Working with hooks

Hooks let you plug custom logic into every stage of a Второй пилот SDK session—from the moment it starts, through each user prompt and tool call, to the moment it ends. You can use hooks to implement permissions, auditing, notifications, and more without modifying the core agent behavior.

Overview

A hook is a callback you register once when creating a session. The SDK invokes it at a well-defined point in the conversation lifecycle, passes contextual input, and optionally accepts output that modifies the session's behavior. For a detailed sequence diagram of the session flow, see the github/copilot-sdk repository.

Hook	When it fires	What you can do
onSessionStart	Session begins (new or resumed)	Inject context, load preferences
onUserPromptSubmitted	User sends a message	Rewrite prompts, add context, filter input
onPreToolUse	Before a tool executes	Allow, deny, or modify the call
onPostToolUse	After a tool returns	Transform results, redact secrets, audit
onSessionEnd	Session ends	Clean up, record metrics
onErrorOccurred	An error is raised	Custom logging, retry logic, alerts

All hooks are optional—register only the ones you need. Returning null (or the language equivalent) from any hook tells the SDK to continue with default behavior.

Registering hooks

Pass a hooks object when you create (or resume) a session. Every example below follows this pattern.

import { CopilotClient } from "@github/copilot-sdk";

const client = new CopilotClient();
await client.start();

const session = await client.createSession({
    hooks: {
        onSessionStart: async (input, invocation) => { /* ... */ },
        onPreToolUse:   async (input, invocation) => { /* ... */ },
        onPostToolUse:  async (input, invocation) => { /* ... */ },
        // ... add only the hooks you need
    },
    onPermissionRequest: async () => ({ kind: "approved" }),
});

For examples in Python, Go, and .NET, see the github/copilot-sdk repository.

Permission control

Use onPreToolUse to build a permission layer that decides which tools the agent may run, what arguments are allowed, and whether the user should be prompted before execution.

Allow-list a safe set of tools

const READ_ONLY_TOOLS = ["read_file", "glob", "grep", "view"];

const session = await client.createSession({
    hooks: {
        onPreToolUse: async (input) => {
            if (!READ_ONLY_TOOLS.includes(input.toolName)) {
                return {
                    permissionDecision: "deny",
                    permissionDecisionReason:
                        `Only read-only tools are allowed. "${input.toolName}" was blocked.`,
                };
            }
            return { permissionDecision: "allow" };
        },
    },
    onPermissionRequest: async () => ({ kind: "approved" }),
});

For examples in Python, Go, and .NET, see the github/copilot-sdk repository.

Restrict file access to specific directories

const ALLOWED_DIRS = ["/home/user/projects", "/tmp"];

const session = await client.createSession({
    hooks: {
        onPreToolUse: async (input) => {
            if (["read_file", "write_file", "edit"].includes(input.toolName)) {
                const filePath = (input.toolArgs as { path: string }).path;
                const allowed = ALLOWED_DIRS.some((dir) => filePath.startsWith(dir));

                if (!allowed) {
                    return {
                        permissionDecision: "deny",
                        permissionDecisionReason:
                            `Access to "${filePath}" is outside the allowed directories.`,
                    };
                }
            }
            return { permissionDecision: "allow" };
        },
    },
    onPermissionRequest: async () => ({ kind: "approved" }),
});

Ask the user before destructive operations

const DESTRUCTIVE_TOOLS = ["delete_file", "shell", "bash"];

const session = await client.createSession({
    hooks: {
        onPreToolUse: async (input) => {
            if (DESTRUCTIVE_TOOLS.includes(input.toolName)) {
                return { permissionDecision: "ask" };
            }
            return { permissionDecision: "allow" };
        },
    },
    onPermissionRequest: async () => ({ kind: "approved" }),
});

Returning "ask" delegates the decision to the user at runtime—useful for destructive actions where you want a human in the loop.

Auditing and compliance

Combine onPreToolUse, onPostToolUse, and the session lifecycle hooks to build a complete audit trail that records every action the agent takes.

Structured audit log

interface AuditEntry {
    timestamp: number;
    sessionId: string;
    event: string;
    toolName?: string;
    toolArgs?: unknown;
    toolResult?: unknown;
    prompt?: string;
}

const auditLog: AuditEntry[] = [];

const session = await client.createSession({
    hooks: {
        onSessionStart: async (input, invocation) => {
            auditLog.push({
                timestamp: input.timestamp,
                sessionId: invocation.sessionId,
                event: "session_start",
            });
            return null;
        },
        onUserPromptSubmitted: async (input, invocation) => {
            auditLog.push({
                timestamp: input.timestamp,
                sessionId: invocation.sessionId,
                event: "user_prompt",
                prompt: input.prompt,
            });
            return null;
        },
        onPreToolUse: async (input, invocation) => {
            auditLog.push({
                timestamp: input.timestamp,
                sessionId: invocation.sessionId,
                event: "tool_call",
                toolName: input.toolName,
                toolArgs: input.toolArgs,
            });
            return { permissionDecision: "allow" };
        },
        onPostToolUse: async (input, invocation) => {
            auditLog.push({
                timestamp: input.timestamp,
                sessionId: invocation.sessionId,
                event: "tool_result",
                toolName: input.toolName,
                toolResult: input.toolResult,
            });
            return null;
        },
        onSessionEnd: async (input, invocation) => {
            auditLog.push({
                timestamp: input.timestamp,
                sessionId: invocation.sessionId,
                event: "session_end",
            });

            // Persist the log — swap this with your own storage backend
            await fs.promises.writeFile(
                `audit-${invocation.sessionId}.json`,
                JSON.stringify(auditLog, null, 2),
            );
            return null;
        },
    },
    onPermissionRequest: async () => ({ kind: "approved" }),
});

For an example in Python, see the github/copilot-sdk repository.

Redact secrets from tool results

const SECRET_PATTERNS = [
    /(?:api[_-]?key|token|secret|password)\s*[:=]\s*["']?[\w\-\.]+["']?/gi,
];

const session = await client.createSession({
    hooks: {
        onPostToolUse: async (input) => {
            if (typeof input.toolResult !== "string") return null;

            let redacted = input.toolResult;
            for (const pattern of SECRET_PATTERNS) {
                redacted = redacted.replace(pattern, "[REDACTED]");
            }

            return redacted !== input.toolResult
                ? { modifiedResult: redacted }
                : null;
        },
    },
    onPermissionRequest: async () => ({ kind: "approved" }),
});

Notifications

Hooks fire in your application's process, so you can trigger any side-effect such as desktop notifications, sounds, Slack messages, or webhook calls.

Desktop notification on session events

import notifier from "node-notifier"; // npm install node-notifier

const session = await client.createSession({
    hooks: {
        onSessionEnd: async (input, invocation) => {
            notifier.notify({
                title: "Copilot Session Complete",
                message: `Session ${invocation.sessionId.slice(0, 8)} finished (${input.reason}).`,
            });
            return null;
        },
        onErrorOccurred: async (input) => {
            notifier.notify({
                title: "Copilot Error",
                message: input.error.slice(0, 200),
            });
            return null;
        },
    },
    onPermissionRequest: async () => ({ kind: "approved" }),
});

For an example in Python, see the github/copilot-sdk repository.

Play a sound when a tool finishes

import { exec } from "node:child_process";

const session = await client.createSession({
    hooks: {
        onPostToolUse: async (input) => {
            // macOS: play a system sound after every tool call
            exec("afplay /System/Library/Sounds/Pop.aiff");
            return null;
        },
        onErrorOccurred: async () => {
            exec("afplay /System/Library/Sounds/Basso.aiff");
            return null;
        },
    },
    onPermissionRequest: async () => ({ kind: "approved" }),
});

Post to Slack on errors

const SLACK_WEBHOOK_URL = process.env.SLACK_WEBHOOK_URL!;

const session = await client.createSession({
    hooks: {
        onErrorOccurred: async (input, invocation) => {
            if (!input.recoverable) {
                await fetch(SLACK_WEBHOOK_URL, {
                    method: "POST",
                    headers: { "Content-Type": "application/json" },
                    body: JSON.stringify({
                        text: `🚨 Unrecoverable error in session \`${invocation.sessionId.slice(0, 8)}\`:\n\`\`\`${input.error}\`\`\``,
                    }),
                });
            }
            return null;
        },
    },
    onPermissionRequest: async () => ({ kind: "approved" }),
});

Prompt enrichment

Use onSessionStart and onUserPromptSubmitted to automatically inject context so users don't have to repeat themselves.

Inject project metadata at session start

import * as fs from "node:fs";

const session = await client.createSession({
    hooks: {
        onSessionStart: async (input) => {
            const pkg = JSON.parse(
                await fs.promises.readFile("package.json", "utf-8"),
            );
            return {
                additionalContext: [
                    `Project: ${pkg.name} v${pkg.version}`,
                    `Node: ${process.version}`,
                    `CWD: ${input.cwd}`,
                ].join("\n"),
            };
        },
    },
    onPermissionRequest: async () => ({ kind: "approved" }),
});

Expand shorthand commands in prompts

const SHORTCUTS: Record<string, string> = {
    "/fix":      "Find and fix all errors in the current file",
    "/test":     "Write comprehensive unit tests for this code",
    "/explain":  "Explain this code in detail",
    "/refactor": "Refactor this code to improve readability",
};

const session = await client.createSession({
    hooks: {
        onUserPromptSubmitted: async (input) => {
            for (const [shortcut, expansion] of Object.entries(SHORTCUTS)) {
                if (input.prompt.startsWith(shortcut)) {
                    const rest = input.prompt.slice(shortcut.length).trim();
                    return { modifiedPrompt: rest ? `${expansion}: ${rest}` : expansion };
                }
            }
            return null;
        },
    },
    onPermissionRequest: async () => ({ kind: "approved" }),
});

Error handling and recovery

The onErrorOccurred hook gives you a chance to react to failures—whether that means retrying, notifying a human, or gracefully shutting down.

Retry transient model errors

const session = await client.createSession({
    hooks: {
        onErrorOccurred: async (input) => {
            if (input.errorContext === "model_call" && input.recoverable) {
                return {
                    errorHandling: "retry",
                    retryCount: 3,
                    userNotification: "Temporary model issue—retrying…",
                };
            }
            return null;
        },
    },
    onPermissionRequest: async () => ({ kind: "approved" }),
});

Friendly error messages

const FRIENDLY_MESSAGES: Record<string, string> = {
    model_call:      "The AI model is temporarily unavailable. Please try again.",
    tool_execution:  "A tool encountered an error. Check inputs and try again.",
    system:          "A system error occurred. Please try again later.",
};

const session = await client.createSession({
    hooks: {
        onErrorOccurred: async (input) => {
            return {
                userNotification: FRIENDLY_MESSAGES[input.errorContext] ?? input.error,
            };
        },
    },
    onPermissionRequest: async () => ({ kind: "approved" }),
});

Session metrics

Track how long sessions run, how many tools are invoked, and why sessions end—useful for dashboards and cost monitoring.

const metrics = new Map<string, { start: number; toolCalls: number; prompts: number }>();

const session = await client.createSession({
    hooks: {
        onSessionStart: async (input, invocation) => {
            metrics.set(invocation.sessionId, {
                start: input.timestamp,
                toolCalls: 0,
                prompts: 0,
            });
            return null;
        },
        onUserPromptSubmitted: async (_input, invocation) => {
            metrics.get(invocation.sessionId)!.prompts++;
            return null;
        },
        onPreToolUse: async (_input, invocation) => {
            metrics.get(invocation.sessionId)!.toolCalls++;
            return { permissionDecision: "allow" };
        },
        onSessionEnd: async (input, invocation) => {
            const m = metrics.get(invocation.sessionId)!;
            const durationSec = (input.timestamp - m.start) / 1000;

            console.log(
                `Session ${invocation.sessionId.slice(0, 8)}: ` +
                `${durationSec.toFixed(1)}s, ${m.prompts} prompts, ` +
                `${m.toolCalls} tool calls, ended: ${input.reason}`,
            );

            metrics.delete(invocation.sessionId);
            return null;
        },
    },
    onPermissionRequest: async () => ({ kind: "approved" }),
});

For an example in Python, see the github/copilot-sdk repository.

Combining hooks

Hooks compose naturally. A single hooks object can handle permissions, auditing, and notifications—each hook does its own job.

const session = await client.createSession({
    hooks: {
        onSessionStart: async (input) => {
            console.log(`[audit] session started in ${input.cwd}`);
            return { additionalContext: "Project uses TypeScript and Vitest." };
        },
        onPreToolUse: async (input) => {
            console.log(`[audit] tool requested: ${input.toolName}`);
            if (input.toolName === "shell") {
                return { permissionDecision: "ask" };
            }
            return { permissionDecision: "allow" };
        },
        onPostToolUse: async (input) => {
            console.log(`[audit] tool completed: ${input.toolName}`);
            return null;
        },
        onErrorOccurred: async (input) => {
            console.error(`[alert] ${input.errorContext}: ${input.error}`);
            return null;
        },
        onSessionEnd: async (input, invocation) => {
            console.log(`[audit] session ${invocation.sessionId.slice(0, 8)} ended: ${input.reason}`);
            return null;
        },
    },
    onPermissionRequest: async () => ({ kind: "approved" }),
});

Best practices

• Keep hooks fast. Every hook runs inline—slow hooks delay the conversation. Offload heavy work (database writes, HTTP calls) to a background queue when possible.
• Return null when you have nothing to change. This tells the SDK to proceed with defaults and avoids unnecessary object allocation.
• Be explicit with permission decisions. Returning { permissionDecision: "allow" } is clearer than returning null, even though both allow the tool.
• Don't swallow critical errors. It's fine to suppress recoverable tool errors, but always log or alert on unrecoverable ones.
• Use additionalContext instead of modifiedPrompt when possible. Appending context preserves the user's original intent while still guiding the model.
• Scope state by session ID. If you track per-session data, key it on invocation.sessionId and clean up in onSessionEnd.

Further reading

For more information, see the Hooks reference in the github/copilot-sdk repository.