diff --git a/OpenNest.Server.Tests/NestUploadValidationTests.cs b/OpenNest.Server.Tests/NestUploadValidationTests.cs
new file mode 100644
index 00000000..4256fe02
--- /dev/null
+++ b/OpenNest.Server.Tests/NestUploadValidationTests.cs
@@ -0,0 +1,152 @@
+using System.Net;
+using System.Net.Http.Json;
+using System.Text;
+using System.Text.Json;
+using OpenNest.Data;
+
+namespace OpenNest.Server.Tests;
+
+/// <summary>
+/// Client-allocated ids and uploaded nest metadata: conflicts are 409 and invalid
+/// metadata is 400, never a 500, and neither changes stored records.
+/// </summary>
+public sealed class NestUploadValidationTests
+{
+    private static readonly JsonSerializerOptions Options = new(JsonSerializerDefaults.Web)
+    { Converters = { new System.Text.Json.Serialization.JsonStringEnumConverter() } };
+
+    [Fact]
+    public async Task Create_WithExistingId_Returns409WithCurrentRecordAndKeepsIt()
+    {
+        using var factory = new ServerFactory();
+        using var client = factory.CreateClient();
+        var id = Guid.NewGuid();
+        using var created = await client.PostAsync("/api/nests", NestRevisionTests.Body(new NestRecord { Id = id, Name = "Original" }, [1]));
+        Assert.Equal(HttpStatusCode.OK, created.StatusCode);
+
+        using var duplicate = await client.PostAsync("/api/nests", NestRevisionTests.Body(new NestRecord { Id = id, Name = "Replacement" }, [2]));
+
+        Assert.Equal(HttpStatusCode.Conflict, duplicate.StatusCode);
+        var current = (await duplicate.Content.ReadFromJsonAsync<NestRecord>(Options))!;
+        Assert.Equal(id, current.Id);
+        Assert.Equal("Original", current.Name);
+        Assert.Equal(new byte[] { 1 }, await client.GetByteArrayAsync($"/api/nests/{id}/file"));
+        using var reopened = new NestDatabase(factory.DatabasePath, pooling: false);
+        Assert.Equal(1, reopened.History(id, 0, 100)!.Total);
+    }
+
+    [Fact]
+    public async Task Create_WithIdOfDeletedNest_Returns409AndKeepsItRecoverable()
+    {
+        using var factory = new ServerFactory();
+        using var client = factory.CreateClient();
+        var id = Guid.NewGuid();
+        using var created = await client.PostAsync("/api/nests", NestRevisionTests.Body(new NestRecord { Id = id, Name = "Original" }, [1]));
+        using var delete = new HttpRequestMessage(HttpMethod.Delete, $"/api/nests/{id}");
+        delete.Headers.TryAddWithoutValidation("If-Match", "\"1\"");
+        using var deleted = await client.SendAsync(delete);
+        Assert.Equal(HttpStatusCode.OK, deleted.StatusCode);
+
+        using var duplicate = await client.PostAsync("/api/nests", NestRevisionTests.Body(new NestRecord { Id = id, Name = "Replacement" }, [2]));
+
+        Assert.Equal(HttpStatusCode.Conflict, duplicate.StatusCode);
+        var current = (await duplicate.Content.ReadFromJsonAsync<NestRecord>(Options))!;
+        Assert.True(current.Deleted);
+        Assert.Equal(2, current.Revision);
+        Assert.Equal(HttpStatusCode.NotFound, (await client.GetAsync($"/api/nests/{id}")).StatusCode);
+        Assert.Equal(new byte[] { 1 }, await client.GetByteArrayAsync($"/api/nests/deleted/{id}/file"));
+    }
+
+    [Theory]
+    [InlineData("""{"name":null}""")]
+    [InlineData("""{"name":"A","customer":null}""")]
+    [InlineData("""{"name":"A","material":null}""")]
+    [InlineData("""{"name":"A","comments":null}""")]
+    [InlineData("""{"name":"A","madeBy":null}""")]
+    [InlineData("""{"name":"A","status":99}""")]
+    [InlineData("""{"name":"A","status":"cancelled"}""")]
+    public async Task Create_WithInvalidMetadata_Returns400AndStoresNothing(string metadata)
+    {
+        using var factory = new ServerFactory();
+        using var client = factory.CreateClient();
+
+        using var response = await client.PostAsync("/api/nests", RawBody(metadata, [1]));
+
+        Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
+        using var reopened = new NestDatabase(factory.DatabasePath, pooling: false);
+        Assert.Empty(reopened.List());
+    }
+
+    [Theory]
+    [InlineData("""{"name":"A","status":"toBeCut"}""", NestStatus.ToBeCut)]
+    [InlineData("""{"name":"A","status":2}""", NestStatus.HasBeenCut)]
+    public async Task Create_WithDefinedStatus_StoresIt(string metadata, NestStatus expected)
+    {
+        using var factory = new ServerFactory();
+        using var client = factory.CreateClient();
+
+        using var response = await client.PostAsync("/api/nests", RawBody(metadata, [1]));
+
+        Assert.Equal(HttpStatusCode.OK, response.StatusCode);
+        Assert.Equal(expected, (await response.Content.ReadFromJsonAsync<NestRecord>(Options))!.Status);
+    }
+
+    [Theory]
+    [InlineData("""{"name":null}""")]
+    [InlineData("""{"name":"A","status":99}""")]
+    public async Task UpdateFile_WithInvalidMetadata_Returns400AndLeavesRecordUnchanged(string metadata)
+    {
+        using var factory = new ServerFactory();
+        using var client = factory.CreateClient();
+        var id = await Create(client);
+        client.DefaultRequestHeaders.TryAddWithoutValidation("If-Match", "\"1\"");
+
+        using var response = await client.PutAsync($"/api/nests/{id}/file", RawBody(metadata, [2]));
+
+        Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
+        await AssertUnchanged(factory, client, id);
+    }
+
+    [Theory]
+    [InlineData("""{"name":null}""")]
+    [InlineData("""{"name":"A","comments":null}""")]
+    [InlineData("""{"name":"A","status":99}""")]
+    public async Task UpdateMetadata_WithInvalidMetadata_Returns400AndLeavesRecordUnchanged(string metadata)
+    {
+        using var factory = new ServerFactory();
+        using var client = factory.CreateClient();
+        var id = await Create(client);
+        client.DefaultRequestHeaders.TryAddWithoutValidation("If-Match", "\"1\"");
+
+        using var response = await client.PutAsync($"/api/nests/{id}/metadata",
+            new StringContent(metadata, Encoding.UTF8, "application/json"));
+
+        Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
+        await AssertUnchanged(factory, client, id);
+    }
+
+    private static async Task<Guid> Create(HttpClient client)
+    {
+        using var created = await client.PostAsync("/api/nests", NestRevisionTests.Body(new NestRecord { Name = "Original" }, [1]));
+        Assert.Equal(HttpStatusCode.OK, created.StatusCode);
+        return (await created.Content.ReadFromJsonAsync<NestRecord>(Options))!.Id;
+    }
+
+    private static async Task AssertUnchanged(ServerFactory factory, HttpClient client, Guid id)
+    {
+        Assert.Equal(new byte[] { 1 }, await client.GetByteArrayAsync($"/api/nests/{id}/file"));
+        using var reopened = new NestDatabase(factory.DatabasePath, pooling: false);
+        var current = reopened.Get(id)!;
+        Assert.Equal("Original", current.Name);
+        Assert.Equal(1, current.Revision);
+        Assert.Equal(1, reopened.History(id, 0, 100)!.Total);
+    }
+
+    private static MultipartFormDataContent RawBody(string metadata, byte[] bytes)
+    {
+        var body = new MultipartFormDataContent();
+        body.Add(new StringContent(metadata), "metadata");
+        body.Add(new ByteArrayContent(bytes), "file", "fixture.nest");
+        return body;
+    }
+}
diff --git a/OpenNest.Server.Tests/ServerOptionsTests.cs b/OpenNest.Server.Tests/ServerOptionsTests.cs
new file mode 100644
index 00000000..b82a0c33
--- /dev/null
+++ b/OpenNest.Server.Tests/ServerOptionsTests.cs
@@ -0,0 +1,34 @@
+namespace OpenNest.Server.Tests;
+
+public sealed class ServerOptionsTests
+{
+    private static readonly string Default = Path.Combine("data", "nests.db");
+
+    [Theory]
+    [InlineData(new[] { "--database=/srv/a.db" }, "/srv/a.db")]
+    [InlineData(new[] { "--database", "/srv/a.db" }, "/srv/a.db")]
+    [InlineData(new[] { "--urls=http://127.0.0.1:8090", "--database", "/srv/a.db" }, "/srv/a.db")]
+    [InlineData(new[] { "--database=/srv/a=b.db" }, "/srv/a=b.db")]
+    public void DatabasePath_AcceptsBothForms_AndOverridesEnvironment(string[] args, string expected)
+    {
+        Assert.Equal(expected, ServerOptions.DatabasePath(args, "/env/nests.db"));
+    }
+
+    [Fact]
+    public void DatabasePath_WithoutArgument_UsesEnvironmentThenDefault()
+    {
+        Assert.Equal("/env/nests.db", ServerOptions.DatabasePath(["--urls=http://127.0.0.1:8090"], "/env/nests.db"));
+        Assert.Equal(Default, ServerOptions.DatabasePath([], null));
+    }
+
+    [Theory]
+    [InlineData("--database")]
+    [InlineData("--database", "--urls=http://127.0.0.1:8090")]
+    [InlineData("--database=")]
+    [InlineData("--database=  ")]
+    [InlineData("--database=/a.db", "--database", "/b.db")]
+    public void DatabasePath_MissingOrRepeated_FailsInsteadOfFallingBack(params string[] args)
+    {
+        Assert.Throws<ArgumentException>(() => ServerOptions.DatabasePath(args, "/env/nests.db"));
+    }
+}
diff --git a/OpenNest.Server/NestDatabase.cs b/OpenNest.Server/NestDatabase.cs
index 1456cc41..2c982ccc 100644
--- a/OpenNest.Server/NestDatabase.cs
+++ b/OpenNest.Server/NestDatabase.cs
@@ -142,6 +142,9 @@ public sealed partial class NestDatabase : IDisposable
         lock (_sync)
         {
             using var transaction = _connection.BeginTransaction();
+            // Ids are client-allocated: an existing id, active or deleted, is a conflict, not a replacement.
+            if ((Get(id) ?? Get(id, deleted: true)) is { } existing)
+                throw new RevisionConflictException(existing, "A nest with this id already exists.");
             using var command = _connection.CreateCommand();
             command.Transaction = transaction;
             command.CommandText = """
@@ -488,7 +491,8 @@ public sealed partial class NestDatabase : IDisposable
     }
 }
 
-public sealed class RevisionConflictException(NestRecord current) : Exception("The nest has changed. Reload or Save As.")
+public sealed class RevisionConflictException(NestRecord current, string message = "The nest has changed. Reload or Save As.")
+    : Exception(message)
 {
     public NestRecord Current { get; } = current;
 }
diff --git a/OpenNest.Server/NestMetadata.cs b/OpenNest.Server/NestMetadata.cs
new file mode 100644
index 00000000..9a7d53de
--- /dev/null
+++ b/OpenNest.Server/NestMetadata.cs
@@ -0,0 +1,60 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using OpenNest.Data;
+
+namespace OpenNest.Server;
+
+/// <summary>
+/// Reads and checks uploaded nest metadata. Text fields must be present and the status
+/// must be a defined value, so invalid metadata is a 400 and never reaches storage.
+/// </summary>
+internal static class NestMetadata
+{
+    // Integer statuses stay accepted for existing callers; GetError refuses undefined ones.
+    public static readonly JsonSerializerOptions Options = new(JsonSerializerDefaults.Web)
+    {
+        PropertyNameCaseInsensitive = true,
+        Converters = { new JsonStringEnumConverter(JsonNamingPolicy.CamelCase) },
+    };
+
+    public static bool TryRead(string? json, out NestRecord record, out string error)
+    {
+        record = new NestRecord();
+        if (string.IsNullOrWhiteSpace(json))
+        {
+            error = "Missing 'metadata' part.";
+            return false;
+        }
+
+        try
+        {
+            if (JsonSerializer.Deserialize<NestRecord>(json, Options) is not { } parsed)
+            {
+                error = "Missing metadata.";
+                return false;
+            }
+
+            record = parsed;
+        }
+        catch (JsonException)
+        {
+            error = "Invalid metadata JSON.";
+            return false;
+        }
+
+        error = GetError(record) ?? "";
+        return error.Length == 0;
+    }
+
+    public static string? GetError(NestRecord record)
+    {
+        if (record.Name is null) return "name must be a string.";
+        if (record.Customer is null) return "customer must be a string.";
+        if (record.Material is null) return "material must be a string.";
+        if (record.Comments is null) return "comments must be a string.";
+        if (record.MadeBy is null) return "madeBy must be a string.";
+        if (!Enum.IsDefined(record.Status)) return "status must be quote, toBeCut or hasBeenCut.";
+        if (!double.IsFinite(record.Thickness)) return "thickness must be a finite number.";
+        return null;
+    }
+}
diff --git a/OpenNest.Server/Program.cs b/OpenNest.Server/Program.cs
index eebe89ba..677ea6ff 100644
--- a/OpenNest.Server/Program.cs
+++ b/OpenNest.Server/Program.cs
@@ -20,11 +20,8 @@ builder.Services.ConfigureHttpJsonOptions(options =>
         options.SerializerOptions.Converters.Add(converter);
 });
 
-// Storage path: --database <path> or OPENNEST_DB, default ./data/nests.db (a Docker volume).
-var databasePath =
-    args.FirstOrDefault(a => a.StartsWith("--database=", StringComparison.Ordinal))?.Split('=', 2)[1]
-    ?? Environment.GetEnvironmentVariable("OPENNEST_DB")
-    ?? Path.Combine("data", "nests.db");
+// Storage path: --database=<path> or --database <path>, then OPENNEST_DB, default ./data/nests.db (a Docker volume).
+var databasePath = ServerOptions.DatabasePath(args, Environment.GetEnvironmentVariable(ServerOptions.DatabaseVariable));
 builder.Services.AddSingleton(_ => new NestDatabase(databasePath));
 
 // Listen port: --urls or ASPNETCORE_URLS; Dockerfile defaults to 8090.
@@ -60,7 +57,8 @@ app.MapPost("/api/nests", async (HttpContext http, NestDatabase db) =>
         return error;
 
     var id = record!.Id == Guid.Empty ? Guid.NewGuid() : record.Id;
-    return Results.Ok(db.Insert(id, record, file!));
+    try { return Results.Ok(db.Insert(id, record, file!)); }
+    catch (RevisionConflictException conflict) { return Results.Json(conflict.Current, statusCode: 409); }
 });
 app.MapGet("/api/nests/{id:guid}/revisions", (Guid id, HttpRequest request, NestDatabase db) =>
     ReadHistory(id, request, db));
@@ -90,10 +88,11 @@ app.MapPut("/api/nests/{id:guid}/metadata", async (Guid id, HttpRequest request,
     var error = IfMatch.Read(request, out var expected);
     if (error is not null) return error;
     NestRecord? record;
-    try { record = await request.ReadFromJsonAsync<NestRecord>(); }
+    try { record = await request.ReadFromJsonAsync<NestRecord>(NestMetadata.Options); }
     catch (Exception ex) when (ex is JsonException or InvalidOperationException or BadHttpRequestException)
     { return Results.BadRequest("Invalid metadata JSON."); }
     if (record is null) return Results.BadRequest("Missing metadata.");
+    if (NestMetadata.GetError(record) is { } invalid) return Results.BadRequest(invalid);
     try
     {
         var updated = db.UpdateMetadata(id, record, expected);
@@ -184,22 +183,8 @@ static async Task<(NestRecord? Record, byte[]? File, IResult? Error)> ReadMultip
         return (null, null, Results.BadRequest("Malformed multipart body."));
     }
 
-    var metadataPart = form["metadata"].FirstOrDefault();
-    if (string.IsNullOrWhiteSpace(metadataPart))
-        return (null, null, Results.BadRequest("Missing 'metadata' part."));
-
-    NestRecord? record;
-    try
-    {
-        record = JsonSerializer.Deserialize<NestRecord>(metadataPart, MultipartJson.Options);
-    }
-    catch (JsonException)
-    {
-        return (null, null, Results.BadRequest("Invalid metadata JSON."));
-    }
-
-    if (record is null)
-        return (null, null, Results.BadRequest("Missing metadata."));
+    if (!NestMetadata.TryRead(form["metadata"].FirstOrDefault(), out var record, out var metadataError))
+        return (null, null, Results.BadRequest(metadataError));
 
     var filePart = form.Files["file"];
     if (filePart is null || filePart.Length == 0)
@@ -211,14 +196,5 @@ static async Task<(NestRecord? Record, byte[]? File, IResult? Error)> ReadMultip
     return (record, buffer.ToArray(), null);
 }
 
-internal static class MultipartJson
-{
-    public static readonly JsonSerializerOptions Options = new(JsonSerializerDefaults.Web)
-    {
-        PropertyNameCaseInsensitive = true,
-        Converters = { new JsonStringEnumConverter(JsonNamingPolicy.CamelCase) },
-    };
-}
-
 // Entry-point type for in-memory integration tests (WebApplicationFactory<Program>).
 public partial class Program { }
diff --git a/OpenNest.Server/ServerOptions.cs b/OpenNest.Server/ServerOptions.cs
new file mode 100644
index 00000000..849b5cef
--- /dev/null
+++ b/OpenNest.Server/ServerOptions.cs
@@ -0,0 +1,43 @@
+namespace OpenNest.Server;
+
+/// <summary>Startup settings read from the command line and environment.</summary>
+public static class ServerOptions
+{
+    public const string DatabaseVariable = "OPENNEST_DB";
+
+    /// <summary>
+    /// The database path: <c>--database=&lt;path&gt;</c> or <c>--database &lt;path&gt;</c>, then
+    /// <c>OPENNEST_DB</c>, then <c>./data/nests.db</c>. A <c>--database</c> without a path,
+    /// or given twice, fails startup instead of silently storing nests elsewhere.
+    /// </summary>
+    public static string DatabasePath(IReadOnlyList<string> args, string? environmentPath)
+    {
+        string? path = null;
+        for (var i = 0; i < args.Count; i++)
+        {
+            string value;
+            if (args[i] == "--database")
+            {
+                if (i + 1 >= args.Count || args[i + 1].StartsWith("--", StringComparison.Ordinal))
+                    throw new ArgumentException("--database requires a path.", nameof(args));
+                value = args[++i];
+            }
+            else if (args[i].StartsWith("--database=", StringComparison.Ordinal))
+            {
+                value = args[i]["--database=".Length..];
+            }
+            else
+            {
+                continue;
+            }
+
+            if (string.IsNullOrWhiteSpace(value))
+                throw new ArgumentException("--database requires a path.", nameof(args));
+            if (path is not null)
+                throw new ArgumentException("--database was given more than once.", nameof(args));
+            path = value;
+        }
+
+        return path ?? environmentPath ?? Path.Combine("data", "nests.db");
+    }
+}
diff --git a/docs/nest-storage.md b/docs/nest-storage.md
index 3295b94b..b5ffc165 100644
--- a/docs/nest-storage.md
+++ b/docs/nest-storage.md
@@ -65,7 +65,9 @@ installs are unaffected until an operator opts in via **File > Storage Mode...**
 (`RemoteNestRepository`) and the server (`OpenNest.Server`). JSON is camelCase
 with enums serialized as strings (`JsonSerializerDefaults.Web` +
 `JsonStringEnumConverter(JsonNamingPolicy.CamelCase)`), matching the rest of
-`OpenNest.Data`'s local JSON stores.
+`OpenNest.Data`'s local JSON stores. Uploaded metadata (POST and both PUTs) must
+carry string text fields (`null` is 400) and a defined `status`: a name, or for
+existing callers its integer value 0–2; an undefined value is 400 and stores nothing.
 
 | Field | Type | Notes |
 |---|---|---|
@@ -94,7 +96,7 @@ with enums serialized as strings (`JsonSerializerDefaults.Web` +
 | GET | `/api/nests/query?search=&sort=&order=&offset=&limit=` | — | `{ "items": NestRecord[], "total", "offset", "limit" }`: one bounded page filtered in SQL (see below), or 400 for an invalid parameter |
 | GET | `/api/nests/{id}` | — | `NestRecord` or 404 |
 | GET | `/api/nests/{id}/file` | — | `.nest` archive bytes (`application/zip`) or 404 |
-| POST | `/api/nests` | multipart: `metadata` (JSON `NestRecord`) + `file` (`.nest` bytes) | `NestRecord` with server-assigned `id` when the client sends an empty guid |
+| POST | `/api/nests` | multipart: `metadata` (JSON `NestRecord`) + `file` (`.nest` bytes) | `NestRecord` with server-assigned `id` when the client sends an empty guid; 409 with the current record (active or deleted) when the `id` already exists, storing nothing |
 | PUT | `/api/nests/{id}/file` | multipart: `metadata` + `file` | Updated `NestRecord`, or 404 if `id` is unknown |
 | PUT | `/api/nests/{id}/metadata` | JSON `NestRecord` | Updated `NestRecord` (archive untouched), or 404 |
 | DELETE | `/api/nests/{id}` | — | Updated hidden `NestRecord`, or 404 |
@@ -291,8 +293,9 @@ revision; save the nest to keep it.
 
 `OpenNest.Server.NestDatabase` uses SQLite (`Microsoft.Data.Sqlite`) with
 `journal_mode=WAL`. Each row holds the metadata columns plus the `.nest`
-archive as a `BLOB`. The database file path comes from `--database=<path>`,
-then `OPENNEST_DB`, defaulting to `./data/nests.db`. The server opens the
+archive as a `BLOB`. The database file path comes from `--database=<path>` or
+`--database <path>`, then `OPENNEST_DB`, defaulting to `./data/nests.db`; a
+`--database` without a path, or given twice, fails startup. The server opens the
 database at startup, so an unusable path fails startup rather than the first request.
 
 Run **one service instance with its database on a local filesystem**. Multiple PCs