fix(providers/openai): blank line between reasoning summary parts

providers/openai/responses_language_model.go

@@ -1196,7 +1196,7 @@ func (o responsesLanguageModel) Generate(ctx context.Context, call fantasy.Call)
 			}
 
 			content = append(content, fantasy.ReasoningContent{
-				Text: strings.Join(metadata.Summary, "\n"),
+				Text: strings.Join(metadata.Summary, "\n\n"),
 				ProviderMetadata: fantasy.ProviderMetadata{
 					Name: metadata,
 				},
@@ -1526,10 +1526,14 @@ func (o responsesLanguageModel) Stream(ctx context.Context, call fantasy.Call) (
 				if state != nil {
 					state.metadata.Summary = append(state.metadata.Summary, "")
 					activeReasoning[added.ItemID] = state
+					// Summary parts open with a bold title that Markdown keeps
+					// inline after a single newline. The blank line also goes
+					// before the first part, so consumers that join consecutive
+					// reasoning items still get a paragraph break between them.
 					if !yield(fantasy.StreamPart{
 						Type:  fantasy.StreamPartTypeReasoningDelta,
 						ID:    added.ItemID,
-						Delta: "\n",
+						Delta: "\n\n",
 						ProviderMetadata: fantasy.ProviderMetadata{
 							Name: state.metadata,
 						},

providers/openai/responses_reasoning_summary_test.go

@@ -0,0 +1,78 @@
+package openai
+
+import (
+	"context"
+	"testing"
+
+	"charm.land/fantasy"
+	"github.com/stretchr/testify/require"
+)
+
+func TestResponsesStream_ReasoningSummaryPartsStartWithBlankLine(t *testing.T) {
+	t.Parallel()
+
+	partAdded := func(itemID, index string) string {
+		return responsesSSEEvent("response.reasoning_summary_part.added", `{"type":"response.reasoning_summary_part.added","item_id":"`+itemID+`","output_index":0,"summary_index":`+index+`,"part":{"type":"summary_text","text":""}}`)
+	}
+	textDelta := func(itemID, index, delta string) string {
+		return responsesSSEEvent("response.reasoning_summary_text.delta", `{"type":"response.reasoning_summary_text.delta","item_id":"`+itemID+`","output_index":0,"summary_index":`+index+`,"delta":"`+delta+`"}`)
+	}
+	sms := newStreamingMockServer()
+	defer sms.close()
+	sms.chunks = []string{
+		responsesSSEEvent("response.output_item.added", `{"type":"response.output_item.added","output_index":0,"item":{"id":"rs_1","type":"reasoning","summary":[]}}`),
+		partAdded("rs_1", "0"),
+		textDelta("rs_1", "0", `**A**\n\na`),
+		partAdded("rs_1", "1"),
+		textDelta("rs_1", "1", `**B**\n\nb`),
+		responsesSSEEvent("response.output_item.done", `{"type":"response.output_item.done","output_index":0,"item":{"id":"rs_1","type":"reasoning","summary":[{"type":"summary_text","text":"**A**\n\na"},{"type":"summary_text","text":"**B**\n\nb"}]}}`),
+		responsesSSEEvent("response.output_item.added", `{"type":"response.output_item.added","output_index":1,"item":{"id":"rs_2","type":"reasoning","summary":[]}}`),
+		partAdded("rs_2", "0"),
+		textDelta("rs_2", "0", `**C**\n\nc`),
+		responsesSSEEvent("response.output_item.done", `{"type":"response.output_item.done","output_index":1,"item":{"id":"rs_2","type":"reasoning","summary":[{"type":"summary_text","text":"**C**\n\nc"}]}}`),
+		responsesSSEEvent("response.completed", `{"type":"response.completed","response":{"id":"resp_1","status":"completed","output":[],"usage":{"input_tokens":1,"output_tokens":1,"total_tokens":2}}}`),
+	}
+
+	stream, err := newResponsesProvider(t, sms.server.URL).Stream(context.Background(), fantasy.Call{Prompt: testPrompt})
+	require.NoError(t, err)
+	parts, err := collectStreamParts(stream)
+	require.NoError(t, err)
+
+	text := map[string]string{}
+	for _, part := range parts {
+		if part.Type == fantasy.StreamPartTypeReasoningDelta {
+			text[part.ID] += part.Delta
+		}
+	}
+	require.Equal(t, map[string]string{
+		"rs_1": "\n\n**A**\n\na\n\n**B**\n\nb",
+		"rs_2": "\n\n**C**\n\nc",
+	}, text)
+}
+
+func TestResponsesGenerate_ReasoningSummaryPartsJoinWithBlankLine(t *testing.T) {
+	t.Parallel()
+
+	server := newMockServer()
+	defer server.close()
+	server.response = map[string]any{
+		"id":     "resp_1",
+		"object": "response",
+		"model":  "gpt-4.1",
+		"status": "completed",
+		"output": []any{
+			map[string]any{"type": "reasoning", "id": "rs_1", "summary": []any{
+				map[string]any{"type": "summary_text", "text": "**A**\n\na"},
+				map[string]any{"type": "summary_text", "text": "**B**\n\nb"},
+			}},
+		},
+		"usage": map[string]any{"input_tokens": 1, "output_tokens": 1, "total_tokens": 2},
+	}
+
+	resp, err := newResponsesProvider(t, server.server.URL).Generate(context.Background(), fantasy.Call{Prompt: testPrompt})
+	require.NoError(t, err)
+
+	reasoning := resp.Content.Reasoning()
+	require.Len(t, reasoning, 1)
+	require.Equal(t, "**A**\n\na\n\n**B**\n\nb", reasoning[0].Text)
+}