Composed Secrets

Composed secrets derive one exported value from other secrets in the active profile. They are useful for connection strings, command arguments, and other formats whose components should remain independently stored:

[profiles.default]
DB_USER = { description = "Database user" }
DB_PASSWORD = { description = "Database password" }
DB_HOST = { description = "Database host" }

DATABASE_URL = {
  description = "PostgreSQL connection string",
  composed = "postgres://{DB_USER}:{DB_PASSWORD}@{DB_HOST}/app"
}

DB_USER, DB_PASSWORD, and DB_HOST resolve through their ordinary providers. DATABASE_URL is then rendered in memory and exported alongside them. The composed result is never read from or written to a provider.

Static dependency graph

Every {NAME} must name a secret declared in the effective profile. SecretSpec validates the complete graph while loading secretspec.toml, before accessing a provider:

declaration order does not matter;
a composition may reference another composition;
unknown references are errors;
dependency cycles are errors.

[profiles.default]
USER = { description = "Database user" }
PASSWORD = { description = "Database password" }
HOST = { description = "Database host" }

AUTHORITY = { description = "Database authority", composed = "{USER}:{PASSWORD}" }
DATABASE_URL = { description = "Database URL", composed = "postgres://{AUTHORITY}@{HOST}/app" }

This differs deliberately from dotenv expansion, where behavior can depend on file order, process environment, and how a particular parser handles undefined or recursive variables.

Template syntax

Composition is a small, strict language:

Syntax	Meaning
`{SECRET_NAME}`	Insert one declared secret’s exported value
`{{`	Insert a literal `{`
`}}`	Insert a literal `}`

The following are intentionally unsupported:

${NAME} and shell-style expressions such as ${NAME:-fallback};
ambient environment-variable lookup;
command substitution;
recursive expansion.

Substitution is one pass. If PASSWORD contains the literal text {HOST}, inserting {PASSWORD} produces {HOST}; it is not scanned again. This keeps secret bytes opaque and prevents values from unexpectedly becoming executable template syntax.

Missing, empty, and optional values

Missing and empty are different:

an empty dependency inserts an empty string;
a missing dependency makes a required composition missing;
when the composed secret sets required = false, a missing dependency omits the composed result instead.

SecretSpec never silently replaces a missing reference with empty text. Interactive secretspec check prompts for the unresolved provider-backed dependencies, not for the derived result.

Read-only behavior

A composed secret cannot also declare default, providers, ref, type, or enabled generate. These fields would give the same name two competing value sources.

get resolves the target’s transitive dependencies and prints the result;
set rejects the composed name as read-only;
import skips composed names because there is no stored value to copy;
check, run, export, and SDK resolution include the composed value like any other resolved secret.

Profiles and inheritance

References are checked against the effective profile after default profile inheritance. A profile may override the template while inheriting component declarations:

[profiles.default]
DB_USER = { description = "Database user" }
DB_PASSWORD = { description = "Database password" }
DB_HOST = { description = "Database host" }
DATABASE_URL = { description = "Database URL", composed = "postgres://{DB_USER}:{DB_PASSWORD}@{DB_HOST}/app" }

[profiles.development]
DATABASE_URL = { composed = "postgres://{DB_USER}:{DB_PASSWORD}@{DB_HOST}/app_dev" }

Profile-level storage defaults do not apply to composed secrets, because their source is the dependency graph rather than a provider. Profile-level required defaults still apply.

Paths and encoding

When a dependency uses as_path = true, its exported temporary-file path is inserted. Setting as_path = true on the composed secret instead writes the final rendered value to a temporary file.

Composition performs raw string concatenation. It does not URL-encode values: SecretSpec cannot infer whether a component is a username, password, host, path, or query parameter. Store each component in the representation required by the destination format.

See the composed configuration reference for the field-level constraints.