googleapis/mcp-toolbox · error

failed to write content to object %q in bucket %q: %w

Error message

failed to write content to object %q in bucket %q: %w

What it means

WriteObject writes a string to a GCS object through storage.Writer; this error wraps a failure returned by io.WriteString BEFORE Close is attempted. It means data could not be handed to the underlying upload stream — typically context cancellation, a broken HTTP body, or an already-failed writer — so the object was never finalized.

Source

Thrown at internal/sources/cloudstorage/cloudstorage.go:522

}

// WriteObject writes text content directly into a GCS object. When contentType
// is empty, the writer's ContentType is left unset so Cloud Storage detects it
// from the first 512 bytes. The returned contentType is the post-Close value
// from w.Attrs(), i.e. what GCS actually recorded.
func (s *Source) WriteObject(ctx context.Context, bucket, object, content, contentType string) (map[string]any, error) {
	if err := s.validateBucket(bucket); err != nil {
		return nil, err
	}
	w := s.client.Bucket(bucket).Object(object).NewWriter(ctx)
	if contentType != "" {
		w.ContentType = contentType
	}

	n, err := io.WriteString(w, content)
	if err != nil {
		_ = w.Close()
		return nil, fmt.Errorf("failed to write content to object %q in bucket %q: %w", object, bucket, err)
	}
	if err := w.Close(); err != nil {
		return nil, fmt.Errorf("failed to finalize write to %q/%q: %w", bucket, object, err)
	}

	attrs := w.Attrs()
	finalContentType := ""
	if attrs != nil {
		finalContentType = attrs.ContentType
	}
	return map[string]any{
		"bucket":      bucket,
		"object":      object,
		"bytes":       n,
		"contentType": finalContentType,
	}, nil
}

View on GitHub (pinned to 8cc6e09de2)

Solutions

1. Check the wrapped error for context cancellation: create a fresh context with an adequate timeout and retry the WriteObject call.
2. Verify credentials are valid and not expired (gcloud auth / ADC configuration); 401/403 surfaces at first write for small payloads.
3. Confirm the service account has storage.objects.create on the bucket.
4. For very large content, prefer UploadObject from a temp file or raise the context deadline.
5. Check bucket existence and org policy restrictions before writing.

Example fix

// before: single long deadline for everything
ctx := context.Background()
// after: bounded deadline sized for the payload
ctx, cancel := context.WithTimeout(context.Background(), 2*time.Minute)
defer cancel()
_, err := src.WriteObject(ctx, bucket, object, content, "text/plain")
if errors.Is(err, context.DeadlineExceeded) {
    // retry with larger timeout or chunk the content
}

// Go: validate before WriteObject
func precheckWrite(ctx context.Context, src *cloudstorage.Source, bucket, object, content string) error {
    if content == "" {
        return errors.New("refusing to write empty content")
    }
    if _, err := src.Client.Bucket(bucket).Attrs(ctx); err != nil {
        return fmt.Errorf("bucket %q unavailable: %w", bucket, err)
    }
    return nil
}

// Go: detect context issues before/at write time
func isContextFailure(err error) bool {
    return errors.Is(err, context.Canceled) || errors.Is(err, context.DeadlineExceeded)
}

_, err := src.WriteObject(ctx, bucket, object, content, "application/json")
if err != nil {
    if isContextFailure(err) {
        ctx, cancel := context.WithTimeout(context.Background(), 5*time.Minute)
        defer cancel()
        _, err = src.WriteObject(ctx, bucket, object, content, "application/json")
    }
    if err != nil {
        return fmt.Errorf("write to gs://%s/%s failed: %w", bucket, object, err)
    }
}

When it happens

Trigger: Calling WriteObject(bucket, object, content, contentType) where io.WriteString returns an error: ctx cancelled/expired during the write, GCS returned an error on the first request flush, or the writer's internal pipe broke (e.g. authentication failure surfaced at first write).

Common situations: Very large payloads exceeding request deadlines, cancelled HTTP requests from MCP clients, credential expiry mid-request (401 surfaced at write), writing to a bucket whose location/permissions reject the stream.

Related errors

• failed to finalize upload of %q to %q/%q: %w
• failed to finalize write to %q/%q: %w
• failed to list objects in bucket %q: %w
• failed to open object %q in bucket %q: %w
• failed to read object %q in bucket %q: %w

AI-assisted analysis of googleapis/mcp-toolbox@8cc6e09de2 (2026-09-05). Data as JSON: /api/errors/9e089233364e1c14. Report an issue: GitHub.