fix(provider): handle write errors in streaming responses

- Capture and return io.WriteString errors in CompletionStream for Anthropic and Gemini providers
- Stop writing to the writer after the first error occurs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: appleboy

provider/anthropic/anthropic.go

@@ -74,6 +74,7 @@ func (c *Client) CompletionStream(
 	w io.Writer,
 ) (*core.Response, error) {
 	var sb strings.Builder
+	var writeErr error
 	resp, err := c.client.CreateMessagesStream(ctx, anthropic.MessagesStreamRequest{
 		MessagesRequest: anthropic.MessagesRequest{
 			Model: c.model,
@@ -85,9 +86,11 @@ func (c *Client) CompletionStream(
 			TopP:        convert.ToPtr(c.topP),
 		},
 		OnContentBlockDelta: func(data anthropic.MessagesEventContentBlockDeltaData) {
-			if data.Delta.Text != nil {
+			if data.Delta.Text != nil && writeErr == nil {
 				sb.WriteString(*data.Delta.Text)
-				_, _ = io.WriteString(w, *data.Delta.Text)
+				if _, err := io.WriteString(w, *data.Delta.Text); err != nil {
+					writeErr = err
+				}
 			}
 		},
 	})
@@ -101,6 +104,10 @@ func (c *Client) CompletionStream(
 		return nil, err
 	}
 
+	if writeErr != nil {
+		return nil, fmt.Errorf("failed to write streaming response: %w", writeErr)
+	}
+
 	usage := core.Usage{
 		PromptTokens:     resp.Usage.InputTokens,
 		CompletionTokens: resp.Usage.OutputTokens,

provider/gemini/gemini.go

@@ -91,6 +91,7 @@ func (c *Client) CompletionStream(
 	}
 
 	var sb strings.Builder
+	var writeErr error
 	usage := core.Usage{}
 	for resp, err := range c.client.Models.GenerateContentStream(ctx, c.model, data, cfg) {
 		if err != nil {
@@ -112,12 +113,20 @@ func (c *Client) CompletionStream(
 			for _, part := range resp.Candidates[0].Content.Parts {
 				if part.Text != "" {
 					sb.WriteString(part.Text)
-					_, _ = io.WriteString(w, part.Text)
+					if writeErr == nil {
+						if _, err := io.WriteString(w, part.Text); err != nil {
+							writeErr = err
+						}
+					}
 				}
 			}
 		}
 	}
 
+	if writeErr != nil {
+		return nil, fmt.Errorf("failed to write streaming response: %w", writeErr)
+	}
+
 	return &core.Response{
 		Content: sb.String(),
 		Usage:   usage,