MTPLX vs Spanlens

MTPLX and Spanlens are both inference engines & infra tracked by AIDiveForge. Below is a side-by-side comparison of pricing, capabilities, platforms, and ownership — sourced from each tool's live website and verified before publishing.

MTPLX

The vendor states a 2.24× decode speedup on Qwen3-27B running on an M5 Max MacBook Pro, achieved by using the model's own built-in MTP heads as the drafter — no second model loaded, no external checkpoint to maintain. Acceptance is handled via Leviathan–Chen rejection sampling with a residual (p − q)+ correction, verified bit-exact against single-token autoregressive output. It serves an OpenAI- and Anthropic-compatible API, so downstream tooling like Claude Code, Cline, or the openai-python SDK connects without shims. The wall appears immediately if you leave Apple Silicon: the runtime is explicitly Apple Silicon only, and the custom Metal kernels have no CUDA path.

Spanlens

Spanlens sits in front of your LLM provider via a single baseURL change, recording every call's cost, latency, tokens, and full request-response body with no SDK rewrite required. Agent runs surface as waterfall span trees so you can identify the one step consuming 80% of wall-clock time. The model recommender flags GPT-4o calls that look like classification tasks and shows the cost delta if you swap — with numbers from your own traffic, not benchmarks. The eval and experiment layer lets you replay a fixed dataset across prompt versions before you ship, so quality regressions don't surprise you in production. PII scanning and anomaly detection run at log time, which matters when sensitive data crosses the wire at 3 a.m. with nobody watching.

Attribute	MTPLX	Spanlens
Pricing	Free	Paid
Price	—	$29/mo
Free trial	No	No
Open source	Yes	Yes
Has API	Yes	Yes
Self-hosted option	Yes	Yes
Platforms	macOS (Apple Silicon)	Node.js, Python, Next.js, Edge, self-hosted
Released	2025	—
Pros	Leviathan–Chen rejection sampling with residual correction produces bit-exact output at temperature > 0, so agent workflows that depend on non-greedy sampling get the correct distribution instead of a silent approximation that drifts results unpredictably. The drafter lives inside the target checkpoint's own MTP heads, which means no second model in memory — on a MacBook with 64–128 GB unified memory, that headroom stays available for context or parallel sessions rather than a dedicated draft model. OpenAI- and Anthropic-compatible API endpoints with streaming SSE, so tools like Claude Code, Cline, Continue, and the openai-python SDK connect without a translation layer or custom adapter. The vendor reports 2.24× decode speed on Qwen3-27B at temperature 0.6/top_p 0.95 on an M5 Max — meaning you get more tokens per second without switching to a smaller model or lowering temperature to approximate greedy. Apache-2.0 license with no cloud tier or usage telemetry mentioned in the docs, which means inference stays entirely on local hardware — no prompt data leaves the machine.	Proxy-layer instrumentation via a single baseURL change, so existing code requires no structural rewrite and every provider call is captured from day one rather than after a manual instrumentation sprint. Per-user and per-route cost attribution, which means you can identify the specific customer or endpoint burning disproportionate budget before it compounds across a billing cycle. Agent waterfall trace trees with critical-path highlighting, so a slow or expensive step in a multi-agent run is pinpointed in seconds instead of reproduced manually in a staging environment. Experiment runner replays a fixed dataset across prompt versions and models with quality, cost, and latency compared side by side, which means you ship with evidence that v8 is better than v7 rather than finding out the hard way in production. Self-hosted deployment via Docker Compose under MIT license, so teams with data residency or audit requirements can run the full platform without sending trace data to a third-party cloud.
Cons	The runtime is Apple Silicon only, with custom Metal kernels and no CUDA path: the moment your deployment target is a Linux server, a cloud VM, or a Windows workstation, this tool is not an option and teams move to vLLM or llama.cpp instead. MTP speculative decoding requires models that ship with native MTP heads in their checkpoint — models without those heads get no speedup and fall back to standard autoregressive decode, which means the 2.24× figure applies only to a specific subset of supported architectures. The project is at v0.1.0-preview.1 and built by a single developer: production teams that need an SLA-backed issue resolution path, a security response process, or a multi-maintainer commit history will hit that wall before they finish the proof-of-concept.	PII detection is regex-based and runs at log time as a flag — not a pre-storage redaction guarantee. Teams operating under HIPAA or SOC 2 controls where sensitive data must never reach a log store, even briefly, need a dedicated redaction layer upstream of Spanlens or a different architecture entirely. The LLM-as-judge eval scoring is a single 0–1 scalar per response. Teams needing structured, multi-criteria evaluation rubrics — for example, factual accuracy scored separately from tone and policy compliance — hit the ceiling of what the built-in scorer expresses and end up maintaining a custom eval harness alongside Spanlens. At high request volumes where the proxy layer adds measurable latency to every call, teams running latency-sensitive production paths at scale have moved to SDK-side instrumentation tools or full APM platforms with LLM plugins, where the observability path is out of band rather than in the critical path.

Bottom line

MTPLX is free while Spanlens is paid. Choose based on which difference matters most for your workflow.

Frequently asked questions

What is the difference between MTPLX and Spanlens?

MTPLX is Free and open source, while Spanlens is Paid and open source. Compare pricing, free trial, API, platforms, and pros/cons in the table above on AIDiveForge.

Is MTPLX better than Spanlens?

It depends on your workflow. Use the side-by-side attributes (pricing, open source, API, self-hosted, platforms) to decide. AIDiveForge does not rank a universal winner — we publish verified facts so you can choose.