diff --git a/core/agent-runtime/src/AgentRuntime.ts b/core/agent-runtime/src/AgentRuntime.ts
index 07791a16b..a2bbbd457 100644
--- a/core/agent-runtime/src/AgentRuntime.ts
+++ b/core/agent-runtime/src/AgentRuntime.ts
@@ -361,8 +361,8 @@ export class AgentRuntime {
         const contentBlocks = msg.message
           ? MessageConverter.toContentBlocks(msg.message)
           : [];
-        // Only accumulate content blocks for text/tool content (for storage)
-        if (contentBlocks.length > 0) {
+        // Only accumulate when accumulate !== false (defaults to true)
+        if (contentBlocks.length > 0 && msg.accumulate !== false) {
           content.push(...contentBlocks);
         }
         writer.writeEvent(msg.type, {
@@ -389,7 +389,7 @@ export class AgentRuntime {
     }
 
     return {
-      content,
+      content: MessageConverter.mergeContentBlocks(content),
       usage: hasUsage ? { promptTokens, completionTokens, totalTokens: promptTokens + completionTokens } : undefined,
       aborted: false as const,
     };
diff --git a/core/agent-runtime/src/MessageConverter.ts b/core/agent-runtime/src/MessageConverter.ts
index 67bbe2471..4dbe57f59 100644
--- a/core/agent-runtime/src/MessageConverter.ts
+++ b/core/agent-runtime/src/MessageConverter.ts
@@ -77,14 +77,14 @@ export class MessageConverter {
       output: MessageObject[];
       usage?: RunUsage;
     } {
-    const output: MessageObject[] = [];
+    const contentBlocks: MessageContentBlock[] = [];
     let promptTokens = 0;
     let completionTokens = 0;
     let hasUsage = false;
 
     for (const msg of messages) {
-      if (msg.message) {
-        output.push(MessageConverter.toMessageObject(msg.message, runId));
+      if (msg.message && msg.accumulate !== false) {
+        contentBlocks.push(...MessageConverter.toContentBlocks(msg.message));
       }
       if (msg.usage) {
         hasUsage = true;
@@ -93,6 +93,19 @@ export class MessageConverter {
       }
     }
 
+    const mergedContent = MessageConverter.mergeContentBlocks(contentBlocks);
+    const output: MessageObject[] = mergedContent.length > 0
+      ? [{
+        id: newMsgId(),
+        object: AgentObjectType.ThreadMessage,
+        createdAt: nowUnix(),
+        runId,
+        role: MessageRole.Assistant,
+        status: MessageStatus.Completed,
+        content: mergedContent,
+      }]
+      : [];
+
     let usage: RunUsage | undefined;
     if (hasUsage) {
       usage = {
@@ -105,6 +118,63 @@ export class MessageConverter {
     return { output, usage };
   }
 
+  /**
+   * Merge accumulated content blocks into a clean final form:
+   * 1. Consecutive text blocks are merged into a single text block.
+   * 2. Text blocks immediately following a tool_use block (before the next
+   *    tool_result) are treated as input_json_delta fragments — they are
+   *    concatenated, JSON-parsed, and written into the tool_use block's input.
+   */
+  static mergeContentBlocks(blocks: MessageContentBlock[]): MessageContentBlock[] {
+    if (blocks.length === 0) return blocks;
+
+    const merged: MessageContentBlock[] = [];
+
+    for (let i = 0; i < blocks.length; i++) {
+      const block = blocks[i];
+
+      if (isToolUseBlock(block)) {
+        // Collect subsequent text blocks as input_json_delta fragments
+        const inputFragments: string[] = [];
+        let next = blocks[i + 1];
+        while (next && isTextBlock(next)) {
+          i++;
+          inputFragments.push(next.text.value);
+          next = blocks[i + 1];
+        }
+        if (inputFragments.length > 0) {
+          const raw = inputFragments.join('');
+          let parsedInput: Record<string, unknown>;
+          try {
+            parsedInput = JSON.parse(raw);
+          } catch {
+            parsedInput = {};
+          }
+          merged.push({ ...block, input: { ...block.input, ...parsedInput } });
+        } else {
+          merged.push(block);
+        }
+      } else if (isTextBlock(block)) {
+        // Merge consecutive text blocks
+        const parts: string[] = [ block.text.value ];
+        let next = blocks[i + 1];
+        while (next && isTextBlock(next)) {
+          i++;
+          parts.push(next.text.value);
+          next = blocks[i + 1];
+        }
+        merged.push({
+          type: ContentBlockType.Text,
+          text: { value: parts.join(''), annotations: [] },
+        });
+      } else {
+        merged.push(block);
+      }
+    }
+
+    return merged;
+  }
+
   /**
    * Produce a completed copy of a streaming MessageObject with final content.
    */
diff --git a/core/agent-runtime/test/AgentRuntime.test.ts b/core/agent-runtime/test/AgentRuntime.test.ts
index 53cff757b..424c447c0 100644
--- a/core/agent-runtime/test/AgentRuntime.test.ts
+++ b/core/agent-runtime/test/AgentRuntime.test.ts
@@ -530,6 +530,68 @@ describe('test/AgentRuntime.test.ts', () => {
       assert(deltaEvents.length > 0);
     });
 
+    it('should respect accumulate=false to exclude content from completed message', async () => {
+      executor.execRun = async function* (): AsyncGenerator<AgentStreamMessage> {
+        yield { type: 'assistant.thinking.delta', message: { content: 'thinking...' }, accumulate: false };
+        yield { type: 'assistant.text.delta', message: { content: 'Hello' }, accumulate: true };
+        yield { type: 'tool.delta', message: { content: '{"partial":true}' }, accumulate: false };
+        yield { type: 'assistant.text.delta', message: { content: ' world' }, accumulate: true };
+      };
+
+      const writer = new MockSSEWriter();
+      await runtime.streamRun({ input: { messages: [{ role: 'user', content: 'Hi' }] } }, writer);
+
+      // All custom events are still forwarded
+      const customEvents = writer.events.filter(e =>
+        !e.event.startsWith('thread.') && e.event !== 'done',
+      );
+      assert.equal(customEvents.length, 4);
+
+      // But completed message only has accumulated content (merged text)
+      const completedEvent = writer.events.find(e => e.event === AgentSSEEvent.ThreadMessageCompleted);
+      assert.ok(completedEvent);
+      const content = (completedEvent.data as any).content;
+      assert.equal(content.length, 1);
+      assert(isTextBlock(content[0]));
+      assert.equal(content[0].text.value, 'Hello world');
+    });
+
+    it('should merge text fragments and backfill tool_use input in completed message', async () => {
+      executor.execRun = async function* (): AsyncGenerator<AgentStreamMessage> {
+        yield { type: 'assistant.text.delta', message: { content: 'I will ' }, accumulate: true };
+        yield { type: 'assistant.text.delta', message: { content: 'help you.' }, accumulate: true };
+        yield {
+          type: 'tool.started',
+          message: { content: [{ type: 'tool_use', id: 'fc-1', name: 'Bash', input: {} }] as any },
+          accumulate: true,
+        };
+        yield { type: 'tool.delta', message: { content: '{"command":"ls"}' }, accumulate: true };
+        yield {
+          type: 'tool.completed',
+          message: { content: [{ type: 'tool_result', tool_use_id: 'fc-1', content: 'file1' }] as any },
+          accumulate: true,
+        };
+      };
+
+      const writer = new MockSSEWriter();
+      await runtime.streamRun({ input: { messages: [{ role: 'user', content: 'Hi' }] } }, writer);
+
+      const completedEvent = writer.events.find(e => e.event === AgentSSEEvent.ThreadMessageCompleted);
+      assert.ok(completedEvent);
+      const content = (completedEvent.data as any).content;
+
+      // Text fragments merged into one
+      assert(isTextBlock(content[0]));
+      assert.equal(content[0].text.value, 'I will help you.');
+
+      // tool_use input backfilled from text delta
+      assert.equal(content[1].type, 'tool_use');
+      assert.deepStrictEqual(content[1].input, { command: 'ls' });
+
+      // tool_result preserved
+      assert.equal(content[2].type, 'tool_result');
+    });
+
     it('should emit failed event when execRun throws', async () => {
       executor.execRun = async function* (): AsyncGenerator<AgentStreamMessage> {
         throw new Error('model unavailable');
diff --git a/core/agent-runtime/test/MessageConverter.test.ts b/core/agent-runtime/test/MessageConverter.test.ts
index a09c8aa0d..de77f694a 100644
--- a/core/agent-runtime/test/MessageConverter.test.ts
+++ b/core/agent-runtime/test/MessageConverter.test.ts
@@ -186,18 +186,18 @@ describe('test/MessageConverter.test.ts', () => {
   });
 
   describe('extractFromStreamMessages', () => {
-    it('should extract messages and accumulate usage', () => {
+    it('should merge chunks into single MessageObject and accumulate usage', () => {
       const messages: AgentStreamMessage[] = [
         { message: { content: 'chunk1' }, usage: { promptTokens: 10, completionTokens: 5 } },
         { message: { content: 'chunk2' }, usage: { promptTokens: 0, completionTokens: 8 } },
       ];
       const { output, usage } = MessageConverter.extractFromStreamMessages(messages, 'run_1');
 
-      assert.equal(output.length, 2);
+      assert.equal(output.length, 1);
+      assert.equal(output[0].content.length, 1);
       assert(isTextBlock(output[0].content[0]));
-      assert.equal(output[0].content[0].text.value, 'chunk1');
-      assert(isTextBlock(output[1].content[0]));
-      assert.equal(output[1].content[0].text.value, 'chunk2');
+      assert.equal(output[0].content[0].text.value, 'chunk1chunk2');
+      assert.equal(output[0].runId, 'run_1');
       assert.ok(usage);
       assert.equal(usage.promptTokens, 10);
       assert.equal(usage.completionTokens, 13);
@@ -222,6 +222,19 @@ describe('test/MessageConverter.test.ts', () => {
       assert(isToolUseBlock(output[0].content[1]));
     });
 
+    it('should skip messages with accumulate=false', () => {
+      const messages: AgentStreamMessage[] = [
+        { message: { content: 'thinking...' }, accumulate: false },
+        { message: { content: 'visible text' }, accumulate: true },
+        { message: { content: 'tool delta' }, accumulate: false },
+      ];
+      const { output } = MessageConverter.extractFromStreamMessages(messages, 'run_1');
+      assert.equal(output.length, 1);
+      assert.equal(output[0].content.length, 1);
+      assert(isTextBlock(output[0].content[0]));
+      assert.equal(output[0].content[0].text.value, 'visible text');
+    });
+
     it('should return undefined usage when no usage info', () => {
       const messages: AgentStreamMessage[] = [{ message: { content: 'data' } }];
       const { output, usage } = MessageConverter.extractFromStreamMessages(messages);
@@ -332,6 +345,107 @@ describe('test/MessageConverter.test.ts', () => {
     });
   });
 
+  describe('mergeContentBlocks', () => {
+    it('should return empty array for empty input', () => {
+      assert.deepStrictEqual(MessageConverter.mergeContentBlocks([]), []);
+    });
+
+    it('should merge consecutive text blocks into one', () => {
+      const blocks = [
+        { type: ContentBlockType.Text, text: { value: 'Hello', annotations: [] } },
+        { type: ContentBlockType.Text, text: { value: ' ', annotations: [] } },
+        { type: ContentBlockType.Text, text: { value: 'world', annotations: [] } },
+      ] as any;
+      const result = MessageConverter.mergeContentBlocks(blocks);
+      assert.equal(result.length, 1);
+      assert(isTextBlock(result[0]));
+      assert.equal(result[0].text.value, 'Hello world');
+    });
+
+    it('should not merge text blocks separated by non-text blocks', () => {
+      const blocks = [
+        { type: ContentBlockType.Text, text: { value: 'before', annotations: [] } },
+        { type: ContentBlockType.ToolResult, tool_use_id: 'toolu_1', content: 'result' },
+        { type: ContentBlockType.Text, text: { value: 'after', annotations: [] } },
+      ] as any;
+      const result = MessageConverter.mergeContentBlocks(blocks);
+      assert.equal(result.length, 3);
+      assert(isTextBlock(result[0]));
+      assert.equal(result[0].text.value, 'before');
+      assert(isToolResultBlock(result[1]));
+      assert(isTextBlock(result[2]));
+      assert.equal(result[2].text.value, 'after');
+    });
+
+    it('should backfill tool_use input from subsequent text blocks', () => {
+      const blocks = [
+        { type: ContentBlockType.ToolUse, id: 'toolu_1', name: 'search', input: {} },
+        { type: ContentBlockType.Text, text: { value: '{"command":', annotations: [] } },
+        { type: ContentBlockType.Text, text: { value: ' "curl -s"}', annotations: [] } },
+        { type: ContentBlockType.ToolResult, tool_use_id: 'toolu_1', content: 'ok' },
+      ] as any;
+      const result = MessageConverter.mergeContentBlocks(blocks);
+      assert.equal(result.length, 2);
+      assert(isToolUseBlock(result[0]));
+      assert.deepStrictEqual(result[0].input, { command: 'curl -s' });
+      assert(isToolResultBlock(result[1]));
+    });
+
+    it('should keep tool_use as-is when no text blocks follow', () => {
+      const blocks = [
+        { type: ContentBlockType.ToolUse, id: 'toolu_1', name: 'search', input: { q: 'test' } },
+        { type: ContentBlockType.ToolResult, tool_use_id: 'toolu_1', content: 'result' },
+      ] as any;
+      const result = MessageConverter.mergeContentBlocks(blocks);
+      assert.equal(result.length, 2);
+      assert(isToolUseBlock(result[0]));
+      assert.deepStrictEqual(result[0].input, { q: 'test' });
+    });
+
+    it('should fallback to empty object when JSON parse fails', () => {
+      const blocks = [
+        { type: ContentBlockType.ToolUse, id: 'toolu_1', name: 'search', input: {} },
+        { type: ContentBlockType.Text, text: { value: 'not valid json', annotations: [] } },
+        { type: ContentBlockType.ToolResult, tool_use_id: 'toolu_1', content: 'result' },
+      ] as any;
+      const result = MessageConverter.mergeContentBlocks(blocks);
+      assert.equal(result.length, 2);
+      assert(isToolUseBlock(result[0]));
+      assert.deepStrictEqual(result[0].input, {});
+    });
+
+    it('should handle realistic stream scenario: text + tool_use + input deltas + tool_result + text', () => {
+      const blocks = [
+        { type: ContentBlockType.Text, text: { value: 'I will ', annotations: [] } },
+        { type: ContentBlockType.Text, text: { value: 'help you.', annotations: [] } },
+        { type: ContentBlockType.ToolUse, id: 'fc-1', name: 'Bash', input: {} },
+        { type: ContentBlockType.Text, text: { value: '{"command":"ls -la"}', annotations: [] } },
+        { type: ContentBlockType.ToolResult, tool_use_id: 'fc-1', content: 'file1\nfile2' },
+        { type: ContentBlockType.Text, text: { value: 'Here are', annotations: [] } },
+        { type: ContentBlockType.Text, text: { value: ' the results.', annotations: [] } },
+      ] as any;
+      const result = MessageConverter.mergeContentBlocks(blocks);
+      assert.equal(result.length, 4);
+      assert(isTextBlock(result[0]));
+      assert.equal(result[0].text.value, 'I will help you.');
+      assert(isToolUseBlock(result[1]));
+      assert.deepStrictEqual(result[1].input, { command: 'ls -la' });
+      assert(isToolResultBlock(result[2]));
+      assert(isTextBlock(result[3]));
+      assert.equal(result[3].text.value, 'Here are the results.');
+    });
+
+    it('should pass through generic blocks unchanged', () => {
+      const blocks = [
+        { type: 'thinking', thinking: 'let me think...' },
+      ] as any;
+      const result = MessageConverter.mergeContentBlocks(blocks);
+      assert.equal(result.length, 1);
+      assert.equal(result[0].type, 'thinking');
+      assert.equal((result[0] as any).thinking, 'let me think...');
+    });
+  });
+
   describe('type guards', () => {
     it('isTextBlock should narrow to TextContentBlock', () => {
       const block = { type: ContentBlockType.Text, text: { value: 'hello', annotations: [] } };
diff --git a/core/mcp-client/src/HeaderUtil.ts b/core/mcp-client/src/HeaderUtil.ts
index 5d2306eb0..36017ad4c 100644
--- a/core/mcp-client/src/HeaderUtil.ts
+++ b/core/mcp-client/src/HeaderUtil.ts
@@ -4,7 +4,7 @@ export function mergeHeaders(...headersInits: Array<HeadersInit | undefined>): H
   const res = {};
   for (const headersInit of headersInits) {
     if (!headersInit) continue;
-    const headers = new Headers(headersInit);
+    const headers = new Headers(headersInit as Record<string, string>);
     for (const key of headers.keys()) {
       res[key] = headers.get(key);
     }
diff --git a/core/types/agent-runtime/AgentRuntime.ts b/core/types/agent-runtime/AgentRuntime.ts
index 779efd772..c1665b983 100644
--- a/core/types/agent-runtime/AgentRuntime.ts
+++ b/core/types/agent-runtime/AgentRuntime.ts
@@ -116,4 +116,6 @@ export interface AgentStreamMessage {
   type?: string;
   message?: AgentStreamMessagePayload;
   usage?: AgentRunUsage;
+  /** Whether to accumulate message content into the final completed message. Defaults to true. */
+  accumulate?: boolean;
 }