TL;DR
Anthropic changed its API on Jan. 9, 2026 to reject requests from third‑party coding clients that had been using Claude Pro/Max subscriptions, provoking user complaints. Rivals including OpenAI moved quickly to support those third‑party harnesses, widening a competitive divide.
What happened
Beginning in 2025, terminal‑based coding agents — sometimes called “vibe coding” — gained traction after Anthropic launched Claude Code and rivals released similar CLIs. Many third‑party tools (notably OpenCode) enabled users to authenticate with Anthropic subscriptions to access Claude models from their preferred harnesses. That access relied on an OAuth/log‑in pathway that effectively let subscription holders use Claude via external clients. On January 9, 2026, Anthropic altered its API to detect and reject requests originating from those third‑party clients. The change was not accompanied by a formal company announcement; customers learned about it after service disruptions and an employee thread on a social platform. Anthropic cited operational concerns about unusual traffic patterns and supportability. The move prompted public complaints, a GitHub issue demanding reversal, and immediate competitive responses — OpenAI announced and shipped support for connecting ChatGPT Pro/Plus subscriptions to the same third‑party harnesses.
Why it matters
- Blocking third‑party access challenges developer workflows and can erode customer goodwill toward a paid product.
- The change highlights tensions between model providers who want to control the user experience and independent harnesses that aggregate models.
- A rapid competitive response from rivals can accelerate customer migration and reshape market dynamics.
- The decision exposes strategic incentives at providers with large fundraising ambitions to protect pricing and distribution.
Key facts
- Anthropic closed an OAuth/third‑party client loophole on January 9, 2026 that had allowed external coding tools to use Claude subscriptions.
- Claude Code first appeared as a research preview in early 2025; the full product launched in June 2025 and was included in Pro and Max subscription plans.
- Anthropic’s subscription plans reportedly reached $1 billion in annualized revenue six months after the Claude Code launch (reported in the source).
- OpenCode grew to over 50,000 GitHub stars and claimed more than 650,000 monthly active users according to the source.
- Some third‑party tools relied on a client‑supplied system prompt containing a specific phrase to access Anthropic subscriptions.
- Anthropic did not issue a formal ToS enforcement announcement before or immediately after the API change; a company employee posted a thread in response to complaints.
- Anthropic said the motivation was operational — third‑party harnesses allegedly created unusual traffic patterns and made debugging rate limits and account bans harder.
- OpenAI publicly supported OpenCode and other open‑source coding harnesses and shipped support for connecting ChatGPT Pro/Plus subscriptions with OpenCode.
What to watch next
- Whether Anthropic will issue a formal policy statement or reverse the API enforcement change — not confirmed in the source.
- The scale and direction of any subscription cancellations or customer migrations away from Anthropic — not confirmed in the source.
- Whether rival providers expand native support for third‑party coding harnesses beyond the initial OpenAI actions described in the source — not confirmed in the source.
Quick glossary
- Vibe coding: A colloquial term used to describe interactive, agentic coding workflows where an LLM can iteratively modify files and run commands in response to prompts.
- Terminal‑based coding agent: A command‑line tool that sends user prompts to a language model, executes tool calls such as edits or commands, and feeds results back into the model in a loop.
- OAuth token: An authorization credential that allows third‑party applications to access a service on behalf of a user without sharing the user's password.
- LLM (large language model): A machine learning model trained on large amounts of text that can generate human‑like language and provide reasoning or coding assistance.
Reader FAQ
Did Anthropic announce this API change in advance?
The source says Anthropic did not make a formal announcement ahead of the change; only an employee thread appeared after complaints.
Why did Anthropic block third‑party clients?
Anthropic has said the change addressed unusual traffic patterns and supportability challenges from third‑party harnesses, per the employee thread cited in the source.
Did OpenAI respond to Anthropic’s change?
Yes — the source reports OpenAI extended support to OpenCode and other open‑source harnesses and shipped support for connecting ChatGPT Pro/Plus with OpenCode.
Will Anthropic’s valuation or fundraising be affected?
Not confirmed in the source.
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