Reactive Agents vs the OpenAI / Claude Agent SDKs
A real 2026 question: vendor Agent SDKs (OpenAI Agents SDK, the Claude Agent SDK) got good, model APIs converged, and “just call the SDK” is now legitimate advice. So before reaching for any framework — including this one — it’s worth being honest about when you don’t need one.
This page is that honest take.
The altitude difference
Section titled “The altitude difference”A vendor Agent SDK is a first-party toolkit for one provider: a clean tool-calling loop, structured output, handoffs, and tracing, tuned for that vendor’s models. Reactive Agents is a vendor-neutral agent harness: a typed runtime and execution engine that wraps any provider and adds the production layers an SDK leaves to you.
Neither is strictly “better” — they sit at different altitudes. The SDK is the right floor for a lot of apps. The question is whether your problem needs the floor above it.
At a glance
Section titled “At a glance”| Reactive Agents | Vendor Agent SDK | |
|---|---|---|
| Scope | Multi-provider harness | Single vendor, first-party |
| Provider lock-in | None (Anthropic, OpenAI, Gemini, Ollama, LiteLLM) | Tied to the vendor |
| Local models | First-class — same code on 4B Ollama and frontier | Generally not a goal |
| Type model | Typed effect runtime (errors as values, structured concurrency) | Idiomatic SDK types |
| Execution model | Deterministic 12-phase engine + per-phase hooks | The SDK’s loop |
| Reasoning strategies | 6 (ReAct, Reflexion, Plan-Execute, ToT, Adaptive, Code-Action) | The SDK’s loop |
| Tools | MCP-native + typed builder | Vendor tools (+ MCP on some) |
| Guardrails / cost routing / budgets | Built in | Bring your own |
| Durable execution + crash-resume | Built in | Bring your own |
| Observability | OpenTelemetry + local studio, no SaaS tether | Vendor tracing (often hosted) |
| Best at | Portable, governed, multi-step agents | Fast first-party loops on one vendor |
“Vendor Agent SDK” generalizes the OpenAI Agents SDK and the Claude Agent SDK; specifics differ and both evolve fast. Corrections welcome via PR.
When a vendor SDK is the right call
Section titled “When a vendor SDK is the right call”Be honest with yourself — reach for the raw SDK when:
- You’re committed to one provider and happy there. The first-party SDK will always track that vendor’s newest features first.
- Your loop is simple — a few tools, a few steps, no durability or governance requirements.
- You want the fewest dependencies and the most direct path to that vendor’s models.
- You’re prototyping. Start with the SDK; reach for a harness when the production requirements show up.
If that’s you, use the SDK. A framework would be overhead.
When Reactive Agents earns its keep
Section titled “When Reactive Agents earns its keep”The harness pays for itself when you need things the SDK leaves to you:
- Portability across providers — including local. The same agent code runs on a 4B Ollama model on your laptop and on Claude/GPT/Gemini, one line different. Develop and test locally and privately; swap to a frontier model for production. A single-vendor SDK can’t do this by design.
- A typed runtime, not just typed calls. Built on Effect-TS: an LLM or tool failure is a value in an explicit error channel, concurrency is structured, retries and fallbacks compose. You inherit those guarantees through a plain async API — you don’t write Effect.
- Determinism and inspectability without a SaaS. Every run is a 12-phase lifecycle with before/after/error hooks on each phase, inspectable locally. No hosted tracing subscription required.
- Production layers built in. Guardrails, cost routing and budgets, durable crash-resume, human-in-the-loop approvals, and multi-agent (A2A) — composable layers, not things you reassemble per project.
In short: use the SDK for a loop on one vendor; use Reactive Agents when you need a portable, typed, governed agent across vendors and local hardware.
You can use both
Section titled “You can use both”These aren’t mutually exclusive. A vendor SDK is a fine way to talk to one provider; Reactive Agents is how you make an agent out of it that’s portable, observable, and safe to run unattended. Many teams start on an SDK and adopt a harness when the production requirements arrive.
Honest note: Reactive Agents is early access (v0.12.0, MIT). The vendor SDKs are backed by their providers and move fast. The bet here is the architecture — typed runtime, local-to-frontier portability, observable-by-construction — and it’s real and testable today.
Ready to try it? Start with the Quickstart, or see Build AI Agents in TypeScript for the full picture.