diff --git a/eval/tests/test_runner_hardening.py b/eval/tests/test_runner_hardening.py
index 8245fdf82..6905891c0 100644
--- a/eval/tests/test_runner_hardening.py
+++ b/eval/tests/test_runner_hardening.py
@@ -262,3 +262,37 @@ def test_phase_workspace_accepts_new_regular_review_output(tmp_path):
     artifact.write_text("new review")
 
     runner_artifacts.enforce_phase_workspace(tmp_path, before, allowed_artifact=artifact)
+
+
+def test_phase_workspace_ignores_claude_sandbox_bootstrap_noise(tmp_path):
+    # Reproduced empirically: Claude Code's own enableWeakerNestedSandbox
+    # bootstrap creates this exact set of paths on every session regardless
+    # of task or model output (a trivial "say OK" prompt was enough). None
+    # of it is something the model decided to write, so it must not read as
+    # an unauthorized planning-phase change.
+    before = runner_artifacts.workspace_snapshot(tmp_path)
+    (tmp_path / ".claude" / "agents").mkdir(parents=True)
+    (tmp_path / ".claude" / "commands").mkdir(parents=True)
+    (tmp_path / ".claude" / ".cc-writes").write_text("{}")
+    (tmp_path / ".env").write_text("")
+    (tmp_path / ".env.development.local").write_text("")
+    (tmp_path / ".npmrc").write_text("")
+    (tmp_path / "package.json").write_text("{}")
+    (tmp_path / "node_modules").mkdir()
+    (tmp_path / "node_modules" / ".bin").mkdir()
+    artifact = tmp_path / "review-output.md"
+    artifact.write_text("new review")
+
+    runner_artifacts.enforce_phase_workspace(tmp_path, before, allowed_artifact=artifact)
+
+
+def test_phase_workspace_still_rejects_a_genuinely_unauthorized_change(tmp_path):
+    # The bootstrap-noise exclusion must stay narrow: an actual source-file
+    # edit outside the allowed artifact still has to be caught.
+    before = runner_artifacts.workspace_snapshot(tmp_path)
+    (tmp_path / "src.py").write_text("changed")
+    artifact = tmp_path / "review-output.md"
+    artifact.write_text("new review")
+
+    with pytest.raises(ValueError, match="unauthorized workspace path"):
+        runner_artifacts.enforce_phase_workspace(tmp_path, before, allowed_artifact=artifact)
diff --git a/eval/workflow_bench/runner_artifacts.py b/eval/workflow_bench/runner_artifacts.py
index 0b56a0c9a..da6b9ae58 100644
--- a/eval/workflow_bench/runner_artifacts.py
+++ b/eval/workflow_bench/runner_artifacts.py
@@ -21,6 +21,40 @@ MAX_WORKSPACE_SNAPSHOT_ENTRIES = 100_000
 MAX_WORKSPACE_SNAPSHOT_PATH_BYTES = 16 * 1024 * 1024
 MAX_WORKSPACE_SNAPSHOT_FILE_BYTES = 1024 * 1024 * 1024
 
+# Claude Code's own enableWeakerNestedSandbox bootstrap creates these paths on
+# EVERY session regardless of task or model output -- reproduced empirically
+# with a trivial "say OK" prompt: a synthetic package.json/lockfiles/
+# node_modules, a full set of .env variants, and .claude/agents,
+# .claude/commands, .claude/.cc-writes. None of this is something the model
+# decided to write, so it must not count as an "unauthorized" workspace
+# change during the planning-phase boundary check (the one thing this
+# snapshot is used for -- see workspace_snapshot's callers). Mirrors the
+# pre-existing .git exclusion below, which is the same kind of harness/tool
+# noise rather than substantive diff.
+WORKSPACE_SNAPSHOT_BOOTSTRAP_NOISE = frozenset(
+    {
+        ".claude",
+        ".env",
+        ".env.development",
+        ".env.development.local",
+        ".env.local",
+        ".env.production",
+        ".env.production.local",
+        ".env.test",
+        ".env.test.local",
+        ".gitmodules",
+        ".npmrc",
+        ".yarnrc",
+        ".yarnrc.yml",
+        "bunfig.toml",
+        "node_modules",
+        "package-lock.json",
+        "package.json",
+        "pnpm-lock.yaml",
+        "yarn.lock",
+    }
+)
+
 IMPLEMENTATION_ARMS = frozenset(
     {
         "workflow",
@@ -53,7 +87,9 @@ class VerificationResult:
 
 
 def workspace_snapshot(worktree: Path) -> dict[str, str]:
-    """Hash the workspace without following links, excluding Git internals."""
+    """Hash the workspace without following links, excluding Git internals
+    and Claude Code's own sandbox-bootstrap noise (see
+    WORKSPACE_SNAPSHOT_BOOTSTRAP_NOISE)."""
 
     root = worktree.expanduser().absolute()
     mode = root.lstat().st_mode
@@ -74,7 +110,7 @@ def workspace_snapshot(worktree: Path) -> dict[str, str]:
             raise ValueError(f"workspace snapshot directory is unreadable: {directory}: {exc}") from exc
         for entry in children:
             relative = relative_dir / entry.name
-            if relative.parts[0] == ".git":
+            if relative.parts[0] == ".git" or relative.parts[0] in WORKSPACE_SNAPSHOT_BOOTSTRAP_NOISE:
                 continue
             entry_count += 1
             path_bytes += len(relative.as_posix().encode())