diff --git a/GerbilManager.Tests/PartialUpdateTests.cs b/GerbilManager.Tests/PartialUpdateTests.cs
new file mode 100644
index 0000000..60f14f8
--- /dev/null
+++ b/GerbilManager.Tests/PartialUpdateTests.cs
@@ -0,0 +1,143 @@
+using System.Net;
+using System.Net.Http.Json;
+using System.Text.Json;
+using System.Text.RegularExpressions;
+
+namespace GerbilManager.Tests;
+
+/// <summary>CR-2 regression: PUT /gerbils/{id} must not silently destroy fields that
+/// the caller omits (PATCH semantics). The ForSale-toggle and Charakterbogen-save
+/// send partial bodies — all other fields must be preserved unchanged.</summary>
+public class PartialUpdateTests : IClassFixture<ApiFactory>
+{
+    private readonly HttpClient _client;
+
+    public PartialUpdateTests(ApiFactory factory) => _client = factory.CreateClient();
+
+    // ------------------------------------------------------------------ helpers
+
+    private static Guid ExtractId(string json) =>
+        Guid.Parse(Regex.Match(json, "\"id\":\"([^\"]+)\"").Groups[1].Value);
+
+    private static string? GetStr(JsonElement el, string prop) =>
+        el.TryGetProperty(prop, out var v) && v.ValueKind == JsonValueKind.String
+            ? v.GetString() : null;
+
+    private static string? GetNested(JsonElement el, string prop) =>
+        el.TryGetProperty(prop, out var v) ? v.ToString() : null;
+
+    /// <summary>Create a fully-populated gerbil and return its id.</summary>
+    private async Task<Guid> CreateFullGerbil(string name = "TestTier")
+    {
+        var resp = await _client.PostAsync("/gerbils", JsonContent.Create(new
+        {
+            name,
+            gender = "female",
+            genotype = "aa CC DD ee GG PP spsp rere",
+            originBreeder = "Zucht der Kleinen Chaoten",
+            isDeaf = false,
+            characterTraits = new[] { "neugierig", "zutraulich" },
+            characterNote = "Liebling der Familie",
+            notes = "Eine Notiz",
+            externalRef = "ext-001",
+        }));
+        Assert.Equal(HttpStatusCode.Created, resp.StatusCode);
+        return ExtractId(await resp.Content.ReadAsStringAsync());
+    }
+
+    // ------------------------------------------------------------------ tests
+
+    [Fact]
+    public async Task ForSale_toggle_does_not_wipe_other_fields()
+    {
+        // Arrange: create a fully-populated gerbil
+        var id = await CreateFullGerbil("ForSale-Tier");
+
+        // Act: send only { status: 'ForSale' } — exactly what the detail page does
+        var put = await _client.PutAsync($"/gerbils/{id}",
+            JsonContent.Create(new { status = "ForSale" }));
+        Assert.Equal(HttpStatusCode.NoContent, put.StatusCode);
+
+        // Assert: re-read and check all fields survive
+        var json = JsonDocument.Parse(await _client.GetStringAsync($"/gerbils/{id}")).RootElement;
+
+        Assert.Equal("ForSale", GetStr(json, "status"));
+        Assert.Equal("ForSale-Tier", GetStr(json, "name"));
+        Assert.Equal("female", GetStr(json, "gender"));
+        Assert.Equal("aa CC DD ee GG PP spsp rere", GetStr(json, "genotype"));
+        Assert.Equal("Zucht der Kleinen Chaoten", GetStr(json, "originBreeder"));
+        Assert.Equal("Eine Notiz", GetStr(json, "notes"));
+        Assert.Equal("ext-001", GetStr(json, "externalRef"));
+        Assert.Contains("neugierig", GetNested(json, "characterTraits") ?? "");
+        Assert.Equal("Liebling der Familie", GetStr(json, "characterNote"));
+    }
+
+    [Fact]
+    public async Task CharacterSave_does_not_wipe_other_fields()
+    {
+        // Arrange
+        var id = await CreateFullGerbil("Charakter-Tier");
+
+        // Act: send only { characterTraits, characterNote } — exactly what the Charakterbogen save does
+        var put = await _client.PutAsync($"/gerbils/{id}", JsonContent.Create(new
+        {
+            characterTraits = new[] { "handzahm" },
+            characterNote = "Neue Notiz",
+        }));
+        Assert.Equal(HttpStatusCode.NoContent, put.StatusCode);
+
+        // Assert: name / genotype / originBreeder / status / externalRef all intact
+        var json = JsonDocument.Parse(await _client.GetStringAsync($"/gerbils/{id}")).RootElement;
+
+        Assert.Equal("Active", GetStr(json, "status"));
+        Assert.Equal("Charakter-Tier", GetStr(json, "name"));
+        Assert.Equal("female", GetStr(json, "gender"));
+        Assert.Equal("aa CC DD ee GG PP spsp rere", GetStr(json, "genotype"));
+        Assert.Equal("Zucht der Kleinen Chaoten", GetStr(json, "originBreeder"));
+        Assert.Equal("Eine Notiz", GetStr(json, "notes"));
+        Assert.Equal("ext-001", GetStr(json, "externalRef"));
+        Assert.Contains("handzahm", GetNested(json, "characterTraits") ?? "");
+        Assert.Equal("Neue Notiz", GetStr(json, "characterNote"));
+    }
+
+    [Fact]
+    public async Task EditForm_omitting_import_fields_keeps_them()
+    {
+        // Arrange: gerbil with import-set fields
+        var id = await CreateFullGerbil("EditForm-Tier");
+
+        // Act: send a typical edit-form body that omits originBreeder/isDeaf/externalRef/characterTraits/characterNote
+        var put = await _client.PutAsync($"/gerbils/{id}", JsonContent.Create(new
+        {
+            name = "EditForm-Tier (umbenannt)",
+            gender = "female",
+            genotype = "aa CC DD ee GG PP spsp rere",
+            notes = "Aktualisierte Notiz",
+        }));
+        Assert.Equal(HttpStatusCode.NoContent, put.StatusCode);
+
+        // Assert: name updated, import fields survive
+        var json = JsonDocument.Parse(await _client.GetStringAsync($"/gerbils/{id}")).RootElement;
+
+        Assert.Equal("EditForm-Tier (umbenannt)", GetStr(json, "name"));
+        Assert.Equal("Aktualisierte Notiz", GetStr(json, "notes"));
+        Assert.Equal("Zucht der Kleinen Chaoten", GetStr(json, "originBreeder"));
+        Assert.Equal("ext-001", GetStr(json, "externalRef"));
+        Assert.Contains("neugierig", GetNested(json, "characterTraits") ?? "");
+        Assert.Equal("Liebling der Familie", GetStr(json, "characterNote"));
+    }
+
+    [Fact]
+    public async Task Create_with_minimal_body_uses_safe_defaults()
+    {
+        // A minimal POST (only name) must still produce a valid gerbil
+        var resp = await _client.PostAsync("/gerbils", JsonContent.Create(new { name = "Minimal" }));
+        Assert.Equal(HttpStatusCode.Created, resp.StatusCode);
+
+        var json = JsonDocument.Parse(await resp.Content.ReadAsStringAsync()).RootElement;
+        Assert.Equal("Minimal", GetStr(json, "name"));
+        Assert.Equal("Active", GetStr(json, "status"));
+        // characterTraits defaults to [] on create when not provided
+        Assert.Contains("[]", GetNested(json, "characterTraits") ?? "");
+    }
+}
diff --git a/GerbilManagerWebAPI/Dtos/ApiDtos.cs b/GerbilManagerWebAPI/Dtos/ApiDtos.cs
index 9d2a66c..983583c 100644
--- a/GerbilManagerWebAPI/Dtos/ApiDtos.cs
+++ b/GerbilManagerWebAPI/Dtos/ApiDtos.cs
@@ -59,8 +59,8 @@ namespace GerbilManagerWebAPI.Dtos
     // Request DTOs -----------------------------------------------------------
 
     public record GerbilInput(
-        string Name,
-        Gender Gender,
+        string? Name,
+        Gender? Gender,
         GerbilStatus? Status,
         Guid? LitterId,
         Guid? OriginContactId,
diff --git a/GerbilManagerWebAPI/Endpoints/GerbilEndpoints.cs b/GerbilManagerWebAPI/Endpoints/GerbilEndpoints.cs
index ee47ff4..b62dd8e 100644
--- a/GerbilManagerWebAPI/Endpoints/GerbilEndpoints.cs
+++ b/GerbilManagerWebAPI/Endpoints/GerbilEndpoints.cs
@@ -39,19 +39,18 @@ namespace GerbilManagerWebAPI.Endpoints
                 if (string.IsNullOrWhiteSpace(input.Name))
                     return TypedResults.ValidationProblem(new Dictionary<string, string[]> { ["name"] = ["Name is required."] });
 
-                var g = new Gerbil { Id = Guid.NewGuid(), Name = input.Name };
+                var g = new Gerbil { Id = Guid.NewGuid(), Name = input.Name! };
                 Apply(g, input, isCreate: true);
                 db.Gerbils.Add(g);
                 await db.SaveChangesAsync();
                 return TypedResults.Created($"/gerbils/{g.Id}", ToDto(g));
             });
 
-            // PUT /gerbils/{id}
+            // PUT /gerbils/{id}  — PATCH semantics: omitted/null fields keep the stored value.
             group.MapPut("/{id:guid}", async Task<Results<NoContent, NotFound>> (Guid id, GerbilInput input, ApplicationContext db) =>
             {
                 var g = await db.Gerbils.FirstOrDefaultAsync(x => x.Id == id);
                 if (g is null) return TypedResults.NotFound();
-                g.Name = input.Name;
                 Apply(g, input, isCreate: false);
                 await db.SaveChangesAsync();
                 return TypedResults.NoContent();
@@ -77,27 +76,34 @@ namespace GerbilManagerWebAPI.Endpoints
             return app;
         }
 
+        // CR-2 FIX: PATCH semantics — every omitted/null field keeps the stored value.
+        // Prevents silent data loss when the frontend sends partial bodies (ForSale toggle,
+        // Charakterbogen save, any partial updateGerbil call). On create, supply safe defaults
+        // for fields the frontend omits. A non-null input value always wins (including explicit
+        // nulls — callers that want to clear a nullable field must send a full object; a
+        // dedicated PATCH endpoint can be added later if point-clear is needed).
         private static void Apply(Gerbil g, GerbilInput i, bool isCreate)
         {
-            g.Gender = i.Gender;
+            if (!string.IsNullOrWhiteSpace(i.Name)) g.Name = i.Name!;
+            g.Gender = i.Gender ?? (isCreate ? Gender.unknown : g.Gender);
             g.Status = i.Status ?? (isCreate ? GerbilStatus.Active : g.Status);
-            g.LitterId = i.LitterId;
-            g.OriginContactId = i.OriginContactId;
-            g.ReceiverContactId = i.ReceiverContactId;
-            g.EnclosureId = i.EnclosureId;
-            g.ColorVarietyId = i.ColorVarietyId;
-            g.DateOfBirth = i.DateOfBirth;
-            g.DateOfDeath = i.DateOfDeath;
-            g.CauseOfDeath = i.CauseOfDeath;
-            g.GoHomeDate = i.GoHomeDate;
-            g.Genotype = i.Genotype;
-            g.Notes = i.Notes;
-            g.ImportSource = i.ImportSource;
-            g.ExternalRef = i.ExternalRef;
-            g.OriginBreeder = i.OriginBreeder;
-            g.CharacterTraits = i.CharacterTraits ?? new List<string>();
-            g.CharacterNote = i.CharacterNote;
-            g.IsDeaf = i.IsDeaf;
+            g.LitterId = i.LitterId ?? g.LitterId;
+            g.OriginContactId = i.OriginContactId ?? g.OriginContactId;
+            g.ReceiverContactId = i.ReceiverContactId ?? g.ReceiverContactId;
+            g.EnclosureId = i.EnclosureId ?? g.EnclosureId;
+            g.ColorVarietyId = i.ColorVarietyId ?? g.ColorVarietyId;
+            g.DateOfBirth = i.DateOfBirth ?? g.DateOfBirth;
+            g.DateOfDeath = i.DateOfDeath ?? g.DateOfDeath;
+            g.CauseOfDeath = i.CauseOfDeath ?? g.CauseOfDeath;
+            g.GoHomeDate = i.GoHomeDate ?? g.GoHomeDate;
+            g.Genotype = i.Genotype ?? g.Genotype;
+            g.Notes = i.Notes ?? g.Notes;
+            g.ImportSource = i.ImportSource ?? g.ImportSource;
+            g.ExternalRef = i.ExternalRef ?? g.ExternalRef;
+            g.OriginBreeder = i.OriginBreeder ?? g.OriginBreeder;
+            g.CharacterTraits = i.CharacterTraits ?? g.CharacterTraits;
+            g.CharacterNote = i.CharacterNote ?? g.CharacterNote;
+            g.IsDeaf = i.IsDeaf ?? g.IsDeaf;
             g.IsResident = i.IsResident ?? (isCreate ? true : g.IsResident);
         }