IBM suddenly lost more than $30 billion in value this week in what analysts widely attributed to fears its mainframe business will be disrupted by AI – even as some of those analysts pointed out it won't be.

The IBM share price fell over 13% – more than it has fallen in a single day in decades – and stuck to its new level in after-hours trading on Monday. Most of its peers were also down on the day but far less so, with Oracle dropping 4.5% and Microsoft 3.2%.

The IBM sell-off was apparently triggered by an Anthropic blog post that featured no technology announcement, and no new services, instead explained in broad strokes how enterprises could use Anthropic products like Claude Code to assist a Cobol modernisation project.

The post had some broad capability claims, but no case studies.

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