Hermes For Function Calling: Tool-Use Without OpenAI

For most production workloads, the model has to do something — call an API, query a database, write to a calendar. That is function calling. A model that handles it reliably is more valuable in agent stacks than one that writes prettier prose. Hermes was tuned with this in mind, and it shows.

What the Hermes format looks like

Hermes uses a tool-use grammar that the model card documents in detail. The exact tags vary by version, but the pattern is consistent: the system prompt declares the available tools as JSON schemas, the model emits a structured tool-call block, your harness executes it and returns results, and the model continues. This is the same shape OpenAI and Anthropic use — Hermes just exposes it for an open-weight model.

Pattern (illustrative, not exact tags):

[SYSTEM]
You have access to the following tools:
  search_inventory(query: string) -> list of items
  send_email(to: string, subject: string, body: string)

[USER]
Find us 5 size 10 hiking boots and email John the list.

[MODEL]
<tool_call>
  search_inventory({"query": "size 10 hiking boots"})
</tool_call>

[HARNESS RETURNS]
[ {sku: ..., name: ...}, ... ]

[MODEL]
<tool_call>
  send_email({"to": "john@...", "subject": "Boots", "body": "..."})
</tool_call>

Where it shines

• Building agents that should work without an OpenAI dependency.
• Self-hosted assistants where the tool list is sensitive (internal databases, private endpoints).
• Cost-sensitive agent workloads — Hermes 8B handles a lot of routine tool calls cheaply.
• Latency-sensitive use cases on dedicated hardware — local inference avoids round-trip to a cloud API.

Where to be careful

Applied exercise

1. 1Read the function-calling section of the Hermes model card for the version you run.
2. 2Implement a single tool — a fake 'lookup_user(id)' that returns canned data.
3. 3Wire Hermes to call it and verify the model emits the documented format.
4. 4Add a deliberately invalid tool call to your test set and confirm your harness rejects it gracefully.

The big idea: Hermes makes open-weight tool use practical. The work is in the harness — validate everything, allowlist always.