diff --git a/docker/jenkins/Dockerfile.ubuntu-20.04 b/docker/jenkins/Dockerfile.ubuntu-20.04
index 21b744b4..f625587f 100644
--- a/docker/jenkins/Dockerfile.ubuntu-20.04
+++ b/docker/jenkins/Dockerfile.ubuntu-20.04
@@ -39,13 +39,6 @@ RUN add-apt-repository "deb https://cloud.r-project.org/bin/linux/ubuntu $(lsb_r
 RUN apt-get update && apt-get install -y cmake r-base
 RUN cmake --version
 
-# The real-R integration tier (test/integration-r, run via `npm run test:r`)
-# launches actual Shiny apps, so the image needs the shiny package, not just
-# r-base. Without it that tier skips itself and the build looks green while
-# testing nothing.
-RUN Rscript -e 'install.packages("shiny", repos = "https://packagemanager.posit.co/cran/__linux__/focal/latest")'
-RUN Rscript -e 'stopifnot(requireNamespace("shiny", quietly = TRUE))'
-
 ARG JENKINS_GID=999
 ARG JENKINS_UID=999
 RUN groupadd -g $JENKINS_GID jenkins && \
diff --git a/memory-bank/testingGuide.md b/memory-bank/testingGuide.md
index c15fb196..0949fd9c 100644
--- a/memory-bank/testingGuide.md
+++ b/memory-bank/testingGuide.md
@@ -21,11 +21,20 @@ be named explicitly wherever tests are invoked:
 never loads it directly.
 
 There is no watch mode, no coverage tooling, and no linting step. Two CI systems run
-this: `.github/workflows/ci.yml` (Linux + macOS, plus a build-freshness check and the
-real-R tier) and Jenkins, which uses the vendored interpreter —
-`Jenkinsfile:108` runs `./bin/node ./node_modules/mocha/bin/mocha test test/integration`.
+this: `.github/workflows/ci.yml` (Linux + macOS, plus a build-freshness check) and
+Jenkins, which uses the vendored interpreter — `Jenkinsfile:108` runs
+`./bin/node ./node_modules/mocha/bin/mocha test test/integration`.
 **Keep that list of directories in sync with `package.json`'s `test` script**; a new
-test directory that isn't added in both places silently doesn't run.
+test directory that isn't added in both places silently doesn't run. (Note
+`Jenkinsfile.internal:138` still runs `mocha test` only, so the internal build skips
+the integration tier.)
+
+**The real-R tier runs on GitHub Actions only.** The Jenkins images deliberately do not
+install the `shiny` R package: Jenkins runs `mocha test test/integration`, never
+`npm run test:r`, so provisioning R packages there would build a toolchain nothing
+uses — and it broke the ubuntu-20.04 image, whose R falls back to compiling from
+source, where `shiny -> bslib -> sass -> fs` needs libuv. Jenkins is expected to be
+retired after the current round of refactors; don't add R provisioning back to it.
 
 `.mocharc.json` auto-requires three modules before any test file, and sets a timeout
 that a server boot can survive (mocha's 2s default cannot):