Core Dagger Types

Core Dagger types are the vocabulary of module APIs. They represent values the engine understands deeply: content, execution, services, secrets, generated changes, environment, and model interaction.

Use these types to make boundaries explicit and composition natural.

Types Every Module Author Should Know

• Container: a filesystem, environment, execution, and publish pipeline.
• Directory: a content-addressed tree.
• File: a content-addressed file.
• Secret: sensitive data passed without exposing plaintext.
• Service: a long-running endpoint for other work to use.
• Changeset: a proposed diff against a directory or workspace.
• Env: a set of values and resources that can pass through workflows.
• LLM: model interaction when enabled for a module.

Type Design Principles

Prefer	Because
Directory or File over string paths.	Dagger can track content, cache accurately, and make host boundaries explicit.
Secret over string credentials.	Callers and traces can preserve secrecy by construction.
Service over localhost strings.	Startup, ports, and dependencies become part of the graph.
Changeset over silent writes.	Generated changes become reviewable and composable.
Structured objects over JSON strings.	Callers can discover fields and compose results without parsing.

Reference Boundary

This chapter should teach how module authors think with core types. It should not duplicate every field from the Types Reference.

TODO: Link each core type to its reference page and add author-focused guidance for when to accept it, when to return it, and common mistakes.