From 5b18619730dba18a8de0021d1010a521126b2c32 Mon Sep 17 00:00:00 2001
From: YashpalLohan <btechincollege@gmail.com>
Date: Wed, 3 Jun 2026 02:34:55 +0530
Subject: [PATCH] fix(compilation): raise clear error when managed_section file
 is missing

---
 src/apm_cli/compilation/agents_compiler.py    |  6 ++++-
 .../unit/compilation/test_managed_section.py  | 24 +++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/src/apm_cli/compilation/agents_compiler.py b/src/apm_cli/compilation/agents_compiler.py
index 35986c63b..963ab3259 100644
--- a/src/apm_cli/compilation/agents_compiler.py
+++ b/src/apm_cli/compilation/agents_compiler.py
@@ -1249,8 +1249,12 @@ def _write_output_file_with_config(
 
         if config.agents_md_mode == "managed_section":
             target = Path(output_path)
-            existing = target.read_text(encoding="utf-8") if target.exists() else ""
             try:
+                if not target.exists():
+                    raise ManagedSectionError(
+                        f"{target.name} does not exist yet. Create it with markers first, or use mode: full for initial generation."
+                    )
+                existing = target.read_text(encoding="utf-8")
                 content = apply_managed_section(
                     existing,
                     content,
diff --git a/tests/unit/compilation/test_managed_section.py b/tests/unit/compilation/test_managed_section.py
index 5704a66b6..cc269cab7 100644
--- a/tests/unit/compilation/test_managed_section.py
+++ b/tests/unit/compilation/test_managed_section.py
@@ -324,3 +324,27 @@ def test_write_reraise_uses_bracket_format(self, tmp_path):
         # filename must be wrapped in square brackets: [AGENTS.md] ...
         assert msg.startswith("[")
         assert "] " in msg
+
+    def test_write_output_file_managed_section_missing_file(self, tmp_path):
+        """When mode=managed_section and the output file is missing, error is raised with a clear message."""
+        from apm_cli.compilation.agents_compiler import AgentsCompiler, CompilationConfig
+        from apm_cli.compilation.managed_section import ManagedSectionError
+
+        output_file = tmp_path / "AGENTS.md"
+        # Ensure it does not exist
+        if output_file.exists():
+            output_file.unlink()
+
+        config = CompilationConfig(
+            output_path=str(output_file),
+            agents_md_mode="managed_section",
+            dry_run=False,
+        )
+
+        compiler = AgentsCompiler(str(tmp_path))
+        with pytest.raises(ManagedSectionError) as exc_info:
+            compiler._write_output_file_with_config(str(output_file), "New content.\n", config)
+        
+        msg = str(exc_info.value)
+        assert "AGENTS.md does not exist yet. Create it with markers first, or use mode: full for initial generation." in msg
+        assert msg.startswith("[")