Freelance Full Stack Developer Jobs in Milton, ON

Saatchi & Saatchi is an advertising agency with the belief that creativity, data, media and technology should all work together, and we use that to influence human behavior and drive success for clients. S&S is one of the world's largest agency networks with 114 offices and more than 6000 employees globally. 

Here in our Toronto office we work with some of the countries most valued brands including Toyota, Buckley's, and Children's Advil, as well as several under the Mondelez banner (Crispers, Oreo, and Chips Ahoy just to name a few). We're an award winning agency in both creativity and effectiveness, so it's really important for us here to convert that innovation and the great ideas into real tangible business results for the organization.

Saatchi & Saatchi is seeking a Lead Full Stack Developer for an innovative AI platform initiative.You have 8+ years building production software systems and at least a few years leading or anchoring a team technically. You are a hands-on builder and you hold the architectural picture of a multi-tier platform - React frontend, Spring Boot backend, FastAPI agentic services, agent SDKs on top of LLM providers. You are equally comfortable reviewing a database migration, debugging a prod incident, mentoring a senior engineer, or sitting in a client architecture conversation. You thrive at the intersection of architecture, engineering, and program reality.

Responsibilities

• Own the technical direction across the platform's tiers - frontend, backend, agentic/AI services, data, and integrations - and hold the shared mental model of how the system actually fits together.
• Pair closely with the Senior Technical PM and Program Director to translate product intent into a credible technical plan, and to surface what is genuinely hard vs. what is just unfamiliar.
• Lead architecture and design reviews, write the deciding ADR when needed, and keep a small set of vendor-neutral patterns the team can lean on instead of a sprawl of frameworks.
• Stay hands-on in the code - pick up meaningful slices of work, especially across tier boundaries (e.g. backend agentic, frontend auth/RBAC, infra-aware app changes).
• Be a first-class code reviewer across the stack: you set the bar for clarity, testability, security, and operability, and you raise the team's bar by reviewing thoughtfully.
• Mentor mid-level and senior engineers; coach them through technical decisions, on-call experiences, and difficult code reviews.
• Partner with the Platform Engineer on production reliability - capacity, scaling, observability, and incident response - and own application-side reliability concerns.
• Drive integration architecture with internal and external systems using API-first and event-driven patterns; ensure identity, authorization, data protection, and auditability are built in, not bolted on.
• Champion the LLM/agent layer - agent orchestration, tool/plugin design, prompt and evaluation loops, provider abstraction (e.g. Vertex AI, Claude on Vertex, OpenAI) - and keep us out of vendor lock-in we cannot pay for.
• Build redundancy on yourself - pair, document, rotate ownership, and ensure no critical system has exactly one person who understands it.

• 8+ years in software engineering, with meaningful time as a hands-on lead, staff engineer, or solutions architect on production systems.
• Proven experience designing and delivering complex, multi-service platforms - not just one service or one tier.
• Strong backend depth in at least one modern stack (Java/Spring Boot, Python/FastAPI, Node, Go, .NET); able to ramp on the others.
• Solid frontend literacy - comfortable reading and reviewing modern React/TypeScript code and reasoning about state management, auth, and performance.
• Strong cloud architecture background, preferably GCP (we run primarily on GKE); willing to deepen wherever the platform takes us.
• Deep understanding of API design, integration patterns, distributed systems, and the failure modes that come with them.
• Experience with containerization, Kubernetes, and modern CI/CD; you do not need to be a DevOps specialist, but you must operate well in that world.
• Real production experience with AI / LLM systems - agents, RAG, evaluation, tool integration - or a strong, demonstrable plan for getting there.
• Comfortable working within Agile delivery models (Scrum, Kanban, or whatever variant actually ships).
• Excellent communication: you can explain a complex architecture to a non-technical executive and disagree with a senior engineer respectfully and with evidence.

Required Soft Skills

• Hands-on bias - you reach for the code, the logs, and the diagram before you reach for the meeting invite.
• Calm, decisive judgment when there is no obviously right answer.
• Strong written communication - you can write a one-page architecture rationale that future-you will still understand.
• Generous mentorship instinct - you make the people around you better, on purpose.
• Comfortable with ambiguity and willing to make reversible decisions quickly while flagging the irreversible ones for broader input.
• Pragmatic about technology choices - you optimize for the team's ability to operate the system, not for personal stack preferences.
• Honest about what you do and do not know; willing to say 'I'll find out' and then actually find out.

Nice to Have

• Prior experience working inside Publicis Groupe or another large holding-company agency network.
• Production experience with Google ADK, LangGraph, LlamaIndex, or comparable agent frameworks.
• Experience with event-driven architectures, domain-driven design, and contract-first integration patterns.
• Background in regulated or privacy-sensitive environments (identity, healthcare, financial, marketing-compliance).
• Experience supporting multiple products or clients on a shared platform (multi-tenant, multi-brand, white-label).
• Comfort with Terraform, Kustomize, or other IaC tooling at a 'can read and improve' level.
• Track record contributing to internal developer experience - patterns, scaffolds, golden paths.

Location & Eligibility: Candidates must be based in Canada, with valid Canadian work authorization of at least 12 months.

Hybrid Work: This role is remote. If you are GTA-based, the team meets once a month at our Toronto office (111 Queen St. E, Suite 200, Toronto, ON M5C 1S2).

Time Off:

- Up to 3 weeks vacation, with additional paid closure between Christmas and New Year's

- Extended long weekends for provincial holidays - we give you both the Monday and Friday so you get a full 4-day break

- 10 personal days per year 

Flexibility & Global Mobility: After completing one year, work remotely for up to 6 weeks per year from any of our 50+ global offices through our Work Your World program.

Benefits: Comprehensive group coverage including:

- Medical, dental, and vision care

- Psychological and paramedical services

- Disability insurance

- Fertility support and gender-affirming care

- Dedicated internal guidance programs for employees navigating cancer, fertility treatments, or gender transition

Parental Leave Top up: A 100% salary top up program once approved for Employment Insurance for the first 6 weeks of your parental Leave.

Defined Contribution Pension Plan: Employees have the option to sign up to voluntarily contribute to a DCPP where Saatchi will match the employee contributions up to 5% after completing one year.

Gym Membership: Corporate Membership rates at Good Life Fitness

Free access to a financial planner & more... 

Compensation: The salary range for this position is $130,000-$150,000 per year, based on experience, skills, and relevant certifications. We believe in pay transparency and are committed to offering competitive, market-aligned compensation.

We use artificial intelligence (AI) tools to support parts of our hiring process, such as reviewing applications or analyzing resumes. These tools assist our recruitment team but never replace human decision-making. We believe in a human-first approach, where your experience and potential are recognized by people.

Saatchi & Saatchi is committed to building a diverse workforce representative of our community. We encourage and are pleased to consider all qualified candidates, without regard to race, colour, citizenship, religion, sex, marital / family status, sexual orientation, gender identity, aboriginal status, age, disability or persons who may require an accommodation, to apply. If you require a specific accommodation please contact Human Resources at 416-925-7733 or by email at inquiries@publicisna.com.