Hermes Agent API Client

Hermes Agent API Client is a typed async Python client for authenticated capability discovery and streaming Chat Completions from the Hermes Agent API Server. The package requires Python 3.13 or later.

Installation

Install the package from PyPI:

uv add hermes-agent-api-client

Usage

HermesAgentApiClient is a single-use async context manager. Without an injected HTTP client, it creates and closes its own httpx.AsyncClient:

from hermes_agent_api_client import HermesAgentApiClient, TerminalEvent


async def ask_hermes(request: dict[str, object]) -> None:
    async with HermesAgentApiClient(
        "https://hermes.example",
        "bearer-key",
    ) as client:
        capabilities = await client.probe_capabilities()
        assert capabilities.chat_completions_streaming

        async for event in client.stream_chat_events(
            request,
            session_id="profile.chat-001",
            session_key="home-assistant-instance-001",
        ):
            if isinstance(event, TerminalEvent):
                print(event.outcome)

The default verify=None uses httpx's verified TLS default. An owned client may instead receive verify=False or an ssl.SSLContext. Disabling certificate verification weakens transport security and should be limited to controlled environments.

To share a caller-managed client, inject it explicitly:

import httpx

from hermes_agent_api_client import HermesAgentApiClient


async def probe_with_shared_transport() -> None:
    async with httpx.AsyncClient(verify=True) as http_client:
        async with HermesAgentApiClient(
            "https://hermes.example",
            "bearer-key",
            http_client=http_client,
        ) as client:
            await client.probe_capabilities()

The caller owns an injected client, so exiting HermesAgentApiClient does not close it. TLS verification must be configured on the injected client; passing both http_client and a non-None verify value is rejected.

Capability discovery can distinguish a non-Hermes endpoint from a Hermes Agent that lacks required Chat Completions support using package-root imports only:

from hermes_agent_api_client import (
    HermesAgentApiClient,
    HermesCapabilityError,
    HermesIdentityError,
)


async def discover_hermes(base_url: str, bearer_key: str) -> str:
    try:
        async with HermesAgentApiClient(base_url, bearer_key) as client:
            capabilities = await client.probe_capabilities()
    except HermesIdentityError:
        return "not a Hermes Agent endpoint"
    except HermesCapabilityError:
        return "Hermes Agent lacks Chat Completions support"
    return capabilities.model

Supported contract

The package supports only these Hermes operations:

• GET /v1/capabilities through probe_capabilities().
• Streaming POST /v1/chat/completions through stream_chat_events().

The exact OpenAI-compatible request document remains a caller-owned mapping; the client does not define additional request-model semantics.

Each stream accepts independent keyword-only session_id and session_key values. Non-None values map only to X-Hermes-Session-Id and X-Hermes-Session-Key; the client never derives identifiers or accepts an arbitrary headers mapping. Values must be exact built-in strings containing 1 through 256 visible ASCII characters. Session IDs additionally reject path shapes (.., /, \, or a leading drive-letter prefix). Invalid values fail locally on the stream's first iteration as a non-retryable HermesTransportError, before network dispatch.

A successful capability probe returns an immutable HermesCapabilities value with this supported shape:

• object: exactly hermes.api_server.capabilities.
• platform: exactly hermes-agent.
• model: the advertised API/profile name.
• auth_type: exactly bearer.
• auth_required: the literal boolean True.
• chat_completions: the literal boolean True.
• chat_completions_streaming: the literal boolean True.

At the immutable upstream compatibility target, model is resolved in order from an explicit API-server model override, a non-default active profile, or the hermes-agent fallback. The client preserves the advertised string exactly: it does not trim, normalize, replace, or truncate leading, trailing, or internal whitespace. Strict validation requires an actual string containing 1 through 255 Unicode code points and at least one non-whitespace code point; empty, whitespace-only, non-string, and over-limit values are rejected.

The stream yields immutable AssistantDeltaEvent, ToolProgressEvent, UsageEvent, KeepaliveEvent, and TerminalEvent values. Terminal outcomes are success, length, or upstream_error. Tool progress exposes only a bounded tool_call_id, tool_name, and the closed running/completed status. Terminal events expose only outcome, partial, and the optional closed failure reasons output_truncated, agent_error, or unknown; additional metadata passed when constructing a public TerminalEvent is rejected. Additive upstream wire fields remain ignored by the private decoder.

Public failures derive from HermesContractError and expose only safe category, status_code, and retryable metadata. HermesIdentityError and HermesCapabilityError both inherit HermesProtocolError, so existing callers that catch HermesProtocolError or HermesContractError continue to catch them. Both subclasses have protocol category, no status code, and retryable=False, with no additional instance state.

Capability-probe classification is staged and deterministic:

Identity failures take precedence over required-feature failures. Capability failures are classified only after both identity discriminators are valid, and all remaining wire failures use the generic HermesProtocolError fallback. Authentication HTTP status, other HTTP status, and transport failures remain HermesAuthenticationError, HermesHttpStatusError, and HermesTransportError, respectively. Callers should not expect upstream response bodies, bearer credentials, request payloads, URLs, or raw validation details in public error text.

Compatibility targets the immutable Hermes Agent tag v2026.7.7.2 at commit 9de9c25f620ff7f1ce0fd5457d596052d5159596. Evidence is derived from a capability fixture captured from that tagged handler and local protocol/transport tests; it is not evidence from a live Hermes server or a moving upstream branch.

Security: Hermes API access exposes the configured agent toolset. Protect the bearer key, restrict network access, and grant the Hermes agent only the tools appropriate for the calling application.

Development

Install the exact locked development environment and the commit hook:

uv sync --locked --all-groups
prek install

prek runs the complete quality gate automatically before every Git commit and blocks the commit when any check fails.

License

Licensed under the Universal Permissive License 1.0 (UPL-1.0). See LICENSE.