Repository: ggml-org/llama.cpp
Pull request: server : add thinking content blocks to Anthropic Messages API

Hey team!

Following up on my initial fix for thinking content blocks, I found an additional issue while testing with different reasoning models.

### What was happening

The original fix added thinking content blocks, but there were two problems:

**Missing signature field** - The Anthropic API requires a `signature` field in thinking blocks (non-streaming) and a `signature_delta` event before closing thinking blocks (streaming). Without this, some clients like Claude Code were failing to parse responses correctly.

**Duplicate content_block_start events** - In streaming mode, each token was generating a new `content_block_start` event instead of just one per content block. This was because the `thinking_block_started` and `text_block_started` flags were local variables that reset on each chunk.

### The fix

For the signature issue:
- Added empty `signature` field in non-streaming thinking blocks
- Added `signature_delta` event before `content_block_stop` in streaming

For the duplicate events, I moved the block tracking state to `task_result_state` which persists across streaming chunks. This ensures we only emit one `content_block_start` per content block.

### Testing

Tested with multiple reasoning models to ensure compatibility:

- DeepSeek-R1-Distill-Qwen-7B: non-streaming and streaming working
- Qwen3-4B-Thinking-2507: non-streaming and streaming working

All 27 Anthropic compatibility tests pass, including the new signature verification.

@hksdpc255 I couldn't test with MiniMax-M2 due to hardware limitations (the smallest quantization is 64GB). Could you give it a try with this update? If it still doesn't work, please share the raw API response (a curl to `/v1/messages` with `stream: true`) so I can see what's happening.

Thanks for reporting the Claude Code compatibility issue and helping debug!

Changed files:
filename: tools/server/server-task.cpp
patch: @@ -814,6 +814,15 @@ json server_task_result_cmpl_final::to_json_anthropic() {
         msg.content = content;
     }
 
+    // thinking block comes first (Anthropic extended thinking format)
+    if (!msg.reasoning_content.empty()) {
+        content_blocks.push_back({
+            {"type", "thinking"},
+            {"thinking", msg.reasoning_content},
+            {"signature", ""}  // empty signature for local models (no cryptographic verification)
+        });
+    }
+
     if (!msg.content.empty()) {
         content_blocks.push_back({
             {"type", "text"},
@@ -862,20 +871,57 @@ json server_task_result_cmpl_final::to_json_anthropic_stream() {
         stop_reason = oaicompat_msg.tool_calls.empty() ? "end_turn" : "tool_use";
     }
 
-    bool has_text = !oaicompat_msg.content.empty();
+    bool has_thinking = !oaicompat_msg.reasoning_content.empty();
+    bool has_text     = !oaicompat_msg.content.empty();
     size_t num_tool_calls = oaicompat_msg.tool_calls.size();
 
-    bool text_block_started = false;
+    // content block indices: thinking (0) -> text (0 or 1) -> tool_use (n+)
+    size_t thinking_block_index = 0;
+    size_t text_block_index     = has_thinking ? 1 : 0;
+
+    bool thinking_block_started = false;
+    bool text_block_started     = false;
     std::unordered_set<size_t> tool_calls_started;
 
     for (const auto & diff : oaicompat_msg_diffs) {
+        // handle thinking/reasoning content
+        if (!diff.reasoning_content_delta.empty()) {
+            if (!thinking_block_started) {
+                events.push_back({
+                    {"event", "content_block_start"},
+                    {"data", {
+                        {"type", "content_block_start"},
+                        {"index", thinking_block_index},
+                        {"content_block", {
+                            {"type", "thinking"},
+                            {"thinking", ""}
+                        }}
+                    }}
+                });
+                thinking_block_started = true;
+            }
+
+            events.push_back({
+                {"event", "content_block_delta"},
+                {"data", {
+                    {"type", "content_block_delta"},
+                    {"index", thinking_block_index},
+                    {"delta", {
+                        {"type", "thinking_delta"},
+                        {"thinking", diff.reasoning_content_delta}
+                    }}
+                }}
+            });
+        }
+
+        // handle regular text content
         if (!diff.content_delta.empty()) {
             if (!text_block_started) {
                 events.push_back({
                     {"event", "content_block_start"},
                     {"data", {
                         {"type", "content_block_start"},
-                        {"index", 0},
+                        {"index", text_block_index},
                         {"content_block", {
                             {"type", "text"},
                             {"text", ""}
@@ -889,7 +935,7 @@ json server_task_result_cmpl_final::to_json_anthropic_stream() {
                 {"event", "content_block_delta"},
                 {"data", {
                     {"type", "content_block_delta"},
-                    {"index", 0},
+                    {"index", text_block_index},
                     {"delta", {
                         {"type", "text_delta"},
                         {"text", diff.content_delta}
@@ -898,8 +944,9 @@ json server_task_result_cmpl_final::to_json_anthropic_stream() {
             });
         }
 
+        // handle tool calls
         if (diff.tool_call_index != std::string::npos) {
-            size_t content_block_index = (has_text ? 1 : 0) + diff.tool_call_index;
+            size_t content_block_index = (has_thinking ? 1 : 0) + (has_text ? 1 : 0) + diff.tool_call_index;
 
             if (tool_calls_started.find(diff.tool_call_index) == tool_calls_started.end()) {
                 const auto & full_tool_call = oaicompat_msg.tool_calls[diff.tool_call_index];
@@ -935,18 +982,42 @@ json server_task_result_cmpl_final::to_json_anthropic_stream() {
         }
     }
 
+    // close content blocks in order
+    if (has_thinking) {
+        // Anthropic API requires a signature_delta before closing thinking blocks
+        // We use an empty signature since we can't generate a cryptographic signature for local models
+        events.push_back({
+            {"event", "content_block_delta"},
+            {"data", {
+                {"type", "content_block_delta"},
+                {"index", thinking_block_index},
+                {"delta", {
+                    {"type", "signature_delta"},
+                    {"signature", ""}
+                }}
+            }}
+        });
+        events.push_back({
+            {"event", "content_block_stop"},
+            {"data", {
+                {"type", "content_block_stop"},
+                {"index", thinking_block_index}
+            }}
+        });
+    }
+
     if (has_text) {
         events.push_back({
             {"event", "content_block_stop"},
             {"data", {
                 {"type", "content_block_stop"},
-                {"index", 0}
+                {"index", text_block_index}
             }}
         });
     }
 
     for (size_t i = 0; i < num_tool_calls; i++) {
-        size_t content_block_index = (has_text ? 1 : 0) + i;
+        size_t content_block_index = (has_thinking ? 1 : 0) + (has_text ? 1 : 0) + i;
         events.push_back({
             {"event", "content_block_stop"},
             {"data", {
@@ -1154,11 +1225,10 @@ json server_task_result_rerank::to_json() {
 json server_task_result_cmpl_partial::to_json_anthropic() {
     json events = json::array();
     bool first = (n_decoded == 1);
-    bool text_block_started = false;
+    // use member variables to track block state across streaming calls
+    // (anthropic_thinking_block_started, anthropic_text_block_started)
 
     if (first) {
-        text_block_started = false;
-
         events.push_back({
             {"event", "message_start"},
             {"data", {
@@ -1180,28 +1250,69 @@ json server_task_result_cmpl_partial::to_json_anthropic() {
         });
     }
 
+    // content block indices: thinking (0) -> text (0 or 1) -> tool_use (n+)
+    size_t thinking_block_index = 0;
+    // use anthropic_has_reasoning (set in update()) to know if ANY reasoning was generated
+    size_t text_block_index     = anthropic_has_reasoning ? 1 : 0;
+
+    // use local copies of streaming state (copied from task_result_state in update())
+    // these reflect the state BEFORE this chunk was processed
+    bool thinking_started = anthropic_thinking_block_started;
+    bool text_started     = anthropic_text_block_started;
+
     for (const auto & diff : oaicompat_msg_diffs) {
+        // handle thinking/reasoning content
+        if (!diff.reasoning_content_delta.empty()) {
+            if (!thinking_started) {
+                events.push_back({
+                    {"event", "content_block_start"},
+                    {"data", {
+                        {"type", "content_block_start"},
+                        {"index", thinking_block_index},
+                        {"content_block", {
+                            {"type", "thinking"},
+                            {"thinking", ""}
+                        }}
+                    }}
+                });
+                thinking_started = true;
+            }
+
+            events.push_back({
+                {"event", "content_block_delta"},
+                {"data", {
+                    {"type", "content_block_delta"},
+                    {"index", thinking_block_index},
+                    {"delta", {
+                        {"type", "thinking_delta"},
+                        {"thinking", diff.reasoning_content_delta}
+                    }}
+                }}
+            });
+        }
+
+        // handle regular text content
         if (!diff.content_delta.empty()) {
-            if (!text_block_started) {
+            if (!text_started) {
                 events.push_back({
                     {"event", "content_block_start"},
                     {"data", {
                         {"type", "content_block_start"},
-                        {"index", 0},
+                        {"index", text_block_index},
                         {"content_block", {
                             {"type", "text"},
                             {"text", ""}
                         }}
                     }}
                 });
-                text_block_started = true;
+                text_started = true;
             }
 
             events.push_back({
                 {"event", "content_block_delta"},
                 {"data", {
                     {"type", "content_block_delta"},
-                    {"index", 0},
+                    {"index", text_block_index},
                     {"delta", {
                         {"type", "text_delta"},
                         {"text", diff.content_delta}
@@ -1210,8 +1321,10 @@ json server_task_result_cmpl_partial::to_json_anthropic() {
             });
         }
 
+        // handle tool calls
         if (diff.tool_call_index != std::string::npos) {
-            size_t content_block_index = (text_block_started ? 1 : 0) + diff.tool_call_index;
+            // use anthropic_has_reasoning for thinking block count (persists across calls)
+            size_t content_block_index = (anthropic_has_reasoning ? 1 : 0) + (text_started ? 1 : 0) + diff.tool_call_index;
 
             if (!diff.tool_call_delta.name.empty()) {
                 events.push_back({

filename: tools/server/server-task.h
patch: @@ -96,6 +96,10 @@ struct task_result_state {
     std::string generated_text; // append new chunks of generated text here
     std::vector<std::string> generated_tool_call_ids;
 
+    // for Anthropic API streaming: track content block state across chunks
+    bool anthropic_thinking_block_started = false;
+    bool anthropic_text_block_started = false;
+
     task_result_state(const common_chat_syntax & oaicompat_chat_syntax)
         : oaicompat_chat_syntax(oaicompat_chat_syntax) {}
 
@@ -337,6 +341,12 @@ struct server_task_result_cmpl_partial : server_task_result {
     std::vector<common_chat_msg_diff> oaicompat_msg_diffs; // to be populated by update()
     bool is_updated = false;
 
+    // for Anthropic API: track if any reasoning content has been generated
+    bool anthropic_has_reasoning = false;
+    // Streaming state copied from task_result_state for this chunk
+    bool anthropic_thinking_block_started = false;
+    bool anthropic_text_block_started = false;
+
     virtual bool is_stop() override {
         return false; // in stream mode, partial responses are not considered stop
     }
@@ -346,6 +356,22 @@ struct server_task_result_cmpl_partial : server_task_result {
     virtual void update(task_result_state & state) override {
         is_updated = true;
         state.update_chat_msg(content, true, oaicompat_msg_diffs);
+        // track if the accumulated message has any reasoning content
+        anthropic_has_reasoning = !state.chat_msg.reasoning_content.empty();
+
+        // Copy current state for use in to_json_anthropic() (reflects state BEFORE this chunk)
+        anthropic_thinking_block_started = state.anthropic_thinking_block_started;
+        anthropic_text_block_started = state.anthropic_text_block_started;
+
+        // Pre-compute state updates based on diffs (for next chunk)
+        for (const auto & diff : oaicompat_msg_diffs) {
+            if (!diff.reasoning_content_delta.empty() && !state.anthropic_thinking_block_started) {
+                state.anthropic_thinking_block_started = true;
+            }
+            if (!diff.content_delta.empty() && !state.anthropic_text_block_started) {
+                state.anthropic_text_block_started = true;
+            }
+        }
     }
 
     json to_json_non_oaicompat();

filename: tools/server/tests/unit/test_compat_anthropic.py
patch: @@ -805,3 +805,92 @@ def test_anthropic_vs_openai_different_response_format():
     assert "input_tokens" in anthropic_res.body["usage"]
     assert "completion_tokens" in openai_res.body["usage"]
     assert "output_tokens" in anthropic_res.body["usage"]
+
+
+# Extended thinking tests with reasoning models
+
+@pytest.mark.slow
+@pytest.mark.parametrize("stream", [False, True])
+def test_anthropic_thinking_with_reasoning_model(stream):
+    """Test that thinking content blocks are properly returned for reasoning models"""
+    global server
+    server = ServerProcess()
+    server.model_hf_repo = "bartowski/DeepSeek-R1-Distill-Qwen-7B-GGUF"
+    server.model_hf_file = "DeepSeek-R1-Distill-Qwen-7B-Q4_K_M.gguf"
+    server.reasoning_format = "deepseek"
+    server.jinja = True
+    server.n_ctx = 8192
+    server.n_predict = 1024
+    server.server_port = 8084
+    server.start(timeout_seconds=600)  # large model needs time to download
+
+    if stream:
+        res = server.make_stream_request("POST", "/v1/messages", data={
+            "model": "test",
+            "max_tokens": 1024,
+            "thinking": {
+                "type": "enabled",
+                "budget_tokens": 500
+            },
+            "messages": [
+                {"role": "user", "content": "What is 2+2?"}
+            ],
+            "stream": True
+        })
+
+        events = list(res)
+
+        # should have thinking content block events
+        thinking_starts = [e for e in events if
+            e.get("type") == "content_block_start" and
+            e.get("content_block", {}).get("type") == "thinking"]
+        assert len(thinking_starts) > 0, "Should have thinking content_block_start event"
+        assert thinking_starts[0]["index"] == 0, "Thinking block should be at index 0"
+
+        # should have thinking_delta events
+        thinking_deltas = [e for e in events if
+            e.get("type") == "content_block_delta" and
+            e.get("delta", {}).get("type") == "thinking_delta"]
+        assert len(thinking_deltas) > 0, "Should have thinking_delta events"
+
+        # should have signature_delta event before thinking block closes (Anthropic API requirement)
+        signature_deltas = [e for e in events if
+            e.get("type") == "content_block_delta" and
+            e.get("delta", {}).get("type") == "signature_delta"]
+        assert len(signature_deltas) > 0, "Should have signature_delta event for thinking block"
+
+        # should have text block after thinking
+        text_starts = [e for e in events if
+            e.get("type") == "content_block_start" and
+            e.get("content_block", {}).get("type") == "text"]
+        assert len(text_starts) > 0, "Should have text content_block_start event"
+        assert text_starts[0]["index"] == 1, "Text block should be at index 1 (after thinking)"
+    else:
+        res = server.make_request("POST", "/v1/messages", data={
+            "model": "test",
+            "max_tokens": 1024,
+            "thinking": {
+                "type": "enabled",
+                "budget_tokens": 500
+            },
+            "messages": [
+                {"role": "user", "content": "What is 2+2?"}
+            ]
+        })
+
+        assert res.status_code == 200
+        assert res.body["type"] == "message"
+
+        content = res.body["content"]
+        assert len(content) >= 2, "Should have at least thinking and text blocks"
+
+        # first block should be thinking
+        thinking_blocks = [b for b in content if b.get("type") == "thinking"]
+        assert len(thinking_blocks) > 0, "Should have thinking content block"
+        assert "thinking" in thinking_blocks[0], "Thinking block should have 'thinking' field"
+        assert len(thinking_blocks[0]["thinking"]) > 0, "Thinking content should not be empty"
+        assert "signature" in thinking_blocks[0], "Thinking block should have 'signature' field (Anthropic API requirement)"
+
+        # should also have text block
+        text_blocks = [b for b in content if b.get("type") == "text"]
+        assert len(text_blocks) > 0, "Should have text content block"