allow a max_subagents of 0: no subagents for the agent

frontend/packages/swarm-ui/src/agent/detail/AgentInference.tsx

@@ -21,6 +21,7 @@ import { SelectField } from "../../ui/select-field/SelectField.js";
 import { TextField } from "../../ui/text-field/TextField.js";
 import { Button } from "../../ui/button/Button.js";
 import "./AgentInference.css";
+import { MAX_SUBAGENTS, maxSubagentsValue } from "./max-subagents.js";
 
 type Source = "swarm" | "agent_nix" | "local_file" | "default";
 
@@ -67,18 +68,6 @@ function modelNameProblem(name: string): string | null {
   return null;
 }
 
-// The highest `max_subagents` the swarm accepts.
-const MAX_SUBAGENTS = 32;
-
-// `max_subagents` as typed: empty is no cap, else a whole number in range.
-function maxSubagentsValue(text: string): number | null | undefined {
-  const t = text.trim();
-  if (t === "") return undefined;
-  if (!/^\d+$/.test(t)) return null;
-  const n = Number(t);
-  return n >= 1 && n <= MAX_SUBAGENTS ? n : null;
-}
-
 // Subagent models are typed as one list, split on commas and whitespace.
 function parseModels(text: string): string[] {
   return text.split(/[\s,]+/).filter((m) => m !== "");
@@ -112,7 +101,7 @@ export function draftProblem(d: InferenceDraft): string | null {
       .map(modelNameProblem)
       .find((p) => p !== null) ??
     (maxSubagentsValue(d.maxSubagents) === null
-      ? `max subagents is a whole number from 1 to ${MAX_SUBAGENTS}, or empty for no cap`
+      ? `max subagents is a whole number from 0 (no subagents) to ${MAX_SUBAGENTS}, or empty for no cap`
       : null)
   );
 }
@@ -273,7 +262,11 @@ export function AgentInference({ agent }: { agent: string }) {
         <li>
           <Badge
             label="max subagents"
-            value={String(configured?.max_subagents ?? "no cap")}
+            value={
+              configured?.max_subagents === 0
+                ? "0 (no subagents)"
+                : String(configured?.max_subagents ?? "no cap")
+            }
           />
         </li>
       </ul>

frontend/packages/swarm-ui/src/agent/detail/max-subagents.ts

@@ -0,0 +1,17 @@
+// The create form's and the inference dialog's `max_subagents` field, kept out
+// of the component so `npm test` can run it.
+
+/** The highest `max_subagents` the swarm accepts. */
+export const MAX_SUBAGENTS = 32;
+
+/**
+ * `max_subagents` as typed: `undefined` for empty (no cap), the number for a
+ * whole number from 0 (no subagents) to [`MAX_SUBAGENTS`], else `null`.
+ */
+export function maxSubagentsValue(text: string): number | null | undefined {
+  const t = text.trim();
+  if (t === "") return undefined;
+  if (!/^\d+$/.test(t)) return null;
+  const n = Number(t);
+  return n <= MAX_SUBAGENTS ? n : null;
+}

frontend/packages/swarm-ui/test/max-subagents.test.mjs

@@ -0,0 +1,26 @@
+// The max-subagents field: empty is no cap, 0 is no subagents, and anything
+// that is not a whole number up to the limit is refused. Run by `npm test`.
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import {
+  MAX_SUBAGENTS,
+  maxSubagentsValue,
+} from "../src/agent/detail/max-subagents.ts";
+
+test("empty is no cap, and 0 is a cap of none rather than empty", () => {
+  assert.equal(maxSubagentsValue(""), undefined);
+  assert.equal(maxSubagentsValue("  "), undefined);
+  assert.equal(maxSubagentsValue("0"), 0);
+});
+
+test("a whole number up to the limit is taken as typed", () => {
+  assert.equal(maxSubagentsValue("1"), 1);
+  assert.equal(maxSubagentsValue(" 3 "), 3);
+  assert.equal(maxSubagentsValue(String(MAX_SUBAGENTS)), MAX_SUBAGENTS);
+});
+
+test("above the limit, negative, fractional or not a number is refused", () => {
+  for (const text of [String(MAX_SUBAGENTS + 1), "-1", "1.5", "abc", "1e1"]) {
+    assert.equal(maxSubagentsValue(text), null, text);
+  }
+});