From 1bfabb21fe56e21adce3aed945ab74aff27b67d2 Mon Sep 17 00:00:00 2001
From: Angad Singh <angad@openai.com>
Date: Wed, 19 Aug 2026 18:29:59 +0000
Subject: [PATCH] Raise the MCP tool name limit to 128 bytes (#39594)

## Why

The Responses API accepts tool names up to 128 bytes, but MCP tool name
normalization limited model-visible names to 64 bytes.

## What changed

- Preserve MCP tool names up to the 128-byte Responses API limit.
- Continue shortening and hashing names that exceed the limit so generated
  names remain bounded and unique.

## Testing

- Cover names at the 128-byte boundary and immediately above it.
- Verify distinct long names remain 128 bytes and code-mode compatible.

GitOrigin-RevId: 75544d00e75dd0e9328b3e2ac763d26f9cc99a48
---
 .../codex-mcp/src/connection_manager_tests.rs | 37 ++++++++++++++-----
 codex-rs/codex-mcp/src/tools.rs               |  4 +-
 2 files changed, 30 insertions(+), 11 deletions(-)

diff --git a/codex-rs/codex-mcp/src/connection_manager_tests.rs b/codex-rs/codex-mcp/src/connection_manager_tests.rs
index fac9f6731b..a33ddf3323 100644
--- a/codex-rs/codex-mcp/src/connection_manager_tests.rs
+++ b/codex-rs/codex-mcp/src/connection_manager_tests.rs
@@ -1440,19 +1440,38 @@ fn test_normalize_tools_duplicated_names_skipped() {
     );
 }
 
+#[test]
+fn test_normalize_tools_respects_responses_api_name_length_boundaries() {
+    let namespace = "mcp__codex_apps";
+    let namespace_len = namespace.len() + "__".len();
+
+    for total_len in [128, 129] {
+        let tool_name = "a".repeat(total_len - namespace_len);
+        let model_tools = normalize_tools_for_model_with_prefix(
+            vec![create_test_tool("codex_apps", &tool_name)],
+            /*prefix_mcp_tool_names*/ true,
+            &[],
+        );
+        let model_name = model_tools[0].canonical_tool_name();
+
+        assert_eq!(model_tool_name_len(&model_name), 128);
+        if total_len == 128 {
+            assert_eq!(model_name, ToolName::namespaced(namespace, tool_name));
+        } else {
+            assert_ne!(model_name.name, tool_name);
+        }
+    }
+}
+
 #[test]
 fn test_normalize_tools_long_names_same_server() {
     let server_name = "my_server";
+    let first_name = "a".repeat(128);
+    let second_name = "b".repeat(128);
 
     let tools = vec![
-        create_test_tool(
-            server_name,
-            "extremely_lengthy_function_name_that_absolutely_surpasses_all_reasonable_limits",
-        ),
-        create_test_tool(
-            server_name,
-            "yet_another_extremely_lengthy_function_name_that_absolutely_surpasses_all_reasonable_limits",
-        ),
+        create_test_tool(server_name, &first_name),
+        create_test_tool(server_name, &second_name),
     ];
 
     let model_tools =
@@ -1462,7 +1481,7 @@ fn test_normalize_tools_long_names_same_server() {
 
     let names = model_tool_names(&model_tools);
 
-    assert!(names.iter().all(|name| model_tool_name_len(name) == 64));
+    assert!(names.iter().all(|name| model_tool_name_len(name) == 128));
     assert!(
         names
             .iter()
diff --git a/codex-rs/codex-mcp/src/tools.rs b/codex-rs/codex-mcp/src/tools.rs
index a7ae51925d..51d3f05aac 100644
--- a/codex-rs/codex-mcp/src/tools.rs
+++ b/codex-rs/codex-mcp/src/tools.rs
@@ -106,7 +106,7 @@ pub(crate) fn filter_tools(tools: Vec<ToolInfo>, filter: &ToolFilter) -> Vec<Too
 ///
 /// Raw MCP server/tool names are kept on each [`ToolInfo`] for protocol calls, while
 /// `callable_namespace` / `callable_name` are sanitized and, when necessary, hashed so
-/// every model-visible name is unique and <= 64 bytes.
+/// every model-visible name is unique and <= 128 bytes.
 ///
 /// When `prefix_mcp_tool_names` is true, the historical `mcp__` namespace
 /// prefix is added except for tools from `non_prefixed_mcp_tool_servers`.
@@ -223,7 +223,7 @@ struct CallableToolCandidate {
 }
 
 const MCP_TOOL_NAME_DELIMITER: &str = "__";
-const MAX_TOOL_NAME_LENGTH: usize = 64;
+const MAX_TOOL_NAME_LENGTH: usize = 128;
 const CALLABLE_NAME_HASH_LEN: usize = 12;
 fn callable_namespace_with_prefix(namespace: &str, prefix_mcp_tool_names: bool) -> String {
     if !prefix_mcp_tool_names || namespace.starts_with(LEGACY_MCP_TOOL_NAME_PREFIX) {