diff --git a/cron/scheduler.py b/cron/scheduler.py
index d9ed063f7b..6a3984235c 100644
--- a/cron/scheduler.py
+++ b/cron/scheduler.py
@@ -2373,9 +2373,10 @@ def run_job(
         # No audit row when we failed before the agent existed; the audit write must never raise.
         if _audit is not None:
             _audit.write({}, error_msg)
+        from cron.scheduler_diagnostics import format_run_error
         output = (
             _run_doc_header(job, f"{job_name} (FAILED)", job_id, prompt)
-            + f"## Error\n\n```\n{error_msg}\n```\n"
+            + format_run_error(e)
         )
         return False, output, "", error_msg
 
diff --git a/cron/scheduler_diagnostics.py b/cron/scheduler_diagnostics.py
new file mode 100644
index 0000000000..e2160b14f0
--- /dev/null
+++ b/cron/scheduler_diagnostics.py
@@ -0,0 +1,13 @@
+"""Private run-document diagnostics, kept separate from delivery summaries."""
+
+from traceback import format_exception
+
+from agent.redact import redact_sensitive_text
+
+
+def format_run_error(exc: BaseException) -> str:
+    """Retain chained causes without capturing locals or exposing URL credentials."""
+    traceback_text = redact_sensitive_text(
+        "".join(format_exception(exc)), force=True, redact_url_credentials=True,
+    )
+    return f"## Error\n\n```\n{traceback_text}\n```\n"
diff --git a/evals/cron_error_diagnostics.py b/evals/cron_error_diagnostics.py
new file mode 100644
index 0000000000..668f08b5df
--- /dev/null
+++ b/evals/cron_error_diagnostics.py
@@ -0,0 +1,78 @@
+"""Local-I/O A/B for #104538; no inference, external APIs, or deliveries.
+
+Run with the project Python and pass a checkout to probe (defaults to this tree).
+Exit 1 on the unfixed checkout: the persisted traceback and listed reason are absent.
+The injected provider-setup seam performs a real SDK connection to a non-listening
+loopback socket; this does not diagnose the original macOS/Gmail incident.
+"""
+
+import contextlib
+import io
+import json
+import os
+from pathlib import Path
+import socket
+import sys
+import tempfile
+from unittest.mock import patch
+
+
+def main():
+    checkout = Path(sys.argv[1]).resolve() if len(sys.argv) > 1 else Path(__file__).resolve().parents[1]
+    for key in list(os.environ):
+        if key.startswith(("HERMES_", "OPENAI_", "OPENROUTER_", "ANTHROPIC_")) or any(
+            part in key for part in ("API_KEY", "TOKEN", "SECRET", "PASSWORD")
+        ):
+            os.environ.pop(key, None)
+    with tempfile.TemporaryDirectory(prefix="cron-diagnostics-") as home:
+        os.environ.update(HOME=home, HERMES_HOME=home)
+        sys.path.insert(0, str(checkout))
+        from cron import jobs, scheduler
+        from hermes_cli.cli_commands_mixin import CLICommandsMixin
+        from openai import OpenAI
+        from tools.cronjob_job_args import _format_job
+
+        with socket.socket() as held:
+            held.bind(("127.0.0.1", 0))
+            port = held.getsockname()[1]
+
+            def connection_failure(*args):
+                with OpenAI(api_key="local-probe", base_url=f"http://127.0.0.1:{port}/v1", max_retries=0, timeout=1) as client:
+                    try:
+                        client.chat.completions.create(model="local-probe", messages=[{"role": "user", "content": "probe"}])
+                    except Exception as exc:
+                        raise RuntimeError("Connection error.") from exc
+
+            job = jobs.create_job(prompt="Check status", schedule="every 1h", model="local-probe", deliver="local")
+            with patch.object(scheduler, "_resolve_cron_agent_setup", connection_failure):
+                success, output, response, error = scheduler.run_job(job)
+            path = jobs.save_job_output(job["id"], output)
+            saved = path.read_text(encoding="utf-8")
+            jobs.mark_job_run(job["id"], success, error=error)
+            formatted = _format_job(jobs.get_job(job["id"]))
+            console = io.StringIO()
+            with contextlib.redirect_stdout(console):
+                CLICommandsMixin._cron_list(object(), "list", {"all": False})
+            checks = {
+                "persisted_traceback": "Traceback (most recent call last)" in saved,
+                "persisted_connection_cause": "Connection refused" in saved,
+                "listed_run_reason": formatted.get("last_error") == error,
+                "slash_list_reason": f"error: {error}" in console.getvalue(),
+                "concise_return": not success and not response and error == "RuntimeError: Connection error.",
+                "separate_failure_fields": formatted["last_delivery_error"] is None and formatted["last_fire_error"] is None,
+                "private_output": os.name == "nt" or path.stat().st_mode & 0o077 == 0,
+            }
+            script = Path(home) / "scripts" / "healthy.py"
+            script.parent.mkdir(parents=True, exist_ok=True)
+            script.write_text("print('healthy local control')\n", encoding="utf-8")
+            healthy = jobs.create_job(prompt="Local control", schedule="every 1h", script=str(script), no_agent=True, deliver="local")
+            ok, _, final, healthy_error = scheduler.run_job(healthy)
+            checks["healthy_run"] = ok and "healthy local control" in final and healthy_error is None
+            jobs.mark_job_run(job["id"], True)
+            checks["success_clears_error"] = jobs.get_job(job["id"])["last_error"] is None
+            print(json.dumps({"checkout": str(checkout), "checks": checks}, indent=2))
+            assert all(checks.values()), checks
+
+
+if __name__ == "__main__":
+    main()
diff --git a/hermes_cli/cli_commands_mixin.py b/hermes_cli/cli_commands_mixin.py
index 7061fdcc56..4355234651 100644
--- a/hermes_cli/cli_commands_mixin.py
+++ b/hermes_cli/cli_commands_mixin.py
@@ -1686,6 +1686,8 @@ class CLICommandsMixin:
                 # in last_delivery_error (last_error is None).
                 if status == "delivery_failed" and job.get("last_delivery_error"):
                     status = f"delivery_failed: {job['last_delivery_error']}"
+                elif status == "error" and job.get("last_error"):
+                    status = f"error: {job['last_error']}"
                 print(f"  Last run: {job['last_run_at']} ({status})")
             print()
 
diff --git a/tests/cron/test_run_error_diagnostics.py b/tests/cron/test_run_error_diagnostics.py
new file mode 100644
index 0000000000..f35ae3c673
--- /dev/null
+++ b/tests/cron/test_run_error_diagnostics.py
@@ -0,0 +1,40 @@
+"""Failure detail belongs in the local audit, not the delivery summary."""
+from cron import jobs, scheduler
+
+
+def test_run_error_persists_redacted_cause_but_returns_summary(tmp_path, monkeypatch):
+    def fail_setup(*args):
+        try:
+            raise ConnectionError("https://user:password@localhost/api?token=secret-token")
+        except ConnectionError as cause:
+            raise RuntimeError("Connection error.") from cause
+
+    monkeypatch.setattr(scheduler, "_resolve_cron_agent_setup", fail_setup)
+    with jobs.use_cron_store(tmp_path):
+        job = jobs.create_job(prompt="Check status", schedule="every 1h", model="local-probe", deliver="local")
+        success, output, response, error = scheduler.run_job(job)
+        saved = jobs.save_job_output(job["id"], output).read_text(encoding="utf-8")
+        assert not success and response == "" and error == "RuntimeError: Connection error."
+        assert "Traceback (most recent call last)" in saved and "fail_setup" in saved
+        assert "ConnectionError" in saved and "localhost" in saved
+        assert "password" not in saved and "secret-token" not in saved
+
+
+def test_list_exposes_run_error_and_clears_it_after_success(tmp_path, capsys):
+    from hermes_cli.cli_commands_mixin import CLICommandsMixin
+    from tools.cronjob_job_args import _format_job
+
+    with jobs.use_cron_store(tmp_path):
+        job = jobs.create_job(prompt="Check status", schedule="every 1h", model="local-probe", deliver="local")
+        reason = "RuntimeError: https://user:password@localhost/api?token=secret-token"
+        jobs.mark_job_run(job["id"], False, error=reason)
+        displayed = _format_job(jobs.get_job(job["id"]))
+        assert displayed["last_error"].startswith("RuntimeError:") and "localhost" in displayed["last_error"]
+        assert "password" not in displayed["last_error"] and "secret-token" not in displayed["last_error"]
+        assert displayed["last_delivery_error"] is None and displayed["last_fire_error"] is None
+        CLICommandsMixin._cron_list(object(), "list", {"all": False})
+        console = capsys.readouterr().out
+        assert f"error: {displayed['last_error']}" in console
+        assert "password" not in console and "secret-token" not in console
+        jobs.mark_job_run(job["id"], True)
+        assert _format_job(jobs.get_job(job["id"]))["last_error"] is None
diff --git a/tools/cronjob_job_args.py b/tools/cronjob_job_args.py
index 11ad235cb7..aaa9503f77 100644
--- a/tools/cronjob_job_args.py
+++ b/tools/cronjob_job_args.py
@@ -344,6 +344,8 @@ _FORMAT_JOB_OPTIONAL_KEYS = (
 
 
 def _format_job(job: Dict[str, Any]) -> Dict[str, Any]:
+    from agent.redact import redact_sensitive_text
+
     prompt = str(job.get("prompt") or "")
     skills = _canonical_skills(job.get("skill"), job.get("skills"))
     job_id = str(job.get("id") or "unknown")
@@ -366,6 +368,9 @@ def _format_job(job: Dict[str, Any]) -> Dict[str, Any]:
         "last_delivery_error": job.get("last_delivery_error"),
         "last_delivery_unverified": job.get("last_delivery_unverified"),
         "last_fire_error": job.get("last_fire_error"),
+        "last_error": redact_sensitive_text(
+            job["last_error"], force=True, redact_url_credentials=True,
+        ) if job.get("last_error") else job.get("last_error"),
         "enabled": job.get("enabled", True),
         # Derive from enabled so half-paused records never render as paused.
         "state": effective_job_state(job),
diff --git a/website/docs/user-guide/features/cron.md b/website/docs/user-guide/features/cron.md
index 397d06a5e0..e6b521984f 100644
--- a/website/docs/user-guide/features/cron.md
+++ b/website/docs/user-guide/features/cron.md
@@ -837,6 +837,20 @@ Cron jobs inherit your configured fallback providers and credential pool rotatio
 
 This means cron jobs that run at high frequency or during peak hours are more resilient — a single rate-limited key won't fail the entire run.
 
+## Run failures (`last_error`)
+
+A failed agent run records a concise `last_error`, visible in job listings and `/cron list`
+with credential patterns and URL credentials redacted (including previously stored errors).
+This is separate from `last_fire_error` (scheduler handoff) and `last_delivery_error` (delivery).
+Those fields can correctly be empty when the agent itself failed.
+
+For a connection failure, inspect the run document under `cron/output/<job_id>/` in the active
+Hermes home. Its `## Error` section includes the chained traceback, with credential patterns
+and URL credentials redacted. The file uses the existing private output-file permissions;
+traceback locals are not captured. Delivery notices and `last_error` retain the concise error,
+not the full traceback. Review diagnostics before sharing: redaction is not a guarantee that
+arbitrary application data is non-sensitive.
+
 ## Missed scheduled fires (`last_fire_error`)
 
 On hosted (managed-cron) deployments, a scheduled fire travels from the platform scheduler through the dashboard to the gateway's internal API server. If that final hand-off fails — the gateway process is down, or its API-server listener never started — the run never begins, so there is no execution record and no `last_status` to inspect. The tell-tale shape: the job works every time you trigger it manually, but never auto-fires.