Best AI GTM and Lead Generation Agencies in 2026: 12 Compared

"GTM engineering" has become the label for agencies that do more than send campaigns. They wire up the data sources, buying signals, enrichment and CRM logic that decide who gets contacted and when, and more of them now build with AI agents and Claude Code alongside Clay. Lead generation agencies sit next to them: they run the whole outbound motion and hand you conversations.

We supply sending infrastructure to a lot of these agencies, so we see which setups hold up. Below are twelve worth a look, split into the two groups. Every fact comes from the agency's own website, its Clay partner profile or a named source, checked in November 2026, with sources at the end. Results are as each agency reports them.

The short version

GTM engineering agencies

1. OneGTM

OneGTM is a founder-led GTM engineering agency run by Garrett Wolfe, who was employee #9 at Unify and says he helped take its ARR from $400K to $6M+. It builds custom outbound, signal, CRM and AI systems for seed to Series C companies, and Garrett scopes every build himself. New builds run on Claude Code, with Clay and Attio in the stack, and the agency shows a Clay Artisan Solutions Partner badge.

Its strongest proof is the Antimetal case study: an automated outbound system that created 17 deals and $2.57M in pipeline, with 10 of those deals reaching BDR qualified and 6 reaching technical evaluation. The setup ran on about 250 mailboxes across 37 lookalike domains, sending around 1,225 emails a day.

Best for: lean, early and growth-stage teams that want signal-driven systems they own, not generic outbound.

2. The Kiln

The Kiln describes itself as "expert GTM engineers, at your fingertips", building inbound, outbound and RevOps systems mostly on Clay, with a team that includes early Clay employees. It is a Clay Elite Studio partner. Its Azuga case study is headlined "From 640+ Hours of Manual Work Per Week to a Scalable Data Engine".

Best for: Series B to C, mid-market and enterprise teams with the budget for a large Clay build. Its Clay profile lists $10k+ a month with a 3-month minimum.

3. RevPartners

RevPartners is a RevOps and GTM engineering firm for HubSpot and Clay businesses, offering Clay implementations and GTM engineering as a service. It is both a HubSpot Elite partner and a Clay Elite Studio partner, and reports 1,000+ CRM and RevOps deployments.

Best for: B2B companies that run on HubSpot and want GTM engineering inside that ecosystem.

4. Growth Today

Growth Today positions itself as an "AI-native GTM engine" that connects content, outbound and ads into one allbound motion, with signal-based outbound as one of its services. Its case studies include 111 demos in 120 days for Cavalry, and its Clay profile quotes Rokt on around $40,000 of pipeline in 40 days. It works with a wide stack, including Clay, HubSpot, Instantly, Smartlead, EmailBison and HeyReach.

Best for: B2B teams with $1M+ in revenue that want outbound tied into content and ads. It offers a 90-day pilot; its Clay profile lists $5k+ a month.

5. Scalantec

Scalantec is a DACH-based GTM engineering agency that builds lead generation systems on the client's existing CRM, with the aim to "scale revenue without scaling headcount". It reports €2.6M+ in qualified pipeline for Seven Senders in a single quarter. Its stack includes Clay, Claude, AI Ark, HeyReach, Instantly and EmailBison.

Best for: B2B companies of roughly 50 to 2,000 employees with an offer that already converts and a CRM in place (HubSpot, Salesforce or Dynamics).

6. FullFunnel

FullFunnel provides managed GTM, demand generation and RevOps operators alongside applied AI agents, and builds and runs a client's Clay data engine. It is a Clay Elite Studio partner and says it has served 1,000+ B2B organisations over 12+ years.

Best for: GTM teams that need more capacity without adding headcount. Its Clay profile lists $5k+ a month with a 1-month minimum.

Done-for-you lead generation agencies

7. Shields Outbound

Shields Outbound is a lean, founder-led cold email agency run by James Shields. It builds the full outbound system end to end, from sending domains and authentication to warm-up, lead verification and daily deliverability monitoring, then hands warm, qualified leads to the client to close. It works on a pay per qualified lead model with no retainer, and agrees what counts as qualified (company size, budget, decision-maker, fit) before launch. In James's words: "If we don't hit it, you don't pay."

Published results include 78 booked calls and over $50k MRR closed in 120 days for an AEO and SEO agency, a first client signed at $6,500 a month for a new AEO consultancy, and 1,082 replies from 105,460 sends for CLPR Media.

Best for: teams that want qualified leads without paying a monthly retainer for activity.

8. Growth Engine X

Growth Engine X, founded by ex-Clay employee Eric Nowoslawski, is a Clay Elite Studio partner focused on cold email lead generation at scale. It says it sends 8 million+ emails a month, and reports 62K+ positive responses across clients in 2025. Published client results include 552 positive responses in 4 months for Heritage Wealth Capital.

Best for: B2B companies with a market of 100K+ contacts and a sales team ready to take the calls. It says it is usually not a fit below 40K contacts.

9. Leadbird

Leadbird, founded by Nick Abraham, bills itself as "B2B lead generation, 100% done for you", led by cold email. A 2026 Smartlead case study reports 120+ active clients and 800+ meetings a month booked across them, with 10M+ emails sent through Smartlead.

Best for: companies that want a high-volume cold email programme run entirely by someone else.

10. Belkins

Belkins has run B2B outbound and appointment setting since 2017, across cold email, calling, LinkedIn and paid channels, with outsourced SDRs and deliverability consulting alongside. It says it has generated $2B+ in revenue for clients, launches campaigns in 14 days and books first appointments within 30. Case studies include 330 appointments in 15 months for IGS and 320+ in 12 months for Omnicharge. It is rated 4.9 on Clutch from 230 reviews.

Best for: established B2B sales and marketing teams that want a large, proven appointment-setting operation.

11. Cleverly

Cleverly runs done-for-you lead generation across LinkedIn, cold email and cold calling. It reports $312M in pipeline generated for clients, and case studies including 307 meetings and 49 closed deals in 3 months for VIRC Insurance, and $1 million closed in 12 months for Blox.

Best for: teams that want LinkedIn, email and calling handled by one provider.

12. Martal

Martal is an outbound sales agency with 15 years in business and 200+ onshore sales executives, combining cold email, calling, LinkedIn and outsourced sales teams with its own AI outreach system. Its case studies include 880 booked meetings over 24 months for a financial services client. It is rated 4.9 on Clutch.

Best for: companies from startup to enterprise that want outsourced sales reps, not just campaigns.

How to choose an outbound agency

• Decide whether you want a system or a service. GTM engineering agencies build something your team keeps. Lead generation agencies run it for you, and the pipeline usually stops when the contract does.
• Look at how they get paid. Retainers pay for activity. Pay per lead or pay per meeting models shift the risk to the agency, but agree the definition of "qualified" before launch.
• Ask for pipeline, not activity. Emails sent and replies are inputs. Deals created, qualified pipeline and closed revenue tell you whether the targeting works.
• Check the sending setup. Every one of these models still lands in an inbox. Ask how many mailboxes and domains the plan needs and who maintains them when they decay.

If your agency is building the outbound side, our guide to cold email infrastructure for agencies covers the mailbox and domain decisions, and How Many Mailboxes Do You Need for Cold Email? covers the maths. If you want the infrastructure set up for you, book a call with Frostmailer.

Sources, checked November 2026: getaob.com and getaob.com/antimetal-case-study, shieldsoutbound.com, thekiln.com, revpartners.io, growthtoday.co, scalantec.com, fullfunnel.co, coldoutbound.com, belkins.io, cleverly.co, martal.ca, each agency's profile at clay.com/experts, and for Leadbird, smartlead.ai's Leadbird case study (March 2026). Results and pricing are as published by each agency and change often, so check with the agency before hiring.