You have strategy.matrix.os: [ubuntu-latest, windows-latest]. You need one additional job that runs ubuntu-latest with an extra experimental: true flag without rewriting the whole matrix by hand. Which key adds that combination?
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Short Explanation and Infographic
matrix.include adds or expands combinations (here ubuntu + experimental). exclude removes combinations; fail-fast and concurrency are unrelated to injecting matrix axes.
Full explanation below image
Full Explanation
strategy.matrix.include can add new combinations or attach extra variables to specific legs. Listing os: ubuntu-latest with experimental: true creates (or enriches) that job while leaving the base cartesian product intact depending on how include is used. exclude removes combinations from the product. fail-fast controls cancellation after failures. concurrency coordinates simultaneous workflow runs. For monorepos, include is often paired with a generated matrix from a planning job, but the static include pattern remains exam-relevant. Keep matrix sizes within platform limits and name jobs clearly when experimental flags change behavior.