Sidecars: A low-latency trust boundary for Sandboxes | Modal Blog

Modal Blog · Publicado el 1 oct 2026

Olivia Johnston introduces Modal Sidecars as isolated containers alongside a main Sandbox. She discusses the trust boundary for agent-generated code, reports faster cross-boundary communication in the stated comparison, and explains the risks of hosting an agent harness and its tool calls together. Lee 3 puntos de vista con sus evidencias y enlaces a las fuentes.

Olivia Johnston

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Existing isolation uses an outdated unit of trust
Johnston says technologies such as gVisor and Firecracker isolate the platform from users and users from one another. She calls that unit of trust outdated and asks how to protect users from their “own” code.
Ver el momento de apoyo · Párrafo 1
Sidecars provide 3x faster cross-boundary communication in the stated comparison
Johnston describes Sidecars as isolated containers that run alongside a main Sandbox on the same host. She reports 3x faster communication across trust boundaries than using separate Sandboxes, particularly for operation-heavy workloads.
Ver el momento de apoyo · Párrafo 2
Agent harness in same Sandbox creates lethal trifecta
Johnston cites Simon Willison’s “lethal trifecta”: access to private data, exposure to untrusted content and the ability to communicate externally can allow a tricked agent to leak data. She says a coding agent whose harness runs in its Sandbox has all three by default.
Ver el momento de apoyo · Párrafo 8

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performance-security tradeoff

Sidecars provide 3x faster cross-boundary communication in the stated comparison

Extracto original

Sidecars enable 3x faster communication across trust boundaries than using separate Sandboxes — which is particularly helpful for operation-heavy workloads.

Olivia Johnston·1 oct 2026

Contexto

Today we’re excited to introduce Sidecars, which are our broader answer to this problem. Sidecars are isolated containers that run alongside your main Sandbox on the same host and provide a real security boundary between trusted or untrusted code.

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trust boundary design

Existing isolation uses an outdated unit of trust

Extracto original

technologies like gVisor and Firecracker “solved” “isolation” nearly eight years ago. Unfortunately for us, they solved it for an now-outdated unit of trust. How do you protect users from their “own” code?

Olivia Johnston·1 oct 2026

Contexto

At Modal, our customers rely on Sandboxes to execute untrusted code written by their downstream users or, almost exclusively now, by agents. Running untrusted code isn’t a new problem: every cloud provider has to do this from day 1 to isolate their platform from their user and their users from each other. Fortunately,

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agent security risk

Agent harness in same Sandbox creates lethal trifecta

Extracto original

Simon Willison calls it the lethal trifecta : an agent with access to private data, exposure to untrusted content, and the ability to communicate externally can be tricked into leaking that data. A coding agent whose harness runs in its Sandbox has all three by default.

Olivia Johnston·1 oct 2026

Contexto

Hosting the harness and the tool calls together is a security risk. The harness's credentials sit next to generated code, the agent can read untrusted content from the web, like packages and repos, and anything in the Sandbox can make network calls..

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