diff --git a/packages/__tests__/llm-mapper/anthropic-stream-cache-tokens.test.ts b/packages/__tests__/llm-mapper/anthropic-stream-cache-tokens.test.ts
index 6b4071f9be..5a0f926005 100644
--- a/packages/__tests__/llm-mapper/anthropic-stream-cache-tokens.test.ts
+++ b/packages/__tests__/llm-mapper/anthropic-stream-cache-tokens.test.ts
@@ -114,6 +114,93 @@ data: {"type":"message_stop"}`;
     expect(usageChunk.usage.prompt_tokens).toBe(1000);
     expect(usageChunk.usage.total_tokens).toBe(1000 + 50);
   });
+
+  it("should handle tool call streaming where message_delta only has output_tokens", () => {
+    const converter = new AnthropicToOpenAIStreamConverter();
+
+    // From Anthropic docs: tool call streaming - message_delta only has output_tokens
+    const nativeResponse = `event: message_start
+data: {"type":"message_start","message":{"id":"msg_014p7gG3wDgGV9EUtLvnow3U","type":"message","role":"assistant","model":"claude-opus-4-6","stop_sequence":null,"usage":{"input_tokens":472,"output_tokens":2},"content":[],"stop_reason":null}}
+
+event: content_block_start
+data: {"type":"content_block_start","index":0,"content_block":{"type":"tool_use","id":"toolu_01T1x1fJ34qAmk2tNTrN7Up6","name":"get_weather","input":{}}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"input_json_delta","partial_json":""}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"input_json_delta","partial_json":"{\\"location\\": \\"San Francisco, CA\\"}"}}
+
+event: content_block_stop
+data: {"type":"content_block_stop","index":0}
+
+event: message_delta
+data: {"type":"message_delta","delta":{"stop_reason":"tool_use","stop_sequence":null},"usage":{"output_tokens":89}}
+
+event: message_stop
+data: {"type":"message_stop"}`;
+
+    const chunks: any[] = [];
+    converter.processLines(nativeResponse, (chunk) => {
+      chunks.push(chunk);
+    });
+
+    const usageChunk = chunks.find((c) => c.usage);
+    expect(usageChunk).toBeDefined();
+
+    // prompt_tokens should come from message_start input_tokens (472), NOT be 0 or 1
+    expect(usageChunk.usage.prompt_tokens).toBe(472);
+    expect(usageChunk.usage.completion_tokens).toBe(89);
+  });
+
+  it("should handle extended thinking streaming where message_start has no usage", () => {
+    const converter = new AnthropicToOpenAIStreamConverter();
+
+    // From Anthropic docs: extended thinking - message_start has no usage field
+    const nativeResponse = `event: message_start
+data: {"type":"message_start","message":{"id":"msg_01","type":"message","role":"assistant","content":[],"model":"claude-opus-4-6","stop_reason":null,"stop_sequence":null}}
+
+event: content_block_start
+data: {"type":"content_block_start","index":0,"content_block":{"type":"thinking","thinking":""}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"thinking_delta","thinking":"Let me think..."}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":0,"delta":{"type":"signature_delta","signature":"EqQBCg..."}}
+
+event: content_block_stop
+data: {"type":"content_block_stop","index":0}
+
+event: content_block_start
+data: {"type":"content_block_start","index":1,"content_block":{"type":"text","text":""}}
+
+event: content_block_delta
+data: {"type":"content_block_delta","index":1,"delta":{"type":"text_delta","text":"The answer is 21."}}
+
+event: content_block_stop
+data: {"type":"content_block_stop","index":1}
+
+event: message_delta
+data: {"type":"message_delta","delta":{"stop_reason":"end_turn","stop_sequence":null}}
+
+event: message_stop
+data: {"type":"message_stop"}`;
+
+    const chunks: any[] = [];
+    converter.processLines(nativeResponse, (chunk) => {
+      chunks.push(chunk);
+    });
+
+    // Should not crash and should produce chunks
+    expect(chunks.length).toBeGreaterThan(0);
+
+    // Usage chunk should have 0 tokens (no usage data available)
+    const usageChunk = chunks.find((c) => c.usage);
+    expect(usageChunk).toBeDefined();
+    expect(usageChunk.usage.prompt_tokens).toBe(0);
+    expect(usageChunk.usage.completion_tokens).toBe(0);
+  });
 });
 
 describe("toOpenAI (non-streaming) - Cache Token Handling", () => {
diff --git a/packages/llm-mapper/transform/providers/anthropic/streamedResponse/toOpenai.ts b/packages/llm-mapper/transform/providers/anthropic/streamedResponse/toOpenai.ts
index 0e90adfb3b..04657392b2 100644
--- a/packages/llm-mapper/transform/providers/anthropic/streamedResponse/toOpenai.ts
+++ b/packages/llm-mapper/transform/providers/anthropic/streamedResponse/toOpenai.ts
@@ -77,12 +77,13 @@ export class AnthropicToOpenAIStreamConverter {
       case "message_start":
         this.messageId = event.message.id;
         this.model = event.message.model;
-        this.inputTokens = event.message.usage.input_tokens ?? 0;
+        // usage may be missing (e.g. extended thinking streams on Claude 4.6)
+        this.inputTokens = event.message.usage?.input_tokens ?? 0;
         this.cacheReadInputTokens =
-          event.message.usage.cache_read_input_tokens ?? 0;
+          event.message.usage?.cache_read_input_tokens ?? 0;
         this.cacheCreationInputTokens =
-          event.message.usage.cache_creation_input_tokens ?? 0;
-        this.cacheCreationDetails = event.message.usage.cache_creation
+          event.message.usage?.cache_creation_input_tokens ?? 0;
+        this.cacheCreationDetails = event.message.usage?.cache_creation
           ? {
               ephemeral_5m_input_tokens:
                 event.message.usage.cache_creation.ephemeral_5m_input_tokens ??
@@ -303,20 +304,26 @@ export class AnthropicToOpenAIStreamConverter {
         this.finalizePendingToolCalls(chunks);
 
         // Cache tokens may come from message_start (stored in instance vars) or message_delta
+        // usage may be missing (e.g. extended thinking streams on Claude 4.6)
         const cachedTokens =
-          event.usage.cache_read_input_tokens ?? this.cacheReadInputTokens;
+          event.usage?.cache_read_input_tokens ?? this.cacheReadInputTokens;
         const cacheWriteTokens =
-          event.usage.cache_creation_input_tokens ??
+          event.usage?.cache_creation_input_tokens ??
           this.cacheCreationInputTokens;
         const webSearchRequests =
-          event.usage.server_tool_use?.web_search_requests ?? 0;
+          event.usage?.server_tool_use?.web_search_requests ?? 0;
+
+        // Prefer message_delta input_tokens (cumulative/final) over message_start value
+        const finalInputTokens =
+          event.usage?.input_tokens ?? this.inputTokens;
+        const finalOutputTokens = event.usage?.output_tokens ?? 0;
 
         this.finalUsage = {
-          prompt_tokens: event.usage.input_tokens ?? this.inputTokens,
-          completion_tokens: event.usage.output_tokens,
+          prompt_tokens: finalInputTokens,
+          completion_tokens: finalOutputTokens,
           total_tokens:
-            (event.usage.input_tokens ?? this.inputTokens) +
-            event.usage.output_tokens +
+            finalInputTokens +
+            finalOutputTokens +
             (cachedTokens ?? 0) +
             (cacheWriteTokens ?? 0),
           ...((cachedTokens > 0 || cacheWriteTokens > 0) && {