feat(plugins): add AuthorityRoutingPlugin (ADVISE/EXECUTE/DEFER/STOP)

[Palo-Alto-AI-Research-Lab]

Closes #171.

What

A self-contained ADK BasePlugin that decides an authority posture for every tool call before it runs. Tool schemas govern access; this plugin governs authorization — the layer where "advice became action", "one bounded edit became a workflow", and "an approval was required but the agent proceeded" failures live.

Two stages produce the final posture:

1. Posture router — optional, caller-supplied callable (typically a model call) returning a validated ADVISE / EXECUTE / DEFER / STOP verdict with a scope statement.
2. Deterministic guard — an irreversibility keyword tripwire (scanning tool name + arg values) plus an approval-state check on session context, composed with the router under most-restrictive-wins: plain code can only make a posture stricter, never looser.

Design properties:

• Fail-closed — an unparseable or raised router verdict defaults to DEFER (an unreadable authorization is not an authorization).
• Guard-only mode (no router) is fully deterministic, needs no model, and adds no dependency.
• Complements AgentGovernancePlugin (policy-as-code allow/deny) and the HITL approval gateway rather than duplicating them — those answer "is this call permitted by policy?"; this answers "was the agent authorized to act at all, and within what scope?".

When the final posture is anything other than EXECUTE, before_tool_callback returns a block dict that short-circuits the tool (per the ADK plugin contract).

Files

• src/google/adk_community/plugins/authority_routing_plugin.py — the plugin (self-contained, no new deps)
• src/google/adk_community/plugins/__init__.py — exports AuthorityRoutingPlugin, Posture, PostureVerdict, AuthorityRequest, stricter
• tests/plugins/test_authority_routing_plugin.py — 32 deterministic unit tests
• contributing/samples/authority_routing/{main.py,README.md} — runnable sample

Testing plan

Unit tests — 32 deterministic tests (no credentials, no network) covering the posture ladder, guard-only mode, router mode, most-restrictive-wins composition, fail-closed behavior, verdict coercion, customization, and the block-response shape:

$ pytest tests/plugins/test_authority_routing_plugin.py -q
32 passed
$ pytest tests/ -q
106 passed, 14 skipped

End-to-end sample — python contributing/samples/authority_routing/main.py wires the plugin into a Runner and runs a live walkthrough of the decision layer (deterministic demo router; tool-call logs go to stderr):

CASE                                          TOOL             POSTURE  OUTCOME
------------------------------------------------------------------------------------------------
authorized read — router EXECUTE, guard clear web_search       EXECUTE  tool runs
advice requested — agent must NOT execute     update_ticket    ADVISE   blocked — user asked for a recommendation, not an action
irreversible call — guard overrides EXECUTE   delete_records   DEFER    blocked — deterministic guard: irreversibility tripwire: 'delete'
approval pending — guard floors to DEFER      send_email       DEFER    blocked — deterministic guard: approval-state marker in context: 'awaiting approval'
disallowed — STOP survives strictest-wins     read_vault       STOP     blocked — blocked by standing policy

Row 3 is the key case: the router authorized EXECUTE, but the tool call is irreversible, so the deterministic guard floored it to DEFER. Row 5 shows the opposite — the router's STOP survives even though the guard raised no concern.

Formatting — isort + pyink clean on the added files (./autoformat.sh).

Type of change

New community plugin. Small, focused, one concern; no changes to existing modules beyond the __init__ export.

Provenance / AI-authorship disclosure

The four-posture pattern, the most-restrictive-wins composition, and the tripwire come from a live multi-agent system and were first published as an Anthropic cookbook notebook. Design and validation are mine; the implementation was drafted with AI assistance, then reviewed and tested by hand. Happy to adjust the API surface, naming, or placement to match maintainer preferences.

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