Add OpenAI fallback: gpt-* LLM_MODEL names route to ChatOpenAI

Production safety net for the Fable 5 cutover: flipping LLM_MODEL to a
gpt-* name in the Render dashboard falls back to OpenAI without a code
change. OPENAI_API_KEY re-declared in render.yaml (sync: false) so the
existing dashboard value survives blueprint sync.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: jerelvelarde

.env.example

@@ -2,7 +2,7 @@ ANTHROPIC_API_KEY=
 
 # Claude model — strong models are required for reliable UI generation
 # Recommended: claude-fable-5 (default), claude-opus-4-6
-# To use a different provider, swap the chat model in apps/agent/src/model.py
+# Fallback: gpt-* names route to OpenAI (requires OPENAI_API_KEY)
 LLM_MODEL=claude-fable-5
 
 # Rate limiting (per IP) — disabled by default

README.md

@@ -32,7 +32,7 @@ make dev      # Start all services
 > | `claude-fable-5` | Default |
 > | `claude-opus-4-6` | Strong alternative |
 >
-> To use a different provider, swap the chat model in `apps/agent/src/model.py` (see [docs/bring-to-your-app.md](docs/bring-to-your-app.md)). Smaller or weaker models will produce broken layouts, missing interactivity, or incomplete visualizations.
+> Setting `LLM_MODEL` to a `gpt-*` name routes to OpenAI instead (requires `OPENAI_API_KEY`). For other providers, swap the chat model in `apps/agent/src/model.py` (see [docs/bring-to-your-app.md](docs/bring-to-your-app.md)). Smaller or weaker models will produce broken layouts, missing interactivity, or incomplete visualizations.
 
 - **App**: http://localhost:3000
 - **Agent**: http://localhost:8123

apps/agent/pyproject.toml

@@ -15,6 +15,7 @@ dependencies = [
     "langchain-mcp-adapters>=0.2.1",
     "deepagents>=0.1.0",
     "langchain-anthropic>=1.4.0",
+    "langchain-openai>=1.3.0",
 ]
 
 [dependency-groups]

apps/agent/src/model.py

@@ -3,6 +3,7 @@
 import os
 
 from langchain_anthropic import ChatAnthropic
+from langchain_core.language_models.chat_models import BaseChatModel
 
 DEFAULT_MODEL = "claude-fable-5"
 
@@ -13,8 +14,16 @@
 MAX_TOKENS = 64000
 
 
-def build_model() -> ChatAnthropic:
+def build_model() -> BaseChatModel:
+    model_name = os.environ.get("LLM_MODEL", DEFAULT_MODEL)
+    if model_name.startswith("gpt-"):
+        # Production fallback: gpt-* names route to OpenAI so LLM_MODEL can be
+        # flipped in the deploy dashboard without a code change. No max_tokens
+        # override here — OpenAI's default matches pre-migration behavior.
+        from langchain_openai import ChatOpenAI
+
+        return ChatOpenAI(model=model_name)
     return ChatAnthropic(
-        model=os.environ.get("LLM_MODEL", DEFAULT_MODEL),
+        model=model_name,
         max_tokens=MAX_TOKENS,
     )

apps/agent/tests/test_model.py

@@ -1,12 +1,15 @@
-"""The agent runs on Anthropic Claude — Fable 5 by default, LLM_MODEL overridable."""
+"""The agent runs on Anthropic Claude — Fable 5 by default, LLM_MODEL overridable.
+gpt-* model names route to ChatOpenAI as a production fallback."""
 
 import os
 
 import pytest
 
 os.environ.setdefault("ANTHROPIC_API_KEY", "test-key")
+os.environ.setdefault("OPENAI_API_KEY", "test-key")
 
 from langchain_anthropic import ChatAnthropic
+from langchain_openai import ChatOpenAI
 
 from src.model import build_model
 
@@ -23,6 +26,20 @@ def test_llm_model_env_override(monkeypatch: pytest.MonkeyPatch):
     assert model.model == "claude-opus-4-6"
 
 
+def test_gpt_model_names_route_to_openai_fallback(monkeypatch: pytest.MonkeyPatch):
+    # Production fallback: flipping LLM_MODEL to a gpt-* name in the Render
+    # dashboard must work without a code change.
+    monkeypatch.setenv("LLM_MODEL", "gpt-5.4-2026-03-05")
+    model = build_model()
+    assert isinstance(model, ChatOpenAI)
+    assert model.model_name == "gpt-5.4-2026-03-05"
+
+
+def test_claude_model_names_route_to_anthropic(monkeypatch: pytest.MonkeyPatch):
+    monkeypatch.setenv("LLM_MODEL", "claude-opus-4-6")
+    assert isinstance(build_model(), ChatAnthropic)
+
+
 def test_max_tokens_fits_full_widget_generation():
     # langchain-anthropic's default (4096) truncates generateSandboxedUi args
     # mid-stream: css+html arrive but jsFunctions/jsExpressions are cut off, so