diff --git a/agent_loop.py b/agent_loop.py
index a744f0d26..90266a390 100644
--- a/agent_loop.py
+++ b/agent_loop.py
@@ -41,12 +41,29 @@ def get_pretty_json(data):
 
 def agent_runner_loop(client, system_prompt, user_input, handler, tools_schema, 
                       max_turns=40, verbose=True, initial_user_content=None, yield_info=False):
+    # messages[0] = system, messages[1] = user; agent_before hooks may
+    # override either via the returned dict. Index positions are part of the
+    # contract — if a system message is ever prepended in index 0 (e.g. by a
+    # future tool/system split), update these indices accordingly.
     messages = [
         {"role": "system", "content": system_prompt},
         {"role": "user", "content": initial_user_content if initial_user_content is not None else user_input}
     ]
     turn = 0;  handler.max_turns = max_turns
-    _hook('agent_before', locals())
+    _ctx = _hook('agent_before', locals())
+    if isinstance(_ctx, dict):
+        if 'system_prompt' in _ctx:
+            messages[0]['content'] = _ctx['system_prompt']
+        # initial_user_content takes precedence over user_input when both
+        # are returned by the hook. The caller passed `initial_user_content`
+        # explicitly as the resolved first-turn content; if a plugin
+        # override disagrees, the plugin override wins (plugin is the
+        # last word). This matches the default precedence at call site:
+        # `initial_user_content if initial_user_content is not None else user_input`.
+        if _ctx.get('initial_user_content') is not None:
+            messages[1]['content'] = _ctx['initial_user_content']
+        elif 'user_input' in _ctx and initial_user_content is None:
+            messages[1]['content'] = _ctx['user_input']
     while turn < handler.max_turns:
         turn += 1; turnstr = f'LLM Running (Turn {turn}) ...'
         if handler.parent.task_dir: turnstr = f'Turn {turn} ...'