009. Breaking Removal of Custom Provider at v0.4.0

Date: 2026-02-21 Status: Accepted Issue: F070

Context#

AgentConfig.Command and CustomProvider are the only code path for shell-based agent invocation. With type: step covering deterministic CLI tools and openai_compatible covering LLM APIs, custom has no unique purpose and its shell-template model conflates execution paradigms. Keeping dead code violates Principle 6 (Minimal Abstraction).

Decision#

Delete custom_provider.go and its test files, remove AgentConfig.Command field from the domain model, and add a validation error for provider: custom with migration guidance pointing users to type: step or provider: openai_compatible. Released as a breaking change at v0.4.0.

Alternatives rejected:

• Deprecation warning first — v0.4.0 is already a breaking version boundary; parallel maintenance adds cost with no benefit.
• Keep Command as unused field — dead fields violate Minimal Abstraction and confuse future readers.

Consequences#

Positive#

• Eliminates ~820 LOC of dead code and associated maintenance burden.
• Enforces strict semantic separation: CLI execution vs. LLM API invocation.
• Validation error provides actionable migration guidance at configuration time.

Negative#

• Users with provider: custom workflows get a hard failure requiring migration.
• Breaking change must be prominently documented in CHANGELOG and migration guide.