diff --git a/bin/session-start-hook.sh b/bin/session-start-hook.sh
new file mode 100755
index 0000000..58f0da6
--- /dev/null
+++ b/bin/session-start-hook.sh
@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+# PostSessionStart hook: auto-start autoresearch when API budget is sufficient.
+# Runs async — must exit quickly.
+set -euo pipefail
+
+CONFIG="${HOME}/.claude/autoresearch.yaml"
+SESSION="autoresearch"
+LOG="${HOME}/.claude/autoresearch/hook.log"
+
+# Only run if config exists
+[[ -f "$CONFIG" ]] || exit 0
+
+# Skip if autoresearch tmux session already running
+tmux has-session -t "$SESSION" 2>/dev/null && exit 0
+
+# Read budget limit from config
+BUDGET_LIMIT=$(python3 -c "
+import yaml, os, sys
+try:
+    cfg = yaml.safe_load(open('${CONFIG}'))
+    print(cfg.get('token_threshold', {}).get('api_budget_usd', 5.0))
+except Exception:
+    print(5.0)
+" 2>/dev/null || echo "5.0")
+
+# Check budget (exits 0 = OK, 1 = low, or fails open)
+python3 "${HOME}/work/autoresearch/bin/check_budget.py" "$BUDGET_LIMIT" || exit 0
+
+# Budget OK — start autoresearch session in background
+mkdir -p "$(dirname "$LOG")"
+nohup "${HOME}/work/autoresearch/bin/start.sh" start \
+    >> "$LOG" 2>&1 &
+
+exit 0