xAI staff discovered this week that Anthropic's models stopped responding in Cursor, the popular AI coding IDE. The cutoff follows a now-familiar playbook: Anthropic has been aggressively moving throughout 2025 to wall off major competitors and third-party tools from accessing Claude.
According to an internal Slack message I viewed, xAI cofounder Tony Wu told employees on Wednesday:
"Hi team, I believe many of you have already discovered that anthropic models are not responding on cursor. According to cursor this is a new policy anthropic is enforcing for all its major competitors. This is a both bad and good news. We will get a hit on productivity, but it rly pushes us to develop our own coding product / models."
The "rly" is doing a lot of work there.
The pattern
This is the third time in seven months that Anthropic has severed access to a major player. In June, Windsurf CEO Varun Mohan revealed that Anthropic had cut off nearly all of Windsurf's first-party access to Claude 3.x models with less than five days' notice. The timing was convenient: Bloomberg had just reported OpenAI was acquiring Windsurf for $3 billion. Anthropic's chief scientist Jared Kaplan was blunt about it. "I think it would be odd for us to be selling Claude to OpenAI," he told TechCrunch.
Then in late July, Anthropic revoked OpenAI's access to the Claude API. The stated reason: OpenAI was allegedly plugging Claude into internal tools to benchmark its own models before the GPT-5 launch. Anthropic's commercial terms forbid using Claude to build competing services.
OpenAI called the cutoff "disappointing" and noted its own API remains available to Anthropic. Make of that what you will.
What this means for xAI
Wu's message hints at what everyone already suspected: xAI engineers were relying on Claude through Cursor to accelerate their own development. Not ideal when you're racing to compete with the very company whose tools you're using.
The company already has Grok Code Fast 1, a coding model it released in August. xAI partnered with Cursor, GitHub Copilot, and others to offer it free for a limited time. But Wu's message acknowledges they'll "get a hit on productivity" from losing Claude access.
xAI is burning nearly a billion dollars a month building data centers. Its Q3 revenue was $107 million. The losses were $1.46 billion. So, you know. Productivity hits matter.
The Claude Code factor
The timing is linked to the massive surge in popularity for Claude Code, Anthropic's native terminal environment. The company announced this week that Claude Code hit $1 billion in run-rate revenue. That's real money, and it explains why Anthropic is suddenly very interested in who's accessing their models through third-party tools.
Anthropic also cracked down on "harnesses," which are software wrappers that let developers use their Claude subscription to run automated workflows. An Anthropic staffer acknowledged they'd "tightened safeguards against spoofing the Claude Code harness" and that some legitimate users got caught in the crossfire. The company is reversing those bans.
But the blocks on competitors appear intentional.
The broader picture
The AI coding market is consolidating fast. Cursor reached $1 billion in annualized revenue and a $29.3 billion valuation in November. Anthropic has Claude Code. OpenAI bought Windsurf. Google partnered with Replit. Everyone wants the vibe-coding money.
Which means everyone is also locking down access. Anthropic's abrupt decision to cut partners could either prove its models are so good that developers will follow them anywhere, or hand market share to Google and OpenAI. We'll find out.
Wu's message ended with an attempt at optimism: "This coming year is rly going to be wildly exciting for all of us." The team is "rapidly developing" their own models and products. "We will have something to share with everyone soon."
Anthropic declined to comment. Cursor directed me to Anthropic. xAI didn't respond.
