diff --git a/codex-rs/core/src/hook_runtime.rs b/codex-rs/core/src/hook_runtime.rs
index 5f27a7fc30..9a92854515 100644
--- a/codex-rs/core/src/hook_runtime.rs
+++ b/codex-rs/core/src/hook_runtime.rs
@@ -231,7 +231,6 @@ pub(crate) async fn run_post_tool_use_hooks(
     tool_input: Value,
     tool_response: Value,
 ) -> PostToolUseOutcome {
-    let is_mcp_tool = tool_name.starts_with("mcp__");
     let request = PostToolUseRequest {
         session_id: sess.conversation_id,
         turn_id: turn_context.sub_id.clone(),
@@ -244,7 +243,6 @@ pub(crate) async fn run_post_tool_use_hooks(
         tool_use_id,
         tool_input,
         tool_response,
-        is_mcp_tool,
     };
     let hooks = sess.hooks();
     let preview_runs = hooks.preview_post_tool_use(&request);
diff --git a/codex-rs/core/src/tools/registry.rs b/codex-rs/core/src/tools/registry.rs
index 84d10fe77d..7c04e12c29 100644
--- a/codex-rs/core/src/tools/registry.rs
+++ b/codex-rs/core/src/tools/registry.rs
@@ -482,7 +482,7 @@ impl ToolRegistry {
                 let mut guard = response_cell.lock().await;
                 if let Some(result) = guard.as_mut() {
                     result.result = Box::new(FunctionToolOutput::from_text(
-                        post_tool_use_output_text(updated_tool_output),
+                        post_tool_use_output_to_model_text(updated_tool_output),
                         Some(true),
                     ));
                 }
@@ -522,7 +522,12 @@ impl ToolRegistry {
     }
 }
 
-fn post_tool_use_output_text(output: &Value) -> String {
+/// Converts hook-facing JSON output into the text-only function output sent to the model.
+///
+/// Hook authors may return either plain strings or structured JSON values. Preserve
+/// strings without JSON quoting, and serialize structured values so the model still
+/// sees the replacement faithfully through the text-only response channel.
+fn post_tool_use_output_to_model_text(output: &Value) -> String {
     match output {
         Value::String(text) => text.clone(),
         _ => output.to_string(),
diff --git a/codex-rs/hooks/src/events/post_tool_use.rs b/codex-rs/hooks/src/events/post_tool_use.rs
index 7961a4261d..5d3de74241 100644
--- a/codex-rs/hooks/src/events/post_tool_use.rs
+++ b/codex-rs/hooks/src/events/post_tool_use.rs
@@ -31,7 +31,6 @@ pub struct PostToolUseRequest {
     pub tool_use_id: String,
     pub tool_input: Value,
     pub tool_response: Value,
-    pub is_mcp_tool: bool,
 }
 
 #[derive(Debug)]
@@ -118,7 +117,7 @@ pub(crate) async fn run(
 
     let mut results = results;
     let updated_tool_output =
-        select_updated_tool_output(&mut results, request.is_mcp_tool, &request.tool_response);
+        select_updated_tool_output(&mut results, &request.tool_name, &request.tool_response);
     let additional_contexts = common::flatten_additional_contexts(
         results
             .iter()
@@ -332,11 +331,18 @@ fn serialization_failure_outcome(hook_events: Vec<HookCompletedEvent>) -> PostTo
     }
 }
 
+/// Selects the final hook-provided replacement that the model should see.
+///
+/// For each handler, `updatedToolOutput` takes precedence over
+/// `updatedMCPToolOutput`. MCP-specific replacements only apply to MCP tools,
+/// MCP outputs bypass built-in shape checks, and later valid replacements win
+/// over earlier ones in configured hook order.
 fn select_updated_tool_output(
     results: &mut [dispatcher::ParsedHandler<PostToolUseHandlerData>],
-    is_mcp_tool: bool,
+    tool_name: &str,
     original_tool_response: &Value,
 ) -> Option<Value> {
+    let is_mcp_tool = tool_name.starts_with("mcp__");
     let mut selected = None;
 
     for result in results {
@@ -648,11 +654,7 @@ mod tests {
         ];
 
         assert_eq!(
-            super::select_updated_tool_output(
-                &mut results,
-                /*is_mcp_tool*/ false,
-                &json!("old")
-            ),
+            super::select_updated_tool_output(&mut results, "Bash", &json!("old")),
             Some(json!("second"))
         );
     }
@@ -670,11 +672,7 @@ mod tests {
         )];
 
         assert_eq!(
-            super::select_updated_tool_output(
-                &mut results,
-                /*is_mcp_tool*/ false,
-                &json!("old")
-            ),
+            super::select_updated_tool_output(&mut results, "Bash", &json!("old")),
             None
         );
         assert_eq!(
@@ -700,7 +698,7 @@ mod tests {
         )];
 
         assert_eq!(
-            super::select_updated_tool_output(&mut results, /*is_mcp_tool*/ true, &json!({})),
+            super::select_updated_tool_output(&mut results, "mcp__memory__lookup", &json!({})),
             Some(json!({"source": "generic"}))
         );
     }
@@ -718,7 +716,7 @@ mod tests {
         )];
 
         assert_eq!(
-            super::select_updated_tool_output(&mut results, /*is_mcp_tool*/ true, &json!({})),
+            super::select_updated_tool_output(&mut results, "mcp__memory__lookup", &json!({})),
             Some(json!({"source": "mcp"}))
         );
     }
@@ -762,7 +760,6 @@ mod tests {
             tool_use_id: tool_use_id.to_string(),
             tool_input: json!({ "command": "echo hello" }),
             tool_response: json!({"ok": true}),
-            is_mcp_tool: false,
         }
     }
 }