diff --git a/public/index.html b/public/index.html
index f06519d58..071a6efa4 100644
--- a/public/index.html
+++ b/public/index.html
@@ -2011,7 +2011,7 @@
                                             <strong data-i18n="enable_functions_desc_4">Not supported when Prompt Post-Processing with "no tools" is used!</strong>
                                         </div>
                                     </div>
-                                    <div class="range-block" data-source="openrouter">
+                                    <div class="range-block" data-source="openrouter,custom">
                                         <div class="flex-container flexFlowColumn wide100p textAlignCenter marginTop10">
                                             <div class="flex-container oneline-dropdown">
                                                 <label for="tool_reasoning_mode" data-i18n="Interleaved Thinking">
diff --git a/public/scripts/openai.js b/public/scripts/openai.js
index 5571bf7f9..d50f22fe9 100644
--- a/public/scripts/openai.js
+++ b/public/scripts/openai.js
@@ -257,6 +257,7 @@ export const tool_reasoning_modes = {
 // Providers that support interleaved reasoning forwarding in tool-call chains.
 const interleaved_reasoning_providers = [
     chat_completion_sources.OPENROUTER,
+    chat_completion_sources.CUSTOM,
 ];
 
 export const ZAI_ENDPOINT = {
@@ -1019,8 +1020,8 @@ async function populateChatHistory(messages, prompts, chatCompletion, type = nul
                 const clone = structuredClone(invocation);
                 if (!reasoningIsEligible) {
                     delete clone.reasoning;
-                } else if (previousAssistantReasoning) {
-                    // Prefer currently editable assistant-text reasoning based on forwarding mode over invocation snapshot.
+                } else if (previousAssistantReasoning && !clone.reasoning) {
+                    // Fall back to adjacent assistant-text reasoning only when the invocation has none of its own.
                     clone.reasoning = previousAssistantReasoning;
                 }
                 return clone;