{"meta":{"title":"Pre-tool use hook","intro":"The onPreToolUse hook is called before a tool executes. Use it to:","product":"GitHub Copilot","breadcrumbs":[{"href":"/en/enterprise-cloud@latest/copilot","title":"GitHub Copilot"},{"href":"/en/enterprise-cloud@latest/copilot/how-tos","title":"How-tos"},{"href":"/en/enterprise-cloud@latest/copilot/how-tos/copilot-sdk","title":"Copilot SDK"},{"href":"/en/enterprise-cloud@latest/copilot/how-tos/copilot-sdk/hooks","title":"Use hooks"},{"href":"/en/enterprise-cloud@latest/copilot/how-tos/copilot-sdk/hooks/pre-tool-use","title":"Pre Tool Use"}],"documentType":"article"},"body":"# Pre-tool use hook\n\nThe onPreToolUse hook is called before a tool executes. Use it to:\n\n\n\n\n\n* Approve or deny tool execution\n* Modify tool arguments\n* Add context for the tool\n* Suppress tool output from the conversation\n\n## Hook signature\n\n
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TypeScript
\n\n```typescript\ntype PreToolUseHandler = (\n input: PreToolUseHookInput,\n invocation: HookInvocation\n) => Promise;\n```\n\n
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Python
\n\n```python\nPreToolUseHandler = Callable[\n [PreToolUseHookInput, dict[str, str]],\n Awaitable[PreToolUseHookOutput | None]\n]\n```\n\n
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Go
\n\n```golang\ntype PreToolUseHandler func(\n input PreToolUseHookInput,\n invocation HookInvocation,\n) (*PreToolUseHookOutput, error)\n```\n\n
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.NET
\n\n```csharp\npublic delegate Task PreToolUseHandler(\n PreToolUseHookInput input,\n HookInvocation invocation);\n```\n\n
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Java
\n\n```java\n@FunctionalInterface\npublic interface PreToolUseHandler {\n CompletableFuture handle(\n PreToolUseHookInput input,\n HookInvocation invocation);\n}\n```\n\n
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\n\n## Input\n\n| Field | Type | Description |\n| ----------- | ------ | ------------------------------------------ |\n| `timestamp` | number | Unix timestamp when the hook was triggered |\n| `cwd` | string | Current working directory |\n| `toolName` | string | Name of the tool being called |\n| `toolArgs` | object | Arguments passed to the tool |\n\n## Output\n\nReturn `null` or `undefined` to allow the tool to execute with no changes. Otherwise, return an object with any of these fields:\n\n| Field | Type | Description |\n| -------------------------- | -------------------------------- | ------------------------------------------------- |\n| `permissionDecision` | `\"allow\"` \\| `\"deny\"` \\| `\"ask\"` | Whether to allow the tool call |\n| `permissionDecisionReason` | string | Explanation shown to user (for deny/ask) |\n| `modifiedArgs` | object | Modified arguments to pass to the tool |\n| `additionalContext` | string | Extra context injected into the conversation |\n| `suppressOutput` | boolean | If true, tool output won't appear in conversation |\n\n### Permission decisions\n\n| Decision | Behavior |\n| --------- | ---------------------------------------------- |\n| `\"allow\"` | Tool executes normally |\n| `\"deny\"` | Tool is blocked, reason shown to user |\n| `\"ask\"` | User is prompted to approve (interactive mode) |\n\n### Skipping permission prompts for trusted custom tools\n\nIf you define a custom tool that is safe to run without prompting, set `skipPermission: true` on the tool definition. Use this for trusted, app-owned tools whose inputs are already constrained by your application; use `onPreToolUse` when you need per-call policy checks or argument validation.\n\n```typescript\nconst getWeather = defineTool(\"get_weather\", {\n description: \"Get weather for a location.\",\n parameters: {\n type: \"object\",\n properties: { location: { type: \"string\" } },\n required: [\"location\"],\n },\n skipPermission: true,\n handler: async ({ location }) => ({ forecast: `Sunny in ${location}` }),\n});\n```\n\n## Examples\n\n### Allow all tools (logging only)\n\n
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TypeScript
\n\n```typescript\nconst session = await client.createSession({\n hooks: {\n onPreToolUse: async (input, invocation) => {\n console.log(`[${invocation.sessionId}] Calling ${input.toolName}`);\n console.log(` Args: ${JSON.stringify(input.toolArgs)}`);\n return { permissionDecision: \"allow\" };\n },\n },\n});\n```\n\n
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Python
\n\n```python\nfrom copilot.session import PermissionHandler\n\nasync def on_pre_tool_use(input_data, invocation):\n print(f\"[{invocation['session_id']}] Calling {input_data['toolName']}\")\n print(f\" Args: {input_data['toolArgs']}\")\n return {\"permissionDecision\": \"allow\"}\n\nsession = await client.create_session(on_permission_request=PermissionHandler.approve_all, hooks={\"on_pre_tool_use\": on_pre_tool_use})\n```\n\n
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Go
\n\n```golang\nsession, _ := client.CreateSession(context.Background(), &copilot.SessionConfig{\n Hooks: &copilot.SessionHooks{\n OnPreToolUse: func(input copilot.PreToolUseHookInput, inv copilot.HookInvocation) (*copilot.PreToolUseHookOutput, error) {\n fmt.Printf(\"[%s] Calling %s\\n\", inv.SessionID, input.ToolName)\n fmt.Printf(\" Args: %v\\n\", input.ToolArgs)\n return &copilot.PreToolUseHookOutput{\n PermissionDecision: \"allow\",\n }, nil\n },\n },\n})\n```\n\n
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.NET
\n\n```csharp\nvar session = await client.CreateSessionAsync(new SessionConfig\n{\n Hooks = new SessionHooks\n {\n OnPreToolUse = (input, invocation) =>\n {\n Console.WriteLine($\"[{invocation.SessionId}] Calling {input.ToolName}\");\n Console.WriteLine($\" Args: {input.ToolArgs}\");\n return Task.FromResult(\n new PreToolUseHookOutput { PermissionDecision = \"allow\" }\n );\n },\n },\n});\n```\n\n
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Java
\n\n\n\n```java\nimport com.github.copilot.*;\nimport com.github.copilot.rpc.*;\nimport java.util.concurrent.CompletableFuture;\n\nvar hooks = new SessionHooks()\n .setOnPreToolUse((input, invocation) -> {\n System.out.println(\"[\" + invocation.getSessionId() + \"] Calling \" + input.getToolName());\n System.out.println(\" Args: \" + input.getToolArgs());\n return CompletableFuture.completedFuture(PreToolUseHookOutput.allow());\n });\n\nvar session = client.createSession(\n new SessionConfig()\n .setOnPermissionRequest(PermissionHandler.APPROVE_ALL)\n .setHooks(hooks)\n).get();\n```\n\n
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\n\n### Block specific tools\n\n```typescript\nconst BLOCKED_TOOLS = [\"shell\", \"bash\", \"write_file\", \"delete_file\"];\n\nconst session = await client.createSession({\n hooks: {\n onPreToolUse: async (input) => {\n if (BLOCKED_TOOLS.includes(input.toolName)) {\n return {\n permissionDecision: \"deny\",\n permissionDecisionReason: `Tool '${input.toolName}' is not permitted in this environment`,\n };\n }\n return { permissionDecision: \"allow\" };\n },\n },\n});\n```\n\n### Modify tool arguments\n\n```typescript\nconst session = await client.createSession({\n hooks: {\n onPreToolUse: async (input) => {\n // Add a default timeout to all shell commands\n if (input.toolName === \"shell\" && input.toolArgs) {\n const args = input.toolArgs as { command: string; timeout?: number };\n return {\n permissionDecision: \"allow\",\n modifiedArgs: {\n ...args,\n timeout: args.timeout ?? 30000, // Default 30s timeout\n },\n };\n }\n return { permissionDecision: \"allow\" };\n },\n },\n});\n```\n\n### Restrict file access to specific directories\n\n```typescript\nconst ALLOWED_DIRECTORIES = [\"/home/user/projects\", \"/tmp\"];\n\nconst session = await client.createSession({\n hooks: {\n onPreToolUse: async (input) => {\n if (input.toolName === \"read_file\" || input.toolName === \"write_file\") {\n const args = input.toolArgs as { path: string };\n const isAllowed = ALLOWED_DIRECTORIES.some(dir => \n args.path.startsWith(dir)\n );\n \n if (!isAllowed) {\n return {\n permissionDecision: \"deny\",\n permissionDecisionReason: `Access to '${args.path}' is not permitted. Allowed directories: ${ALLOWED_DIRECTORIES.join(\", \")}`,\n };\n }\n }\n return { permissionDecision: \"allow\" };\n },\n },\n});\n```\n\n### Suppress verbose tool output\n\n```typescript\nconst VERBOSE_TOOLS = [\"list_directory\", \"search_files\"];\n\nconst session = await client.createSession({\n hooks: {\n onPreToolUse: async (input) => {\n return {\n permissionDecision: \"allow\",\n suppressOutput: VERBOSE_TOOLS.includes(input.toolName),\n };\n },\n },\n});\n```\n\n### Add context based on tool\n\n```typescript\nconst session = await client.createSession({\n hooks: {\n onPreToolUse: async (input) => {\n if (input.toolName === \"query_database\") {\n return {\n permissionDecision: \"allow\",\n additionalContext: \"Remember: This database uses PostgreSQL syntax. Always use parameterized queries.\",\n };\n }\n return { permissionDecision: \"allow\" };\n },\n },\n});\n```\n\n## Best practices\n\n1. **Always return a decision** - Returning `null` allows the tool, but being explicit with `{ permissionDecision: \"allow\" }` is clearer.\n\n2. **Provide helpful denial reasons** - When denying, explain why so users understand:\n\n ```typescript\n return {\n permissionDecision: \"deny\",\n permissionDecisionReason: \"Shell commands require approval. Please describe what you want to accomplish.\",\n };\n ```\n\n3. **Be careful with argument modification** - Ensure modified args maintain the expected schema for the tool.\n\n4. **Consider performance** - Pre-tool hooks run synchronously before each tool call. Keep them fast.\n\n5. **Use `suppressOutput` judiciously** - Suppressing output means the model won't see the result, which may affect conversation quality.\n\n## See also\n\n* [Use hooks](/en/enterprise-cloud@latest/copilot/how-tos/copilot-sdk/hooks)\n* [Post-tool use hook](/en/enterprise-cloud@latest/copilot/how-tos/copilot-sdk/hooks/post-tool-use)\n* [Debugging guide](/en/enterprise-cloud@latest/copilot/how-tos/copilot-sdk/troubleshooting/debugging)"}