feat: suppress output of partial XML tool tag fragments in stream processing

internal/adapter/openai/tool_sieve_core.go

@@ -130,8 +130,14 @@ func flushToolSieve(state *toolStreamSieveState, toolNames []string) []toolStrea
 	}
 	if state.pending.Len() > 0 {
 		content := state.pending.String()
-		state.noteText(content)
-		events = append(events, toolStreamEvent{Content: content})
+		// Safety: if pending contains XML tool tag fragments (e.g. "tool_calls>"
+		// from a split closing tag), swallow them instead of leaking.
+		if hasOpenXMLToolTag(content) || looksLikeXMLToolTagFragment(content) {
+			// Drop it — likely an incomplete tool call fragment.
+		} else {
+			state.noteText(content)
+			events = append(events, toolStreamEvent{Content: content})
+		}
 		state.pending.Reset()
 	}
 	return events

internal/adapter/openai/tool_sieve_xml.go

@@ -104,3 +104,27 @@ func findPartialXMLToolTagStart(s string) int {
 	}
 	return -1
 }
+
+// looksLikeXMLToolTagFragment returns true if s looks like a fragment from a
+// split XML tool call tag — for example "tool_calls>" or "/tool_call>\n".
+// These fragments arise when '<' was consumed separately and the tail remains.
+func looksLikeXMLToolTagFragment(s string) bool {
+	trimmed := strings.TrimSpace(s)
+	if trimmed == "" {
+		return false
+	}
+	lower := strings.ToLower(trimmed)
+	// Check for closing tag tails like "tool_calls>" or "/tool_calls>"
+	fragments := []string{
+		"tool_calls>", "tool_call>", "/tool_calls>", "/tool_call>",
+		"function_calls>", "function_call>", "/function_calls>", "/function_call>",
+		"invoke>", "/invoke>", "tool_use>", "/tool_use>",
+		"tool_name>", "/tool_name>", "parameters>", "/parameters>",
+	}
+	for _, f := range fragments {
+		if strings.Contains(lower, f) {
+			return true
+		}
+	}
+	return false
+}