Model Context Protocol (MCP) vs function calls vs OpenAPI tools – when to use?

• MCP (Model Context Protocol): Open, transport-impossible protocol that standardizes host and server discovery and invocation across hosts/resources. best Portable, multifunctional, multi-discount system.
• Function call: Provider functions where the model selects a declared function (JSON schema), returns parameters, and your runtime executes. best Single application, low latency integrated.
• OpenAPI tools: use OpenAPI Specification (OAS) 3.1 Acts as a contract for HTTP services; the proxy/tool ​​layer automatically generates callable tools. best Management, service network integrated.

comparison table

Advantages and Limitations

MCP

• Advantages: Standardized discovery; reusable servers; multi-union orchestration; growing host support (e.g. semantic kernel, cursor; Windows integration plan).
• limit: Required to run server and host policies (identity, consent, sandbox). The host must implement session lifecycle and routing.

function call

• Advantages: Minimal integration overhead; fast control loops; direct validation via JSON schema.
• limit: Application catalog; portability requires redefinition per vendor; limited built-in discovery/governance.

OpenAPI tools

• Advantages: Mature contracts; security schemes (OAUTH2, keys) in specification; rich tools (proxy from OAS).
• limit: OAS defines the HTTP contract, not the proxy control loop – you still need the orchestration/host.

Security and Governance

• MCP: Enforce host policies (allow servers, user consent), per-tool scope and ephemeral credentials. Platform adoption (such as Windows) emphasizes registry controls and consent prompts.
• Function call: Validate model produced ARG against schema; maintain allowlist; log call audit.
• OpenAPI tools: Using OAS security schemes, gateways and schema-driven authentication; constraint allows arbitrary requests in the toolkit.

Ecosystem signals (portability/adoption)

• MCP host/server: support Microsoft Semantic Core (host + server role) and cursor (MCP directory, IDE integration); Microsoft signals Windows-level support.
• Function call: Widely available on the main LLM API (OpenAI Docs shown here) with similar patterns (mode, selection, tool results).
• OpenAPI tools: Multiple proxy stacks come from OAS (Langchain Python/js) automatic generation tool.

Decision Rules (When to Use)

1. Application automation with few actions and tight latency goals → function call. Keep definitions small and validation loops strict.
2. Cross-venue portability and shared integration (agent, IDE, desktop, backend) → MCP. Standardized discovery and invocation between hosts; reuse servers across products.
3. Enterprise estates for HTTP services require contracts, security plans and governance → OpenAPI tools with the arranger. Use OA as a source of truth; build tools, execution gateways.
4. Blending modes (common): Keep OAS as a service to you; through a MCP server for portability, or install a subset as function call Used on critical delay product surfaces.

refer to:

MCP (Model Context Protocol)

Function call (LLM tool calls function)

OpenAPI (spec + LLM toolchain)

Michal Sutter is a data science professional with a Master of Science in Data Science from the University of Padua. With a strong foundation in statistical analysis, machine learning, and data engineering, Michal excels at transforming complex data sets into actionable insights.