Launch HN: Runtime (YC P26) – Sandboxed coding agents for everyone on a team

Gus and Carlos, the founders behind Runtime, ran into the same wall a lot of technical teams hit once AI coding agents got good enough to actually ship code. After their previous startup Mentum got acquired, Gus used agents to build four full-stack products in three months, solo. Great result. Then he tried to hand that workflow to the rest of the team and it fell apart almost immediately — PRs came back unmergeable, every repo needed an engineer to do bespoke local setup, and the tribal knowledge about how to actually run these agents lived entirely in his head. Carlos saw a similar breakdown at Modern Treasury, where letting the support team loose with Devin created more mess than it solved.

So they built Runtime, which is basically an attempt to turn "one person's ad hoc agent setup" into shared infrastructure the whole company can use. Engineers define the system instructions, skills, and integrations exactly once, installable through CLI, mise, or npm. Runtime then snapshots the entire running environment — multi-service Docker Compose stacks, Kafka, Redis, seeded databases, the works — so a fresh sandbox boots in milliseconds with every service already live. No more re-explaining the stack to a PM who just wants to try something.

Under the hood, Runtime routes sandboxes across providers like E2B, Daytona, EC2, or self-hosted Kubernetes, whatever a team already runs on. Secrets never touch the agent directly; they pass through a managed proxy instead. And the guardrails sit at the infrastructure layer itself — command allow/deny lists, network egress rules, role-based access scoped per human and per agent — rather than relying on the agent to behave itself. Every session gets a shareable preview link too, so a build can go from sandbox to teammate review without anyone needing production access.

The platform is agent-agnostic, working with Claude Code, Codex, Cursor, Copilot, Gemini, and Devin, and can be triggered from Slack, Linear, GitHub, the CLI, or a web app. Real use cases are already showing up: one customer built an on-call bot that watches PagerDuty and Sentry, finds the root cause of an alert, and opens a PR with a test attached before a human even gets paged. Another runs financial reconciliations in a private Slack channel by pulling Stripe, NetSuite, and Snowflake data straight into an agent session.

Runtime's core is open source, the hosted product has a free tier, and pricing is a flat platform fee plus compute — no markup on tokens. A fintech unicorn and a handful of YC scaleups are already running on it, including some teams that built similar internal tooling themselves and just handed the keys over to Runtime instead of maintaining it.