diff --git a/.github/workflows/build-macos-arm64.yaml b/.github/workflows/build-macos-arm64.yaml
index dab3b92c98..d44039b0a3 100644
--- a/.github/workflows/build-macos-arm64.yaml
+++ b/.github/workflows/build-macos-arm64.yaml
@@ -105,8 +105,10 @@ jobs:
         run: |
           perl -pli -e 's/magicmime: false/magicmime: true/' stack.yaml
           # lts-24 caps `magic` at 1.1 while some transitive dep pulls 1.1.2;
-          # pin the exact-1.1 hackage revision that stack itself recommends.
-          echo '- magic-1.1@sha256:3a31ca1c49fa5f6a857f647471739bd240a6020af6ae43e71fb72201fc482fa2,928' >> stack.yaml
+          # pin the exact-1.1 hackage revision (idempotent: upstream may
+          # already ship a `- magic-*` entry, avoid the stack S-5470 dup).
+          grep -qE '^- magic(-|@|$)' stack.yaml \
+            || echo '- magic-1.1@sha256:3a31ca1c49fa5f6a857f647471739bd240a6020af6ae43e71fb72201fc482fa2,928' >> stack.yaml
 
       - name: stack setup
         run: stack setup
@@ -468,9 +470,11 @@ jobs:
 
       - name: Run datalad tests
         run: |
-          mkdir -p __testhome__
-          cd __testhome__
-          python -m pytest -c ../datalad/tox.ini -s -v --pyargs datalad
+          testhome="$PWD/__testhome__"
+          toxini="../datalad/tox.ini"
+          mkdir -p "$testhome"
+          cd "$testhome"
+          python -m pytest -c "$toxini" -s -v --pyargs datalad
 
       - name: Set final PR status
         if: always() && github.event.inputs.pr != ''
diff --git a/.github/workflows/build-macos.yaml b/.github/workflows/build-macos.yaml
index f0c9659ade..d97fac0eb0 100644
--- a/.github/workflows/build-macos.yaml
+++ b/.github/workflows/build-macos.yaml
@@ -105,8 +105,10 @@ jobs:
         run: |
           perl -pli -e 's/magicmime: false/magicmime: true/' stack.yaml
           # lts-24 caps `magic` at 1.1 while some transitive dep pulls 1.1.2;
-          # pin the exact-1.1 hackage revision that stack itself recommends.
-          echo '- magic-1.1@sha256:3a31ca1c49fa5f6a857f647471739bd240a6020af6ae43e71fb72201fc482fa2,928' >> stack.yaml
+          # pin the exact-1.1 hackage revision (idempotent: upstream may
+          # already ship a `- magic-*` entry, avoid the stack S-5470 dup).
+          grep -qE '^- magic(-|@|$)' stack.yaml \
+            || echo '- magic-1.1@sha256:3a31ca1c49fa5f6a857f647471739bd240a6020af6ae43e71fb72201fc482fa2,928' >> stack.yaml
 
       - name: stack setup
         run: stack setup
@@ -467,9 +469,11 @@ jobs:
 
       - name: Run datalad tests
         run: |
-          mkdir -p __testhome__
-          cd __testhome__
-          python -m pytest -c ../datalad/tox.ini -s -v --pyargs datalad
+          testhome="$PWD/__testhome__"
+          toxini="../datalad/tox.ini"
+          mkdir -p "$testhome"
+          cd "$testhome"
+          python -m pytest -c "$toxini" -s -v --pyargs datalad
 
       - name: Set final PR status
         if: always() && github.event.inputs.pr != ''
diff --git a/.github/workflows/build-ubuntu.yaml b/.github/workflows/build-ubuntu.yaml
index b8ee4daf27..98b8c76824 100644
--- a/.github/workflows/build-ubuntu.yaml
+++ b/.github/workflows/build-ubuntu.yaml
@@ -510,9 +510,11 @@ jobs:
 
       - name: Run datalad tests
         run: |
-          mkdir -p __testhome__
-          cd __testhome__
-          python -m pytest -c ../datalad/tox.ini -s -v --pyargs datalad
+          testhome="$PWD/__testhome__"
+          toxini="../datalad/tox.ini"
+          mkdir -p "$testhome"
+          cd "$testhome"
+          python -m pytest -c "$toxini" -s -v --pyargs datalad
 
       - name: Set final PR status
         if: always() && github.event.inputs.pr != ''
diff --git a/.github/workflows/build-windows.yaml b/.github/workflows/build-windows.yaml
index e7e259a44f..2053e7934f 100644
--- a/.github/workflows/build-windows.yaml
+++ b/.github/workflows/build-windows.yaml
@@ -101,8 +101,10 @@ jobs:
         run: |
           perl -pli -e 's/magicmime: false/magicmime: true/' stack.yaml
           # lts-24 caps `magic` at 1.1 while some transitive dep pulls 1.1.2;
-          # pin the exact-1.1 hackage revision that stack itself recommends.
-          echo '- magic-1.1@sha256:3a31ca1c49fa5f6a857f647471739bd240a6020af6ae43e71fb72201fc482fa2,928' >> stack.yaml
+          # pin the exact-1.1 hackage revision (idempotent: upstream may
+          # already ship a `- magic-*` entry, avoid the stack S-5470 dup).
+          grep -qE '^- magic(-|@|$)' stack.yaml \
+            || echo '- magic-1.1@sha256:3a31ca1c49fa5f6a857f647471739bd240a6020af6ae43e71fb72201fc482fa2,928' >> stack.yaml
 
       - name: Set UPGRADE_LOCATION
         run: |
@@ -442,9 +444,17 @@ jobs:
 
       - name: Run datalad tests
         run: |
-          mkdir -p __testhome__
-          cd __testhome__
-          python -m pytest -c ../datalad/tox.ini -s -v --pyargs datalad
+          # windows-2025: HOME and default tempfile live on C: while the
+          # actions workspace is on D:.  Any datalad code that computes
+          # `os.path.relpath(some_C_path, some_D_path)` raises
+          # `ValueError: path is on mount 'D:', start on mount 'C:'`.
+          # Run pytest from a C: directory so tempdir + HOME + CWD all
+          # share one drive.  See commit message for context.
+          testhome="$HOME/dl-testhome"
+          toxini="$GITHUB_WORKSPACE/datalad/tox.ini"
+          mkdir -p "$testhome"
+          cd "$testhome"
+          python -m pytest -c "$toxini" -s -v --pyargs datalad
 
       - name: Set final PR status
         if: always() && github.event.inputs.pr != ''
diff --git a/.github/workflows/template/build-{{ostype}}.yaml.j2 b/.github/workflows/template/build-{{ostype}}.yaml.j2
index 8c885f7c62..d2d7e15d5a 100644
--- a/.github/workflows/template/build-{{ostype}}.yaml.j2
+++ b/.github/workflows/template/build-{{ostype}}.yaml.j2
@@ -197,8 +197,10 @@ jobs:
         run: |
           perl -pli -e 's/magicmime: false/magicmime: true/' stack.yaml
           # lts-24 caps `magic` at 1.1 while some transitive dep pulls 1.1.2;
-          # pin the exact-1.1 hackage revision that stack itself recommends.
-          echo '- magic-1.1@sha256:3a31ca1c49fa5f6a857f647471739bd240a6020af6ae43e71fb72201fc482fa2,928' >> stack.yaml
+          # pin the exact-1.1 hackage revision (idempotent: upstream may
+          # already ship a `- magic-*` entry, avoid the stack S-5470 dup).
+          grep -qE '^- magic(-|@|$)' stack.yaml \
+            || echo '- magic-1.1@sha256:3a31ca1c49fa5f6a857f647471739bd240a6020af6ae43e71fb72201fc482fa2,928' >> stack.yaml
 
       - name: stack setup
         run: stack setup
@@ -238,8 +240,10 @@ jobs:
         run: |
           perl -pli -e 's/magicmime: false/magicmime: true/' stack.yaml
           # lts-24 caps `magic` at 1.1 while some transitive dep pulls 1.1.2;
-          # pin the exact-1.1 hackage revision that stack itself recommends.
-          echo '- magic-1.1@sha256:3a31ca1c49fa5f6a857f647471739bd240a6020af6ae43e71fb72201fc482fa2,928' >> stack.yaml
+          # pin the exact-1.1 hackage revision (idempotent: upstream may
+          # already ship a `- magic-*` entry, avoid the stack S-5470 dup).
+          grep -qE '^- magic(-|@|$)' stack.yaml \
+            || echo '- magic-1.1@sha256:3a31ca1c49fa5f6a857f647471739bd240a6020af6ae43e71fb72201fc482fa2,928' >> stack.yaml
 
       - name: Set UPGRADE_LOCATION
         run: |
@@ -778,9 +782,22 @@ jobs:
     {% endif %}
       - name: Run datalad tests
         run: |
-          mkdir -p __testhome__
-          cd __testhome__
-          python -m pytest -c ../datalad/tox.ini -s -v --pyargs datalad
+          {% if ostype == "windows" %}
+          # windows-2025: HOME and default tempfile live on C: while the
+          # actions workspace is on D:.  Any datalad code that computes
+          # `os.path.relpath(some_C_path, some_D_path)` raises
+          # `ValueError: path is on mount 'D:', start on mount 'C:'`.
+          # Run pytest from a C: directory so tempdir + HOME + CWD all
+          # share one drive.  See commit message for context.
+          testhome="$HOME/dl-testhome"
+          toxini="$GITHUB_WORKSPACE/datalad/tox.ini"
+          {% else %}
+          testhome="$PWD/__testhome__"
+          toxini="../datalad/tox.ini"
+          {% endif %}
+          mkdir -p "$testhome"
+          cd "$testhome"
+          python -m pytest -c "$toxini" -s -v --pyargs datalad
 
       - name: Set final PR status
         if: always() && github.event.inputs.pr != ''