Event Hooks

Hermes has four hook systems that run custom code at key lifecycle points:

Hook callback errors are isolated and logged rather than crashing the agent. Hooks are not all passive: directive/control hooks can change flow, transforms can replace content, and a shell pre_tool_call hook can block or fail closed.

Gateway Event Hooks

Gateway hooks fire automatically during gateway operation (Telegram, Discord, Slack, WhatsApp, Teams) without blocking the main agent pipeline.

Creating a Hook

Each hook is a directory under ~/.hermes/hooks/ containing two files:

~/.hermes/hooks/
└── my-hook/
    ├── HOOK.yaml      # Declares which events to listen for
    └── handler.py     # Python handler function

HOOK.yaml

name: my-hook
description: Log all agent activity to a file
events:
  - agent:start
  - agent:end
  - agent:step

The events list determines which events trigger your handler. You can subscribe to any combination of events, including wildcards like command:*.

handler.py

import json
from datetime import datetime
from pathlib import Path

LOG_FILE = Path.home() / ".hermes" / "hooks" / "my-hook" / "activity.log"

async def handle(event_type: str, context: dict):
    """Called for each subscribed event. Must be named 'handle'."""
    entry = {
        "timestamp": datetime.now().isoformat(),
        "event": event_type,
        **context,
    }
    with open(LOG_FILE, "a") as f:
        f.write(json.dumps(entry) + "\n")

Handler rules:

• Must be named handle
• Receives event_type (string) and context (dict)
• Can be async def or regular def — both work
• Errors are caught and logged, never crashing the agent

Available Events

Wildcard Matching

Handlers registered for command:* fire for any command: event (command:model, command:reset, etc.). Monitor all slash commands with a single subscription.

:::tip Threaded replies A handler posting a follow-up message into the same Telegram forum topic should include message_thread_id=int(thread_id) when chat_type == "forum" and thread_id is non-empty. :::

Examples

Telegram Alert on Long Tasks

Send yourself a message when the agent takes more than 10 steps:

# ~/.hermes/hooks/long-task-alert/HOOK.yaml
name: long-task-alert
description: Alert when agent is taking many steps
events:
  - agent:step

# ~/.hermes/hooks/long-task-alert/handler.py
import os
import httpx

THRESHOLD = 10
BOT_TOKEN = os.getenv("TELEGRAM_BOT_TOKEN")
CHAT_ID = os.getenv("TELEGRAM_HOME_CHANNEL")

async def handle(event_type: str, context: dict):
    iteration = context.get("iteration", 0)
    if iteration == THRESHOLD and BOT_TOKEN and CHAT_ID:
        tools = ", ".join(context.get("tool_names", []))
        text = f"⚠️ Agent has been running for {iteration} steps. Last tools: {tools}"
        async with httpx.AsyncClient() as client:
            await client.post(
                f"https://api.telegram.org/bot{BOT_TOKEN}/sendMessage",
                json={"chat_id": CHAT_ID, "text": text},
            )

Command Usage Logger

Track which slash commands are used:

# ~/.hermes/hooks/command-logger/HOOK.yaml
name: command-logger
description: Log slash command usage
events:
  - command:*

# ~/.hermes/hooks/command-logger/handler.py
import json
from datetime import datetime
from pathlib import Path

LOG = Path.home() / ".hermes" / "logs" / "command_usage.jsonl"

def handle(event_type: str, context: dict):
    LOG.parent.mkdir(parents=True, exist_ok=True)
    entry = {
        "ts": datetime.now().isoformat(),
        "command": context.get("command"),
        "args": context.get("args"),
        "platform": context.get("platform"),
        "user": context.get("user_id"),
    }
    with open(LOG, "a") as f:
        f.write(json.dumps(entry) + "\n")

Session Start Webhook

POST to an external service on new sessions:

# ~/.hermes/hooks/session-webhook/HOOK.yaml
name: session-webhook
description: Notify external service on new sessions
events:
  - session:start
  - session:reset

# ~/.hermes/hooks/session-webhook/handler.py
import httpx

WEBHOOK_URL = "https://your-service.example.com/hermes-events"

async def handle(event_type: str, context: dict):
    async with httpx.AsyncClient() as client:
        await client.post(WEBHOOK_URL, json={
            "event": event_type,
            **context,
        }, timeout=5)

Tutorial: BOOT.md — Run a Startup Checklist on Every Gateway Boot

A popular pattern from the community: drop a Markdown checklist at ~/.hermes/BOOT.md, and have the agent run it once every time the gateway starts. Useful for "on every boot, check overnight cron failures and ping me on Discord if anything failed," or "summarize the last 24h of deploy.log and post it to Slack #ops."

This tutorial shows how to build it yourself as a user-defined hook. Hermes does not ship a built-in BOOT.md hook — you wire up exactly the behavior you want.

What we're building

1. A file at ~/.hermes/BOOT.md with natural-language startup instructions.
2. A gateway hook that fires on gateway:startup, spawns a one-shot agent with your gateway's resolved model/credentials, and runs the BOOT.md instructions.
3. A [SILENT] convention so the agent can opt out of sending a message when there's nothing to report.

Step 1: Write your checklist

Create ~/.hermes/BOOT.md. Write it as if you were giving instructions to a human assistant:

# Startup Checklist

1. Run `hermes cron list` and check if any scheduled jobs failed overnight.
2. If any failed, summarize them for Discord #ops (the hook delivers your final response to its configured target).
3. Check if `/opt/app/deploy.log` has any ERROR lines from the last 24 hours. If yes, summarize them and include in the same report.
4. If nothing went wrong, reply with only `[SILENT]` so no message is sent.

The agent sees this as part of its prompt, so anything you can describe in plain language works — tool calls, shell commands, sending messages, summarizing files.

Step 2: Create the hook

~/.hermes/hooks/boot-md/
├── HOOK.yaml
└── handler.py

~/.hermes/hooks/boot-md/HOOK.yaml

name: boot-md
description: Run ~/.hermes/BOOT.md on gateway startup
events:
  - gateway:startup

~/.hermes/hooks/boot-md/handler.py

"""Run ~/.hermes/BOOT.md on every gateway startup."""

import logging
import threading
from pathlib import Path

logger = logging.getLogger("hooks.boot-md")

BOOT_FILE = Path.home() / ".hermes" / "BOOT.md"


def _build_prompt(content: str) -> str:
    return (
        "You are running a startup boot checklist. Follow the instructions "
        "below exactly.\n\n"
        "---\n"
        f"{content}\n"
        "---\n\n"
        "Execute each instruction. Put any user-facing summary in your "
        "final response — the hook delivers it to the configured channel "
        "(e.g. Discord or Slack); you do not send messages yourself.\n"
        "If nothing needs attention and there is nothing to report, reply "
        "with ONLY: [SILENT]"
    )


def _run_boot_agent(content: str) -> None:
    """Spawn a one-shot agent and execute the checklist.

    Uses the gateway's resolved model and runtime credentials so this works
    against custom endpoints, aggregators, and OAuth-based providers alike.
    """
    try:
        from gateway.run import _resolve_gateway_model, _resolve_runtime_agent_kwargs
        from run_agent import AIAgent

        agent = AIAgent(
            model=_resolve_gateway_model(),
            **_resolve_runtime_agent_kwargs(),
            platform="gateway",
            quiet_mode=True,
            skip_context_files=True,
            skip_memory=True,
            max_iterations=20,
        )
        result = agent.run_conversation(_build_prompt(content))
        response = (result.get("final_response", "") or "").strip()
        if response.upper() not in {"[SILENT]", "SILENT", "NO_REPLY", "NO REPLY"}:
            logger.info("boot-md completed: %s", response[:200])
        else:
            logger.info("boot-md completed (nothing to report)")
    except Exception as e:
        logger.error("boot-md agent failed: %s", e)


async def handle(event_type: str, context: dict) -> None:
    if not BOOT_FILE.exists():
        return
    content = BOOT_FILE.read_text(encoding="utf-8").strip()
    if not content:
        return

    logger.info("Running BOOT.md (%d chars)", len(content))

    # Background thread so gateway startup isn't blocked on a full agent turn.
    thread = threading.Thread(
        target=_run_boot_agent,
        args=(content,),
        name="boot-md",
        daemon=True,
    )
    thread.start()

The two key lines:

• _resolve_gateway_model() reads the gateway's currently-configured model.
• _resolve_runtime_agent_kwargs() resolves provider credentials the same way a normal gateway turn does — including API keys, base URLs, OAuth tokens, and credential pools.

Without these, a bare AIAgent() falls back to built-in defaults and will 401 against any non-default endpoint.

Step 3: Test it

Restart the gateway:

hermes gateway restart

Watch the logs:

hermes logs --follow --level INFO | grep boot-md

You should see Running BOOT.md (N chars) followed by either boot-md completed: ... (summary of what the agent did) or boot-md completed (nothing to report) when the agent replied with an exact silence token such as [SILENT].

Delete ~/.hermes/BOOT.md to disable the checklist — the hook stays loaded but silently skips when the file isn't there.

Extending the pattern

• Schedule-aware checklists: key off datetime.now().weekday() inside BOOT.md's instructions ("if it's Monday, also check the weekly deploy log"). The instructions are free-form text, so anything the agent can reason about is fair game.
• Multiple checklists: point the hook at a different file (STARTUP.md, MORNING.md, etc.) and register separate hook directories for each.
• Non-agent variant: if you don't need a full agent loop, skip AIAgent entirely and have the handler post a fixed notification directly via httpx. Cheaper, faster, and has no provider dependency.

Why this isn't a built-in

An earlier version of Hermes shipped this as a built-in hook and silently spawned an agent with bare defaults on every gateway boot. That surprised users with custom endpoints and made the feature invisible to users who didn't know it was running. Keeping it as a documented pattern — built by you, in your hooks directory — means you see exactly what it does and opt in by writing the files.

How It Works

1. On gateway startup, HookRegistry.discover_and_load() scans ~/.hermes/hooks/
2. Each subdirectory with HOOK.yaml + handler.py is loaded dynamically
3. Handlers are registered for their declared events
4. At each lifecycle point, hooks.emit() fires all matching handlers
5. Errors in any handler are caught and logged — a broken hook never crashes the agent

:::info Gateway hooks only fire in the gateway (Telegram, Discord, Slack, WhatsApp, Teams). The CLI does not load gateway hooks. For hooks that work everywhere, use plugin hooks. :::

Plugin Hooks

Plugins can register hooks that fire in both CLI and gateway sessions. These are registered programmatically via ctx.register_hook() in your plugin's register() function.

For plugin packaging and registration details, see the Plugins guide.

def register(ctx):
    ctx.register_hook("pre_tool_call", my_tool_observer)
    ctx.register_hook("post_tool_call", my_tool_logger)
    ctx.register_hook("pre_llm_call", my_memory_callback)
    ctx.register_hook("post_llm_call", my_sync_callback)
    ctx.register_hook("on_session_start", my_init_callback)
    ctx.register_hook("on_session_end", my_cleanup_callback)
    # Kanban board lifecycle (dependency-wait blocking may fire inside its transaction):
    ctx.register_hook("kanban_task_claimed", my_claim_callback)     # dispatcher process
    ctx.register_hook("kanban_task_completed", my_done_callback)    # worker process
    ctx.register_hook("kanban_task_blocked", my_blocked_callback)   # worker process

General rules for all hooks:

• Callbacks receive keyword arguments. Always accept **kwargs for forward compatibility.
• Callback exceptions are logged and skipped; later callbacks continue.
• The catalog below is descriptive: observers ignore returns, transforms accept the first valid string replacement, and directive/control hooks consume documented return shapes. Plugin middleware is a separate registry and surface, not another hook category.
• Correlation fields such as turn_id, api_request_id, task_id, session_id, and api_call_count are hook-specific and may be absent. Treat IDs as opaque.
• Runtime event-name validity comes from hermes_cli.plugins.VALID_HOOKS. hermes hooks list lists configured shell/outbound hooks, not every available event; hermes hooks test <event> reports the valid set only when an invalid event is supplied.

Cache-safe system prompt sections

Plugins that need durable, always-on guidance can register a bounded system prompt section instead of injecting the same text through pre_llm_call on every turn:

def board_rules(session_info):
    return f"Apply the worker rules for profile {session_info['profile_name']}."

def register(ctx):
    ctx.register_system_prompt_section(
        "kanban-advanced.worker-rules",
        board_rules,                       # a string is also accepted
        position="after_memory",
        max_chars=4000,
    )

The contract is deliberately narrow:

• IDs are global, stable, 1–128 character lowercase identifiers using only letters, numbers, ., _, and -. Duplicate IDs are rejected.
• after_memory is the only placement anchor. Sections are sorted by ID, rendered after memory/profile context and before session metadata; plugins cannot reorder or replace core prompt content.
• A callable receives a read-only mapping with session_id, model, provider, platform, profile_name, and cwd. It runs once for a new session. Its rendered bytes are frozen on compression and recovered from the already-persisted full system prompt after a process restart/resume; plugin state is not re-read for an existing session.
• max_chars is capped at 4,000 characters. All plugin sections together, including their audit headings, are capped at 8,000 characters and 32 sections. Empty, non-string, oversized, aggregate-over-budget, or raising sections are skipped with a warning; prompt construction continues.
• Every accepted section is named in the prompt and logged at session start with its plugin, position, and character count.

Use pre_llm_call for truly dynamic per-turn context. There is intentionally no plugin environment-hints hook in this contract: changing cwd, branch, or other environment data must not silently mutate a session's cached prompt. Such a hook needs a concrete consumer and the same frozen/resume-safe semantics before it can be added.

Shipped plugin-hook catalog

Payload fields below are the exact event-specific fields supplied by each call site. For backward compatibility, PluginManager also adds telemetry_schema_version="hermes.observer.v1" to every plugin-hook callback. That legacy envelope marker does not mean all hook payloads share one semantic schema; new versioned contracts belong to their concrete event or capability family.

Streaming output hooks

These observer-only hooks let plugins consume streaming LLM output for telemetry, live dashboards, or TTS pipelines without changing the response. They are delivered through host-owned bounded queues with one background worker per registered callback, so plugin callbacks never run inline on the token path. If one callback stalls, only that callback's queue can fill and drop its oldest pending observer event; other observers continue receiving events independently.

Register them like any other plugin hook:

def on_delta(delta, kind, model, provider, **kwargs):
    if kind == "text":
        print(delta, end="", flush=True)

def register(ctx):
    ctx.register_hook("on_stream_delta", on_delta)

Common fields for all four hooks:

Additional fields:

on_interim_message can also fire after a non-streaming response, so registering only that hook does not force a provider call onto streaming transport.

Reasoning deltas are not exposed to plugins by default. Opt in explicitly:

plugins:
  stream_reasoning_deltas: true

Return values are ignored. To keep the stream fast, callbacks should enqueue their own work and return quickly. Exceptions are logged and do not stop the stream.

pre_tool_call

Fires immediately before every tool execution — built-in tools and plugin tools alike.

Callback signature:

def my_callback(tool_name: str, args: dict, task_id: str, **kwargs):

Fires: In model_tools.py, inside handle_function_call(), before the tool's handler runs. Fires once per tool call — if the model calls 3 tools in parallel, this fires 3 times.

Return value — block or require approval:

return {"action": "block", "message": "Reason the tool call was blocked"}
# or
return {"action": "approve", "message": "Why approval is required", "rule_key": "optional:scope"}

The first valid directive wins. block requires a non-empty message and short-circuits the tool with that text as the error returned to the model. approve escalates the call to the existing human-approval gate; message and rule_key are optional, and denial, timeout, or gate error fails closed. Other return values are ignored.

Use cases: Logging, audit trails, tool call counters, blocking dangerous operations, rate limiting, per-user policy enforcement.

Example — tool call audit log:

import json, logging
from datetime import datetime

logger = logging.getLogger(__name__)

def audit_tool_call(tool_name, args, task_id, **kwargs):
    logger.info("TOOL_CALL session=%s tool=%s args=%s",
                task_id, tool_name, json.dumps(args)[:200])

def register(ctx):
    ctx.register_hook("pre_tool_call", audit_tool_call)

Example — warn on dangerous tools:

DANGEROUS = {"terminal", "write_file", "patch"}

def warn_dangerous(tool_name, **kwargs):
    if tool_name in DANGEROUS:
        print(f"⚠ Executing potentially dangerous tool: {tool_name}")

def register(ctx):
    ctx.register_hook("pre_tool_call", warn_dangerous)

post_tool_call

Fires immediately after every tool execution returns.

Callback signature:

def my_callback(tool_name: str, args: dict, result: str, task_id: str,
                duration_ms: int, **kwargs):

Fires: In model_tools.py, inside handle_function_call(), after the tool's handler returns. Fires once per tool call. Does not fire if the tool raised an unhandled exception (the error is caught and returned as an error JSON string instead, and post_tool_call fires with that error string as result).

Return value: Ignored.

Use cases: Logging tool results, metrics collection, tracking tool success/failure rates, latency dashboards, per-tool budget alerts, sending notifications when specific tools complete.

Example — track tool usage metrics:

from collections import Counter, defaultdict
import json

_tool_counts = Counter()
_error_counts = Counter()
_latency_ms = defaultdict(list)

def track_metrics(tool_name, result, duration_ms=0, **kwargs):
    _tool_counts[tool_name] += 1
    _latency_ms[tool_name].append(duration_ms)
    try:
        parsed = json.loads(result)
        if "error" in parsed:
            _error_counts[tool_name] += 1
    except (json.JSONDecodeError, TypeError):
        pass

def register(ctx):
    ctx.register_hook("post_tool_call", track_metrics)

pre_llm_call

Fires once per turn, before the tool-calling loop begins. All valid callback returns are aggregated in plugin order and injected into the current turn's user message.

Callback signature:

def my_callback(session_id: str, user_message: str, conversation_history: list,
                is_first_turn: bool, model: str, platform: str, **kwargs):

Fires: In run_agent.py, inside run_conversation(), after context compression but before the main while loop. Fires once per run_conversation() call (i.e. once per user turn), not once per API call within the tool loop.

Return value: If the callback returns a dict with a "context" key, or a plain non-empty string, the text is appended to the current turn's user message. Return None for no injection.

# Inject context
return {"context": "Recalled memories:\n- User likes Python\n- Working on hermes-agent"}

# Plain string (equivalent)
return "Recalled memories:\n- User likes Python"

# No injection
return None

Where context is injected: Always the user message, never the system prompt. This preserves the prompt cache — the system prompt stays identical across turns, so cached tokens are reused. The system prompt is Hermes's territory (model guidance, tool enforcement, personality, skills). Plugins contribute context alongside the user's input.

The clean user-message content remains unchanged. For replay and prompt-cache stability, Hermes may persist the exact API-bound message, including plugin-injected context, in the row's api_content sidecar.

When multiple plugins return context, their outputs are joined with double newlines in plugin discovery order (alphabetical by directory name).

Use cases: Memory recall, RAG context injection, guardrails, per-turn analytics.

Example — memory recall:

import httpx

MEMORY_API = "https://your-memory-api.example.com"

def recall(session_id, user_message, is_first_turn, **kwargs):
    try:
        resp = httpx.post(f"{MEMORY_API}/recall", json={
            "session_id": session_id,
            "query": user_message,
        }, timeout=3)
        memories = resp.json().get("results", [])
        if not memories:
            return None
        text = "Recalled context:\n" + "\n".join(f"- {m['text']}" for m in memories)
        return {"context": text}
    except Exception:
        return None

def register(ctx):
    ctx.register_hook("pre_llm_call", recall)

Example — guardrails:

POLICY = "Never execute commands that delete files without explicit user confirmation."

def guardrails(**kwargs):
    return {"context": POLICY}

def register(ctx):
    ctx.register_hook("pre_llm_call", guardrails)

post_llm_call

Fires once per turn, after the tool-calling loop completes and the agent has produced a final response. Only fires on successful turns — does not fire if the turn was interrupted.

Callback signature:

def my_callback(session_id: str, user_message: str, assistant_response: str,
                conversation_history: list, model: str, platform: str, **kwargs):

Fires: In run_agent.py, inside run_conversation(), after the tool loop exits with a final response. Guarded by if final_response and not interrupted — so it does not fire when the user interrupts mid-turn or the agent hits the iteration limit without producing a response.

Return value: Ignored.

Use cases: Syncing conversation data to an external memory system, computing response quality metrics, logging turn summaries, triggering follow-up actions.

Example — sync to external memory:

import httpx

MEMORY_API = "https://your-memory-api.example.com"

def sync_memory(session_id, user_message, assistant_response, **kwargs):
    try:
        httpx.post(f"{MEMORY_API}/store", json={
            "session_id": session_id,
            "user": user_message,
            "assistant": assistant_response,
        }, timeout=5)
    except Exception:
        pass  # best-effort

def register(ctx):
    ctx.register_hook("post_llm_call", sync_memory)

Example — track response lengths:

import logging
logger = logging.getLogger(__name__)

def log_response_length(session_id, assistant_response, model, **kwargs):
    logger.info("RESPONSE session=%s model=%s chars=%d",
                session_id, model, len(assistant_response or ""))

def register(ctx):
    ctx.register_hook("post_llm_call", log_response_length)

pre_verify

Fires once per turn when the agent edited code, just before it finishes (after the built-in verify-on-stop guard). This is a user/plugin policy gate: a callback can keep the agent going — run a check, defer it, tidy the diff — instead of letting it stop.

Hermes' shipped verification guidance is not a default pre_verify hook. It is appended to the evidence-based verify-on-stop nudge when edited code lacks fresh verification evidence, so it does not create a second default continuation path. Set agent.verify_guidance: false to keep that built-in evidence nudge terse.

Callback signature:

def my_callback(session_id: str, platform: str, model: str, coding: bool,
                attempt: int, final_response: str, changed_paths: list, **kwargs):

Scope a hook to the coding context by checking coding and make it one-shot with attempt (shell hooks read both from .extra), the same way a pre_tool_call hook scopes on tool_name — so you can register several pre_verify hooks, each firing only where it should.

Fires: In agent/conversation_loop.py, at the point the agent would accept a final answer, immediately after the verify-on-stop check — but only when the agent edited code this turn and at least one pre_verify hook is registered.

Return value — keep the agent going:

return {"action": "continue", "message": "Run the formatter on your changes, then finish."}

The message is appended as a synthetic user turn and the loop runs again. The Claude-Code Stop shape ({"decision": "block", "reason": "..."}, where blocking the stop means keep going) is accepted too. A directive with no message — or any other return — lets the turn finish.

Bounded: consecutive continue directives in one turn are capped by agent.max_verify_nudges (default 3), so a hook that always says continue can never trap the loop. The attempted answer is kept in history but not surfaced to the user while the agent is being nudged.

Make it idempotent: the hook re-fires after each nudge, so gate on attempt (if attempt: return None) — otherwise it just nudges until the bound is hit.

Use cases: defer tests/lints during creative iteration, require green checks for certain paths, block "done" until a changelog entry exists, run a project-specific verification checklist.

Example — defer checks on creative UI work, scoped + one-shot:

UI = (".tsx", ".jsx", ".css", ".scss")

def defer_ui_checks(coding, attempt, changed_paths, **kwargs):
    if attempt or not coding:
        return None  # one-shot, coding only
    if not all(p.endswith(UI) for p in changed_paths):
        return None  # only pure-UI edits
    return {
        "action": "continue",
        "message": "This is UI work — don't run tests/lints yet; ask the user to "
                   "eyeball it first, and clean the diff before any commit.",
    }

def register(ctx):
    ctx.register_hook("pre_verify", defer_ui_checks)

For standing guidance that should shape the built-in missing-evidence nudge, use agent.verify_guidance. For broader coding posture rules that don't need to gate verification, prefer agent.coding_instructions in config.yaml — it rides the coding brief and costs no extra turn.

transform_api_error_classification

Fires once per failed API call, at the top of agent/error_classifier.classify_api_error(), before the built-in pipeline. Provider plugins use it to own their provider's error quirks without core patches. It is behavior-changing (transform family): the returned classification drives retry, compression, credential rotation, and fallback routing.

Callbacks receive the parsed error context as kwargs — provider (self-scope on this), model, status_code, error_type, error_code, error_message, error_body, error, approx_tokens, context_length, num_messages. Return None to decline, or a dict to claim the error:

return {"reason": "model_not_found",   # required: a FailoverReason name
        "retryable": False, "should_fallback": True}  # optional recovery-hint overrides

Dispatch is run-all-then-pick-first: every callback runs, failures are isolated, and the first valid result in registration order wins (valid-but-losing results log a runtime warning). Invalid dicts and unknown reasons are skipped, so a broken plugin can never break classification.

Privacy: error_message and error_body may carry unredacted provider data. Python plugins only — shell registrations are refused at config parse with a warning.

on_session_start

Fires once when a brand-new session is created. Does not fire on session continuation (when the user sends a second message in an existing session).

Callback signature:

def my_callback(session_id: str, model: str, platform: str, **kwargs):

Fires: In run_agent.py, inside run_conversation(), during the first turn of a new session — specifically after the system prompt is built but before the tool loop starts. The check is if not conversation_history (no prior messages = new session).

Return value: Ignored.

Use cases: Initializing session-scoped state, warming caches, registering the session with an external service, logging session starts.

Example — initialize a session cache:

_session_caches = {}

def init_session(session_id, model, platform, **kwargs):
    _session_caches[session_id] = {
        "model": model,
        "platform": platform,
        "tool_calls": 0,
        "started": __import__("datetime").datetime.now().isoformat(),
    }

def register(ctx):
    ctx.register_hook("on_session_start", init_session)

on_session_end

Fires at the very end of every run_conversation() call, regardless of outcome. Also fires from the CLI's exit handler if the agent was mid-turn when the user quit.

Callback signature:

def my_callback(session_id: str, completed: bool, interrupted: bool,
                model: str, platform: str, **kwargs):

Fires: In two places:

1. run_agent.py — at the end of every run_conversation() call, after all cleanup. Always fires, even if the turn errored.
2. cli.py — in the CLI's atexit handler, but only if the agent was mid-turn (_agent_running=True) when the exit occurred. This catches Ctrl+C and /exit during processing. In this case, completed=False and interrupted=True.

Return value: Ignored.

Use cases: Flushing buffers, closing connections, persisting session state, logging session duration, cleanup of resources initialized in on_session_start.

Example — flush and cleanup:

_session_caches = {}

def cleanup_session(session_id, completed, interrupted, **kwargs):
    cache = _session_caches.pop(session_id, None)
    if cache:
        # Flush accumulated data to disk or external service
        status = "completed" if completed else ("interrupted" if interrupted else "failed")
        print(f"Session {session_id} ended: {status}, {cache['tool_calls']} tool calls")

def register(ctx):
    ctx.register_hook("on_session_end", cleanup_session)

Example — session duration tracking:

import time, logging
logger = logging.getLogger(__name__)

_start_times = {}

def on_start(session_id, **kwargs):
    _start_times[session_id] = time.time()

def on_end(session_id, completed, interrupted, **kwargs):
    start = _start_times.pop(session_id, None)
    if start:
        duration = time.time() - start
        logger.info("SESSION_DURATION session=%s seconds=%.1f completed=%s interrupted=%s",
                     session_id, duration, completed, interrupted)

def register(ctx):
    ctx.register_hook("on_session_start", on_start)
    ctx.register_hook("on_session_end", on_end)

on_session_finalize

Fires when the CLI or gateway tears down an active session — for example, when the user runs /new, the gateway GC'd an idle session, or the CLI quit with an active agent. Use it to flush state tied to the outgoing session ID. On gateway reset, the replacement session already exists before this callback runs.

Callback signature:

def my_callback(session_id: str | None, platform: str, **kwargs):

Fires: In CLI/TUI teardown and in gateway reset, shutdown, or idle-expiry paths. Gateway shutdown and expiry can finalize without a matching on_session_reset.

Return value: Ignored.

Use cases: Persist final session metrics before the session ID is discarded, close per-session resources, emit a final telemetry event, drain queued writes.

on_session_reset

Fires at a CLI or TUI session boundary, or when the gateway swaps in a new session key for an active chat. This lets plugins react to cleared conversation state without waiting for the next on_session_start.

Callback signature:

def my_callback(session_id: str, platform: str, **kwargs):

Fires: CLI supplies session_id, platform, and reason; TUI supplies session_id and platform; gateway adds reason, old_session_id, and new_session_id after allocating the replacement key. On gateway reset, the order is: create and persist the replacement → on_session_finalize(old_id) → on_session_reset(new_id) → on_session_start(new_id) on the first inbound turn.

Return value: Ignored.

Use cases: Reset per-session caches keyed by session_id, emit "session rotated" analytics, prime a fresh state bucket.

See the Build a Plugin guide for the full walkthrough including tool schemas, handlers, and advanced hook patterns.

subagent_start

Fires once per child agent after delegate_task has constructed the child AIAgent and before that child is run. Whether you delegate a single task or a batch of three, this hook fires once for each child.

This hook is specific to delegation/subagent lifecycle. It is not a universal "before any agent invocation" gate for gateway, CLI, cron, batch, MoA, or other runner-originated agent executions.

Callback signature:

def my_callback(parent_session_id: str | None,
                parent_turn_id: str,
                parent_subagent_id: str | None,
                child_session_id: str | None,
                child_subagent_id: str,
                child_role: str,
                child_goal: str,
                **kwargs):

Fires: In tools/delegate_tool.py, inside _build_child_agent(), after the child AIAgent has been constructed and annotated with subagent identity metadata, and before _run_single_child() runs the child.

Return value: Ignored. This is an observer hook only; returning a value does not block or mutate the child agent run.

Use cases: Logging subagent creation, mapping parent/child session relationships, tracking nested delegation trees, emitting pre-run audit records, pre-allocating per-child observability resources.

Example — log subagent creation:

import logging

logger = logging.getLogger(__name__)

def log_subagent_start(
    parent_session_id,
    parent_turn_id,
    child_session_id,
    child_subagent_id,
    child_role,
    child_goal,
    **kwargs,
):
    logger.info(
        "SUBAGENT_START parent=%s turn=%s child_session=%s child=%s role=%s goal=%r",
        parent_session_id,
        parent_turn_id,
        child_session_id,
        child_subagent_id,
        child_role,
        child_goal[:200],
    )

def register(ctx):
    ctx.register_hook("subagent_start", log_subagent_start)

:::info subagent_start is useful for delegation observability, but it is not a blocking policy hook. To block delegation before a child is built, use pre_tool_call to block the delegate_task tool call. :::

subagent_stop

Fires once per child agent after delegate_task finishes. Whether you delegated a single task or a batch of three, this hook fires once for each child, serialised on the parent thread.

Callback signature:

def my_callback(parent_session_id: str, child_role: str | None,
                child_summary: str | None, child_status: str,
                tool_call_history: list[dict], duration_ms: int, **kwargs):

Fires: In tools/delegate_tool.py, after ThreadPoolExecutor.as_completed() drains all child futures. Firing is marshalled to the parent thread so hook authors don't have to reason about concurrent callback execution.

Return value: Ignored.

Use cases: Logging orchestration activity, accumulating child durations for billing, writing post-delegation audit records.

Example — log orchestrator activity:

import logging
logger = logging.getLogger(__name__)

def log_subagent(parent_session_id, child_role, child_status, duration_ms, **kwargs):
    logger.info(
        "SUBAGENT parent=%s role=%s status=%s duration_ms=%d",
        parent_session_id, child_role, child_status, duration_ms,
    )

def register(ctx):
    ctx.register_hook("subagent_stop", log_subagent)

:::info With heavy delegation (e.g. orchestrator roles × 5 leaves × nested depth), subagent_stop fires many times per turn. Keep your callback fast; push expensive work to a background queue. :::

pre_gateway_dispatch

Fires once per incoming MessageEvent in the gateway, after the internal-event guard but before auth/pairing and agent dispatch. This is the interception point for gateway-level message-flow policies (listen-only windows, human handover, per-chat routing, etc.) that don't fit cleanly into any single platform adapter.

Callback signature:

def my_callback(event, gateway, session_store, **kwargs):

Fires: In gateway/run.py, inside GatewayRunner._handle_message(), immediately after is_internal is computed. Internal events skip the hook entirely (they are system-generated — background-process completions, etc. — and must not be gate-kept by user-facing policy).

Return value: None or a dict. The first recognized action dict wins; remaining plugin results are ignored. Exceptions in plugin callbacks are caught and logged; the gateway always falls through to normal dispatch on error.

Use cases: Listen-only group chats (only respond when tagged; buffer ambient messages into context); human handover (silent-ingest customer messages while owner handles the chat manually); per-profile rate limiting; policy-driven routing.

Example — drop unauthorized DMs silently without triggering the pairing code:

def deny_unauthorized_dms(event, **kwargs):
    src = event.source
    if src.chat_type == "dm" and not _is_approved_user(src.user_id):
        return {"action": "skip", "reason": "unauthorized-dm"}
    return None

def register(ctx):
    ctx.register_hook("pre_gateway_dispatch", deny_unauthorized_dms)

Example — rewrite an ambient-message buffer into a single prompt on mention:

_buffers = {}

def buffer_or_rewrite(event, **kwargs):
    key = (event.source.platform, event.source.chat_id)
    buf = _buffers.setdefault(key, [])
    if _bot_mentioned(event.text):
        combined = "\n".join(buf + [event.text])
        buf.clear()
        return {"action": "rewrite", "text": combined}
    buf.append(event.text)
    return {"action": "skip", "reason": "ambient-buffered"}

def register(ctx):
    ctx.register_hook("pre_gateway_dispatch", buffer_or_rewrite)

gateway_platform_event

Fires for supported platform-native events only after the gateway's normal, profile-scoped authorization check succeeds. The callback receives plain dictionaries; raw SDK objects, adapter handles, bot clients, and callback contexts are never part of this stable contract.

Telegram message reactions were the first supported event; message edits, deletes, and thread lifecycle events followed:

def on_platform_event(platform, event_type, payload, **kwargs):
    if platform == "telegram" and event_type == "reaction":
        print(payload["chat_id"], payload["message_id"], payload["emojis"])
    elif event_type == "message_edited":
        print(platform, payload["chat_id"], payload["message_id"], payload["text"])

def register(ctx):
    ctx.register_hook("gateway_platform_event", on_platform_event)

Every payload is additive and event-specific; there is no monolithic gateway payload version. All ids are strings; missing/unavailable fields are None, never guessed. Malformed events and events whose source cannot be authorized are dropped (fail closed). A transient Telegram Application rebuild re-registers the observer together with the core handlers.

Per-event payload contracts (v1, additive):

The bot's own progressive message edits (streaming) never fire message_edited on Discord — bot-authored events are dropped at the fire-site.

This hook is observer-only: it does not add raw-event access or adapter access. Raw SDK payload access is deliberately not shipped — adapter SDK objects change shape without notice and would become un-evolvable API surface; where genuinely needed it requires its own explicit capability (gateway.raw_events) with a "no stability guarantee" label and its own design (tracked in #64228). For acting on a platform (adding a reaction, renaming a thread), use the capability-gated ctx.platform_actions facade documented in the plugins guide — it is gated off by default behind the gateway.platform_actions capability. PluginContext.dispatch_tool() can only call tools registered in the tool registry; send_message is intentionally not registered there (its transport is reserved for explicit CLI, cron, kanban, and MCP delivery paths). A future outbound-delivery contract must first provide stable delivered content/handles across all adapters; this slice does not pre-register an inert gateway_message_delivered hook.

pre_approval_request

Fires before an approval decision is requested. It covers prompted surfaces—interactive CLI, Ink TUI, gateway platforms, and ACP clients—and approvals.mode=smart decisions made without a human prompt (surface="smart"). In smart mode, the hook runs before the auxiliary LLM is called.

This is the right place to wire a custom notifier — for example, a macOS menu-bar app that pops an allow/deny notification, or an audit log that records every approval request with context.

Callback signature:

def my_callback(
    command: str,
    description: str,
    pattern_key: str,
    pattern_keys: list[str],
    session_key: str,
    surface: str,
    **kwargs,
):

Return value: ignored. Hooks here are observer-only; they cannot veto or pre-answer the approval. Use pre_tool_call to block a tool before it reaches the approval system.

Use cases: Desktop notifications, push alerts, audit logging, Slack webhooks, escalation routing, metrics.

Example — desktop notification on macOS:

import subprocess

def notify_approval(command, description, session_key, **kwargs):
    title = "Hermes needs approval"
    body = f"{description}: {command[:80]}"
    subprocess.Popen([
        "osascript", "-e",
        f'display notification "{body}" with title "{title}"',
    ])

def register(ctx):
    ctx.register_hook("pre_approval_request", notify_approval)

post_approval_response

Fires after a prompted or smart approval decision, after a prompt times out, or when the gateway cannot deliver the approval notification. Notification failure emits choice="notify_failed" before any approval decision exists.

Callback signature:

def my_callback(
    command: str,
    description: str,
    pattern_key: str,
    pattern_keys: list[str],
    session_key: str,
    surface: str,
    choice: str,
    **kwargs,
):

Same kwargs as pre_approval_request, plus:

Return value: ignored.

Use cases: Close the matching desktop notification, record the final decision in an audit log, update metrics, roll forward a rate limiter.

def log_decision(command, choice, session_key, **kwargs):
    logger.info("approval %s: %s for session %s", choice, command[:60], session_key)

def register(ctx):
    ctx.register_hook("post_approval_response", log_decision)

pre_transcription

Fires inside the STT dispatcher (tools.transcription_tools.transcribe_audio) after the provider has been resolved and before any backend is invoked, whether that backend is built-in, a type: command provider, or a plugin-registered provider. Lets a plugin steer the transcription request itself instead of only observing the transcript afterwards.

Callback signature:

def my_callback(
    file_path: str,
    provider: str,
    model: str | None,
    language: str | None,
    prompt: str | None,
    source: str | None,
    **kwargs,
) -> dict | None:

Return value: a dict with any of "prompt", "language", "model" mapped to strings, or None to leave the request unchanged. Non-string values, unknown keys, and file_path are ignored (file_path attempts are logged as a warning). Results are applied in registration order, last-writer-wins per field, on top of the stt.prompt config value. Returning "" for prompt clears the configured prompt for that request.

Use cases: Inject a per-user or per-chat vocabulary list before the audio is uploaded, force language from the caller's locale, downgrade model for long recordings, route noisy sources to a different model.

VOCAB = "Hermes, Teknium, Nous Research, kanban"

def add_vocab(provider, prompt, source, **kwargs):
    if source != "gateway":
        return None
    return {"prompt": f"{prompt}. {VOCAB}" if prompt else VOCAB}

def register(ctx):
    ctx.register_hook("pre_transcription", add_vocab)

Not every backend accepts a prompt. local maps it to faster-whisper's initial_prompt; openai, groq, mistral, and deepinfra send it as prompt; xai, elevenlabs, local_command, and type: command providers log at DEBUG and transcribe without it. See the provider support table for the full matrix and the privacy boundary. Hook-plumbing errors are fail-open: the dispatch continues with the unmodified request.

transform_tool_result

Fires after a tool returns and before the result is appended to the conversation. Lets a plugin rewrite ANY tool's result string — not just terminal output — before the model sees it.

Callback signature:

def my_callback(tool_name: str, args: dict, result: str, task_id: str, **kwargs) -> str | None:

The full payload also includes session_id, tool_call_id, turn_id, api_request_id, duration_ms, status, error_type, and error_message. result is the final result returned by tool dispatch; it and args can contain arbitrary user/tool content and secrets.

Return value: The first str replaces the result (including an empty string); None leaves it unchanged.

Use cases: Redact organization-specific PII from web_extract output, wrap long JSON tool responses in a summary header, inject retrieval-augmented hints into read_file results, rewrite delegate_task subagent reports into a project-specific schema.

import re
SECRET = re.compile(r"sk-[A-Za-z0-9]{32,}")

def redact_secrets(tool_name, result, **kwargs):
    if SECRET.search(result):
        return SECRET.sub("[REDACTED]", result)
    return None

def register(ctx):
    ctx.register_hook("transform_tool_result", redact_secrets)

Applies to every tool. For terminal-only rewriting see transform_terminal_output below — it is narrower, runs before transform_tool_result, and its replacement is still subject to the terminal tool's final output limit.

transform_terminal_output

Fires inside the terminal tool after foreground process capture has already been bounded by the environment, and before the final output limit. It lets plugins replace the captured stdout/stderr; the replacement is still subject to the final output limit.

Callback signature:

def my_callback(
    command: str,
    output: str,
    returncode: int,
    task_id: str,
    env_type: str,
    **kwargs,
) -> str | None:

Return value: First str replaces the output; None leaves it unchanged. Command and output can contain credentials or other sensitive data.

def summarize_find(command, output, **kwargs):
    if command.startswith("find ") and len(output) > 50_000:
        lines = output.count("\n")
        head = "\n".join(output.splitlines()[:40])
        return f"{head}\n\n[summary: {lines} paths total, showing first 40]"
    return None

def register(ctx):
    ctx.register_hook("transform_terminal_output", summarize_find)

Pairs with transform_tool_result, which runs afterward for every tool, including terminal.

transform_llm_output

Fires once per turn after the tool-calling loop completes and the model has produced a final response, before that response is delivered to the user (CLI, gateway, or programmatic caller). Lets a plugin rewrite the assistant's final text using classical-programming methods — no extra inference tokens burned on SOUL flavor text or a skill-driven transform.

Callback signature:

def my_callback(
    response_text: str,
    session_id: str,
    model: str,
    platform: str,
    **kwargs,
) -> str | None:

Return value: Non-empty str to replace the response text, None or empty string to leave it unchanged. First non-empty string wins when multiple plugins register. Unlike the tool and terminal transforms, an empty string is not accepted as a replacement.

Use cases: Apply a personality/vocabulary transform (pirate-speak, Spongebob), redact user-specific identifiers from the final text, append a project-specific signature footer, enforce a house style guide without burning tokens on SOUL instructions.

When CLI streaming is enabled, an append-only transform is printed after the streamed body. A transform that replaces the response is printed in full after the streamed body, labeled as a post-stream transformation, so replacement content is never silently lost.

import os, re

def spongebob(response_text, **kwargs):
    if os.environ.get("SPONGEBOB_MODE") != "on":
        return None  # pass through unchanged
    return re.sub(r"!", "!! Tartar sauce!", response_text)

def register(ctx):
    ctx.register_hook("transform_llm_output", spongebob)

The hook is guarded on a non-empty, non-interrupted response — it will not fire on stop-button interrupts or empty turns. Exceptions are logged as warnings and do not break agent execution.

API-request observer hooks

pre_api_request

Fires for each provider attempt immediately before sending it. This is observer-only. The legacy user_message, conversation_history, and request_messages fields are raw and intentionally unsanitized for compatibility; new consumers should prefer the sanitized request envelope.

post_api_request

Fires after a provider response has been normalized successfully. This is observer-only. Prefer the sanitized response; assistant_message is the raw normalized message, and usage contains accounting data.

api_request_error

Fires for a failed provider attempt with status/retry timing, an error object, and sanitized request. This is observer-only. Error messages may still contain provider or user data.

on_skill_lifecycle

Fires after an authoritative skill-usage state change. It is observer-only and exposes the local skill_name, provenance, correlation IDs, usage count, and reuse flags.

Kanban lifecycle observers

kanban_task_claimed

Fires after the claim commit in the dispatcher process, immediately before worker spawn.

kanban_task_completed

Fires after completion and cleanup, usually in the worker process. Its summary can contain project or user content.

kanban_task_blocked

Fires after a normal blocked transition. The dependency-wait path invokes it before that write transaction exits. Its reason can contain project or user content.

All three kanban hooks are observer-only and carry task_id, profile_name, board, assignee, and run_id; completed adds summary, and blocked adds reason.

Kanban worker-lifecycle, task-mutation, and dispatch observers

Five additional observers (RFC #58548) extend the kanban family. All are observer-only, fire after the relevant transaction commits, and short-circuit on has_hook — with no subscriber, dispatch behavior is unchanged. Task-scoped hooks carry the same common fields as the hooks above.

• on_kanban_worker_spawned — after spawn_fn returns and the worker PID is persisted. Adds worker_pid (may be None) and workspace_path. Runs inside the dispatch lock; keep callbacks fast.
• on_kanban_worker_exited — tick-derived, when detect_crashed_workers reclaims a dead-PID task. Adds worker_pid, exit_kind, exit_code, outcome, retry_status.
• on_kanban_worker_stale_claim — when a TTL-expired claim is reclaimed; live-PID extensions don't fire. Adds worker_pid, heartbeat_stale, retry_status.
• on_kanban_task_updated — after a committed task-field write outside the claim/complete/block lifecycle (assign_task, model/reasoning overrides, dashboard editors). Adds changed_fields — field names only, never values.
• on_kanban_dispatch_tick — once per dispatcher tick, strictly after the dispatch lock is released, including idle and lock-contended ticks. Payload: board, profile_name, dry_run, outcome, result.

Shell Hooks

Declare shell-script hooks in your ~/.hermes/config.yaml and Hermes will run them as subprocesses whenever the corresponding plugin-hook event fires — in both CLI and gateway sessions. No Python plugin authoring required.

Use shell hooks when you want a drop-in, single-file script (Bash, Python, anything with a shebang) to:

• Block a tool call — reject dangerous terminal commands, enforce per-directory policies, require approval for destructive write_file / patch operations.
• Run after a tool call — auto-format Python or TypeScript files that the agent just wrote, log API calls, trigger a CI workflow.
• Inject context into the next LLM turn — prepend git status output, the current weekday, or retrieved documents to the user message (see pre_llm_call).
• Observe lifecycle events — write a log line when a subagent completes (subagent_stop) or a session starts (on_session_start).

Shell hooks are registered by calling agent.shell_hooks.register_from_config(cfg) at both CLI startup (hermes_cli/main.py) and gateway startup (gateway/run.py). They compose naturally with Python plugin hooks — both flow through the same dispatcher.

Comparison at a glance

Configuration schema

hooks:
  <event_name>:                  # Must be in VALID_HOOKS
    - matcher: "<regex>"         # Optional; used for pre/post_tool_call only
      command: "<shell command>" # Required; runs via shlex.split, shell=False
      timeout: <seconds>         # Optional; default 60, capped at 300
      fail_closed: <bool>        # Optional; default false. pre_tool_call only.
                                 # `failClosed` also accepted (Cursor/Claude Code compat)

hooks_auto_accept: false         # See "Consent model" below

Event names must be one of the plugin hook events; typos produce a "Did you mean X?" warning and are skipped. Unknown keys inside a single entry are ignored; missing command is a skip-with-warning. timeout > 300 is clamped with a warning. fail_closed: true on an event other than pre_tool_call warns and is ignored (only blocking-capable events can fail closed).

JSON wire protocol

Each time the event fires, Hermes spawns a subprocess for every matching hook (matcher permitting), pipes a JSON payload to stdin, and reads stdout back as JSON.

stdin — payload the script receives:

{
  "hook_event_name": "pre_tool_call",
  "tool_name":       "terminal",
  "tool_input":      {"command": "rm -rf /"},
  "session_id":      "sess_abc123",
  "cwd":             "/home/user/project",
  "extra":           {"task_id": "...", "tool_call_id": "..."}
}

tool_name and tool_input are null for non-tool events (pre_llm_call, subagent_stop, session lifecycle). The extra dict carries all event-specific kwargs (user_message, conversation_history, child_role, duration_ms, …). Unserialisable values are stringified rather than omitted.

stdout — optional response:

// Block a pre_tool_call (both shapes accepted; normalised internally):
{"decision": "block", "reason":  "Forbidden: rm -rf"}   // Claude-Code style
{"action":   "block", "message": "Forbidden: rm -rf"}   // Hermes-canonical

// Inject context for pre_llm_call:
{"context": "Today is Friday, 2026-04-17"}

// Keep the agent going at the verify gate (pre_verify); both shapes accepted:
{"action": "continue", "message": "Run the formatter, then finish."}
{"decision": "block",  "reason":  "Run the formatter, then finish."}

// Silent no-op — any empty / non-matching output is fine:

Malformed JSON, non-zero exit codes, and timeouts log a warning but never abort the agent loop.

Exit code 2 = block (Claude Code / Cursor compatible)

A pre_tool_call hook that exits with code 2 blocks the tool call even when its stdout carries no block JSON. The block message is resolved in priority order:

1. stdout block JSON (reason / message), when present;
2. the first 400 characters of stderr;
3. a generic "Blocked by shell hook." default.

So the simplest possible blocking hook is:

#!/usr/bin/env bash
echo "policy violation: rm -rf is not permitted" >&2
exit 2

For events whose block directive is not honored (everything except pre_tool_call), exit 2 is treated like any other non-zero exit: a warning is logged and stdout is still parsed.

Fail-open vs fail-closed

By default shell hooks fail open: a spawn error, timeout, or unparseable stdout logs a warning and the action proceeds. That is the right default for observability hooks — but wrong for security gates. A crashed secret-scanner must not silently allow the tool call it was supposed to vet.

Set fail_closed: true (or failClosed: true, the Cursor/Claude Code spelling) on a pre_tool_call entry to invert that:

hooks:
  pre_tool_call:
    - matcher: "terminal|write_file|patch"
      command: "~/.hermes/agent-hooks/secret-scan.sh"
      timeout: 10
      fail_closed: true

With fail_closed: true, each of these now blocks the tool call with hook <command> failed closed: <reason>:

fail_closed only applies to blocking-capable events (pre_tool_call today); setting it on any other event logs a warning at config-parse time and is ignored. hermes hooks test reflects these semantics — the parsed line shows exactly the block shape the dispatcher would receive.

Worked examples

1. Auto-format Python files after every write

# ~/.hermes/config.yaml
hooks:
  post_tool_call:
    - matcher: "write_file|patch"
      command: "~/.hermes/agent-hooks/auto-format.sh"

#!/usr/bin/env bash
# ~/.hermes/agent-hooks/auto-format.sh
payload="$(cat -)"
path=$(echo "$payload" | jq -r '.tool_input.path // empty')
[[ "$path" == *.py ]] && command -v black >/dev/null && black "$path" 2>/dev/null
printf '{}\n'

The agent's in-context view of the file is not re-read automatically — the reformat only affects the file on disk. Subsequent read_file calls pick up the formatted version.

2. Block destructive terminal commands

hooks:
  pre_tool_call:
    - matcher: "terminal"
      command: "~/.hermes/agent-hooks/block-rm-rf.sh"
      timeout: 5

#!/usr/bin/env bash
# ~/.hermes/agent-hooks/block-rm-rf.sh
payload="$(cat -)"
cmd=$(echo "$payload" | jq -r '.tool_input.command // empty')
if echo "$cmd" | grep -qE 'rm[[:space:]]+-rf?[[:space:]]+/'; then
  printf '{"decision": "block", "reason": "blocked: rm -rf / is not permitted"}\n'
else
  printf '{}\n'
fi

3. Inject git status into every turn (Claude-Code UserPromptSubmit equivalent)

hooks:
  pre_llm_call:
    - command: "~/.hermes/agent-hooks/inject-cwd-context.sh"

#!/usr/bin/env bash
# ~/.hermes/agent-hooks/inject-cwd-context.sh
cat - >/dev/null   # discard stdin payload
if status=$(git status --porcelain 2>/dev/null) && [[ -n "$status" ]]; then
  jq --null-input --arg s "$status" \
     '{context: ("Uncommitted changes in cwd:\n" + $s)}'
else
  printf '{}\n'
fi

Claude Code's UserPromptSubmit event is intentionally not a separate Hermes event — pre_llm_call fires at the same place and already supports context injection. Use it here.

4. Log every subagent completion

hooks:
  subagent_stop:
    - command: "~/.hermes/agent-hooks/log-orchestration.sh"

#!/usr/bin/env bash
# ~/.hermes/agent-hooks/log-orchestration.sh
log=~/.hermes/logs/orchestration.log
jq -c '{ts: now, parent: .session_id, extra: .extra}' < /dev/stdin >> "$log"
printf '{}\n'

Consent model

Each unique (event, command) pair prompts the user for approval the first time Hermes sees it, then persists the decision to ~/.hermes/shell-hooks-allowlist.json. Subsequent runs (CLI or gateway) skip the prompt.

Three escape hatches bypass the interactive prompt — any one is sufficient:

1. --accept-hooks flag on the CLI (e.g. hermes --accept-hooks chat)
2. HERMES_ACCEPT_HOOKS=1 environment variable
3. hooks_auto_accept: true in ~/.hermes/config.yaml

Non-TTY runs (gateway, cron, CI) need one of these three — otherwise any newly-added hook silently stays un-registered and logs a warning.

Script edits are silently trusted. The allowlist keys on the exact command string, not the script's hash, so editing the script on disk does not invalidate consent. hermes hooks doctor flags mtime drift so you can spot edits and decide whether to re-approve.

Manual allowlisting

Manual allowlisting is useful for non-TTY or service-account deployments where an operator cannot answer the first-use prompt interactively. The allowlist file is ~/.hermes/shell-hooks-allowlist.json, and the expected format is an approvals array. Each approval records the hook event and the exact command string:

{
  "approvals": [
    {
      "event": "post_llm_call",
      "command": "/home/hermes/.hermes/hooks/my-hook.py"
    }
  ]
}

The command string must match the configured hook command exactly. A path-keyed object with a sha256 field is not the expected format and will not approve the hook. Verify manual entries with hermes hooks list.

The hermes hooks CLI

Security

Shell hooks run with your full user credentials — same trust boundary as a cron entry or a shell alias. Treat the hooks: block in config.yaml as privileged configuration:

• Only reference scripts you wrote or fully reviewed.
• Keep scripts inside ~/.hermes/agent-hooks/ so the path is easy to audit.
• Re-run hermes hooks doctor after you pull a shared config to spot newly-added hooks before they register.
• If your config.yaml is version-controlled across a team, review PRs that change the hooks: section the same way you'd review CI config.

Ordering and precedence

Both Python plugin hooks and shell hooks flow through the same invoke_hook() dispatcher. Python plugins are registered first (discover_and_load()), shell hooks second (register_from_config()), so Python pre_tool_call block decisions take precedence in tie cases. The first valid block wins — the aggregator returns as soon as any callback produces {"action": "block", "message": str} with a non-empty message.

Outbound Webhooks

Outbound webhooks are the push-side mirror of the inbound webhook platform: inbound webhooks wake Hermes when the world changes; outbound webhooks tell the world when Hermes does something. Configure a list of HTTP endpoints and the lifecycle events they care about, and Hermes POSTs a signed JSON payload to each endpoint whenever a matching event fires — no polling on the receiving end.

Typical uses:

• Notify a CI system or dashboard when an agent turn finishes (on_session_end)
• Track subagent completions across a fleet (subagent_stop)
• Feed tool activity into external monitoring (post_tool_call with a matcher)
• Wake another Hermes instance: point the URL at that instance's inbound webhook

Configuration

Add a hooks.outbound: list to ~/.hermes/config.yaml:

hooks:
  outbound:
    - name: ci-notify                       # optional label for logs
      url: https://ci.example.com/hermes-events
      events: [on_session_end, subagent_stop]
      secret_env: HERMES_OUTBOUND_WEBHOOK_SECRET   # env var holding the HMAC secret
      timeout: 10                           # per-attempt seconds (1–60)

    - name: tool-monitor
      url: https://metrics.example.com/hooks/hermes
      events: [post_tool_call]
      matcher: "terminal|delegate_task"     # regex, tool-scoped events only

Any event from the plugin-hook set is valid (pre_tool_call, post_tool_call, pre_llm_call, post_llm_call, on_session_start, on_session_end, subagent_start, subagent_stop, ...). Malformed entries warn and are skipped — a broken webhook never crashes the agent. Changes take effect on the next CLI session / gateway restart.

Secrets: prefer secret_env (the name of an environment variable, typically set in ~/.hermes/.env) over an inline secret: literal, so the config file stays free of credentials. Entries without a secret are delivered unsigned (flagged as UNSIGNED by hermes hooks list).

Wire format

Each firing POSTs a JSON body with the same top-level shape as shell hooks' stdin, plus delivery metadata:

{
  "hook_event_name": "on_session_end",
  "tool_name": null,
  "tool_input": null,
  "session_id": "sess_abc123",
  "cwd": "/home/user/project",
  "extra": {"completed": true, "interrupted": false, "model": "...", "platform": "cli"},
  "delivery_id": "3f2c9a...",
  "timestamp": "2026-07-22T14:00:00Z"
}

Headers:

Verify the signature exactly as you would a GitHub webhook:

import hashlib, hmac

def verify(body: bytes, header: str, secret: str) -> bool:
    expected = "sha256=" + hmac.new(secret.encode(), body, hashlib.sha256).hexdigest()
    return hmac.compare_digest(expected, header)

Because delivery_id and timestamp live inside the signed body, a verified receiver also gets replay protection for free:

• Dedupe on delivery_id (or the matching X-Hermes-Delivery header) — remember recently seen ids and skip duplicates. Hermes retries failed deliveries once, so the same id can legitimately arrive twice.
• Reject stale events by checking timestamp against your clock with a tolerance window (5 minutes is the common default). An attacker replaying a captured request can't forge a fresh timestamp without the secret.

Delivery semantics

• Fire-and-forget, off the hot path. Events are serialized and queued instantly; a single background thread performs the HTTP POSTs. A slow or dead endpoint can never stall a tool call or an agent turn.
• Notify-only. Unlike shell hooks, outbound webhooks cannot block tool calls or inject context — the response body is ignored. They observe, never steer.
• Bounded retries. Connection errors and 5xx responses are retried once with backoff; 4xx responses are not retried (the receiver said the request itself is wrong). Failures are logged and dropped — delivery is best-effort, not guaranteed.
• Redirects are never followed. A 3xx response is treated as a misconfiguration and logged — following a redirected POST would silently drop the signed payload. Point the url at the final endpoint.
• Bounded queue. If the queue backs up (dead endpoint, event storm), new events are dropped with a warning rather than consuming unbounded memory.
• No consent prompt. Outbound targets execute no code on your machine — they receive data at a URL you configured. HERMES_SAFE_MODE=1 still skips registration, same as plugins and shell hooks. Note that payloads include tool inputs and event metadata, so only point targets at endpoints you trust, and prefer https://.

hermes hooks list shows configured outbound targets alongside shell hooks, including whether each target is signed.