fable-pin

fable-pin makes every subagent of a Claude Code session run on one model, whatever the prompt asked for. “fable” here is the model alias for Claude Fable, Anthropic’s most recent model, and the mod rewrites the model field of every agent.spawn to it. Forks are left alone, because a fork inherits its parent’s model anyway. It is for anyone who has written “use Fable for all agents” into a settings file and watched the model pick something else.

That is the whole point of doing this as a hook. A rule in CLAUDE.md is text the model reads and sometimes ignores. A hook on agent.spawn runs in the engine and rewrites the Agent tool’s own model parameter, so the subagent’s transcript shows the pinned model as its own. It makes no model call itself and costs no tokens.

Install

Needs Claude Code 2.1.287 or later.

claude plugin marketplace add karanb192/claude-code-mods
claude plugin install fable-pin@claude-code-mods

The same marketplace carries the author’s cache-tax mod and mod-builder, a skill for building mods. Restart Claude Code or run /reload-plugins. Pinning is on from the first spawn. /fable-pin off turns it off and remembers that across sessions, /fable-pin on turns it back on, /fable-pin status says which. To pin a different model, edit the TARGET constant at the top of hooks/register.ts to any alias or full model id the Agent tool accepts.

How it works

Three hooks. session.start reads one boolean from the plugin store and registers the command. command.run matches the argument against on and off and writes the boolean back. agent.spawn returns the event unchanged when pinning is off, when the spawn is a fork or when it already asks for the target, and otherwise passes it on with model replaced. Nothing else is read, run, sent or stored. If the hook throws, the engine skips it and the spawn goes through unpinned with one dim line in the transcript.

The author ships two tests on the official kit, run with claude plugin test plugins/fable-pin: a subagent asking for another model comes out pinned and a fork keeps its model, and after /fable-pin off the next spawn is left alone.

If you want more than one rule, agent-router routes subagents to models by conditions; fable-pin is the one-rule version.