diff --git a/src/harbor_hypeman/environment.py b/src/harbor_hypeman/environment.py
index e24243e..bae0c82 100644
--- a/src/harbor_hypeman/environment.py
+++ b/src/harbor_hypeman/environment.py
@@ -110,12 +110,16 @@ async def start(self, force_build: bool) -> None:
             raise RuntimeError("Hypeman environment is already started.")
 
         image = await self._prepare_image(force_build)
+        instance_name = self._instance_name()
         instance = await self._client.instances.create(
             image=image,
-            name=self._instance_name(),
+            name=instance_name,
             entrypoint=["/bin/sh", "-c"],
             cmd=["while true; do sleep 3600; done"],
-            env=self._startup_env(),
+            env={
+                **self._startup_env(),
+                "HYPEMAN_INSTANCE_NAME": instance_name,
+            },
             network={"enabled": self.network_policy.network_mode == NetworkMode.PUBLIC},
             tags={
                 "harbor.managed": "true",
@@ -183,11 +187,15 @@ async def _prepare_image(self, force_build: bool) -> str:
         ):
             if docker_image is None:
                 raise RuntimeError("Prebuilt image selection requires docker_image.")
-            image = await self._client.images.create(
-                name=docker_image,
-                tags={_BUILD_TAG: self.environment_id},
-            )
-            return image.name
+            try:
+                await self._client.images.get(docker_image)
+                return docker_image
+            except NotFoundError:
+                image = await self._client.images.create(
+                    name=docker_image,
+                    tags={_BUILD_TAG: self.environment_id},
+                )
+                return image.name
 
         if not force_build:
             cached_image = await self._cached_build_image()
diff --git a/tests/test_environment.py b/tests/test_environment.py
index 1f018bc..65b9e29 100644
--- a/tests/test_environment.py
+++ b/tests/test_environment.py
@@ -8,13 +8,14 @@
 from typing import Any, cast
 from unittest.mock import AsyncMock, MagicMock
 
+import httpx
 import pytest
 from harbor.environments.base import SandboxBuildFailedError
 from harbor.environments.factory import EnvironmentFactory
 from harbor.models.task.config import EnvironmentConfig, NetworkMode, NetworkPolicy
 from harbor.models.trial.config import EnvironmentConfig as TrialEnvironmentConfig
 from harbor.models.trial.paths import TrialPaths
-from hypeman import AsyncHypeman
+from hypeman import AsyncHypeman, NotFoundError
 
 import harbor_hypeman.environment as environment_module
 from harbor_hypeman import HypemanEnvironment
@@ -25,7 +26,10 @@ def _client() -> Any:
     client.images.create = AsyncMock(
         return_value=SimpleNamespace(name="docker.io/library/alpine:latest")
     )
-    client.images.get = AsyncMock()
+    response = httpx.Response(404, request=httpx.Request("GET", "https://example.com"))
+    client.images.get = AsyncMock(
+        side_effect=NotFoundError("missing", response=response, body=None)
+    )
     client.builds.list = AsyncMock(return_value=[])
     client.builds.create = AsyncMock()
     client.builds.get = AsyncMock()
@@ -131,8 +135,11 @@ async def test_start_prebuilt_image_maps_resources_and_network(tmp_path: Path) -
     assert create_kwargs["size"] == "2048MB"
     assert create_kwargs["overlay_size"] == "4096MB"
     assert create_kwargs["network"] == {"enabled": False}
-    assert create_kwargs["env"] == {"TASK_ENV": "value"}
     assert create_kwargs["name"].startswith("harbor-test-task-abc123-env-")
+    assert create_kwargs["env"] == {
+        "TASK_ENV": "value",
+        "HYPEMAN_INSTANCE_NAME": create_kwargs["name"],
+    }
     client.instances.wait.assert_awaited_once_with(
         "instance-1",
         state="Running",
@@ -141,6 +148,26 @@ async def test_start_prebuilt_image_maps_resources_and_network(tmp_path: Path) -
     )
 
 
+async def test_start_uses_existing_prebuilt_image(tmp_path: Path) -> None:
+    client = _client()
+    client.images.get.side_effect = None
+    client.images.get.return_value = SimpleNamespace(
+        name="docker.io/builds/build-1:latest",
+        status="ready",
+    )
+    environment = _environment(
+        tmp_path,
+        task_config=EnvironmentConfig(docker_image="builds/build-1"),
+        client=client,
+    )
+
+    await environment.start(force_build=False)
+
+    client.images.get.assert_awaited_once_with("builds/build-1")
+    client.images.create.assert_not_awaited()
+    assert client.instances.create.await_args.kwargs["image"] == "builds/build-1"
+
+
 async def test_start_creates_configured_workdir(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch
 ) -> None:
@@ -247,6 +274,7 @@ async def test_start_failure_deletes_created_instance(tmp_path: Path) -> None:
 
 async def test_start_reuses_ready_build(tmp_path: Path) -> None:
     client = _client()
+    client.images.get.side_effect = None
     client.builds.list.return_value = [
         SimpleNamespace(
             status="ready",