diff --git a/connect/README.md b/connect/README.md
index b95a85864..7083b62a1 100644
--- a/connect/README.md
+++ b/connect/README.md
@@ -168,9 +168,17 @@ const processId = await spawn({
   signer: createSigner(wallet),
   tags,
   data,
+  // `mainnet` mode only: the execution device the process runs on. Defaults to
+  // "genesis-wasm@1.0"; set e.g. "lua@5.3a" to spawn a native Lua process.
+  // May also be set via the EXECUTION_DEVICE environment variable.
+  executionDevice,
 });
 ```
 
+> In `mainnet` mode, `module` must be a transaction id (a string) referencing an
+> uploaded module.  `executionDevice` selects the process's execution
+> device (default `genesis-wasm@1.0`).
+
 #### `connect`
 
 If you would like the connect to use ao components other than the defaults, you
diff --git a/connect/src/client/ao-core.js b/connect/src/client/ao-core.js
index 5f23c85e7..9a08a1d65 100644
--- a/connect/src/client/ao-core.js
+++ b/connect/src/client/ao-core.js
@@ -88,8 +88,13 @@ export function spawnWith(deps) {
 
     if (!scheduler) throw new Error('No scheduler provided')
     if (!module) throw new Error('No module provided')
+    if (typeof module !== 'string') {
+      throw new Error('module must be a transaction-id string')
+    }
 
     const authority = process.env.AUTHORITY || args.authority || scheduler
+    const executionDevice =
+      process.env.EXECUTION_DEVICE || args.executionDevice || 'genesis-wasm@1.0'
 
     debugLog('info', 'Node URL:', deps.url)
     debugLog('info', 'Scheduler:', scheduler)
@@ -102,7 +107,7 @@ export function spawnWith(deps) {
         device: 'process@1.0',
         'scheduler-device': 'scheduler@1.0',
         'push-device': 'push@1.0',
-        'execution-device': 'genesis-wasm@1.0',
+        'execution-device': executionDevice,
         Authority: authority,
         Scheduler: scheduler,
         Module: module,
diff --git a/connect/src/client/ao-core.spawn.unit.test.js b/connect/src/client/ao-core.spawn.unit.test.js
new file mode 100644
index 000000000..40288cbce
--- /dev/null
+++ b/connect/src/client/ao-core.spawn.unit.test.js
@@ -0,0 +1,64 @@
+import { describe, test } from 'node:test'
+import * as assert from 'node:assert'
+import { spawnWith } from './ao-core.js'
+
+// Unit tests for mainnet spawn param-building — no live node. A fake `deps`
+// captures the params the client would POST.
+const makeDeps = () => {
+  const captured = {}
+  return {
+    captured,
+    scheduler: 'SCHED_ADDR',
+    aoCore: {
+      request: async (params) => {
+        captured.params = params
+        return { headers: { get: (k) => (k === 'process' ? 'PID123' : null) } }
+      }
+    }
+  }
+}
+
+describe('mainnet spawn: execution-device override', () => {
+  test('defaults to genesis-wasm@1.0 when not provided', async () => {
+    const deps = makeDeps()
+    await spawnWith(deps)({ module: 'MODTX' })
+    assert.equal(deps.captured.params['execution-device'], 'genesis-wasm@1.0')
+  })
+
+  test('honors args.executionDevice', async () => {
+    const deps = makeDeps()
+    await spawnWith(deps)({ module: 'MODTX', executionDevice: 'lua@5.3a' })
+    assert.equal(deps.captured.params['execution-device'], 'lua@5.3a')
+  })
+
+  test('honors EXECUTION_DEVICE env', async () => {
+    const prev = process.env.EXECUTION_DEVICE
+    process.env.EXECUTION_DEVICE = 'lua@5.3a'
+    try {
+      const deps = makeDeps()
+      await spawnWith(deps)({ module: 'MODTX' })
+      assert.equal(deps.captured.params['execution-device'], 'lua@5.3a')
+    } finally {
+      if (prev === undefined) delete process.env.EXECUTION_DEVICE
+      else process.env.EXECUTION_DEVICE = prev
+    }
+  })
+})
+
+describe('mainnet spawn: non-string module is rejected', () => {
+  test('throws on a map-valued module instead of "[object Object]"', async () => {
+    const deps = makeDeps()
+    await assert.rejects(
+      () => spawnWith(deps)({ module: { 'content-type': 'application/lua', body: 'x' } }),
+      /module must be a transaction-id string/
+    )
+    assert.equal(deps.captured.params, undefined, 'request must not be sent for a bad module')
+  })
+
+  test('still spawns for a valid string module', async () => {
+    const deps = makeDeps()
+    const pid = await spawnWith(deps)({ module: 'MODTX' })
+    assert.equal(pid, 'PID123')
+    assert.equal(deps.captured.params.Module, 'MODTX')
+  })
+})