What
An agent is a helper that Claude Code can hand a task to, and plugins (add-on packages) can supply their own. A plugin agent can now provide a way to load its definition, meaning its instructions and settings, at the moment it is launched instead of using the version listed earlier.
When that happens, Claude Code checks that the loaded definition is the same kind of agent and still comes from a plugin. If either check fails, the launch stops with an error saying the agent could not be loaded from its plugin.
Why
This suggests plugin agents may be loaded only when they are needed rather than all at startup. If a plugin agent fails with the new error, the definition it loaded did not match what was expected.
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The writer flagged doubt
It is unclear whether any plugin agent actually uses this loading path yet, or whether it is switched on.