diff --git a/src/preset_manager.py b/src/preset_manager.py
index c694ca1..34233f1 100644
--- a/src/preset_manager.py
+++ b/src/preset_manager.py
@@ -92,6 +92,18 @@ Use precise language. Show causal relationships explicitly. Quantify uncertainty
                     custom.setdefault("inject_prefix", "")
                     custom.setdefault("inject_suffix", "")
                     self.save(presets)
+            # Heal a forward-incompatible file the same way the legacy `custom`
+            # migration above does: fill in any built-in presets an older or
+            # partial presets.json is missing, so they reach existing installs
+            # (a missing built-in is otherwise silently absent from the picker
+            # served by GET /api/presets). There is no delete path for the
+            # built-in keys, so this never clobbers an intentional removal.
+            # Defaults first, loaded values win — user edits are preserved.
+            if isinstance(presets, dict) and any(
+                k not in presets for k in self.DEFAULT_PRESETS
+            ):
+                presets = {**self.DEFAULT_PRESETS, **presets}
+                self.save(presets)
             return presets
         except Exception as e:
             logger.error(f"Error loading presets: {e}")
diff --git a/tests/test_preset_fill_missing_defaults.py b/tests/test_preset_fill_missing_defaults.py
new file mode 100644
index 0000000..04fd620
--- /dev/null
+++ b/tests/test_preset_fill_missing_defaults.py
@@ -0,0 +1,78 @@
+"""An older / partial presets.json must be healed forward on load: built-in
+presets that are missing get filled in, WITHOUT clobbering user edits.
+
+This extends the adjacent legacy `custom`-shape migration in
+`PresetManager.load`, which already repairs forward-incompatible files and
+re-saves them. A missing built-in is never an intentional user action — there
+is no delete path for the built-in keys (only `user_templates` entries can be
+deleted), and presets are hidden via an `enabled: False` flag, not removal — so
+filling them back in is safe.
+"""
+import json
+import os
+import tempfile
+
+from src.preset_manager import PresetManager
+
+
+def _write_presets(data: dict) -> str:
+    d = tempfile.mkdtemp()
+    with open(os.path.join(d, "presets.json"), "w", encoding="utf-8") as f:
+        json.dump(data, f)
+    return d
+
+
+def test_missing_builtin_presets_are_filled_in():
+    # Partial file: has code_analyze + brainstorm, missing reason + custom.
+    data_dir = _write_presets({
+        "code_analyze": {"name": "Code Analyze", "temperature": 0.2,
+                         "max_tokens": 8000, "system_prompt": "analyze"},
+        "brainstorm": {"name": "Brainstorm", "temperature": 0.9,
+                       "max_tokens": 4096, "system_prompt": "ideate"},
+    })
+    pm = PresetManager(data_dir)
+    for key in PresetManager.DEFAULT_PRESETS:
+        assert key in pm.presets, f"built-in preset {key!r} should be present"
+    # The fill is persisted so the next load is already complete.
+    with open(os.path.join(data_dir, "presets.json"), encoding="utf-8") as f:
+        on_disk = json.load(f)
+    assert "reason" in on_disk and "custom" in on_disk
+
+
+def test_fill_does_not_clobber_user_edits():
+    # An edited `custom` (enabled, bespoke prompt) plus a missing `reason`.
+    edited_custom = {
+        "name": "My Persona",
+        "character_name": "My Persona",
+        "temperature": 0.55,
+        "max_tokens": 1234,
+        "system_prompt": "You are my bespoke assistant.",
+        "inject_prefix": "PRE",
+        "inject_suffix": "SUF",
+        "enabled": True,
+    }
+    data_dir = _write_presets({
+        "code_analyze": {"name": "Code Analyze", "temperature": 0.2,
+                         "max_tokens": 8000, "system_prompt": "analyze"},
+        "brainstorm": {"name": "Brainstorm", "temperature": 0.9,
+                       "max_tokens": 4096, "system_prompt": "ideate"},
+        "custom": edited_custom,
+        "user_templates": [{"id": "t1", "name": "Tmpl"}],
+        # missing: reason
+    })
+    pm = PresetManager(data_dir)
+    # reason was filled...
+    assert "reason" in pm.presets
+    # ...but the user's edited custom + templates are untouched.
+    assert pm.presets["custom"] == edited_custom
+    assert pm.presets["user_templates"] == [{"id": "t1", "name": "Tmpl"}]
+
+
+def test_complete_file_is_not_rewritten_needlessly():
+    # A file that already has every built-in must be returned unchanged.
+    full = {k: dict(v) for k, v in PresetManager.DEFAULT_PRESETS.items()}
+    full["custom"]["enabled"] = True  # a user edit that must survive
+    data_dir = _write_presets(full)
+    pm = PresetManager(data_dir)
+    assert pm.presets["custom"]["enabled"] is True
+    assert set(PresetManager.DEFAULT_PRESETS) <= set(pm.presets)