diff --git a/.agents/scripts/full-loop-helper.sh b/.agents/scripts/full-loop-helper.sh
index 6e464c518..33526f61c 100755
--- a/.agents/scripts/full-loop-helper.sh
+++ b/.agents/scripts/full-loop-helper.sh
@@ -89,6 +89,85 @@ is_headless() {
     [[ "$HEADLESS" == "true" ]]
 }
 
+# Install a git pre-commit hook that blocks TODO.md changes in headless mode (t173)
+# This is a hard guard — even if the AI agent tries to commit TODO.md, git rejects it.
+install_headless_todo_guard() {
+    local git_dir
+    git_dir=$(git rev-parse --git-dir 2>/dev/null || echo "")
+    if [[ -z "$git_dir" ]]; then
+        return 0
+    fi
+
+    local hooks_dir="$git_dir/hooks"
+    local hook_file="$hooks_dir/pre-commit"
+    local guard_marker="# t173-headless-todo-guard"
+
+    mkdir -p "$hooks_dir"
+
+    # If a pre-commit hook already exists, append our guard (if not already present)
+    if [[ -f "$hook_file" ]]; then
+        if grep -q "$guard_marker" "$hook_file" 2>/dev/null; then
+            return 0  # Already installed
+        fi
+        # Append to existing hook
+        cat >> "$hook_file" << 'GUARD'
+
+# t173-headless-todo-guard
+# Block TODO.md and planning file commits in headless worker mode
+if [[ "${FULL_LOOP_HEADLESS:-false}" == "true" ]]; then
+    if git diff --cached --name-only | grep -qE '^(TODO\.md|todo/)'; then
+        echo "[t173 GUARD] BLOCKED: Headless workers must not commit TODO.md or todo/ files."
+        echo "[t173 GUARD] The supervisor owns all TODO.md updates. Put notes in commit messages or PR body."
+        exit 1
+    fi
+fi
+GUARD
+    else
+        # Create new hook
+        cat > "$hook_file" << 'GUARD'
+#!/usr/bin/env bash
+# t173-headless-todo-guard
+# Block TODO.md and planning file commits in headless worker mode
+if [[ "${FULL_LOOP_HEADLESS:-false}" == "true" ]]; then
+    if git diff --cached --name-only | grep -qE '^(TODO\.md|todo/)'; then
+        echo "[t173 GUARD] BLOCKED: Headless workers must not commit TODO.md or todo/ files."
+        echo "[t173 GUARD] The supervisor owns all TODO.md updates. Put notes in commit messages or PR body."
+        exit 1
+    fi
+fi
+GUARD
+        chmod +x "$hook_file"
+    fi
+
+    return 0
+}
+
+# Remove the t173 headless guard from pre-commit hook (cleanup)
+remove_headless_todo_guard() {
+    local git_dir
+    git_dir=$(git rev-parse --git-dir 2>/dev/null || echo "")
+    if [[ -z "$git_dir" ]]; then
+        return 0
+    fi
+
+    local hook_file="$git_dir/hooks/pre-commit"
+    if [[ ! -f "$hook_file" ]]; then
+        return 0
+    fi
+
+    # Remove the guard block (from marker to end of guard)
+    if grep -q "t173-headless-todo-guard" "$hook_file" 2>/dev/null; then
+        # Use sed to remove the guard block
+        local tmp_file
+        tmp_file=$(mktemp)
+        awk '/# t173-headless-todo-guard/{skip=1} /^fi$/ && skip{skip=0; next} !skip' "$hook_file" > "$tmp_file"
+        mv "$tmp_file" "$hook_file"
+        chmod +x "$hook_file"
+    fi
+
+    return 0
+}
+
 print_phase() {
     local phase="$1"
     local description="$2"
@@ -656,6 +735,12 @@ cmd_start() {
         return 1
     fi
 
+    # Install git pre-commit guard to block TODO.md commits in headless mode (t173)
+    if is_headless; then
+        install_headless_todo_guard
+        print_info "HEADLESS: Installed TODO.md commit guard (t173)"
+    fi
+
     # Pre-flight GitHub auth check — workers spawned via nohup/cron may lack
     # SSH keys or valid gh tokens. Fail fast before burning compute.
     if ! gh auth status >/dev/null 2>&1; then
@@ -873,6 +958,9 @@ cmd_cancel() {
     rm -f ".agents/loop-state/quality-loop.local.state" 2>/dev/null
     rm -f ".claude/ralph-loop.local.state" 2>/dev/null
     rm -f ".claude/quality-loop.local.state" 2>/dev/null
+
+    # Remove headless TODO.md guard if installed (t173)
+    remove_headless_todo_guard
     
     print_success "Full loop cancelled"
     return 0
@@ -929,6 +1017,9 @@ cmd_complete() {
     echo ""
     
     clear_state
+
+    # Remove headless TODO.md guard if installed (t173)
+    remove_headless_todo_guard
     
     echo "<promise>FULL_LOOP_COMPLETE</promise>"
     
diff --git a/.agents/scripts/loop-common.sh b/.agents/scripts/loop-common.sh
index 1cd1bfb55..d908f33fc 100755
--- a/.agents/scripts/loop-common.sh
+++ b/.agents/scripts/loop-common.sh
@@ -335,16 +335,23 @@ loop_generate_reanchor() {
     local git_branch
     git_branch=$(git branch --show-current 2>/dev/null || echo "unknown")
     
-    # Get TODO.md in-progress tasks
+    # Detect headless worker mode (t173: workers must not interact with TODO.md)
+    local is_headless="false"
+    if [[ "${FULL_LOOP_HEADLESS:-false}" == "true" ]]; then
+        is_headless="true"
+    fi
+
+    # Get TODO.md in-progress tasks (read-only context, skip in headless mode - t173)
     local todo_in_progress=""
-    if [[ -f "TODO.md" ]]; then
+    if [[ "$is_headless" == "false" && -f "TODO.md" ]]; then
         todo_in_progress=$(grep -A10 "## In Progress" TODO.md 2>/dev/null | head -15 || echo "No tasks in progress")
     fi
     
     # Extract single next task (the "pin" concept from Loom)
     # Focus on ONE task per iteration to reduce context drift
+    # Skip in headless mode — workers work on their assigned task only (t173)
     local next_task=""
-    if [[ -f "TODO.md" ]]; then
+    if [[ "$is_headless" == "false" && -f "TODO.md" ]]; then
         # Get first unchecked task from In Progress section, or first from Backlog
         next_task=$(awk '
             /^## In Progress/,/^##/ { if (/^- \[ \]/) { print; exit } }
@@ -382,12 +389,35 @@ loop_generate_reanchor() {
         latest_receipt=$(cat "$latest_receipt_file")
     fi
     
+    # Build headless worker restriction block (t173)
+    local headless_restriction=""
+    if [[ "$is_headless" == "true" ]]; then
+        headless_restriction="
+## MANDATORY Worker Restrictions (t173 - Headless Mode)
+
+- **Do NOT edit, commit, or push TODO.md** — the supervisor owns all TODO.md updates.
+- **Do NOT edit todo/PLANS.md or todo/tasks/*** — these are supervisor-managed.
+- Report status via exit code, log output, and PR creation only.
+- Put task notes in commit messages or PR body, never in TODO.md.
+- Work ONLY on the assigned task described above. Do not pick tasks from TODO.md.
+"
+    fi
+
+    # Build TODO.md section (omitted in headless mode - t173)
+    local todo_section=""
+    if [[ "$is_headless" == "false" ]]; then
+        todo_section="### TODO.md In Progress
+\`\`\`
+$todo_in_progress
+\`\`\`"
+    fi
+
     # Generate re-anchor prompt with single-task focus
     cat > "$LOOP_REANCHOR_FILE" << EOF
 # Re-Anchor Context (MANDATORY - Read Before Any Work)
 
 **Loop:** $task_id | **Iteration:** $iteration | **Branch:** $git_branch
-
+${headless_restriction}
 ## Original Task
 
 $prompt
@@ -396,7 +426,7 @@ $prompt
 
 Choose the single most important next action. Do NOT try to do everything at once.
 
-${next_task:-"No specific task found in TODO.md - work on the original task above."}
+${next_task:-"Work on the original task above."}
 
 ## Current State
 
@@ -410,10 +440,7 @@ $git_status
 $git_log
 \`\`\`
 
-### TODO.md In Progress
-\`\`\`
-$todo_in_progress
-\`\`\`
+${todo_section}
 
 ## Guardrails (Do Not Repeat These Mistakes)
 
diff --git a/.agents/scripts/supervisor-helper.sh b/.agents/scripts/supervisor-helper.sh
index 46aa9a7d3..42e85116c 100755
--- a/.agents/scripts/supervisor-helper.sh
+++ b/.agents/scripts/supervisor-helper.sh
@@ -2574,10 +2574,21 @@ build_dispatch_cmd() {
     # Include task description in the prompt so the worker knows what to do
     # even if TODO.md doesn't have an entry for this task (t158)
     # Always pass --headless for supervisor-dispatched workers (t174)
+    # Inject explicit TODO.md restriction into worker prompt (t173)
     local prompt="/full-loop $task_id --headless"
     if [[ -n "$description" ]]; then
         prompt="/full-loop $task_id --headless -- $description"
     fi
+
+    # t173: Explicit worker restriction — prevents TODO.md race condition
+    prompt="$prompt
+
+## MANDATORY Worker Restrictions (t173)
+- Do NOT edit, commit, or push TODO.md — the supervisor owns all TODO.md updates.
+- Do NOT edit todo/PLANS.md or todo/tasks/* — these are supervisor-managed.
+- Report status via exit code, log output, and PR creation only.
+- Put task notes in commit messages or PR body, never in TODO.md."
+
     if [[ -n "$memory_context" ]]; then
         prompt="$prompt