Tool-Call Flow

Coverage

The protocol-level cycle by which a language model uses external capabilities. Covers the four phases (declaration, request, execution, continuation), the message-history state model that ties them together, the structural differences between vendor protocols (Anthropic tool-use, OpenAI function-calling, Model Context Protocol, Gemini function calling), parallel tool calls, streaming tool calls, the runtime's role as orchestrator, error encoding inside the cycle, and the boundary between model-side intent and runtime-side execution.

Philosophy

A tool-call flow is the smallest unit of agentic capability. Strip away orchestration patterns, multi-agent coordination, evaluation harnesses — what remains is a single language model alternating turns with a runtime that executes capabilities on its behalf. Understanding this cycle precisely is the foundation for understanding everything that builds on it.

The cycle's defining property is the separation of planning from execution. The model produces structured intent; the runtime carries it out. This separation is not a workaround for current model capabilities; it is a deliberate design choice that makes the system auditable, composable, and recoverable. A system that fuses the two — by letting the model execute code directly, or by letting the runtime make decisions — gains expressiveness and loses every benefit the separation provides.

The four-phase structure is identical across every current vendor protocol. The names differ, the message shapes differ, the encoding of parallelism differs, but the cycle — declare, request, execute, continue — is the same. A practitioner who understands the cycle can move between Anthropic, OpenAI, MCP, and Gemini at the cost of a translation layer; a practitioner who understands only one vendor's encoding cannot.

The Four Phases