Summary

Adds optional per-task model/provider overrides to delegate_task (each tasks[] entry), with precedence per-task > delegation.* config > parent inherit. Along the way it fixes two latent bugs that silently made delegation.* config inert, so the new feature actually takes effect.

Scope is deliberately tight: the diff touches only tools/delegate_tool.py and tests/tools/test_delegate.py — no top-level params, no run_agent.py dispatch changes. Per-task overrides ride the existing tasks forwarding path.

Problem

Three distinct defects, all in the delegation path:

• A — silent config swallow. _load_config caught any persistent-load failure with a bare except: return {}, so a malformed config silently dropped all delegation.* settings with zero signal — subagents just inherited the parent and nobody knew why.
• B — credential pool clobbers a pinned endpoint. A child configured with an explicit delegation.base_url was still handed a credential pool. At runtime _swap_credential (run_agent.py:3164/3182/3186) overwrites self.base_url from the leased entry, silently redirecting the child off its configured endpoint.
• Feature gap. The build loop fed every child the same batch-wide credential bundle and the tasks[] schema had no model/provider field, so subagents could never run on a different model/provider per task.

Solution

• A: keep the graceful degrade (still returns {} → parent inherit) but logger.warning(...) first so the failure is observable. This is defense-in-depth — the common corrupt-YAML case already surfaces via _warn_config_parse_failure in hermes_cli/config.py; this catches the residual unexpected-failure path that previously went dark.
• B: skip the credential pool only for a literal config endpoint. _resolve_delegation_credentials now tags its result with base_url_is_explicit — True only on the literal delegation.base_url branch, False for a provider-resolved endpoint. _build_child_agent gates the skip on a new override_base_url_explicit flag. A provider-resolved base_url (e.g. delegation.provider: openrouter) keeps its pool, because that pool's entries carry the same provider's endpoint — a swap there is legitimate same-provider key rotation, not an off-endpoint redirect.
• Feature: new _resolve_task_credentials(task, cfg, batch_creds, parent_agent) returns the batch bundle unchanged when a task carries no override; otherwise it overlays the per-task model/provider onto a dict(cfg) copy and re-resolves through the existing resolver. A per-task provider switch drops base_url/api_key/api_mode from the copy so the new provider re-derives its own endpoint. Resolution runs in an up-front pre-pass (before any child is built), so an invalid per-task provider fails the whole call cleanly with a tool_error rather than leaving half-built children. The tasks.items schema gains model and provider string properties.

Deliberate non-goals: per-task overrides expose model and provider only. base_url and api_key are intentionally not per-task — an LLM-emitted tool call must not be able to route a subagent to an arbitrary endpoint or inject a key. There is no batch-wide override arg (a shared value is just the same field on each task), no delegate_task signature/dispatch change, and _swap_credential is untouched.

Testing

• tests/tools/test_delegate.py → 146 passed.
• tests/agent/test_credential_pool_routing.py tests/run_agent/test_fallback_credential_isolation.py → 17 passed (confirms the guard-narrowing didn't regress pool/isolation behavior).

Coverage: TestLoadConfig (Bug A, warn + degrade); TestExplicitEndpointSkipsPool — incl. test_provider_resolved_base_url_still_attaches_pool and test_literal_base_url_is_flagged_explicit — (Bug B + guard, both directions); TestPerTaskCredentialResolution (resolver/precedence/endpoint re-derivation); TestPerTaskOverrideReachesChild (un-mocked e2e — a per-task model actually reaches child construction through the build loop, vs the mocked units that isolate the resolver).

Reviewer focus

• Pool guard: tools/delegate_tool.py:1161 (one-line override_base_url_explicit gate).
• _resolve_task_credentials: :2489+ (early by-reference return for no-override tasks — safe, consumers are read-only and acp_args is copied at :1043; provider switch drops the endpoint).
• Pre-pass: :2048 (resolves all per-task creds before any child is built → one bad provider fails the batch cleanly).
• Flag origin: :2440 (base_url_is_explicit=True only on the literal config base_url branch).

Issues & related PRs

Reconciled item-by-item against the full delegation cluster (18 active issues + 2 active PRs, plus stale/closed context). This PR ships per-task tasks[].model/provider only — no top-level delegate_task(model=…) argument — which is what splits Closes from Refs below.

Closes (reported defect fully resolved):

Closes #7833 — Bug B: a child pinned to an explicit delegation.base_url is no longer rebound to the parent's pool via _swap_credential; the pool is skipped for a literal config endpoint.
Closes #17685 — per-task tasks[].model is now in the schema and forwarded end-to-end (tested); it is no longer silently ignored.
Closes #18591 — per-task model with the exact fallback chain it requests (per-task > delegation.model > parent); this issue asks for no top-level arg.
Closes #27294, Closes #27295, Closes #27296 — same explicit-delegation.base_url clobber (filed three times); fixed by the pool-skip.
Closes #12440 — the blocking impact (sub-agents stuck on a small-context default) is resolved via the working, tested per-task model path plus the now-visible config-load failure. (Caveat: for a config-only delegation.model on the same provider as the parent with no base_url, see the known limitation under #31155; HERMES_MODEL env-override is out of scope.)
Closes #15789 — its primary ask is per-task tasks[].model/provider (the detailed schema it proposes); that surface is delivered and tested. The issue mentions a top-level single-task parameter only in passing, and that surface was intentionally removed upstream (see fb0f579b1 under Refs), so the substantive request is fully met.

Refs (related but not fully resolved here):

Refs #5012, #6306, #10995, #17732, #23467, #32711 — each of these requests a top-level / per-call delegate_task(model=…) argument. This PR deliberately does not add it: the top-level model parameter was intentionally removed by the maintainer in fb0f579b1 ("refactor: remove model parameter from delegate_task function … simplifies the function signature and enforces consistent behavior across task delegation"), which stripped it from the signature, schema, and handler together. Re-introducing it would revert that decision, so these remain Refs pending a maintainer call on whether to reverse the refactor. The per-task (tasks[]) surface this PR adds is a different, nested surface that does not conflict with that removal, and it satisfies the underlying routing need (a model string per child) for all of these. (#6306 asks model-only across three levels; #23467/#32711 are explicitly about the removed top-level parameter.)
Refs #14974 — tracking issue for per-task endpoint overrides; per-task model/provider is delivered, but per-task base_url/api_key/profile are deliberately not exposed.
Refs #31155 — known limitation. config-only delegation.model (no base_url) where parent and child share the same provider: our guard intentionally keeps the pool for a provider-resolved endpoint (to preserve key rotation), so the child can still inherit the parent's pooled model. The root fix is endpoint/model-identity keying in _resolve_child_credential_pool, which this PR does not change.
Refs #32671 — Bug A: a silent _load_config persistent-load failure is now observable (warns instead of swallowing), but config-read semantics are otherwise unchanged — surfaced, not fully closed.
Refs #26482 — if the reported symptom is the explicit-delegation.base_url pool clobber, our pool-skip resolves it. Downgraded from Closes to Refs because competing PR #26486 (which asserts Fixes #26482) attributes this issue to provider relabeling — children relabeled custom, losing provider identity — fixed in hermes_cli/gateway.py, which this PR does not touch. Defer to the maintainer on which mechanism #26482 actually reports.

Related / competing PRs (overlap — not cleanly superseded; recommend maintainer consolidation):

• PR fix(delegation): accept per-call model override on delegate_task #23769 — adds a top-level model param + per-task. The top-level half reverts fb0f579b1 (the maintainer's deliberate removal of that param); it does not fix Bug A or Bug B. Its claim that delegation.model was already plumbed matches our Bug A finding.
• PR fix(delegate_task): accept caller-supplied model override #34472 — adds a top-level model param, passes it through the handler lambda, and overrides creds['model'] after resolution (a post-resolution patch, vs. our up-front single-resolver pre-pass), plus per-task model/provider. Also reverts fb0f579b1, and likewise fixes neither Bug A nor Bug B. Its premise ("schema advertises a model the function ignores") is stale — fb0f579b1 removed the schema property too, so no schema/impl mismatch exists today.
• PR fix(delegation): honor provider overrides for subagents #26486 — provider-override relabeling for subagents; overlaps Bug B / the provider path (and is the PR that asserts Fixes #26482 via the gateway relabel path this PR does not touch).
• Stale open PRs attempting the same fixes: fix(delegate): respect model selection for sub-agents (fixes #12440) #12503 (fixes [BUG] Sub-agent model configuration is ignored, causing persistent context window errors #12440), Fix delegate_task model/provider precedence and expose per-task overrides #17581, fix: honor per-task model override in delegate_task #17718, fix(delegation): respect delegation.model config for subagents (fixes #12440) #18649 (fixes [BUG] Sub-agent model configuration is ignored, causing persistent context window errors #12440).