diff --git a/ui/src/components/agents/AgentsHome.tsx b/ui/src/components/agents/AgentsHome.tsx
index c1a0b5b2..1fd4174a 100644
--- a/ui/src/components/agents/AgentsHome.tsx
+++ b/ui/src/components/agents/AgentsHome.tsx
@@ -7,7 +7,7 @@ import type { AgentThread, ImageChunk } from "@/lib/agents/types"
 import type { CreateAgentThreadVariables } from "@/lib/agents/queries"
 import type { ModelSelection } from "@/lib/agents/provider/useModelOptions"
 import { AgentPromptBar } from "@/components/agents/AgentPromptBar"
-import { SlackConnectDialog } from "@/components/agents/SlackConnectDialog"
+import { OnboardingDialog } from "@/components/agents/OnboardingDialog"
 import { Logo } from "@/components/agents/ported/Logo"
 import {
   agentThreadKeys,
@@ -107,7 +107,7 @@ export function AgentsHome() {
 
   return (
     <div className="flex min-w-0 flex-1 flex-col overflow-y-auto px-6 py-8">
-      <SlackConnectDialog />
+      <OnboardingDialog />
       <div className="mx-auto flex w-full max-w-2xl flex-1 flex-col items-center justify-center">
         <div className="flex w-full flex-col items-center gap-6">
           <Logo />
diff --git a/ui/src/components/agents/OnboardingDialog.tsx b/ui/src/components/agents/OnboardingDialog.tsx
new file mode 100644
index 00000000..457d7361
--- /dev/null
+++ b/ui/src/components/agents/OnboardingDialog.tsx
@@ -0,0 +1,205 @@
+import { Dialog } from "@base-ui/react/dialog"
+import { useQuery } from "@tanstack/react-query"
+import { useEffect, useState } from "react"
+import { IoLogoSlack } from "react-icons/io5"
+
+import type { ModelOption } from "@/lib/api"
+import { Button } from "@/components/ui/button"
+import {
+  Select,
+  SelectContent,
+  SelectItem,
+  SelectTrigger,
+  SelectValue,
+} from "@/components/ui/select"
+import { api, slackConnectUrl } from "@/lib/api"
+import {
+  buildProfileUpdate,
+  useOptions,
+  useProfile,
+  useSaveProfile,
+} from "@/lib/profile"
+import { useSession } from "@/lib/session"
+
+/**
+ * First-run onboarding modal: pick a default agent model, then connect Slack.
+ *
+ * The model step shows until the user has saved a default model; the Slack step
+ * shows (where Sign in with Slack is enabled) until their Slack account is
+ * linked. Both steps live in the same dialog so a new user is walked through
+ * picking a model and connecting Slack in one place. Dismissing hides it for
+ * the session; an incomplete step reappears on the next login.
+ */
+export function OnboardingDialog() {
+  const session = useSession()
+  const mapping = useQuery({ queryKey: ["myMapping"], queryFn: api.myMapping })
+  const profile = useProfile()
+  const options = useOptions()
+  const save = useSaveProfile()
+  const [dismissed, setDismissed] = useState(false)
+  const [error, setError] = useState<string | null>(null)
+
+  const firstModel: ModelOption | undefined = options.data?.models[0]
+  const defaultModel = options.data?.default_agent_model ?? firstModel?.id ?? ""
+  const defaultEffort =
+    options.data?.default_agent_reasoning_effort ??
+    firstModel?.default_effort ??
+    ""
+  const [modelId, setModelId] = useState("")
+  const [effort, setEffort] = useState("")
+
+  useEffect(() => {
+    if (!modelId && defaultModel) setModelId(defaultModel)
+  }, [modelId, defaultModel])
+
+  const currentModel: ModelOption | undefined =
+    options.data?.models.find((m) => m.id === modelId) ?? firstModel
+
+  useEffect(() => {
+    if (!currentModel) return
+    if (!effort || !currentModel.efforts.includes(effort)) {
+      setEffort(currentModel.default_effort)
+    }
+  }, [currentModel, effort])
+
+  const slackEnabled = session.data?.slack_oauth_enabled ?? false
+  const slackConnected = !!mapping.data?.slack_user_id
+  const hasDefaultModel = !!profile.data?.default_model
+
+  const needsModel =
+    !profile.isLoading && profile.data !== undefined && !hasDefaultModel
+  const needsSlack =
+    slackEnabled && !slackConnected && !mapping.isLoading && !mapping.isError
+  const open = !dismissed && (needsModel || needsSlack)
+  const step: "model" | "slack" = needsModel ? "model" : "slack"
+
+  const handleSaveModel = () => {
+    if (!modelId) return
+    setError(null)
+    save
+      .mutateAsync(
+        buildProfileUpdate(
+          profile.data,
+          { default_model: modelId, reasoning_effort: effort },
+          defaultModel,
+          defaultEffort
+        )
+      )
+      .catch((e: Error) => setError(e.message))
+  }
+
+  return (
+    <Dialog.Root
+      open={open}
+      onOpenChange={(next) => {
+        if (!next) setDismissed(true)
+      }}
+    >
+      <Dialog.Portal>
+        <Dialog.Backdrop className="fixed inset-0 z-50 bg-black/50 data-open:animate-in data-open:fade-in-0 data-closed:animate-out data-closed:fade-out-0" />
+        <Dialog.Popup className="fixed top-1/2 left-1/2 z-50 w-[min(28rem,calc(100vw-2rem))] -translate-x-1/2 -translate-y-1/2 rounded-lg bg-popover p-6 text-popover-foreground shadow-md ring-1 ring-foreground/10 data-open:animate-in data-open:fade-in-0 data-open:zoom-in-95 data-closed:animate-out data-closed:fade-out-0 data-closed:zoom-out-95">
+          {step === "model" ? (
+            <div className="flex flex-col gap-4">
+              <Dialog.Title className="text-base font-semibold">
+                Choose your default model
+              </Dialog.Title>
+              <Dialog.Description className="text-sm text-muted-foreground">
+                Pick the model Open SWE uses when you don't specify one. You can
+                change this anytime in your settings.
+              </Dialog.Description>
+              <div className="flex flex-col gap-3">
+                <div className="flex items-center justify-between gap-3">
+                  <span className="text-sm font-medium">Default model</span>
+                  <Select
+                    value={modelId}
+                    onValueChange={(v) => v && setModelId(v)}
+                  >
+                    <SelectTrigger className="w-48">
+                      <SelectValue placeholder="Pick a model" />
+                    </SelectTrigger>
+                    <SelectContent>
+                      {options.data?.models.map((m) => (
+                        <SelectItem key={m.id} value={m.id}>
+                          {m.label}
+                        </SelectItem>
+                      ))}
+                    </SelectContent>
+                  </Select>
+                </div>
+                <div className="flex items-center justify-between gap-3">
+                  <span className="text-sm font-medium">Reasoning effort</span>
+                  <Select
+                    value={effort}
+                    onValueChange={(v) => v && setEffort(v)}
+                  >
+                    <SelectTrigger className="w-48">
+                      <SelectValue />
+                    </SelectTrigger>
+                    <SelectContent>
+                      {currentModel?.efforts.map((e) => (
+                        <SelectItem key={e} value={e}>
+                          {e}
+                        </SelectItem>
+                      ))}
+                    </SelectContent>
+                  </Select>
+                </div>
+              </div>
+              {error && <p className="text-xs text-destructive">{error}</p>}
+              <div className="mt-2 flex justify-end gap-2">
+                <Button
+                  variant="outline"
+                  size="sm"
+                  onClick={() => setDismissed(true)}
+                >
+                  Maybe later
+                </Button>
+                <Button
+                  size="sm"
+                  onClick={handleSaveModel}
+                  disabled={!modelId || save.isPending}
+                >
+                  {save.isPending
+                    ? "Saving…"
+                    : needsSlack
+                      ? "Save & continue"
+                      : "Save"}
+                </Button>
+              </div>
+            </div>
+          ) : (
+            <div className="flex flex-col gap-4">
+              <div className="flex items-center gap-3">
+                <IoLogoSlack className="size-6 shrink-0 text-muted-foreground" />
+                <Dialog.Title className="text-base font-semibold">
+                  Connect your Slack account
+                </Dialog.Title>
+              </div>
+              <Dialog.Description className="text-sm text-muted-foreground">
+                Connect Slack so that when you tag Open SWE, it can resolve your
+                GitHub account. We use the email Slack verifies, which also lets
+                Linear mentions resolve to you.
+              </Dialog.Description>
+              <div className="mt-2 flex justify-end gap-2">
+                <Button
+                  variant="outline"
+                  size="sm"
+                  onClick={() => setDismissed(true)}
+                >
+                  Maybe later
+                </Button>
+                <Button
+                  size="sm"
+                  onClick={() => window.location.assign(slackConnectUrl())}
+                >
+                  <IoLogoSlack className="size-4" />
+                  Connect Slack
+                </Button>
+              </div>
+            </div>
+          )}
+        </Dialog.Popup>
+      </Dialog.Portal>
+    </Dialog.Root>
+  )
+}
diff --git a/ui/src/components/agents/SlackConnectDialog.tsx b/ui/src/components/agents/SlackConnectDialog.tsx
deleted file mode 100644
index 6ddcbb86..00000000
--- a/ui/src/components/agents/SlackConnectDialog.tsx
+++ /dev/null
@@ -1,71 +0,0 @@
-import { Dialog } from "@base-ui/react/dialog"
-import { useQuery } from "@tanstack/react-query"
-import { useState } from "react"
-import { IoLogoSlack } from "react-icons/io5"
-
-import { Button } from "@/components/ui/button"
-import { api, slackConnectUrl } from "@/lib/api"
-import { useSession } from "@/lib/session"
-
-/**
- * Modal shown on first login (and until connected) prompting the user to link
- * Slack so Open SWE can resolve their GitHub account when tagged in Slack.
- * ``open`` is derived from the mapping query, so it appears once the data
- * resolves to "not connected" and closes itself once Slack is linked; dismissing
- * it hides it for the session.
- */
-export function SlackConnectDialog() {
-  const session = useSession()
-  const mapping = useQuery({ queryKey: ["myMapping"], queryFn: api.myMapping })
-  const [dismissed, setDismissed] = useState(false)
-
-  const slackEnabled = session.data?.slack_oauth_enabled ?? false
-  const connected = !!mapping.data?.slack_user_id
-  const shouldShow =
-    slackEnabled && !connected && !mapping.isLoading && !mapping.isError
-  const open = shouldShow && !dismissed
-
-  return (
-    <Dialog.Root
-      open={open}
-      onOpenChange={(next) => {
-        if (!next) setDismissed(true)
-      }}
-    >
-      <Dialog.Portal>
-        <Dialog.Backdrop className="fixed inset-0 z-50 bg-black/50 data-open:animate-in data-open:fade-in-0 data-closed:animate-out data-closed:fade-out-0" />
-        <Dialog.Popup className="fixed top-1/2 left-1/2 z-50 w-[min(28rem,calc(100vw-2rem))] -translate-x-1/2 -translate-y-1/2 rounded-lg bg-popover p-6 text-popover-foreground shadow-md ring-1 ring-foreground/10 data-open:animate-in data-open:fade-in-0 data-open:zoom-in-95 data-closed:animate-out data-closed:fade-out-0 data-closed:zoom-out-95">
-          <div className="flex flex-col gap-4">
-            <div className="flex items-center gap-3">
-              <IoLogoSlack className="size-6 shrink-0 text-muted-foreground" />
-              <Dialog.Title className="text-base font-semibold">
-                Connect your Slack account
-              </Dialog.Title>
-            </div>
-            <Dialog.Description className="text-sm text-muted-foreground">
-              Connect Slack so that when you tag Open SWE, it can resolve your
-              GitHub account. We use the email Slack verifies, which also lets
-              Linear mentions resolve to you.
-            </Dialog.Description>
-            <div className="mt-2 flex justify-end gap-2">
-              <Button
-                variant="outline"
-                size="sm"
-                onClick={() => setDismissed(true)}
-              >
-                Maybe later
-              </Button>
-              <Button
-                size="sm"
-                onClick={() => window.location.assign(slackConnectUrl())}
-              >
-                <IoLogoSlack className="size-4" />
-                Connect Slack
-              </Button>
-            </div>
-          </div>
-        </Dialog.Popup>
-      </Dialog.Portal>
-    </Dialog.Root>
-  )
-}