From 4b330c93131f93f77dae5fa295dafe532325b8d2 Mon Sep 17 00:00:00 2001
From: Will Willems <rut.willems@gmail.com>
Date: Fri, 20 Feb 2026 16:39:18 +0100
Subject: [PATCH] fix(ai-client): prevent continuation stall when multiple
 client tools complete in the same round

Add re-entrancy guard on drainPostStreamActions to prevent nested drains
from stealing queued continuation checks, and add a tool-result type guard
in checkForContinuation to prevent spurious continuation requests after the
model has already produced its text response.

Closes #302
---
 .../fix-client-tool-continuation-stall.md     |  7 +++++++
 .../typescript/ai-client/src/chat-client.ts   | 20 ++++++++++++++++---
 2 files changed, 24 insertions(+), 3 deletions(-)
 create mode 100644 .changeset/fix-client-tool-continuation-stall.md

diff --git a/.changeset/fix-client-tool-continuation-stall.md b/.changeset/fix-client-tool-continuation-stall.md
new file mode 100644
index 00000000..b330e26e
--- /dev/null
+++ b/.changeset/fix-client-tool-continuation-stall.md
@@ -0,0 +1,7 @@
+---
+'@tanstack/ai-client': patch
+---
+
+fix: prevent client tool continuation stall when multiple tools complete in the same round
+
+When an LLM response triggers multiple client-side tool calls simultaneously, the chat would permanently stall after all tools completed. This was caused by nested `drainPostStreamActions` calls stealing queued continuation checks from the outer drain. Added a re-entrancy guard on `drainPostStreamActions` and a `tool-result` type check in `checkForContinuation` to prevent both the structural and semantic causes of the stall.
diff --git a/packages/typescript/ai-client/src/chat-client.ts b/packages/typescript/ai-client/src/chat-client.ts
index 27078d5e..a89eedb5 100644
--- a/packages/typescript/ai-client/src/chat-client.ts
+++ b/packages/typescript/ai-client/src/chat-client.ts
@@ -40,6 +40,8 @@ export class ChatClient {
   private pendingToolExecutions: Map<string, Promise<void>> = new Map()
   // Flag to deduplicate continuation checks during action draining
   private continuationPending = false
+  // Re-entrancy guard for drainPostStreamActions
+  private draining = false
 
   private callbacksRef: {
     current: {
@@ -619,9 +621,15 @@ export class ChatClient {
    * Drain and execute all queued post-stream actions
    */
   private async drainPostStreamActions(): Promise<void> {
-    while (this.postStreamActions.length > 0) {
-      const action = this.postStreamActions.shift()!
-      await action()
+    if (this.draining) return
+    this.draining = true
+    try {
+      while (this.postStreamActions.length > 0) {
+        const action = this.postStreamActions.shift()!
+        await action()
+      }
+    } finally {
+      this.draining = false
     }
   }
 
@@ -634,6 +642,12 @@ export class ChatClient {
       return
     }
 
+    const messages = this.processor.getMessages()
+    const lastPart = messages.at(-1)?.parts.at(-1)
+    if (lastPart?.type !== 'tool-result') {
+      return
+    }
+
     if (this.shouldAutoSend()) {
       this.continuationPending = true
       try {