diff --git a/.github/workflows/lint.yml b/.github/workflows/lint.yml
index 129fc766458c..69402439d08a 100644
--- a/.github/workflows/lint.yml
+++ b/.github/workflows/lint.yml
@@ -12,7 +12,55 @@ name: lint
 permissions:
   contents: read
 
+env:
+  NOX_REUSE_EXISTING_VIRTUALENVS: "true"
+  NOX_ENVDIR: "/tmp/shared_nox_envs"
+  NOX_DEFAULT_VENV_BACKEND: "uv"
+  UV_VENV_SEED: "1"
+  # Run checks in parallel using 4 cores
+  PARALLEL_WORKERS: "4"
+
 jobs:
+  initialize:
+    runs-on: ubuntu-latest
+    outputs:
+      matrix: ${{ steps.set-matrix.outputs.matrix }}
+      is_full_run: ${{ steps.check-label.outputs.is_full_run }}
+    env:
+      MAX_SHARDS: 4
+    steps:
+    - name: Checkout
+      uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+      # Use a fetch-depth of 2 to avoid error `fatal: origin/main...HEAD: no merge base`
+      # See https://github.com/googleapis/google-cloud-python/issues/12013
+      # and https://github.com/actions/checkout#checkout-head.
+      with:
+        fetch-depth: 2
+        persist-credentials: false
+    - name: Check for unit_test:all_packages label
+      id: check-label
+      run: |
+        if [[ "${{ contains(github.event.pull_request.labels.*.name, 'unit_test:all_packages') }}" == "true" ]]; then
+          echo "is_full_run=true" >> $GITHUB_OUTPUT
+        else
+          echo "is_full_run=false" >> $GITHUB_OUTPUT
+        fi
+    - name: Setup Python
+      uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6
+      with:
+        python-version: "3.10"
+    - name: Get package shards
+      id: set-matrix
+      env:
+        BUILD_TYPE: presubmit
+        TARGET_BRANCH: ${{ github.base_ref || github.event.merge_group.base_ref }}
+        TEST_ALL_PACKAGES: ${{ steps.check-label.outputs.is_full_run }}
+      run: |
+        if [ -n "$TARGET_BRANCH" ]; then
+          git fetch origin "$TARGET_BRANCH" --depth=1 || true
+        fi
+        python3 ci/get_package_shards.py
+
   lint:
     runs-on: ubuntu-latest
     steps:
@@ -24,6 +72,14 @@ jobs:
       with:
         fetch-depth: 2
         persist-credentials: false
+    - name: Check for unit_test:all_packages label
+      id: check-label
+      run: |
+        if [[ "${{ contains(github.event.pull_request.labels.*.name, 'unit_test:all_packages') }}" == "true" ]]; then
+          echo "is_full_run=true" >> $GITHUB_OUTPUT
+        else
+          echo "is_full_run=false" >> $GITHUB_OUTPUT
+        fi
     - name: Setup Python
       uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6
       with:
@@ -31,11 +87,12 @@ jobs:
     - name: Install nox
       run: |
         python -m pip install --upgrade setuptools pip wheel
-        python -m pip install nox
+        python -m pip install nox uv
     - name: Run lint
       env:
         BUILD_TYPE: presubmit
         TARGET_BRANCH: ${{ github.base_ref || github.event.merge_group.base_ref }}
+        TEST_ALL_PACKAGES: ${{ steps.check-label.outputs.is_full_run }}
         TEST_TYPE: lint
         # TODO(https://github.com/googleapis/google-cloud-python/issues/13775): Specify `PY_VERSION` rather than relying on the default python version of the nox session.
         PY_VERSION: "unused"
@@ -45,13 +102,22 @@ jobs:
       env:
         BUILD_TYPE: presubmit
         TARGET_BRANCH: ${{ github.base_ref || github.event.merge_group.base_ref }}
+        TEST_ALL_PACKAGES: ${{ steps.check-label.outputs.is_full_run }}
         TEST_TYPE: lint_setup_py
         # TODO(https://github.com/googleapis/google-cloud-python/issues/13775): Specify `PY_VERSION` rather than relying on the default python version of the nox session.
         PY_VERSION: "unused"
       run: |
         ci/run_conditional_tests.sh
-  mypy:
+
+  mypy-shard:
+    needs: initialize
+    if: needs.initialize.outputs.matrix != '[]' && needs.initialize.outputs.matrix != ''
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: true
+      matrix:
+        package_shard: ${{ fromJson(needs.initialize.outputs.matrix) }}
+    name: ${{ matrix.package_shard.is_sharded && format('mypy ({0})', matrix.package_shard.name) || format('mypy ({0})', matrix.package_shard.description) }}
     steps:
     - name: Checkout
       uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
@@ -68,13 +134,28 @@ jobs:
     - name: Install nox
       run: |
         python -m pip install --upgrade setuptools pip wheel
-        python -m pip install nox
-    - name: Run mypy
+        python -m pip install nox uv
+    - name: Run mypy for ${{ matrix.package_shard.description }}
       env:
         BUILD_TYPE: presubmit
         TARGET_BRANCH: ${{ github.base_ref || github.event.merge_group.base_ref }}
         TEST_TYPE: mypy
+        PACKAGE_LIST: ${{ matrix.package_shard.packages }}
         # TODO(https://github.com/googleapis/google-cloud-python/issues/13775): Specify `PY_VERSION` rather than relying on the default python version of the nox session.
         PY_VERSION: "unused"
       run: |
         ci/run_conditional_tests.sh
+
+  mypy-status:
+    if: always()
+    needs: [initialize, mypy-shard]
+    runs-on: ubuntu-latest
+    name: mypy-status
+    steps:
+    - name: Check mypy job status
+      run: |
+        if [[ "${{ needs['mypy-shard'].result }}" != "success" && "${{ needs['mypy-shard'].result }}" != "skipped" ]]; then
+          echo "mypy failed with result: ${{ needs['mypy-shard'].result }}"
+          exit 1
+        fi
+        echo "All mypy shards passed successfully!"
diff --git a/.github/workflows/unittest.yml b/.github/workflows/unittest.yml
index 392f5ece6270..c9e8f5239bf1 100644
--- a/.github/workflows/unittest.yml
+++ b/.github/workflows/unittest.yml
@@ -12,13 +12,80 @@ name: unittest
 permissions:
   contents: read
 
+env:
+  PACKAGE_DIRS: packages preview-packages
+  NOX_REUSE_EXISTING_VIRTUALENVS: "true"
+  NOX_ENVDIR: "/tmp/shared_nox_envs"
+  NOX_DEFAULT_VENV_BACKEND: "uv"
+  UV_VENV_SEED: "1"
+  # Note: PARALLEL_WORKERS must remain "1" for unit tests because running pytest concurrently
+  # on the same runner VM can cause socket/IPC/process deadlocks across packages.
+  PARALLEL_WORKERS: "1"
+
 jobs:
+  initialize:
+    runs-on: ubuntu-latest
+    outputs:
+      matrix: ${{ steps.set-matrix.outputs.matrix }}
+      is_full_run: ${{ steps.check-label.outputs.is_full_run }}
+    env:
+      MAX_SHARDS: 16
+      # Define weights for long-running unit tests to balance shard execution time
+      # Each weight is roughly 1 minute of expected execution time
+      # Default for unset packages is 1
+      PACKAGE_WEIGHTS: |
+        bigframes: 6
+        google-ai-generativelanguage: 4
+        google-auth: 5
+        google-cloud-compute: 12
+        google-cloud-compute-v1beta: 12
+        google-cloud-dialogflow: 6
+        google-cloud-dialogflow-cx: 6
+        google-cloud-discoveryengine: 8
+        google-cloud-retail: 5
+        google-shopping-merchant-accounts: 4
+    steps:
+    - name: Checkout
+      uses: actions/checkout@11d5960a326750d5838078e36cf38b85af677262 # v4
+      with:
+        fetch-depth: 2
+        persist-credentials: false
+    - name: Check for unit_test:all_packages label
+      id: check-label
+      run: |
+        if [[ "${{ contains(github.event.pull_request.labels.*.name, 'unit_test:all_packages') }}" == "true" ]]; then
+          echo "is_full_run=true" >> $GITHUB_OUTPUT
+        else
+          echo "is_full_run=false" >> $GITHUB_OUTPUT
+        fi
+    - name: Setup Python
+      uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5
+      with:
+        python-version: "3.10"
+    - name: Get package shards
+      id: set-matrix
+      env:
+        BUILD_TYPE: presubmit
+        TARGET_BRANCH: ${{ github.base_ref || github.event.merge_group.base_ref }}
+        TEST_ALL_PACKAGES: ${{ steps.check-label.outputs.is_full_run }}
+        MAX_SHARDS: ${{ env.MAX_SHARDS }}
+        PACKAGE_WEIGHTS: ${{ env.PACKAGE_WEIGHTS }}
+      run: |
+        if [ -n "$TARGET_BRANCH" ]; then
+          git fetch origin "$TARGET_BRANCH" --depth=1 || true
+        fi
+        python3 ci/get_package_shards.py
+
   unit:
+    needs: initialize
+    if: needs.initialize.outputs.matrix != '[]' && needs.initialize.outputs.matrix != ''
     runs-on: ubuntu-22.04
     strategy:
       fail-fast: true
       matrix:
         python: ['3.10', "3.11", "3.12", "3.13", "3.14"]
+        package_shard: ${{ fromJson(needs.initialize.outputs.matrix) }}
+    name: ${{ matrix.package_shard.is_sharded && format('unit ({0}, {1})', matrix.python, matrix.package_shard.name) || format('unit ({0})', matrix.python) }}
     steps:
     - name: Checkout
       uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
@@ -32,30 +99,52 @@ jobs:
       uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1 # v6
       with:
         python-version: ${{ matrix.python }}
+        cache: 'pip'
     - name: Install nox
       run: |
         python -m pip install --upgrade setuptools pip wheel
-        python -m pip install nox
-    - name: Run unit tests
+        python -m pip install nox uv
+    - name: Run unit tests for ${{ matrix.package_shard.description }}
       env:
-        COVERAGE_FILE: ${{ github.workspace }}/.coverage-${{ matrix.python }}
         BUILD_TYPE: presubmit
         TARGET_BRANCH: ${{ github.base_ref || github.event.merge_group.base_ref }}
         TEST_TYPE: unit
         PY_VERSION: ${{ matrix.python }}
+        PACKAGE_LIST: ${{ matrix.package_shard.packages }}
       run: |
         ci/run_conditional_tests.sh
     - name: Upload coverage results
       uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
       with:
-        name: coverage-artifact-${{ matrix.python }}
-        path: .coverage-${{ matrix.python }}
+        name: coverage-artifact-${{ matrix.python }}-${{ matrix.package_shard.index }}
+        path: .coverage.${{ matrix.python }}.*
         include-hidden-files: true
 
+  all-tests:
+    needs: [initialize, unit]
+    if: always()
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check unit test results
+        run: |
+          # 1. Check initialize job
+          if [[ "${{ needs.initialize.result }}" != "success" ]]; then
+            echo "Error: The initialize job status was: ${{ needs.initialize.result }}"
+            exit 1
+          fi
+          # 2. Check unit test shards
+          if [[ "${{ needs.unit.result }}" != "success" && "${{ needs.unit.result }}" != "skipped" ]]; then
+            echo "Unit tests failed"
+            exit 1
+          fi
+          echo "All unit tests passed or were skipped"
+
   cover:
+    if: always() && !cancelled() && needs.all-tests.result == 'success'
     runs-on: ubuntu-latest
     needs:
-        - unit
+        - all-tests
+        - initialize
     steps:
     - name: Checkout
       uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
@@ -69,116 +158,43 @@ jobs:
       uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5
       with:
         python-version: "3.10"
-    - name: Set number of files changes in packages and preview-packages directories
+    - name: Determine if coverage evaluation is required
       id: packages
+      env:
+        TEST_ALL_PACKAGES: ${{ needs.initialize.outputs.is_full_run }}
+        TARGET_BRANCH: ${{ github.base_ref || github.event.merge_group.base_ref }}
       run: |
-        git diff HEAD~1 -- packages preview-packages > /dev/null
-        num_files_changed=$(git diff HEAD~1 -- packages preview-packages | wc -l | tr -d ' ')
-        echo "num_files_changed=${num_files_changed}" >> "$GITHUB_OUTPUT"
+        if [[ "${TEST_ALL_PACKAGES}" == "true" ]]; then
+          echo "should_evaluate_coverage=true" >> "$GITHUB_OUTPUT"
+        else
+          TARGET_BRANCH="${TARGET_BRANCH:-main}"
+          git fetch origin "$TARGET_BRANCH" --depth=1 || true
+          num_files_changed=$(git diff --name-only "origin/${TARGET_BRANCH}" -- ${PACKAGE_DIRS} | wc -l | tr -d ' ')
+          if [[ "${num_files_changed}" -gt 0 ]]; then
+            echo "should_evaluate_coverage=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "should_evaluate_coverage=false" >> "$GITHUB_OUTPUT"
+          fi
+        fi
     - name: Install coverage
-      if: ${{ steps.packages.outputs.num_files_changed > 0 }}
+      if: ${{ steps.packages.outputs.should_evaluate_coverage == 'true' }}
       run: |
         python -m pip install --upgrade setuptools pip wheel
         python -m pip install coverage
     - name: Download coverage results
-      if: ${{ steps.packages.outputs.num_files_changed > 0 }}
+      if: ${{ steps.packages.outputs.should_evaluate_coverage == 'true' }}
       uses: actions/download-artifact@634f93cb2916e3fdff6788551b99b062d0335ce0 # v5
       with:
-        path: .coverage-results/
+        path: /dev/shm/.coverage-results/
+        merge-multiple: true
     - name: Report coverage results
-      if: ${{ steps.packages.outputs.num_files_changed > 0 }}
+      if: ${{ steps.packages.outputs.should_evaluate_coverage == 'true' }}
       env:
         # TODO: default to 100% coverage after next gapic-generator release
         # https://github.com/googleapis/google-cloud-python/issues/17459
         DEFAULT_FAIL_UNDER: 99
+        TEST_ALL_PACKAGES: ${{ needs.initialize.outputs.is_full_run }}
+        TARGET_BRANCH: ${{ github.base_ref || github.event.merge_group.base_ref }}
+        BUILD_TYPE: presubmit
       run: |
-        # Find all modified packages
-        modified_packages=$(git diff --name-only HEAD~1 -- packages preview-packages | cut -d/ -f1,2 | sort -u)
-
-        existing_modified_packages=()
-        for pkg in ${modified_packages}; do
-          if [ -d "${pkg}" ]; then
-            existing_modified_packages+=("${pkg}")
-          fi
-        done
-
-        if [ ${#existing_modified_packages[@]} -eq 0 ]; then
-          echo "All modified packages were deleted. Skipping coverage evaluation."
-          exit 0
-        fi
-
-        if [ -d .coverage-results ]; then
-          # Unzip any zipped coverage results
-          find .coverage-results -type f -name '*.zip' -exec unzip -o {} \;
-
-          # Find all coverage files and combine them.
-          # We find files starting with .coverage (excluding .coveragerc files and templates)
-          coverage_files=$(find .coverage-results . -type f -name '.coverage*' ! -name '.coveragerc*')
-          if [ -n "${coverage_files}" ]; then
-            coverage combine ${coverage_files}
-          else
-            echo "Error: No coverage files found to combine."
-            exit 1
-          fi
-
-          # Find all modified packages
-          modified_packages=$(git diff --name-only HEAD~1 -- packages preview-packages | cut -d/ -f1,2 | sort -u)
-
-          failed_packages=()
-          passed_packages=()
-
-          for pkg in ${modified_packages}; do
-            if [ -d "${pkg}" ]; then
-              echo "============================================================"
-              echo "Evaluating coverage for package: ${pkg}"
-              echo "============================================================"
-
-              set +e
-              pushd "${pkg}" > /dev/null
-              if [ -f ".coveragerc" ]; then
-                echo "Using package-specific configuration: ${pkg}/.coveragerc"
-                # If fail_under is specified in the package-specific .coveragerc, coverage report
-                # will automatically enforce it. Otherwise, we enforce the default.
-                if grep -q "fail_under" ".coveragerc"; then
-                  COVERAGE_FILE=../../.coverage coverage report --include="$PWD/**"
-                else
-                  echo "No fail_under specified in ${pkg}/.coveragerc, enforcing default"
-                  COVERAGE_FILE=../../.coverage coverage report --include="$PWD/**" --fail-under="${DEFAULT_FAIL_UNDER}"
-                fi
-              else
-                echo "No .coveragerc found for ${pkg}, enforcing default"
-                COVERAGE_FILE=../../.coverage coverage report --include="$PWD/**" --fail-under="${DEFAULT_FAIL_UNDER}"
-              fi
-              status=$?
-              popd > /dev/null
-              set -e
-
-              if [ ${status} -ne 0 ]; then
-                failed_packages+=("${pkg}")
-              else
-                passed_packages+=("${pkg}")
-              fi
-            fi
-          done
-
-          echo "============================================================"
-          echo "Coverage Evaluation Summary"
-          echo "============================================================"
-          if [ ${#passed_packages[@]} -gt 0 ]; then
-            echo "Passed packages:"
-            for pkg in "${passed_packages[@]}"; do
-              echo "  - ${pkg}"
-            done
-          fi
-          if [ ${#failed_packages[@]} -gt 0 ]; then
-            echo "Failed packages:"
-            for pkg in "${failed_packages[@]}"; do
-              echo "  - ${pkg}"
-            done
-            exit 1
-          fi
-        else
-          echo "Error: No coverage results were downloaded from the unit test jobs."
-          echo "This usually means the unit tests did not run or failed to upload their coverage files."
-          exit 1
-        fi
+        ci/report_coverage.sh
diff --git a/ci/get_package_shards.py b/ci/get_package_shards.py
new file mode 100644
index 000000000000..77cc191cc3ac
--- /dev/null
+++ b/ci/get_package_shards.py
@@ -0,0 +1,233 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Script to group changed packages into balanced shards for CI testing.
+
+This script identifies which packages have changed compared to a target branch
+(or since the last commit) and groups them into a fixed number of shards.
+It uses package weights (configured via environment variables) to balance the
+execution time across shards while maintaining alphabetical order and
+contiguous grouping. All directory variants of a package (e.g. packages/foo and
+preview-packages/foo) are kept aligned in the exact same shard.
+"""
+
+import os
+import subprocess
+import json
+import math
+import sys
+import collections
+
+
+def get_package_directories():
+    """Parses package directory roots from the PACKAGE_DIRS environment variable.
+
+    Defaults to ['packages', 'preview-packages'] if not set.
+    """
+    env_dirs = os.environ.get("PACKAGE_DIRS", "")
+    if env_dirs:
+        dirs = [d.strip() for d in env_dirs.replace('\n', ' ').split(' ') if d.strip()]
+        if dirs:
+            return dirs
+    return ["packages", "preview-packages"]
+
+
+def get_package_weights():
+    """Parses package weights from the PACKAGE_WEIGHTS environment variable.
+    Package weights represent the relative size of the test, where the weight
+    approximately represents the minutes the test takes to run. This is used
+    for distributing work properly across shards.
+
+    The environment variable is expected to be a multiline string where each line
+    is in the format 'package_name: weight'. Lines starting with '#' are ignored.
+
+    Returns:
+        dict: A mapping of package names to their integer weights.
+    """
+    weights = {}
+    env_weights = os.environ.get("PACKAGE_WEIGHTS", "")
+    for line in env_weights.splitlines():
+        line = line.strip()
+        if not line or line.startswith("#"):
+            continue
+        if ":" in line:
+            try:
+                pkg, weight = line.split(":", 1)
+                weights[pkg.strip()] = int(weight.strip())
+            except ValueError:
+                continue
+    return weights
+
+
+def get_packages():
+    """Lists all package directory paths in the repository grouped by package name.
+
+    Returns:
+        dict: A dictionary mapping package_name -> list of relative directory paths.
+    """
+    subdirs = get_package_directories()
+    packages_map = collections.defaultdict(list)
+    for subdir in subdirs:
+        if not os.path.exists(subdir):
+            continue
+        for d in os.listdir(subdir):
+            full_path = os.path.join(subdir, d) + '/'
+            if os.path.isdir(full_path):
+                packages_map[d].append(full_path)
+    return packages_map
+
+
+def get_packages_to_test():
+    """Determines the dictionary of package names to directory paths that need to be tested.
+
+    This is based on git diffs against the target branch (presubmit) or the
+    previous commit (continuous). If TEST_ALL_PACKAGES is set to true,
+    all packages are returned.
+
+    Returns:
+        dict: A dictionary mapping package_name -> list of relative directory paths to be tested.
+    """
+    build_type = os.environ.get('BUILD_TYPE', 'presubmit')
+    target_branch = os.environ.get('TARGET_BRANCH', 'main')
+    test_all_packages = os.environ.get('TEST_ALL_PACKAGES', 'false').lower() == 'true'
+
+    all_packages = get_packages()
+
+    if test_all_packages:
+        return all_packages
+
+    if build_type == 'presubmit':
+        git_diff_arg = f"origin/{target_branch}"
+    elif build_type == 'continuous':
+        git_diff_arg = "HEAD~1.."
+    else:
+        return all_packages
+
+    try:
+        res = subprocess.check_output(['git', 'diff', '--name-only', git_diff_arg]).decode('utf-8')
+        changed_files = res.splitlines()
+    except subprocess.CalledProcessError:
+        # If change detection fails, fall back to all packages
+        return all_packages
+
+    package_dirs = set(get_package_directories())
+    to_test_paths = collections.defaultdict(list)
+    for f in changed_files:
+        parts = f.split('/')
+        if len(parts) >= 2 and parts[0] in package_dirs:
+            pkg_name = parts[1]
+            full_path = f"{parts[0]}/{parts[1]}/"
+            if pkg_name in all_packages and full_path in all_packages[pkg_name]:
+                if full_path not in to_test_paths[pkg_name]:
+                    to_test_paths[pkg_name].append(full_path)
+
+    return dict(to_test_paths)
+
+
+def group_packages(packages_map):
+    """Groups packages by package name into balanced shards.
+
+    All directory variants of a package (e.g. packages/foo and preview-packages/foo)
+    are kept together in the exact same shard.
+
+    Args:
+        packages_map (dict): Dictionary mapping package_name -> list of directory paths.
+
+    Returns:
+        list: A list of dictionaries, each representing a shard with its name,
+              index, description, and the space-separated list of packages.
+    """
+    if not packages_map:
+        return []
+
+    package_weights_map = get_package_weights()
+    sorted_pkg_names = sorted(packages_map.keys())
+
+    pkg_items = []
+    total_weight = 0
+    for name in sorted_pkg_names:
+        paths = packages_map[name]
+        # Multiply base weight by number of directory variants (e.g. standard + preview)
+        weight = package_weights_map.get(name, 1) * len(paths)
+        pkg_items.append((name, paths, weight))
+        total_weight += weight
+
+    # Dynamically determine target weight to balance across max shards.
+    max_shards = int(os.environ.get("MAX_SHARDS", 16))
+    target_weight = max(10, math.ceil(total_weight / max_shards))
+
+    shards_list = []
+    current_shard_items = []
+    current_shard_weight = 0
+
+    # Pack packages alphabetically by package name.
+    for name, paths, weight in pkg_items:
+        # If adding this package would exceed target weight AND we haven't reached the 
+        # shard limit, start a new shard. Otherwise, keep "stuffing" the current one.
+        if current_shard_items and (current_shard_weight + weight > target_weight) and len(shards_list) < max_shards - 1:
+            shards_list.append(current_shard_items)
+            current_shard_items = [(name, paths, weight)]
+            current_shard_weight = weight
+        else:
+            current_shard_items.append((name, paths, weight))
+            current_shard_weight += weight
+
+    if current_shard_items:
+        shards_list.append(current_shard_items)
+
+    # Construct the final shards output list
+    shards = []
+    for i, shard_items in enumerate(shards_list):
+        index = i + 1
+        name = f"Shard {index}"
+        num_in_shard = len(shard_items)
+
+        # Calculate contiguous range description using package names
+        first_pkg_name = shard_items[0][0]
+        last_pkg_name = shard_items[-1][0]
+        if num_in_shard == 1:
+            desc = first_pkg_name
+        else:
+            desc = f"{first_pkg_name}...{last_pkg_name} ({num_in_shard} packages)"
+
+        all_paths = []
+        for _, paths, _ in shard_items:
+            all_paths.extend(paths)
+
+        shards.append({
+            "name": name,
+            "index": index,
+            "description": desc,
+            "packages": " ".join(all_paths),
+            "is_sharded": True
+        })
+
+    # Set is_sharded dynamically based on the total number of shards
+    total_shards = len(shards)
+    for shard in shards:
+        shard["is_sharded"] = total_shards > 1
+
+    return shards
+
+
+if __name__ == "__main__":
+    packages = get_packages_to_test()
+    shards = group_packages(packages)
+    shards_json = json.dumps(shards)
+    print(shards_json)
+
+    github_output = os.environ.get("GITHUB_OUTPUT")
+    if github_output:
+        with open(github_output, "a") as f:
+            f.write(f"matrix={shards_json}\n")
diff --git a/ci/report_coverage.sh b/ci/report_coverage.sh
new file mode 100755
index 000000000000..f204bc45cdcc
--- /dev/null
+++ b/ci/report_coverage.sh
@@ -0,0 +1,164 @@
+#!/bin/bash
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -eo pipefail
+
+# This script generates coverage reports for modified packages.
+# It assumes coverage databases have been downloaded to RESULTS_DIR.
+# It uses /dev/shm for high-performance processing.
+
+RESULTS_DIR="${RESULTS_DIR:-/dev/shm/.coverage-results}"
+LOG_DIR="/dev/shm/coverage-logs"
+DEFAULT_FAIL_UNDER="${DEFAULT_FAIL_UNDER:-99}"
+MAX_JOBS=$(nproc)
+
+mkdir -p "${LOG_DIR}"
+
+if [ ! -d "${RESULTS_DIR}" ]; then
+    echo "Error: No coverage results found in ${RESULTS_DIR}."
+    exit 1
+fi
+
+# Unzip any zipped coverage results
+find "$RESULTS_DIR" -type f -name '*.zip' -print0 | xargs -0 -P "${MAX_JOBS}" -I {} unzip -q -o {} -d "$RESULTS_DIR"
+
+# Identify modified packages
+BUILD_TYPE="${BUILD_TYPE:-presubmit}"
+TARGET_BRANCH="${TARGET_BRANCH:-main}"
+
+PACKAGE_DIRS="${PACKAGE_DIRS:-packages preview-packages}"
+
+if [[ "${TEST_ALL_PACKAGES}" == "true" ]]; then
+    # Test all packages mode: evaluate coverage for every package in the repository
+    modified_packages=$(for dir in ${PACKAGE_DIRS}; do ls -d ${dir}/*/ 2>/dev/null; done | cut -d/ -f1,2 | sort -u)
+elif [[ "${BUILD_TYPE}" == "presubmit" ]]; then
+    # Presubmit build: evaluate coverage only for packages modified relative to the target branch
+    modified_packages=$(git diff --name-only "origin/${TARGET_BRANCH}" -- ${PACKAGE_DIRS} 2>/dev/null | cut -d/ -f1,2 | sort -u)
+else
+    # Continuous build (post-merge on main): evaluate coverage for packages modified in the last commit
+    modified_packages=$(git diff --name-only HEAD~1 -- ${PACKAGE_DIRS} 2>/dev/null | cut -d/ -f1,2 | sort -u)
+fi
+
+# Function to report coverage for a single package
+report_package_coverage() {
+    local pkg=$1
+    local pkg_name_clean=$(echo "${pkg}" | sed 's|/$||' | sed 's|/|_|g')
+    local pkg_log="${LOG_DIR}/${pkg_name_clean}.log"
+    local pkg_status="${LOG_DIR}/${pkg_name_clean}.status"
+    
+    # Use /dev/shm for the combined coverage file to reduce disk I/O
+    local pkg_coverage_db="/dev/shm/.coverage.${pkg_name_clean}"
+
+    set +e
+    (
+        # Find coverage databases belonging specifically to this package
+        shopt -s nullglob
+        local pkg_files=(
+            "${RESULTS_DIR}/.coverage."*"."${pkg_name_clean}
+            "${RESULTS_DIR}/.coverage."*"."${pkg_name_clean}"."*
+            "${RESULTS_DIR}"/*/".coverage."*"."${pkg_name_clean}
+            "${RESULTS_DIR}"/*/".coverage."*"."${pkg_name_clean}"."*
+        )
+        shopt -u nullglob
+
+        # Handle explicit skip via .coveragerc
+        if [ -f "${pkg}/.coveragerc" ] && grep -Eq "fail_under\s*=\s*0" "${pkg}/.coveragerc"; then
+            echo "Package has fail_under = 0, passing instantly with 100% success (even without coverage files)" > "${pkg_log}"
+            echo 0 > "${pkg_status}"
+            return
+        fi
+
+        if [ ${#pkg_files[@]} -eq 0 ]; then
+            echo "Warning: No coverage results found for ${pkg}" > "${pkg_log}"
+            echo 1 > "${pkg_status}"
+            return
+        fi
+
+        pushd "${pkg}" > /dev/null
+
+        # Combine databases
+        COVERAGE_FILE="${pkg_coverage_db}" coverage combine "${pkg_files[@]}" >> "${pkg_log}" 2>&1
+
+        # Generate report
+        if [ -f ".coveragerc" ]; then
+            echo "Using package-specific configuration: ${pkg}/.coveragerc" >> "${pkg_log}"
+            if grep -q "fail_under" ".coveragerc"; then
+                COVERAGE_FILE="${pkg_coverage_db}" coverage report --rcfile=".coveragerc" --include="$PWD/**" >> "${pkg_log}" 2>&1
+            else
+                echo "No fail_under specified in ${pkg}/.coveragerc, enforcing default" >> "${pkg_log}"
+                COVERAGE_FILE="${pkg_coverage_db}" coverage report --rcfile=".coveragerc" --include="$PWD/**" --fail-under="${DEFAULT_FAIL_UNDER}" >> "${pkg_log}" 2>&1
+            fi
+        else
+            echo "No .coveragerc found for ${pkg}, enforcing default" >> "${pkg_log}"
+            COVERAGE_FILE="${pkg_coverage_db}" coverage report --include="$PWD/**" --fail-under="${DEFAULT_FAIL_UNDER}" >> "${pkg_log}" 2>&1
+        fi
+        
+        echo $? > "${pkg_status}"
+        popd > /dev/null
+    )
+    set -e
+}
+
+# Run coverage reporting in parallel
+for pkg in ${modified_packages}; do
+    if [ -d "${pkg}" ]; then
+        # Limit concurrent jobs
+        while [ $(jobs -r | wc -l) -ge "${MAX_JOBS}" ]; do
+            sleep 0.1
+        done
+        report_package_coverage "${pkg}" &
+    fi
+done
+
+wait
+
+# Summary and exit
+failed_packages=()
+passed_packages=()
+
+for pkg in ${modified_packages}; do
+    if [ -d "${pkg}" ]; then
+        pkg_name_clean=$(echo "${pkg}" | sed 's|/$||' | sed 's|/|_|g')
+        pkg_log="${LOG_DIR}/${pkg_name_clean}.log"
+        pkg_status="${LOG_DIR}/${pkg_name_clean}.status"
+
+        echo "============================================================"
+        echo "Evaluating coverage for package: ${pkg}"
+        echo "============================================================"
+        [ -f "${pkg_log}" ] && cat "${pkg_log}"
+
+        status=$(cat "${pkg_status}" 2>/dev/null || echo 1)
+        if [ "${status}" -ne 0 ]; then
+            failed_packages+=("${pkg}")
+        else
+            passed_packages+=("${pkg}")
+        fi
+    fi
+done
+
+echo "============================================================"
+echo "Coverage Evaluation Summary"
+echo "============================================================"
+if [ ${#passed_packages[@]} -gt 0 ]; then
+    echo "✅ Passed: ${#passed_packages[@]} packages"
+fi
+
+if [ ${#failed_packages[@]} -gt 0 ]; then
+    echo "❌ Failed: ${#failed_packages[@]} packages"
+    for pkg in "${failed_packages[@]}"; do
+        echo "  - ${pkg}"
+    done
+    exit 1
+fi
diff --git a/ci/run_conditional_tests.sh b/ci/run_conditional_tests.sh
index b4df9cb923f4..241de51a4f88 100755
--- a/ci/run_conditional_tests.sh
+++ b/ci/run_conditional_tests.sh
@@ -47,77 +47,94 @@ git config --global url."${PROJECT_ROOT}".insteadOf "https://github.com/googleap
 # A script file for running the test in a sub project.
 test_script="${PROJECT_ROOT}/ci/run_single_test.sh"
 
-if [[ ${BUILD_TYPE} == "presubmit" ]]; then
+if [ "${TEST_ALL_PACKAGES}" = "true" ]; then
+    GIT_DIFF_ARG=""
+
+elif [[ ${BUILD_TYPE} == "presubmit" ]]; then
     # For presubmit build, we want to know the difference from the
     # common commit in the target branch.
-    GIT_DIFF_ARG="origin/$TARGET_BRANCH..."
-
-    # Then fetch enough history for finding the common commit.
-    git fetch origin "$TARGET_BRANCH" --deepen=200
+    if [ -n "${TARGET_BRANCH}" ]; then
+        git fetch origin "${TARGET_BRANCH}" --depth=1 || true
+    fi
+    GIT_DIFF_ARG="origin/${TARGET_BRANCH}"
 
 elif [[ ${BUILD_TYPE} == "continuous" ]]; then
     # For continuous build, we want to know the difference in the last
     # commit. This assumes we use squash commit when merging PRs.
-    GIT_DIFF_ARG="HEAD~.."
-
-    # Then fetch one last commit for getting the diff.
-    git fetch origin "$TARGET_BRANCH" --deepen=1
+    GIT_DIFF_ARG="HEAD~1.."
 
 else
     # Run everything.
     GIT_DIFF_ARG=""
 fi
 
-# Then detect changes in the test scripts.
-
-set +e
-git diff --quiet ${GIT_DIFF_ARG} ci
-changed=$?
-set -e
-
-# Now we have a fixed list, but we can change it to autodetect if
-# necessary.
-
-subdirs=(
-    packages
-    preview-packages
-)
-
-RETVAL=0
-
-for subdir in ${subdirs[@]}; do
-    for d in `ls -d ${subdir}/*/`; do
-        should_test=false
-        if [ -n "${GIT_DIFF_ARG}" ]; then
-            echo "checking changes with 'git diff --quiet ${GIT_DIFF_ARG} ${d}'"
-            set +e
-            git diff --quiet ${GIT_DIFF_ARG} ${d}
-            changed=$?
-            set -e
-            if [[ "${changed}" -eq 0 ]]; then
-                echo "no change detected in ${d}, skipping"
+run_test_in_dir() {
+    local d=$1
+    local pkg_name_clean=$(echo ${d} | sed 's|/$||' | sed 's|/|_|g')
+    local log_file="/tmp/test_log_${PY_VERSION}_${pkg_name_clean}.log"
+    export COVERAGE_FILE="${PROJECT_ROOT}/.coverage.${PY_VERSION}.${pkg_name_clean}"
+
+    pushd ${d} > /dev/null
+    set +e
+    ${test_script} > "${log_file}" 2>&1
+    local ret=$?
+    set -e
+    popd > /dev/null
+
+    echo "============================================================"
+    echo "Running tests in ${d}"
+    echo "============================================================"
+    cat "${log_file}"
+    rm -f "${log_file}"
+
+    if [ ${ret} -ne 0 ]; then
+        exit ${ret}
+    fi
+}
+export -f run_test_in_dir
+export test_script PROJECT_ROOT PY_VERSION TEST_TYPE
+
+dirs_to_test=()
+
+if [ -n "${PACKAGE_LIST}" ]; then
+    echo "Using provided PACKAGE_LIST"
+    dirs_to_test=(${PACKAGE_LIST})
+else
+    subdirs=(${PACKAGE_DIRS:-packages preview-packages})
+
+    for subdir in ${subdirs[@]}; do
+        if [ ! -d "${subdir}" ]; then continue; fi
+        for d in `ls -d ${subdir}/*/ 2>/dev/null`; do
+            should_test=false
+            if [ -n "${GIT_DIFF_ARG}" ]; then
+                set +e
+                git diff --quiet ${GIT_DIFF_ARG} -- "${d}"
+                changed=$?
+                set -e
+                if [[ "${changed}" -ne 0 ]]; then
+                    should_test=true
+                fi
             else
-                echo "change detected in ${d}"
+                # If GIT_DIFF_ARG is empty, run all the tests.
                 should_test=true
             fi
-        else
-            # If GIT_DIFF_ARG is empty, run all the tests.
-            should_test=true
-        fi
-        if [ "${should_test}" = true ]; then
-            echo "running test in ${d}"
-            pushd ${d}
-            # Temporarily allow failure.
-            set +e
-            ${test_script}
-            ret=$?
-            set -e
-            if [ ${ret} -ne 0 ]; then
-                RETVAL=${ret}
+            if [ "${should_test}" = true ]; then
+                dirs_to_test+=("${d}")
             fi
-            popd
-        fi
+        done
     done
-done
+fi
+
+if [ ${#dirs_to_test[@]} -eq 0 ]; then
+    echo "No packages to test."
+    exit 0
+fi
+
+PARALLEL_WORKERS="${PARALLEL_WORKERS:-1}"
+AVAIL_CORES=$(nproc 2>/dev/null || echo 4)
+if [ "${PARALLEL_WORKERS}" -gt "${AVAIL_CORES}" ]; then
+    PARALLEL_WORKERS="${AVAIL_CORES}"
+fi
 
-exit ${RETVAL}
+echo "Running tests across ${#dirs_to_test[@]} package(s) using ${PARALLEL_WORKERS} parallel worker(s)..."
+printf "%s\0" "${dirs_to_test[@]}" | xargs -0 -P "${PARALLEL_WORKERS}" -I {} bash -c 'run_test_in_dir "$@"' _ {}