diff --git a/server.py b/server.py
index 7994650..3a17593 100644
--- a/server.py
+++ b/server.py
@@ -45,7 +45,8 @@ from audio import find_wem_files, convert_wem
 from tunings import (
     DEFAULT_REFERENCE_PITCH, DEFAULT_TUNINGS, PROFILE_IDS, PROFILE_PATHWAYS,
     apply_flat_instrument_patch_to_profiles, apply_reference_pitch,
-    normalize_instrument_profiles, settings_with_instrument_profiles, tuning_name,
+    normalize_instrument_profile, normalize_instrument_profiles,
+    settings_with_instrument_profiles, tuning_name,
 )
 import sloppak as sloppak_mod
 import drums as drums_mod
@@ -9331,13 +9332,25 @@ def save_settings(data: dict):
                 return {"error": "pathway must be one of songs, practice, learn, studio"}
             updates["pathway"] = raw
 
+    _profile_patch = None
     if "instrument_profiles" in data:
         raw = data["instrument_profiles"]
         if raw is not None:
-            profiles, error = normalize_instrument_profiles(raw)
-            if error:
-                return {"error": error}
-            updates["instrument_profiles"] = profiles
+            if not isinstance(raw, dict):
+                return {"error": "instrument_profiles must be an object"}
+            # Validate each PROVIDED profile individually and keep the patch
+            # PARTIAL — /api/settings is a partial-merge endpoint, so updating one
+            # profile must NOT reset the others to defaults. Merged over the
+            # persisted profiles inside the lock below (not via the wholesale
+            # `updates` merge, which would clobber the unspecified ones).
+            _profile_patch = {}
+            for _pid, _praw in raw.items():
+                if _pid not in PROFILE_IDS:
+                    return {"error": f"unknown instrument profile: {_pid}"}
+                _prof, _perr = normalize_instrument_profile(_pid, _praw)
+                if _perr:
+                    return {"error": _perr}
+                _profile_patch[_pid] = _prof
     if "active_instrument_profile" in data:
         raw = data["active_instrument_profile"]
         if raw is not None:
@@ -9360,6 +9373,15 @@ def save_settings(data: dict):
         if cfg is None:
             cfg = _default_settings()
         cfg.update(updates)
+        if _profile_patch is not None:
+            # Merge the validated partial over the persisted profiles so a
+            # single-profile update leaves the others intact (a fresh config
+            # falls back to the built-in defaults for the unspecified ones).
+            _existing, _ = normalize_instrument_profiles(cfg.get("instrument_profiles"))
+            if _existing is None:
+                _existing = {}
+            _existing.update(_profile_patch)
+            cfg["instrument_profiles"] = _existing
         # Only canonicalize/persist the instrument profiles when this save
         # actually touches them (or the config already carries them). GET always
         # virtualizes profiles via settings_with_instrument_profiles, so a save
diff --git a/static/v3/badges.js b/static/v3/badges.js
index 20e94a1..c67b3e1 100644
--- a/static/v3/badges.js
+++ b/static/v3/badges.js
@@ -500,8 +500,17 @@
             renderInstrument(); keepOpen();
         }));
         menu.querySelectorAll('[data-pill="strings"]').forEach((b) => b.addEventListener('click', async () => {
-            await saveSettings({ string_count: Number(b.getAttribute('data-val')) });
-            setWorkingInstrument(settings.instrument, settings.string_count);
+            const newSc = Number(b.getAttribute('data-val'));
+            // Clamp the tuning to one valid for the new string count and post it
+            // alongside string_count — otherwise the backend silently resets a
+            // now-invalid tuning to Standard while this UI keeps showing the old
+            // one (settings/tuner desync). Mirrors the instrument-switch clamp.
+            const tunings = _tuningsForInstrument(settings.instrument, newSc);
+            await saveSettings({
+                string_count: newSc,
+                tuning: tunings.includes(settings.tuning) ? settings.tuning : (tunings[0] || settings.tuning),
+            });
+            setWorkingInstrument(settings.instrument, newSc);
             renderInstrument(); keepOpen();
         }));
         menu.querySelector('[data-inst-tuning]').addEventListener('change', (e) => saveSettings({ tuning: e.target.value }));
diff --git a/tests/test_settings_api.py b/tests/test_settings_api.py
index 2428929..0574ade 100644
--- a/tests/test_settings_api.py
+++ b/tests/test_settings_api.py
@@ -834,6 +834,23 @@ def test_reset_clears_requested_keys(client, tmp_path):
     assert cfg["demucs_server_url"] == "http://demucs.example:9000"
 
 
+def test_partial_instrument_profiles_update_preserves_others(client, tmp_path):
+    # /api/settings is a partial-merge endpoint, so a POST that carries only ONE
+    # instrument profile must not reset the others to defaults.
+    gl = client.get("/api/settings").json()["instrument_profiles"]["guitar-lead"]
+    gl = dict(gl); gl["tuning"] = "Drop D"
+    client.post("/api/settings", json={"instrument_profiles": {"guitar-lead": gl}})
+    assert (client.get("/api/settings").json()["instrument_profiles"]
+            ["guitar-lead"]["tuning"] == "Drop D")
+    # Now update ONLY bass (Drop D is valid for a 4-string bass).
+    bass = client.get("/api/settings").json()["instrument_profiles"]["bass"]
+    bass = dict(bass); bass["tuning"] = "Drop D"
+    client.post("/api/settings", json={"instrument_profiles": {"bass": bass}})
+    out = client.get("/api/settings").json()["instrument_profiles"]
+    assert out["guitar-lead"]["tuning"] == "Drop D", "the untouched profile survived"
+    assert out["bass"]["tuning"] == "Drop D"
+
+
 def test_active_profile_switch_on_fresh_config(client, tmp_path):
     # A fresh config has no instrument_profiles; an explicit active-profile
     # switch must be honored, not overwritten by the profile inferred from the