Synth
Synth is a macOS workbench plus hosted compute for optimizing coding agents and long-horizon agent workflows.
Pick Synth if you are hill-climbing a coding agent and have been burned by eval runs you cannot reproduce. Paired held-out evaluation and fail-closed evidence contracts make results defensible, and the Sidecar monitor in Stack's Goal mode means you stop reading every tool call on long runs. The setup cost is real: CLI, Python, and container work before anything useful appears, and Workshop is an Apple silicon macOS build. If you only need plain LLM observability with no optimization loop, LangSmith or Weights & Biases are easier to adopt; if you need governed prompt experiments inside a lakehouse, note that MLflow covers that ground.
Verified 20h ago · liveness 72/100 · cite: rightaichoice.com/tools/synth
- Research engineers optimizing coding-agent prompts, contexts, and scaffolds
- Teams running long-horizon sparse-reward agents that need auditable receipts
- CLI- and SDK-comfortable groups wanting local-first work with optional hosted compute
- Labs hill-climbing evals with GELO, GEPA, and benchmark-owned grading
- Non-technical users who want a no-code eval or observability dashboard
- Beginners without prior experience building agent pipelines or running evals
- Windows or Linux desktop users expecting a Workshop build for their platform
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Skip Synth if you want a point-and-click eval or observability dashboard and have no appetite for CLI, Python, and container work, or if your team works on Windows or Linux desktops rather than Apple silicon Macs.
Included usage is a weekly allowance ($10 on Free, $60 on Standard, $300 on Max) that renews every week rather than monthly, so a burst that exceeds it bills before you expect a reset.
At $20/mo Standard (weekly renewal, $60/week included usage) Synth sits in the same band as a typical individual developer tool, and at $200/mo Max it competes with team-tier eval platforms rather than individual seats. Cheaper entry points exist if you do not need optimization loops: local-only use with your own provider keys costs nothing to Synth. The Free tier's $10/week allowance is enough for evaluation, not for hosted long-horizon runs.
In short
Synth — Synth is a macOS workbench plus hosted compute for optimizing coding agents and long-horizon agent workflows. Best for Research engineers optimizing coding-agent prompts, contexts, and scaffolds, Teams running long-horizon sparse-reward agents that need auditable receipts, CLI- and SDK-comfortable groups wanting local-first work with optional hosted compute. Free to start; paid plans from $20/mo.
What's new in Synth
Checked todayAcross the latest 5 updates: 1 feature update, 2 launches, 1 pricing change and 1 changelog entry.
Synth v0.23: Index Search from the SDK and MCP, and weekly included usage
Index Search now works from the Python SDK, HTTP, and hosted MCP, and plans include weekly usage starting with Free at $0.00/mo with $10.00 of usage every week.
Synth Index alpha v0.1
Search reviewed research with Fast and Deep Search from the API, Python SDK, or MCP. Sign up, get an API key, and pay per search.
Managed Research: Direct xAI Grok Build
Hosted SMR runs on Grok Build over direct xAI, with a simplified public Grok catalog and synth-ai 0.11.7.
Research Factory adds typed research cycles and held-out proof
Research Factory ships in synth-ai 0.15.0 with typed research and maintenance cycles, immutable candidates, and benchmark-owned grading.
Stack releases split into Stable and nightly channels
Synth publishes Stable and nightly Stack releases with per-channel notes on what shipped in each.
What people actually say about Synth — is it worth it?
We scanned public community sources for Synth on Jul 3, 2026 and could not establish that the discussion we found is about this tool rather than something else sharing its name. Our own analysis of that scan says the posts were off-subject. Rather than publish a sentiment score built on the wrong subject, we publish nothing here and re-run the scan.
Viability Score
How well maintained and how widely used is Synth? Built from what the vendor actually publishes (docs, changelog, tutorials, integrations, pricing), whether the site is live, and how much real users discuss it. How we calculate this
Last calculated: October 2026
How we score →Key Features
- Local-first macOS Workshop workbench for Apple silicon
- Stack harness with live runs, traces, scores, candidates, and handoffs
- Sidecar monitor agent in Stack Goal mode that narrates progress and steers workers
- GELO (Go-Explore Long-Horizon) optimizer for sparse-reward agent tasks
- GEPA (Gradient-Free Evolutionary Prompt Adaptation) for coding-agent prompts
- Paired held-out evaluation with fail-closed evidence contracts
- Research Factory with typed research cycles, immutable candidates, and benchmark-owned grading
- Synth Index alpha v0.1 Fast and Deep Search over reviewed research
- Index Search via Python SDK, HTTP API, or hosted MCP
- Managed Research hosted runs on direct xAI Grok Build
- Local Qwen MLX SFT fine-tuning with training-workload selection
- Graphs API and Graph Evolve for automatically evolving multi-step LLM workflow graphs
- Jesterky native runtime that replays agent workflow orchestration
- Stack Stable and nightly release channels with per-channel notes
- CLI and SDK interface with open-source Containers and Optimizers installed via uv
About Synth
Synth is research infrastructure for engineers who iterate on coding agents and want reproducible evidence rather than impressions. Workshop is a local-first macOS app for Apple silicon where you build containers, run evals and optimizers, inspect traces, and compare policy changes in one workspace. Managed Research runs the same loops on Synth-hosted compute, and since June 2026 hosted runs go through direct xAI Grok Build on synth-ai 0.11.7. Optimizers GELO (Go-Explore Long-Horizon) and GEPA (Gradient-Free Evolutionary Prompt Adaptation) hill-climb agent behavior, while paired held-out evaluation with fail-closed evidence contracts keeps results defensible. A Sidecar monitor agent, default in Stack's Goal mode, narrates progress and audits done-claims mid-run. Research Factory turns a bounded objective and budget into typed research cycles with immutable candidates and benchmark-owned grading. Synth Index alpha v0.1 adds Fast and Deep Search over reviewed research from the API, Python SDK, or MCP, pay per search. Pricing as of the v0.23 release: Free at $0.00/mo with $10.00/week included usage, Standard at $20.00/mo with $60.00/week, Max at $200.00/mo with $300.00/week, all weekly renewal. No Synth account is needed for local models, local data, and local execution; ChatGPT login and direct provider APIs are equally valid entry points billed by those providers. Containers and Optimizers ship as standalone open-source projects you add with uv, and Stack publishes Stable and nightly channels. Expect CLI, Python, and container work: this is aimed at agent builders and research engineers, not at buyers of turnkey eval dashboards.
Behind the Verdict
Synth's argument is that agent quality work fails not on ideas but on receipts. The pieces that back that up are specific. GEPA does gradient-free evolutionary prompt adaptation for coding-agent prompts, GELO does go-explore long-horizon search on sparse-reward tasks, and both feed paired held-out evaluation with fail-closed evidence contracts, so a run that cannot produce evidence is treated as failed rather than as a win. Research Factory, shipped in synth-ai 0.15.0, converts a bounded objective and budget into typed research and maintenance cycles with immutable candidates and benchmark-owned grading, which is the part most eval stacks leave to spreadsheets. The daily workflow pieces matter more than the optimizer names for most buyers. Stack's Goal mode defaults to a Sidecar events feed where a monitor agent narrates progress, audits done-claims, and steers the worker; Stack ships on Stable and nightly channels with per-channel notes. Interactive trace inspection, local reports, and a CLI/SDK surface keep the loop on your machine, with hosted capacity optional. Where it gets thin: the surface is broad and some of it is launch-stage. Synth Index is alpha v0.1 and pay-per-search, hosted GELO relaunched in June 2026 behind a 72-hour proposer-spend promo and an invite-only plugin lane, and Swarms is still listed as coming in a later release. Several blog posts are explicitly marked historical. Workshop is Apple silicon macOS only, so Windows and Linux desktop users are out of luck for the local workbench, and the docs, SDK, and CLI assume you are comfortable with Python and containers. The docs reference synth-ai 0.23.1. Fit: research engineers and agent teams who already run evals and want them to hill-climb reproducibly, plus groups wiring hosted optimizers into CI. Not a fit: anyone who wants a no-code dashboard, or a team that needs an observability product rather than an optimization loop.
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Real-world workflow fit
Concrete scenarios for the personas Synth actually fits — and what changes day-one when you adopt it.
You install Workshop on an Apple silicon Mac, point GEPA at your coding-agent prompts, and run paired held-out evaluation with fail-closed evidence contracts.
Outcome: You get a candidate prompt set that beat the baseline on held-out tasks, with the receipts to show why the result is trustworthy.
You configure Stack in Goal mode so the Sidecar monitor narrates progress and audits done-claims while the worker runs, then inspect traces and artifacts locally.
Outcome: You stop reading every tool call and catch stalled or falsely-completed runs before they ship.
You hand Research Factory a bounded objective and budget via the synth-ai SDK, let it run typed research cycles, then search reviewed write-ups through Synth Index MCP.
Outcome: You end up with immutable candidates and benchmark-owned grading plus cited research you can reference in a decision memo.
Use Cases
- Optimize prompts for a coding agent to improve multi-turn code generation accuracy.
- Run automated evaluation hill-climbs on agent prompts with GEPA or GELO.
- Stand up a Stack harness to monitor and debug a long-running agent with live artifact feeds.
- Delegate research tasks through the synth-ai SDK or hosted MCP for parallel experimentation.
- Publish reproducible runs with receipts and artifact bundles via the Open Research Factory.
- Wire hosted optimizers into CI/CD with GitHub Actions for continuous prompt improvement.
- Fine-tune locally on Qwen with MLX SFT while sizing the exact training workload.
- Search reviewed research with Synth Index Fast Search and cite the exact write-up version.
Models Under the Hood
as of 2026-10-18
Limitations
- Synth is a local-first macOS workbench for Apple silicon plus hosted compute, accessed via Python SDK, HTTP API, CLI, and hosted MCP servers (synth-ai 0.23.1).
- Hosted Managed Research runs on direct xAI Grok Build, while local fine-tuning uses Qwen MLX SFT.
- Several surfaces are early-stage: Synth Index is alpha v0.1 with Fast and Deep Search billed per search, and Swarms is listed as coming in a later release.
- Pricing is usage-based with weekly included usage across Free, Standard ($20/mo), and Max ($200/mo) plans.
as of 2026-10-18
Verification history
We have re-verified Synth 10 times since . Each pass re-reads the vendor's own pages and re-checks every listed field against that evidence; passes where nothing had changed are marked as such.
- — re-verified summary, description, our verdict, our analysis, pricing model, pricing tiers, features, integrations, who it suits, who should skip it
- — re-verified summary, description, our verdict, our analysis, pricing model, pricing tiers, features, integrations, who it suits, who should skip it
- — re-verified summary, description, our verdict, our analysis, pricing model, pricing tiers, features, integrations, who it suits, who should skip it
- — re-verified summary, description, our verdict, our analysis, pricing model, pricing tiers, features, integrations, who it suits, who should skip it
- — re-verified summary, description, our verdict, our analysis, pricing model, pricing tiers, features, integrations, who it suits, who should skip it
- — re-verified summary, description, our verdict, our analysis, pricing model, pricing tiers, features, integrations, who it suits, who should skip it
Showing the 6 most recent of 10 verification passes.
Free to cite with attribution — this page re-verifies continuously.
12-month cost
Project the real annual outlay, including the implied monthly cost when only an annual tier is published.
Vendor list price only. Add-on usage, seat overages, and contract minimums are surfaced under Hidden costs & gotchas.
Plans compared
For each published Synth tier: who it actually fits, and what it adds vs. the previous tier. Cross-reference the cost calculator above for projected annual outlay.
Free
$0.00/mo
Ideal for
An individual engineer evaluating Synth locally on an Apple silicon Mac, or trying Index Search before committing to a paid allowance.
What this tier adds
Starting tier: $10.00/week of included usage at $0.00/mo, weekly renewal, with organization signup and local-first Workshop access.
Standard
$20.00/mo
Ideal for
A research engineer or small team running hosted evaluation and optimization loops regularly rather than only local runs.
What this tier adds
Raises included usage from $10.00 to $60.00 per week at $20.00/mo, covering hosted capacity for research runs.
Max
$200.00/mo
Ideal for
A lab running long-horizon agent optimization and hosted research runs most days of the week.
What this tier adds
Raises included usage to $300.00 per week at $200.00/mo for sustained long-horizon agent work.
Where the pricing makes sense
The company stage and team size where Synth's pricing actually pencils out — and where peers do it cheaper.
At $20/mo Standard (weekly renewal, $60/week included usage) Synth sits in the same band as a typical individual developer tool, and at $200/mo Max it competes with team-tier eval platforms rather than individual seats. Cheaper entry points exist if you do not need optimization loops: local-only use with your own provider keys costs nothing to Synth. The Free tier's $10/week allowance is enough for evaluation, not for hosted long-horizon runs.
Setup time & first value
How long it actually takes to get something useful out of Synth — broken out by persona, not the marketing-page minute.
Individual research engineer: an afternoon to install the synth-ai SDK and CLI (0.23.1), authenticate, and get a first local run into Workshop; a day or two before GEPA or GELO produces a usable candidate. Team wiring hosted runs and GitHub Actions: roughly a week, mostly spent fitting your agent harness into a container and defining the grading contract. Local-only use with your own provider
Switching to or from Synth
How to bring data in from common predecessors and how to get it back out — written for the switcher, not the buyer.
- →From LangSmith: import your traces into Stack and re-express existing evaluators as paired held-out scores with evidence contracts.
- →From Weights & Biases: replace run tracking with Stack traces plus local reports, and move prompt sweeps onto GEPA for automated adaptation.
- →From hand-rolled prompt spreadsheets: point GEPA at the prompt and let hill-climbing replace manual trial-and-error.
- →From older synth-ai SDK versions: follow the docs' "Migrate from 0.22" guide to move to synth-ai 0.23.1.
- ↗To LangSmith: export Stack traces and rebuild the optimization loop as evaluators plus datasets inside LangSmith.
- ↗To Weights & Biases: move prompt experiments into W&B sweeps and keep reports there instead of local Stack runs.
- ↗To a plain git-and-notebooks workflow: pull the open-source Containers and Optimizers packages and drop the hosted pieces.
Integrations
Resources & Guides
- Documentationusesynth.ai
Docs · Synth
Full product docs from usesynth.ai
- Examplesusesynth.ai
Examples · Synth
Working sample projects from usesynth.ai
- API Referenceusesynth.ai
Reference · Synth
Methods, params, types from usesynth.ai
- Documentationusesynth.ai
Releases · Synth
Full product docs from usesynth.ai
- Resourceusesynth.ai
Changelog · Synth
Helpful link from usesynth.ai
- Documentationusesynth.ai
Llms · Synth
Full product docs from usesynth.ai
Tutorials & Learning
YouTube returned 6 videos for “Synth”, and we withheld 6: 6 could not be judged, because “Synth” is a single word that other videos use for other things. We are showing none, because we could not prove any of them are about Synth.
Official links
Tools that pair well with Synth
Common stack mates teams adopt alongside Synth, with the specific reason each pairing earns its keep.
Arena AI
Arena AI ranks chat models, coding agents and fullstack builds on live head-to-head human votes.
Imbue
Imbue builds inspectable coding-agent tools, from mngr's parallel agent runner to Latchkey's HTTP permissions
Zhipu GLM
Zhipu GLM is a full-stack large-model platform with open-weights GLM-5.3 coding models, MaaS APIs and autonomous agents.
Featured Head-to-Head Comparisons
Synth vs Presto Voice
If you run a QSR chain and need to automate drive-thru ordering with proven revenue uplift, Presto Voice is the clear choice. For developers optimizing coding agent prompts and workflows, Synth’s free tier and hosted optimizers like GELO (currently with a 72-hour free promo) are purpose-built. These tools serve entirely different domains — choose based on whether your bottleneck is the drive-thru or the agent pipeline.
Synth vs Locus Robotics
Two separate worlds. Locus Robotics is a proven warehouse automation solution delivering 2-3x productivity gains via AMRs and the LocusONE platform — ideal for high-volume fulfillment centers. Synth is a developer-centric research platform for optimizing coding agent prompts and workflows, with a free tier and recent GELO optimizer promo. Choose Locus for physical operations, Synth for AI agent engineering.
Synth vs Truleo
Truleo and Synth serve completely different audiences. Truleo is purpose-built for law enforcement agencies to surface leads from siloed data, dramatically reducing report writing time and connecting RMS, CAD, jail calls, and BWC. Synth is a developer tool for AI researchers optimizing coding agent prompts and workflows, with recent Stack sidecar monitor and GELO optimizer updates. Choose based on your domain: police intelligence or coding agent R&D.
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