diff --git a/platform-integrations/install.sh b/platform-integrations/install.sh
index c09db6c2..67dbb67d 100755
--- a/platform-integrations/install.sh
+++ b/platform-integrations/install.sh
@@ -342,7 +342,6 @@ class FileOps:
         mode_block = "\n".join(mode_lines).strip()
         start = _sentinel_start(slug)
         end   = _sentinel_end(slug)
-        block = f"\n{start}\n  {mode_block.replace(chr(10), chr(10) + '  ')}\n{end}\n"
 
         try:
             with open(target_yaml_path) as f:
@@ -353,9 +352,34 @@ class FileOps:
         if not existing.strip() or "customModes:" not in existing:
             existing = "customModes:\n"
 
-        if start in existing:
-            pattern = re.compile(re.escape(start) + r".*?" + re.escape(end), re.DOTALL)
-            new_content = pattern.sub(block.strip(), existing)
+        # Match the list-item indentation already used under `customModes:` so the
+        # inserted block doesn't mix 0-indent and 2-indent sequence items (which is
+        # invalid YAML). The source uses 2-space items; a target written by
+        # yaml.safe_dump (Bob/marketplace tooling) may use 0-space. Detect and match.
+        item_indent = "  "
+        seen_modes = False
+        for ln in existing.splitlines():
+            if ln.strip() == "customModes:":
+                seen_modes = True
+                continue
+            if seen_modes and ln.lstrip().startswith("- "):
+                item_indent = ln[: len(ln) - len(ln.lstrip())]
+                break
+        block_body = "\n".join(item_indent + ln if ln else ln for ln in mode_block.split("\n"))
+        block = f"\n{start}\n{block_body}\n{end}\n"
+
+        # Match a *real* sentinel block only: the start and end markers must each
+        # sit at the beginning of a line. A bare sentinel substring inside another
+        # mode's quoted scalar (e.g. the install-evolve-lite mode documents the
+        # literal `# >>>evolve:evolve-lite<<<` in its customInstructions) must NOT
+        # be treated as an existing block — otherwise the replace finds no matching
+        # end, no-ops, and the merge is silently dropped while still reporting ✓.
+        block_re = re.compile(
+            r"^[ \t]*" + re.escape(start) + r".*?^[ \t]*" + re.escape(end) + r"[^\n]*$",
+            re.DOTALL | re.MULTILINE,
+        )
+        if block_re.search(existing):
+            new_content = block_re.sub(lambda _m: block.strip(), existing)
         else:
             new_content = existing.rstrip() + block
 
@@ -370,9 +394,11 @@ class FileOps:
             text = f.read()
         start = _sentinel_start(slug)
         end   = _sentinel_end(slug)
+        # Line-anchored so a sentinel literal mentioned inside another mode's
+        # quoted text is never mistaken for a real block (see merge above).
         pattern = re.compile(
-            r"\n?" + re.escape(start) + r".*?" + re.escape(end) + r"\n?",
-            re.DOTALL
+            r"^[ \t]*" + re.escape(start) + r".*?" + re.escape(end) + r"[^\n]*$\n?",
+            re.DOTALL | re.MULTILINE,
         )
         self.atomic_write_text(target_yaml_path, pattern.sub("", text))
 
diff --git a/tests/platform_integrations/test_idempotency.py b/tests/platform_integrations/test_idempotency.py
index 8ca06f78..02ebbb83 100644
--- a/tests/platform_integrations/test_idempotency.py
+++ b/tests/platform_integrations/test_idempotency.py
@@ -3,6 +3,7 @@
 """
 
 import json
+import re
 
 import pytest
 
@@ -140,6 +141,47 @@ def test_uninstall_removes_namespaced_shared_lib(self, temp_project_dir, install
 
         file_assertions.assert_dir_not_exists(bob_dir / "lib" / "evolve-lite")
 
+    def test_install_merges_mode_despite_sentinel_literal_in_another_mode(self, temp_project_dir, install_runner, file_assertions):
+        """A sentinel literal quoted inside another mode's text must not block the merge.
+
+        Regression: the install-evolve-lite marketplace mode documents the literal
+        `# >>>evolve:evolve-lite<<<` in its customInstructions. A naive `if start in
+        existing` substring check treated that as an existing block, took the replace
+        branch, found no matching end sentinel, and silently dropped the merge while
+        still reporting success. The sentinel match must be line-anchored.
+        """
+        bob_dir = temp_project_dir / ".bob"
+        modes_file = bob_dir / "custom_modes.yaml"
+        modes_file.parent.mkdir(parents=True, exist_ok=True)
+        # Reproduce the exact user failure: a 0-indent list (as yaml.safe_dump /
+        # Bob marketplace tooling writes it) whose quoted text mentions the
+        # sentinel literal. This trips BOTH the substring false-match and the
+        # 0-indent-vs-2-indent mismatch.
+        modes_file.write_text(
+            "customModes:\n"
+            "- slug: install-evolve-lite\n"
+            "  name: Install Evolve Lite\n"
+            '  customInstructions: "Merged between # >>>evolve:evolve-lite<<< sentinel comments."\n'
+            "  groups:\n"
+            "  - read\n"
+        )
+
+        install_runner.run("install", platform="bob", mode="lite")
+
+        content = modes_file.read_text()
+        # The evolve-lite mode was actually merged in (real sentinel block written).
+        assert "# >>>evolve:evolve-lite<<<" in content
+
+        # All top-level list items share one indentation — a 0-indent/2-indent mix
+        # would be invalid YAML (the indentation-matching fix).
+        indents = set(re.findall(r"(?m)^([ \t]*)- slug:", content))
+        assert len(indents) == 1, f"mixed custom-mode list indentation: {indents}"
+
+        slugs = re.findall(r"(?m)^[ \t]*- slug:\s*(\S+)", content)
+        assert "evolve-lite" in slugs, f"evolve-lite mode not merged; slugs={slugs}"
+        # ...and the pre-existing mode is preserved.
+        assert "install-evolve-lite" in slugs
+
     def test_install_preserves_user_content_during_legacy_purge(self, temp_project_dir, install_runner, bob_fixtures, file_assertions):
         """The legacy purge MUST NOT clobber non-evolve user skills/commands."""
         bob_dir = temp_project_dir / ".bob"