fix(ai-gemini): dedupe functionResponse parts by id, not name

.changeset/gemini-parallel-tool-dedup.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/ai-gemini': patch
+---
+
+Fix `mergeConsecutiveSameRoleMessages` deduplicating `functionResponse` parts by `name` instead of `id`. Two parallel calls to the same tool in one turn share a `name` but have distinct ids, so the second response was silently dropped, leaving Gemini with fewer response parts than call parts on the next request (`400 INVALID_ARGUMENT: ... number of function response parts is equal to the number of function call parts`). Deduping by `id` still collapses a genuine duplicate tool result while preserving both responses for same-tool parallel calls.

packages/ai-gemini/src/adapters/text.ts

@@ -759,7 +759,7 @@ export class GeminiTextAdapter<
    * user messages in multi-turn conversations.
    *
    * Also filters out empty model messages (e.g., from a previous failed request)
-   * and deduplicates functionResponse parts with the same name (tool call ID).
+   * and deduplicates functionResponse parts with the same id (tool call ID).
    */
   private mergeConsecutiveSameRoleMessages(
     messages: Array<Content>,
@@ -790,16 +790,20 @@ export class GeminiTextAdapter<
       }
     }
 
-    // Deduplicate functionResponse parts with the same name (tool call ID)
+    // Deduplicate functionResponse parts with the same id (tool call ID).
+    // Two parallel calls to the *same* tool share a `name` but have distinct
+    // `id`s — keying on `name` dropped every response but the first for
+    // same-tool parallel calls, leaving Gemini with fewer response parts
+    // than call parts and a 400 on the next request.
     for (const msg of merged) {
       if (!msg.parts) continue
-      const seenFunctionResponseNames = new Set<string>()
+      const seenFunctionResponseIds = new Set<string>()
       msg.parts = msg.parts.filter((part) => {
-        if ('functionResponse' in part && part.functionResponse?.name) {
-          if (seenFunctionResponseNames.has(part.functionResponse.name)) {
+        if ('functionResponse' in part && part.functionResponse?.id) {
+          if (seenFunctionResponseIds.has(part.functionResponse.id)) {
             return false
           }
-          seenFunctionResponseNames.add(part.functionResponse.name)
+          seenFunctionResponseIds.add(part.functionResponse.id)
         }
         return true
       })

packages/ai-gemini/tests/gemini-adapter.test.ts

@@ -712,6 +712,79 @@ describe('GeminiAdapter through AI', () => {
     expect(textParts[0].text).toBe("what's a good electric guitar?")
   })
 
+  it('preserves both functionResponse parts when two parallel calls hit the same tool', async () => {
+    const streamChunks = [
+      {
+        candidates: [
+          {
+            content: {
+              parts: [{ text: '50 USD and 30 EUR logged' }],
+            },
+            finishReason: 'STOP',
+          },
+        ],
+        usageMetadata: {
+          promptTokenCount: 10,
+          candidatesTokenCount: 5,
+          totalTokenCount: 15,
+        },
+      },
+    ]
+
+    mocks.generateContentStreamSpy.mockResolvedValue(createStream(streamChunks))
+
+    const adapter = createTextAdapter()
+
+    for await (const _ of chat({
+      adapter,
+      messages: [
+        { role: 'user', content: 'log 50 USD and 30 EUR' },
+        {
+          role: 'assistant',
+          content: null,
+          toolCalls: [
+            {
+              id: 'call_1',
+              type: 'function',
+              function: {
+                name: 'lookupCurrency',
+                arguments: '{"query":"USD"}',
+              },
+            },
+            {
+              id: 'call_2',
+              type: 'function',
+              function: {
+                name: 'lookupCurrency',
+                arguments: '{"query":"EUR"}',
+              },
+            },
+          ],
+        },
+        { role: 'tool', toolCallId: 'call_1', content: '{"rate":1}' },
+        { role: 'tool', toolCallId: 'call_2', content: '{"rate":0.9}' },
+      ],
+      tools: [weatherTool],
+    })) {
+      /* consume */
+    }
+
+    const [payload] = mocks.generateContentStreamSpy.mock.calls[0]!
+    const lastMsg = payload.contents[payload.contents.length - 1]
+    const functionResponses = lastMsg.parts.filter(
+      (p: any) => p.functionResponse,
+    )
+
+    // Two parallel calls to the SAME tool ("lookupCurrency" twice) share a
+    // `name` but have distinct `id`s. Deduping by name (the old behavior)
+    // dropped one response, leaving Gemini with fewer response parts than
+    // call parts and a 400 on the next request.
+    expect(functionResponses).toHaveLength(2)
+    expect(
+      functionResponses.map((p: any) => p.functionResponse.id).sort(),
+    ).toEqual(['call_1', 'call_2'])
+  })
+
   it('reads Part-level thoughtSignature from Gemini 3.x streaming response', async () => {
     const thoughtSig = 'base64-encoded-thought-signature-xyz'