diff --git a/backend/apps/agents/core/error_classify.py b/backend/apps/agents/core/error_classify.py
index 275a4eca..173ece10 100644
--- a/backend/apps/agents/core/error_classify.py
+++ b/backend/apps/agents/core/error_classify.py
@@ -245,6 +245,21 @@ def p_get_transient_exc_types() -> Tuple[type, ...]:
     return p_transient_exc_types
 
 
+# A broken cert chain is deterministic per host (corporate TLS-inspection proxy, clock skew, stale
+# CA bundle): it never heals inside a retry schedule, so it must be checked BEFORE the exception-type
+# tuple, because the raising httpx.ConnectError subclasses the transient httpx.TransportError.
+CERT_FAILURE_PATTERNS = re.compile(
+    r"certificate\s+verify\s+failed|CERTIFICATE_VERIFY_FAILED|unable\s+to\s+get\s+local\s+issuer"
+    r"|self.signed\s+certificate|certificate\s+has\s+expired|hostname\s+mismatch",
+    re.IGNORECASE,
+)
+
+
+@typechecked
+def is_cert_failure(exc: BaseException, extra_text: str = "") -> bool:
+    return bool(CERT_FAILURE_PATTERNS.search(f"{exc!s}\n{extra_text}"))
+
+
 @typechecked
 def is_transient_capacity_error(exc: BaseException, extra_text: str = "") -> bool:
     # The Claude CLI's underlying ProcessError stringifies to a generic "Command failed with exit code 1 / Check stderr output for details"; the real cause (rate_limit_error / No pool capacity available / 429 / overloaded) only surfaces in the subprocess's stderr stream, which we capture via the SDK's `stderr` callback and pass in as extra_text. Classify against both so we catch capacity errors regardless of which channel carried the message.
@@ -252,6 +267,9 @@ def is_transient_capacity_error(exc: BaseException, extra_text: str = "") -> boo
     # An overflow can arrive dressed as a 429 ("request too large"); retrying the identical oversized request is guaranteed futile, the valve owns it.
     if is_context_overflow_error(exc, extra_text):
         return False
+    # 335s of retries cannot fix a certificate; the user has to (ENG-218, reproduced against badssl).
+    if is_cert_failure(exc, extra_text):
+        return False
     # A failure that names its own recovery window ("reset after 1m 57s") heals itself, even when
     # it's dressed as a 401; the reset hint outranks the auth-shaped non-transient veto (caught live).
     if combined and re.search(r"reset\s+after\s+\d", combined, re.IGNORECASE):
diff --git a/backend/apps/agents/manager/run/handle_run_error.py b/backend/apps/agents/manager/run/handle_run_error.py
index 3ebac036..9ab9f5b8 100644
--- a/backend/apps/agents/manager/run/handle_run_error.py
+++ b/backend/apps/agents/manager/run/handle_run_error.py
@@ -18,6 +18,7 @@ from backend.apps.agents.core.error_classify import (
     is_free_trial_exhausted,
     is_out_of_tokens,
     is_auth_error,
+    is_cert_failure,
     is_cli_binary_missing,
     is_unknown_model_error,
     parse_retry_after,
@@ -111,6 +112,22 @@ async def handle_run_error(e: Exception, session: AgentSession, session_id: str,
             })
         except Exception:
             logger.debug("submit_diagnostic for context_overflow failed", exc_info=True)
+    elif is_cert_failure(e, extra_text=p_stderr_tail):
+        # Deterministic per host: corporate TLS-inspection proxy, clock skew, or a stale CA bundle. Waiting can't fix any of them, so name the real remedies instead of a rate-limit-shaped pill (ENG-218).
+        friendly_msg = (
+            "OpenSwarm couldn't verify the AI provider's security certificate, so the "
+            "connection was refused. This usually means a corporate proxy or security "
+            "tool (Zscaler, Netskope) is inspecting your traffic, or your system clock "
+            "is wrong. Check the clock, try a different network, or ask IT to allow "
+            "api.anthropic.com; retrying won't help until one of those changes."
+        )
+        error_msg = Message(role="system", content=friendly_msg, branch_id=session.active_branch_id)
+        session.messages.append(error_msg)
+        await ws_manager.send_to_session(session_id, "agent:message", {
+            "session_id": session_id,
+            "message": error_msg.model_dump(mode="json"),
+        })
+        p_report_model_error("cert_failure", session_id, session, turn, e, p_stderr_tail)
     elif is_cli_binary_missing(e, extra_text=p_stderr_tail):
         # The bundled CLI vanished from an installed app (Windows AV quarantine class; 22 of 25 field installs never recovered). The raw "not found at: C:\..." card is unactionable; name the likely cause and the two real fixes.
         friendly_msg = (
diff --git a/backend/tests/test_error_classify.py b/backend/tests/test_error_classify.py
index bbc5d61c..afaf8d48 100644
--- a/backend/tests/test_error_classify.py
+++ b/backend/tests/test_error_classify.py
@@ -131,3 +131,16 @@ def test_first_real_exception_all_cancelled_is_none():
 def test_first_real_exception_plain_passthrough():
     boom = RuntimeError("x")
     assert first_real_exception(boom) is boom
+
+
+def test_cert_failure_is_never_transient():
+    import httpx
+    from backend.apps.agents.core.error_classify import is_cert_failure, is_transient_capacity_error
+    exc = httpx.ConnectError("[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self-signed certificate")
+    assert is_cert_failure(exc) is True
+    # httpx.ConnectError subclasses the transient TransportError; the cert check must win (ENG-218).
+    assert is_transient_capacity_error(exc) is False
+    for msg in ("unable to get local issuer certificate", "certificate has expired", "Hostname mismatch"):
+        assert is_transient_capacity_error(httpx.ConnectError(msg)) is False
+    # A cert-free transport hiccup keeps its transient classification.
+    assert is_transient_capacity_error(httpx.ConnectError("Connection refused")) is True