Why a 1M-Token Context Window Makes Your Spec More Important, Not Less
The bigger context window doesn't make your idea clearer
A 1M-token context window means an AI agent can now hold your entire codebase, your notes, and your half-formed ideas in its head at once. That's genuinely new. It also means that if what you hand it is vague, the agent will now build vague things for hours instead of minutes. Longer runs amplify ambiguity — they don't resolve it.
On September 1, 2026, Anthropic made Fable 5.1 generally available with a 1M-token context window and roughly 75% cheaper prompt-cache reads, alongside a gated Mythos 5.1 release for vetted security researchers. The pricing change matters more than it sounds. Cheaper cache reads mean an agent can re-read a large context repeatedly without the cost curve breaking your budget.
For the first time, a solo non-technical founder can economically run a multi-hour agentic coding session. That used to be an enterprise-budget activity. Now it's a Tuesday.
Which raises the question nobody selling you AI tools wants to answer: what, exactly, are you going to put in that window?
