fix(cli): MCP agentChat slim-wire payload

The MCP `send_agent_message` was building wire payloads with a legacy
`messages: [...]` array, but the agent's run loop now destructures
`payload.message` (singular) per the slim-wire contract. Result:
turn 1 entered `ai.streamText` with an empty messages array and
threw `AI_InvalidPromptError: messages must not be empty`.

Migrates all three send paths (session.in append, triggerTask
fallback, and `trigger:upgrade-required` continuation) plus the
`trigger: "close"` chunk to the slim-wire shape, and aligns the
local `ChatInputChunk` type with the canonical one in ai-shared.ts.

Author: ericallam

packages/cli-v3/src/mcp/tools/agentChat.ts

@@ -29,15 +29,23 @@ type ChatSession = {
 const activeSessions = new Map<string, ChatSession>();
 
 // ─── ChatInputChunk serialization (mirrors TriggerChatTransport) ──
-
+//
+// Slim-wire: one delta `message` per chunk, not a full `messages[]` array.
+// The agent's run loop destructures `payload.message` (singular). Sending
+// a `messages: [...]` array makes `message` undefined and turn 1 calls
+// `streamText({ messages: [] })`, which throws
+// `AI_InvalidPromptError: messages must not be empty`.
 type ChatInputChunk =
   | {
       kind: "message";
       payload: {
-        messages: ChatMessage[];
+        message?: ChatMessage;
         chatId: string;
         trigger: "submit-message" | "close" | "preload" | "regenerate-message" | "action";
         metadata?: unknown;
+        sessionId?: string;
+        continuation?: boolean;
+        previousRunId?: string;
       };
     }
   | { kind: "stop"; message?: string };
@@ -203,16 +211,19 @@ export const sendAgentMessageTool = {
     // Track the outgoing user message
     session.messages.push(userMessage);
 
+    // Slim-wire: one delta `message` per trigger. Prior turns live in the
+    // session.out snapshot+replay; we only ship the new user message.
     const wirePayload = {
-      messages: [userMessage],
+      message: userMessage,
       chatId: session.chatId,
       trigger: "submit-message" as const,
       metadata: session.clientData,
     };
 
     // If we have an active run, send via session.in. If that fails
     // (run ended, token expired, etc.) fall back to triggering a new
-    // run on the same session with the full history.
+    // run on the same session — the new run replays prior turns from the
+    // snapshot and picks up `message` as turn N's user delta.
     if (session.runId) {
       try {
         await session.apiClient.appendToSessionStream(
@@ -223,7 +234,7 @@ export const sendAgentMessageTool = {
       } catch (sendErr: any) {
         const result = await session.apiClient.triggerTask(session.agentId, {
           payload: {
-            messages: session.messages,
+            message: userMessage,
             chatId: session.chatId,
             sessionId: session.sessionId,
             trigger: "submit-message",
@@ -298,7 +309,8 @@ export const closeAgentChatTool = {
           serializeInputChunk({
             kind: "message",
             payload: {
-              messages: [],
+              // `trigger: "close"` carries no message delta — the agent
+              // looks at `trigger` and exits without touching `message`.
               chatId: session.chatId,
               trigger: "close",
             },
@@ -368,12 +380,17 @@ async function collectAgentResponse(
         }
 
         if (chunk.type === "trigger:upgrade-required") {
-          // Agent requested upgrade — trigger continuation with full history.
-          // Same session, new run — reuse sessionId, swap runId.
+          // Agent requested upgrade — trigger continuation. Same session,
+          // new run — reuse sessionId, swap runId. Slim-wire: ship only
+          // the latest user message as the turn-N delta; prior turns
+          // come back via snapshot+replay on the new run's boot.
+          const lastUserMessage = [...session.messages]
+            .reverse()
+            .find((m) => m.role === "user");
           const previousRunId = session.runId;
           const result = await session.apiClient.triggerTask(session.agentId, {
             payload: {
-              messages: session.messages,
+              message: lastUserMessage,
               chatId: session.chatId,
               sessionId: session.sessionId,
               trigger: "submit-message",