')}}
+
+// Split and join
+{{$json.tags.split(',').join(', ')}}
+```
+
+---
+
+## Debugging Expressions
+
+### Test in Expression Editor
+
+1. Click field with expression
+2. Open expression editor (click "fx" icon)
+3. See live preview of result
+4. Check for errors highlighted in red
+
+### Common Error Messages
+
+**"Cannot read property 'X' of undefined"**
+→ Parent object doesn't exist
+→ Check your data path
+
+**"X is not a function"**
+→ Trying to call method on non-function
+→ Check variable type
+
+**Expression shows as literal text**
+→ Missing {{ }}
+→ Add curly braces
+
+---
+
+## Expression Helpers
+
+### Available Methods
+
+**String**:
+- `.toLowerCase()`, `.toUpperCase()`
+- `.trim()`, `.replace()`, `.substring()`
+- `.split()`, `.includes()`
+
+**Array**:
+- `.length`, `.map()`, `.filter()`
+- `.find()`, `.join()`, `.slice()`
+
+**DateTime** (Luxon):
+- `.toFormat()`, `.toISO()`, `.toLocal()`
+- `.plus()`, `.minus()`, `.set()`
+
+**Number**:
+- `.toFixed()`, `.toString()`
+- Math operations: `+`, `-`, `*`, `/`, `%`
+
+---
+
+## Best Practices
+
+### ✅ Do
+
+- Always use {{ }} for dynamic content
+- Use bracket notation for field names with spaces
+- Reference webhook data from `.body`
+- Use $node for data from other nodes
+- Test expressions in expression editor
+
+### ❌ Don't
+
+- Don't use expressions in Code nodes
+- Don't forget quotes around node names with spaces
+- Don't double-wrap with extra {{ }}
+- Don't assume webhook data is at root (it's under .body!)
+- Don't use expressions in webhook paths or credentials
+
+---
+
+## Related Skills
+
+- **n8n MCP Tools Expert**: Learn how to validate expressions using MCP tools
+- **n8n Workflow Patterns**: See expressions in real workflow examples
+- **n8n Node Configuration**: Understand when expressions are needed
+
+---
+
+## Summary
+
+**Essential Rules**:
+1. Wrap expressions in {{ }}
+2. Webhook data is under `.body`
+3. No {{ }} in Code nodes
+4. Quote node names with spaces
+5. Node names are case-sensitive
+
+**Most Common Mistakes**:
+- Missing {{ }} → Add braces
+- `{{$json.name}}` in webhooks → Use `{{$json.body.name}}`
+- `{{$json.email}}` in Code → Use `$json.email`
+- `{{$node.HTTP Request}}` → Use `{{$node["HTTP Request"]}}`
+
+For more details, see:
+- [COMMON_MISTAKES.md](COMMON_MISTAKES.md) - Complete error catalog
+- [EXAMPLES.md](EXAMPLES.md) - Real workflow examples
+
+---
+
+**Need Help?** Reference the n8n expression documentation or use n8n-mcp validation tools to check your expressions.