projectdiscovery/nuclei · error

probe concurrency must be at least 1

Error message

probe concurrency must be at least 1

What it means

For raw HTTP requests nuclei bufio-reads the first line; any read error - an empty raw block, or a request line with no trailing newline so ReadString returns io.EOF - aborts with 'could not read request'. Annotation lines (@tls-SNI etc.) are skipped, but a real request line terminated by a newline must follow them.

Source

Thrown at lib/config.go:151

			return errors.New("template threads must be at least 1")
		}
		if opts.HostConcurrency <= 0 {
			return errors.New("host concurrency must be at least 1")
		}
		if opts.HeadlessHostConcurrency <= 0 {
			return errors.New("headless host concurrency must be at least 1")
		}
		if opts.HeadlessTemplateConcurrency <= 0 {
			return errors.New("headless template threads must be at least 1")
		}
		if opts.JavascriptTemplateConcurrency <= 0 {
			return errors.New("js must be at least 1")
		}
		if opts.TemplatePayloadConcurrency <= 0 {
			return errors.New("payload concurrency must be at least 1")
		}
		if opts.ProbeConcurrency <= 0 {
			return errors.New("probe concurrency must be at least 1")
		}
		e.opts.TemplateThreads = opts.TemplateConcurrency
		e.opts.BulkSize = opts.HostConcurrency
		e.opts.HeadlessBulkSize = opts.HeadlessHostConcurrency
		e.opts.HeadlessTemplateThreads = opts.HeadlessTemplateConcurrency
		e.opts.JsConcurrency = opts.JavascriptTemplateConcurrency
		e.opts.PayloadConcurrency = opts.TemplatePayloadConcurrency
		e.opts.ProbeConcurrency = opts.ProbeConcurrency
		return nil
	}
}

// WithResponseReadSize sets the maximum size of response to read in bytes.
// A value of 0 means no limit. Recommended values: 1MB (1048576) to 10MB (10485760).
func WithResponseReadSize(responseReadSize int) NucleiSDKOptions {
	return func(e *NucleiEngine) error {
		if responseReadSize < 0 {
			return errors.New("response read size must be non-negative")

View on GitHub (pinned to 265b3a3dec)

Solutions

1. Make the raw block start with a complete request line that ends with a newline
2. Use a literal block (raw: |) so line breaks survive YAML parsing
3. If only annotations exist, add at least a minimal request line (GET / HTTP/1.1)
4. Validate with nuclei -validate

Example fix

# before
raw:
  - "GET /path HTTP/1.1"
# after
raw:
  - |
    GET /path HTTP/1.1
    Host: {{BaseURL}}

import "strings"

func rawRequestReadable(data string) bool {
    if !strings.Contains(data, "\n") {
        return false // no terminated first line
    }
    for _, line := range strings.Split(data, "\n") {
        if !strings.HasPrefix(line, "@") && strings.TrimSpace(line) != "" {
            return true // a real request line exists
        }
    }
    return false
}

When it happens

Trigger: A raw: block that is empty, contains only @annotation lines then EOF, or a quoted single-line scalar like "GET / HTTP/1.1" whose string lacks a final newline.

Common situations: Truncated raw templates; YAML quoted scalars collapsing newlines; copy-paste dropping the last line break.

Related errors

• response read size must be non-negative
• no new line found at the end of host header
• no host header found
• validation failed for these fields
• both verbose and silent mode specified

AI-assisted analysis of projectdiscovery/nuclei@265b3a3dec (2026-08-15). Data as JSON: /api/errors/af2ed94a335c9e6c. Report an issue: GitHub.