core/agent: rebuild a streamed round's provider blocks

Streaming is the default path: `thinking` fragments, a `server_tool_use`'s `partial_json` query and a `web_search_tool_result`'s entries arrive as separate SSE events, so the blocks the round has to hand back exist only as fragments. The dispatch rebuilds each one from its start payload (field for field, under the stream's own names) plus the fragments that follow, and passes it to the loop as a chunk; the loop copies it onto the round's message, where a round that kept only those blocks counts as payload rather than an empty completion.

The thinking text streams as reasoning as it arrives — the search query and the signature go back with the round only.

tests/test_agent/test_messages_dispatch.py

@@ -8,6 +8,7 @@ from msgspec import UNSET
 
 from plyngent.agent import (
     AssistantMessageEvent,
+    ReasoningDeltaEvent,
     TextDeltaEvent,
     ToolCallEvent,
     ToolRegistry,
@@ -396,9 +397,9 @@ async def test_dispatch_stream_tool_calls_without_stop_yields_no_finish() -> Non
     assert not any(c.choices and c.choices[0].finish_reason not in (None, UNSET) for c in chunks)
 
 
-def _search_stream_events() -> list[AnthropicStreamEvent]:
+def _search_stream_events(*, text: bool = True) -> list[AnthropicStreamEvent]:
     """The stream a DeepSeek search turn sends: thinking, search, results, text."""
-    return [
+    events: list[AnthropicStreamEvent] = [
         AnthropicMessageStart(
             message=AnthropicMessageResponse(
                 id="msg_s",
@@ -407,11 +408,16 @@ def _search_stream_events() -> list[AnthropicStreamEvent]:
                 usage=AnthropicUsage(input_tokens=164, output_tokens=0),
             )
         ),
-        AnthropicContentBlockStart(index=0, content_block=AnthropicRawContentBlock(type="thinking", text="")),
+        AnthropicContentBlockStart(
+            index=0,
+            content_block=AnthropicRawContentBlock(type="thinking", thinking=""),
+        ),
+        AnthropicContentBlockDelta(index=0, delta=AnthropicRawContentBlock(type="thinking_delta", thinking="Search ")),
         AnthropicContentBlockDelta(
             index=0,
-            delta=AnthropicRawContentBlock(type="thinking_delta", text="Search."),
+            delta=AnthropicRawContentBlock(type="thinking_delta", thinking="the docs."),
         ),
+        AnthropicContentBlockDelta(index=0, delta=AnthropicRawContentBlock(type="signature_delta", signature="sig-1")),
         AnthropicContentBlockStop(index=0),
         AnthropicContentBlockStart(
             index=1,
@@ -430,23 +436,39 @@ def _search_stream_events() -> list[AnthropicStreamEvent]:
             index=2,
             content_block=AnthropicRawContentBlock(
                 type="web_search_tool_result",
-                id="call_00_1",
-                input={"content": [{"type": "web_search_result", "url": "https://api-docs.deepseek.com"}]},
+                tool_use_id="call_00_1",
+                content=[
+                    {
+                        "type": "web_search_result",
+                        "url": "https://api-docs.deepseek.com",
+                        "encrypted_content": "sd918tZXSvVp",
+                    }
+                ],
             ),
         ),
         AnthropicContentBlockStop(index=2),
-        AnthropicContentBlockStart(index=3, content_block=AnthropicRawContentBlock(type="text", text="")),
-        AnthropicContentBlockDelta(
-            index=3,
-            delta=AnthropicRawContentBlock(type="text_delta", text="DeepSeek-V4 is out."),
-        ),
-        AnthropicContentBlockStop(index=3),
-        AnthropicMessageDelta(
-            delta={"stop_reason": "end_turn"},
-            usage=AnthropicUsage(input_tokens=0, output_tokens=12),
-        ),
-        AnthropicMessageStop(),
     ]
+    if text:
+        events.extend(
+            [
+                AnthropicContentBlockStart(index=3, content_block=AnthropicRawContentBlock(type="text", text="")),
+                AnthropicContentBlockDelta(
+                    index=3,
+                    delta=AnthropicRawContentBlock(type="text_delta", text="DeepSeek-V4 is out."),
+                ),
+                AnthropicContentBlockStop(index=3),
+            ]
+        )
+    events.extend(
+        [
+            AnthropicMessageDelta(
+                delta={"stop_reason": "end_turn"},
+                usage=AnthropicUsage(input_tokens=0, output_tokens=12),
+            ),
+            AnthropicMessageStop(),
+        ]
+    )
+    return events
 
 
 async def test_search_turn_streams_its_text_without_a_local_tool_call() -> None:
@@ -476,10 +498,58 @@ async def test_search_turn_streams_its_text_without_a_local_tool_call() -> None:
     assert not any(isinstance(event, ToolCallEvent) for event in events)
     assert not any(isinstance(event, ToolResultEvent) for event in events)
     assert any(isinstance(event, TextDeltaEvent) and event.content == "DeepSeek-V4 is out." for event in events)
+    # The chain of thought streams as reasoning, fragment by fragment.
+    assert [event.content for event in events if isinstance(event, ReasoningDeltaEvent)] == ["Search ", "the docs."]
+    # ...and the round keeps the search blocks, rebuilt, for the next request.
+    assistant = next(event.message for event in events if isinstance(event, AssistantMessageEvent))
+    assert assistant.reasoning_content == "Search the docs."
+    assert assistant.provider_blocks == [
+        {"type": "thinking", "thinking": "Search the docs.", "signature": "sig-1"},
+        {
+            "type": "server_tool_use",
+            "id": "call_00_1",
+            "name": "web_search",
+            "input": {"query": "DeepSeek-V4"},
+        },
+        {
+            "type": "web_search_tool_result",
+            "tool_use_id": "call_00_1",
+            "content": [
+                {
+                    "type": "web_search_result",
+                    "url": "https://api-docs.deepseek.com",
+                    "encrypted_content": "sd918tZXSvVp",
+                }
+            ],
+        },
+    ]
     # One round: the search answered inside the response, so nothing was sent back.
     assert len(client.calls) == 1
 
 
+async def test_search_only_round_is_not_an_empty_completion() -> None:
+    """A search turn with no text yet still produced payload.
+
+    A budget-capped or ``max_uses_exceeded`` search can end the turn on its
+    blocks alone; calling that an empty completion failed a round the model had
+    actually used.
+    """
+    client = ScriptedMessagesClient(stream_events=[_search_stream_events(text=False)])
+    events = [
+        event
+        async for event in run_chat_loop(
+            cast("Any", client),
+            [UserChatMessage(content="what is DeepSeek-V4?")],
+            model="deepseek-flash",
+            stream=True,
+        )
+    ]
+    assistant = next(event.message for event in events if isinstance(event, AssistantMessageEvent))
+    assert assistant.content is None
+    assert assistant.provider_blocks is not UNSET
+    assert len(assistant.provider_blocks) == 3
+
+
 async def test_dispatch_messages_sends_provider_tools_as_server_tools() -> None:
     """Hosted tools reach the Anthropic request in that surface's own shape."""
     client = ScriptedMessagesClient(non_stream=[_message_response(text="ok")])