kandev/docs/plans/executor-model-mismatch-fallback/task-01-executor-model-decision.md

id, title, status, wave, depends_on, plan, spec

id	title	status	wave	depends_on	plan	spec
01-executor-model-decision	Define executor model decisions	done	1		plan.md	../../specs/no-silent-model-fallback/spec.md

Task 01: Define executor model decisions

Acceptance

• An unadvertised requested model causes no model-selection call, and the agent default continues.
• An explicit fallback applies only when advertised, while advertised apply errors remain explicit.
• Initial launch, context reset, and workspace rebind use one typed decision helper without duplicate calls.

TDD scenarios

1. RED: Replace strict-gone tests with no-call and provider-default expectations.
2. RED: Add absent fallback, empty catalog, method-not-supported, and auto-fallback cases.
3. RED: Add context-reset and workspace-rebind decision cases.
4. GREEN: Add the typed decision and apply the executor-authoritative order.
5. REFACTOR: Remove duplicated model-selection branches and keep one policy owner.

Verification

• cd apps/backend && go test -tags fts5 -run 'TestApplyStartModelPolicy|TestInitializeAndPrompt.*Model|TestReapplySessionModel|TestWorkspaceRebind.*Model' ./internal/agent/runtime/lifecycle
• cd apps/backend && go test -tags fts5 ./internal/agent/runtime/lifecycle
• git diff --check

Files likely touched

• apps/backend/internal/agent/runtime/lifecycle/start_model.go
• apps/backend/internal/agent/runtime/lifecycle/start_model_test.go
• apps/backend/internal/agent/runtime/lifecycle/session.go
• apps/backend/internal/agent/runtime/lifecycle/session_test.go
• apps/backend/internal/agent/runtime/lifecycle/manager_interaction.go
• apps/backend/internal/agent/runtime/lifecycle/manager_interaction_test.go
• apps/backend/internal/agent/runtime/lifecycle/manager_workspace_rebind.go
• apps/backend/internal/agent/runtime/lifecycle/manager_workspace_rebind_test.go

Dependencies

None.

Parallelism

Sequential. Tasks 02 and 03 consume the typed decision contract.

Inputs

• The amended no-silent-model-fallback specification.
• The executor-authoritative model-selection ADR.
• The current strict applyStartModelPolicy implementation.

Output contract

Report the decision outcomes, model-selection call lists, RED evidence, GREEN evidence, and test results.

Risks

• An empty catalog can mean unsupported selection or incomplete provider data.
• Runtime model overrides must follow the same executor catalog rules.
• A handled default decision must prevent a later profile-layer retry.

Results

Implemented the typed executor-authoritative decision and reused it for initial launch, context reset, and workspace rebind. Lifecycle coverage passed with 1,857 tests.