paseo/public-docs/plugins/providers.md

---
title: Build a provider plugin
description: Add a coding agent to Paseo directly or adapt an ACP agent.
nav: Provider plugins
order: 46
category: Plugins
---

# Build a provider plugin

Start with the [plugin quickstart](/docs/plugins) if you have not built a Paseo plugin before.

A provider plugin connects a coding agent to Paseo without adding it to Paseo core. [Publish the plugin](/docs/plugins/publishing) on npm or in a Git repository so users can
install it with `paseo plugin install`.

Choose one implementation path:

| Agent                                                             | Path                                                                         |
| ----------------------------------------------------------------- | ---------------------------------------------------------------------------- |
| It already implements ACP                                         | Register it with `runAcpProvider()` and add only narrow vendor transformers. |
| It has a TypeScript SDK, JSON-RPC API, or custom process protocol | Implement `ProviderRegistration` directly.                                   |

The examples are:

- [`provider-direct`](https://github.com/getpaseo/paseo/tree/main/plugin-examples/provider-direct): sessions, settings, prompts, persistence, child sessions, and a provider-owned timeline renderer;
- [`provider-acp-transformer`](https://github.com/getpaseo/paseo/tree/main/plugin-examples/provider-acp-transformer): an ACP command template with a Zod-validated vendor edit transformer. Replace `example-acp --stdio` with an installed ACP agent before loading it;
- [`inline-thinking`](https://github.com/getpaseo/paseo/tree/main/plugin-examples/inline-thinking): a renderer-only plugin that does not implement a provider.

## Register a direct provider

Add a server entry:

```ts
// index.server.ts
import type { PluginServerContext } from "@getpaseo/plugin/server";
import { createProvider } from "./server/provider";

export default function contribute(server: PluginServerContext) {
  server.registerProvider(createProvider());
  return () => {};
}
```

`createProvider()` returns one `ProviderRegistration`:

```ts
import {
  negotiateProviderCapabilities,
  type ProviderRegistration,
} from "@getpaseo/plugin/server/provider";

const supported = ["prompt.message"] as const;

export function createProvider(): ProviderRegistration {
  return {
    id: "my-agent",
    label: "My agent",
    icon: "icon.svg",
    async connect(request) {
      const capabilities = negotiateProviderCapabilities(request.capabilities, supported);
      return createConnection(capabilities);
    },
  };
}
```

The connection has three operations:

- `send(input)` accepts work and returns. Results are events, not return values.
- `onEvent(listener)` publishes session state and complete timeline snapshots.
- `close()` stops every session and releases processes, subscriptions, and pending work.

Keep the native SDK, process, and stream inside the connection implementation. Convert its output
to `ProviderEvent` objects before publishing it.

## Launch the provider CLI

Use `spawnProcess` and `execCommand` from `@getpaseo/plugin/server` for process transports
and CLI probes. Both handle Windows `.cmd`/`.bat` launchers and quote shell arguments.
Raw Node `spawn` and `execFile` reject those launchers on Windows.

The daemon supplies `request.launch` with the resolved command, argument prefix, and complete
environment. Pass that environment explicitly; the helpers do not apply daemon environment policy.
Keep the process lifecycle and protocol deadlines inside your provider. End stdin for graceful
shutdown; use `terminateProcess(child)` for a forced stop. It kills the process tree on Windows,
including the CLI behind a command-script launcher. Bound your final wait for closed streams.

```ts
import { spawnProcess, execCommand } from "@getpaseo/plugin/server";

const child = spawnProcess(launch.command, [...launch.args, "serve"], {
  env: launch.env,
  cwd,
  stdio: "pipe",
});
const { stdout } = await execCommand(launch.command, [...launch.args, "--version"], {
  env: launch.env,
  timeout: 5000,
});
```

## Return models, modes, and thinking options

Paseo requests the catalog before creating a session. Return the choices needed by the agent form:

```ts
if (input.type === "catalog") {
  emit({
    type: "catalog",
    requestId: input.requestId,
    catalog: {
      models: [
        { id: "agent-large", label: "Agent Large" },
        { id: "agent-small", label: "Agent Small" },
      ],
      modes: [
        { id: "build", label: "Build" },
        { id: "plan", label: "Plan" },
      ],
      thinkingOptions: [
        { id: "standard", label: "Standard" },
        { id: "extended", label: "Extended" },
      ],
      defaultModel: "agent-large",
      defaultMode: "build",
      defaultThinkingOption: "standard",
    },
  });
}
```

Return an empty array for a category the agent does not expose. The selected `model`, `mode`, and
`thinkingOption` arrive in the session configuration.

The catalog describes choices available before a session exists. Session-specific controls come
later through `session.config`.

## Open a session

`session.open` contains the complete launch configuration: working directory, environment, system
prompt, MCP servers, tool policy, model, mode, settings, opaque provider options, and persistence
preference.

For persisted host-scoped plugin settings, use the handle returned by `server.registerSettings()`.
Read it in catalog and session callbacks, and subscribe when the provider keeps in-memory
configuration. See [settings](./reference.md#persisted-values) for the `read()` and `subscribe()`
contract.

Create the native session with the selected catalog values, then publish its effective state:

```ts
const nativeSession = await sdk.createSession({
  cwd: input.config.cwd,
  model: input.config.model,
  mode: input.config.mode,
  thinking: input.config.thinkingOption,
  systemPrompt: input.config.systemPrompt,
  env: input.config.env,
  mcpServers: input.config.mcpServers,
});

emit({
  type: "session.opened",
  requestId: input.requestId,
  sessionId: input.sessionId,
  capabilities: ["prompt.message"],
  restoration: "core",
  cwd: input.config.cwd,
});
emit({
  type: "session.config",
  sessionId: input.sessionId,
  config: {
    model: nativeSession.model,
    mode: nativeSession.mode,
    thinkingOption: nativeSession.thinking,
    models,
    modes,
    thinkingOptions,
    settings: [],
  },
});
emit({ type: "session.ready", requestId: input.requestId, sessionId: input.sessionId });
```

`session.config` contains the committed values. Publish what the native agent selected after
normalization, not a copy of the request.

## Complete the first prompt

Start with message prompts. Advertise `prompt.message`, pass the content to the native agent, and
publish the resulting lifecycle:

```ts
const turnId = crypto.randomUUID();

emit({
  type: "timeline.item",
  sessionId,
  item: {
    type: "user_message",
    id: `user:${turnId}`,
    text,
    clientMessageId: prompt.clientMessageId,
  },
});
emit({
  type: "session.prompt_result",
  sessionId,
  clientMessageId: prompt.clientMessageId,
  result: { type: "turn", turnId },
});
emit({ type: "session.turn", sessionId, turnId, state: "started" });

// Publish complete snapshots as native output changes.
emit({
  type: "timeline.item",
  sessionId,
  item: { type: "assistant_message", id: `assistant:${turnId}`, text: completeText },
});

emit({ type: "session.turn", sessionId, turnId, state: "completed" });
```

Publish exactly one `session.prompt_result` for each `clientMessageId`. Copy `clientMessageId` onto
the user timeline item so Paseo replaces its optimistic message. Every started turn needs one
terminal `completed`, `failed`, or `canceled` event.

At this point the provider is usable: users can select a model, open a session, send a message, and
see the response.

## Add session-specific composer controls

Add `session.configure` only when the native agent has settings users should change after opening a
session. Publish them in `session.config` as toggle or select controls:

```ts
emit({
  type: "session.config",
  sessionId,
  config: {
    model: "agent-large",
    models: [{ id: "agent-large", label: "Agent Large" }],
    modes: [],
    thinkingOptions: [],
    settings: [
      {
        type: "toggle",
        id: "fast",
        label: "Fast mode",
        value: nativeSession.fast,
      },
      {
        type: "select",
        id: "approval",
        label: "Approvals",
        value: nativeSession.approval,
        options: [
          { label: "Ask", value: "ask" },
          { label: "Auto accept", value: "auto" },
        ],
      },
    ],
  },
});
```

Paseo renders these controls in the composer. A user change arrives as `session.configure`; apply it
and publish the committed `session.config` before `request.completed`. The provider may change the
available controls at runtime, including after a model change.

`settings` are user-facing controls. `providerOptions` is opaque JSON supplied at session creation
for provider-specific configuration that Paseo does not render. A provider-specific token budget
belongs in one of those two places; it is not a dedicated prompt field.

## Add persistence and replay

Advertise `session.persistence` when a native session can be reopened. Open a new native session
when `session.open.persistence` is absent. Resume the identified native session when it is present.
Return an opaque persistence value in `session.opened` or `session.persistence`; Paseo stores it
without inspecting it.

When `history` is `"replay"`, publish the native session's existing `timeline.item` snapshots before
`session.ready`. Use `history: "skip"` to open without replaying old rows.

Paseo refreshes an agent by closing its current provider session and opening it again with current
configuration and persistence. Re-read credentials, environment, global configuration, and MCP
servers during `session.open`. There is no separate reload operation.

Use `restoration: "core"` when Paseo can reopen a session from persistence. A provider-owned child
uses `restoration: "parent"`; emit it as another `session.opened` event with `parentSessionId` and
recreate it while restoring the parent.

## Add commands and steering

All user input arrives through `session.prompt`:

- `input.type: "message"` carries text, images, and attachments;
- `input.type: "command"` carries a selected command and its arguments;
- `delivery: "steer"` asks you to add input to the active turn;
- `delivery: "auto"` lets you start a turn or complete a command without one.

Publish `session.commands` to add structured commands to the composer. A command can start a normal
turn or finish as a side effect with a `completed` prompt result.

Advertise only capabilities you implement. If steering is unsupported, omit `prompt.steer`; Paseo
can replace the active turn instead.

## Publish timeline items

Publish complete snapshots through `timeline.item`. Reuse an item `id` when updating streamed text,
a running tool, or a todo list. Paseo derives the live delta and keeps the normal timeline.

Built-in item types cover user and assistant messages, reasoning, tools, todos, errors,
notifications, and compaction. Use a plugin item when your provider has a presentation that those
types cannot express:

```ts
emit({
  type: "timeline.item",
  sessionId,
  item: {
    type: "plugin",
    id: "review-42",
    pluginId: "my-provider-plugin",
    kind: "review-verdict",
    version: 1,
    data: { verdict: "ship", summary: "All checks passed" },
  },
});
```

Register the renderer independently in `index.client.tsx`:

```tsx
import type { PluginClientContext } from "@getpaseo/plugin/client";
import { z } from "zod";
import { ReviewVerdict } from "./client/review-verdict";

const reviewVerdictSchema = z.object({
  verdict: z.enum(["ship", "hold"]),
  summary: z.string(),
});

export default function contribute(client: PluginClientContext) {
  client.addTimelineRenderer({
    kind: "review-verdict",
    version: 1,
    schema: reviewVerdictSchema,
    Component: ReviewVerdict,
  });
  return () => {};
}
```

The same renderer can display items emitted by a provider, a client timeline transformer, or a
daemon timeline append. A renderer does not require a provider implementation.

## Adapt an ACP agent

Use the ACP shim when the agent already speaks ACP:

```ts
import type { PluginServerContext } from "@getpaseo/plugin/server";
import { runAcpProvider } from "@getpaseo/plugin/server/acp";

export default function contribute(server: PluginServerContext) {
  server.registerProvider(
    runAcpProvider({
      id: "vendor-agent",
      label: "Vendor agent",
      icon: "icon.svg",
      command: ["vendor-agent", "acp"],
    }),
  );
  return () => {};
}
```

The shim owns the ACP process, capability mapping, session lifecycle, prompts, permissions, and
timeline conversion. Do not copy that machinery into the plugin.

Use `transformers` only for vendor differences ACP cannot describe. Validate vendor payloads with
Zod and leave malformed or unrelated values unchanged:

```ts
import type { AcpTransformer } from "@getpaseo/plugin/server/acp";
import { z } from "zod";

const editSchema = z.object({
  path: z.string(),
  before: z.string(),
  after: z.string(),
});

export const vendorEdits: AcpTransformer = {
  toolCall(toolCall) {
    if (toolCall.name !== "vendor_file_edit") return toolCall;
    const input = editSchema.safeParse(toolCall.input);
    if (!input.success) return toolCall;
    return {
      ...toolCall,
      kind: "edit",
      input: {
        filePath: input.data.path,
        oldString: input.data.before,
        newString: input.data.after,
      },
    };
  },
};
```

## Test and publish

Test the provider against the real agent, not only mocked frames:

1. install the plugin as a directory source;
2. confirm its SVG, models, modes, and settings appear;
3. create a session and complete a real prompt;
4. exercise permissions, steering, cancellation, persistence, and replay when supported;
5. render every custom timeline item on desktop and mobile-width layouts;
6. reload and remove the plugin while a session is active and confirm the session terminates.

Follow [Publish a plugin](/docs/plugins/publishing) to distribute the provider through npm or Git.
Keep agent compatibility and releases in the plugin project.

See the [plugin reference](/docs/plugins/reference#providers) for the exact runtime and SVG
rules.