fix: distinguish transient errors from missing sessions in crash detection

src/features/background-agent/session-existence.ts

@@ -0,0 +1,50 @@
+import type { OpencodeClient } from "./opencode-client"
+
+export const MIN_SESSION_GONE_POLLS = 3
+
+function extractErrorMessage(error: unknown): string | undefined {
+  if (typeof error === "string") {
+    return error
+  }
+
+  if (typeof error !== "object" || error === null || !("message" in error)) {
+    return undefined
+  }
+
+  return typeof error.message === "string" ? error.message : undefined
+}
+
+function extractErrorStatus(error: unknown): number | undefined {
+  if (typeof error !== "object" || error === null || !("status" in error)) {
+    return undefined
+  }
+
+  return typeof error.status === "number" ? error.status : undefined
+}
+
+function isSessionNotFoundError(error: unknown): boolean {
+  if (extractErrorStatus(error) === 404) {
+    return true
+  }
+
+  const message = extractErrorMessage(error)?.toLowerCase()
+  if (!message) {
+    return false
+  }
+
+  return message.includes("not found") || message.includes("missing")
+}
+
+export async function verifySessionExists(client: OpencodeClient, sessionID: string): Promise<boolean> {
+  try {
+    const response = await client.session.get({ path: { id: sessionID } })
+
+    if (response.error !== undefined && response.error !== null) {
+      return !isSessionNotFoundError(response.error)
+    }
+
+    return response.data != null
+  } catch (error) {
+    return !isSessionNotFoundError(error)
+  }
+}