Current as of April 2026. GPT-5.1-Codex is OpenAI’s high-reasoning model optimized for complex tool execution, despite the coding-centric branding. For Hermes Agent, it provides a massive 400K context window and reliable function calling that excels in multi-step autonomous workflows across different messaging platforms.
Specs
| Provider | OpenAI |
| Input cost | $1.25 / M tokens |
| Output cost | $10 / M tokens |
| Context window | 400K tokens |
| Max output | 128K tokens |
| Parameters | N/A |
| Features | function_calling, vision, reasoning |
What it’s good at
Reliable Tool Execution
The model handles complex MCP tool chains with high precision, rarely hallucinating parameters during long autonomous runs.
Massive 400K Context Window
It maintains persistent memory across weeks of Slack and Discord logs without needing to constantly prune the message history.
Advanced Reasoning Chains
The reasoning capabilities allow it to plan multi-platform operations, such as monitoring a webhook and executing shell commands on Modal, with minimal failure.
Where it falls short
High Output Costs
At $10 per million output tokens, it is significantly more expensive than competitors for high-volume automated messaging.
Inherent Latency
The internal reasoning process adds noticeable delay, which can make real-time interactions on WhatsApp feel sluggish.
Verbose Responses
The model tends to over-explain its logic before calling a tool, which consumes unnecessary output tokens in a closed learning loop.
Best use cases with Hermes Agent
- Cross-Platform Orchestration — It effectively manages workflows spanning Discord, SSH, and Docker while keeping track of complex state changes across 47 built-in tools.
- Deep History Analysis — The 400K window allows Hermes to ingest months of platform data to inform its autonomous decisions without losing the thread.
Not ideal for
- Simple Notification Bots — Using a $10/1M output token model for basic Telegram alerts is a waste of budget when cheaper models exist.
- Low-Latency Chat — If your Hermes instance requires instant replies for human interaction, the reasoning overhead will frustrate users.
Hermes Agent setup
Configure the provider as OpenAI and set the model ID to openai/gpt-5.1-codex. You must increase the request timeout in your Hermes config to at least 60 seconds to prevent drops during complex reasoning phases.
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
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 Sonnet — Sonnet is faster and cheaper at $3/$15, but GPT-5.1-Codex handles the 400K context window with much better recall for long-term memory.
- vs GPT-4o — GPT-4o is a better generalist for chat, but Codex is more rigid and reliable when executing strict MCP protocols and shell commands.
Bottom line
GPT-5.1-Codex is the premium choice for Hermes users who need rock-solid tool reliability and massive context windows, provided they can stomach the high output costs and latency.
For more, see our Hermes local-LLM setup guide.