diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 66ced103..7517d98c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,7 +21,14 @@ jobs:
         include:
           - platform: windows-latest
             formats: nsis
-          - platform: ubuntu-22.04
+          # Pinned rather than `ubuntu-latest` so the glibc floor of the .deb
+          # and .AppImage is a decision, not whatever GitHub last moved the
+          # label to. Was ubuntu-22.04, which begins deprecation on 2026-09-17
+          # (brownout periods fail the job outright) and is fully unsupported
+          # from 2027-04-17. Moving raises the floor from glibc 2.35 to 2.39,
+          # so the Linux artifacts no longer run on Ubuntu 22.04 or Debian 12 —
+          # stated in the README's Install section.
+          - platform: ubuntu-24.04
             formats: deb,appimage
           - platform: macos-latest
             formats: dmg
@@ -33,7 +40,7 @@ jobs:
       - uses: actions/checkout@v5
 
       - name: Install Linux build dependencies
-        if: matrix.platform == 'ubuntu-22.04'
+        if: matrix.platform == 'ubuntu-24.04'
         run: |
           sudo apt-get update
           sudo apt-get install -y \
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 2401d3d7..5df43dbd 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -118,7 +118,10 @@ jobs:
             artifacts: |
               app/target/release/*-setup.exe
               app/target/release/*-portable.exe
-          - platform: ubuntu-22.04
+          # Pinned deliberately; ubuntu-22.04 is being retired (deprecation
+          # from 2026-09-17). See the matching note in ci.yml for the glibc
+          # 2.39 floor this puts on the .deb and .AppImage.
+          - platform: ubuntu-24.04
             formats: deb,appimage
             artifacts: |
               app/target/release/*.deb
@@ -139,7 +142,7 @@ jobs:
           ref: ${{ needs.prepare.outputs.tag }}
 
       - name: Install Linux build dependencies
-        if: matrix.platform == 'ubuntu-22.04'
+        if: matrix.platform == 'ubuntu-24.04'
         run: |
           sudo apt-get update
           sudo apt-get install -y \
diff --git a/README.md b/README.md
index 9fc84d17..cf154a66 100644
--- a/README.md
+++ b/README.md
@@ -257,6 +257,12 @@ carries that sidecar inside the binary and unpacks it to
 several times the size of the installer, and why it works from a USB stick with
 nothing beside it. Delete that folder to reclaim the space once you are done.
 
+**Linux glibc requirement:** the `.deb` and `.AppImage` are built on Ubuntu
+24.04, so they need **glibc 2.39 or newer** — Ubuntu 24.04+, Debian 13+,
+Fedora 40+. They will not run on Ubuntu 22.04 or Debian 12; build from source
+there. (The build moved off the Ubuntu 22.04 runner because GitHub retires that
+image from 2026-09-17.)
+
 **macOS first launch:** the `.dmg` is currently unsigned and un-notarized, so
 Gatekeeper will block it. Open **System Settings → Privacy & Security**, find the
 blocked-app notice, and choose **Open Anyway**.