diff --git a/packages/paperclip-runner/runner/crates/process-table/src/lib.rs b/packages/paperclip-runner/runner/crates/process-table/src/lib.rs
index 99d3af9d337..01dd6d51bd7 100644
--- a/packages/paperclip-runner/runner/crates/process-table/src/lib.rs
+++ b/packages/paperclip-runner/runner/crates/process-table/src/lib.rs
@@ -68,6 +68,16 @@ pub fn read(pid: i32) -> Read {
     platform::read(pid)
 }
 
+/// The session a process belongs to. `None` when the process cannot be read,
+/// or when its session is not visible here: a kernel thread, or a process
+/// whose session leader is outside this PID namespace, reports session 0.
+pub fn session(pid: i32) -> Option<i32> {
+    if pid <= 0 {
+        return None;
+    }
+    platform::session(pid).filter(|sid| *sid > 0)
+}
+
 #[cfg(target_os = "macos")]
 mod platform {
     use super::{ProcessEntry, Read};
@@ -137,6 +147,12 @@ mod platform {
             zombie: info.pbi_status == SZOMB,
         })
     }
+
+    pub(super) fn session(pid: i32) -> Option<i32> {
+        // SAFETY: getsid reads no caller memory; it returns -1 on error.
+        let sid = unsafe { libc::getsid(pid) };
+        (sid > 0).then_some(sid)
+    }
 }
 
 #[cfg(target_os = "linux")]
@@ -181,6 +197,17 @@ mod platform {
             zombie: state == "Z" || state == "X",
         })
     }
+
+    pub(super) fn session(pid: i32) -> Option<i32> {
+        parse_session(&std::fs::read_to_string(format!("/proc/{pid}/stat")).ok()?)
+    }
+
+    /// The session field of `/proc/<pid>/stat`. The kernel writes 0 when the
+    /// session is not visible in the reader's PID namespace.
+    pub(super) fn parse_session(stat: &str) -> Option<i32> {
+        let rest = &stat[stat.rfind(')')? + 1..];
+        rest.split_whitespace().nth(3)?.parse().ok()
+    }
 }
 
 #[cfg(not(any(target_os = "linux", target_os = "macos")))]
@@ -194,6 +221,10 @@ mod platform {
     pub(super) fn read(_pid: i32) -> Read {
         Read::Unreadable
     }
+
+    pub(super) fn session(_pid: i32) -> Option<i32> {
+        None
+    }
 }
 
 #[cfg(test)]
@@ -239,5 +270,31 @@ mod tests {
             (parsed.ppid, parsed.pgid, parsed.started, parsed.zombie),
             (1, 4200, 987654, false)
         );
+        assert_eq!(platform::parse_session(stat), Some(4200));
+    }
+
+    #[cfg(target_os = "linux")]
+    #[test]
+    fn reports_no_session_for_a_kernel_thread() {
+        // A kernel thread, or a process whose session leader is outside this
+        // PID namespace, reports session 0. rustix's getsid asserts a nonzero
+        // session in debug builds, which panicked runnerd's lineage sampler.
+        let stat = "2 (kthreadd) S 0 0 0 0 -1 2129984 0 0 0 0 0 0 0 0 20 0 1 0 9 0 0";
+        assert_eq!(platform::parse_session(stat), Some(0));
+        let kthreadd = std::fs::read_to_string("/proc/2/stat")
+            .ok()
+            .filter(|stat| stat.contains("(kthreadd)"));
+        if kthreadd.is_some() {
+            assert_eq!(session(2), None);
+        }
+    }
+
+    #[cfg(any(target_os = "linux", target_os = "macos"))]
+    #[test]
+    fn reads_this_process_session() {
+        let me = i32::try_from(std::process::id()).unwrap();
+        assert!(session(me).is_some_and(|sid| sid > 0));
+        assert_eq!(session(0), None);
+        assert_eq!(session(-1), None);
     }
 }
diff --git a/packages/paperclip-runner/runner/crates/runner-core/src/process_lineage.rs b/packages/paperclip-runner/runner/crates/runner-core/src/process_lineage.rs
index 04893cfb35a..db9bfa68aea 100644
--- a/packages/paperclip-runner/runner/crates/runner-core/src/process_lineage.rs
+++ b/packages/paperclip-runner/runner/crates/runner-core/src/process_lineage.rs
@@ -59,10 +59,10 @@ impl ProcessTable for LiveProcessTable {
     }
 
     fn session_of(&self, pid: i32) -> Option<i32> {
-        let pid = rustix::process::Pid::from_raw(pid)?;
-        rustix::process::getsid(Some(pid))
-            .ok()
-            .map(|sid| sid.as_raw_nonzero().get())
+        // Not rustix's getsid: it asserts a nonzero session, and a kernel
+        // thread or a process whose session leader is outside this PID
+        // namespace reports 0. That assertion killed the sampler thread.
+        paperclip_process_table::session(pid)
     }
 }
 
diff --git a/server/src/services/native-runtime/native-session-executor.test.ts b/server/src/services/native-runtime/native-session-executor.test.ts
index 5a577b4ab17..c08d42ab0c9 100644
--- a/server/src/services/native-runtime/native-session-executor.test.ts
+++ b/server/src/services/native-runtime/native-session-executor.test.ts
@@ -20548,6 +20548,22 @@ it("saves a runner-marked model rejection from a provider terminal as a model ch
   expect(nativeSessionFailureSourceCode(new NativeProviderTerminalFailure("provider_turn_failed", false,
     "openrouter/unavailable-model is not a valid model ID"))).toBe("native_provider_terminal_failed");
 });
+it("records a runner process exit that closed the provider event stream as runner loss", () => {
+  // Shape the Codex/OpenCode drivers write when runnerd exits mid-turn.
+  const lost = new NativeProviderTerminalFailure("notification_transport_failed", false,
+    "Notification transport failed closed: native_runner_process_exited: runnerd exited unexpectedly with code 137");
+  const code = nativeSessionFailureSourceCode(lost);
+  expect(code).toBe("native_runner_process_exited");
+  expect(nativeSessionFailureDisposition(1, new Date(), code)).toMatchObject({ phase: "retryable_failure", failureCode: "native_session_interrupted" });
+  expect(nativeSessionFailureDisposition(3, new Date(), code)).toEqual({ phase: "terminal_failure", failureCode: "native_session_retry_exhausted", nextAttemptAt: null });
+  // Other closed-stream causes, and the runner code only inside provider text, stay provider failures.
+  expect(nativeSessionFailureSourceCode(new NativeProviderTerminalFailure("notification_transport_failed", false,
+    "Notification transport failed closed: provider stream ended"))).toBe("native_provider_terminal_failed");
+  expect(nativeSessionFailureSourceCode(new NativeProviderTerminalFailure("provider_turn_failed", false,
+    "Notification transport failed closed: native_runner_process_exited: runnerd exited unexpectedly"))).toBe("native_provider_terminal_failed");
+  expect(nativeSessionFailureSourceCode(new NativeProviderTerminalFailure("notification_transport_failed", false,
+    "Notification transport failed closed: tool said native_runner_process_exited: oops"))).toBe("native_provider_terminal_failed");
+});
 it("keeps an ACPX missing runtime package failure actionable and stops automatic retries", () => {
   const failure = new Error('PRP command session.open failed: {"result":{"code":"command_execution_failed","message":"failed to start ACPX provider: ACPX sidecar command session.open was rejected (retryable=false, classification=runtime_package_missing)"},"status":"failed"}');
   const code = nativeSessionFailureSourceCode(failure);
diff --git a/server/src/services/native-runtime/native-session-executor.ts b/server/src/services/native-runtime/native-session-executor.ts
index 9dfa84c231a..223f2b69a79 100644
--- a/server/src/services/native-runtime/native-session-executor.ts
+++ b/server/src/services/native-runtime/native-session-executor.ts
@@ -8078,6 +8078,12 @@ export function nativeSessionFailureSourceCode(
     // terminal message; it needs a model choice, not a generic provider failure.
     if (!error.recoverable && /^native_provider_model_rejected:/.test(error.message))
       return "native_provider_model_rejected";
+    // The event stream closed because the runner process itself exited. The
+    // runner transport names that cause first in the closed-stream message; it
+    // is runner loss, not a provider verdict on its session.
+    if (error.providerCode === "notification_transport_failed"
+      && /^Notification transport failed closed: native_runner_process_exited: /.test(error.message))
+      return "native_runner_process_exited";
     return "native_provider_terminal_failed";
   }
   if (error instanceof NativeSessionProtocolIntegrityError)