diff --git a/changelog.d/unreleased/3200.security.md b/changelog.d/unreleased/3200.security.md
new file mode 100644
index 0000000000..88d5a47d36
--- /dev/null
+++ b/changelog.d/unreleased/3200.security.md
@@ -0,0 +1,16 @@
+---
+category: security
+issues:
+  - 3200
+affected:
+  - src/CodeIndex/Lsp/LspServer.cs
+  - tests/CodeIndex.Tests/LspServerTests.cs
+---
+
+## English
+
+- **LSP errors no longer expose raw exception messages (#3200)** — LSP invalid-params and internal-failure responses now return stable JSON-RPC messages instead of forwarding implementation exception text to clients.
+
+## 日本語
+
+- **LSP エラーが raw 例外メッセージを公開しなくなりました (#3200)** — LSP の invalid params / internal failure 応答は、実装内部の例外文をクライアントへ転送せず、安定した JSON-RPC メッセージを返すようになりました。
diff --git a/src/CodeIndex/Lsp/LspServer.cs b/src/CodeIndex/Lsp/LspServer.cs
index dd86becd23..401d62623e 100644
--- a/src/CodeIndex/Lsp/LspServer.cs
+++ b/src/CodeIndex/Lsp/LspServer.cs
@@ -16,6 +16,10 @@ internal sealed class LspServer : IDisposable
     internal const int MaxLspHeaderLineBytes = 8 * 1024;
     internal const int MaxPositionDocumentBytes = 4 * 1024 * 1024;
     internal const int MaxJsonDepth = 32;
+    private const int JsonRpcInvalidParamsCode = -32602;
+    private const int JsonRpcInternalErrorCode = -32603;
+    private const string JsonRpcInvalidParamsMessage = "Invalid params";
+    private const string JsonRpcInternalErrorMessage = "Internal error";
     private static readonly JsonDocumentOptions LspJsonDocumentOptions = new()
     {
         MaxDepth = MaxJsonDepth,
@@ -98,9 +102,13 @@ public int Run(Stream input, Stream output)
                     _ => hasId ? Error(id, -32601, $"Method not found: {method}") : null,
                 };
             }
-            catch (Exception ex) when (ex is ArgumentException or InvalidOperationException or JsonException or IOException)
+            catch (Exception ex) when (ex is ArgumentException or JsonException)
             {
-                return hasId ? Error(id, -32602, ex.Message) : null;
+                return hasId ? Error(id, JsonRpcInvalidParamsCode, JsonRpcInvalidParamsMessage) : null;
+            }
+            catch (Exception ex) when (ex is InvalidOperationException or IOException)
+            {
+                return hasId ? Error(id, JsonRpcInternalErrorCode, JsonRpcInternalErrorMessage) : null;
             }
         }
     }
diff --git a/tests/CodeIndex.Tests/LspServerTests.cs b/tests/CodeIndex.Tests/LspServerTests.cs
index 013906373e..808fe62609 100644
--- a/tests/CodeIndex.Tests/LspServerTests.cs
+++ b/tests/CodeIndex.Tests/LspServerTests.cs
@@ -105,6 +105,75 @@ public void HandleMessage_TooDeepJson_ReturnsParseError_Issue3021()
         }
     }
 
+    [Fact]
+    public void HandleMessage_InvalidParams_ReturnsStableErrorMessage_Issue3200()
+    {
+        var projectRoot = TestProjectHelper.CreateTempProject("cdidx_lsp_invalid_params");
+        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 = 20,
+                method = "textDocument/documentSymbol",
+                @params = new
+                {
+                    textDocument = new { uri = string.Empty },
+                },
+            });
+
+            var response = server.HandleMessage(request);
+
+            Assert.NotNull(response);
+            Assert.Equal(-32602, response!["error"]!["code"]!.GetValue<int>());
+            var message = response["error"]!["message"]!.GetValue<string>();
+            Assert.Equal("Invalid params", message);
+            Assert.DoesNotContain("textDocument.uri", message, StringComparison.Ordinal);
+        }
+        finally
+        {
+            TestProjectHelper.DeleteDirectory(projectRoot);
+        }
+    }
+
+    [Fact]
+    public void HandleMessage_InternalFailure_ReturnsStableErrorMessage_Issue3200()
+    {
+        var projectRoot = TestProjectHelper.CreateTempProject("cdidx_lsp_internal_error");
+        try
+        {
+            var dbPath = TestProjectHelper.CreateProjectDb(projectRoot);
+            var db = new DbContext(dbPath);
+            using var server = new LspServer(new DbReader(db), "1.2.3", ProgramRunner.CreateDefaultJsonOptions(), projectRoot);
+            db.Dispose();
+            var request = JsonSerializer.Serialize(new
+            {
+                jsonrpc = "2.0",
+                id = 21,
+                method = "workspace/symbol",
+                @params = new
+                {
+                    query = "Needle",
+                },
+            });
+
+            var response = server.HandleMessage(request);
+
+            Assert.NotNull(response);
+            Assert.Equal(-32603, response!["error"]!["code"]!.GetValue<int>());
+            var message = response["error"]!["message"]!.GetValue<string>();
+            Assert.Equal("Internal error", message);
+            Assert.DoesNotContain(nameof(ObjectDisposedException), message, StringComparison.Ordinal);
+        }
+        finally
+        {
+            TestProjectHelper.DeleteDirectory(projectRoot);
+        }
+    }
+
     [Fact]
     public void Run_MalformedJsonFrame_WritesParseErrorAndContinues()
     {