From 67be1216aa507bd8049c4b7f54360eaa88335590 Mon Sep 17 00:00:00 2001
From: Doron Kopit <doron@traceloop.com>
Date: Thu, 5 Nov 2026 15:09:03 +0200
Subject: [PATCH] fix(anthropic): record finish reasons for streamed turns with
 empty content

Fixes #4362
---
 .../instrumentation/anthropic/span_utils.py   | 10 +++--
 .../instrumentation/anthropic/streaming.py    |  5 ++-
 .../tests/test_semconv_span_attrs.py          | 38 +++++++++++++++++++
 3 files changed, 49 insertions(+), 4 deletions(-)

diff --git a/packages/opentelemetry-instrumentation-anthropic/opentelemetry/instrumentation/anthropic/span_utils.py b/packages/opentelemetry-instrumentation-anthropic/opentelemetry/instrumentation/anthropic/span_utils.py
index 61ecb32bc8..bc6bf2b52b 100644
--- a/packages/opentelemetry-instrumentation-anthropic/opentelemetry/instrumentation/anthropic/span_utils.py
+++ b/packages/opentelemetry-instrumentation-anthropic/opentelemetry/instrumentation/anthropic/span_utils.py
@@ -392,17 +392,17 @@ def set_response_attributes(span, response):
 
 
 @dont_throw
-def set_streaming_response_attributes(span, complete_response_events):
+def set_streaming_response_attributes(span, complete_response_events, stop_reason=None):
     from opentelemetry.instrumentation.anthropic import set_span_attribute
 
-    if not span.is_recording() or not complete_response_events:
+    if not span.is_recording() or not (complete_response_events or stop_reason):
         return
 
     # Collect all parts and determine finish_reason
     parts = []
     finish_reasons = []
 
-    for event in complete_response_events:
+    for event in complete_response_events or []:
         finish_reason = event.get("finish_reason")
         if finish_reason:
             mapped = _map_finish_reason(finish_reason)
@@ -436,6 +436,10 @@ def set_streaming_response_attributes(span, complete_response_events):
                     "content": event.get("text"),
                 })
 
+    # A turn with no content blocks only carries the message-level stop_reason
+    if not finish_reasons and stop_reason:
+        finish_reasons.append(_map_finish_reason(stop_reason))
+
     if finish_reasons:
         span.set_attribute(GenAIAttributes.GEN_AI_RESPONSE_FINISH_REASONS, finish_reasons)
 
diff --git a/packages/opentelemetry-instrumentation-anthropic/opentelemetry/instrumentation/anthropic/streaming.py b/packages/opentelemetry-instrumentation-anthropic/opentelemetry/instrumentation/anthropic/streaming.py
index bd2d6dc15d..6c74dfc1e5 100644
--- a/packages/opentelemetry-instrumentation-anthropic/opentelemetry/instrumentation/anthropic/streaming.py
+++ b/packages/opentelemetry-instrumentation-anthropic/opentelemetry/instrumentation/anthropic/streaming.py
@@ -58,6 +58,7 @@ def _process_response_item(item, complete_response):
                 if event.get("type") == "tool_use":
                     event["input"] = event.get("input", "") + item.delta.partial_json
     elif item.type == "message_delta":
+        complete_response["stop_reason"] = item.delta.stop_reason
         for event in complete_response.get("events", []):
             event["finish_reason"] = item.delta.stop_reason
         if item.usage:
@@ -175,7 +176,9 @@ def _handle_streaming_response(span, event_logger, complete_response):
     else:
         if not span.is_recording():
             return
-        set_streaming_response_attributes(span, complete_response.get("events"))
+        set_streaming_response_attributes(
+            span, complete_response.get("events"), complete_response.get("stop_reason")
+        )
 
 
 class AnthropicStream(ObjectProxy):
diff --git a/packages/opentelemetry-instrumentation-anthropic/tests/test_semconv_span_attrs.py b/packages/opentelemetry-instrumentation-anthropic/tests/test_semconv_span_attrs.py
index f99385702f..60f9ab86d7 100644
--- a/packages/opentelemetry-instrumentation-anthropic/tests/test_semconv_span_attrs.py
+++ b/packages/opentelemetry-instrumentation-anthropic/tests/test_semconv_span_attrs.py
@@ -1062,6 +1062,44 @@ def test_streaming_finish_reason_none_does_not_set_span_attr():
     assert GenAIAttributes.GEN_AI_RESPONSE_FINISH_REASONS not in span.attributes
 
 
+@pytest.mark.parametrize(
+    "with_content, expected_output",
+    [
+        (False, None),
+        (True, [{"role": "assistant", "parts": [{"type": "text", "content": "hi"}], "finish_reason": "stop"}]),
+    ],
+)
+def test_streaming_finish_reason_recorded_from_message_delta(with_content, expected_output):
+    """The message_delta stop_reason must be recorded even when the streamed
+    turn has no content blocks (content == [])."""
+    from opentelemetry.instrumentation.anthropic.streaming import (
+        _handle_streaming_response,
+        _process_response_item,
+    )
+
+    items = []
+    if with_content:
+        items += [
+            SimpleNamespace(type="content_block_start", index=0, content_block=SimpleNamespace(type="text")),
+            SimpleNamespace(
+                type="content_block_delta", index=0, delta=SimpleNamespace(type="text_delta", text="hi")
+            ),
+        ]
+    items.append(
+        SimpleNamespace(type="message_delta", delta=SimpleNamespace(stop_reason="end_turn"), usage=None)
+    )
+    complete_response = {"events": [], "model": "", "usage": {}, "id": ""}
+    for item in items:
+        _process_response_item(item, complete_response)
+
+    span = make_span()
+    _handle_streaming_response(span, None, complete_response)
+
+    assert span.attributes[GenAIAttributes.GEN_AI_RESPONSE_FINISH_REASONS] == ["stop"]
+    output = span.attributes.get(GenAIAttributes.GEN_AI_OUTPUT_MESSAGES)
+    assert (json.loads(output) if output else None) == expected_output
+
+
 # ---------------------------------------------------------------------------
 # P2 #1 — Streaming: unknown block types (e.g. redacted_thinking) must be skipped
 # ---------------------------------------------------------------------------