Spin up a background agent to do the research, so you keep working while it reads. When you are already a subagent, do the research yourself with the same job.

Its job:

Investigate the question against primary sources (official documentation, source code, specifications, first-party APIs), not a secondary write-up of them. Follow every claim back to the source that owns it.
Write the findings to a single Markdown file, citing each claim's source.
Save it where the repository already keeps such notes, as AGENTS.md names them; match the existing convention. If there is none, put it somewhere sensible, say where, and add one line to the nearest AGENTS.md saying what the notes hold and when to read them.