diff --git a/server/agentModels.test.js b/server/agentModels.test.js
index 98a1445..420082f 100644
--- a/server/agentModels.test.js
+++ b/server/agentModels.test.js
@@ -31,6 +31,7 @@ const ok = (c, m) => {
 // ── optionsForBackend: raw lists incl. the "" CLI-default row ──
 ok(optionsForBackend("codex").some((o) => o.value === ""), "optionsForBackend(codex) includes the (CLI default) row");
 ok(optionsForBackend("codex").some((o) => o.value === "gpt-5.4"), "optionsForBackend(codex) lists codex models");
+ok(["gpt-5.6-sol", "gpt-5.6-terra", "gpt-5.6-luna"].every((s) => optionsForBackend("codex").some((o) => o.value === s)), "#999: optionsForBackend(codex) includes the GPT-5.6 Sol/Terra/Luna slugs");
 ok(optionsForBackend("nope").length === 1 && optionsForBackend("nope")[0].value === "", "optionsForBackend(unknown) → single CLI-default row");
 
 // ── modelsForBackend: concrete (non-"") options, claude fallback ──
diff --git a/src/lib/agentModels.ts b/src/lib/agentModels.ts
index 073ddde..7b4e4cd 100644
--- a/src/lib/agentModels.ts
+++ b/src/lib/agentModels.ts
@@ -13,6 +13,12 @@ export const MODEL_OPTIONS: Record<string, { value: string; label: string }[]> =
     { value: "gpt-5.4", label: "gpt-5.4" },
     { value: "gpt-5", label: "gpt-5" },
     { value: "gpt-4o", label: "gpt-4o" },
+    // #999: GPT-5.6 Sol/Terra/Luna — new concrete slugs. Appended AFTER the
+    // existing rows so modelsForBackend("codex")[0] stays "gpt-5.4" (the #931
+    // default/heal target). Same append pattern as the #958 Fable-5 row.
+    { value: "gpt-5.6-sol", label: "gpt-5.6-sol" },
+    { value: "gpt-5.6-terra", label: "gpt-5.6-terra" },
+    { value: "gpt-5.6-luna", label: "gpt-5.6-luna" },
   ],
   claude: [
     { value: "", label: "(CLI default)" },