Current as of April 2026. GPT-5.1-Codex-Mini is the efficiency king for Hermes Agent users who need high-frequency tool calling without the premium price tag of flagship models. Its 400K context window is the highlight, allowing for massive persistent memory across long-running autonomous sessions.
Specs
| Provider | OpenAI |
| Input cost | $0.25 / M tokens |
| Output cost | $2.00 / M tokens |
| Context window | 400K tokens |
| Max output | 100K tokens |
| Parameters | N/A |
| Features | function_calling, vision, reasoning |
What it’s good at
Reliable Tool Execution
The function calling is precise, rarely hallucinating JSON structures even when Hermes is juggling 40+ built-in tools simultaneously.
Massive Context Window
A 400K token limit allows the agent to maintain deep historical context of Slack and Discord conversations without needing frequent memory pruning.
Cost-Effective Autonomy
At $0.25 per million input tokens, you can keep an agent active 24/7 on multiple messaging platforms for a fraction of the cost of GPT-4o.
Where it falls short
Reasoning Loops
It occasionally gets stuck in repetitive logic cycles when navigating complex MCP protocols compared to full-sized models.
Nuance Handling
The ‘Mini’ architecture can struggle to distinguish between subtle conversational tones when managing multi-channel relays across Slack and Telegram.
Best use cases with Hermes Agent
- Cross-Platform Orchestration — It excels at monitoring Slack triggers to execute shell commands and post formatted updates to Discord in real-time.
- Persistent Background Automation — The low $2/1M output cost and 100K output limit make it perfect for agents that need to generate long reports or logs autonomously.
Not ideal for
- High-Stakes Decision Making — The reduced parameter count means it lacks the deep reasoning required for complex financial or safety-critical automation.
- Real-time Vision Monitoring — While it has vision capabilities, the latency in processing visual data through Hermes can be too slow for high-speed monitoring.
Hermes Agent setup
Set your model ID to openai/gpt-5.1-codex-mini and ensure your OpenAI API key is exported in your environment. You should enable the ‘function_calling’ feature in your Hermes config to utilize the 47 built-in tools effectively.
Hermes makes custom endpoints easy. Run:
hermes model
Choose Custom endpoint from the menu. Enter the base URL and model identifier when prompted:
- Base URL:
https://api.haimaker.ai/v1 - Model:
openai/gpt-5.1-codex-mini
Hermes stores the selection and uses it for all subsequent agent runs across whatever platforms you have wired up (Telegram, Discord, Slack, etc.). Tune HERMES_STREAM_READ_TIMEOUT and related env vars if you’re hitting slow providers.
How it compares
- vs Claude 3.5 Haiku — Haiku is faster for basic text replies, but Codex-Mini’s 400K context window is vastly superior for agents requiring long-term memory.
- vs GPT-4o-mini — Codex-Mini costs slightly more but offers much better reliability for complex tool sequences and MCP integration in autonomous workflows.
Bottom line
For developers building autonomous agents that need to live in messaging platforms and handle complex tool-use on a budget, this model is the most logical choice.
TRY GPT-5.1-CODEX-MINI IN HERMES
For more, see our Hermes local-LLM setup guide.