fix: make toolset switching atomic to prevent tool desync race (#648)

Co-authored-by: Letta <noreply@letta.com>

src/tools/toolset.ts

@@ -2,7 +2,7 @@ import { getClient } from "../agent/client";
 import { resolveModel } from "../agent/model";
 import { toolFilter } from "./filter";
 import {
-  clearTools,
+  clearToolsWithLock,
   GEMINI_PASCAL_TOOLS,
   getToolNames,
   isOpenAIModel,
@@ -121,14 +121,14 @@ export async function forceToolsetSwitch(
   toolsetName: ToolsetName,
   agentId: string,
 ): Promise<void> {
-  // Clear currently loaded tools
-  clearTools();
-
   // Load the appropriate toolset
-  // Map toolset name to a model identifier for loading
+  // Note: loadTools/loadSpecificTools acquire a switch lock that causes
+  // sendMessageStream to wait, preventing messages from being sent with
+  // stale or partial tools during the switch.
   let modelForLoading: string;
   if (toolsetName === "none") {
-    // Just clear tools, no loading needed
+    // Clear tools with lock protection so sendMessageStream() waits
+    clearToolsWithLock();
     return;
   } else if (toolsetName === "codex") {
     await loadSpecificTools([...CODEX_TOOLS]);
@@ -174,8 +174,9 @@ export async function switchToolsetForModel(
   // Resolve model ID to handle when possible so provider checks stay consistent
   const resolvedModel = resolveModel(modelIdentifier) ?? modelIdentifier;
 
-  // Clear currently loaded tools and load the appropriate set for the target model
-  clearTools();
+  // Load the appropriate set for the target model
+  // Note: loadTools acquires a switch lock that causes sendMessageStream to wait,
+  // preventing messages from being sent with stale or partial tools during the switch.
   await loadTools(resolvedModel);
 
   // If no tools were loaded (e.g., unexpected handle or edge-case filter),