Document v1.1.5 Twig filter bug fix in CLAUDE.md

Added comprehensive session history for v1.1.5 including:
- Root cause analysis of Twig filter vs function mismatch
- Error log evidence and backtrace analysis
- Technical solution with code examples
- Lessons learned about multi-plugin Twig compatibility
- Updated "Bugs found" section marking Twig bug as fixed

Also fixed markdown linting warnings:
- Added blank line before fenced code block
- Added language specification (text) to code fence
- Added blank line around list

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -250,7 +250,7 @@ unzip -l wc-composable-product-vX.X.X.zip
 
 ## Bugs found
 
-- There is a bug related to twig in the frontend area. Documented in `logs/fatal-errors*.log`
+- ✅ ~~There is a bug related to twig in the frontend area. Documented in `logs/fatal-errors*.log`~~ **FIXED in v1.1.5**
 - Translate the admin area, too
 
 ## Session History
@@ -741,6 +741,92 @@ Added a dedicated fixed price field to the Composable Options tab that appears/h
 
 ---
 
+### v1.1.5 - Critical Twig Filter Bug Fix (2025-12-31)
+
+#### Session 8: Twig Template Compatibility Fix
+
+**Critical bug fix** resolving template rendering errors when other plugins use Twig.
+
+**The bug:**
+
+Plugin crashed with `Twig\Error\SyntaxError: Unknown "esc_attr" filter` when rendering the product selector template on the frontend.
+
+**Root cause analysis:**
+
+1. **Filter vs Function mismatch**: The template used filter syntax (`{{ product.name|esc_attr }}`), but WordPress escaping functions were only registered as Twig **functions**, not **filters**
+2. **Plugin conflict**: When another plugin (e.g., WooCommerce Tier and Package Prices) bundles its own Twig installation, it may parse our templates with its Twig instance
+3. **Missing registrations**: That external Twig instance didn't have our custom filters registered, causing the "Unknown filter" error
+
+**Error log evidence:**
+
+From [logs/fatal-errors-2025-12-31.log:5](logs/fatal-errors-2025-12-31.log#L5):
+
+```text
+Uncaught Twig\Error\SyntaxError: Unknown "esc_attr" filter in "product-selector.twig" at line 26
+```
+
+The backtrace showed the error originated from `/wp-content/plugins/wc-tier-and-package-prices/vendor/twig/twig/`, proving another plugin's Twig instance was parsing our template.
+
+**The fix:**
+
+Added Twig filter registrations alongside existing function registrations in [includes/Plugin.php:88-91](includes/Plugin.php#L88-L91):
+
+```php
+// Add WordPress escaping functions as Twig filters
+$this->twig->addFilter(new \Twig\TwigFilter('esc_html', 'esc_html'));
+$this->twig->addFilter(new \Twig\TwigFilter('esc_attr', 'esc_attr'));
+$this->twig->addFilter(new \Twig\TwigFilter('esc_url', 'esc_url'));
+```
+
+This allows both syntaxes to work:
+
+- Filter syntax: `{{ product.name|esc_attr }}` ✅
+- Function syntax: `{{ esc_attr(product.name) }}` ✅
+
+**Files modified:**
+
+- includes/Plugin.php:
+  - Lines 88-91: Added TwigFilter registrations for WordPress escaping functions
+- wc-composable-product.php:
+  - Lines 6, 22: Version bump to 1.1.5
+- CHANGELOG.md: Added v1.1.5 release notes with technical details
+
+**What works (v1.1.5):**
+
+Everything from v1.1.4 plus:
+
+- Product selector template renders without errors ✅
+- Compatible with plugins that bundle Twig (e.g., pricing plugins) ✅
+- WordPress escaping works with both filter and function syntax ✅
+- No more "Unknown filter" errors ✅
+
+**Key lessons learned:**
+
+1. **Filter vs Function Registration**: In Twig, `{{ value|filter }}` requires `TwigFilter`, while `{{ function(value) }}` requires `TwigFunction` - they're not interchangeable
+2. **Multiple Twig Instances**: WordPress plugins may bundle their own Twig installations that can parse other plugins' templates
+3. **Template Syntax Matters**: Using filter syntax in templates requires filter registration, even if function registration exists
+4. **Defensive Compatibility**: Register WordPress functions as BOTH filters and functions for maximum compatibility
+5. **Error Log Investigation**: Backtrace reveals which Twig instance is parsing the template, crucial for diagnosing multi-plugin conflicts
+6. **Template Location Doesn't Matter**: Even though our template is in our plugin directory, other Twig instances can still parse it during rendering
+
+**Debugging approach:**
+
+1. User mentioned Twig bug in CLAUDE.md "Bugs found" section
+2. Checked `logs/fatal-errors-2025-12-31.log` and found the exact error
+3. Analyzed backtrace showing external Twig instance from another plugin
+4. Examined template and found filter syntax (`|esc_attr`)
+5. Checked Plugin.php and discovered only function registrations existed
+6. Added filter registrations alongside function registrations
+7. Committed fix with detailed explanation
+
+**Impact:**
+
+This was a **critical bug** preventing the plugin from working on sites with certain other WooCommerce extensions installed. Users would see a blank page or error when viewing composable products.
+
+**Status:** Fixed and committed to dev branch (8fc0614)
+
+---
+
 **For AI Assistants:**
 
 When starting a new session on this project: