diff --git a/changelog.d/unreleased/3127.security.md b/changelog.d/unreleased/3127.security.md
new file mode 100644
index 0000000000..e29338e2e8
--- /dev/null
+++ b/changelog.d/unreleased/3127.security.md
@@ -0,0 +1,16 @@
+---
+category: security
+issues:
+  - 3127
+affected:
+  - src/CodeIndex/Lsp/LspServer.cs
+  - tests/CodeIndex.Tests/LspServerTests.cs
+---
+
+## English
+
+- **LSP unknown method diagnostics truncate method names (#3127)** — unknown JSON-RPC method names now use bounded diagnostic text before being echoed in `Method not found` responses.
+
+## 日本語
+
+- **LSP の未知 method 診断で method 名を切り詰めるようになりました (#3127)** — 未知 JSON-RPC method 名は `Method not found` response に埋め込む前に bounded diagnostic text へ変換されます。
diff --git a/src/CodeIndex/Lsp/LspServer.cs b/src/CodeIndex/Lsp/LspServer.cs
index ca69278320..d706360d08 100644
--- a/src/CodeIndex/Lsp/LspServer.cs
+++ b/src/CodeIndex/Lsp/LspServer.cs
@@ -18,6 +18,7 @@ internal sealed class LspServer : IDisposable
     internal const int MaxPositionDocumentBytes = 4 * 1024 * 1024;
     internal const int MaxTextDocumentUriChars = McpBoundedText.MaxResourceUriChars;
     internal const int MaxJsonDepth = 32;
+    internal const int MaxUnknownMethodDiagnosticChars = 240;
     private const int JsonRpcInvalidParamsCode = -32602;
     private const int JsonRpcInternalErrorCode = -32603;
     private const string JsonRpcInvalidParamsMessage = "Invalid params";
@@ -101,7 +102,7 @@ public int Run(Stream input, Stream output)
                     "textDocument/documentSymbol" => Result(id, DocumentSymbol(root)),
                     "textDocument/definition" => Result(id, Definition(root)),
                     "textDocument/references" => Result(id, References(root)),
-                    _ => hasId ? Error(id, -32601, $"Method not found: {method}") : null,
+                    _ => hasId ? Error(id, -32601, $"Method not found: {SanitizeUnknownMethod(method)}") : null,
                 };
             }
             catch (Exception ex) when (ex is ArgumentException or JsonException)
@@ -115,6 +116,23 @@ public int Run(Stream input, Stream output)
         }
     }
 
+    private static string SanitizeUnknownMethod(string method)
+    {
+        var wasTruncated = method.Length > MaxUnknownMethodDiagnosticChars;
+        var boundedMethod = wasTruncated ? method[..MaxUnknownMethodDiagnosticChars] : method;
+        var sanitized = boundedMethod
+            .Replace('\r', ' ')
+            .Replace('\n', ' ')
+            .Replace('\t', ' ')
+            .Trim();
+        return AppendEllipsisIfNeeded(sanitized, wasTruncated);
+    }
+
+    private static string AppendEllipsisIfNeeded(string value, bool wasTruncated)
+        => wasTruncated && !value.EndsWith("...", StringComparison.Ordinal)
+            ? value + "..."
+            : value;
+
     private JsonObject HandleShutdown(JsonNode? id)
     {
         _shutdownRequested = true;
diff --git a/tests/CodeIndex.Tests/LspServerTests.cs b/tests/CodeIndex.Tests/LspServerTests.cs
index 7b316afd2a..8706c17011 100644
--- a/tests/CodeIndex.Tests/LspServerTests.cs
+++ b/tests/CodeIndex.Tests/LspServerTests.cs
@@ -105,6 +105,68 @@ public void HandleMessage_TooDeepJson_ReturnsParseError_Issue3021()
         }
     }
 
+    [Fact]
+    public void HandleMessage_UnknownMethod_TruncatesMethodName_Issue3127()
+    {
+        var projectRoot = TestProjectHelper.CreateTempProject("cdidx_lsp_unknown_method");
+        try
+        {
+            var dbPath = TestProjectHelper.CreateProjectDb(projectRoot);
+            using var db = new DbContext(dbPath);
+            using var server = new LspServer(new DbReader(db), "1.2.3", ProgramRunner.CreateDefaultJsonOptions(), projectRoot);
+            var method = new string('m', LspServer.MaxLspFrameBytes - 4096) + "UNBOUNDED_SENTINEL";
+            var request = JsonSerializer.Serialize(new
+            {
+                jsonrpc = "2.0",
+                id = 1,
+                method,
+            });
+
+            var response = server.HandleMessage(request);
+
+            Assert.NotNull(response);
+            var error = response!["error"]!;
+            Assert.Equal(-32601, error["code"]!.GetValue<int>());
+            var message = error["message"]!.GetValue<string>();
+            Assert.StartsWith("Method not found: ", message, StringComparison.Ordinal);
+            Assert.EndsWith("...", message, StringComparison.Ordinal);
+            Assert.DoesNotContain("UNBOUNDED_SENTINEL", message, StringComparison.Ordinal);
+            Assert.True(message.Length <= "Method not found: ".Length + LspServer.MaxUnknownMethodDiagnosticChars + "...".Length);
+        }
+        finally
+        {
+            TestProjectHelper.DeleteDirectory(projectRoot);
+        }
+    }
+
+    [Fact]
+    public void HandleMessage_UnknownMethod_PreservesSlashDelimitedMethodName_Issue3127()
+    {
+        var projectRoot = TestProjectHelper.CreateTempProject("cdidx_lsp_unknown_method_slash");
+        try
+        {
+            var dbPath = TestProjectHelper.CreateProjectDb(projectRoot);
+            using var db = new DbContext(dbPath);
+            using var server = new LspServer(new DbReader(db), "1.2.3", ProgramRunner.CreateDefaultJsonOptions(), projectRoot);
+            var request = JsonSerializer.Serialize(new
+            {
+                jsonrpc = "2.0",
+                id = 1,
+                method = "textDocument/hover",
+            });
+
+            var response = server.HandleMessage(request);
+
+            Assert.NotNull(response);
+            Assert.Equal(-32601, response!["error"]!["code"]!.GetValue<int>());
+            Assert.Equal("Method not found: textDocument/hover", response["error"]!["message"]!.GetValue<string>());
+        }
+        finally
+        {
+            TestProjectHelper.DeleteDirectory(projectRoot);
+        }
+    }
+
     [Fact]
     public void HandleMessage_InvalidParams_ReturnsStableErrorMessage_Issue3200()
     {