How AI Is Transforming COBOL Modernization and Why Businesses Are Taking Notice

It's an open secret — though surprisingly few people seem aware of it — that the institutions keeping the global financial system running rely on code that is ancient, barely understood, and frighteningly difficult to replace. Now, artificial intelligence is finally making that problem solvable. And the market has responded with a sharp reality check for one of technology's oldest names.

IBM shares recorded their worst single-day drop in more than 25 years earlier this week, plunging 13% after AI startup Anthropic announced that its Claude Code tool can dramatically accelerate COBOL modernisation — the kind of painstaking, expensive legacy work that has long underpinned a significant portion of IBM's consulting revenue.

"Modernising a COBOL system once required armies of consultants spending years mapping workflows."

— Anthropic, Official Blog Post

Anthropic argued that tools like Claude Code can now automate the exploration and analysis phases that consume the majority of effort in COBOL modernisation projects. That single claim was enough to send investors reaching for the sell button.

📌 COBOL Is Bigger Than Most People Realise

To understand why the market reaction was so sharp, it helps to appreciate just how entrenched COBOL remains in global infrastructure. Hundreds of billions of lines of COBOL code run in production every single day, powering critical systems across finance and government. The language handles an estimated 95% of ATM transactions in the United States alone.

The deeper problem, however, isn't the code itself — it's the people who understand it. The number of developers with COBOL expertise continues to shrink as the workforce that built these systems has largely retired. That talent scarcity is precisely what made COBOL modernisation so expensive for so long, and what made large consulting engagements — the kind IBM and rivals like Accenture and Cognizant built profitable practices around — essentially unavoidable.

Anthropic argues that AI flips this equation entirely. Claude Code works by:

• 🔍 Mapping dependencies across thousands of lines of code
• 📄 Documenting workflows automatically
• ⚡ Identifying risks faster than human analysts
• 📊 Providing teams with deep insights for informed decision-making

The company claims teams can now modernise COBOL codebases in quarters, not years.

🏢 IBM Was Already Here

What the market's reaction may be overlooking is that IBM itself has been making a very similar argument for years. Anthropic's announcement comes roughly three years after IBM suggested using AI to rewrite COBOL as Java and launched a dedicated product — watsonx Code Assistant for Z — to do exactly that.

IBM CEO Arvind Krishna stated as recently as July 2025 that the company's AI coding assistant for mainframes "has got very strong adoption," with the majority of customers using it to understand their COBOL codebases and determine what to modernise.

"Clients already had the option to migrate from the mainframe, yet they are sticking with the platform."

— Amit Daryanani, Evercore ISI Analyst

IBM defended its position following the selloff, stating that its mainframe platform delivers the same quality of performance and security regardless of programming language — COBOL or otherwise. Analysts were also quick to add nuance to the panic, with Evercore ISI's Daryanani suggesting that fear of displacement may be outrunning the reality on the ground.

📉 The Broader Market Pattern

IBM wasn't alone in taking a hit. Accenture and Cognizant also declined following the news — a clear signal that investors are reassessing the entire consulting model built around legacy modernisation, not just IBM's mainframe hardware business.

Just the previous week, cybersecurity stocks sold off sharply after Anthropic announced Claude Code Security, a tool that scans codebases for vulnerabilities. The pattern is becoming familiar: each new AI capability announcement triggers an immediate reassessment of which existing revenue streams might be compressed, and the market prices in that fear without delay.

💬 IBM's Counterargument: Code Translation ≠ Platform Modernisation

IBM didn't stay quiet. Rob Thomas, the company's Senior Vice President and Chief Commercial Officer, pushed back directly, drawing a distinction the market appeared to have missed:

"Translating code is one thing. Modernising a platform is something else entirely. The two are not the same, and the gap between them is where most enterprises run into trouble."

— Rob Thomas, IBM SVP & Chief Commercial Officer

Thomas's argument is worth considering carefully. The value IBM's mainframe delivers, he contends, has nothing to do with COBOL as a language. It lives in the vertically integrated stack underneath it:

• 🖥️ z/OS — IBM's mainframe operating system
• ⚙️ Transaction processing architecture
• 🔐 Quantum-safe encryption
• 🔧 Decades of hardware-software optimisation

None of these are touched by a code translation tool. Thomas also raised a point that further complicates the headline narrative: roughly 40% of COBOL actually runs on Windows, Linux, and other distributed platforms — not mainframes at all. Much of what is being framed as an IBM mainframe story is, in part, a distributed systems problem that has been folded into a mainframe headline.

✅ Real-World Results From IBM's Existing Clients

IBM's own clients are already making the case with concrete outcomes:

🔎 The Bottom Line

Whether Monday's selloff was a fair verdict or a reflexive one, the underlying shift is real: AI is making COBOL modernisation economically viable for the first time in decades. The question IBM is asking — and one the market has not yet fully answered — is whether that represents a threat to its business, or an acceleration of the transformation it is already leading.

🔗 See also: Related coverage on AI and enterprise technology transformation