diff --git a/src/agent_learner/adapters/hermes.py b/src/agent_learner/adapters/hermes.py
index 12ef87d..7eb4683 100644
--- a/src/agent_learner/adapters/hermes.py
+++ b/src/agent_learner/adapters/hermes.py
@@ -296,15 +296,20 @@ def _render_hooks_block(*, prompt_command: str, auto_command: str) -> str:
 
 def _merge_hooks_section(existing_text: str, *, prompt_command: str, auto_command: str) -> str:
     lines = existing_text.splitlines()
-    desired = {
-        "pre_llm_call": [f"    - command: {prompt_command!r}", "      timeout: 15"],
-        "on_session_end": [f"    - command: {auto_command!r}", "      timeout: 15"],
+    desired_commands = {
+        "pre_llm_call": prompt_command,
+        "on_session_end": auto_command,
     }
     if not lines:
         return _render_hooks_block(prompt_command=prompt_command, auto_command=auto_command).rstrip("\n")
     if len(lines) == 1 and lines[0].strip() == "hooks: {}":
         return _render_hooks_block(prompt_command=prompt_command, auto_command=auto_command).rstrip("\n")
 
+    def _render_entry(command: str, *, compact: bool) -> list[str]:
+        if compact:
+            return [f"  - command: {command!r}", "    timeout: 15"]
+        return [f"    - command: {command!r}", "      timeout: 15"]
+
     merged: list[str] = ["hooks:"]
     index = 1
     seen_events: set[str] = set()
@@ -323,9 +328,13 @@ def _merge_hooks_section(existing_text: str, *, prompt_command: str, auto_comman
                 index += 1
             block = lines[start:index]
             block_text = "\n".join(block)
-            addition = desired.get(event)
-            if addition and addition[0] not in block_text:
-                block = block + addition
+            command = desired_commands.get(event)
+            if command:
+                compact = any(existing_line.startswith("  - ") for existing_line in block[1:])
+                command_match = f"command: {command!r}"
+                command_match_alt = f"command: {command}"
+                if command_match not in block_text and command_match_alt not in block_text:
+                    block = block + _render_entry(command, compact=compact)
             merged.extend(block)
             seen_events.add(event)
             continue
@@ -335,7 +344,7 @@ def _merge_hooks_section(existing_text: str, *, prompt_command: str, auto_comman
         if event in seen_events:
             continue
         merged.append(f"  {event}:")
-        merged.extend(desired[event])
+        merged.extend(_render_entry(desired_commands[event], compact=False))
     return "\n".join(merged)
 
 
diff --git a/tests/test_installers.py b/tests/test_installers.py
index 8c89a85..df6c194 100644
--- a/tests/test_installers.py
+++ b/tests/test_installers.py
@@ -139,6 +139,32 @@ def test_install_hermes_user_scope_merges_hooks_into_existing_config_without_dup
     assert second_written
 
 
+def test_install_hermes_user_scope_preserves_compact_yaml_hook_indentation(tmp_path: Path) -> None:
+    hermes_root = tmp_path / ".hermes"
+    hermes_root.mkdir(parents=True, exist_ok=True)
+    config_path = hermes_root / "config.yaml"
+    config_path.write_text(
+        "model:\n"
+        "  provider: openai-codex\n"
+        "hooks:\n"
+        "  pre_llm_call:\n"
+        "  - command: /tmp/existing_prompt.py\n"
+        "    timeout: 5\n"
+        "hooks_auto_accept: false\n",
+        encoding="utf-8",
+    )
+
+    install_hermes_adapter_with_scope(tmp_path, scope="user")
+    install_hermes_adapter_with_scope(tmp_path, scope="user")
+
+    config_text = config_path.read_text(encoding="utf-8")
+    assert config_text.count("hermes_prompt_context.py") == 1
+    assert config_text.count("auto_session_learning.py") == 1
+    assert "  - command: /tmp/existing_prompt.py" in config_text
+    assert config_text.count("  - command: /tmp/existing_prompt.py") == 1
+    assert "    - command: '/tmp/existing_prompt.py'" not in config_text
+
+
 def test_installers_are_independent(tmp_path: Path) -> None:
     install_codex_adapter(tmp_path)
     assert not (tmp_path / ".claude").exists()