fix(reasoning): never cut a lead-in that carries an image

internal/reasoningmode/answer_observation.go

@@ -63,6 +63,12 @@ func SplitReasoningAnswerObserved(content string, observation NarrationObservati
 			// The stream showed no narration at all: the whole answer is body.
 			return content, true
 		}
+		// An image reference in front of the boundary is answer content (the
+		// agent published a picture in its lead-in), never step narration, so
+		// the safe answer is to keep everything.
+		if strings.Contains(content[:offset], "![") {
+			return content, true
+		}
 		body := strings.TrimLeft(content[offset:], "\r\n")
 		if strings.TrimSpace(body) != "" {
 			return body, true

internal/reasoningmode/answer_observation_test.go

@@ -68,6 +68,19 @@ func TestNarrationObservationFallsBackWithoutUsableBoundary(t *testing.T) {
 	}
 }
 
+func TestNarrationObservationKeepsImageLeadIn(t *testing.T) {
+	// The reported production shape: the lead-in carries the answer prose plus
+	// the picture, and the boundary observed by the stream only lands after it.
+	content := "根据知识库中《末端负载曲线-CRA系列》文档，以下是 **CR5A 的末端负载曲线图**：\n\n" +
+		"![CR5A 末端负载曲线](resource://FsVe56UYMz64sm8NsUZNrg)\n\n" +
+		"### 如何查看该负载曲线\n负载能力与偏心距离成负相关。\n"
+	offset := strings.Index(content, "### 如何查看该负载曲线")
+	observation := NarrationObservation{Observed: true, AnswerRuns: 4, ToolEvents: 6, LastRunStart: offset, LastRunClean: true}
+	if got := TrimReasoningNarrationObserved(content, observation); got != content {
+		t.Fatalf("image lead-in must survive the trim, got %q", got)
+	}
+}
+
 func TestNarrationObservationKeepsSingleRunAnswer(t *testing.T) {
 	// The quick-answer agent publishes the whole answer as a single answer run
 	// (with tool_call/tool_result steps around it). Nothing in that stream is