Programmatic Integration

Hermes ships three protocols for driving the agent from external programs — IDE plugins, custom UIs, CI pipelines, embedded sub-agents. Pick the one that matches your transport and consumer.

All three drive the same AIAgent core. They differ only in wire format and which set of features they expose.

ACP (Agent Client Protocol)

hermes acp starts a stdio JSON-RPC server speaking ACP. Used in production by VS Code (Zed Industries' ACP extension), Zed, and any JetBrains IDE with an ACP plugin.

Capabilities exposed: session creation, prompt submission, streaming agent message chunks, tool-call events, permission requests, session fork, cancel, and authentication. Tool output is rendered into ACP Diff/ToolCall content blocks the IDE understands.

Full lifecycle, event bridge, and approval flow: ACP Internals.

hermes acp                  # serve ACP on stdio
hermes acp --check          # verify ACP dependencies and adapter imports
hermes acp --setup          # interactive provider/model setup for ACP terminal auth

TUI Gateway JSON-RPC

tui_gateway/server.py is the protocol the Ink TUI (hermes --tui) and the embedded dashboard PTY bridge talk to. Any external host can speak the same protocol over stdio (or WebSocket via tui_gateway/ws.py).

Method catalog (selected)

prompt.submit           prompt.background       session.steer
session.create          session.list            session.active_list
session.activate        session.close           session.interrupt
session.history         session.compress        session.branch
session.title           session.usage           session.status
clarify.respond         sudo.respond            secret.respond
approval.respond        config.set / config.get commands.catalog
command.resolve         command.dispatch        cli.exec
reload.mcp              reload.env              process.stop
delegation.status       subagent.interrupt      subagent.steer
spawn_tree.save / list / load
terminal.resize         clipboard.paste         image.attach

session.active_list, session.activate, and session.close are the process-local live-session controls used by the TUI session switcher. Use session.list / /resume for saved transcript discovery; use the active-session methods only for sessions that are currently open in the TUI gateway process.

Rewinding history on prompt.submit

A rewind / edit / regenerate is a prompt.submit that drops part of the stored transcript before running the new turn. Because that write is a destructive rewrite of the session's durable rows, the gateway honors it only when the client states its intent:

A truncation parameter without confirm_truncate is refused with code 4004 or 4029 and nothing is written. Hosts that implement rewind must set the flag at the moment the user asks for it, and must never keep truncation parameters in state across ordinary submits. Prefer truncate_before_row_id (from resume row_id / _row_id) over ordinals; keep the ordinal as a back-compat / optimistic-row path only when no durable id is available yet.

On a successful truncating submit against a durable session, the prompt.submit result additionally carries survivor_user_row_ids — the fresh post-rewrite row IDs of the surviving user turns, in visible-user-ordinal order. The rewrite re-inserts the kept prefix as new rows, so every row ID the host cached before the rewind is stale afterward; rebind cached IDs from this list (a null entry means that turn has no durable ID — drop the cached one) or the next rewind targeting an older surviving turn will be refused with 4018.

Events streamed back

message.delta, message.complete, tool.start, tool.progress, tool.complete, approval.request, clarify.request, sudo.request, sudo.expire, secret.request, secret.expire, gateway.ready, plus session lifecycle and error events. Expiry events carry the original { request_id }; external hosts should clear only the matching pending prompt.

Pi-style RPC mapping

Every command in the Pi-mono RPC spec (issue #360) has a TUI-gateway equivalent:

OpenAI-Compatible API Server

gateway/platforms/api_server.py exposes hermes over HTTP for any client that already speaks the OpenAI format. Useful when you want a web frontend, a curl-driven CI runner, or a non-Python consumer.

Endpoints:

POST /v1/chat/completions        OpenAI Chat Completions (streaming via SSE)
POST /v1/responses               OpenAI Responses API (stateful)
POST /v1/runs                    Start a run, returns run_id (202)
GET  /v1/runs/{id}               Run status
GET  /v1/runs/{id}/events        SSE stream of lifecycle events
POST /v1/runs/{id}/approval      Resolve a pending approval
POST /v1/runs/{id}/steer         Inject mid-run guidance at the next tool boundary
POST /v1/runs/{id}/stop          Interrupt the run
GET  /v1/capabilities            Machine-readable feature flags
GET  /v1/models                  Lists hermes-agent
GET  /api/model/options          Provider-aware picker inventory
GET  /health, /health/detailed

Setup, headers (X-Hermes-Session-Id, X-Hermes-Session-Key), and frontend wiring: API Server.

Model catalog surfaces

The OpenAI-compatible API intentionally keeps GET /v1/models minimal: it is the compatibility endpoint frontends expect, not the full Hermes provider/model picker catalog.

If an external control plane needs Hermes' curated provider rows, per-model pricing, or capability hints, use one of the authenticated picker surfaces:

• API server REST: GET /api/model/options with the API-server bearer key
• Dashboard backend REST: GET /api/model/options with X-Hermes-Session-Token
• TUI gateway RPC: model.options

Those surfaces share the same payload builder and the same custom-provider probe policy:

• Normal open: probe only the current custom provider so offline saved endpoints do not stall the picker.
• Explicit refresh (refresh=1 or refresh: true): bust the provider-model cache and probe all saved custom providers so live catalogs repopulate fully.

Use /v1/models for OpenAI-client compatibility. Use /api/model/options or model.options when you are building a Hermes-aware model picker.

POST /v1/runs/{id}/steer is the HTTP equivalent of Hermes /steer: it does not create a new user turn or immediately rewrite the assistant output already in flight. Instead, the text is appended to the live run and becomes visible to the agent after the next tool boundary, so it can course-correct without discarding the current tool-calling loop.

/v1/runs/{id}/steer is only accepted while the run status is running. Queued, approval-paused, stopping, cancelled, failed, and completed runs return 409 run_not_accepting_steer, even if the server still retains internal agent references during cooperative shutdown.

A 200 (and the run.steered event) means the text was queued, not that the agent consumed it. If a steer lands after the agent's final response — with no later tool boundary to deliver it at — the undelivered text is returned as pending_steer on the terminal run.completed event and run status, so the client can replay it as the next user turn instead of losing it.

Which one should I use?

• You're writing an IDE plugin and the IDE already speaks ACP → ACP. Zero protocol work on the IDE side.
• You're writing a custom desktop / web / TUI host and want every Hermes feature (slash commands, approvals, clarify, multi-agent, session branching) → TUI gateway JSON-RPC.
• You want any OpenAI-compatible frontend, a language-agnostic HTTP client, or curl-driven automation → API server.
• You want a Python in-process embed without a subprocess → import run_agent.AIAgent directly. See Agent Loop.

Model hot-swapping

Mid-session model switching works on every surface — it's the /model slash command under the hood.

• CLI / TUI: /model claude-sonnet-4 or /model openrouter:anthropic/claude-sonnet-4.6
• TUI gateway RPC: command.dispatch with {"command": "/model claude-sonnet-4"}
• ACP: the IDE sends the slash command as a prompt; the agent dispatches it
• API server: include a model field in the request body

Provider-aware resolution (the same model name picks the right format for whatever provider you're on) is built in. See hermes_cli/model_switch.py.

A note on --mode rpc

Hermes does not have a --mode rpc flag. The three protocols above already cover the use cases — ACP for IDE-protocol clients, the TUI gateway for stdio JSON-RPC hosts, and the API server for HTTP. If you find a real gap that none of them fill, open an issue with the concrete consumer you're building.