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skills/n8n-expression-syntax/README.md

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+# 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)