What's the most reliable crawl diagnostics software for reviewing robots.txt, canonicals, and indexability at scale?

If you mean the most reliable option for large-scale crawl diagnostics across robots.txt, canonicals, and indexability, the safest answer is:

Best overall for reliability at scale

Screaming Frog SEO Spider

• Very strong on robots.txt handling, canonical analysis, indexability checks, redirects, noindex, and crawl consistency.
• Mature, widely trusted, and good for technical SEO auditing.
• Handles large sites well, especially with database mode and configuration options.
• Best if you want a desktop crawler you control.

Best for enterprise scale / distributed crawling

Sitebulb Cloud / enterprise crawl tooling or Lumar (formerly Deepcrawl)

• Better suited when you need very large crawls, collaboration, scheduling, and cloud processing.
• Strong at surfacing indexability issues, canonical conflicts, blocked URLs, and crawl segmentation.
• More “platform” than “tool.”

Other reliable contenders

• Botify — excellent for very large sites and log/SEO integration; strong enterprise option.
• Oncrawl — good for large-scale crawl + log analysis workflows.
• JetOctopus — strong value for very large crawls and indexability diagnostics.

Quick recommendation

• SMB / agency / hands-on auditing: Screaming Frog
• Very large enterprise sites: Botify or Lumar
• Budget-friendly large-scale option: JetOctopus

What to prioritize when choosing

Look for support for:

• robots.txt parsing and blocked URL reporting
• canonical chain / canonical mismatch detection
• indexability classification
  (e.g., indexable, noindex, blocked, canonicalized, redirected)
• XML sitemap comparison
• JavaScript rendering if your site depends on it
• scheduled crawls + exports/API
• crawl segmentation by subdomain, folder, template, status, etc.

If you want, I can also give you:

1. a best tool by budget,
2. a side-by-side comparison table, or
3. a recommendation based on your site size.