diff --git a/pkg/providers/http_provider.go b/pkg/providers/http_provider.go
index ddc549a..0342c71 100644
--- a/pkg/providers/http_provider.go
+++ b/pkg/providers/http_provider.go
@@ -65,7 +65,21 @@ func (p *HTTPProvider) Chat(ctx context.Context, messages []Message, tools []Too
 			return nil, err
 		}
 		if statusCode != http.StatusOK {
-			return nil, fmt.Errorf("API error (status %d, content-type %q): %s", statusCode, contentType, previewResponseBody(body))
+			preview := previewResponseBody(body)
+			if statusCode == http.StatusBadRequest && strings.Contains(strings.ToLower(preview), "no tool call found for function call output") {
+				// Retry once with sanitized history to avoid orphaned tool outputs causing hard-fail.
+				safeMessages := sanitizeResponsesRetryMessages(messages)
+				body2, status2, ctype2, err2 := p.callResponses(ctx, safeMessages, tools, model, options)
+				if err2 == nil && status2 == http.StatusOK && json.Valid(body2) {
+					logger.InfoCF("provider", "Recovered responses 400 by sanitizing tool outputs", map[string]interface{}{"messages_before": len(messages), "messages_after": len(safeMessages)})
+					return parseResponsesAPIResponse(body2)
+				}
+				if err2 != nil {
+					return nil, err2
+				}
+				return nil, fmt.Errorf("API error (status %d, content-type %q): %s", status2, ctype2, previewResponseBody(body2))
+			}
+			return nil, fmt.Errorf("API error (status %d, content-type %q): %s", statusCode, contentType, preview)
 		}
 		if !json.Valid(body) {
 			return nil, fmt.Errorf("API error (status %d, content-type %q): non-JSON response: %s", statusCode, contentType, previewResponseBody(body))
@@ -207,6 +221,22 @@ func toChatCompletionsContent(msg Message) []map[string]interface{} {
 	return content
 }
 
+func sanitizeResponsesRetryMessages(messages []Message) []Message {
+	out := make([]Message, 0, len(messages))
+	for _, m := range messages {
+		if strings.EqualFold(strings.TrimSpace(m.Role), "tool") {
+			text := strings.TrimSpace(m.Content)
+			if text == "" {
+				continue
+			}
+			out = append(out, Message{Role: "user", Content: "[tool_result_fallback] " + text})
+			continue
+		}
+		out = append(out, m)
+	}
+	return out
+}
+
 func toResponsesInputItems(msg Message) []map[string]interface{} {
 	return toResponsesInputItemsWithState(msg, nil)
 }