diff --git a/.github/workflows/opencode-review-dispatch.yml b/.github/workflows/opencode-review-dispatch.yml
index 3909ea3b9c..62afd062d3 100644
--- a/.github/workflows/opencode-review-dispatch.yml
+++ b/.github/workflows/opencode-review-dispatch.yml
@@ -595,6 +595,37 @@ jobs:
             exit 1
           fi
 
+      - name: Reclaim stale coverage images
+        # One OpenCode job runs per runner at a time, so coverage images older
+        # than two hours belong to finished or abandoned runs. Runs dispatched
+        # before per-run cleanup existed left them behind and filled the
+        # self-hosted guest disk. Failure here never blocks measurement.
+        run: |
+          set -uo pipefail
+          reclaim() {
+            docker image ls --filter reference=opencode-coverage-tools --filter until=2h --format '{{.ID}}' |
+              sort -u | while read -r image; do docker image rm -f "$image" || return 1; done &&
+              docker image prune -f &&
+              docker builder prune -f --filter until=24h
+          }
+          if ! reclaim; then
+            echo "::warning::Could not reclaim stale coverage images; continuing."
+          fi
+
+      - name: Expose runner Rust toolchain
+        # Self-hosted runners keep rustup's cargo in ~/.cargo/bin, which the
+        # runner service does not put on PATH. Hosted images already have it.
+        run: |
+          set -euo pipefail
+          if command -v cargo >/dev/null 2>&1; then
+            exit 0
+          fi
+          if [ -x "$HOME/.cargo/bin/cargo" ]; then
+            echo "$HOME/.cargo/bin" >>"$GITHUB_PATH"
+            exit 0
+          fi
+          echo "::warning::cargo is not installed on this runner; Rust coverage will report a missing toolchain."
+
       - name: Measure test and docstring evidence
         id: measure
         env:
diff --git a/CHANGELOG.d/20260929-coverage-runner-hygiene.md b/CHANGELOG.d/20260929-coverage-runner-hygiene.md
new file mode 100644
index 0000000000..1c37aeee44
--- /dev/null
+++ b/CHANGELOG.d/20260929-coverage-runner-hygiene.md
@@ -0,0 +1,8 @@
+## Fixed
+
+- Rust coverage on the self-hosted OpenCode runner can find `cargo` again. The
+  runner keeps it in `~/.cargo/bin`, which its service does not put on PATH,
+  so every Rust coverage gate failed and no fast-mlsirm review could approve.
+  A missing toolchain is now reported as such instead of a bare exception name.
+- Coverage jobs remove coverage images older than two hours before measuring.
+  Runs dispatched before per-run cleanup existed had filled the runner disk.
diff --git a/scripts/ci/materialize_base_rust_dependencies.py b/scripts/ci/materialize_base_rust_dependencies.py
index 4a46f232c8..f56871a083 100644
--- a/scripts/ci/materialize_base_rust_dependencies.py
+++ b/scripts/ci/materialize_base_rust_dependencies.py
@@ -446,6 +446,12 @@ def _lock_for(root: str) -> str:
             vendor_dir = output_dir / "vendor"
             try:
                 completed = _run_cargo_vendor(manifest_path, vendor_dir, sync_manifests)
+            except FileNotFoundError as exc:
+                raise RuntimeError(
+                    f"could not run trusted cargo vendor for base manifest {lock_path}: "
+                    "cargo is not installed or not on PATH on this runner "
+                    "(self-hosted runners keep it in ~/.cargo/bin)"
+                ) from exc
             except (OSError, subprocess.TimeoutExpired) as exc:
                 raise RuntimeError(
                     f"could not run trusted cargo vendor for base manifest {lock_path}: "
diff --git a/tests/test_materialize_base_rust_dependencies.py b/tests/test_materialize_base_rust_dependencies.py
index b912c4ea4c..d8b409d342 100644
--- a/tests/test_materialize_base_rust_dependencies.py
+++ b/tests/test_materialize_base_rust_dependencies.py
@@ -540,3 +540,33 @@ def test_materialize_rejects_bad_sha_and_symlinked_output_dir(tmp_path: Path) ->
     linked_output.symlink_to(real_dir)
     with pytest.raises(ValueError, match="must not be a symlink"):
         materializer.materialize(Path("/unused"), "a" * 40, linked_output)
+
+
+def test_missing_cargo_is_reported_as_a_runner_toolchain_gap(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch, capsys: pytest.CaptureFixture[str]
+) -> None:
+    """A runner without cargo on PATH gets an actionable message, not a bare exception name."""
+    repo = tmp_path / "repo"
+    _init_repo(repo)
+    (repo / "Cargo.toml").write_text(
+        '[package]\nname = "foo"\nversion = "0.1.0"\nedition = "2021"\n', encoding="utf-8"
+    )
+    (repo / "src").mkdir()
+    (repo / "src" / "lib.rs").write_text("pub fn x() {}\n", encoding="utf-8")
+    (repo / "Cargo.lock").write_text(
+        'version = 3\n\n[[package]]\nname = "foo"\nversion = "0.1.0"\n', encoding="utf-8"
+    )
+    base_sha = _commit_all(repo)
+
+    def no_cargo(*_args, **_kwargs):
+        raise FileNotFoundError(2, "No such file or directory", "cargo")
+
+    monkeypatch.setattr(materializer, "_run_cargo_vendor", no_cargo)
+    exit_code = materializer.main(
+        ["--repo-root", str(repo), "--base-sha", base_sha, "--output-dir", str(tmp_path / "out")]
+    )
+
+    assert exit_code == 1
+    err = capsys.readouterr().err
+    assert "cargo is not installed or not on PATH" in err
+    assert "~/.cargo/bin" in err
diff --git a/tests/test_opencode_coverage_runner_hygiene.py b/tests/test_opencode_coverage_runner_hygiene.py
new file mode 100644
index 0000000000..a1c1f74dcd
--- /dev/null
+++ b/tests/test_opencode_coverage_runner_hygiene.py
@@ -0,0 +1,97 @@
+"""Keep the self-hosted OpenCode coverage runner usable between jobs.
+
+On 2026-09-29 `cwlab-s1-04` filled its disk twice with per-job coverage images,
+and every Rust coverage run failed because `cargo` lives in `~/.cargo/bin`,
+which the runner service does not put on PATH.
+"""
+
+from __future__ import annotations
+
+import os
+import subprocess
+from pathlib import Path
+
+import yaml
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+WORKFLOW = REPO_ROOT / ".github/workflows/opencode-review-dispatch.yml"
+
+
+def _steps() -> list[dict]:
+    return yaml.safe_load(WORKFLOW.read_text(encoding="utf-8"))["jobs"]["coverage-evidence"]["steps"]
+
+
+def _step(name: str) -> dict:
+    return next(s for s in _steps() if s.get("name") == name)
+
+
+def _index(name: str) -> int:
+    return next(i for i, s in enumerate(_steps()) if s.get("name") == name)
+
+
+def _run(script: str, tmp_path: Path, *, home: Path, path_dirs: list[Path]) -> subprocess.CompletedProcess:
+    github_path = tmp_path / "github_path"
+    github_path.touch()
+    env = {
+        "HOME": str(home),
+        "PATH": os.pathsep.join([*map(str, path_dirs), "/usr/bin", "/bin"]),
+        "GITHUB_PATH": str(github_path),
+    }
+    return subprocess.run(["bash", "-c", script], env=env, capture_output=True, text=True)
+
+
+def _fake(bin_dir: Path, name: str, body: str) -> None:
+    bin_dir.mkdir(parents=True, exist_ok=True)
+    tool = bin_dir / name
+    tool.write_text("#!/bin/sh\n" + body, encoding="utf-8")
+    tool.chmod(0o755)
+
+
+def test_both_steps_run_before_coverage_measurement() -> None:
+    measure = _index("Measure test and docstring evidence")
+    assert _index("Reclaim stale coverage images") < measure
+    assert _index("Expose runner Rust toolchain") < measure
+
+
+def test_image_reclaim_targets_only_old_coverage_images(tmp_path: Path) -> None:
+    calls = tmp_path / "calls"
+    fake_bin = tmp_path / "bin"
+    _fake(fake_bin, "docker", f'echo "$*" >> {calls}\n[ "$1 $2" = "image ls" ] && echo abc123\nexit 0\n')
+    result = _run(_step("Reclaim stale coverage images")["run"], tmp_path, home=tmp_path, path_dirs=[fake_bin])
+    assert result.returncode == 0, result.stderr
+    log = calls.read_text(encoding="utf-8").splitlines()
+    assert "image ls --filter reference=opencode-coverage-tools --filter until=2h --format {{.ID}}" in log
+    assert "image rm -f abc123" in log
+    assert "image prune -f" in log
+    assert "builder prune -f --filter until=24h" in log
+    assert not any("prune -a" in line or "system prune" in line for line in log)
+
+
+def test_image_reclaim_failure_warns_without_blocking_coverage(tmp_path: Path) -> None:
+    fake_bin = tmp_path / "bin"
+    _fake(fake_bin, "docker", "exit 1\n")
+    result = _run(_step("Reclaim stale coverage images")["run"], tmp_path, home=tmp_path, path_dirs=[fake_bin])
+    assert result.returncode == 0
+    assert "::warning::" in result.stdout
+
+
+def test_home_cargo_is_added_to_github_path(tmp_path: Path) -> None:
+    home = tmp_path / "home"
+    _fake(home / ".cargo" / "bin", "cargo", "exit 0\n")
+    result = _run(_step("Expose runner Rust toolchain")["run"], tmp_path, home=home, path_dirs=[])
+    assert result.returncode == 0, result.stderr
+    assert (tmp_path / "github_path").read_text(encoding="utf-8") == f"{home}/.cargo/bin\n"
+
+
+def test_cargo_already_on_path_is_left_alone(tmp_path: Path) -> None:
+    fake_bin = tmp_path / "bin"
+    _fake(fake_bin, "cargo", "exit 0\n")
+    result = _run(_step("Expose runner Rust toolchain")["run"], tmp_path, home=tmp_path / "home", path_dirs=[fake_bin])
+    assert result.returncode == 0
+    assert (tmp_path / "github_path").read_text(encoding="utf-8") == ""
+
+
+def test_missing_cargo_warns_readably(tmp_path: Path) -> None:
+    result = _run(_step("Expose runner Rust toolchain")["run"], tmp_path, home=tmp_path / "home", path_dirs=[])
+    assert result.returncode == 0
+    assert "::warning::cargo is not installed" in result.stdout
diff --git a/tests/test_pr_review_autofix_nvidia_nim_contract.py b/tests/test_pr_review_autofix_nvidia_nim_contract.py
index 3c0594c6aa..a24da5731e 100644
--- a/tests/test_pr_review_autofix_nvidia_nim_contract.py
+++ b/tests/test_pr_review_autofix_nvidia_nim_contract.py
@@ -17,7 +17,7 @@
 DOCTORING_RECORD = Path("docs/doctoring/hourly-nvidia-nim-autofix.md")
 CHANGELOG = Path("CHANGELOG.md")
 REVIEW_DISPATCH_WORKFLOW = Path(".github/workflows/opencode-review-dispatch.yml")
-REVIEW_DISPATCH_BLOB_SHA = "3909ea3b9c285ba3dac09e0cccb6618fdb9adad6"
+REVIEW_DISPATCH_BLOB_SHA = "62afd062d3bc8571fb0d350572d80ce81a9ae048"
 
 
 def _workflow_text(path: Path) -> str: