Support for Codex via Plano (#808)

* Add Codex CLI support; xAI response improvements

* Add native Plano running check and update CLI agent error handling

* adding PR suggestions for transformations and code quality

* message extraction logic in ResponsesAPIRequest

* xAI support for Responses API by routing to native endpoint + refactor code

demos/llm_routing/codex_router/README.md

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+# Codex Router - Multi-Model Access with Intelligent Routing
+
+Plano extends Codex CLI to access multiple LLM providers through a single interface. This gives you:
+
+1. **Access to Models**: Connect to OpenAI, Anthropic, xAI, Gemini, and local models via Ollama
+2. **Intelligent Routing via Preferences for Coding Tasks**: Configure which models handle specific development tasks:
+   - Code generation and implementation
+   - Code understanding and analysis
+   - Debugging and optimization
+   - Architecture and system design
+
+Uses a [1.5B preference-aligned router LLM](https://arxiv.org/abs/2506.16655) to automatically select the best model based on your request type.
+
+## Benefits
+
+- **Single Interface**: Access multiple LLM providers through the same Codex CLI
+- **Task-Aware Routing**: Requests are analyzed and routed to models based on task type (code generation vs code understanding)
+- **Provider Flexibility**: Add or remove providers without changing your workflow
+- **Routing Transparency**: See which model handles each request and why
+
+## Quick Start
+
+### Prerequisites
+
+```bash
+# Install Codex CLI
+npm install -g @openai/codex
+
+# Install Plano CLI
+pip install planoai
+```
+
+### Step 1: Open the Demo
+
+```bash
+git clone https://github.com/katanemo/arch.git
+cd arch/demos/llm_routing/codex_router
+```
+
+### Step 2: Set API Keys
+
+```bash
+export OPENAI_API_KEY="your-openai-key-here"
+export ANTHROPIC_API_KEY="your-anthropic-key-here"
+export XAI_API_KEY="your-xai-key-here"
+export GEMINI_API_KEY="your-gemini-key-here"
+```
+
+### Step 3: Start Plano
+
+```bash
+planoai up
+# or: uvx planoai up
+```
+
+### Step 4: Launch Codex Through Plano
+
+```bash
+planoai cli-agent codex
+# or: uvx planoai cli-agent codex
+```
+
+By default, `planoai cli-agent codex` starts Codex with `gpt-5.3-codex`. With this demo config:
+
+- `code understanding` prompts are routed to `gpt-5-2025-08-07`
+- `code generation` prompts are routed to `gpt-5.3-codex`
+
+## Monitor Routing Decisions
+
+In a second terminal:
+
+```bash
+sh pretty_model_resolution.sh
+```
+
+This shows each request model and the final model selected by Plano's router.
+
+## Configuration Highlights
+
+`config.yaml` demonstrates:
+
+- OpenAI default model for Codex sessions (`gpt-5.3-codex`)
+- Routing preference override for code understanding (`gpt-5-2025-08-07`)
+- Additional providers (Anthropic, xAI, Gemini, Ollama local) to show cross-provider routing support
+
+## Optional Overrides
+
+Set a different Codex session model:
+
+```bash
+planoai cli-agent codex --settings='{"CODEX_MODEL":"gpt-5-2025-08-07"}'
+```