Senior Language Engineer

About this role

ABOUT RETELL AI

Retell AI is using first-principles thinking to reimagine the call center with cutting-edge voice AI. Thousands of companies now use Retell's AI voice agents to handle sales, support, and logistics calls that once required large teams of human agents. Backed by Y Combinator, Alt Capital, and other leading investors, we've scaled to $80M in ARR with a team of 50, up from $5M at the start of 2025, and are now valued at over $1.5B.

Our vision for 2026 is to build a modern CX platform where entire contact centers are powered by AI. Instead of basic automation that needs constant human tuning, we're creating intelligent AI “workers” that act as frontline agents, QA analysts, and managers, continuously executing, monitoring, and improving every customer interaction.

We're growing fast and looking for ambitious builders who want to tackle hard technical problems, move quickly, and have a real impact on one of the fastest-growing voice AI companies in the world.

Let's build the future together.

Recent recognition:

- No. 1 Best Places to Work in the Bay Area, San Francisco Business Times 2026 https://finance.yahoo.com/technology/ai/articles/voice-ai-startup-retell-ai-145200025.html

- Top 50 AI Apps, a16z (2025) https://a16z.com/the-ai-application-spending-report-where-startup-dollars-really-go/

- #3 Fastest-Growing Software Company, G2 Best Software Awards 2026 https://company.g2.com/news/g2s-2026-best-software-awards

- Best Agentic AI Software, G2 Best Software Awards 2026 https://www.retellai.com/blog/retell-ai-recognized-as-the-best-agentic-ai-software-by-g2

- #4 Fastest-Growing Software Vendor, Brex Benchmark 2025 https://www.brex.com/journal/brex-benchmark-december-2025

- Enterprise Tech 30 Class of 2026, Nasdaq & Wing VC https://www.nasdaq.com/videos/retell-ai

- Top-Ranked Startup, Lean AI Leaderboard https://leanaileaderboard.com/

- Backed by Y Combinator https://www.ycombinator.com/companies/retell-ai

ABOUT THE ROLE

Retell AI transforms customer experience with voice AI for enterprises, including customers like CVS/Aetna, American Airlines, Lenovo, and Grab. We have more customer stories than we can tell!

This role sits at the intersection of frontier AI systems, high-quality data, and core product requirements. You will own the full data stack, from data creation, scraping, and synthesis, to managing an annotation workforce, to data cleaning, processing, evaluation, and analysis, applying sharp linguistic judgment throughout. You will play a critical role in driving modeling innovation, working closely with researchers, ML engineers, and product managers to ensure our AI systems sound natural, humanlike, and aligned with human preferences.

KEY RESPONSIBILITIES

- Define requirements and own the end-to-end pipeline for creating high-quality datasets for voice agent use cases, across both text and audio modalities.

- Engineer web-scale data pipelines and apply synthetic generation techniques to produce high-quality training and evaluation data.

- Coordinate and manage a human annotation workforce: author guidelines, define quality targets, and QA annotator output.

- Build data processing and cleaning pipelines that align datasets to production needs, balancing coverage across use cases, languages, and domains.

- Analyze production logs, curated datasets, and other sources to surface failure patterns and identify high-leverage areas for targeted data collection.

- Apply your linguistic taste to judge which outputs are more natural, conversational, and humanlike, and produce preference data that encodes that judgment.

- Partner with researchers and engineers to drive each modeling iteration.

REQUIREMENTS

- 2+ years of experience in computational linguistics, language data processing, or a similar field, including hands-on work with large-scale text and audio datasets.

- Highly technical: fluent at writing scripts for data processing and at leveraging models for synthetic data generation.

- Native-level command of English, with the confidence to make opinionated linguistic calls about what sounds natural in voice agent conversations.

YOU MIGHT THRIVE IF YOU

- A strong applied ML background in language or audio modeling — ideally having contributed to the data pipelines behind a well-known audio or language model.

- A PhD in Computational Linguistics or an equivalent field with computational emphasis.

- Experience managing human annotation and evaluation teams.

- Excitement for building scalable systems that bridge research and production.

JOB DETAILS

- Cash: 200k - 290k

- Equity: Equity Provided

- Location: Redwood City, CA, US

- US Visas: Retell AI is open to sponsoring work authorization for qualified candidates, including H1B/H-1B, TN, L-1, E-3, F-1 (OPT/CPT).

OTHER BENEFITS

- 100% coverage for medical, dental, and vision insurance

- $70/day DoorDash credit for unlimited breakfast, lunch, dinner, and snacks

- $200/month wellness reimbursement (gym, fitness classes, etc.)

- $300/month commuter reimbursement (gas, Caltrain, etc.)

- $75/month phone bill reimbursement

- $50/month internet reimbursement

COMPENSATION PHILOSOPHY

- Best Offer Upfront: Choose from three cash-equity balance options; no negotiation needed.

- Top 1% Talent: Above-market pay (top 5 percentile) to attract exceptional builders.

- High Ownership: Small teams, >$1M revenue/employee, and significant equity.

- Performance-Based: Offers tied to interview performance, not experience or past salaries.

INTERVIEW PROCESS:

- Talent Screen (15min): chat with our recruiter to get a better sense of the role, the team, and what it’s like to work here.

- Technical Interview (45 min): practical coding for data processing (scraping, cleaning, analysis)

- Technical Interview (45 min): linguistic judgement and preference alignment

- Onsite/Virtual Interviews (2 hrs): Hosted in our office if…