diff --git a/src/cli.tsx b/src/cli.tsx
index 99c460b..6b90b7d 100644
--- a/src/cli.tsx
+++ b/src/cli.tsx
@@ -46,8 +46,11 @@ registerPing(program)
 // Any invocation that isn't a known subcommand or a top-level help/version
 // request is shorthand for `agent session start --connect ...`. So a bare
 // `agent`, and `agent "fix the tests" --model ...`, both forward the prompt
-// and every trailing flag through to a fresh connected session. `agent --help`,
-// `agent --version`, `agent help`, and every subcommand dispatch unchanged.
+// and every trailing flag through to a fresh connected session. A bare
+// `agent` starts the session idle (idle_start): the sandbox spins up and
+// Claude Code waits for the first thing typed into the composer, like a
+// local `claude`. `agent --help`, `agent --version`, `agent help`, and every
+// subcommand dispatch unchanged.
 const topLevelCommands = new Set([
   'help',
   ...program.commands.flatMap((c) => [c.name(), ...c.aliases()]),
diff --git a/src/commands/session.tsx b/src/commands/session.tsx
index 7ab9b7c..efd79ad 100644
--- a/src/commands/session.tsx
+++ b/src/commands/session.tsx
@@ -220,6 +220,12 @@ export function registerSession(program: Command): void {
           // Appended to the initial user query at build time; gives this
           // session instructions on top of the config's shared system prompt.
           if (promptText) req.prompt = promptText
+          // A promptless connect (a bare `agent`) starts the session idle:
+          // no fabricated kickoff message, Claude Code waits at the prompt
+          // for whatever is typed into the composer, like a local `claude`.
+          // A promptless --watch/--detach start still runs the config's
+          // workflow, so this only applies to the interactive open.
+          if (opts.connect && !promptText) req.idle_start = true
 
           const api = new ApiClient()
           const session = await api.startAgentSession(req)
diff --git a/src/lib/types.ts b/src/lib/types.ts
index 5bb0c39..26e07bf 100644
--- a/src/lib/types.ts
+++ b/src/lib/types.ts
@@ -197,6 +197,12 @@ export interface StartAgentSessionRequest {
   // after the config's shared `claude.system` system prompt. Distinct from the
   // system prompt, which is identical for every session of a config.
   prompt?: string
+  // Start with no initial message: the sandbox spins up, Claude Code sits idle
+  // at the prompt, and the first message sent to the session opens turn 0,
+  // exactly like a local `claude`. Sent for a promptless --connect start (a
+  // bare `agent`). Mutually exclusive with prompt; the server ignores it when
+  // the resolved config is not interactive (that session runs its workflow).
+  idle_start?: boolean
 }
 
 // Replay payload for POST /v1/sessions/{id}/replay. Re-runs an existing