[oldsj-mainloop]

Summary

• Add optional model and effort to TaskCreate and the delegate MCP tool, validated at task creation against the resolved provider profile's models/effort_levels (explicit provider_profile_id, else the project's or installation's default). Either named but not offered answers 422 with a TaskError whose detail carries allowed_models/allowed_effort_levels, via a new apierr.ReasonDetail/tasks.Error.Detail. Leaving both unset is unchanged behavior.
• Persist a valid choice on the task, copy it onto the attempt when the attempt is made, and apply it as the new session's own /model//effort override (SetModelOverride/SetEffortOverride, the same storage the owner's routes use) before the attempt's first brief is sent in sendBrief — never by a call after the fact that could race the first turn.
• Show the chosen model and effort on the task view (Task/TaskAttempt.model/.effort); a task-owned session's own view already shows them automatically, since they're the same override storage the chat composer already reads.
• Expand the delegate tool's description with short, provider-neutral guidance for choosing a tier: scope/risk/reversibility, mechanical vs. judgment work, a cost bias toward the cheapest choice likely to succeed, and preferring a higher tier when re-delegating a failure. It names no provider's models, pointing at the resolved profile's own list instead.
• New migration 00340_task_model_effort.sql adds model/effort columns to tasks and task_attempts, and includes the attempt's columns in the existing routing-immutability trigger.
• Docs: docs/specs/delegation.md (new "Choosing a worker's model and effort" section), docs/specs/sessions.md, docs/specs/chat.md (cross-reference).

Note: task_retry/task_reassign (handoff) remain stubbed/unavailable in this codebase, so there's no live "resume" path to exercise yet; the attempt-level copy (beginCreate) is the inheritance primitive the issue asks for, ready for when handoff lands.

Test plan

• go build ./..., go vet, golangci-lint run clean on touched packages
• go generate ./... produces no further diff (contract/codegen drift check)
• New test TestCreateValidatesModelAndEffort (server/internal/tasks/service_test.go): validation against the resolved profile, persistence, 422 allowed-values detail, unchanged default — passing against a real test Postgres
• New integration test TestDelegateAppliesModelAndEffortBeforeTheFirstTurn (server/internal/tasks/provision_test.go): attempt persistence + override applied before the first brief + unchanged default when unset — passing
• Existing server/internal/tasks, server/internal/sessions, server/internal/mcpserver, server/internal/app (apicheck) suites pass against a real test Postgres
• Web: tsc -b, eslint, vitest run src/lib/taskState.test.ts, vite build all clean; new modelEffortLabel test
• pnpm dlx redocly lint api/openapi.yaml valid
• Synced to mainloop-dev: sync id 6f03c26a77bd16bbddd4a049f544653a50e1b0ed6126416841d3318da7c35eed, health_ok: true, smoke 10 ok/0 fail/1 skip

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