MCP vs Function Calling vs OpenAPI: Which Tool-Invocation Pattern Fits Your LLM Integration?

What these approaches do

MCP, function calling, and OpenAPI tools are three patterns for letting large models interact with external capabilities. Each defines how an LLM discovers, invokes, and composes calls to services or local code, but they differ in scope, portability, and operational requirements.

What each approach is

• MCP (Model Context Protocol): An open, transport-agnostic protocol that standardizes discovery and invocation of tools, resources, and prompts across hosts and servers. MCP shines when you need portable, cross-host tooling and a catalog of reusable servers.

• Function Calling: A vendor-driven feature where the model selects a declared function based on a provided JSON Schema and returns a structured argument payload. Your runtime executes the function. This pattern is optimal for tight, low-latency app-local integrations.

• OpenAPI Tools: Use the OpenAPI Specification 3.1 as the contract for HTTP services and let agent/tooling layers generate callable tools. This is best when you have an estate of HTTP services that need governed contracts, security schemes, and discoverable operations.

Key differences at a glance

• Interface contract: MCP uses a protocol data model for tools/resources/prompts, function calling relies on per-function JSON Schema, and OpenAPI Tools use OAS 3.1 documents.
• Discovery: MCP supports dynamic discovery via tools/list, function calling requires a static list passed to the model, and OpenAPI derives tools from OAS catalogs.
• Invocation: MCP invokes via tools/call over a JSON-RPC session, function calling has the model select a function and return arguments for the app to execute, while OpenAPI Tools call HTTP endpoints per OAS operations.
• Orchestration: MCP supports host-level routing across servers and tools, function calling focuses on app-local chaining, and agent/toolkits route intents to operations for OpenAPI Tools.
• Transport and portability: MCP works over stdio or HTTP variants and is built for cross-host reuse; function calling is often in-band via an LLM API and vendor-specific; OpenAPI uses standard HTTP(S) contracts and is vendor-neutral but requires orchestration.

Strengths and limits

MCP

• Strengths: Standardized discovery and invocation, reuse across servers and products, good for multi-tool orchestration, and growing host support in projects like Semantic Kernel and Cursor.
• Limits: Requires running servers, host policy for identity and consent, sandboxing, and an implemented session lifecycle and routing layer on the host.

Function Calling

• Strengths: Minimal integration overhead, fast control loop for low latency, and straightforward validation using JSON Schema.
• Limits: Catalogs are app-local and nonportable without redefinition per vendor, with limited built-in discovery and governance features.

OpenAPI Tools

• Strengths: Mature, well-understood contracts; built-in security schemes such as OAuth2 and API keys; and a rich ecosystem of tooling and agent generators from OAS.
• Limits: OAS specifies HTTP contracts but does not provide agentic control loops; you still need an orchestrator or host to route intents and manage sessions.

Security and governance

• MCP: Enforce host-level policy such as allowed servers, per-tool scopes, consent prompts, and ephemeral credentials. Platform-level adoption often emphasizes registry control and explicit user consent.
• Function Calling: Rely on strict JSON Schema validation of model-produced arguments, maintain allowlists, and log function calls for auditability.
• OpenAPI Tools: Use OAS security schemes, API gateways, and schema-driven validation. Constrain any agent toolkits to prevent arbitrary outbound requests and centralize governance.

Ecosystem signals and portability

• MCP: Supported by projects like Microsoft Semantic Kernel and Cursor, with signals of broader platform interest including Windows-level integration plans.
• Function Calling: Widely available across major LLM providers with similar request patterns and schema-based validation, making it easy to adopt for single-app integrations.
• OpenAPI Tools: Many agent stacks and toolkits can auto-generate tools from OAS, and OpenAPI remains the lingua franca for HTTP service contracts.

Decision rules: when to use which

• Use Function Calling when you have app-local automations with a handful of deterministic actions and tight latency targets. Keep schemas small, validate strictly, and unit-test the loop.
• Use MCP when you need cross-runtime portability, shared integrations across agents, IDEs, desktop apps, and backends. MCP is the right fit for standardized discovery and reuse of servers across products.
• Use OpenAPI Tools when you manage an enterprise estate of HTTP services that need formal contracts, security schemes, and governance. Treat OAS as the source of truth, generate tools from it, and enforce policies via gateways and orchestrators.

Hybrid patterns and practical tips

A common practical pattern is hybrid. Maintain OAS specs for your services so they remain the canonical contract. Then expose services via an MCP server to enable cross-host portability, and for latency-critical surfaces, mount a subset as function calls inside specific products. This lets you balance governance, portability, and performance without committing exclusively to one approach.