> ## Documentation Index
> Fetch the complete documentation index at: https://docs.interchange.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Choose your operating loop

> Decide whether to work through Apostra, run your own application, or combine an agent with durable application control.

Start with the work you want done. A marketer can give an authorized assistant
a goal and guide the workflow in Apostra. A developer can build software that
chooses when and how to call its tools. A team can also bring business data in
and read results out without putting a model in the loop.

| Approach                       | You own                                                           | Apostra provides                                                                                          | Choose it when                                                                      |
| ------------------------------ | ----------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------- |
| Managed, human-guided workflow | Goals, source data, business rules, and approvals                 | The application and assistant experience, advertising records, and supported buying or selling operations | A person wants to operate campaigns without building an application.                |
| Composable application         | Interface, decisions, scheduling, credentials, and recovery       | Tools and APIs, canonical advertising state, seller connections, execution, and delivery reads            | Your product already knows the sequence or needs to move data between systems.      |
| Hybrid agent application       | Durable state, allowed actions, approval checkpoints, and runtime | The same tools, transaction state, seller connections, and reporting                                      | A model helps interpret goals or compare offers, while your code controls progress. |

“Managed” here means using Apostra's supported product workflows. It does not
mean you can upload arbitrary software and have Apostra host its runtime. A
custom application or external agent owns its own scheduling, retries, and
long-running workflow state today.

## Keep business decisions separate from transaction state

For a hybrid application, keep your customer's request, approval records,
workflow checkpoints, and Apostra object IDs in your application's durable
store. Let Apostra own the saved campaign, creative, media buy, and delivery
facts. Re-read those objects when resuming instead of treating a model's prior
answer as their current state.

An agent can propose a budget or choose among returned offers. Your application
decides whether that recommendation is within policy and who must approve it.
Apostra still applies account permissions, seller readiness, creative review,
funding, and transaction checks. Passing your application's approval does not
skip those checks.

This division works for a small loop that reads data and recommends an action,
and for a larger application that also manages creative production or external
business systems. Add those responsibilities when your product needs them;
they do not require a different Apostra campaign model.

## Choose MCP or REST for each job

| Job                                                            | Starting point                                                       |
| -------------------------------------------------------------- | -------------------------------------------------------------------- |
| Let a coding agent explore or operate the account              | v3 MCP, interactive OAuth, `get_status`, then current `tools/list`.  |
| Build an external agent that uses typed advertising tools      | v3 MCP plus an application-owned runtime and durable store.          |
| Send business events or run a scheduled HTTP integration       | v2 REST with a credential authorized for that API.                   |
| Combine agent decisions with deterministic imports and exports | v3 MCP for the agent and v2 REST where the required operation lives. |

The transport does not decide who owns the loop. Code can be an MCP client, and
an agent can call REST through application tools. Prefer MCP when discovery and
model-facing schemas help; prefer REST when direct HTTP requests fit the job.

MCP OAuth credentials are bound to their endpoint. A token issued for
`/mcp/v3` cannot be reused on REST or another MCP endpoint. See
[Authentication](/v3/authentication) before combining surfaces.

## Decide whether the software needs its own identity

A coding agent using a person's OAuth connection acts in that authorized
session. It does not need to be registered as a buyer agent. An application
integration also does not automatically need a separate buyer-agent identity.

Register an external buyer agent when software needs its own credentials,
advertiser grants, lifecycle, and machine notifications. An administrator uses
the [buyer-agent management flow](/v2/setup/buyer-agent-credentials) to establish
those separately. Registration alone grants no advertiser access or spending
authority. Hosted-agent creation is not currently available through the public
v3 catalog.

Continue with [Connect in five minutes](/v3/quickstart), then
[Build an agent](/v3/build-an-agent) or
[Build a reporting pipeline](/v3/reporting-pipeline).