diff --git a/src/openultrasast/cpg/backend.py b/src/openultrasast/cpg/backend.py
index dffead0..c0979cd 100644
--- a/src/openultrasast/cpg/backend.py
+++ b/src/openultrasast/cpg/backend.py
@@ -27,6 +27,7 @@ import shutil
 import signal
 import subprocess
 import tempfile
+import time
 from collections.abc import Callable, Mapping, Sequence
 from dataclasses import dataclass, field
 from pathlib import Path
@@ -188,6 +189,10 @@ class CpgResult:
     # remainder is a scan of LESS CODE THAN IT WAS ASKED ABOUT, and silence about those files is the same
     # quiet failure as an empty query result. The names travel with the CPG so the driver can say so.
     unparsed: tuple[str, ...] = ()
+    # Removes the scratch tree this CPG lives in. A build allocates a temp directory holding the graph, its
+    # request files and joern's own workspace copy; nothing reclaimed it, and a day of scanning left 1,055
+    # directories and 355MB behind on a disk that was already at 95%. The driver disposes in a `finally`.
+    cleanup: Callable[[], None] | None = None
 
 
 class CpgBackend(Protocol):
@@ -236,8 +241,10 @@ class JoernBackend:
         parse = shutil.which("joern-parse")
         if parse is None and os.environ.get("OPENULTRASAST_JOERN_PROBE", "").strip().lower() not in {"1", "on", "true", "yes"}:
             return None
-        scratch = Path(tempfile.mkdtemp(prefix="ousast-cpg-"))
+        _sweep_stale_scratch()
+        scratch = Path(tempfile.mkdtemp(prefix=SCRATCH_PREFIX))
         cpg_path = scratch / "cpg.bin"
+        dispose = lambda: shutil.rmtree(scratch, ignore_errors=True)  # noqa: E731 -- one expression, named
 
         # THE PRECONDITION. Before anything is built, make the toolchain open one file from the tree and say
         # how big it is. Every instrument failure this project has had takes the same shape -- the input was
@@ -269,6 +276,7 @@ class JoernBackend:
                     run=lambda query, params: self.query_across(shards, query, params),
                     run_batch=lambda query, requests: self.query_batch_across(shards, query, requests),
                     unparsed=unparsed,
+                    cleanup=dispose,
                 )
         command = [parse or "joern-parse", self._heap_flag(), str(root), "--output", str(cpg_path)]
         completed = self._run(command, timeout=self.build_timeout, cwd=scratch)
@@ -287,6 +295,7 @@ class JoernBackend:
             run=lambda query, params: self.query(cpg_path, query, params),
             run_batch=lambda query, requests: self.query_batch(cpg_path, query, requests),
             unparsed=unparsed,
+            cleanup=dispose,
         )
 
     def _files_with_frontend_defect(self, root: Path, language: str) -> tuple[str, ...]:
@@ -692,6 +701,33 @@ class JoernBackend:
             return None
 
 
+SCRATCH_PREFIX = "ousast-cpg-"
+# How long an abandoned scratch tree may survive. Long enough that a scan running in another process is never
+# disturbed, short enough that a machine scanning all day does not fill its disk.
+SCRATCH_MAX_AGE_SECONDS = 6 * 60 * 60
+
+
+def _sweep_stale_scratch() -> None:
+    """Remove scratch trees older than `SCRATCH_MAX_AGE_SECONDS`, best effort and never fatal.
+
+    Belt as well as braces: `cleanup` disposes of a scan's own tree, but a process killed mid-scan -- which
+    is how most of them died here -- never runs its `finally`. Only directories under the system temp root
+    carrying this module's own prefix are touched.
+    """
+    root = Path(tempfile.gettempdir())
+    cutoff = time.time() - SCRATCH_MAX_AGE_SECONDS
+    try:
+        candidates = list(root.glob(f"{SCRATCH_PREFIX}*"))
+    except OSError:
+        return
+    for path in candidates:
+        try:
+            if path.is_dir() and path.stat().st_mtime < cutoff:
+                shutil.rmtree(path, ignore_errors=True)
+        except OSError:  # noqa: PERF203 -- one unreadable entry must not stop the sweep
+            continue
+
+
 def _terminate_group(process: subprocess.Popen[str]) -> None:
     """SIGTERM the process group, then SIGKILL what is left. Never raises: this is cleanup, not logic."""
     for signal_number in (signal.SIGTERM, signal.SIGKILL):
diff --git a/src/openultrasast/model/scan.py b/src/openultrasast/model/scan.py
index a37a693..2bbba2c 100644
--- a/src/openultrasast/model/scan.py
+++ b/src/openultrasast/model/scan.py
@@ -277,6 +277,9 @@ def scan_repository(
     scanned = len(judged)
 
     findings = _ordered(_deduplicated(collected))
+    # The graph has answered everything it is going to. Its scratch tree holds the CPG, the request files and
+    # joern's own working copy -- tens of megabytes per scan -- and nothing else reclaims it.
+    _dispose(cpg)
     return ModelScanResult(
         findings=findings,
         by_rung=_tally(findings),
@@ -293,6 +296,16 @@ def scan_repository(
     )
 
 
+def _dispose(cpg: object) -> None:
+    """Release a built CPG's scratch tree. Never raises: cleanup must not turn a good scan into a failure."""
+    cleanup = getattr(cpg, "cleanup", None)
+    if callable(cleanup):
+        try:
+            cleanup()
+        except Exception as exc:  # noqa: BLE001 -- a scan that found things must not fail while tidying up
+            logger.warning("could not remove the cpg scratch directory: %s", exc)
+
+
 def _build(backend: Any, root: Path, language: str) -> Any:
     """Build through the backend, passing the language when the backend can use it."""
     try:
diff --git a/tests/test_cpg_backend.py b/tests/test_cpg_backend.py
index 3b2c4d5..f3ad8f9 100644
--- a/tests/test_cpg_backend.py
+++ b/tests/test_cpg_backend.py
@@ -55,6 +55,7 @@ def test_the_seam_imports_nothing_outside_the_standard_library() -> None:
         "signal",
         "subprocess",
         "tempfile",
+        "time",
         "dataclasses",
         "pathlib",
         "typing",
@@ -725,3 +726,62 @@ def test_a_timed_out_query_kills_the_jvm_not_just_the_script(tmp_path: Path) ->
         with contextlib.suppress(ProcessLookupError, PermissionError):
             os.kill(child, 9)
     assert not alive, "the grandchild JVM must die with the script, or every timeout leaks a gigabyte"
+
+
+def test_a_built_cpg_can_dispose_of_its_scratch_tree(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+    """Every build allocates a temp directory for the graph, its request files and joern's own working copy.
+
+    Nothing reclaimed it. A day of scanning left 1,055 directories and 355MB behind, on a disk already at
+    95%, and the scans that leaked hardest were the ones that failed -- which is the wrong way round.
+    """
+    import subprocess
+
+    from openultrasast.cpg.backend import JoernBackend
+
+    (tmp_path / "a.php").write_text("<?php function a() {}")
+
+    def runner(command, **kwargs):  # type: ignore[no-untyped-def]
+        if "-r" in command:
+            return subprocess.CompletedProcess(args=command, returncode=0, stdout="", stderr="")
+        if "--script" in command:
+            body = '---OUSAST-CPG-BEGIN---\n{"files": "1", "methods": "1"}\n---OUSAST-CPG-END---\n'
+            return subprocess.CompletedProcess(args=command, returncode=0, stdout=body, stderr="")
+        Path(command[command.index("-o") + 1]).write_text("cpg")
+        return subprocess.CompletedProcess(args=command, returncode=0, stdout="", stderr="")
+
+    monkeypatch.setenv("OPENULTRASAST_JOERN_PROBE", "on")
+    monkeypatch.setattr("shutil.which", lambda name: f"/opt/bin/{name}")
+
+    result = JoernBackend(runner=runner).build(tmp_path, language="php")
+    assert result is not None and callable(result.cleanup)
+
+    scratch = result.cpg_path.parent
+    assert scratch.is_dir()
+    result.cleanup()
+    assert not scratch.exists(), "the scratch tree is gone once the caller is done with the graph"
+
+
+def test_the_sweep_removes_only_old_scratch_and_only_ours(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+    """A process killed mid-scan never runs its cleanup, which is how most of the 1,055 died. The sweep is
+    the backstop -- and it must not touch a scan running in another process, nor anything that is not ours."""
+    import os
+    import time
+
+    from openultrasast.cpg.backend import SCRATCH_MAX_AGE_SECONDS, SCRATCH_PREFIX, _sweep_stale_scratch
+
+    monkeypatch.setattr("tempfile.gettempdir", lambda: str(tmp_path))
+    old = tmp_path / f"{SCRATCH_PREFIX}old"
+    fresh = tmp_path / f"{SCRATCH_PREFIX}fresh"
+    foreign = tmp_path / "someone-elses-work"
+    for path in (old, fresh, foreign):
+        path.mkdir()
+        (path / "cpg.bin").write_text("x")
+    stale = time.time() - SCRATCH_MAX_AGE_SECONDS - 60
+    os.utime(old, (stale, stale))
+    os.utime(foreign, (stale, stale))
+
+    _sweep_stale_scratch()
+
+    assert not old.exists(), "an abandoned tree older than the ceiling is reclaimed"
+    assert fresh.is_dir(), "a scan running right now is left alone"
+    assert foreign.is_dir(), "and nothing outside this module's own prefix is ever touched"