fix(sdk): an empty or null concurrency value is rejected, not silently dropped

Truthiness guards at the trigger call sites swallowed falsey concurrency
values before validation, so a JavaScript caller passing an empty string or
null started the session without the intended limits. Only undefined counts
as absent now; everything else flows into validation and throws.

Author: matt-aitken

packages/trigger-sdk/src/v3/ai.ts

@@ -11822,7 +11822,7 @@ function createChatStartSessionAction<TChat extends AnyTask = AnyTask>(
       ...(options?.triggerConfig?.queue || params.triggerConfig?.queue
         ? { queue: params.triggerConfig?.queue ?? options?.triggerConfig?.queue }
         : {}),
-      ...(concurrency ? triggerConcurrencyBody(concurrency) : {}),
+      ...(concurrency !== undefined ? triggerConcurrencyBody(concurrency) : {}),
       ...(concurrencyKey !== undefined ? { concurrencyKey } : {}),
       tags,
       ...(maxAttempts !== undefined ? { maxAttempts } : {}),

packages/trigger-sdk/src/v3/chat-client.ts

@@ -672,7 +672,7 @@ export class AgentChat<TAgent = unknown> {
       },
       ...(this.triggerConfigDefault?.machine ? { machine: this.triggerConfigDefault.machine } : {}),
       ...(this.triggerConfigDefault?.queue ? { queue: this.triggerConfigDefault.queue } : {}),
-      ...(this.triggerConfigDefault?.concurrency
+      ...(this.triggerConfigDefault?.concurrency !== undefined
         ? triggerConcurrencyBody(this.triggerConfigDefault.concurrency)
         : {}),
       ...(this.triggerConfigDefault?.concurrencyKey !== undefined

packages/trigger-sdk/src/v3/chat-server.ts

@@ -545,7 +545,7 @@ async function openHandoverSession(opts: {
     },
     ...(opts.triggerConfig?.machine ? { machine: opts.triggerConfig.machine } : {}),
     ...(opts.triggerConfig?.queue ? { queue: opts.triggerConfig.queue } : {}),
-    ...(opts.triggerConfig?.concurrency
+    ...(opts.triggerConfig?.concurrency !== undefined
       ? triggerConcurrencyBody(opts.triggerConfig.concurrency)
       : {}),
     ...(opts.triggerConfig?.concurrencyKey !== undefined

packages/trigger-sdk/src/v3/concurrency-shared.ts

@@ -11,7 +11,7 @@
 export function triggerConcurrencyBody(concurrency: string | string[] | undefined): {
   concurrency?: string[];
 } {
-  if (!concurrency) {
+  if (concurrency === undefined) {
     return {};
   }
   const limits = Array.isArray(concurrency) ? concurrency : [concurrency];

packages/trigger-sdk/src/v3/createStartSessionAction.test.ts

@@ -211,6 +211,27 @@ describe("chat.createStartSessionAction — runtime", () => {
     expect(lastStartBody).toBeUndefined();
   });
 
+  it("rejects an empty-string concurrency instead of silently dropping it", async () => {
+    installStartFixture();
+
+    const emptyDefault = chat.createStartSessionAction("fake-chat", {
+      triggerConfig: { concurrency: "" as unknown as string[] },
+    });
+    await expect(emptyDefault({ chatId: "chat-empty-limit" })).rejects.toThrow(
+      /non-empty strings/
+    );
+
+    const emptyPerCall = chat.createStartSessionAction("fake-chat");
+    await expect(
+      emptyPerCall({
+        chatId: "chat-empty-limit-2",
+        triggerConfig: { concurrency: "" as unknown as string[] },
+      })
+    ).rejects.toThrow(/non-empty strings/);
+
+    expect(lastStartBody).toBeUndefined();
+  });
+
   it("server-mints override tokens for additional API keys", async () => {
     const requests: Array<{ url: string; body: unknown }> = [];
     const start = chat.createStartSessionAction("fake-chat", {