A setup is a machine, and it carries what that machine can run

Providers and hosts were two independent lists, and a session named one of
each. They were never independent: a provider only exists on a machine
where that program is installed, so the spawn screen offered the whole
cross-product, including "the Claude CLI on the box that hasn't got it".
The picker could not know, because nothing in the model said.

Now a setup is a machine -- optional ssh, plus the providers it has -- and
spawning is two choices in order: pick a setup, then one of its providers.
The impossible pairs stop being expressible rather than being validated
against. Provider names are unique within a setup and only within one, so
two machines can each have a `claude-cli`, which was previously either a
name collision or two entries called things like "claude" and "claude on
the vm".

It also settles the "Run on" problem properly. That control was offered for
every provider but honoured only by the Claude driver -- an echo session
sent to a host ran locally and said otherwise. There is no such control
now: the machine is chosen first, and echo is a provider of the setup with
no ssh, where it belongs, since it runs in-process and has no transport to
cross.

The built-in echo provider is gone as a concept. It used to be conjured at
read time and never written to the file, which meant a provider nobody
could see or edit; it is now seeded into the config on first run alongside
claude-cli. What the file says is what there is, and deleting it is a
choice rather than a state to be repaired.

A config in the old shape is refused with instructions rather than loaded.
`Config` defaults unknown fields away, so `providers:` and `hosts:` would
otherwise have vanished into an empty config that was then seeded over --
a migration nobody would notice until their setups were gone.

Verified against a running server and on the emulator: a fresh install
seeds "this machine" with echo and claude-cli and the file reads cleanly;
a two-setup config lists both with their own providers; spawning on a
setup works and the session row names it; asking for a provider a setup
lacks says which it offers, and an unknown setup says which exist. On the
phone, selecting "dev vm" narrows the provider chips to that machine's one
and shows its address.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017xn8nHw1tw1R6PtiY1eEtw

app/androidApp/src/main/kotlin/com/example/aiapp/SpawnScreen.kt

@@ -39,7 +39,6 @@ import kotlinx.coroutines.withContext
 private val PERMISSION_MODES = listOf("manual", "acceptEdits", "auto", "bypassPermissions", "plan")
 
 /** Runs on the backend machine itself -- the "no host" case. */
-private const val LOCAL_HOST_LABEL = "backend"
 
 /**
  * The spawn screen: what to run, where to run it, and the per-kind fields.
@@ -57,10 +56,14 @@ fun SpawnScreen(
     // What the form is made of, and whether we have it yet. A failure here
     // is not the same as a server with nothing to offer, so it must not
     // reach the pickers as empty lists -- see LoadState.
-    var options by remember { mutableStateOf<LoadState<SpawnOptions>>(LoadState.Loading) }
+    var options by remember { mutableStateOf<LoadState<List<Setup>>>(LoadState.Loading) }
 
-    var provider by remember { mutableStateOf<Provider?>(null) }
-    var host by remember { mutableStateOf<String?>(null) }
+    // Setup first, then one of its providers. Choosing a setup can
+    // invalidate the provider, so the provider is stored by name and
+    // resolved against the current setup rather than held as an object
+    // that could outlive the list it came from.
+    var setupName by remember { mutableStateOf<String?>(null) }
+    var providerName by remember { mutableStateOf<String?>(null) }
     var title by remember { mutableStateOf("") }
     var model by remember { mutableStateOf("") }
     var cwd by remember { mutableStateOf("") }
@@ -74,23 +77,16 @@ fun SpawnScreen(
     LaunchedEffect(Unit) {
         options =
             try {
-                val fetched =
-                    withContext(Dispatchers.IO) {
-                        SpawnOptions(fetchProviders(settings), fetchHosts(settings))
-                    }
-                provider = fetched.providers.firstOrNull()
+                val fetched = withContext(Dispatchers.IO) { fetchSetups(settings) }
+                val first = fetched.firstOrNull()
+                setupName = first?.name
+                providerName = first?.providers?.firstOrNull()?.name
                 LoadState.Loaded(fetched)
             } catch (e: ApiException) {
                 LoadState.failed(e)
             }
     }
 
-    val current = provider
-    // Only the Claude CLI has models, a working directory, and permission
-    // modes; keying the extra fields on the kind rather than the provider
-    // name keeps a second Claude provider from needing anything here.
-    val isClaude = current?.kind == "claude_cli"
-
     Column(Modifier.fillMaxSize().verticalScroll(rememberScrollState()).padding(16.dp)) {
         Row(verticalAlignment = Alignment.CenterVertically, modifier = Modifier.fillMaxWidth()) {
             Text(
@@ -106,7 +102,7 @@ fun SpawnScreen(
         // failure to fetch them leaves no form worth showing -- so this
         // reports and stops, rather than offering empty pickers under an
         // error message.
-        val (providers, hosts) =
+        val setups =
             when (val state = options) {
                 is LoadState.Loading -> {
                     CircularProgressIndicator()
@@ -118,33 +114,56 @@ fun SpawnScreen(
                 }
                 is LoadState.Loaded -> state.value
             }
+        val setup = setups.firstOrNull { it.name == setupName }
+        val current = setup?.providers?.firstOrNull { it.name == providerName }
+        // Only the Claude CLI has models, a working directory and
+        // permission modes; keying the extra fields on the kind rather
+        // than the provider name keeps a second Claude provider from
+        // needing anything here.
+        val isClaude = current?.kind == "claude_cli"
 
+        // The machine first, because it decides what can be run at all.
         ChipGroup(
-            label = "Provider",
-            options = providers.map { it.name },
-            selected = current?.name,
-            onSelect = { name -> provider = providers.first { it.name == name } },
+            label = "Setup",
+            options = setups.map { it.name },
+            selected = setupName,
+            onSelect = { name ->
+                setupName = name
+                // The provider list changes with the machine, so a name
+                // carried over from the previous one would be a selection
+                // that isn't in the picker. Take that machine's first.
+                providerName =
+                    setups.firstOrNull { it.name == name }?.providers?.firstOrNull()?.name
+            },
         )
-
-        // Always offered, whatever the provider: where a session runs is
-        // independent of what runs it.
-        ChipGroup(
-            label = "Run on",
-            options = listOf(LOCAL_HOST_LABEL) + hosts.map { it.name },
-            selected = host ?: LOCAL_HOST_LABEL,
-            onSelect = { name -> host = name.takeIf { it != LOCAL_HOST_LABEL } },
-        )
-        host?.let { chosen ->
-            hosts
-                .firstOrNull { it.name == chosen }
-                ?.let {
-                    Text(
-                        it.address,
-                        style = MaterialTheme.typography.bodySmall,
-                        color = MaterialTheme.colorScheme.onSurfaceVariant,
-                    )
-                }
+        setup?.address?.let {
+            Text(
+                it,
+                style = MaterialTheme.typography.bodySmall,
+                color = MaterialTheme.colorScheme.onSurfaceVariant,
+            )
+            // The address belongs to the setup above it, not to the
+            // provider label below; without this they read as one block.
+            Spacer(Modifier.height(8.dp))
         }
+
+        // Only what this machine actually has. A setup with none says so
+        // rather than showing an empty row that reads as a failure.
+        if (setup != null && setup.providers.isEmpty()) {
+            Text(
+                "\"${setup.name}\" has no providers configured.",
+                style = MaterialTheme.typography.bodyMedium,
+                color = MaterialTheme.colorScheme.onSurfaceVariant,
+            )
+        } else {
+            ChipGroup(
+                label = "Provider",
+                options = setup?.providers?.map { it.name }.orEmpty(),
+                selected = providerName,
+                onSelect = { providerName = it },
+            )
+        }
+
         Spacer(Modifier.height(16.dp))
 
         OutlinedTextField(
@@ -210,9 +229,9 @@ fun SpawnScreen(
                             withContext(Dispatchers.IO) {
                                 spawnSession(
                                     settings,
+                                    setup = setup?.name.orEmpty(),
                                     provider = chosen.name,
                                     title = title.trim(),
-                                    host = host,
                                     model = model.trim().takeIf { isClaude },
                                     cwd = cwd.trim().takeIf { isClaude },
                                     permissionMode = permissionMode.takeIf { isClaude },
@@ -232,9 +251,6 @@ fun SpawnScreen(
     }
 }
 
-/** What the spawn form is built from, fetched as one thing. */
-private data class SpawnOptions(val providers: List<Provider>, val hosts: List<RemoteHost>)
-
 /**
  * A labeled row of choices that wraps onto as many lines as it needs.
  *