Era V · 2023–2024
The Tool-Use Inflection
By C.W. Jameson · Published 19 May 2026 · Last reviewed 19 May 2026
Tool-use turned a chatbot into a runtime. Function-calling shipped in OpenAI's API in June 2023, in Anthropic's a few months later, and across Gemini and the open-weights ecosystem within a year. The change was architectural, not merely additive: a model that can call tools is a different shape of program. The first generation of serious agentic harnesses emerged around it.
Function calling
OpenAI shipped a 'functions' parameter in June 2023. The model emitted structured arguments instead of prose, the harness executed, the result returned. The pattern was older than the API; chain-of-thought reasoning had pointed toward it. The formalisation mattered.
The first agent harnesses
BabyAGI and AutoGPT went viral in 2023 as demos and faltered as production tools. The next generation (Aider, Cursor, Cline, Claude Code) built on the same primitives but with discipline. By 2024 the coding-agent market existed.
JSON schemas as contracts
Structured outputs went from prompting trick to first-class API feature. By mid-2024 every frontier provider supported strict JSON schema enforcement at the API layer.
Signature models of the era
- GPT-4-turbo (the first tool-use refinement)
- Claude 3 family
- Gemini 1.5
Technical shifts
- Function calling becomes universal
- JSON-schema strict mode ships across providers
- Parallel tool-call emission
Market shifts
- First $100M ARR coding-agent companies
- MCP (Model Context Protocol) emerges as a tool-server standard
- The agent framework market fragments
Authentication — is the document from this era?
| Tell | Meaning |
|---|---|
| Mentions 'function calling' rather than 'tool use' | Document predates Anthropic's terminology unification — early 2024 or before. |
Agents catalogued in this era
- GitHub Copilot — The original. Tab-completion that started the coding-agent market.
- OpenAI Assistants API — OpenAI's hosted agent runtime. Threads, tools, file search, code interpreter — managed for you.
- Perplexity — Search-first answer engine. Citations on every claim, model-agnostic, the first credible Google alternative for fact retrieval.
- GPT-4o — The omni model. Text, image, audio natively in one system. Speed doubled vs. GPT-4.
- Whisper — Open-weights speech recognition. The transcription backbone for most AI pipelines.
- LangChain — The framework that defined agent chaining before 'agent' was a product category.
- Amazon Bedrock — AWS's model marketplace. Run Claude, Llama, Titan — whatever model you need — inside your VPC.
Primary sources
- [1] OpenAI: Function calling — 2023-06-13
From the Almanac shop
The AI Eras — Pocket Field Guide
Ten eras of AI on a single foldable. The Almanac in your pocket.
$19 — Coming soon