diff --git a/.changeset/tool-call-manager-repeat-start.md b/.changeset/tool-call-manager-repeat-start.md
new file mode 100644
index 0000000000..bccb6fe17a
--- /dev/null
+++ b/.changeset/tool-call-manager-repeat-start.md
@@ -0,0 +1,5 @@
+---
+'@tanstack/ai': patch
+---
+
+Fix `ToolCallManager.addToolCallStartEvent` running a tool call twice (or wiping its accumulated arguments) when a producer sends a repeat `TOOL_CALL_START` for a `toolCallId` that is already tracked. AG-UI's `TOOL_CALL_START` carries no `index`, so a custom/malformed stream that re-sends START for the same id could either overwrite the tracked entry's arguments back to `''` (same index) or insert a duplicate row that `getToolCalls()` returned twice (missing/different index). Repeats for an already-tracked id are now ignored; first-party adapters, which only emit START once per call, are unaffected.
diff --git a/packages/ai/src/activities/chat/tools/tool-calls.ts b/packages/ai/src/activities/chat/tools/tool-calls.ts
index 78637427ac..4d5cf811f6 100644
--- a/packages/ai/src/activities/chat/tools/tool-calls.ts
+++ b/packages/ai/src/activities/chat/tools/tool-calls.ts
@@ -233,6 +233,15 @@ export class ToolCallManager<
    * Add a TOOL_CALL_START event to begin tracking a tool call (AG-UI)
    */
   addToolCallStartEvent(event: ToolCallStartEvent): void {
+    // AG-UI's TOOL_CALL_START carries no index, and a non-first-party or
+    // malformed producer can send a second START for a toolCallId that is
+    // already tracked. Without this guard, a repeat with the same index
+    // overwrites the slot (wiping any TOOL_CALL_ARGS already accumulated),
+    // and a repeat with a missing/different index inserts a duplicate row
+    // that getToolCalls() returns twice, running the tool twice.
+    for (const toolCall of this.toolCallsMap.values()) {
+      if (toolCall.id === event.toolCallId) return
+    }
     const index = (event as AdapterYieldChunk).index ?? this.toolCallsMap.size
     const name = event.toolCallName ?? event.toolName
     this.toolCallsMap.set(index, {
diff --git a/packages/ai/tests/tool-call-manager.test.ts b/packages/ai/tests/tool-call-manager.test.ts
index f1e45c691e..26158a66f4 100644
--- a/packages/ai/tests/tool-call-manager.test.ts
+++ b/packages/ai/tests/tool-call-manager.test.ts
@@ -491,6 +491,52 @@ describe('ToolCallManager', () => {
       expect(toolCalls).toHaveLength(1)
       expect(toolCalls[0]?.function.arguments).toBe('{"location":"New York"}')
     })
+
+    it('should ignore a repeat TOOL_CALL_START for an already-tracked toolCallId (same explicit index)', () => {
+      const manager = new ToolCallManager([mockWeatherTool])
+
+      // AG-UI's TOOL_CALL_START carries no `index`, but first-party adapter
+      // yields still attach one (see AdapterYieldChunk). Build the event
+      // directly so the explicit index survives, matching this issue's repro.
+      const startWithIndex = (index: number) =>
+        ({
+          ...toolCallStart({
+            toolCallId: 'call_123',
+            toolCallName: 'get_weather',
+          }),
+          index,
+        }) as ToolCallStartEvent
+
+      manager.addToolCallStartEvent(startWithIndex(0))
+      manager.addToolCallArgsEvent(
+        toolCallArgs({ toolCallId: 'call_123', delta: '{"location":"Paris"}' }),
+      )
+      // A malformed/custom-server stream re-sends START for the same id/index.
+      manager.addToolCallStartEvent(startWithIndex(0))
+
+      const toolCalls = manager.getToolCalls()
+      expect(toolCalls).toHaveLength(1)
+      // Accumulated arguments must survive the repeat START, not reset to ''.
+      expect(toolCalls[0]?.function.arguments).toBe('{"location":"Paris"}')
+    })
+
+    it('should ignore a repeat TOOL_CALL_START for an already-tracked toolCallId (no index)', () => {
+      const manager = new ToolCallManager([mockWeatherTool])
+
+      manager.addToolCallStartEvent(
+        toolCallStart({ toolCallId: 'call_123', toolCallName: 'get_weather' }),
+      )
+      // Repeat START with no index — must not insert a second row, which
+      // would otherwise make getToolCalls() return the id twice and run
+      // the tool twice.
+      manager.addToolCallStartEvent(
+        toolCallStart({ toolCallId: 'call_123', toolCallName: 'get_weather' }),
+      )
+
+      const toolCalls = manager.getToolCalls()
+      expect(toolCalls).toHaveLength(1)
+      expect(toolCalls.filter((tc) => tc.id === 'call_123')).toHaveLength(1)
+    })
   })
 })