diff --git a/products/tasks/backend/exceptions.py b/products/tasks/backend/exceptions.py
index edd9940b8c17..b60c9740ef0c 100644
--- a/products/tasks/backend/exceptions.py
+++ b/products/tasks/backend/exceptions.py
@@ -266,6 +266,18 @@ class RepositoryCloneError(ProcessTaskTransientError):
     pass
 
 
+class RepositoryAccessError(ProcessTaskFatalError, NonReportableError):
+    """GitHub refused to serve the repository with the credential the sandbox has.
+
+    The run has no token, or its token does not cover the repository. A retry reuses the same
+    credential, so it cannot succeed. This is an expected customer setup state, so it is not
+    captured to error tracking.
+    """
+
+    def __init__(self, message: str, context: dict[str, Any]):
+        ProcessTaskError.__init__(self, message, context, None, capture=False, non_retryable=True)
+
+
 class RetryableRepositorySetupError(ProcessTaskTransientError):
     """Failed to setup repository (install dependencies, etc)."""
 
diff --git a/products/tasks/backend/temporal/process_task/activities/provision_sandbox.py b/products/tasks/backend/temporal/process_task/activities/provision_sandbox.py
index 560d096baa23..e38a1cee89ca 100644
--- a/products/tasks/backend/temporal/process_task/activities/provision_sandbox.py
+++ b/products/tasks/backend/temporal/process_task/activities/provision_sandbox.py
@@ -37,6 +37,7 @@
     OAuthTokenError,
     ProcessTaskError,
     ProcessTaskTransientError,
+    RepositoryAccessError,
     RepositoryCloneError,
     SandboxNetworkPolicyError,
     TaskInvalidStateError,
@@ -1229,6 +1230,18 @@ def clone_repository_in_sandbox(input: CloneRepositoryInSandboxInput) -> CloneRe
                     blobless=blobless_clone,
                 )
 
+            if _is_repository_access_denied_clone_error(clone_result):
+                raise RepositoryAccessError(
+                    f"Could not clone {input.repository}: GitHub denied access. Check that the PostHog "
+                    "GitHub app is installed on this repository's account and can read this repository.",
+                    {
+                        "repository": input.repository,
+                        "sandbox_id": input.sandbox_id,
+                        "exit_code": clone_result.exit_code,
+                        "has_github_token": bool(input.github_token),
+                    },
+                )
+
             if clone_result.exit_code != 0:
                 error_output = clone_result.stderr or clone_result.stdout or clone_result.error or "No output captured"
                 raise RepositoryCloneError(
@@ -1254,6 +1267,20 @@ def clone_repository_in_sandbox(input: CloneRepositoryInSandboxInput) -> CloneRe
         return CloneRepositoryInSandboxOutput(clone_ms=clone_timer.elapsed_ms)
 
 
+def _is_repository_access_denied_clone_error(result: ExecutionResult) -> bool:
+    if result.exit_code == 0:
+        return False
+
+    # Git asks for a username when GitHub answers 401, which is how GitHub hides a private
+    # repository from a request without a usable token.
+    output = f"{result.stdout}\n{result.stderr}".casefold()
+    return (
+        "could not read username" in output
+        or "authentication failed for" in output
+        or "remote: repository not found" in output
+    )
+
+
 def _is_missing_remote_branch_clone_error(result: ExecutionResult) -> bool:
     if result.exit_code == 0:
         return False
diff --git a/products/tasks/backend/temporal/process_task/activities/tests/test_provision_sandbox.py b/products/tasks/backend/temporal/process_task/activities/tests/test_provision_sandbox.py
index ccf6d04e303c..4d6a64e263de 100644
--- a/products/tasks/backend/temporal/process_task/activities/tests/test_provision_sandbox.py
+++ b/products/tasks/backend/temporal/process_task/activities/tests/test_provision_sandbox.py
@@ -13,10 +13,13 @@
 
 from asgiref.sync import async_to_sync
 
+from posthog.temporal.common.posthog_client import is_expected_activity_failure
+
 from products.tasks.backend.constants import SNAPSHOT_KIND_DIRECTORY, SNAPSHOT_KIND_FILESYSTEM
 from products.tasks.backend.exceptions import (
     ComputeBillingLimitError,
     OrganizationExecutionError,
+    RepositoryAccessError,
     RepositoryCloneError,
     SandboxCleanupError,
     SandboxRateLimitedError,
@@ -610,6 +613,63 @@ def test_clone_failure_records_failed_latency_and_captures_command_result(mocker
     assert str(capture_exception.call_args.args[0]) == "clone output"
 
 
+@pytest.mark.parametrize(
+    "stderr, expected_error",
+    [
+        (
+            "Cloning into 'repo'...\nfatal: could not read Username for 'https://github.com': No such device or address\n",
+            RepositoryAccessError,
+        ),
+        (
+            "Cloning into 'repo'...\nremote: Repository not found.\nfatal: repository 'https://github.com/acme/repo.git/' not found\n",
+            RepositoryAccessError,
+        ),
+        (
+            "Cloning into 'repo'...\nfatal: unable to access 'https://github.com/acme/repo.git/': Could not resolve host: github.com\n",
+            RepositoryCloneError,
+        ),
+    ],
+)
+def test_clone_failure_stops_retries_only_when_github_denies_access(
+    mocker, activity_environment, stderr, expected_error
+):
+    context = TaskProcessingContext(
+        task_id="task-id",
+        run_id="run-id",
+        team_id=1,
+        team_uuid="team-uuid",
+        organization_id="organization-id",
+        github_integration_id=123,
+        repository="acme/repo",
+        distinct_id="distinct-id",
+        state={},
+    )
+    sandbox = mocker.Mock()
+    sandbox.clone_repository.return_value = ExecutionResult(stdout="", stderr=stderr, exit_code=128, error=None)
+    mocker.patch(
+        "products.tasks.backend.temporal.process_task.activities.provision_sandbox.get_sandbox_class_for_sandbox_id",
+        return_value=mocker.Mock(get_by_id=mocker.Mock(return_value=sandbox)),
+    )
+    capture_exception = mocker.patch("products.tasks.backend.exceptions.capture_exception")
+
+    with pytest.raises(expected_error) as error:
+        async_to_sync(activity_environment.run)(
+            clone_repository_in_sandbox,
+            CloneRepositoryInSandboxInput(
+                context=context,
+                sandbox_id="sandbox-id",
+                repository="acme/repo",
+                github_token="",
+                shallow_clone=True,
+            ),
+        )
+
+    access_denied = expected_error is RepositoryAccessError
+    assert error.value.non_retryable is access_denied
+    assert is_expected_activity_failure(error.value) is access_denied
+    assert capture_exception.called is not access_denied
+
+
 def _prepared_for_create() -> PrepareSandboxForRepositoryOutput:
     return PrepareSandboxForRepositoryOutput(
         sandbox_name="sandbox-name",