diff --git a/harness-engineering-bench/tau3/baseline/build.yaml b/harness-engineering-bench/tau3/baseline/build.yaml
index 8e07caa..beacdb4 100644
--- a/harness-engineering-bench/tau3/baseline/build.yaml
+++ b/harness-engineering-bench/tau3/baseline/build.yaml
@@ -49,6 +49,12 @@ model: fireworks_ai/deepseek-v4-flash
 environment_name: ${inner_env:-modal}
 # inner eval sandboxes share a dedicated Modal app instead of the __harbor__ default
 extra_harbor_args: ["--ek", "app_name=harness-engineering-bench", "--ek", "sandbox_idle_timeout_secs=3600"]
+# NOTE: tau3's optimizer trial reliably dies at ~38-40 min with grpclib
+# StreamTerminatedError ("Connection lost"). Four live experiments (default
+# DinD, --ek modal_vm_runtime=true, --ek modal_sandbox_v2=true, and
+# --max-retries 1) all failed identically, so no `optimizer_harbor_args` value
+# is set here: none of them is a fix, and a config value that claims to be one
+# is worse than nothing. Tracked upstream; see the PR that added this field.
 harbor_python_version: "3.12"
 n_attempts: 1
 max_retries: 1
diff --git a/vero/src/vero/harbor/build/config.py b/vero/src/vero/harbor/build/config.py
index d6bc857..1463098 100644
--- a/vero/src/vero/harbor/build/config.py
+++ b/vero/src/vero/harbor/build/config.py
@@ -424,6 +424,10 @@ class HarborBuildConfig(StrictModel):
             task_environment, which is that sub-run's environment. Use it for
             things like UV_TOOL_BIN_DIR, so `uv tool install` targets a writable
             directory on a non-root sandbox.
+        optimizer_harbor_args: Extra flags for the outer harbor run that hosts
+            the optimizer trial. Distinct from extra_harbor_args, which tunes
+            the nested evaluation sub-run. Rejected if they override a flag
+            `vero harbor run` controls.
         timeout_seconds: Wall clock for one evaluation.
         case_timeout_seconds: Wall clock for one case.
         task_agent_timeout_seconds: Wall clock declared for the target agent.
@@ -534,8 +538,16 @@ class HarborBuildConfig(StrictModel):
     feedback_transcripts: bool = False
     feedback_max_bytes: int = Field(default=3000, ge=0)
     expose_attempt_detail: bool = False
+    # Extra flags for the NESTED `harbor run` that evaluates a candidate.
     extra_harbor_args: list[str] = Field(default_factory=list)
     agent_env: dict[str, str] = Field(default_factory=dict)
+    # Extra flags for the OUTER `harbor run` that hosts the optimizer trial
+    # (`vero harbor run`). Distinct from `extra_harbor_args`: that one tunes the
+    # nested eval sub-run, this one tunes the environment the optimizer itself
+    # lives in. A build declares here what its optimizer trial needs to survive,
+    # e.g. `--ek modal_vm_runtime=true` for a long trial whose teardown keeps
+    # losing the DinD gRPC stream.
+    optimizer_harbor_args: list[str] = Field(default_factory=list)
 
     timeout_seconds: float = Field(default=1800.0, gt=0)
     case_timeout_seconds: float = Field(default=180.0, gt=0)
@@ -651,6 +663,22 @@ def validate_extra_harbor_args(cls, value: list[str]) -> list[str]:
             )
         return value
 
+    @field_validator("optimizer_harbor_args")
+    @classmethod
+    def validate_optimizer_harbor_args(cls, value: list[str]) -> list[str]:
+        # `vero harbor run` owns the outer command's task, agent, environment and
+        # model; a build must not fight the CLI over them.
+        controlled = {"-a", "-e", "-m", "-p"}
+        conflicts = [
+            argument for argument in value if argument.split("=", 1)[0] in controlled
+        ]
+        if conflicts:
+            raise ValueError(
+                "optimizer_harbor_args override controlled flags: "
+                + ", ".join(conflicts)
+            )
+        return value
+
     @field_validator("partitions")
     @classmethod
     def validate_partitions(
diff --git a/vero/src/vero/harbor/cli.py b/vero/src/vero/harbor/cli.py
index 8b70018..72730a6 100644
--- a/vero/src/vero/harbor/cli.py
+++ b/vero/src/vero/harbor/cli.py
@@ -437,6 +437,9 @@ def run_command(config_path, agent, model, environment, params, env_file, extra)
         # for a deterministic command line.
         for key in sorted(config.agent_env):
             command.extend(["--ae", f"{key}={config.agent_env[key]}"])
+        # Build-declared outer-trial flags first, so a command-line arg can still
+        # override them (harbor's `--ek` takes the last value for a key).
+        command.extend(config.optimizer_harbor_args)
         command.extend(extra)
         click.echo(shlex.join(command))
         completed = subprocess.run(
diff --git a/vero/tests/test_v05_cli.py b/vero/tests/test_v05_cli.py
index d18534c..5d48c9a 100644
--- a/vero/tests/test_v05_cli.py
+++ b/vero/tests/test_v05_cli.py
@@ -309,3 +309,62 @@ def test_cli_rejects_options_that_do_not_apply(tmp_path: Path):
     assert "are only valid with --produce" in producer_timeout.output
     assert wandb.exit_code == 2
     assert "require --wandb-project" in wandb.output
+
+
+def test_harbor_run_forwards_build_declared_optimizer_args(tmp_path, monkeypatch):
+    """`optimizer_harbor_args` tunes the OUTER trial, `extra_harbor_args` the nested eval.
+
+    Regression guard for the tau3 teardown failure: the build declared
+    `--ek modal_vm_runtime=true` and it has to reach the `harbor run` that hosts
+    the optimizer, not the `harbor run` that scores a candidate.
+    """
+    from vero.harbor import build as harbor_build
+    from vero.harbor import cli as harbor_cli
+
+    config_path = tmp_path / "build.yaml"
+    config_path.write_text("name: org/task\n", encoding="utf-8")
+
+    class _Config:
+        harbor_requirement = "harbor[modal]==0.20.0"
+        agent_env: dict[str, str] = {}
+        optimizer_harbor_args = ["--ek", "modal_vm_runtime=true"]
+        extra_harbor_args = ["--ek", "app_name=nested-only"]
+
+    monkeypatch.setattr(harbor_build, "load_harbor_build_config", lambda *a, **k: _Config())
+    monkeypatch.setattr(harbor_build, "compile_harbor_task", lambda config, output: output)
+    monkeypatch.setattr(harbor_cli.shutil, "which", lambda name: "/usr/bin/uvx")
+    monkeypatch.setattr(
+        harbor_cli, "_compiled_run_environment", lambda task, overrides: {}
+    )
+
+    recorded: list[list[str]] = []
+
+    def _record(command, env=None):
+        recorded.append(command)
+        return subprocess.CompletedProcess(command, 0)
+
+    monkeypatch.setattr(harbor_cli.subprocess, "run", _record)
+
+    result = CliRunner().invoke(
+        main,
+        [
+            "harbor",
+            "run",
+            "--config",
+            str(config_path),
+            "--agent",
+            "codex",
+            "--model",
+            "gpt-5.3-codex",
+            "--yes",
+        ],
+    )
+
+    assert result.exit_code == 0, result.output
+    assert len(recorded) == 1
+    command = recorded[0]
+    assert "modal_vm_runtime=true" in command
+    # The nested-eval flags stay out of the outer command.
+    assert "app_name=nested-only" not in command
+    # Build-declared flags come first so a command-line `--ek` can override them.
+    assert command.index("modal_vm_runtime=true") < command.index("--yes")
diff --git a/vero/tests/test_v05_harbor_build.py b/vero/tests/test_v05_harbor_build.py
index 43b9c61..c3770ee 100644
--- a/vero/tests/test_v05_harbor_build.py
+++ b/vero/tests/test_v05_harbor_build.py
@@ -391,6 +391,12 @@ def test_build_config_requires_pins_and_valid_partition_references(tmp_path):
         _config(tmp_path / "unknown", selection_partition="missing")
     with pytest.raises(ValidationError, match="controlled flags"):
         _config(tmp_path / "flags", extra_harbor_args=["--jobs-dir=/forged"])
+    with pytest.raises(ValidationError, match="controlled flags"):
+        _config(tmp_path / "outer-flags", optimizer_harbor_args=["-a", "forged"])
+    assert _config(
+        tmp_path / "outer-ek",
+        optimizer_harbor_args=["--ek", "modal_vm_runtime=true"],
+    ).optimizer_harbor_args == ["--ek", "modal_vm_runtime=true"]
     with pytest.raises(ValidationError, match="explicit version"):
         _config(tmp_path / "source", task_source="org/unversioned")
 
diff --git a/vero/tests/test_v05_harbor_http.py b/vero/tests/test_v05_harbor_http.py
index a4e17a5..0cbc1ff 100644
--- a/vero/tests/test_v05_harbor_http.py
+++ b/vero/tests/test_v05_harbor_http.py
@@ -260,7 +260,11 @@ def test_harbor_run_uses_current_python_and_pinned_harbor_extra(tmp_path, monkey
 
     config_path = tmp_path / "build.yaml"
     config_path.write_text("task_name: unused\n")
-    config = SimpleNamespace(harbor_requirement="harbor[modal]==0.20.0", agent_env={})
+    config = SimpleNamespace(
+        harbor_requirement="harbor[modal]==0.20.0",
+        agent_env={},
+        optimizer_harbor_args=[],
+    )
     observed = {}
 
     def compile_task(_config, output):
@@ -313,7 +317,11 @@ def test_harbor_run_env_file_secrets_reach_subprocess_not_command_line(
     config_path.write_text("task_name: unused\n")
     env_file = tmp_path / "secrets.env"
     env_file.write_text("MODAL_TOKEN_ID=mt-id\nMODAL_TOKEN_SECRET=mt-secret\n")
-    config = SimpleNamespace(harbor_requirement="harbor[modal]==0.20.0", agent_env={})
+    config = SimpleNamespace(
+        harbor_requirement="harbor[modal]==0.20.0",
+        agent_env={},
+        optimizer_harbor_args=[],
+    )
     observed = {}
 
     def compile_task(_config, output):