diff --git a/USER_GUIDE.md b/USER_GUIDE.md
index 1c7cf7b0e4..e4ab3c7e65 100644
--- a/USER_GUIDE.md
+++ b/USER_GUIDE.md
@@ -1968,6 +1968,9 @@ server over stdio. It reuses the existing CodeIndex database and exposes
 `initialize`, `workspace/symbol`, `textDocument/documentSymbol`,
 `textDocument/definition`, and `textDocument/references` for editors that can
 launch an arbitrary LSP command but do not speak MCP.
+Incoming `textDocument.uri` values are rejected before URI parsing when they
+exceed 4096 characters, matching the MCP resource URI limit and keeping error
+responses bounded.
 
 Tool results include structured JSON in `structuredContent` plus a short text summary in `content`, so AI tools can parse typed data without scraping large text blocks.
 
@@ -4230,6 +4233,8 @@ cdidxには**MCP（Model Context Protocol）サーバー**が組み込まれて
 任意の LSP command を起動できるが MCP には対応していない editor 向けに
 `initialize`、`workspace/symbol`、`textDocument/documentSymbol`、
 `textDocument/definition`、`textDocument/references` を公開します。
+受信した `textDocument.uri` は 4096 文字を超える場合、URI parse の前に拒否されます。
+これは MCP resource URI の上限と揃えており、エラー応答が過大にならないようにします。
 
 ツール結果は `structuredContent` に構造化JSON、`content` に短い要約テキストを返すため、AIツールは巨大なテキストをパースせずに型付きデータを扱えます。
 
diff --git a/changelog.d/unreleased/3129.security.md b/changelog.d/unreleased/3129.security.md
new file mode 100644
index 0000000000..3d0eb17df1
--- /dev/null
+++ b/changelog.d/unreleased/3129.security.md
@@ -0,0 +1,17 @@
+---
+category: security
+issues:
+  - 3129
+affected:
+  - src/CodeIndex/Lsp/LspServer.cs
+  - tests/CodeIndex.Tests/LspServerTests.cs
+  - USER_GUIDE.md
+---
+
+## English
+
+- **LSP text-document URIs are length-capped before parsing (#3129)** — LSP handlers now reject oversized `textDocument.uri` values before URI parsing or path normalization and keep the JSON-RPC error bounded.
+
+## 日本語
+
+- **LSP の text-document URI を parse 前に長さ制限するようになりました (#3129)** — LSP handler は過大な `textDocument.uri` を URI parse や path normalization の前に拒否し、JSON-RPC error を bounded に保ちます。
diff --git a/src/CodeIndex/Lsp/LspServer.cs b/src/CodeIndex/Lsp/LspServer.cs
index 401d62623e..ca69278320 100644
--- a/src/CodeIndex/Lsp/LspServer.cs
+++ b/src/CodeIndex/Lsp/LspServer.cs
@@ -5,6 +5,7 @@
 using System.Text.Json.Nodes;
 using CodeIndex.Cli;
 using CodeIndex.Database;
+using CodeIndex.Mcp;
 using CodeIndex.Models;
 
 namespace CodeIndex.Lsp;
@@ -15,6 +16,7 @@ internal sealed class LspServer : IDisposable
     internal const int MaxLspFrameBytes = 8 * 1024 * 1024;
     internal const int MaxLspHeaderLineBytes = 8 * 1024;
     internal const int MaxPositionDocumentBytes = 4 * 1024 * 1024;
+    internal const int MaxTextDocumentUriChars = McpBoundedText.MaxResourceUriChars;
     internal const int MaxJsonDepth = 32;
     private const int JsonRpcInvalidParamsCode = -32602;
     private const int JsonRpcInternalErrorCode = -32603;
@@ -480,6 +482,9 @@ private static string GetDocumentPath(JsonElement root)
         var uri = GetString(root, "params", "textDocument", "uri");
         if (string.IsNullOrWhiteSpace(uri))
             throw new ArgumentException("textDocument.uri is required.");
+        if (uri.Length > MaxTextDocumentUriChars)
+            throw new ArgumentException(
+                $"textDocument.uri is too long. Max length is {MaxTextDocumentUriChars} characters; actual length is {uri.Length}.");
         return UriToPath(uri);
     }
 
diff --git a/tests/CodeIndex.Tests/LspServerTests.cs b/tests/CodeIndex.Tests/LspServerTests.cs
index 808fe62609..7b316afd2a 100644
--- a/tests/CodeIndex.Tests/LspServerTests.cs
+++ b/tests/CodeIndex.Tests/LspServerTests.cs
@@ -345,6 +345,43 @@ public void HandleMessage_DocumentSymbol_ResolvesDuplicateBasenamesByRelativePat
         }
     }
 
+    [Fact]
+    public void HandleMessage_DocumentSymbol_RejectsOversizedTextDocumentUri_Issue3129()
+    {
+        var projectRoot = TestProjectHelper.CreateTempProject("cdidx_lsp_document_symbol_long_uri");
+        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 oversizedUri = "file:///" + new string('a', LspServer.MaxTextDocumentUriChars);
+            var request = JsonSerializer.Serialize(new
+            {
+                jsonrpc = "2.0",
+                id = 3129,
+                method = "textDocument/documentSymbol",
+                @params = new
+                {
+                    textDocument = new { uri = oversizedUri },
+                },
+            });
+
+            var response = server.HandleMessage(request);
+
+            Assert.NotNull(response);
+            var error = response!["error"]!;
+            Assert.Equal(-32602, error["code"]!.GetValue<int>());
+            var message = error["message"]!.GetValue<string>();
+            Assert.Equal("Invalid params", message);
+            Assert.True(message.Length < 120);
+            Assert.DoesNotContain(oversizedUri, message, StringComparison.Ordinal);
+        }
+        finally
+        {
+            TestProjectHelper.DeleteDirectory(projectRoot);
+        }
+    }
+
     [Fact]
     public void HandleMessage_Definition_ReturnsLocationForTokenAtPosition()
     {