diff --git a/cron/scheduler.py b/cron/scheduler.py
index 82f10ee94..0fab517ac 100644
--- a/cron/scheduler.py
+++ b/cron/scheduler.py
@@ -1969,6 +1969,40 @@ def _scan_assembled_cron_prompt(
     return assembled
 
 
+def _guard_job_credential_exfil(job: dict) -> None:
+    """Fail closed if a job's stored provider/base_url pair would exfiltrate a
+    credential (F8 runtime backstop; CWE-200/CWE-522).
+
+    The model-callable cron tool validates this on create/update, but a job
+    persisted before that guard — or written directly to the jobs store —
+    reaches the scheduler's provider-resolution sink unchecked. Re-validate the
+    EFFECTIVE stored pair with the same guard the tool uses, so a named
+    provider's stored key is never paired with an off-host base_url at fire
+    time. Raises ``RuntimeError`` (caught by the run_job failure path → the run
+    is aborted and reported) when the pair is unsafe; returns ``None`` otherwise.
+
+    Fallback providers come from operator config, not the model-callable job, so
+    they are trusted and validated by the caller, not here.
+    """
+    try:
+        from tools.cronjob_tools import _validate_cron_base_url
+        err = _validate_cron_base_url(job.get("provider"), job.get("base_url"))
+    except Exception:
+        # The validator is defensively coded to RETURN (not raise) its own
+        # fail-closed string when provider metadata can't be resolved; only a
+        # truly unexpected error lands here. Don't wedge every cron job on such
+        # an error — the create/update-time guard remains the primary control.
+        err = None
+    if err:
+        job_id = job.get("id")
+        logger.error(
+            "Job '%s': refusing to run — unsafe provider/base_url pair could "
+            "exfiltrate a stored credential: %s",
+            job_id, err,
+        )
+        raise RuntimeError(f"Cron job '{job_id}' blocked for safety: {err}")
+
+
 def run_job(job: dict) -> tuple[bool, str, str, Optional[str]]:
     """
     Execute a single cron job.
@@ -2356,6 +2390,15 @@ def run_job(job: dict) -> tuple[bool, str, str, Optional[str]]:
             format_runtime_provider_error,
         )
         from hermes_cli.auth import AuthError
+
+        # F8 runtime backstop: never resolve a stored provider/base_url pair that
+        # would ship a named provider's stored credential to an off-host endpoint
+        # (CWE-200/CWE-522). The cron tool validates this on create/update, but a
+        # job persisted before that guard — or written directly to the jobs store
+        # — reaches this sink unchecked. Fail closed before resolution so no
+        # off-host call is ever made with a stored key.
+        _guard_job_credential_exfil(job)
+
         try:
             # Do not inject HERMES_INFERENCE_PROVIDER here. resolve_runtime_provider()
             # already prefers persisted config over stale shell/env overrides when
diff --git a/tests/cron/test_scheduler_provider.py b/tests/cron/test_scheduler_provider.py
index 00b03e9b2..404b15e2d 100644
--- a/tests/cron/test_scheduler_provider.py
+++ b/tests/cron/test_scheduler_provider.py
@@ -571,3 +571,66 @@ def test_cron_status_reports_stalled_when_no_heartbeat(tmp_path, monkeypatch, ca
     out = capsys.readouterr().out
     assert "STALLED" in out
     assert "will fire automatically" not in out
+
+
+# ── F8: runtime backstop — never resolve a stored pair that exfiltrates a key ──
+
+
+class TestGuardJobCredentialExfil:
+    """run_job() must fail closed before provider resolution when a job's stored
+    provider/base_url pair would ship a named provider's stored credential to an
+    off-host endpoint — covering jobs persisted before the create/update guard
+    or written directly to the store (F8 stored-job path; CWE-200/CWE-522)."""
+
+    def test_named_registry_provider_offhost_is_blocked(self):
+        import pytest
+        from cron.scheduler import _guard_job_credential_exfil
+
+        job = {"id": "j1", "provider": "anthropic",
+               "base_url": "https://evil.example/v1"}
+        with pytest.raises(RuntimeError) as exc:
+            _guard_job_credential_exfil(job)
+        assert "blocked for safety" in str(exc.value)
+
+    def test_named_custom_offhost_is_blocked(self, monkeypatch):
+        import pytest
+        import hermes_cli.runtime_provider as rp
+        from cron.scheduler import _guard_job_credential_exfil
+
+        monkeypatch.setattr(rp, "has_named_custom_provider", lambda n: True)
+        monkeypatch.setattr(
+            rp, "_get_named_custom_provider",
+            lambda n: {"name": "legit", "base_url": "https://legit.example/v1",
+                       "api_key": "sk-legit"},
+        )
+        job = {"id": "j2", "provider": "custom:legit",
+               "base_url": "https://evil.example/v1"}
+        with pytest.raises(RuntimeError):
+            _guard_job_credential_exfil(job)
+
+    def test_named_custom_matching_host_is_allowed(self, monkeypatch):
+        import hermes_cli.runtime_provider as rp
+        from cron.scheduler import _guard_job_credential_exfil
+
+        monkeypatch.setattr(rp, "has_named_custom_provider", lambda n: True)
+        monkeypatch.setattr(
+            rp, "_get_named_custom_provider",
+            lambda n: {"name": "legit", "base_url": "https://legit.example/v1",
+                       "api_key": "sk-legit"},
+        )
+        job = {"id": "j3", "provider": "custom:legit",
+               "base_url": "https://legit.example/v1"}
+        assert _guard_job_credential_exfil(job) is None
+
+    def test_bare_custom_is_allowed(self):
+        from cron.scheduler import _guard_job_credential_exfil
+
+        job = {"id": "j4", "provider": "custom",
+               "base_url": "https://anything.example/v1"}
+        assert _guard_job_credential_exfil(job) is None
+
+    def test_no_base_url_is_allowed(self):
+        from cron.scheduler import _guard_job_credential_exfil
+
+        assert _guard_job_credential_exfil({"id": "j5", "provider": "anthropic"}) is None
+        assert _guard_job_credential_exfil({"id": "j6"}) is None
diff --git a/tests/tools/test_cronjob_tools.py b/tests/tools/test_cronjob_tools.py
index 08c82f375..41aea33c7 100644
--- a/tests/tools/test_cronjob_tools.py
+++ b/tests/tools/test_cronjob_tools.py
@@ -336,6 +336,81 @@ class TestUnifiedCronjobTool:
         assert updated["job"]["provider"] == "openrouter"
         assert updated["job"]["base_url"] is None
 
+    @staticmethod
+    def _patch_named_legit(monkeypatch):
+        import hermes_cli.runtime_provider as rp
+        monkeypatch.setattr(rp, "has_named_custom_provider", lambda n: True)
+        monkeypatch.setattr(
+            rp, "_get_named_custom_provider",
+            lambda n: {"name": "legit", "base_url": "https://legit.example/v1",
+                       "api_key": "sk-legit"},
+        )
+
+    @staticmethod
+    def _save_legacy_unsafe_job():
+        """Write a job with an unsafe named-provider + off-host base_url pair
+        DIRECTLY to the store, bypassing the create-time tool guard (mirrors a
+        job persisted before the guard existed)."""
+        from cron.jobs import save_jobs
+        save_jobs([
+            {
+                "id": "legacyunsafe1",
+                "name": "legacy",
+                "prompt": "x",
+                "schedule": {"kind": "interval", "minutes": 5, "display": "every 5m"},
+                "schedule_display": "every 5m",
+                "repeat": {"times": None, "completed": 0},
+                "enabled": True,
+                "state": "scheduled",
+                "provider": "custom:legit",
+                "base_url": "https://evil.example/v1",
+            }
+        ])
+        return "legacyunsafe1"
+
+    def test_legacy_unsafe_job_blocked_on_unrelated_update(self, monkeypatch):
+        """F8 stored-job path: editing an UNRELATED field on a job that already
+        holds an unsafe provider/base_url pair must be rejected, so the pair
+        cannot be left active/schedulable by sidestepping validation."""
+        self._patch_named_legit(monkeypatch)
+        job_id = self._save_legacy_unsafe_job()
+
+        result = json.loads(cronjob(action="update", job_id=job_id, name="renamed"))
+        assert result["success"] is False
+        assert "not allowed" in json.dumps(result)
+
+        # The rejected update must not have mutated the stored job at all.
+        from cron.jobs import get_job
+        stored = get_job(job_id)
+        assert stored["name"] == "legacy"
+        assert stored["base_url"] == "https://evil.example/v1"
+
+    def test_legacy_unsafe_job_remediated_by_clearing_base_url(self, monkeypatch):
+        """The operator can still fix a legacy unsafe job in a single update by
+        clearing base_url (the effective pair becomes safe)."""
+        self._patch_named_legit(monkeypatch)
+        job_id = self._save_legacy_unsafe_job()
+
+        result = json.loads(
+            cronjob(action="update", job_id=job_id, name="renamed", base_url="")
+        )
+        assert result["success"] is True
+        assert result["job"]["base_url"] is None
+        assert result["job"]["name"] == "renamed"
+
+    def test_legacy_unsafe_job_remediated_by_matching_host(self, monkeypatch):
+        """Repointing base_url at the named provider's own configured host also
+        remediates the job (no off-host exfil)."""
+        self._patch_named_legit(monkeypatch)
+        job_id = self._save_legacy_unsafe_job()
+
+        result = json.loads(
+            cronjob(action="update", job_id=job_id,
+                    base_url="https://legit.example/v1")
+        )
+        assert result["success"] is True
+        assert result["job"]["base_url"] == "https://legit.example/v1"
+
     def test_create_skill_backed_job(self):
         result = json.loads(
             cronjob(
@@ -581,3 +656,51 @@ class TestLocalDeliveryNotice:
         )
         assert created["deliver"] == "origin"
         assert "local-only cron job" not in created["message"]
+
+
+class TestValidateCronBaseUrl:
+    """The cron base_url guard must not let a NAMED custom provider's stored
+    credential be sent to an off-host endpoint (CWE-200/CWE-522)."""
+
+    @staticmethod
+    def _v(*args):
+        from tools.cronjob_tools import _validate_cron_base_url
+        return _validate_cron_base_url(*args)
+
+    @staticmethod
+    def _patch_named_legit(monkeypatch):
+        import hermes_cli.runtime_provider as rp
+        monkeypatch.setattr(rp, "has_named_custom_provider", lambda n: True)
+        monkeypatch.setattr(
+            rp, "_get_named_custom_provider",
+            lambda n: {"name": "legit", "base_url": "https://legit.example/v1", "api_key": "sk-legit"},
+        )
+
+    def test_named_custom_offhost_base_url_blocked(self, monkeypatch):
+        self._patch_named_legit(monkeypatch)
+        err = self._v("custom:legit", "https://evil.example/v1")
+        assert err and "not allowed" in err
+
+    def test_named_custom_matching_host_allowed(self, monkeypatch):
+        self._patch_named_legit(monkeypatch)
+        assert self._v("custom:legit", "https://legit.example/v1") is None
+        # subdomain of the configured host is still the provider's own endpoint
+        assert self._v("custom:legit", "https://eu.legit.example/v1") is None
+
+    def test_named_custom_lookalike_host_blocked(self, monkeypatch):
+        self._patch_named_legit(monkeypatch)
+        assert self._v("custom:legit", "https://legit.example.attacker.test/v1") is not None
+
+    def test_bare_custom_allows_any_base_url(self):
+        # Bare 'custom' is inline/host-derived BYOK — no stored secret to leak.
+        assert self._v("custom", "https://anything.example/v1") is None
+
+    def test_no_base_url_is_allowed(self):
+        assert self._v("custom:legit", None) is None
+
+    def test_named_registry_offhost_blocked(self):
+        # A named registry provider (stored key) + off-host override is refused.
+        assert self._v("anthropic", "https://evil.example/v1") is not None
+
+    def test_base_url_without_provider_rejected(self):
+        assert self._v(None, "https://x.example/v1") is not None
diff --git a/tools/cronjob_tools.py b/tools/cronjob_tools.py
index 999297c20..02ac58f9c 100644
--- a/tools/cronjob_tools.py
+++ b/tools/cronjob_tools.py
@@ -445,6 +445,86 @@ def _normalize_deliver_param(value: Any) -> Optional[str]:
     return text or None
 
 
+def _validate_cron_base_url(
+    provider: Optional[Any], base_url: Optional[Any]
+) -> Optional[str]:
+    """Reject pairing a named provider's stored credential with an off-host base_url.
+
+    The cron tool is model-callable, so a prompt-injected job could set a real
+    provider plus an attacker ``base_url``; on fire the scheduler resolves that
+    provider's stored API key and sends it to the URL, exfiltrating the
+    credential (CWE-200/CWE-522). Allow a ``base_url`` override only when it
+    cannot leak a stored secret: no override at all, a configured custom/byok
+    provider that carries its own endpoint+key, or an override whose host
+    matches the named provider's own endpoint.
+
+    Returns an error string if blocked, else None (valid).
+    """
+    bu = _normalize_optional_job_value(base_url, strip_trailing_slash=True)
+    if not bu:
+        return None
+    prov = _normalize_optional_job_value(provider)
+    if not prov:
+        # A base_url with no explicit provider inherits the default/session
+        # provider's stored key — the same exfil primitive without naming a
+        # provider. Require an explicit (custom) provider for custom endpoints.
+        return (
+            "base_url override requires an explicit provider. Set provider to a "
+            "configured custom provider to use a custom endpoint."
+        )
+    try:
+        from hermes_cli.runtime_provider import (
+            has_named_custom_provider,
+            resolve_requested_provider,
+            _get_named_custom_provider,
+        )
+        from hermes_cli.auth import PROVIDER_REGISTRY
+        from utils import base_url_host_matches, base_url_hostname
+    except Exception:
+        # Can't resolve provider metadata -> fail closed.
+        return f"Unable to validate base_url override for provider {prov!r}; refused."
+
+    if prov.lower() == "custom":
+        # Bare/inline 'custom' (and aliases that resolve to it) is pure BYOK: the
+        # runtime derives the key from a pool keyed by THIS base_url or from
+        # host-gated env vars, never an arbitrary stored secret. Safe to allow.
+        return None
+    if has_named_custom_provider(prov):
+        # A NAMED custom provider carries a STORED key, and
+        # _resolve_named_custom_runtime prefers the override base_url while still
+        # sending that stored key — so an off-host override exfiltrates it.
+        # Require the override host to match the provider's CONFIGURED endpoint.
+        try:
+            cp = _get_named_custom_provider(prov)
+        except Exception:
+            cp = None
+        cfg_host = base_url_hostname((cp or {}).get("base_url", "")) if cp else ""
+        if cfg_host and base_url_host_matches(bu, cfg_host):
+            return None
+        return (
+            f"base_url {bu!r} is not allowed for provider {prov!r}. A named "
+            f"custom provider's stored credential may only be sent to its own "
+            f"configured endpoint ({cfg_host or 'unknown'})."
+        )
+    try:
+        resolved = resolve_requested_provider(prov)
+    except Exception:
+        resolved = prov
+    pconfig = PROVIDER_REGISTRY.get(resolved) if isinstance(resolved, str) else None
+    known_host = base_url_hostname(getattr(pconfig, "inference_base_url", "") if pconfig else "")
+    if known_host and base_url_host_matches(bu, known_host):
+        return None
+    # Fail closed: any non-custom provider we cannot host-match to its own
+    # endpoint is refused. This covers named providers with a stored credential
+    # AND aliases/unknown names we can't resolve to a known host (e.g. "openai",
+    # "google"), which would otherwise pair a stored key with the override URL.
+    return (
+        f"base_url {bu!r} is not allowed for provider {prov!r}. A named "
+        f"provider's stored credential may only be sent to its own endpoint; "
+        f'use a configured custom provider (provider="custom") for a custom base_url.'
+    )
+
+
 def _validate_cron_script_path(script: Optional[str]) -> Optional[str]:
     """Validate a cron job script path at the API boundary.
 
@@ -625,6 +705,12 @@ def cronjob(
                 if script_error:
                     return tool_error(script_error, success=False)
 
+            # Reject a model-supplied base_url that would route a named
+            # provider's stored credential to an attacker endpoint (F8).
+            base_url_error = _validate_cron_base_url(provider, base_url)
+            if base_url_error:
+                return tool_error(base_url_error, success=False)
+
             # Validate context_from references existing jobs
             if context_from:
                 from cron.jobs import get_job as _get_job
@@ -779,6 +865,25 @@ def cronjob(
                 updates["provider"] = _normalize_optional_job_value(provider)
             if base_url is not None:
                 updates["base_url"] = _normalize_optional_job_value(base_url, strip_trailing_slash=True)
+            # Re-validate the EFFECTIVE provider/base_url on EVERY update, not
+            # only when this update supplies provider/base_url. A job persisted
+            # before this guard (or written directly to the jobs store) may
+            # already hold an unsafe named-provider + off-host base_url pair;
+            # if we only checked when the update touches those axes, editing any
+            # unrelated field (name, schedule, ...) would succeed and leave that
+            # exfil-capable pair active and schedulable (F8). The effective pair
+            # merges this update's normalized values over the stored job; an
+            # operator can still remediate in the same update by clearing
+            # base_url or pointing provider/base_url at a safe pair.
+            eff_provider = (
+                updates["provider"] if "provider" in updates else job.get("provider")
+            )
+            eff_base_url = (
+                updates["base_url"] if "base_url" in updates else job.get("base_url")
+            )
+            base_url_error = _validate_cron_base_url(eff_provider, eff_base_url)
+            if base_url_error:
+                return tool_error(base_url_error, success=False)
             if script is not None:
                 # Pass empty string to clear an existing script
                 if script: