diff --git a/packages/a2a-server/src/agent/task.ts b/packages/a2a-server/src/agent/task.ts
index ac1ced0482..170f24e67c 100644
--- a/packages/a2a-server/src/agent/task.ts
+++ b/packages/a2a-server/src/agent/task.ts
@@ -1092,19 +1092,21 @@ export class Task {
     logger.info(
       `[Task] Adding ${completedTools.length} tool responses to history without generating a new response.`,
     );
-    const responsesToAdd = completedTools.flatMap(
-      (toolCall) => toolCall.response.responseParts,
-    );
-
-    for (const response of responsesToAdd) {
-      let parts: genAiPart[];
-      if (Array.isArray(response)) {
-        parts = response;
-      } else if (typeof response === 'string') {
-        parts = [{ text: response }];
-      } else {
-        parts = [response];
+    const parts: genAiPart[] = [];
+    for (const toolCall of completedTools) {
+      const response = toolCall.response?.responseParts;
+      if (!response) {
+        continue;
       }
+      if (Array.isArray(response)) {
+        parts.push(...response);
+      } else if (typeof response === 'string') {
+        parts.push({ text: response });
+      } else {
+        parts.push(response);
+      }
+    }
+    if (parts.length > 0) {
       // eslint-disable-next-line @typescript-eslint/no-floating-promises
       this.geminiClient.addHistory({
         role: 'user',
diff --git a/packages/core/src/core/geminiChat.test.ts b/packages/core/src/core/geminiChat.test.ts
index 07fe800f3b..4483585202 100644
--- a/packages/core/src/core/geminiChat.test.ts
+++ b/packages/core/src/core/geminiChat.test.ts
@@ -21,6 +21,7 @@ import {
   type StreamEvent,
   stripToolCallIdPrefixes,
   type HistoryTurn,
+  coalesceConsecutiveRoles,
 } from './geminiChat.js';
 import {
   type CompletedToolCall,
@@ -3137,4 +3138,62 @@ describe('GeminiChat', () => {
       expect(stripped[1].parts![0].functionResponse!.id).toBe('call_123');
     });
   });
+
+  describe('coalesceConsecutiveRoles', () => {
+    it('should return empty history if empty array is passed', () => {
+      expect(coalesceConsecutiveRoles([])).toEqual([]);
+    });
+
+    it('should not modify history when roles alternate correctly', () => {
+      const history: HistoryTurn[] = [
+        { id: '1', content: { role: 'user', parts: [{ text: 'hello' }] } },
+        { id: '2', content: { role: 'model', parts: [{ text: 'hi' }] } },
+        {
+          id: '3',
+          content: { role: 'user', parts: [{ text: 'how are you?' }] },
+        },
+      ];
+      expect(coalesceConsecutiveRoles(history)).toEqual(history);
+    });
+
+    it('should coalesce consecutive user turns', () => {
+      const history: HistoryTurn[] = [
+        { id: '1', content: { role: 'user', parts: [{ text: 'hello' }] } },
+        { id: '2', content: { role: 'user', parts: [{ text: 'world' }] } },
+      ];
+      expect(coalesceConsecutiveRoles(history)).toEqual([
+        {
+          id: '1',
+          content: {
+            role: 'user',
+            parts: [{ text: 'hello' }, { text: 'world' }],
+          },
+        },
+      ]);
+    });
+
+    it('should handle undefined or missing parts gracefully', () => {
+      const history: HistoryTurn[] = [
+        { id: '1', content: { role: 'user' } },
+        { id: '2', content: { role: 'user', parts: [{ text: 'world' }] } },
+      ];
+      expect(coalesceConsecutiveRoles(history)).toEqual([
+        {
+          id: '1',
+          content: {
+            role: 'user',
+            parts: [{ text: 'world' }],
+          },
+        },
+      ]);
+    });
+
+    it('should not coalesce turns if roles are undefined', () => {
+      const history: HistoryTurn[] = [
+        { id: '1', content: { parts: [{ text: 'hello' }] } },
+        { id: '2', content: { parts: [{ text: 'world' }] } },
+      ];
+      expect(coalesceConsecutiveRoles(history)).toEqual(history);
+    });
+  });
 });
diff --git a/packages/core/src/core/geminiChat.ts b/packages/core/src/core/geminiChat.ts
index a95188e1f4..5104f42ff5 100644
--- a/packages/core/src/core/geminiChat.ts
+++ b/packages/core/src/core/geminiChat.ts
@@ -683,10 +683,13 @@ export class GeminiChat {
   ): Promise<AsyncGenerator<GenerateContentResponse>> {
     // Last mile scrubbing to remove internal tracking properties (e.g. callIndex)
     // before sending to the Gemini API. This whitelists only standard Gemini fields.
-    const scrubbedHistory = this.context.config.isContextManagementEnabled()
+    let scrubbedHistory = this.context.config.isContextManagementEnabled()
       ? scrubHistory([...requestHistory])
       : [...requestHistory];
 
+    // Always coalesce consecutive roles to prevent 400 Bad Request errors
+    scrubbedHistory = coalesceConsecutiveRoles(scrubbedHistory);
+
     const scrubbedContents = scrubbedHistory.map((h) => h.content);
 
     const requestContents = apiHistoryOverride
@@ -1472,3 +1475,31 @@ export function stripToolCallIdPrefixes(contents: Content[]): Content[] {
     }),
   }));
 }
+
+export function coalesceConsecutiveRoles(
+  history: HistoryTurn[],
+): HistoryTurn[] {
+  const result: HistoryTurn[] = [];
+  for (const turn of history) {
+    const lastIdx = result.length - 1;
+    const last = result[lastIdx];
+    if (last && last.content.role && last.content.role === turn.content.role) {
+      const hasParts = last.content.parts || turn.content.parts;
+      result[lastIdx] = {
+        id: last.id,
+        content: {
+          ...last.content,
+          parts: hasParts
+            ? [...(last.content.parts || []), ...(turn.content.parts || [])]
+            : undefined,
+        },
+      };
+    } else {
+      result.push({
+        id: turn.id,
+        content: { ...turn.content },
+      });
+    }
+  }
+  return result;
+}