From 18fecc6723718d3abae6dadd3289d4a463bd6f3f Mon Sep 17 00:00:00 2001
From: Mitsuhiro Kotake <kotakem@openai.com>
Date: Tue, 23 Jun 2026 13:58:49 +0900
Subject: [PATCH] Disambiguate flattened MCP tool names

---
 .../codex-mcp/src/connection_manager_tests.rs | 21 +++++++++++++++++++
 codex-rs/codex-mcp/src/tools.rs               | 13 ++++++++----
 2 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/codex-rs/codex-mcp/src/connection_manager_tests.rs b/codex-rs/codex-mcp/src/connection_manager_tests.rs
index 508598d343..9920e71448 100644
--- a/codex-rs/codex-mcp/src/connection_manager_tests.rs
+++ b/codex-rs/codex-mcp/src/connection_manager_tests.rs
@@ -481,6 +481,27 @@ fn test_normalize_tools_disambiguates_sanitized_tool_name_collisions() {
     assert_eq!(callable_tool_names.len(), 2);
 }
 
+#[test]
+fn test_normalize_tools_disambiguates_canonical_flat_name_collisions() {
+    let tools = vec![create_test_tool("a", "b__c"), create_test_tool("a__b", "c")];
+
+    let model_tools =
+        normalize_tools_for_model_with_prefix(tools, /*prefix_mcp_tool_names*/ true);
+
+    assert_eq!(model_tools.len(), 2);
+    assert_eq!(model_tool_names(&model_tools).len(), 2);
+    let canonical_flat_names = model_tools
+        .iter()
+        .map(|tool| {
+            tool.canonical_tool_name()
+                .canonical_flat_name()
+                .into_owned()
+        })
+        .collect::<HashSet<_>>();
+    assert_eq!(canonical_flat_names.len(), 2);
+    assert!(canonical_flat_names.contains("mcp__a__b__c"));
+}
+
 #[test]
 fn tool_filter_allows_by_default() {
     let filter = ToolFilter::default();
diff --git a/codex-rs/codex-mcp/src/tools.rs b/codex-rs/codex-mcp/src/tools.rs
index 83cbfd5e74..906432a0cf 100644
--- a/codex-rs/codex-mcp/src/tools.rs
+++ b/codex-rs/codex-mcp/src/tools.rs
@@ -376,8 +376,8 @@ fn unique_callable_parts(
     used_names: &mut HashSet<String>,
     reserved_len: usize,
 ) -> (String, String) {
-    let model_name = format!("{namespace}{tool_name}");
-    if model_name.len() + reserved_len <= MAX_TOOL_NAME_LENGTH && used_names.insert(model_name) {
+    let canonical_flat_name = canonical_flat_tool_name(namespace, tool_name);
+    if canonical_flat_name.len() <= MAX_TOOL_NAME_LENGTH && used_names.insert(canonical_flat_name) {
         return (namespace.to_string(), tool_name.to_string());
     }
 
@@ -390,10 +390,15 @@ fn unique_callable_parts(
         };
         let (namespace, tool_name) =
             fit_callable_parts_with_hash(namespace, tool_name, &hash_input, reserved_len);
-        let model_name = format!("{namespace}{tool_name}");
-        if used_names.insert(model_name) {
+        if used_names.insert(canonical_flat_tool_name(&namespace, &tool_name)) {
             return (namespace, tool_name);
         }
         attempt = attempt.saturating_add(1);
     }
 }
+
+fn canonical_flat_tool_name(namespace: &str, tool_name: &str) -> String {
+    ToolName::namespaced(namespace, tool_name)
+        .canonical_flat_name()
+        .into_owned()
+}