fix: enable runtime fallback for delegated child sessions (#2357)

src/tools/delegate-task/model-string-parser.ts

@@ -1,10 +1,62 @@
-/**
- * Parse a model string in "provider/model" format.
- */
-export function parseModelString(model: string): { providerID: string; modelID: string } | undefined {
-  const parts = model.split("/")
-  if (parts.length >= 2) {
-    return { providerID: parts[0], modelID: parts.slice(1).join("/") }
+const KNOWN_VARIANTS = new Set([
+  "low",
+  "medium",
+  "high",
+  "xhigh",
+  "max",
+  "none",
+  "auto",
+  "thinking",
+])
+
+function parseVariantFromModelID(rawModelID: string): { modelID: string; variant?: string } {
+  const trimmedModelID = rawModelID.trim()
+  if (!trimmedModelID) {
+    return { modelID: "" }
   }
-  return undefined
+
+  const parenthesizedVariant = trimmedModelID.match(/^(.*)\(([^()]+)\)\s*$/)
+  if (parenthesizedVariant) {
+    const modelID = parenthesizedVariant[1]?.trim() ?? ""
+    const variant = parenthesizedVariant[2]?.trim()
+    return variant ? { modelID, variant } : { modelID }
+  }
+
+  const spaceVariant = trimmedModelID.match(/^(.*\S)\s+([a-z][a-z0-9_-]*)$/i)
+  if (spaceVariant) {
+    const modelID = spaceVariant[1]?.trim() ?? ""
+    const variant = spaceVariant[2]?.trim().toLowerCase()
+    if (variant && KNOWN_VARIANTS.has(variant)) {
+      return { modelID, variant }
+    }
+  }
+
+  return { modelID: trimmedModelID }
+}
+
+export function parseModelString(
+  model: string,
+): { providerID: string; modelID: string; variant?: string } | undefined {
+  const trimmedModel = model.trim()
+  if (!trimmedModel) return undefined
+
+  const parts = trimmedModel.split("/")
+  if (parts.length < 2) {
+    return undefined
+  }
+
+  const providerID = parts[0]?.trim()
+  const rawModelID = parts.slice(1).join("/").trim()
+  if (!providerID || !rawModelID) {
+    return undefined
+  }
+
+  const parsedModel = parseVariantFromModelID(rawModelID)
+  if (!parsedModel.modelID) {
+    return undefined
+  }
+
+  return parsedModel.variant
+    ? { providerID, modelID: parsedModel.modelID, variant: parsedModel.variant }
+    : { providerID, modelID: parsedModel.modelID }
 }