feat(delegation): add per-task model override for delegate_task

[liuhao1024]

Problem

delegate_task currently uses a single global model configured via delegation.model / delegation.provider in config.yaml. All delegated subagents run on the same model regardless of task complexity.

This limits a common pattern: using different model tiers for different subtasks — e.g. a smart/expensive model for hard reasoning tasks, a cheap/fast model for mechanical work, and a medium-tier model as the default.

Solution

Add an optional model parameter to delegate_task that overrides the global delegation.* config for that specific invocation. Supports both top-level (applies to all children) and per-task (individual tasks in batch mode) overrides.

Precedence: per-task model > top-level model > delegation.* config

API

# Top-level: all children use this model
delegate_task(
    goal="Analyze this complex codebase architecture",
    model={"provider": "anthropic", "model": "claude-sonnet-4"}
)

# Per-task: different models for different tasks
delegate_task(
    tasks=[
        {"goal": "Complex reasoning task", "model": {"provider": "anthropic", "model": "claude-sonnet-4"}},
        {"goal": "Format 50 files with black", "model": {"provider": "openrouter", "model": "deepseek/deepseek-chat"}},
    ]
)

# Model-only: inherit parent's provider, just swap the model
delegate_task(goal="Quick task", model={"model": "cheap-model"})

Implementation

• _resolve_model_override() — resolves a model dict into a credential dict (provider, base_url, api_key, api_mode), following the same pattern as cron job model overrides
• When only model is specified (no provider), the child inherits the parent's provider credentials — the common "use a cheaper model" pattern
• When provider is specified, full credentials are resolved via the runtime provider system
• Bare "custom" provider is treated as no provider (inherits parent) — prevents LLM hallucinated custom providers from breaking resolution

Changes

• tools/delegate_tool.py: +122 lines (new _resolve_model_override helper, schema additions, handler update)
• tests/tools/test_delegate.py: +207 lines (12 new tests covering schema validation, credential resolution, precedence ordering)

Tests

tests/tools/test_delegate.py::TestModelOverride::test_schema_has_model_top_level PASSED
tests/tools/test_delegate.py::TestModelOverride::test_schema_has_model_per_task PASSED
tests/tools/test_delegate.py::TestModelOverride::test_resolve_model_override_none_returns_none PASSED
tests/tools/test_delegate.py::TestModelOverride::test_resolve_model_override_model_only_inherits_parent_provider PASSED
tests/tools/test_delegate.py::TestModelOverride::test_resolve_model_override_bare_custom_ignored PASSED
tests/tools/test_delegate.py::TestModelOverride::test_resolve_model_override_with_provider_resolves_credentials PASSED
tests/tools/test_delegate.py::TestModelOverride::test_resolve_model_override_provider_no_key_raises PASSED
tests/tools/test_delegate.py::TestModelOverride::test_resolve_model_override_invalid_provider_raises PASSED
tests/tools/test_delegate.py::TestModelOverride::test_top_level_model_override_used_for_child PASSED
tests/tools/test_delegate.py::TestModelOverride::test_per_task_model_overrides_top_level PASSED
tests/tools/test_delegate.py::TestModelOverride::test_no_model_override_uses_delegation_config PASSED
tests/tools/test_delegate.py::TestModelOverride::test_batch_per_task_model_with_provider PASSED

Fixes #41814

[yimisunrise]

+1 for this feature. Per-task model/provider override would be very useful for routing different subtasks to different cost/quality tiers. Looking forward to seeing this merged.

[jarodtaylor]

For the love of God, PLEASE implement this feature!

[creemer]

+1 Please implement it. It will be great!

[dresraz]

+1 Please implement! would be really useful!

[rigario]

Strong +1 for this PR. Per-task/per-call model overrides are a useful foundation for routing delegated work across cost/quality tiers.

I opened #46981 as a small dependent/additive follow-up that builds on this shape and adds explicit fallback isolation for cost-capped subagents e.g. allowing a cheap child model to avoid silently inheriting an expensive parent fallback chain.

If #41826 lands first, #46981 can be rebased down to just the fallback-isolation delta.

[alt-glitch]

This was generated by AI during triage.
Duplicate of #6771 (earliest open per-task model-override PR). This is part of a heavily saturated cluster (tracking #14974/#15789) with 6+ competing PRs (#6771, #12794, #16163, and others). Marking as duplicate of the earliest canonical open implementation; a maintainer should pick one to land and close the rest.

[teknium1]

We do not want this