diff --git a/services/capture/src/capture-manager.js b/services/capture/src/capture-manager.js
index 5f1315f..bf57fa5 100644
--- a/services/capture/src/capture-manager.js
+++ b/services/capture/src/capture-manager.js
@@ -4,6 +4,48 @@ import { dirname } from 'node:path';
 import { v4 as uuidv4 } from 'uuid';
 import { createUploadStream } from './s3/client.js';
 
+/**
+ * Reads the first line from a spawned process's stderr stream.
+ * Resolves with the parsed JSON object when the first '\n' arrives.
+ * Rejects if the process exits with a non-zero code before emitting a line,
+ * or if timeoutMs elapses.
+ */
+function readFirstStderrLine(proc, timeoutMs = 35_000) {
+  return new Promise((resolve, reject) => {
+    let buf = '';
+    let settled = false;
+    const settle = (fn) => { if (settled) return; settled = true; fn(); };
+
+    const timer = setTimeout(() => {
+      settle(() => reject(new Error(`deltacast-capture: timed out waiting for format JSON after ${timeoutMs}ms`)));
+    }, timeoutMs);
+
+    proc.stderr.setEncoding('utf8');
+    proc.stderr.on('data', (chunk) => {
+      buf += chunk;
+      const nl = buf.indexOf('\n');
+      if (nl === -1) return;
+      const line = buf.slice(0, nl).trim();
+      clearTimeout(timer);
+      try {
+        const parsed = JSON.parse(line);
+        if (parsed.error) {
+          settle(() => reject(new Error(`deltacast-capture: ${parsed.error}`)));
+        } else {
+          settle(() => resolve(parsed));
+        }
+      } catch (e) {
+        settle(() => reject(new Error(`deltacast-capture: invalid JSON on stderr: ${line}`)));
+      }
+    });
+
+    proc.on('exit', (code) => {
+      clearTimeout(timer);
+      settle(() => reject(new Error(`deltacast-capture: exited with code ${code} before emitting format JSON`)));
+    });
+  });
+}
+
 const S3_BUCKET = process.env.S3_BUCKET || 'wild-dragon';
 
 // Growing-files mode: writes the master to a local SMB-backed share that the