Guard all prompt_async paths against empty bodies

scripts/opencode-builder.sh

@@ -254,6 +254,14 @@ send_message_capture() {
   local body="$2"
   local out_file="$3"
 
+  # Refuse to send empty bodies — the server logs them as
+  # "No user message found in stream. This should never happen."
+  if [[ -z "$body" ]]; then
+    log_warn "send_message_capture: refusing to send empty body to ${sid}"
+    : > "$out_file" 2>/dev/null || true
+    return 1
+  fi
+
   # Use prompt_async — fire-and-forget; server returns 204 immediately.
   # The synchronous /message endpoint would block until the agent finishes
   # its full response (potentially many minutes for a busy worker).
@@ -640,6 +648,13 @@ Evaluate the health of this session."
   printf '%s' "$eval_prompt" > "$eval_tmp"
   local eval_body
   eval_body=$(jq -nc --arg a "worker-health-evaluator" --rawfile t "$eval_tmp" '{agent: $a, parts: [{type: "text", text: $t}]}')
+  if [[ -z "$eval_body" ]]; then
+    # jq failed (likely couldn't read eval_tmp) — bail rather than send empty
+    rm -f "$eval_tmp"
+    delete_session "$eval_sid"
+    echo "ok"
+    return
+  fi
   curl -sf -X POST "${BASE}/session/${eval_sid}/prompt_async" -H "Content-Type: application/json" -d "$eval_body" >/dev/null 2>&1
   local eval_deadline=$(( $(date +%s) + 120 ))
   while [[ $(date +%s) -lt $eval_deadline ]]; do
@@ -1204,6 +1219,14 @@ main_loop() {
 
               printf '%s' "$prompt_text" > "$tmp_prompt"
 
+              # Belt-and-braces: confirm the file actually has content on disk
+              # before launching the session.
+              if [[ ! -s "$tmp_prompt" ]]; then
+                log_warn "[pool:${prefix}]   prompt file empty after write for ${kind} ${work_num} (${agent}) — skipping launch"
+                rm -f "$tmp_prompt"
+                continue
+              fi
+
               $SESSION_START --server "${BASE}" --tag "$worker_tag" --agent "$agent" --prompt-file "$tmp_prompt" --restart \
                 >/dev/null 2>&1 &
               pids+=($!)