diff --git a/plugins/contract-validator/hooks/breaking-change-check.sh b/plugins/contract-validator/hooks/breaking-change-check.sh
new file mode 100755
index 0000000..a66664c
--- /dev/null
+++ b/plugins/contract-validator/hooks/breaking-change-check.sh
@@ -0,0 +1,174 @@
+#!/bin/bash
+# contract-validator breaking change detection hook
+# Warns when plugin interface changes might break consumers
+# This is a PostToolUse hook - non-blocking, warnings only
+
+PREFIX="[contract-validator]"
+
+# Check if warnings are enabled (default: true)
+if [[ "${CONTRACT_VALIDATOR_BREAKING_WARN:-true}" != "true" ]]; then
+    exit 0
+fi
+
+# Read tool input from stdin
+INPUT=$(cat)
+
+# Extract file_path from JSON input
+FILE_PATH=$(echo "$INPUT" | grep -o '"file_path"[[:space:]]*:[[:space:]]*"[^"]*"' | head -1 | sed 's/.*"file_path"[[:space:]]*:[[:space:]]*"\([^"]*\)".*/\1/')
+
+# If no file_path found, exit silently
+if [ -z "$FILE_PATH" ]; then
+    exit 0
+fi
+
+# Check if file is a plugin interface file
+is_interface_file() {
+    local file="$1"
+
+    case "$file" in
+        */plugin.json) return 0 ;;
+        */.claude-plugin/plugin.json) return 0 ;;
+        */hooks.json) return 0 ;;
+        */hooks/hooks.json) return 0 ;;
+        */.mcp.json) return 0 ;;
+        */agents/*.md) return 0 ;;
+        */commands/*.md) return 0 ;;
+        */skills/*.md) return 0 ;;
+    esac
+
+    return 1
+}
+
+# Exit if not an interface file
+if ! is_interface_file "$FILE_PATH"; then
+    exit 0
+fi
+
+# Check if file exists and is in a git repo
+if [[ ! -f "$FILE_PATH" ]]; then
+    exit 0
+fi
+
+# Get the directory containing the file
+FILE_DIR=$(dirname "$FILE_PATH")
+FILE_NAME=$(basename "$FILE_PATH")
+
+# Try to get the previous version from git
+cd "$FILE_DIR" 2>/dev/null || exit 0
+
+# Check if we're in a git repo
+if ! git rev-parse --git-dir > /dev/null 2>&1; then
+    exit 0
+fi
+
+# Get previous version (HEAD version before current changes)
+PREV_CONTENT=$(git show HEAD:"$FILE_PATH" 2>/dev/null || echo "")
+
+# If no previous version, this is a new file - no breaking changes possible
+if [ -z "$PREV_CONTENT" ]; then
+    exit 0
+fi
+
+# Read current content
+CURR_CONTENT=$(cat "$FILE_PATH" 2>/dev/null || echo "")
+
+if [ -z "$CURR_CONTENT" ]; then
+    exit 0
+fi
+
+BREAKING_CHANGES=()
+
+# Detect breaking changes based on file type
+case "$FILE_PATH" in
+    */plugin.json|*/.claude-plugin/plugin.json)
+        # Check for removed or renamed fields in plugin.json
+
+        # Check if name changed
+        PREV_NAME=$(echo "$PREV_CONTENT" | grep -o '"name"[[:space:]]*:[[:space:]]*"[^"]*"' | head -1)
+        CURR_NAME=$(echo "$CURR_CONTENT" | grep -o '"name"[[:space:]]*:[[:space:]]*"[^"]*"' | head -1)
+        if [ -n "$PREV_NAME" ] && [ "$PREV_NAME" != "$CURR_NAME" ]; then
+            BREAKING_CHANGES+=("Plugin name changed - consumers may need updates")
+        fi
+
+        # Check if version had major bump (semantic versioning)
+        PREV_VER=$(echo "$PREV_CONTENT" | grep -o '"version"[[:space:]]*:[[:space:]]*"[^"]*"' | sed 's/.*"\([0-9]*\)\..*/\1/')
+        CURR_VER=$(echo "$CURR_CONTENT" | grep -o '"version"[[:space:]]*:[[:space:]]*"[^"]*"' | sed 's/.*"\([0-9]*\)\..*/\1/')
+        if [ -n "$PREV_VER" ] && [ -n "$CURR_VER" ] && [ "$CURR_VER" -gt "$PREV_VER" ] 2>/dev/null; then
+            BREAKING_CHANGES+=("Major version bump detected - verify breaking changes documented")
+        fi
+        ;;
+
+    */hooks.json|*/hooks/hooks.json)
+        # Check for removed hook events
+        PREV_EVENTS=$(echo "$PREV_CONTENT" | grep -oE '"(PreToolUse|PostToolUse|UserPromptSubmit|SessionStart|SessionEnd|Notification|Stop|SubagentStop|PreCompact)"' | sort -u)
+        CURR_EVENTS=$(echo "$CURR_CONTENT" | grep -oE '"(PreToolUse|PostToolUse|UserPromptSubmit|SessionStart|SessionEnd|Notification|Stop|SubagentStop|PreCompact)"' | sort -u)
+
+        # Find removed events
+        REMOVED_EVENTS=$(comm -23 <(echo "$PREV_EVENTS") <(echo "$CURR_EVENTS") 2>/dev/null)
+        if [ -n "$REMOVED_EVENTS" ]; then
+            BREAKING_CHANGES+=("Hook events removed: $(echo $REMOVED_EVENTS | tr '\n' ' ')")
+        fi
+
+        # Check for changed matchers
+        PREV_MATCHERS=$(echo "$PREV_CONTENT" | grep -o '"matcher"[[:space:]]*:[[:space:]]*"[^"]*"' | sort -u)
+        CURR_MATCHERS=$(echo "$CURR_CONTENT" | grep -o '"matcher"[[:space:]]*:[[:space:]]*"[^"]*"' | sort -u)
+        if [ "$PREV_MATCHERS" != "$CURR_MATCHERS" ]; then
+            BREAKING_CHANGES+=("Hook matchers changed - verify tool coverage")
+        fi
+        ;;
+
+    */.mcp.json)
+        # Check for removed MCP servers
+        PREV_SERVERS=$(echo "$PREV_CONTENT" | grep -o '"[^"]*"[[:space:]]*:' | grep -v "mcpServers" | sort -u)
+        CURR_SERVERS=$(echo "$CURR_CONTENT" | grep -o '"[^"]*"[[:space:]]*:' | grep -v "mcpServers" | sort -u)
+
+        REMOVED_SERVERS=$(comm -23 <(echo "$PREV_SERVERS") <(echo "$CURR_SERVERS") 2>/dev/null)
+        if [ -n "$REMOVED_SERVERS" ]; then
+            BREAKING_CHANGES+=("MCP servers removed - tools may be unavailable")
+        fi
+        ;;
+
+    */agents/*.md)
+        # Check if agent file was significantly reduced (might indicate removal of capabilities)
+        PREV_LINES=$(echo "$PREV_CONTENT" | wc -l)
+        CURR_LINES=$(echo "$CURR_CONTENT" | wc -l)
+
+        # If more than 50% reduction, warn
+        if [ "$PREV_LINES" -gt 10 ] && [ "$CURR_LINES" -lt $((PREV_LINES / 2)) ]; then
+            BREAKING_CHANGES+=("Agent definition significantly reduced - capabilities may be removed")
+        fi
+
+        # Check if agent name/description changed in frontmatter
+        PREV_DESC=$(echo "$PREV_CONTENT" | head -20 | grep -i "description" | head -1)
+        CURR_DESC=$(echo "$CURR_CONTENT" | head -20 | grep -i "description" | head -1)
+        if [ -n "$PREV_DESC" ] && [ "$PREV_DESC" != "$CURR_DESC" ]; then
+            BREAKING_CHANGES+=("Agent description changed - verify consumer expectations")
+        fi
+        ;;
+
+    */commands/*.md|*/skills/*.md)
+        # Check if command/skill was significantly changed
+        PREV_LINES=$(echo "$PREV_CONTENT" | wc -l)
+        CURR_LINES=$(echo "$CURR_CONTENT" | wc -l)
+
+        if [ "$PREV_LINES" -gt 10 ] && [ "$CURR_LINES" -lt $((PREV_LINES / 2)) ]; then
+            BREAKING_CHANGES+=("Command/skill significantly reduced - behavior may change")
+        fi
+        ;;
+esac
+
+# Output warnings if any breaking changes detected
+if [[ ${#BREAKING_CHANGES[@]} -gt 0 ]]; then
+    echo ""
+    echo "$PREFIX WARNING: Potential breaking changes in $(basename "$FILE_PATH")"
+    echo "$PREFIX ============================================"
+    for change in "${BREAKING_CHANGES[@]}"; do
+        echo "$PREFIX   - $change"
+    done
+    echo "$PREFIX ============================================"
+    echo "$PREFIX Consider updating CHANGELOG and notifying consumers"
+    echo ""
+fi
+
+# Always exit 0 - non-blocking
+exit 0
diff --git a/plugins/contract-validator/hooks/hooks.json b/plugins/contract-validator/hooks/hooks.json
index baf09c1..a37bf96 100644
--- a/plugins/contract-validator/hooks/hooks.json
+++ b/plugins/contract-validator/hooks/hooks.json
@@ -5,6 +5,17 @@
         "type": "command",
         "command": "${CLAUDE_PLUGIN_ROOT}/hooks/auto-validate.sh"
       }
+    ],
+    "PostToolUse": [
+      {
+        "matcher": "Edit|Write",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/breaking-change-check.sh"
+          }
+        ]
+      }
     ]
   }
 }