get_project / render.backend reported "gpuAcceleration": true with
"active": "cpu" and no reason, which read as a broken GPU. The
headless MCP server only attaches the GPU compositor when started with
--gpu, which docs/agents.md didn't mention, and a failed --gpu said
only "no usable GPU adapter" (the adapter's error went to a log no
one sees). The session now keeps the host's reason there is no GPU
compositor (accel_note): the CLI's "headless: renders on the CPU
unless started with --gpu", or the desktop app's compositor setup or
retirement error. The status reports it as `why` whenever it renders
on the CPU (or that the project's renderer is Software Only). --gpu
failures include the adapter's error (Gpu::try_headless).
docs/agents.md documents `mcp --gpu`.
Test: render_backend_says_why_it_renders_on_the_cpu (engine).
Checked with effectcraft-cli mcp and mcp --gpu.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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