fix(delegation): honor pinned delegation.provider — no silent parent-fallback substitution

[teknium1]

Summary

An explicitly pinned delegation.provider/delegation.model is now honored for the child's whole run — a mid-run failure on the pin fails loudly instead of silently rerouting the quiet-mode child onto the parent's fallback models.

Root cause: _build_child_agent inherited parent._fallback_chain unconditionally (delegate_tool.py:1469→1524), so try_activate_fallback could swap the pinned child's model/provider at runtime with no surfaced signal. The sibling override_provider guard already cleared parent OpenRouter routing filters for exactly this reason — the fallback chain never got the same treatment.

Changes

• tools/delegate_tool.py: parent_fallback = None when override_provider is set (pin honored or loud failure); pinned delegation.command missing from PATH now raises at credential preflight AND in _build_child_agent (was: logger.warning + silent transport fallback); delegate_task catches the build-time ValueError and returns tool_error.
• tests/tools/test_delegate.py: 3 regression tests (pin disables chain inheritance, missing pinned command raises in build, missing pinned command rejected at preflight).

Validation

	Before	After
Pinned provider, mid-run 401/429	child silently hops to parent fallback models	fails loudly, pin respected
Unpinned delegation	inherits parent fallback chain	unchanged
Pinned delegation.command not on PATH	warning + silent default transport	spawn refused with clear message
tests/tools/test_delegate.py	63 passed	66 passed

Fixes #80450. Tracker #79686 audit item ("no-silent-fallback model pinning in delegate_task").

Infographic

[spfcraze]

This was generated by AI during triage.

Summary:
The fallback-chain gate keys on override_provider alone, so a model-only delegation pin still inherits the parent's fallback chain and can be silently swapped mid-run — the case #80450's proposed fix explicitly names alongside override_provider.

Problems:

• tools/delegate_tool.py:3783 — the if not configured_provider: branch returns "provider": None while carrying the configured model, so delegate_task passes override_provider=None and the new parent_fallback = None if override_provider else ... gate keeps parent._fallback_chain for a model-only pin (the configuration.md:2282-documented "Setting just model without provider" combination).
• try_activate_fallback (agent/chat_completion_helpers.py:1730) then swaps the child's model/provider in-place on mid-run auth/429/5xx — the pinned model is silently replaced, the runtime substitution delegate_task: pinned delegation.provider/model silently replaced at runtime by inherited parent fallback chain #80450 describes ("the pin is NOT honored after spawn").

Solution:
Pass fallback_model=None when an explicit delegation.model pin is set, per #80450's own proposed fix — thread a "model explicitly configured" flag from _resolve_delegation_credentials into the parent_fallback decision so the gate covers the model-only case the way it covers provider pins.

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Checked against ee6b581 — the tip of fix/delegation-pin-no-parent-fallback when this was written — and 9d4ef04, main at the same moment.

[github-actions]

૮ >ﻌ< ა ci review

_{ran on 5516ebe — fix(delegation): honor pinned delegation.provider — no silen}

⚠️ Warnings

CI timings · View report · View job

Wall time 21m21s vs 8m (+166.9%). 14 job(s) slower, 9 faster, 1 unchanged.

• Python tests / Run tests slice 7/12: -55.0s
• Python tests / Run tests slice 4/12: -35.0s
• Python tests / Run tests slice 1/12: +31.0s
• Python tests / Run tests slice 3/12: -22.0s
• Python tests / Run tests slice 5/12: -19.0s

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OSV vulnerability scan · View job

5 known vulnerabilities found in pinned dependencies.

• CVE-2026-67213 in package-lock.json
• CVE-2026-67213 in website/package-lock.json
• CVE-2026-70608 in package-lock.json
• CVE-2026-56876 in package-lock.json
• CVE-2026-70606 in package-lock.json

How to fix:

Review the findings in the Security tab. Update the affected dependencies if a patched version is available.