diff --git a/.github/workflows/dotnet-sdk-tests.yml b/.github/workflows/dotnet-sdk-tests.yml
index 37cdc07f7c..b5871897f6 100644
--- a/.github/workflows/dotnet-sdk-tests.yml
+++ b/.github/workflows/dotnet-sdk-tests.yml
@@ -12,7 +12,7 @@ permissions:
 
 jobs:
   test:
-    name: ".NET SDK Tests (${{ matrix.os }}, ${{ matrix.transport }}, ${{ matrix.backend }})"
+    name: ".NET SDK Tests (${{ matrix.os }}, ${{ matrix.transport }}, ${{ matrix.backend }}, ${{ matrix.shard }})"
     if: github.event.repository.fork == false
     env:
       POWERSHELL_UPDATECHECK: Off
@@ -24,22 +24,39 @@ jobs:
         os: [ubuntu-latest, macos-latest, windows-latest]
         transport: ["default", "inprocess"]
         backend: [capi]
+        shard: [full]
         # TODO: Re-enable after fixing in-process sqlite file locking on shutdown on Windows.
         exclude:
           - os: windows-latest
             transport: "inprocess"
+          - os: windows-latest
+            transport: default
+            shard: full
         include:
+          # Keep xUnit serial within each process, but split the slow Windows
+          # default-transport suite across two isolated test hosts.
+          - os: windows-latest
+            transport: default
+            backend: capi
+            shard: "1"
+          - os: windows-latest
+            transport: default
+            backend: capi
+            shard: "2"
           - os: ubuntu-latest
             transport: inprocess
             backend: anthropic-messages
+            shard: full
             test-filter: "FullyQualifiedName~GitHub.Copilot.Test.E2E&E2EBackend!=SelfConfiguredBackend&E2EBackend!=CapiOnly"
           - os: ubuntu-latest
             transport: inprocess
             backend: openai-responses
+            shard: full
             test-filter: "FullyQualifiedName~GitHub.Copilot.Test.E2E&E2EBackend!=SelfConfiguredBackend&E2EBackend!=CapiOnly"
           - os: ubuntu-latest
             transport: inprocess
             backend: openai-completions
+            shard: full
             test-filter: "FullyQualifiedName~GitHub.Copilot.Test.E2E&E2EBackend!=SelfConfiguredBackend&E2EBackend!=CapiOnly"
     runs-on: ${{ matrix.os }}
     defaults:
@@ -92,9 +109,30 @@ jobs:
       - name: Run .NET SDK tests
         env:
           COPILOT_HMAC_KEY: ${{ secrets.COPILOT_DEVELOPER_CLI_INTEGRATION_HMAC_KEY }}
+          DOTNET_TEST_SHARD: ${{ matrix.shard }}
         run: |
           args=(--no-build -v n)
-          if [[ -n "$DOTNET_TEST_FILTER" ]]; then
-            args+=(--filter "$DOTNET_TEST_FILTER")
+
+          filter="$DOTNET_TEST_FILTER"
+          if [[ "$DOTNET_TEST_SHARD" != "full" ]]; then
+            if [[ "$DOTNET_TEST_SHARD" == "1" ]]; then
+              initials=(A C D H I J K L N Q S U W Y)
+              shard_filter="FullyQualifiedName~GitHub.Copilot.Test.ConnectionToken"
+            else
+              initials=(B E F G M O P R T V X Z)
+              shard_filter=""
+            fi
+
+            for namespace in E2E Unit; do
+              for initial in "${initials[@]}"; do
+                clause="FullyQualifiedName~GitHub.Copilot.Test.${namespace}.${initial}"
+                shard_filter="${shard_filter:+${shard_filter}|}${clause}"
+              done
+            done
+            filter="${filter:+(${filter})&}(${shard_filter})"
+          fi
+
+          if [[ -n "$filter" ]]; then
+            args+=(--filter "$filter")
           fi
           dotnet test "${args[@]}"
diff --git a/.github/workflows/python-sdk-tests.yml b/.github/workflows/python-sdk-tests.yml
index 8c4d5806db..1ea9739756 100644
--- a/.github/workflows/python-sdk-tests.yml
+++ b/.github/workflows/python-sdk-tests.yml
@@ -79,4 +79,6 @@ jobs:
       - name: Run Python SDK tests
         env:
           COPILOT_HMAC_KEY: ${{ secrets.COPILOT_DEVELOPER_CLI_INTEGRATION_HMAC_KEY }}
-        run: uv run pytest -v -s
+        # Keep each module's shared E2E client and proxy on one process while
+        # running independent modules concurrently in isolated workers.
+        run: uv run pytest -v -s -n 2 --dist=loadfile
diff --git a/python/pyproject.toml b/python/pyproject.toml
index 7e6274d9c0..e96c587a64 100644
--- a/python/pyproject.toml
+++ b/python/pyproject.toml
@@ -46,6 +46,7 @@ dev = [
     "pytest>=7.0.0",
     "pytest-asyncio>=0.21.0",
     "pytest-timeout>=2.0.0",
+    "pytest-xdist>=3.6.0",
     "websockets>=12.0",
     "opentelemetry-sdk>=1.0.0",
 ]