diff --git a/dev/phase-test.sh b/dev/phase-test.sh
index 3907ee6..438a80c 100755
--- a/dev/phase-test.sh
+++ b/dev/phase-test.sh
@@ -167,31 +167,61 @@ else
   fail "needs_human mtime guard: expected 1 notify call, got $NOTIFY_COUNT"
 fi
 
-# ── Test 9: PostToolUse hook writes marker on phase file reference ────────
+# ── Test 9: PostToolUse hook detects writes, ignores reads ────────────────
 HOOK_SCRIPT="$(dirname "$0")/../lib/hooks/on-phase-change.sh"
 MARKER_FILE="/tmp/phase-changed-test-session.marker"
 rm -f "$MARKER_FILE"
 
 if [ -x "$HOOK_SCRIPT" ]; then
-  # Simulate hook input that references the phase file
-  echo "{\"tool_input\":{\"command\":\"echo PHASE:awaiting_ci > ${PHASE_FILE}\"}}" \
-    | bash "$HOOK_SCRIPT" "$PHASE_FILE" "$MARKER_FILE"
+  # 9a: Bash redirect to phase file → marker written
+  printf '{"tool_name":"Bash","tool_input":{"command":"echo PHASE:awaiting_ci > %s"}}' \
+    "$PHASE_FILE" | "$HOOK_SCRIPT" "$PHASE_FILE" "$MARKER_FILE"
   if [ -f "$MARKER_FILE" ]; then
-    ok "PostToolUse hook writes marker when phase file referenced"
+    ok "PostToolUse hook writes marker on Bash redirect to phase file"
   else
-    fail "PostToolUse hook did not write marker"
+    fail "PostToolUse hook did not write marker on Bash redirect"
   fi
   rm -f "$MARKER_FILE"
 
-  # Simulate hook input that does NOT reference the phase file
-  echo "{\"tool_input\":{\"command\":\"echo hello > /tmp/other-file\"}}" \
-    | bash "$HOOK_SCRIPT" "$PHASE_FILE" "$MARKER_FILE"
+  # 9b: Write tool targeting phase file → marker written
+  printf '{"tool_name":"Write","tool_input":{"file_path":"%s","content":"PHASE:done"}}' \
+    "$PHASE_FILE" | "$HOOK_SCRIPT" "$PHASE_FILE" "$MARKER_FILE"
+  if [ -f "$MARKER_FILE" ]; then
+    ok "PostToolUse hook writes marker on Write tool to phase file"
+  else
+    fail "PostToolUse hook did not write marker on Write tool"
+  fi
+  rm -f "$MARKER_FILE"
+
+  # 9c: Bash read of phase file (cat) → NO marker (not a write)
+  printf '{"tool_name":"Bash","tool_input":{"command":"cat %s"}}' \
+    "$PHASE_FILE" | "$HOOK_SCRIPT" "$PHASE_FILE" "$MARKER_FILE"
+  if [ ! -f "$MARKER_FILE" ]; then
+    ok "PostToolUse hook ignores Bash read of phase file (no false positive)"
+  else
+    fail "PostToolUse hook wrote marker for Bash read (false positive)"
+  fi
+  rm -f "$MARKER_FILE"
+
+  # 9d: Unrelated Bash command → NO marker
+  printf '{"tool_name":"Bash","tool_input":{"command":"echo hello > /tmp/other-file"}}' \
+    | "$HOOK_SCRIPT" "$PHASE_FILE" "$MARKER_FILE"
   if [ ! -f "$MARKER_FILE" ]; then
     ok "PostToolUse hook skips marker for unrelated operations"
   else
     fail "PostToolUse hook wrote marker for unrelated operation (false positive)"
   fi
   rm -f "$MARKER_FILE"
+
+  # 9e: Write tool targeting different file → NO marker
+  printf '{"tool_name":"Write","tool_input":{"file_path":"/tmp/other-file","content":"hello"}}' \
+    | "$HOOK_SCRIPT" "$PHASE_FILE" "$MARKER_FILE"
+  if [ ! -f "$MARKER_FILE" ]; then
+    ok "PostToolUse hook skips marker for Write to different file"
+  else
+    fail "PostToolUse hook wrote marker for Write to different file (false positive)"
+  fi
+  rm -f "$MARKER_FILE"
 else
   fail "PostToolUse hook script not found or not executable: $HOOK_SCRIPT"
 fi
diff --git a/lib/agent-session.sh b/lib/agent-session.sh
index b9b8f7f..95dc7b5 100644
--- a/lib/agent-session.sh
+++ b/lib/agent-session.sh
@@ -110,8 +110,8 @@ create_agent_session() {
           }
         }' > "$settings"
       fi
+      rm -f "$phase_marker"
     fi
-    rm -f "$phase_marker"
   fi
 
   rm -f "$idle_marker"
diff --git a/lib/hooks/on-phase-change.sh b/lib/hooks/on-phase-change.sh
index b402d01..ab2c17f 100755
--- a/lib/hooks/on-phase-change.sh
+++ b/lib/hooks/on-phase-change.sh
@@ -2,9 +2,9 @@
 # on-phase-change.sh — PostToolUse hook for phase file write detection.
 #
 # Called by Claude Code after every Bash|Write tool execution.
-# Checks if the tool input references the phase file path and, if so,
-# writes a "phase-changed" timestamp marker so monitor_phase_loop can
-# react immediately instead of waiting for the next mtime-based poll.
+# Detects writes (not reads) to the phase file and writes a timestamp
+# marker so monitor_phase_loop can react immediately instead of waiting
+# for the next mtime-based poll.
 #
 # Usage (in .claude/settings.json):
 #   {"type": "command", "command": "this-script /path/to/phase-file /path/to/marker"}
@@ -14,11 +14,30 @@
 phase_file="${1:-}"
 marker_file="${2:-}"
 
+[ -z "$phase_file" ] && exit 0
+[ -z "$marker_file" ] && exit 0
+
 input=$(cat)  # consume hook JSON from stdin
 
-[ -z "$phase_file" ] || [ -z "$marker_file" ] && exit 0
+# Fast path: skip if phase file not referenced at all
+printf '%s' "$input" | grep -qF "$phase_file" || exit 0
 
-# Check if the tool input references the phase file path
-if printf '%s' "$input" | grep -qF "$phase_file"; then
-  date +%s > "$marker_file"
-fi
+# Parse tool type and detect writes only (ignore reads like cat/head)
+tool_name=$(printf '%s' "$input" | jq -r '.tool_name // empty' 2>/dev/null)
+
+case "$tool_name" in
+  Write)
+    # Write tool: check if file_path targets the phase file
+    file_path=$(printf '%s' "$input" | jq -r '.tool_input.file_path // empty' 2>/dev/null)
+    [ "$file_path" = "$phase_file" ] && date +%s > "$marker_file"
+    ;;
+  Bash)
+    # Bash tool: check if the decoded command contains a redirect (>)
+    # targeting the phase file — distinguishes writes from reads
+    command_str=$(printf '%s' "$input" | jq -r '.tool_input.command // empty' 2>/dev/null)
+    if printf '%s' "$command_str" | grep -qF "$phase_file" \
+       && printf '%s' "$command_str" | grep -q '>'; then
+      date +%s > "$marker_file"
+    fi
+    ;;
+esac