diff --git a/rewrite-csharp/csharp/OpenRewrite.Tests/Rpc/RecipesNuGetConfigTests.cs b/rewrite-csharp/csharp/OpenRewrite.Tests/Rpc/RecipesNuGetConfigTests.cs
new file mode 100644
index 00000000000..14b0b9cd1fb
--- /dev/null
+++ b/rewrite-csharp/csharp/OpenRewrite.Tests/Rpc/RecipesNuGetConfigTests.cs
@@ -0,0 +1,82 @@
+/*
+ * Copyright 2026 the original author or authors.
+ * <p>
+ * Licensed under the Moderne Source Available License (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * https://docs.moderne.io/licensing/moderne-source-available-license
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+using OpenRewrite.CSharp.Rpc;
+
+namespace OpenRewrite.Tests.Rpc;
+
+/// <summary>
+/// When a local NuGet feed (<c>~/.nuget/local-feed</c>) exists for cross-repo
+/// development, the server makes it additive to the recipe project's
+/// <c>nuget.config</c>: it creates one (public + local feed) when none exists,
+/// and otherwise only appends the local feed — preserving a caller-written
+/// config (e.g. an exclusive configured feed) untouched.
+/// </summary>
+public class RecipesNuGetConfigTests
+{
+    private const string LocalFeed = "/home/dev/.nuget/local-feed";
+
+    [Fact]
+    public void CreatesPublicPlusLocalFeedWhenNoConfigExists()
+    {
+        string xml = RewriteRpcServer.BuildRecipesNuGetConfig(null, LocalFeed);
+
+        Assert.Contains("api.nuget.org", xml);
+        Assert.Contains(LocalFeed, xml);
+    }
+
+    [Fact]
+    public void AppendsOnlyLocalFeedToExistingConfig()
+    {
+        // A caller-written exclusive config: nuget.org cleared, one configured feed.
+        string existing = """
+            <?xml version="1.0" encoding="utf-8"?>
+            <configuration>
+              <packageSources>
+                <clear />
+                <add key="internal" value="http://localhost:9091/nuget-all" allowInsecureConnections="true" />
+              </packageSources>
+            </configuration>
+            """;
+
+        string xml = RewriteRpcServer.BuildRecipesNuGetConfig(existing, LocalFeed);
+
+        // Caller's exclusivity and feed are preserved...
+        Assert.Contains("<clear", xml);
+        Assert.Contains("http://localhost:9091/nuget-all", xml);
+        // ...local feed is added...
+        Assert.Contains(LocalFeed, xml);
+        // ...and nuget.org is NOT reintroduced.
+        Assert.DoesNotContain("api.nuget.org", xml);
+    }
+
+    [Fact]
+    public void IsIdempotentWhenLocalFeedAlreadyPresent()
+    {
+        string existing = $"""
+            <?xml version="1.0" encoding="utf-8"?>
+            <configuration>
+              <packageSources>
+                <add key="local-feed" value="{LocalFeed}" />
+              </packageSources>
+            </configuration>
+            """;
+
+        string xml = RewriteRpcServer.BuildRecipesNuGetConfig(existing, LocalFeed);
+
+        int occurrences = xml.Split(LocalFeed).Length - 1;
+        Assert.Equal(1, occurrences);
+    }
+}
diff --git a/rewrite-csharp/csharp/OpenRewrite.Tool/Program.cs b/rewrite-csharp/csharp/OpenRewrite.Tool/Program.cs
index 1d3ef922e65..cdb1011a684 100644
--- a/rewrite-csharp/csharp/OpenRewrite.Tool/Program.cs
+++ b/rewrite-csharp/csharp/OpenRewrite.Tool/Program.cs
@@ -21,6 +21,9 @@
 var logFile = args.FirstOrDefault(a => a.StartsWith("--log-file="))
     ?.Substring("--log-file=".Length);
 
+var recipeInstallDir = args.FirstOrDefault(a => a.StartsWith("--recipe-install-dir="))
+    ?.Substring("--recipe-install-dir=".Length);
+
 var loggerConfig = new LoggerConfiguration();
 if (logFile != null)
 {
@@ -43,5 +46,5 @@
 Log.Debug("<< Parser warmup ({Elapsed})", sw.Elapsed);
 
 Log.Information("Starting RPC server");
-await RewriteRpcServer.RunAsync();
+await RewriteRpcServer.RunAsync(recipeInstallDir: recipeInstallDir);
 Log.Information("RPC server exited");
diff --git a/rewrite-csharp/csharp/OpenRewrite/CSharp/Rpc/RewriteRpcServer.cs b/rewrite-csharp/csharp/OpenRewrite/CSharp/Rpc/RewriteRpcServer.cs
index c81b4eae878..d3235f81e4c 100644
--- a/rewrite-csharp/csharp/OpenRewrite/CSharp/Rpc/RewriteRpcServer.cs
+++ b/rewrite-csharp/csharp/OpenRewrite/CSharp/Rpc/RewriteRpcServer.cs
@@ -17,6 +17,7 @@
 using System.Diagnostics;
 using System.Reflection;
 using System.Runtime.Loader;
+using System.Xml.Linq;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Converters;
 using Newtonsoft.Json.Linq;
@@ -53,6 +54,7 @@ public class RewriteRpcServer
     private readonly ConcurrentDictionary<string, object?> _recipeAccumulators = new();
     private readonly ConcurrentDictionary<string, ExecutionContext> _executionContexts = new();
     private string? _recipesProjectDir;
+    private readonly string? _recipeInstallDir;
     private JsonRpc? _jsonRpc;
     private DotNetBuildContext? _buildContext;
 
@@ -88,9 +90,10 @@ public void Connect(JsonRpc jsonRpc)
         jsonRpc.StartListening();
     }
 
-    public RewriteRpcServer(RecipeMarketplace marketplace)
+    public RewriteRpcServer(RecipeMarketplace marketplace, string? recipeInstallDir = null)
     {
         _marketplace = marketplace;
+        _recipeInstallDir = recipeInstallDir;
 
         // Register type name overrides for nagoya types that don't match Java names
         RpcSendQueue.RegisterJavaTypeName(typeof(CsLambda),
@@ -636,7 +639,11 @@ private string EnsureRecipesProject()
                 return existing;
         }
 
-        _recipesProjectDir = Path.Combine(Path.GetTempPath(), "rewrite-recipes", Guid.NewGuid().ToString("N")[..8]);
+        // Use the caller-supplied recipe install directory when provided (so a
+        // co-located NuGet.config is found by dotnet's project-directory config
+        // walk); otherwise fall back to a unique temp directory.
+        _recipesProjectDir = _recipeInstallDir
+            ?? Path.Combine(Path.GetTempPath(), "rewrite-recipes", Guid.NewGuid().ToString("N")[..8]);
         Directory.CreateDirectory(_recipesProjectDir);
 
         var csprojPath = Path.Combine(_recipesProjectDir, "Recipes.csproj");
@@ -649,26 +656,69 @@ private string EnsureRecipesProject()
             </Project>
             """);
 
-        // Add local NuGet feed as a package source if it exists, so that
-        // locally-published SDK snapshots are discovered alongside nuget.org
+        // For local cross-repo development, make the local NuGet feed additive to
+        // whatever config already lives in the project dir. A caller (e.g. the Moderne
+        // CLI) may have written its own nuget.config there — possibly an exclusive
+        // configured feed — so we must not clobber it: append only the local feed when
+        // a config is present, and create the standalone dev default (public + local
+        // feed) only when none exists. No-ops in production, where local-feed is absent.
         var localFeed = Path.Combine(
             Environment.GetFolderPath(Environment.SpecialFolder.UserProfile),
             ".nuget", "local-feed");
         if (Directory.Exists(localFeed))
         {
             var nugetConfig = Path.Combine(_recipesProjectDir, "nuget.config");
-            File.WriteAllText(nugetConfig, $"""
+            var existing = File.Exists(nugetConfig) ? File.ReadAllText(nugetConfig) : null;
+            File.WriteAllText(nugetConfig, BuildRecipesNuGetConfig(existing, localFeed));
+        }
+
+        return csprojPath;
+    }
+
+    /// <summary>
+    /// Produces the recipe project's <c>nuget.config</c> with the local development
+    /// feed present. When <paramref name="existingConfigXml"/> is null/empty, creates a
+    /// standalone config with nuget.org + the local feed. Otherwise the caller already
+    /// wrote a config (possibly an exclusive configured feed): only the local feed is
+    /// appended to <c>&lt;packageSources&gt;</c>, preserving the caller's sources and any
+    /// <c>&lt;clear/&gt;</c>, and idempotently (no duplicate if already present).
+    /// </summary>
+    internal static string BuildRecipesNuGetConfig(string? existingConfigXml, string localFeedPath)
+    {
+        if (string.IsNullOrWhiteSpace(existingConfigXml))
+        {
+            return $"""
                 <?xml version="1.0" encoding="utf-8"?>
                 <configuration>
                   <packageSources>
                     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
-                    <add key="local-feed" value="{localFeed}" />
+                    <add key="local-feed" value="{localFeedPath}" />
                   </packageSources>
                 </configuration>
-                """);
+                """;
         }
 
-        return csprojPath;
+        var doc = XDocument.Parse(existingConfigXml);
+        var configuration = doc.Element("configuration")
+                            ?? throw new InvalidOperationException("nuget.config is missing its <configuration> root");
+        var packageSources = configuration.Element("packageSources");
+        if (packageSources == null)
+        {
+            packageSources = new XElement("packageSources");
+            configuration.Add(packageSources);
+        }
+
+        bool alreadyPresent = packageSources.Elements("add").Any(e =>
+            string.Equals((string?)e.Attribute("value"), localFeedPath, StringComparison.OrdinalIgnoreCase));
+        if (!alreadyPresent)
+        {
+            packageSources.Add(new XElement("add",
+                new XAttribute("key", "local-feed"),
+                new XAttribute("value", localFeedPath)));
+        }
+
+        var declaration = doc.Declaration ?? new XDeclaration("1.0", "utf-8", null);
+        return declaration + Environment.NewLine + doc.Root!.ToString();
     }
 
     private static void RunDotnet(string arguments)
@@ -1218,6 +1268,7 @@ public override Marker VisitMarker(Marker marker, int p)
     }
 
     public static async Task RunAsync(RecipeMarketplace? marketplace = null,
+        string? recipeInstallDir = null,
         CancellationToken cancellationToken = default)
     {
         marketplace ??= new RecipeMarketplace();
@@ -1246,7 +1297,7 @@ public static async Task RunAsync(RecipeMarketplace? marketplace = null,
         var handler = new HeaderDelimitedMessageHandler(outputStream, inputStream, formatter);
         using var jsonRpc = new StringErrorDataJsonRpc(handler);
 
-        var server = new RewriteRpcServer(marketplace);
+        var server = new RewriteRpcServer(marketplace, recipeInstallDir);
         server._jsonRpc = jsonRpc;
         _current = server;
         // Allow concurrent request dispatch so reentrant callbacks don't deadlock.
diff --git a/rewrite-csharp/src/main/java/org/openrewrite/csharp/rpc/CSharpRewriteRpc.java b/rewrite-csharp/src/main/java/org/openrewrite/csharp/rpc/CSharpRewriteRpc.java
index 0232f4f921a..055b46a70a2 100644
--- a/rewrite-csharp/src/main/java/org/openrewrite/csharp/rpc/CSharpRewriteRpc.java
+++ b/rewrite-csharp/src/main/java/org/openrewrite/csharp/rpc/CSharpRewriteRpc.java
@@ -191,6 +191,7 @@ public static class Builder implements Supplier<CSharpRewriteRpc> {
         private Duration timeout = Duration.ofSeconds(60);
         private boolean traceRpcMessages;
         private @Nullable Path workingDirectory;
+        private @Nullable Path recipeInstallDir;
         private @Nullable Path profileOutputPath;
 
         public Builder marketplace(RecipeMarketplace marketplace) {
@@ -278,6 +279,20 @@ public Builder workingDirectory(@Nullable Path workingDirectory) {
             return this;
         }
 
+        /**
+         * Directory in which the server creates its recipe project (where
+         * {@code dotnet add package} resolves recipe NuGet packages). Set this so a
+         * caller-written {@code NuGet.config} co-located in the directory is honored
+         * by dotnet's project-directory config discovery.
+         *
+         * @param recipeInstallDir The directory to create the recipe project in
+         * @return This builder
+         */
+        public Builder recipeInstallDir(@Nullable Path recipeInstallDir) {
+            this.recipeInstallDir = recipeInstallDir;
+            return this;
+        }
+
         /**
          * Enable .NET EventPipe profiling on the C# process. Captures CPU sampling,
          * GC events, sampled allocations, and runtime counters into a {@code .nettrace}
@@ -310,7 +325,8 @@ public CSharpRewriteRpc get() {
                             dotnetPath.toString(),
                             csharpServerEntry.toAbsolutePath().normalize().toString(),
                             log == null ? null : "--log-file=" + log.toAbsolutePath().normalize(),
-                            traceRpcMessages ? "--trace-rpc-messages" : null
+                            traceRpcMessages ? "--trace-rpc-messages" : null,
+                            recipeInstallDir == null ? null : "--recipe-install-dir=" + recipeInstallDir.toAbsolutePath().normalize()
                     );
                 }
             } else {
@@ -367,7 +383,8 @@ private CSharpRewriteRpc startProcess(Stream<@Nullable String> cmd) {
                     "--project", csproj.toAbsolutePath().normalize().toString(),
                     "--framework", "net10.0",
                     log == null ? null : "--log-file=" + log.toAbsolutePath().normalize(),
-                    traceRpcMessages ? "--trace-rpc-messages" : null
+                    traceRpcMessages ? "--trace-rpc-messages" : null,
+                    recipeInstallDir == null ? null : "--recipe-install-dir=" + recipeInstallDir.toAbsolutePath().normalize()
             );
         }
 
@@ -393,7 +410,8 @@ private CSharpRewriteRpc startProcess(Stream<@Nullable String> cmd) {
             return Stream.of(
                     toolExecutable.toAbsolutePath().normalize().toString(),
                     log == null ? null : "--log-file=" + log.toAbsolutePath().normalize(),
-                    traceRpcMessages ? "--trace-rpc-messages" : null
+                    traceRpcMessages ? "--trace-rpc-messages" : null,
+                    recipeInstallDir == null ? null : "--recipe-install-dir=" + recipeInstallDir.toAbsolutePath().normalize()
             );
         }