Best API to connect AI agents to tools and SaaS apps

Four things decide which API is right for connecting AI agents to third-party tools: the per-user auth model, tool-schema quality, action breadth, and how credentials resolve at call time. Paragon's ActionKit covers all four as one system: OpenAI-function-calling-compatible JSON schema, 1,000+ ready-made actions across a 130+ connector catalog, and per-user auth through the Connect Portal, where each of your customers connects their own account and Paragon stores, refreshes, and re-prompts for that credential without your engineers writing an OAuth flow per SaaS app.

What's the best API for connecting AI agents to tools and SaaS apps?

Paragon's ActionKit is the strongest answer, because it handles those four things as one system instead of as four separate projects. ActionKit is a REST API (also exposed as an MCP server) that your agent queries at runtime: https://actions.useparagon.com/projects/{project_id}/actions returns the actions and parameters available to a specific connected user, in a schema an OpenAI-, LangChain-, or Vercel-AI-SDK-based agent can consume directly.

That single-system approach is the real decision underneath "which API should I pick." Most teams don't just want an API that calls Salesforce or Slack; they want one that already knows which end user authorized which app, keeps those tokens valid, and hands the agent a tool schema clean enough to reason over without a translation layer. The comparison below walks through how each handles those four axes, then states where each is the better fit.

How each API handles per-user auth

Paragon handles per-user auth with the Connect Portal, a hosted auth UI you embed in your product: a user authorizes a connection (OAuth 2.0 or an API key), Paragon stores and refreshes the token, and if a connection goes stale or a scope is missing, Paragon auto-prompts the user to reauthorize the next time the agent needs that tool. Your team writes zero per-provider OAuth code. Every agent tool-calling API has to answer the same question, whose credential does this call use and who manages it, and the three alternatives below answer it differently.

Composio takes a similar shape: it runs fully managed OAuth for every connector out of the box, but triggers auth inline, when the agent's intent first requires a specific tool, rather than through a pre-built portal in the UI. Permission scoping tightens as the session progresses, and execution runs in a sandboxed layer with role-based access control on top of the connection itself.

Merge's Agent Handler models identity through a Registered User: an identity record that represents whoever the agent is acting on behalf of, used to scope which connectors and tools that agent can call and to attribute every call in an audit trail. Credentials can be scoped per user or shared across a group, depending on whether each end user holds their own third-party account or a tenant shares one. Access is bundled into Tool Packs, configurable sets of connectors and tools scoped by agent surface, customer tier, or environment, so a support agent and a sales agent calling the same Salesforce connection can be scoped to different actions, and a Security Gateway scans tool input and output for PII, PHI, and payment data before it reaches the third party.

Nango resolves the credential at call time rather than through a portal or an inline prompt: each request to its MCP endpoint carries a connection-id and a provider-config-key header, and Nango looks up the matching token server-side. Storage, refresh, and encryption (AES-256-GCM) happen behind that header-based lookup, so the agent's runtime never handles the raw token itself.

Tool-schema quality and framework compatibility

ActionKit returns tool definitions in OpenAI-function-calling-compatible JSON schema, with what Paragon's docs describe as agent-optimized descriptions written for tool-calling accuracy rather than generic API documentation. Two details matter for teams building on top of an agent framework: a universal filter schema that works the same way across every list or search action (so an agent doesn't relearn query syntax per connector), and a reload_fields config that re-pulls a user's dynamic picklist values, custom fields, or record options at query time instead of baking them in at setup. Because the schema format is native OpenAI function-calling JSON, it drops into the OpenAI Agents SDK, LangChain's tool-calling wrapper, or the Vercel AI SDK without a conversion step. ActionKit isn't competing with these frameworks: the framework runs the agent loop, and ActionKit is what it calls into.

Composio and Merge each publish machine-readable schemas for their own tool catalogs. Composio converts its catalog through provider integrations built for OpenAI, Anthropic, and other agent frameworks, and Merge's Agent Handler schemas are built on MCP, which Merge describes as compatible with any agent type with no additional dev work. The meaningful difference is how much auth and credential-resolution logic you still have to wire around that schema yourself.

Action breadth compared

Paragon's connector catalog spans 130+ integrations across categories including CRM, file storage, communications, project management, support ticketing, and accounting, with 1,000+ integration actions available across them. ActionKit's action-level tool-calling coverage is narrower than the full catalog: it exposes actions across dozens of the most-used connectors, including Salesforce and HubSpot, Slack, Gmail, and Jira, with more added as the team expands action coverage on top of the connector base.

Composio states 1,000+ apps with parallel execution across them. Merge's Agent Handler connects across hundreds of MCP-ready third-party services, per its own docs, naming Salesforce, Slack, Jira, GitHub, HubSpot, NetSuite, and Workday among them. Nango states 800+ APIs, the broadest single-number app count among the four, exposed through both its MCP endpoint and its REST/SDK surface for non-agent use cases.

Raw app count answers one question. It doesn't show whether those actions are agent-ready, tested schemas or a generic passthrough, the distinction the comparison below is built around.

ActionKit vs. Composio vs. Merge Agent Handler vs. Nango

Paragon is the clear winner for teams that need per-user auth handled end-to-end and a broad, agent-ready action surface in one API, without stitching a separate auth layer on top of a tool-calling layer.