APIs are point-to-point connections that you maintain. MCP is a standard protocol that vendors maintain. For agentic systems running continuously across multiple tools, this difference is everything. We rebuilt our prospecting pipeline on MCP after custom APIs cost us more maintenance time than improvement time. Here is why the choice matters and when each approach makes sense.
Most companies building agentic systems do not realise they have a problem until it is too late.
They take an API (HubSpot, Salesforce, Slack, whatever), write custom code to connect an agent to it, and think they are done.
Six months later, the integration breaks. The vendor updates the API. Your custom code no longer works. You have to rebuild it.
You were building on sand.
At Rivett, we chose MCP (Model Context Protocol) instead. And it changed how we think about agentic systems entirely.
01What Is an API and Why Does It Break?
An API is a point-to-point connection. You write code that says: "When X happens in System A, tell System B about it."
This works fine for one connection. But when you have many connections, it gets messy.
Example: an SDR agent needs to read leads from LinkedIn, check enrichment data from ZoomInfo, log decisions to HubSpot, send emails via Gmail, and update Slack with status. That is five custom integrations. Five different pieces of code. Five different ways for things to break.
When LinkedIn changes their API, your integration breaks. When ZoomInfo updates their schema, your integration breaks again. Every vendor update is a potential problem.
This is why custom API integrations are fragile. They are point-to-point. They are specific to each vendor. When vendors change, you break.
02What Is MCP and How Is It Different?
MCP is infrastructure for agents - a standard protocol rather than a custom connection.
With MCP, you do not write custom code to connect an agent to HubSpot. You connect the agent to the MCP protocol. HubSpot maintains an MCP server. They handle the details.
If HubSpot changes their API, they update their MCP server. Your agent still works because you are connected to the standard, not to the specific API.
With APIs, you maintain integrations. With MCP, vendors maintain them.
This changes the game for agentic systems.
03Why Does MCP Matter Specifically for Agents?
Agents are not like traditional software. They run continuously, need access to many systems simultaneously, and need to stay working even when vendors change things.
Agents Need Multiple Integrations
A single agent might need access to 5-10 different systems. With custom APIs, that is 5-10 different integrations to maintain, each able to break independently. With MCP, that is one protocol connecting to 5-10 MCP servers. The protocol is stable. The servers are maintained by vendors.
Agents Run Continuously
An agent runs 24/7. It wakes up every 15 minutes. It needs to access your systems constantly. Custom API integrations get stale - rate limits change, authentication tokens expire, endpoints shift. With MCP, the protocol handles this. Rate limiting is built in. Authentication is standardised.
Agents Learn from Feedback
An agent needs to remember what happened, log decisions, and understand patterns from past actions. Custom API integrations are typically one-way. MCP servers are bidirectional - the agent can ask questions and get rich answers back. The agent can learn.
04Why Did We Choose MCP at Rivett?
We built our prospecting pipeline on custom APIs first. It worked for 3 months.
Then Slack changed their API rate limits. Then HubSpot updated their schema. Then Gmail tightened their authentication.
We spent more time maintaining integrations than improving the agent.
We rebuilt on MCP. Now our agent is connected to standard MCP servers for HubSpot, Slack, Gmail, and LinkedIn. When vendors change, they update their MCP servers. Our agent keeps running.
We went from "integration maintenance every week" to "integration maintenance never." This freed us to focus on what actually matters: making the agent smarter, not keeping it alive.
05When Should You Use MCP vs Custom APIs?
The choice depends on what you are building.
Use custom APIs when:
- You need a one-time integration with a single system
- The integration is not core to your operations
- You control both sides of the connection
Use MCP when:
- You are building agentic systems (continuous, multi-system, learning)
- You need the integration to stay working as vendors update
- You want to avoid maintenance overhead
For Rivett, we use MCP exclusively because agents are our core product.
06How Do You Evaluate MCP Readiness?
Three questions to ask before committing:
Do my core vendors support MCP? HubSpot, Salesforce, Slack, and Gmail all have MCP servers or are adding them. Check before you build.
Am I building for continuous operation or one-time tasks? If continuous, MCP wins.
How much integration maintenance can I afford? If none, MCP. If some, custom APIs might work short-term.