diff --git a/src/glmt/delta-accumulator.ts b/src/glmt/delta-accumulator.ts
index 7923a0ae..7915267c 100644
--- a/src/glmt/delta-accumulator.ts
+++ b/src/glmt/delta-accumulator.ts
@@ -284,7 +284,10 @@ export class DeltaAccumulator {
 
   getToolCallBlockIndex(toolCallIndex: number): number {
     const toolCall = this.toolCallsIndex[toolCallIndex];
-    return toolCall?.blockIndex ?? 0;
+    if (!toolCall || toolCall.blockIndex < 0) {
+      throw new Error(`Tool call ${toolCallIndex} does not have an assigned content block`);
+    }
+    return toolCall.blockIndex;
   }
 
   getUnstoppedBlocks(): ContentBlock[] {
diff --git a/src/glmt/pipeline/tool-call-handler.ts b/src/glmt/pipeline/tool-call-handler.ts
index 5bc7f46d..251813a4 100644
--- a/src/glmt/pipeline/tool-call-handler.ts
+++ b/src/glmt/pipeline/tool-call-handler.ts
@@ -5,20 +5,12 @@
  * - Process tool call deltas from OpenAI
  * - Generate tool_use content blocks for Anthropic format
  * - Handle input_json_delta events
- * - Close previous tool_use blocks before starting new ones (Anthropic sequential block requirement)
  */
 
 import type { DeltaAccumulator } from '../delta-accumulator';
 import type { OpenAIToolCallDelta, OpenAIToolCall, ContentBlock, AnthropicSSEEvent } from './types';
-import { ResponseBuilder } from './response-builder';
 
 export class ToolCallHandler {
-  private responseBuilder: ResponseBuilder;
-
-  constructor() {
-    this.responseBuilder = new ResponseBuilder(false);
-  }
-
   processToolCalls(toolCalls: OpenAIToolCall[]): ContentBlock[] {
     const content: ContentBlock[] = [];
 
@@ -54,12 +46,8 @@ export class ToolCallHandler {
       accumulator.addToolCallDelta(toolCallDelta);
 
       if (isNewToolCall) {
-        const previousBlock = accumulator.getCurrentBlock();
-        if (previousBlock && previousBlock.type === 'tool_use' && !previousBlock.stopped) {
-          events.push(this.responseBuilder.createContentBlockStopEvent(previousBlock));
-          previousBlock.stopped = true;
-        }
-
+        // OpenAI may interleave tool_call fragments across chunks, so blocks must stay open
+        // until the stream finalizes. Closing on a later index truncates earlier tool input.
         const block = accumulator.startBlock('tool_use');
         const toolCall = accumulator.getToolCall(toolCallDelta.index);
         accumulator.setToolCallBlockIndex(toolCallDelta.index, block.index);
diff --git a/src/proxy/transformers/request-transformer.ts b/src/proxy/transformers/request-transformer.ts
index a9c6ab69..a8ccb98c 100644
--- a/src/proxy/transformers/request-transformer.ts
+++ b/src/proxy/transformers/request-transformer.ts
@@ -457,9 +457,35 @@ function transformMessages(messagesValue: unknown): OpenAIMessage[] {
     }
 
     if (role === 'user') {
-      const preToolResultParts: OpenAIContentPart[] = [];
+      const userParts: OpenAIContentPart[] = [];
+      const followUpParts: OpenAIContentPart[] = [];
       const resolvedToolUseIds = new Set<string>();
 
+      const handleUserPart = (
+        part: OpenAIContentPart,
+        blockIndex: number,
+        kind: 'text' | 'image'
+      ) => {
+        if (!pendingToolUseIds || pendingToolUseIds.size === 0) {
+          userParts.push(part);
+          return;
+        }
+
+        if (resolvedToolUseIds.size === 0) {
+          throw new Error(
+            `messages[${messageIndex}].content[${blockIndex}] ${kind} is not allowed before tool_result blocks for pending tool_use ids`
+          );
+        }
+
+        if (resolvedToolUseIds.size !== pendingToolUseIds.size) {
+          throw new Error(
+            `messages[${messageIndex}].content[${blockIndex}] ${kind} is not allowed between tool_result blocks for pending tool_use ids`
+          );
+        }
+
+        followUpParts.push(part);
+      };
+
       content.forEach((block, blockIndex) => {
         const parsed = assertObject(
           block,
@@ -472,25 +498,16 @@ function transformMessages(messagesValue: unknown): OpenAIMessage[] {
 
         if (parsed.type === 'text') {
           const text = typeof parsed.text === 'string' ? parsed.text : '';
-          if (pendingToolUseIds && pendingToolUseIds.size > 0 && !resolvedToolUseIds.size) {
-            preToolResultParts.push({ type: 'text', text });
-          } else {
-            translatedMessages.push({ role: 'user', content: text });
-          }
+          handleUserPart({ type: 'text', text }, blockIndex, 'text');
           return;
         }
 
         if (isImageBlock(parsed)) {
-          if (pendingToolUseIds && pendingToolUseIds.size > 0 && !resolvedToolUseIds.size) {
-            preToolResultParts.push(
-              toImagePart(parsed, `messages[${messageIndex}].content[${blockIndex}]`)
-            );
-          } else {
-            translatedMessages.push({
-              role: 'user',
-              content: [toImagePart(parsed, `messages[${messageIndex}].content[${blockIndex}]`)],
-            });
-          }
+          handleUserPart(
+            toImagePart(parsed, `messages[${messageIndex}].content[${blockIndex}]`),
+            blockIndex,
+            'image'
+          );
           return;
         }
 
@@ -555,8 +572,12 @@ function transformMessages(messagesValue: unknown): OpenAIMessage[] {
         );
       }
 
-      if (preToolResultParts.length > 0) {
-        flushUserContent(translatedMessages, preToolResultParts);
+      if (userParts.length > 0) {
+        flushUserContent(translatedMessages, userParts);
+      }
+
+      if (followUpParts.length > 0) {
+        flushUserContent(translatedMessages, followUpParts);
       }
       return;
     }
diff --git a/tests/integration/proxy/messages-endpoint.test.ts b/tests/integration/proxy/messages-endpoint.test.ts
index cf1d408b..90c819eb 100644
--- a/tests/integration/proxy/messages-endpoint.test.ts
+++ b/tests/integration/proxy/messages-endpoint.test.ts
@@ -23,10 +23,34 @@ function startMockUpstream(): Promise<void> {
         body += chunk.toString();
       }
       upstreamBody = JSON.parse(body);
-      const parsed = upstreamBody as { stream?: boolean };
+      const parsed = upstreamBody as {
+        stream?: boolean;
+        messages?: Array<{ role?: string; content?: string | Array<{ type?: string; text?: string }> }>;
+      };
 
       if (parsed.stream) {
         res.writeHead(200, { 'Content-Type': 'text/event-stream' });
+
+        if (parsed.messages?.[0]?.content === 'interleaved tool fragments') {
+          res.write(
+            'data: {"id":"chatcmpl_1","model":"hf-model","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"id":"call_1","type":"function","function":{"name":"search"}}]}}]}\n\n'
+          );
+          res.write(
+            'data: {"id":"chatcmpl_1","model":"hf-model","choices":[{"index":0,"delta":{"tool_calls":[{"index":1,"id":"call_2","type":"function","function":{"name":"open"}}]}}]}\n\n'
+          );
+          res.write(
+            'data: {"id":"chatcmpl_1","model":"hf-model","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"function":{"arguments":"{\\"path\\":\\"a.ts\\"}"}}]}}]}\n\n'
+          );
+          res.write(
+            'data: {"id":"chatcmpl_1","model":"hf-model","choices":[{"index":0,"delta":{"tool_calls":[{"index":1,"function":{"arguments":"{\\"path\\":\\"b.ts\\"}"}}]}}]}\n\n'
+          );
+          res.write(
+            'data: {"id":"chatcmpl_1","model":"hf-model","choices":[{"index":0,"delta":{},"finish_reason":"tool_calls"}],"usage":{"prompt_tokens":9,"completion_tokens":4}}\n\n'
+          );
+          res.end('data: [DONE]\n\n');
+          return;
+        }
+
         res.write(
           'data: {"id":"chatcmpl_1","model":"hf-model","choices":[{"index":0,"delta":{"role":"assistant","content":"Hello"}}]}\n\n'
         );
@@ -356,6 +380,66 @@ describe('openai proxy messages endpoint', () => {
     expect(userAfterTool?.[1]?.content).toBe('What should I do next?');
   });
 
+  it('rejects text before tool_result blocks when tool results are pending', async () => {
+    const response = await requestProxy({
+      model: 'hf-model',
+      stream: false,
+      messages: [
+        { role: 'user', content: 'search docs' },
+        {
+          role: 'assistant',
+          content: [{ type: 'tool_use', id: 'toolu_01', name: 'search', input: { q: 'docs' } }],
+        },
+        {
+          role: 'user',
+          content: [
+            { type: 'text', text: 'before' },
+            { type: 'tool_result', tool_use_id: 'toolu_01', content: 'found 3 docs' },
+          ],
+        },
+      ],
+    });
+
+    const body = (await response.json()) as { error?: { type?: string; message?: string } };
+    expect(response.status).toBe(400);
+    expect(body.error?.type).toBe('invalid_request_error');
+    expect(body.error?.message).toContain(
+      'text is not allowed before tool_result blocks for pending tool_use ids'
+    );
+  });
+
+  it('rejects follow-up text between pending tool_result blocks', async () => {
+    const response = await requestProxy({
+      model: 'hf-model',
+      stream: false,
+      messages: [
+        { role: 'user', content: 'read both files' },
+        {
+          role: 'assistant',
+          content: [
+            { type: 'tool_use', id: 'toolu_01', name: 'Read', input: { file_path: 'a.ts' } },
+            { type: 'tool_use', id: 'toolu_02', name: 'Read', input: { file_path: 'b.ts' } },
+          ],
+        },
+        {
+          role: 'user',
+          content: [
+            { type: 'tool_result', tool_use_id: 'toolu_01', content: 'content of a' },
+            { type: 'text', text: 'Now compare them' },
+            { type: 'tool_result', tool_use_id: 'toolu_02', content: 'content of b' },
+          ],
+        },
+      ],
+    });
+
+    const body = (await response.json()) as { error?: { type?: string; message?: string } };
+    expect(response.status).toBe(400);
+    expect(body.error?.type).toBe('invalid_request_error');
+    expect(body.error?.message).toContain(
+      'text is not allowed between tool_result blocks for pending tool_use ids'
+    );
+  });
+
   it('translates parallel tool calls with streaming', async () => {
     const response = await requestProxy({
       model: 'hf-model',
@@ -409,4 +493,36 @@ describe('openai proxy messages endpoint', () => {
     expect(toolMsgs?.[0]?.tool_call_id).toBe('toolu_01');
     expect(toolMsgs?.[1]?.tool_call_id).toBe('toolu_02');
   });
+
+  it('streams interleaved tool call fragments without premature block stops', async () => {
+    const response = await requestProxy({
+      model: 'hf-model',
+      stream: true,
+      messages: [{ role: 'user', content: [{ type: 'text', text: 'interleaved tool fragments' }] }],
+      tools: [
+        {
+          name: 'search',
+          description: 'Search docs',
+          input_schema: { type: 'object', properties: { path: { type: 'string' } } },
+        },
+        {
+          name: 'open',
+          description: 'Open a file',
+          input_schema: { type: 'object', properties: { path: { type: 'string' } } },
+        },
+      ],
+    });
+
+    const body = await response.text();
+    expect(response.status).toBe(200);
+    expect(body.match(/event: content_block_start/g)?.length).toBe(2);
+    expect(body.match(/event: content_block_stop/g)?.length).toBe(2);
+
+    const stopIndex = body.indexOf('event: content_block_stop');
+    const deltaAIndex = body.indexOf('"partial_json":"{\\"path\\":\\"a.ts\\"}"');
+    const deltaBIndex = body.indexOf('"partial_json":"{\\"path\\":\\"b.ts\\"}"');
+    expect(deltaAIndex).toBeGreaterThan(-1);
+    expect(deltaBIndex).toBeGreaterThan(-1);
+    expect(stopIndex).toBeGreaterThan(deltaBIndex);
+  });
 });
diff --git a/tests/unit/proxy/transformers/request-transformer-regressions.test.ts b/tests/unit/proxy/transformers/request-transformer-regressions.test.ts
index af83395d..77b56294 100644
--- a/tests/unit/proxy/transformers/request-transformer-regressions.test.ts
+++ b/tests/unit/proxy/transformers/request-transformer-regressions.test.ts
@@ -120,8 +120,48 @@ describe('ProxyRequestTransformer regressions', () => {
           },
         ],
       })
+    ).toThrow('text is not allowed before tool_result blocks for pending tool_use ids');
+
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'assistant',
+            content: [{ type: 'tool_use', id: 'toolu_1', name: 'vision', input: { detail: 'high' } }],
+          },
+          {
+            role: 'user',
+            content: [
+              { type: 'tool_result', tool_use_id: 'toolu_1', content: 'result' },
+              { type: 'text', text: 'follow-up' },
+            ],
+          },
+        ],
+      })
     ).not.toThrow();
 
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'assistant',
+            content: [
+              { type: 'tool_use', id: 'toolu_1', name: 'search', input: { q: 'docs' } },
+              { type: 'tool_use', id: 'toolu_2', name: 'open', input: { url: 'https://example.com' } },
+            ],
+          },
+          {
+            role: 'user',
+            content: [
+              { type: 'tool_result', tool_use_id: 'toolu_1', content: 'partial' },
+              { type: 'text', text: 'follow-up' },
+              { type: 'tool_result', tool_use_id: 'toolu_2', content: 'done' },
+            ],
+          },
+        ],
+      })
+    ).toThrow('text is not allowed between tool_result blocks for pending tool_use ids');
+
     expect(() =>
       new ProxyRequestTransformer().transform({
         messages: [