The error usually means two copies of `wp-ai-client` are loaded, not that the API key is wrong. A plugin's bundled Composer copy takes priority over the core copy.
The result is a mixed class graph: `WordPress\AiClient\AiClient` resolves to core while `WordPress\AiClient\Providers\ProviderRegistry` resolves to the plugin's older vendor directory, so key
The upstream repository is archived and the report has been open since March 1, 2026, so the fix has to happen in the plugin, not in core.