diff --git a/assets/templates/browserbase/cua/Dockerfile.runtime b/assets/templates/browserbase/cua/Dockerfile.runtime
index b2d862bfd..90a70a021 100644
--- a/assets/templates/browserbase/cua/Dockerfile.runtime
+++ b/assets/templates/browserbase/cua/Dockerfile.runtime
@@ -16,8 +16,10 @@ FROM --platform=linux/amd64 node:22-slim
 
 WORKDIR /app/cua-server
 
-# Install curl for health checks
-RUN apt-get update -qq && apt-get install -y -qq curl && rm -rf /var/lib/apt/lists/*
+# Install curl for health checks.
+# Acquire::Retries=3 mitigates transient archive.ubuntu.com CDN sync mismatches
+# (launchpad bug #1876035).
+RUN apt-get -o Acquire::Retries=3 update -qq && apt-get -o Acquire::Retries=3 install -y -qq curl && rm -rf /var/lib/apt/lists/*
 
 # Copy pre-built binary
 COPY dist/sea/cua-server-linux-x64 ./cua-server-linux-x64
diff --git a/assets/templates/browserbase/cua/setup-binary.sh b/assets/templates/browserbase/cua/setup-binary.sh
index ba8f73c17..d25c295e2 100644
--- a/assets/templates/browserbase/cua/setup-binary.sh
+++ b/assets/templates/browserbase/cua/setup-binary.sh
@@ -11,10 +11,12 @@ set -e
 
 cd /app/cua-server
 
-# Install curl if not present (needed for health checks)
+# Install curl if not present (needed for health checks).
+# Acquire::Retries=3 mitigates transient archive.ubuntu.com CDN sync mismatches
+# that fail fresh-sandbox apt-get update mid-rollout (launchpad bug #1876035).
 if ! command -v curl &> /dev/null; then
     echo "Installing curl..."
-    apt-get update -qq && apt-get install -y -qq curl
+    apt-get -o Acquire::Retries=3 update -qq && apt-get -o Acquire::Retries=3 install -y -qq curl
 fi
 
 # Set server configuration
diff --git a/assets/templates/browserbase/cua/setup.sh b/assets/templates/browserbase/cua/setup.sh
index 32405f609..b506184a6 100644
--- a/assets/templates/browserbase/cua/setup.sh
+++ b/assets/templates/browserbase/cua/setup.sh
@@ -12,10 +12,12 @@ set -e
 
 cd /app/cua-server
 
-# Install curl if not present (needed for health checks)
+# Install curl if not present (needed for health checks).
+# Acquire::Retries=3 mitigates transient archive.ubuntu.com CDN sync mismatches
+# that fail fresh-sandbox apt-get update mid-rollout (launchpad bug #1876035).
 if ! command -v curl &> /dev/null; then
     echo "Installing curl..."
-    apt-get update -qq && apt-get install -y -qq curl
+    apt-get -o Acquire::Retries=3 update -qq && apt-get -o Acquire::Retries=3 install -y -qq curl
 fi
 
 # Install pnpm if not present
diff --git a/environments/hello_mcp_harbor/hello_mcp_harbor.py b/environments/hello_mcp_harbor/hello_mcp_harbor.py
index 6c3ac3dca..9b9364822 100644
--- a/environments/hello_mcp_harbor/hello_mcp_harbor.py
+++ b/environments/hello_mcp_harbor/hello_mcp_harbor.py
@@ -46,7 +46,9 @@ def _build_run_command(agent_workdir: str) -> str:
     return f"""
 set -e
 
-apt-get update && apt-get install -y curl
+# Acquire::Retries=3 mitigates transient archive.ubuntu.com CDN sync mismatches
+# that fail fresh-sandbox apt-get update mid-rollout (launchpad bug #1876035).
+apt-get -o Acquire::Retries=3 update && apt-get -o Acquire::Retries=3 install -y curl
 
 curl -fsSL https://opencode.ai/install | bash
 export PATH="$HOME/.opencode/bin:$PATH"
diff --git a/environments/openenv_echo/proj/server/Dockerfile b/environments/openenv_echo/proj/server/Dockerfile
index 3b9804056..03e69b44a 100644
--- a/environments/openenv_echo/proj/server/Dockerfile
+++ b/environments/openenv_echo/proj/server/Dockerfile
@@ -32,8 +32,10 @@ RUN if ! command -v uv >/dev/null 2>&1; then \
         mv /root/.local/bin/uvx /usr/local/bin/uvx; \
     fi
 
-# Install git for building from git repos (build-time only)
-RUN apt-get update && apt-get install -y --no-install-recommends \
+# Install git for building from git repos (build-time only).
+# Acquire::Retries=3 mitigates transient archive.ubuntu.com CDN sync mismatches
+# (launchpad bug #1876035).
+RUN apt-get -o Acquire::Retries=3 update && apt-get -o Acquire::Retries=3 install -y --no-install-recommends \
     git \
     && rm -rf /var/lib/apt/lists/*
 
diff --git a/environments/openenv_textarena/proj/server/Dockerfile b/environments/openenv_textarena/proj/server/Dockerfile
index c298c0f32..591ffc7e3 100644
--- a/environments/openenv_textarena/proj/server/Dockerfile
+++ b/environments/openenv_textarena/proj/server/Dockerfile
@@ -32,8 +32,10 @@ RUN if ! command -v uv >/dev/null 2>&1; then \
         mv /root/.local/bin/uvx /usr/local/bin/uvx; \
     fi
 
-# Install git for building from git repos (build-time only)
-RUN apt-get update && apt-get install -y --no-install-recommends \
+# Install git for building from git repos (build-time only).
+# Acquire::Retries=3 mitigates transient archive.ubuntu.com CDN sync mismatches
+# (launchpad bug #1876035).
+RUN apt-get -o Acquire::Retries=3 update && apt-get -o Acquire::Retries=3 install -y --no-install-recommends \
     git \
     && rm -rf /var/lib/apt/lists/*
 
@@ -59,8 +61,8 @@ FROM ${BASE_IMAGE}
 
 WORKDIR /app
 
-# Install runtime system libraries required by TextArena (cv2 needs libGL, glib)
-RUN apt-get update && apt-get install -y --no-install-recommends \
+# Install runtime system libraries required by TextArena (cv2 needs libGL, glib).
+RUN apt-get -o Acquire::Retries=3 update && apt-get -o Acquire::Retries=3 install -y --no-install-recommends \
     libgl1 \
     libglib2.0-0 \
     && rm -rf /var/lib/apt/lists/*
diff --git a/environments/terminus_harbor/terminus_harbor.py b/environments/terminus_harbor/terminus_harbor.py
index 3905d961e..749ea185a 100644
--- a/environments/terminus_harbor/terminus_harbor.py
+++ b/environments/terminus_harbor/terminus_harbor.py
@@ -57,10 +57,13 @@ async def post_sandbox_setup(self, state: vf.State) -> None:
 
         sandbox_id = state["sandbox_id"]
 
-        # Install curl, git, uv, and Python
+        # Install curl, git, uv, and Python.
+        # Acquire::Retries=3 mitigates transient archive.ubuntu.com CDN sync
+        # mismatches that fail fresh-sandbox apt-get update mid-rollout
+        # (launchpad bug #1876035).
         await self.sandbox_client.execute_command(
             sandbox_id,
-            "apt-get update && apt-get install -y curl git 2>&1",
+            "apt-get -o Acquire::Retries=3 update && apt-get -o Acquire::Retries=3 install -y curl git 2>&1",
             working_dir=None,
             timeout=120,
         )
diff --git a/tests/test_opencode_rlm_env.py b/tests/test_opencode_rlm_env.py
index fee3d4095..d51e12f80 100644
--- a/tests/test_opencode_rlm_env.py
+++ b/tests/test_opencode_rlm_env.py
@@ -151,7 +151,10 @@ def test_config_renders_valid_json_after_shell_expansion(self):
 class TestRunCommand:
     def test_run_command_installs_jq(self):
         env = build_env()
-        assert "apt-get install -y curl git unzip jq" in env.run_command
+        assert (
+            "apt-get -o Acquire::Retries=3 install -y curl git unzip jq"
+            in env.run_command
+        )
 
     def test_run_command_installs_bun(self):
         env = build_env()
diff --git a/tests/test_v1_harbor_cli.py b/tests/test_v1_harbor_cli.py
index a314a34b5..67c458f75 100644
--- a/tests/test_v1_harbor_cli.py
+++ b/tests/test_v1_harbor_cli.py
@@ -104,17 +104,23 @@ def test_opencode_config_owns_opencode_harness_fields() -> None:
     program = cast(dict[str, object], harness.program)
     command = cast(list[object], program["command"])
     mcp_setup = cast(dict[str, object], program["tools"])["mcp"]
+    setup = cast(str, program["setup"])
 
     assert harness.config.agent_workdir == "/workspace"
     assert harness.config.disabled_tools == ["webfetch"]
     assert harness.config.system_prompt is None
     assert harness.config.max_turns == 2
+    assert "apt-get -o Acquire::Retries=3 update" in setup
+    assert "apt-get -o Acquire::Retries=3 install" in setup
     assert "/workspace" in cast(str, command[2])
     assert '"webfetch": false' in cast(str, mcp_setup)
     assert "/opencode/system.txt" not in cast(dict[str, object], program["files"])
 
 
 def test_pi_harness_writes_intercepted_model_and_mcp_config() -> None:
+    harness = vf.Pi()
+    program = cast(dict[str, object], harness.program)
+    setup = cast(str, program["setup"])
     models = json.loads(
         pi_models_json(
             {
@@ -127,6 +133,8 @@ def test_pi_harness_writes_intercepted_model_and_mcp_config() -> None:
     )
     mcp = json.loads(pi_mcp_json())
 
+    assert "apt-get -o Acquire::Retries=3 update" in setup
+    assert "apt-get -o Acquire::Retries=3 install" in setup
     provider = models["providers"]["verifiers"]
     assert provider["baseUrl"] == "http://127.0.0.1:1/rollout/key/v1"
     assert provider["api"] == "openai-completions"
diff --git a/tests/test_v1_mini_swe_agent.py b/tests/test_v1_mini_swe_agent.py
index 64b67946b..ffff0d049 100644
--- a/tests/test_v1_mini_swe_agent.py
+++ b/tests/test_v1_mini_swe_agent.py
@@ -40,6 +40,8 @@ def test_mini_swe_agent_builds_sandbox_program():
     assert isinstance(harness, vf.CLIHarness)
     assert program["sandbox"] is not False
     assert "OPENAI_MODEL" in cast(dict[str, object], program["env"])
+    assert "apt-get -o Acquire::Retries=3 update" in cast(str, program["setup"])
+    assert "apt-get -o Acquire::Retries=3 install" in cast(str, program["setup"])
     assert "/mini-swe-agent/prompt.txt" in cast(dict[str, object], program["files"])
     assert "/mini-swe-agent/system.txt" in cast(dict[str, object], program["files"])
     assert "mini_swe_agent_log" in cast(dict[str, object], program["artifacts"])
diff --git a/tests/test_v1_rlm_swe.py b/tests/test_v1_rlm_swe.py
index e69110d17..3382f9b84 100644
--- a/tests/test_v1_rlm_swe.py
+++ b/tests/test_v1_rlm_swe.py
@@ -20,9 +20,13 @@ def test_rlm_harness_builds_sandbox_program_without_eager_checkout():
     program = as_mapping(harness.program)
     program_env = as_mapping(program["env"])
     artifacts = as_mapping(program["artifacts"])
+    setup = program["setup"]
 
     assert isinstance(harness, vf.CLIHarness)
     assert program["sandbox"] is not False
+    assert isinstance(setup, list)
+    assert "apt-get -o Acquire::Retries=3 update" in setup[0]
+    assert "apt-get -o Acquire::Retries=3 install" in setup[0]
     assert "RLM_MODEL" in program_env
     assert "rlm_metrics" in artifacts
 
diff --git a/verifiers/envs/experimental/composable/harnesses/mini_swe_agent.py b/verifiers/envs/experimental/composable/harnesses/mini_swe_agent.py
index 4a90b2586..c6ab44348 100644
--- a/verifiers/envs/experimental/composable/harnesses/mini_swe_agent.py
+++ b/verifiers/envs/experimental/composable/harnesses/mini_swe_agent.py
@@ -39,11 +39,14 @@ def build_mini_swe_agent_install_script(
     """Build the shell script that installs mini-SWE-agent."""
     install_tools = ""
     if install_python:
+        # Acquire::Retries=3 mitigates transient archive.ubuntu.com CDN sync
+        # mismatches that fail fresh-sandbox apt-get update mid-rollout. See
+        # launchpad bug #1876035.
         install_tools = """\
 export DEBIAN_FRONTEND=noninteractive
 if ! command -v python3 >/dev/null 2>&1 || ! python3 -m pip --version >/dev/null 2>&1; then
-  apt-get update -qq
-  apt-get install -y -qq python3 python3-pip ca-certificates
+  apt-get -o Acquire::Retries=3 update -qq
+  apt-get -o Acquire::Retries=3 install -y -qq python3 python3-pip ca-certificates
 fi
 """
 
diff --git a/verifiers/envs/experimental/composable/harnesses/opencode.py b/verifiers/envs/experimental/composable/harnesses/opencode.py
index 8f4d884e9..953816642 100644
--- a/verifiers/envs/experimental/composable/harnesses/opencode.py
+++ b/verifiers/envs/experimental/composable/harnesses/opencode.py
@@ -58,14 +58,17 @@ def build_install_script(
 ) -> str:
     """Build the shell script that installs OpenCode in a sandbox."""
     rg_install = (
-        "apt-get install -y -qq ripgrep > /dev/null 2>&1 || true"
+        "apt-get -o Acquire::Retries=3 install -y -qq ripgrep > /dev/null 2>&1 || true"
         if install_ripgrep
         else ""
     )
     sha256_check = f'echo "{release_sha256}  /tmp/opencode.tar.gz" | sha256sum -c -'
+    # Acquire::Retries=3 mitigates transient archive.ubuntu.com CDN sync mismatches
+    # (e.g. "File has unexpected size ... Mirror sync in progress?"). See launchpad
+    # bug #1876035. apt's default retries is 0, so one bad fetch fails the rollout.
     return f"""\
 set -e
-apt-get update -qq && apt-get install -y -qq curl tar > /dev/null 2>&1
+apt-get -o Acquire::Retries=3 update -qq && apt-get -o Acquire::Retries=3 install -y -qq curl tar > /dev/null 2>&1
 {rg_install}
 
 OPENCODE_RELEASE_REPO="{release_repo}"
diff --git a/verifiers/envs/experimental/composable/tasksets/swe/multi_swe.py b/verifiers/envs/experimental/composable/tasksets/swe/multi_swe.py
index b57c73e0e..c7802a3cd 100644
--- a/verifiers/envs/experimental/composable/tasksets/swe/multi_swe.py
+++ b/verifiers/envs/experimental/composable/tasksets/swe/multi_swe.py
@@ -264,7 +264,9 @@ async def setup(self, state) -> None:
         commands = [
             f"test -d {shlex.quote(repo_path)}",
             "test -f /home/fix-run.sh",
-            "command -v patch || (apt-get update && apt-get install -y patch)",
+            # Acquire::Retries=3: harden against transient archive.ubuntu.com CDN
+            # mirror-sync mismatches mid-rollout (launchpad bug #1876035).
+            "command -v patch || (apt-get -o Acquire::Retries=3 update && apt-get -o Acquire::Retries=3 install -y patch)",
             "rm -f /home/fix.patch /home/test_output.txt /home/create_fix_patch.sh",
         ]
         for command in commands:
diff --git a/verifiers/envs/experimental/opencode_env.py b/verifiers/envs/experimental/opencode_env.py
index 3d8911a14..59861a73c 100644
--- a/verifiers/envs/experimental/opencode_env.py
+++ b/verifiers/envs/experimental/opencode_env.py
@@ -54,7 +54,9 @@
 DEFAULT_RUN_COMMAND_TEMPLATE = """\
 set -eo pipefail
 
-apt-get update && apt-get install -y curl
+# Acquire::Retries=3 mitigates transient archive.ubuntu.com CDN sync mismatches
+# that fail fresh-sandbox apt-get update mid-rollout (launchpad bug #1876035).
+apt-get -o Acquire::Retries=3 update && apt-get -o Acquire::Retries=3 install -y curl
 
 for install_attempt in 1 2 3; do
     if {install_command}; then
diff --git a/verifiers/envs/experimental/opencode_rlm_env.py b/verifiers/envs/experimental/opencode_rlm_env.py
index e37667bf3..1c8715246 100644
--- a/verifiers/envs/experimental/opencode_rlm_env.py
+++ b/verifiers/envs/experimental/opencode_rlm_env.py
@@ -96,7 +96,9 @@ async def metric(state: State) -> float:
 RLM_RUN_COMMAND_TEMPLATE = """\
 set -e
 
-apt-get update && apt-get install -y curl git unzip jq
+# Acquire::Retries=3 mitigates transient archive.ubuntu.com CDN sync mismatches
+# that fail fresh-sandbox apt-get update mid-rollout (launchpad bug #1876035).
+apt-get -o Acquire::Retries=3 update && apt-get -o Acquire::Retries=3 install -y curl git unzip jq
 
 # Install bun (TypeScript runtime required by the RLM plugin)
 curl -fsSL https://bun.sh/install | bash
diff --git a/verifiers/v1/packages/harnesses/mini_swe_agent.py b/verifiers/v1/packages/harnesses/mini_swe_agent.py
index 284e50cae..66d942c76 100644
--- a/verifiers/v1/packages/harnesses/mini_swe_agent.py
+++ b/verifiers/v1/packages/harnesses/mini_swe_agent.py
@@ -123,8 +123,8 @@ def build_mini_swe_agent_install_script(
         install_tools = """\
 export DEBIAN_FRONTEND=noninteractive
 if ! command -v python3 >/dev/null 2>&1 || ! python3 -m pip --version >/dev/null 2>&1; then
-  apt-get update -qq
-  apt-get install -y -qq python3 python3-pip ca-certificates
+  apt-get -o Acquire::Retries=3 update -qq
+  apt-get -o Acquire::Retries=3 install -y -qq python3 python3-pip ca-certificates
 fi
 """
 
diff --git a/verifiers/v1/packages/harnesses/opencode.py b/verifiers/v1/packages/harnesses/opencode.py
index 2980bfb74..47f66339e 100644
--- a/verifiers/v1/packages/harnesses/opencode.py
+++ b/verifiers/v1/packages/harnesses/opencode.py
@@ -153,14 +153,16 @@ def build_install_script(
     install_ripgrep: bool = True,
 ) -> str:
     rg_install = (
-        "apt-get install -y -qq ripgrep > /dev/null 2>&1 || true"
+        "apt-get -o Acquire::Retries=3 install -y -qq ripgrep > /dev/null 2>&1 || true"
         if install_ripgrep
         else ""
     )
     sha256_check = f'echo "{release_sha256}  /tmp/opencode.tar.gz" | sha256sum -c -'
+    # Acquire::Retries=3 mitigates transient archive.ubuntu.com CDN sync
+    # mismatches that fail fresh-sandbox apt-get calls mid-rollout.
     return f"""\
 set -e
-apt-get update -qq && apt-get install -y -qq curl tar ca-certificates > /dev/null 2>&1
+apt-get -o Acquire::Retries=3 update -qq && apt-get -o Acquire::Retries=3 install -y -qq curl tar ca-certificates > /dev/null 2>&1
 {rg_install}
 
 OPENCODE_RELEASE_REPO={shlex.quote(release_repo)}
diff --git a/verifiers/v1/packages/harnesses/pi.py b/verifiers/v1/packages/harnesses/pi.py
index 0822d61d4..2887d4143 100644
--- a/verifiers/v1/packages/harnesses/pi.py
+++ b/verifiers/v1/packages/harnesses/pi.py
@@ -87,7 +87,7 @@ def __init__(
 def build_pi_install_script(package: str = DEFAULT_PI_PACKAGE) -> str:
     return f"""\
 set -e
-apt-get update -qq && apt-get install -y -qq curl ca-certificates nodejs npm > /dev/null 2>&1
+apt-get -o Acquire::Retries=3 update -qq && apt-get -o Acquire::Retries=3 install -y -qq curl ca-certificates nodejs npm > /dev/null 2>&1
 npm install -g {shlex.quote(package)}
 """
 
diff --git a/verifiers/v1/packages/harnesses/rlm.py b/verifiers/v1/packages/harnesses/rlm.py
index be442a23a..f1c321fb8 100644
--- a/verifiers/v1/packages/harnesses/rlm.py
+++ b/verifiers/v1/packages/harnesses/rlm.py
@@ -116,7 +116,8 @@ def __init__(
             },
             dirs=dirs,
             setup=[
-                "apt-get update && apt-get install -y --no-install-recommends "
+                "apt-get -o Acquire::Retries=3 update && "
+                "apt-get -o Acquire::Retries=3 install -y --no-install-recommends "
                 "ca-certificates curl git && rm -rf /var/lib/apt/lists/*",
                 build_install_command(),
             ],
diff --git a/verifiers/v1/utils/sandbox_utils.py b/verifiers/v1/utils/sandbox_utils.py
index cf6c9f3b8..1b1089f49 100644
--- a/verifiers/v1/utils/sandbox_utils.py
+++ b/verifiers/v1/utils/sandbox_utils.py
@@ -453,11 +453,15 @@ def python_package_install_command(package_args: str) -> str:
         "if command -v python3 >/dev/null 2>&1; then PYTHON=python3; "
         "elif command -v python >/dev/null 2>&1; then PYTHON=python; "
         "elif command -v apt-get >/dev/null 2>&1; then "
-        "apt-get update && apt-get install -y python3 python3-pip && PYTHON=python3; "
+        "apt-get -o Acquire::Retries=3 update && "
+        "apt-get -o Acquire::Retries=3 install -y python3 python3-pip && "
+        "PYTHON=python3; "
         "else echo 'python is required to install sandbox packages' >&2; exit 127; fi\n"
         "$PYTHON -m pip --version >/dev/null 2>&1 || "
         "$PYTHON -m ensurepip --upgrade || "
-        "(command -v apt-get >/dev/null 2>&1 && apt-get update && apt-get install -y python3-pip)\n"
+        "(command -v apt-get >/dev/null 2>&1 && "
+        "apt-get -o Acquire::Retries=3 update && "
+        "apt-get -o Acquire::Retries=3 install -y python3-pip)\n"
         "$PYTHON -m pip install --disable-pip-version-check --break-system-packages "
         f"{package_args} || "
         "$PYTHON -m pip install --disable-pip-version-check "
@@ -471,7 +475,8 @@ def python_runtime_setup_command() -> str:
         "if command -v python3 >/dev/null 2>&1; then exit 0; fi\n"
         "if command -v python >/dev/null 2>&1; then exit 0; fi\n"
         "if command -v apt-get >/dev/null 2>&1; then "
-        "apt-get update && apt-get install -y python3; exit 0; fi\n"
+        "apt-get -o Acquire::Retries=3 update && "
+        "apt-get -o Acquire::Retries=3 install -y python3; exit 0; fi\n"
         "echo 'python is required for sandbox Python programs' >&2\n"
         "exit 127"
     )