[varunj-msft]

Description

When a backend scan needs a default adversarial or scorer chat target that the server didn't register as adversarial_chat or objective_scorer_chat, it now fails with that reason instead of quietly running against an OpenAIChatTarget built from the OPENAI_CHAT_* environment variables. Rich suggested this in the review of #3908 (point 3) as its own small fix on main: a misspelled name in a startup script should stop a scan, not swap in a different target.

• require_registered_default_targets() in pyrit.scenario.core is a scope, like override_default_adversarial_target. Inside it, get_default_adversarial_target() and get_default_scorer_target() raise when the registry has no entry. Outside it, the library keeps its fallback, so notebooks and scripts that rely on OPENAI_CHAT_* work as before.
• The scan service resolves every default inside that scope: the launch checks, preparing the scenario, and the run. Scoped and explicitly chosen targets are unaffected.
• Starting a scan that needs a missing default returns 400 with the reason. The backend README says so.

Breaking for the backend: a server that only sets OPENAI_CHAT_* and relied on that fallback now needs to register the targets, for example through the target initializer's ADVERSARIAL_CHAT_* and OBJECTIVE_SCORER_CHAT_* variables, or choose an adversarial target for the scan.

[Copilot]

🟡 Changes recommended

Valid scans with explicit or registered non-chat objective scorers are rejected because introspection and scorer-resolution paths unnecessarily require an unused objective_scorer_chat target.

2 open issues

• Honor selected scorer inputs in scan introspection · New
• Defer unused chat-target lookup for default scorers · New

What changed in this PR

This PR makes backend scans fail when a required default chat target is not registered, rather than silently using an environment-configured OpenAI target. It preserves fallback behavior for notebooks and scripts outside the strict scope.

Changes:

• Adds and exports a scoped requirement for registered default targets.
• Applies strict resolution to backend launch checks, preparation, and execution.
• Adds regression tests and documents the backend configuration requirement.

File	Description
tests/​unit/​scenario/​core/​test_scenario_target_defaults.py	Tests strict resolution, fallback restoration, and target overrides.
tests/​unit/​backend/​test_scenario_run_service.py	Tests launch failure when a required default is missing.
pyrit/​scenario/​core/​scenario_target_defaults.py	Adds scoped enforcement of registered defaults.
pyrit/​scenario/​core/​__init__.py	Exports the new context manager.
pyrit/​backend/​services/​scenario_run_service.py	Enforces registered defaults throughout scan execution.
pyrit/​backend/​README.md	Documents missing-default failures and registration requirements.

_{Review effort: Balanced}