diff --git a/nisse-dev/src/nisse_dev/agent.py b/nisse-dev/src/nisse_dev/agent.py
index 995cc87..935ba72 100644
--- a/nisse-dev/src/nisse_dev/agent.py
+++ b/nisse-dev/src/nisse_dev/agent.py
@@ -217,6 +217,7 @@ def prepare_workspace(
     if not repo.exists():
         repo.clone(remote, credential=credential)
     repo.configure_identity(account, f"{account}@users.noreply.local")
+    repo.install_excludes()  # build artifacts are never the work (#80)
     return repo
 
 
diff --git a/nisse-dev/src/nisse_dev/git.py b/nisse-dev/src/nisse_dev/git.py
index 4df2292..56f5596 100644
--- a/nisse-dev/src/nisse_dev/git.py
+++ b/nisse-dev/src/nisse_dev/git.py
@@ -33,6 +33,35 @@ class GitError(RuntimeError):
     pass
 
 
+# Build artifacts and caches that are never the work (D4, #80). Kept out of
+# every commit the harness makes, tracked or not, and excluded in the clone
+# so they don't even show as changes. Conservative on purpose: a pattern here
+# must never match a project's real source.
+ARTIFACT_PATTERNS = (
+    "__pycache__/",
+    "*.py[cod]",
+    ".pytest_cache/",
+    ".mypy_cache/",
+    ".ruff_cache/",
+    ".tox/",
+    ".nox/",
+    "*.egg-info/",
+    ".coverage",
+    "htmlcov/",
+    ".venv/",
+    "node_modules/",
+    ".DS_Store",
+)
+_EXCLUDES_MARK = "# nisse-dev: build artifacts are never the work (D4)"
+
+
+def _pathspec(pattern: str) -> str:
+    """A pathspec leaving ``pattern`` out wherever it occurs in the tree."""
+    glob = pattern.rstrip("/")
+    tail = "/**" if pattern.endswith("/") else ""
+    return f":(exclude,glob)**/{glob}{tail}"
+
+
 @dataclass(frozen=True)
 class HTTPSToken:
     """A push-only account's token, for HTTPS remotes (D4)."""
@@ -84,6 +113,7 @@ class GitRepo:
         *args: str,
         credential: PushCredential | None = None,
         cwd: str | None = None,
+        raw: bool = False,
     ) -> str:
         quoted = " ".join(shlex.quote(a) for a in args)
         command, stdin, secrets = f"git {quoted}", None, {}
@@ -117,7 +147,8 @@ class GitRepo:
                 f"git {args[0]} failed: "
                 f"{result.stderr.strip() or result.stdout.strip()}"
             )
-        return result.stdout.strip()
+        # Raw for output whose leading whitespace means something (porcelain).
+        return result.stdout if raw else result.stdout.strip()
 
     # -- inspection -------------------------------------------------------
 
@@ -191,15 +222,42 @@ class GitRepo:
             return False
         return True
 
+    def install_excludes(self) -> None:
+        """List the build artifacts in the clone's own exclude file, so they
+        never appear as changes. Local to the clone: nothing in the project
+        changes. Idempotent."""
+        path = shlex.quote(str(self.root / ".git" / "info" / "exclude"))
+        block = "\n".join([_EXCLUDES_MARK, *ARTIFACT_PATTERNS])
+        result = self.host.execute(
+            f'mkdir -p -- "$(dirname -- {path})" && touch -- {path} && '
+            f"grep -qxF {shlex.quote(_EXCLUDES_MARK)} {path} || "
+            f"cat >> {path}",
+            stdin=block,
+            timeout_s=TIMEOUT_S,
+        )
+        if not result.ok:
+            raise GitError(f"could not set excludes: {result.stderr.strip()}")
+
     def commit_all(self, message: str) -> str | None:
-        """Returns the new commit sha, or None when there was nothing to
-        commit — an empty milestone is a fact to report, not an error."""
+        """Commit the work: every change except build artifacts, which are
+        left out even when the project tracks them (#80). Returns the new
+        commit sha, or None when there was nothing to commit — an empty
+        milestone is a fact to report, not an error."""
         if not self.has_changes():
             return None
-        self._git("add", "-A")
+        self._git("add", "-A", "--", ".", *(_pathspec(p) for p in ARTIFACT_PATTERNS))
+        staged = self._git("diff", "--cached", "--name-only")
+        if not staged:
+            return None
         self._git("commit", "-q", "-m", message)
         return self.head_sha()
 
+    def left_out(self) -> list[str]:
+        """Changes not in the last commit: build artifacts the harness left
+        out, to be reported rather than silently dropped (D4)."""
+        out = self._git("status", "--porcelain", "--untracked-files=all", raw=True)
+        return sorted(line[3:] for line in out.splitlines() if line)
+
     def push(
         self, remote_url: str, branch: str, *, credential: PushCredential | None = None
     ) -> None:
diff --git a/nisse-dev/src/nisse_dev/publish.py b/nisse-dev/src/nisse_dev/publish.py
index a23c3ee..219eb50 100644
--- a/nisse-dev/src/nisse_dev/publish.py
+++ b/nisse-dev/src/nisse_dev/publish.py
@@ -7,6 +7,7 @@ PR exists rather than repeating the push-and-open — the spec's own example of
 an idempotent transition reconciled against external truth.
 """
 
+from collections.abc import Sequence
 from dataclasses import dataclass
 
 from nisse_dev.git import GitRepo, PushCredential
@@ -80,6 +81,8 @@ class MilestonePublisher:
                 return PublishResult(None, "no changes to publish")
             # The executor committed its work itself; publish that.
             sha = self._git.head_sha()
+        # D4: what was left out (build artifacts) is reported, not dropped.
+        left_out = self._git.left_out()
 
         intent_id = self._store.begin_intent(
             OPEN_PR_INTENT,
@@ -99,17 +102,19 @@ class MilestonePublisher:
             return PublishResult(
                 existing,
                 f"PR #{existing.number} was already open for this milestone; "
-                f"pushed to it: {existing.url}",
+                f"pushed to it: {existing.url}" + _left_out_note(left_out),
             )
         pr = self._client.open_pull_request(
             self._repo,
             head=branch,
             base=self._base_branch,
             title=f"Milestone {milestone}: {project}",
-            body=_pr_body(milestone, plan, service_url),
+            body=_pr_body(milestone, plan, service_url, left_out),
         )
         self._store.commit_intent(intent_id, cause=f"opened PR #{pr.number}")
-        return PublishResult(pr, f"opened PR #{pr.number}: {pr.url}")
+        return PublishResult(
+            pr, f"opened PR #{pr.number}: {pr.url}" + _left_out_note(left_out)
+        )
 
     def published_pull_request(
         self, task_id: str, milestone: int
@@ -161,7 +166,21 @@ class MilestonePublisher:
         return sha
 
 
-def _pr_body(milestone: int, plan: str, service_url: str | None) -> str:
+def _left_out_note(paths: list[str]) -> str:
+    if not paths:
+        return ""
+    shown = ", ".join(paths[:5]) + (
+        f" and {len(paths) - 5} more" if len(paths) > 5 else ""
+    )
+    return f" (left out {len(paths)} build artifact(s): {shown})"
+
+
+def _pr_body(
+    milestone: int,
+    plan: str,
+    service_url: str | None,
+    left_out: Sequence[str] = (),
+) -> str:
     parts = [
         f"Milestone {milestone}, implemented by nisse-dev against the approved plan.",
         "",
@@ -174,4 +193,13 @@ def _pr_body(milestone: int, plan: str, service_url: str | None) -> str:
         # so a reviewer can exercise it. Nothing reports one yet; the browser
         # tool (D7) is what will.
         parts += ["", "## Running test service", "", service_url]
+    if left_out:
+        parts += [
+            "",
+            "## Left out",
+            "",
+            "Build artifacts in the workspace, never part of the work:",
+            "",
+            *(f"- `{path}`" for path in left_out),
+        ]
     return "\n".join(parts)
diff --git a/nisse-dev/tests/test_git.py b/nisse-dev/tests/test_git.py
index a2b8e5e..369b23a 100644
--- a/nisse-dev/tests/test_git.py
+++ b/nisse-dev/tests/test_git.py
@@ -195,3 +195,52 @@ def test_an_https_token_answers_git_through_the_helper(
     assert asked.stdout == "username=[redacted]\npassword=[redacted]\n"
     stored = ssh_host.execute(f"{body} store", secrets={"NISSE_PUSH_TOKEN": "x"})
     assert stored.stdout == ""
+
+
+# --- build artifacts are never the work (#80, D4) ----------------------------
+
+
+def test_artifacts_are_left_out_even_when_the_project_tracks_them(
+    repo: GitRepo,
+) -> None:
+    # The sandbox's main tracks .pyc files (an earlier run published them).
+    cache = repo.root / "pkg" / "__pycache__"
+    cache.mkdir(parents=True)
+    (repo.root / "pkg" / "m.py").write_text("x = 1\n")
+    (cache / "m.cpython-312.pyc").write_bytes(b"old")
+    repo._git("add", "-A")
+    repo._git("commit", "-qm", "a project that tracks its caches")
+
+    # A test run: the tracked .pyc changes, new caches appear, and the work.
+    (cache / "m.cpython-312.pyc").write_bytes(b"new")
+    (cache / "m.cpython-310.pyc").write_bytes(b"x")
+    (repo.root / ".pytest_cache").mkdir()
+    (repo.root / ".pytest_cache" / "v").write_text("x")
+    (repo.root / "pkg" / "m.py").write_text("x = 2\n")
+
+    assert repo.commit_all("the work") is not None
+    committed = repo._git("show", "--name-only", "--format=", "HEAD").split()
+    assert committed == ["pkg/m.py"]
+    assert repo.left_out() == [
+        ".pytest_cache/v",
+        "pkg/__pycache__/m.cpython-310.pyc",
+        "pkg/__pycache__/m.cpython-312.pyc",
+    ]
+
+
+def test_only_artifacts_changed_means_nothing_to_commit(repo: GitRepo) -> None:
+    (repo.root / "__pycache__").mkdir()
+    (repo.root / "__pycache__" / "a.pyc").write_bytes(b"x")
+    assert repo.commit_all("nothing real") is None
+
+
+def test_excludes_hide_new_artifacts_and_install_once(repo: GitRepo) -> None:
+    repo.install_excludes()
+    repo.install_excludes()
+    exclude = (repo.root / ".git" / "info" / "exclude").read_text()
+    assert exclude.count("__pycache__/") == 1
+    (repo.root / "node_modules").mkdir()
+    (repo.root / "node_modules" / "x.js").write_text("x")
+    (repo.root / "real.py").write_text("x")
+    # Even the model's own `git add -A` now skips them.
+    assert repo._git("status", "--porcelain", "--untracked-files=all") == "?? real.py"
diff --git a/nisse-dev/tests/test_publish.py b/nisse-dev/tests/test_publish.py
index 5e744d2..234158d 100644
--- a/nisse-dev/tests/test_publish.py
+++ b/nisse-dev/tests/test_publish.py
@@ -90,6 +90,9 @@ class FakeGit:
     def commit_all(self, message: str) -> str | None:
         return self.repo.commit_all(message)
 
+    def left_out(self) -> list[str]:
+        return self.repo.left_out()
+
     def push(self, remote_url: str, branch: str, *, credential: object = None) -> None:
         self.pushed.append(branch)
         self.remote[branch] = self.repo.head_sha()
@@ -147,6 +150,23 @@ def test_a_branch_named_after_the_task_does_not_block_publishing(
     assert git.pushed == ["demo-abc-milestone-1"]
 
 
+def test_left_out_artifacts_are_reported_not_published(
+    store: StateStore, tmp_path: Path
+) -> None:
+    git = FakeGit(tmp_path / "work")
+    (tmp_path / "work" / "__pycache__").mkdir()
+    (tmp_path / "work" / "__pycache__" / "work.cpython-310.pyc").write_bytes(b"x")
+    client = FakeGitea()
+    result = build(store, git, client).publish(
+        task_id="t", milestone=1, project="demo", plan="p"
+    )
+    assert "left out 1 build artifact(s): __pycache__/work.cpython-310.pyc" in (
+        result.note
+    )
+    assert "## Left out" in client.opened[0]["body"]
+    assert "`__pycache__/work.cpython-310.pyc`" in client.opened[0]["body"]
+
+
 def test_intent_is_recorded_then_committed(store: StateStore, tmp_path: Path) -> None:
     build(store, FakeGit(tmp_path / "work"), FakeGitea()).publish(
         task_id="t", milestone=1, project="demo", plan="p"