diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index f7100b579..1114d1466 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -10,6 +10,7 @@
 
 - Fixed the bundled `websearch` skill description and missing-key guidance omitting the `/login` → **MCP Connections** step required to configure Serper.
 - Fixed `retry_worker` cancelling its own recovery when a stopped session worker left a saved stop marker behind, leaving the session stuck at "Session worker is not connected".
+- Fixed RLM model discovery omitting OpenAI Codex GPT-5.6 models ([#718](https://github.com/PrimeIntellect-ai/prime-agent/pull/718) by [@nickadminroot](https://github.com/nickadminroot)).
 
 ## [0.7.0] - 2026-08-05
 
diff --git a/packages/coding-agent/src/core/model-registry.ts b/packages/coding-agent/src/core/model-registry.ts
index 8644ff5ad..d7cb73851 100644
--- a/packages/coding-agent/src/core/model-registry.ts
+++ b/packages/coding-agent/src/core/model-registry.ts
@@ -27,7 +27,7 @@ import { dirname, join } from "path";
 import { type Static, type TProperties, Type } from "typebox";
 import type { Validator } from "typebox/compile";
 import type { TLocalizedValidationError } from "typebox/error";
-import { getAgentDir, VERSION } from "../config.js";
+import { getAgentDir } from "../config.js";
 import type { AuthSourceToken, AuthStatus, AuthStorage } from "./auth-storage.js";
 import { PRIME_INFERENCE_PROVIDER_ID } from "./prime-inference-auth.js";
 import {
@@ -372,6 +372,10 @@ function readOpenAICodexAccountId(token: string): string | undefined {
 	}
 }
 
+// Codex model discovery gates its catalog by Codex CLI semver, not the Prime Agent version.
+// 0.144.0 is the first catalog version that advertises GPT-5.6 models.
+const OPENAI_CODEX_DISCOVERY_CLIENT_VERSION = "0.144.0";
+
 function openAICodexModelsUrl(baseUrl: string): string {
 	const normalized = baseUrl.replace(/\/+$/, "");
 	let path: string;
@@ -383,7 +387,7 @@ function openAICodexModelsUrl(baseUrl: string): string {
 		path = `${normalized}/codex/models`;
 	}
 	const url = new URL(path);
-	url.searchParams.set("client_version", VERSION);
+	url.searchParams.set("client_version", OPENAI_CODEX_DISCOVERY_CLIENT_VERSION);
 	return url.toString();
 }
 
diff --git a/packages/coding-agent/test/suite/regressions/639-codex-discovery-client-version.test.ts b/packages/coding-agent/test/suite/regressions/639-codex-discovery-client-version.test.ts
new file mode 100644
index 000000000..8ab92d3b0
--- /dev/null
+++ b/packages/coding-agent/test/suite/regressions/639-codex-discovery-client-version.test.ts
@@ -0,0 +1,41 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { createHarness, type Harness } from "../harness.js";
+
+function openAICodexToken(accountId: string): string {
+	const payload = Buffer.from(
+		JSON.stringify({ "https://api.openai.com/auth": { chatgpt_account_id: accountId } }),
+	).toString("base64url");
+	return `header.${payload}.signature`;
+}
+
+describe("#639 OpenAI Codex model discovery", () => {
+	let harness: Harness | undefined;
+
+	afterEach(() => {
+		vi.unstubAllGlobals();
+		harness?.cleanup();
+		harness = undefined;
+	});
+
+	it("sends a Codex client version that advertises GPT-5.6 models", async () => {
+		harness = await createHarness({
+			provider: "openai-codex",
+			models: [{ id: "gpt-5.6-luna" }],
+		});
+		const fetchModels = vi.fn(async (input: string | URL | Request) => {
+			const url = new URL(input instanceof Request ? input.url : input);
+			expect(url.searchParams.get("client_version")).toBe("0.144.0");
+			return new Response(JSON.stringify({ models: [{ slug: "gpt-5.6-luna" }] }), {
+				status: 200,
+				headers: { "content-type": "application/json" },
+			});
+		});
+		vi.stubGlobal("fetch", fetchModels);
+		harness.authStorage.setRuntimeApiKey("openai-codex", openAICodexToken("account-1"));
+
+		await expect(harness.session.findRlmModels("luna", 8)).resolves.toMatchObject({
+			models: [{ selector: "openai-codex/gpt-5.6-luna" }],
+		});
+		expect(fetchModels).toHaveBeenCalledOnce();
+	});
+});