ADR-0079: A per-provider prompt layer

• Status: Accepted
• Date: 2026-09-08
• Deciders: jka
• Decision: D79

Context

With two providers available, users wanted instructions scoped to one model, such as formatting preferences for Claude or house rules for a local model, in addition to the global and project layers of ADR-0070. The prompt must remain a core concern.

Decision

A fourth layer, providers/<name>.md in the XDG agent directory, is included only while that provider is active. Composition order is base, system, provider, project: project instructions stay last as the most specific.

• The provider contract does not change. compose takes the active provider's name and folds the file into the single system string the provider already receives. A provider never knows the layer exists.
• echo has no provider layer. A missing file or unknown name adds nothing.
• The file name is restricted to [A-Za-z0-9_-]+, the shape of a registered provider name, so it cannot point outside the directory.
• agent.prompt.edit accepts which = provider with a name validated against the registry.
• The Agent Prompts… dialog gains a provider chooser row. The Preferences Agent page gains an Agent Prompts… button and a hint when only echo is installed.

Consequences

• Instructions can target one model without affecting others, with no provider changes.
• The per-provider layer is user-wide. A project-and-provider layer is not built.
• The allow-list of ADR-0084 was given the same three scopes for consistency.