Functions

Everything else curb exports.

Define an agent

Function	What it does
`defineAgent`	Declares the agent
`createAgentStore`	State that lasts the whole call
`defineTool`	Types the props of a `Tool` outside JSX
`z`	Zod, for validating input

`defineAgent`

import { defineAgent } from "curb";

export const agent = defineAgent({ app: App, timeZone: "Europe/Berlin", firstTurn: "agent" });

Option	Type	Default	What it does
`app`	component		The agent’s React tree
`maxLoadedSkills`	`number`	`3`	Skills open at once. The least recently used leaves past this
`maxSteps`	`number`	`8`	Model steps per turn
`maxTranscriptMessages`	`number`	`60`	Messages sent to the model per step. The full transcript is kept
`timeZone`	`string`	`"UTC"`	An IANA time zone. Every request carries today’s date and time in it
`firstTurn`	`"user"`, `"agent"`, `{ say }`	`"user"`	Who opens the call (Who speaks first)
`mcpServers`	array		Remote MCP servers (MCP servers)
`goals`	object		What counts as a successful call (Goals and experiments)
`experiments`	object		A/B experiments, by name
`preview`	`{ suggestedPrompts?, chatPlaceholder? }`		Hints for the preview. Never reach the model

It refuses a time zone that does not exist, a firstTurn no conversation can open with, and goals or experiments a host could not evaluate.

`createAgentStore`

import { createAgentStore } from "curb";

export const useClaim = createAgentStore("claim", (set) => ({
  verified: false,
  verify: () => set({ verified: true }),
}));

createAgentStore(key, initializer) returns a hook on state that lasts the whole call and survives a skill leaving the request. Call it once, at module level. A change re-renders the agent, so a tool gated on a store field appears and leaves on its own (State and conditional tools).

`defineTool`

const openClaim = defineTool({
  name: "open_claim",
  description: "Open a claim.",
  schema: z.object({ policy: z.string() }),
  onSubmit: async ({ policy }) => ({ ok: true, message: `Opened for ${policy}.` }),
});

<Tool {...openClaim} />

`z`

Zod, re-exported so a project needs no dependency of its own.

Errors

Error	Thrown when
`ToolInputError`	A handler refuses its input, with a message for the model
`isToolInputError`	Tests for that error
`AgentAuthoringError`	The agent is authored wrongly, with a code and a fix
`MessageRefusedError`	A message was refused by the app or the host
`HostUnavailableError`	`askHost` found no host connected
`HostTimeoutError`	`askHost` got no answer in time
`SettingsRefusedError`	A settings update is not allowed or not valid

`ToolInputError`

import { ToolInputError } from "curb";

throw new ToolInputError("That policy number has eight digits.");

The model reads the message and can ask the caller again. data is optional: new ToolInputError(message, { data }).

Building on Curb

These are for programs built around Curb. An agent does not need them.

Export	What it does
`AgentSession`	The conversation loop on a runtime
`AgentSiteRuntime`	The headless React host of one conversation
`AgentInfoStore`	The store behind `useAgentInfo`
`RouteProvider`	Provides the current skill’s path to a tree
`AgentRouter`	Removed: renders nothing and warns with `AGENT_ROUTER_REMOVED`
`compileSite`	Compiles a semantic tree into context and tools
`flattenContext`	Compiled context as text lines
`createSemanticRoot`	A React root that renders into a semantic tree
`createSkillRoutes`	Routes from a project’s skill modules
`skillPathFromModuleId`	A module’s skill path
`titleFromSkillPath`	A readable title from a skill path
`listPublicMarkdown`	Lists `context/` files
`preloadPublicMarkdown`	Loads `context/` files
`resolvePublicMarkdown`	Reads one `context/` file
`validateSchemaInput`	Validates input
`zodSchemaToFields`	A Zod object as field descriptions
`diagnoseAgentAuthoring`	Runs the authoring checks
`formatAuthoringDiagnostic`	Prints a diagnostic
`mergeAuthoringDiagnostics`	Merges diagnostics without repeats
`AUTHORING_DIAGNOSTIC_CODES`	Every code (Diagnostics)
`McpManager`	Connects to and calls MCP servers
`configureMcp`	Sets up the MCP manager
`getMcpManager`	The current MCP manager
`preloadMcpTools`	Connects the configured servers early
`resolveMcpServer`	Finds a configured server
`setMcpOAuthCallbackUrl`	Sets where MCP sign-in returns to

Next steps

Components: what an agent renders.
Hooks: what a component can read and do.