1. Configure Codex

Add the following to ~/.codex/config.toml:

profile="cborg-gpt-mini"

[model_providers.cborg]
name = "CBorg API"
base_url = "https://api.cborg.lbl.gov/v1"
env_key = "CBORG_API_KEY"
supports_websockets = false
wire_api = "responses"

[profiles.cborg-gpt-large]
model = "gpt-5.5"
model_reasoning_effort = "high"
model_provider = "cborg"
personality = "pragmatic"

[profiles.cborg-gpt-mini]
model = "gpt-5.4-mini"
model_reasoning_effort = "medium"
model_provider = "cborg"
personality = "pragmatic"

[profiles.cborg-gemini-pro]
model = "gemini-pro-high"
model_provider = "cborg"
personality = "pragmatic"

[profiles.cborg-gemini-flash]
model = "gemini-flash-high"
model_provider = "cborg"
personality = "pragmatic"

[profiles.openai-gpt-large]
model = "gpt-5.5"
model_reasoning_effort = "high"
personality = "pragmatic"

[profiles.openai-gpt-mini]
model = "gpt-5.4-mini"
model_reasoning_effort = "medium"
personality = "pragmatic"

[features]
fast_mode = false

[notice]
fast_default_opt_out = true

The cborg-* profiles route through the CBorg API using your CBORG_API_KEY. The openai-* profiles use OpenAI directly via ChatGPT Enterprise sign-on.

2. Start Codex

cd my-project
codex

Switch profiles using the --profile flag, e.g.:

codex --profile cborg-gpt-large
codex --profile cborg-gemini-pro

Tips for Reducing Costs

1. Disable fast mode

Fast mode costs 2.5x more than standard processing, and is only about 50% faster. The configuration above disables it by default via fast_mode = false in [features]. You can also toggle it at runtime using the /fast command.

2. Use a smaller model for most tasks

Only switch to a large model when necessary:

• gpt-5.4-mini with “medium” reasoning is around 1/10th the cost of gpt-5.5 and is very performant — try to use the smaller model for most situations and switch to the large model only for the most challenging coding or debugging tasks.
• gpt-5.3-codex has nearly the same performance as gpt-5.5 and is around 1/2 of the cost
• Use /new for each new task rather than allowing excessive context to accumulate
• Disable any unnecessary MCP servers and Skills
• Keep AGENTS.md files to a reasonable length and do not include instructions that mirror what models already do by default, such as “Write clean code.”

3. Recommended default configuration

The configuration in step 1 above sets cborg-gpt-mini as the default profile, which uses a cost-effective model with fast mode disabled. Switch to a larger or alternative model only when needed using the --profile flag.