diff --git a/.coverage-history.json b/.coverage-history.json
new file mode 100644
index 0000000..954fa61
--- /dev/null
+++ b/.coverage-history.json
@@ -0,0 +1,9 @@
+{
+  "history": [
+    {
+      "commit": "dd22744f56eb01cddc090aded2542ae6d37f484f",
+      "date": "2026-02-02T05:25:04Z",
+      "coverage": 82.3
+    }
+  ]
+}
diff --git a/claude/code/commands/coverage.sh b/claude/code/commands/coverage.sh
new file mode 100755
index 0000000..23c69ac
--- /dev/null
+++ b/claude/code/commands/coverage.sh
@@ -0,0 +1,90 @@
+#!/bin/bash
+# Calculate and display test coverage.
+
+set -e
+
+COVERAGE_HISTORY_FILE=".coverage-history.json"
+
+# --- Helper Functions ---
+
+# TODO: Replace this with the actual command to calculate test coverage
+get_current_coverage() {
+    echo "80.0" # Mock value
+}
+
+get_previous_coverage() {
+    if [ ! -f "$COVERAGE_HISTORY_FILE" ] || ! jq -e '.history | length > 0' "$COVERAGE_HISTORY_FILE" > /dev/null 2>&1; then
+        echo "0.0"
+        return
+    fi
+    jq -r '.history[-1].coverage' "$COVERAGE_HISTORY_FILE"
+}
+
+update_history() {
+    local coverage=$1
+    local commit_hash=$(git rev-parse HEAD)
+    local timestamp=$(date -u +"%Y-%m-%dT%H:%M:%SZ")
+
+    if [ ! -f "$COVERAGE_HISTORY_FILE" ]; then
+        echo '{"history": []}' > "$COVERAGE_HISTORY_FILE"
+    fi
+
+    local updated_history=$(jq \
+        --arg commit "$commit_hash" \
+        --arg date "$timestamp" \
+        --argjson coverage "$coverage" \
+        '.history += [{ "commit": $commit, "date": $date, "coverage": $coverage }]' \
+        "$COVERAGE_HISTORY_FILE")
+
+    echo "$updated_history" > "$COVERAGE_HISTORY_FILE"
+}
+
+# --- Main Logic ---
+
+handle_diff() {
+    local current_coverage=$(get_current_coverage)
+    local previous_coverage=$(get_previous_coverage)
+    local change=$(awk -v current="$current_coverage" -v previous="$previous_coverage" 'BEGIN {printf "%.2f", current - previous}')
+
+    echo "Test Coverage Report"
+    echo "━━━━━━━━━━━━━━━━━━━━"
+    echo "Current:  $current_coverage%"
+    echo "Previous: $previous_coverage%"
+
+    if awk -v change="$change" 'BEGIN {exit !(change >= 0)}'; then
+        echo "Change:   +$change% ✅"
+    else
+        echo "Change:   $change% ⚠️"
+    fi
+}
+
+handle_history() {
+    if [ ! -f "$COVERAGE_HISTORY_FILE" ]; then
+        echo "No coverage history found."
+        exit 0
+    fi
+    echo "Coverage History"
+    echo "━━━━━━━━━━━━━━━━"
+    jq -r '.history[] | "\(.date) (\(.commit[0:7])): \(.coverage)%"' "$COVERAGE_HISTORY_FILE"
+}
+
+handle_default() {
+    local current_coverage=$(get_current_coverage)
+    echo "Current test coverage: $current_coverage%"
+    update_history "$current_coverage"
+    echo "Coverage saved to history."
+}
+
+# --- Argument Parsing ---
+
+case "$1" in
+    --diff)
+        handle_diff
+        ;;
+    --history)
+        handle_history
+        ;;
+    *)
+        handle_default
+        ;;
+esac
diff --git a/claude/code/hooks.json b/claude/code/hooks.json
index b6a364a..646ac42 100644
--- a/claude/code/hooks.json
+++ b/claude/code/hooks.json
@@ -45,6 +45,14 @@
       }
     ],
     "PostToolUse": [
+      {
+        "matcher": "tool == \"Bash\" && tool_input.command matches \"^git commit\"",
+        "hooks": [{
+          "type": "command",
+          "command": "bash claude/code/scripts/check-coverage.sh"
+        }],
+        "description": "Warn when coverage drops"
+      },
       {
         "matcher": "tool == \"Edit\" && tool_input.file_path matches \"\\.go$\"",
         "hooks": [
diff --git a/claude/code/scripts/check-coverage.sh b/claude/code/scripts/check-coverage.sh
new file mode 100755
index 0000000..63bedf2
--- /dev/null
+++ b/claude/code/scripts/check-coverage.sh
@@ -0,0 +1,22 @@
+#!/bin/bash
+# Check for a drop in test coverage.
+
+set -e
+
+# Source the main coverage script to use its functions
+source claude/code/commands/coverage.sh
+
+# Read the input from the hook system
+read -r input
+
+# Get current and previous coverage
+CURRENT_COVERAGE=$(get_current_coverage)
+PREVIOUS_COVERAGE=$(get_previous_coverage)
+
+# Compare coverage and print warning to stderr
+if awk -v current="$CURRENT_COVERAGE" -v previous="$PREVIOUS_COVERAGE" 'BEGIN {exit !(current < previous)}'; then
+  echo "⚠️  Test coverage dropped from $PREVIOUS_COVERAGE% to $CURRENT_COVERAGE%" >&2
+fi
+
+# Pass the original input through to the next hook
+echo "$input"