An engineering director at a 50-person SaaS company described their monorepo migration in late 2025. They'd been multi-repo since founding — eight separate repositories for different services and shared libraries. The migration to a monorepo took three months of focused work. Six months later, she could finally evaluate the outcome.

The verdict: it was worth it, but not for the reasons she expected, and the trade-offs were real. Her experience matches a pattern that's emerged across engineering organizations through 2025 and 2026. The monorepo question has finally settled into something that can be advised on, rather than just debated.

What Drove the Conversion

The teams that have moved from multi-repo to monorepo cite specific concrete benefits.

Atomic cross-service changes. Changes that affect multiple services no longer require coordinating PRs across repos. One PR, one review, one merge. This is the single benefit most teams cite as worth the trouble.

Shared code without versioning hell. Internal libraries that are used across services don't need to be published, versioned, and consumed through package managers. They're just imported directly. The friction of internal library management drops to near zero.

AI agent coherence. This is the 2026-specific argument. AI agents working across services do dramatically better with a single source of truth than with multiple repos. The agent can read code from any service and reason about cross-cutting changes coherently.

Tooling unification. One linter config, one test framework, one CI/CD setup, one build tool. The tooling chaos that was acceptable across 5 repos is unmanageable across 25.

Search and navigation. Engineers find code faster when it's all in one place. Across multiple repos, the friction of "which repo is that in?" adds up.

What Drives Holding Multi-Repo

The teams that have stayed multi-repo also cite specific reasons.

Clear team ownership boundaries. When team A owns repo X and team B owns repo Y, the boundaries are physical. Permissions, CI, deployment all flow naturally. In a monorepo, ownership boundaries require explicit tooling.

Independent deployment cadence. Multi-repo lends itself to independent deployments. Each service ships when its repo is updated. Monorepos can support this but require deliberate setup.

Build performance at scale. Very large monorepos require sophisticated build systems (Bazel, Buck, Nx, Turborepo). Multi-repo avoids this complexity at the cost of duplication.

Open-source compatibility. Some services need to be open-sourced separately. Monorepos can handle this but require more discipline.

Team autonomy. Some organizations value independent team velocity over coordination. Multi-repo supports this organizationally more naturally.

When Monorepo Is Worth It

A clear set of conditions favor monorepo adoption.

Small to mid-size codebases (under a few million lines). Monorepos work well at this scale without exotic build tooling. The performance and tooling complications kick in mostly above this.

High inter-service coupling. When changes routinely touch multiple services, the monorepo's atomic-change benefits compound.

Heavy use of shared internal libraries. When 5+ services share libraries that change frequently, the version management cost in multi-repo is high.

AI-augmented development workflow. Monorepos let AI agents work coherently across the entire codebase. For teams leaning heavily on agentic development, the monorepo benefit is now particularly large.

Smaller engineering organizations. Teams under 100 engineers usually benefit from monorepo's simplification. Above that, the trade-offs get more complex.

Strong build tooling investment. Monorepos require investing in build systems. Teams without that investment shouldn't try monorepo at scale.

When Multi-Repo Is Worth It

Reverse conditions favor multi-repo.

Very large codebases. Once you're past several million lines, the build tooling needs to be enterprise-grade. Some organizations conclude multi-repo is simpler than enterprise monorepo tooling.

Cross-organizational projects. Open-source projects, customer-facing libraries, joint ventures with other companies. Multi-repo aligns with the organizational boundaries.

Mature, slow-changing services. Services that rarely change and have stable interfaces don't benefit much from co-location. Multi-repo works fine.

Engineering culture that values team autonomy. Some organizations explicitly value the autonomy that physical repo boundaries provide. The cultural fit matters.

Mixed-language codebases without strong tooling. Multi-repo handles polyglot codebases without requiring sophisticated build orchestration.

The Specific Tooling That Makes Monorepos Work

Several tools have matured into reliable options.

Turborepo. Strong default for JavaScript/TypeScript monorepos at small-to-medium scale. Build caching, parallel execution, intelligent dependency tracking. Most new JS/TS monorepos use it.

Nx. More feature-rich than Turborepo. Cross-language support. Strong choice for monorepos that mix JS/TS with other languages.

Bazel. Enterprise-grade. Strong choice for very large monorepos. Steep learning curve; not the right tool for smaller teams.

Buck (Meta). Similar tier to Bazel, less broadly adopted outside Meta.

Lerna and Yarn workspaces. Older but still functional for JS/TS specifically. Less powerful than Turborepo or Nx but lower learning curve.

Choice depends on scale, language mix, and team capability.

How Migrations Work When They Work

The teams that have successfully migrated to monorepo describe specific patterns.

Migrate in phases, not all at once. Move related repos first; gradually pull in others. A big-bang migration usually fails.

Stand up the build tooling first. Get Turborepo, Nx, or your choice working before moving code. The build tooling is the foundation.

Preserve git history. Use git subtree or similar tools to move repos with their history. Lost history is a real cost.

Reorganize as you migrate. The migration is an opportunity to clean up directory structure, eliminate dead code, and refactor across boundaries. Take the opportunity.

Train the team on the new tools. Turborepo, Nx, and friends have learning curves. Plan the upskilling.

Update CI/CD carefully. The CI/CD changes for monorepo are substantial. Plan them as their own project.

What This Means for Architecture Decisions

The clearer monorepo guidance has downstream implications.

Microservices proliferation looks worse than it did in 2020. When the cost of multiple services is amplified by multi-repo overhead, microservices become more expensive. Monorepo softens the cost but doesn't eliminate it. Many organizations are consolidating services that they once split.

Internal library publishing infrastructure is less needed. Monorepos eliminate the need for internal NPM registries, Go module proxies, and similar. The infrastructure simplification is real.

Documentation centralization is more achievable. With code in one place, documentation can be co-located. This is a real benefit that monorepo enables.

Cross-team collaboration patterns shift. Code reviews across team boundaries are easier in monorepo. Teams that used to be siloed by repo boundaries can collaborate more naturally.

What Engineering Leaders Should Do

Three concrete recommendations.

Audit your current setup honestly. If you're multi-repo, are the boundaries serving you or fighting you? If you're monorepo, is the build tooling appropriate for your scale?

Don't migrate without specific concrete benefits. Migration is expensive. Do it because you're hitting specific pain points, not because you read a blog post.

Invest in build tooling proportionate to your scale. Small monorepos can use lightweight tooling (Turborepo). Large ones need investment in Bazel-class tools. Mismatched tooling produces frustration.

Update for AI-augmented development. The monorepo benefit has grown specifically because AI agents work better in monorepos. If your team is heavily AI-augmented, the monorepo case is stronger than it was in 2024.

The monorepo debate consumed conference talks for years. The actual answer is more nuanced than either side claimed but also more knowable than the debate suggested. For most small-to-medium engineering organizations operating with AI-augmented workflows, monorepo is now the better default. For very large organizations or those with specific structural reasons to stay multi-repo, the multi-repo path remains viable.

The decision points are concrete. The trade-offs are well-understood. The tooling exists. What's left is the actual engineering judgment of whether your specific situation favors one or the other — a much easier question to answer than the abstract debate ever was.