kestrel-sovereign-sdk

Lightweight SDK providing base interfaces, protocols, and utilities for Kestrel Sovereign feature package development. Feature packages depend on this SDK instead of the full framework, keeping dependencies minimal and development fast.

Voice provider contracts

kestrel_sdk.voice defines independent TTS, STT, and realtime conversation provider contracts. Realtime providers declare capability metadata and mint a provider-neutral browser bootstrap (WebRTC or WebSocket); voice IDs are scoped to their provider. Tool-call batches pair every governed function result with one continuation, while legacy single-result methods remain available for older adapters.

Installation

uv pip install git+https://github.com/KestrelSovereignAI/kestrel-sovereign-sdk.git

With encryption helpers:

uv pip install "kestrel-sovereign-sdk[crypto] @ git+https://github.com/KestrelSovereignAI/kestrel-sovereign-sdk.git"

Dependencies

• pydantic>=2.0
• Optional: cryptography>=42.0 (via [crypto] extra)

Usage

from kestrel_sdk.features.base import Feature, Tool

class MyFeature(Feature):
    name = "my-feature"

    def get_tools(self):
        return [Tool(name="my-tool", description="Does something", handler=self.handle)]

Host features (host/fleet scope)

Feature is a subagent — each instance is bound to one agent (self.agent), mounts its router under that agent's prefix, and can be called as a tool with its own LLM context. HostFeature is the host/fleet-scoped sibling: it runs once per host, has no agent binding, mounts its router at the host root, and lives across host start/stop rather than agent enable/disable. It is what kestrel-sovereign discovers and mounts, and what fleet-observability host features implement.

from kestrel_sdk import HostFeature, HostContext, UIContributions

class FleetObservability(HostFeature):
    name = "fleet-observability"       # stable slug for discovery / mounting
    capability = "fleet.observe"       # optional capability gate

    def get_router(self):
        # Mounted at the HOST ROOT — no agent prefix, no get_agent dependency.
        from fastapi import APIRouter
        router = APIRouter()
        # ... host-scoped routes ...
        return router

    async def on_host_start(self, ctx: HostContext):
        # Host-scoped store handle built on the SDK's OWN storage layer.
        # The feature layer (entities + a fleet TenantContext) is layered on
        # top of this handle — the SDK stays dependency-free.
        target = self.resolve_host_engine_target(ctx.config["host_db_url"])
        self.db = ctx.db
        await ctx.backplane.subscribe("fleet.events", self._on_event)

    async def _on_event(self, event):
        # Handle a live fleet event (persist, fan out to console, etc.).
        ...

    async def on_host_stop(self, ctx: HostContext):
        await ctx.backplane.close()

    def get_ui_contributions(self):
        return UIContributions(
            static_dir="/pkg/fleet/static",
            modules=["fleet-panel.js"],
            capability=self.capability,
        )

HostContext is a minimal, runtime_checkable Protocol exposing the host db backend, a pub/sub backplane handle, and host config. UIContributions is a pure-data dataclass (static_dir / modules / css / capability) shared by agent and host features, so feature packages never need to import Sovereign or carry a fallback copy just to describe their console assets.

Application extensions

Application packages can customize agent prompt context through the SDK-owned AppExtension contract without importing the Sovereign runtime:

from kestrel_sdk import AppExtension

class CompanionExtension(AppExtension):
    def get_system_prompt_prefix(self) -> str:
        return "You are this application's companion persona."

Sovereign consumes this contract and keeps a compatibility re-export at its historic import path.

Database surface (entity feature packages)

Feature packages that need raw SQL or ORM access (e.g. kestrel-feature-entities) develop against kestrel_sdk.storage.database:

from kestrel_sdk.storage.database import (
    DatabaseBackend,           # async ABC: execute / fetch_* / transaction
    PrivacyMode,               # 6-mode enum
    EngineTarget,              # frozen dataclass: url, persistent, description
    resolve_engine_target,     # PrivacyMode + fallback_url -> EngineTarget
)

target = resolve_engine_target(PrivacyMode.NORMAL, "postgresql+asyncpg://...")
# target.url is the SQLAlchemy URL the feature should bind its ORM engine to.
# Volatile modes (EPHEMERAL/ISOLATED) ignore fallback_url and return
# in-memory or tempfile sqlite URLs with persistent=False.

To get the active DatabaseBackend instance at runtime, features access it through the agent context they already receive in their Feature.__init__:

class MyEntityFeature(Feature):
    def __init__(self, agent):
        super().__init__(agent)
        self.db: DatabaseBackend = agent.db   # provided by sovereign

The SDK declares the DatabaseBackend ABC; sovereign provides the concrete SQLiteBackend / PostgresBackend instance via agent.db. Feature packages should never instantiate their own backend — that creates a parallel connection pool and bypasses the agent's privacy enforcement.

Isolated-feature configuration transitions

An isolated service can opt into a host-only configuration lifecycle request when it needs to clean up resources using its old effective config before a replacement (for example, retiring a Telegram webhook with the old token). This is capability-negotiated: older services advertise no config_transition capability, and hosts must use their existing replacement flow without sending a transition RPC.

from kestrel_sdk.isolated_feature import (
    ConfigTransitionResult,
    IsolatedFeatureService,
)

class TelegramService(IsolatedFeatureService):
    def __init__(self):
        super().__init__(name="telegram", version="1.0.0")
        self.advertise_config_transition()

    async def on_config_transition(self, next_config):
        # self.host_config is still the old effective config here.
        await self.retire_webhook(token=self.host_config["token"])
        # The host must now stop and replace this process with next_config.
        return ConfigTransitionResult.restart_required()

The host checks client.supports_config_transition and calls await client.prepare_config_transition(next_config). A restart result means the hook completed ordered cleanup and the host must replace the process. A service that can atomically switch its own resources may opt in with advertise_config_transition(supports_live_apply=True) and return ConfigTransitionResult.applied(); only then does the SDK update service.host_config to the next config and the host may retain the process. Failures raise ConfigTransitionError and leave the old config active.

If a caller cancels or times out a transition after it has started, the SDK does not attempt to cancel the child hook: the request may already be on the wire. It re-raises the cancellation locally and fences the client for process replacement, so the host must stop and start the child with its known next config rather than issue more tools or transitions against an unknown outcome. SubprocessIsolatedFeatureClient retains that next config before releasing the cancelled call, so its following stop() / start() replacement initializes the new child with the intended effective config. A normal hook failure leaves the old config retained because the existing child remains the known-safe instance. Its process lifecycle, transition, and health calls are serialized to keep a probe from spanning that state change. stop() is the exception: it cancels an in-flight startup, health probe, or transition before taking its bounded shutdown/terminate path, so a wedged child cannot block replacement. Likewise, child/transport failures are reported as the generic typed ConfigTransitionError; no transport or configuration detail is reflected in the public lifecycle message.

The JSON-RPC method is lifecycle/config-transition, not a tools/* method, so it is never agent-callable. The client serializes public transition and shutdown calls: a transition already under way completes or fails before a queued shutdown starts, while a transition begun after shutdown fails locally. The service also processes transition and shutdown requests in stream order; health requests queued behind a transition see its final state. Config values are not logged or reflected in lifecycle error envelopes.

Private host ingress

An isolated service can register bounded JSON callbacks for its trusted host without creating agent-callable tools. Registration advertises the versioned host_ingress capability during initialize; the host can call only the registered names with client.call_host_ingress(...). Legacy or malformed capabilities, unknown names, malformed JSON, and oversized payloads fail closed before the host writes the request.

from kestrel_sdk.isolated_feature import IsolatedFeatureService

class ChannelService(IsolatedFeatureService):
    def __init__(self):
        super().__init__(name="channel", version="1.0.0")
        self.register_host_ingress("routing-update", self.apply_routing_update)

    async def apply_routing_update(self, payload):
        # payload is a strict JSON value, bounded to 64 KiB on both sides.
        self.routes = payload["routes"]
        return {"accepted": True}

# Host side, after initialize:
await client.call_host_ingress("routing-update", {"routes": ["primary"]})

Ingress uses the private host/ingress JSON-RPC method, never tools/*, so it remains absent from tools/list and agent tool inventories. Synchronous handlers run in a worker thread; native async handlers preserve normal request concurrency. Ingress is rejected once shutdown or restart-required lifecycle fencing begins, and public errors never reflect handler exceptions or payload values.

Isolated tool execution context

Hosts can attach trusted, versioned invocation metadata to an isolated tools/call without adding scheduler fields to user tool arguments. New SDK services advertise the tool_execution_context capability; a host that passes context fails closed against legacy services that do not advertise it.

from kestrel_sdk.isolated_feature import (
    ToolExecutionContext,
    ToolExecutionTrigger,
    get_tool_execution_context,
)

# Host side: retain this idempotency key across retry attempts.
context = ToolExecutionContext(
    invocation_id="occurrence-execution-123",
    idempotency_key="payment-effect-123",
    attempt=2,
    trigger=ToolExecutionTrigger(
        kind="scheduler",
        id="occurrence-123",
        source_id="daily-payment-job",
    ),
)
await client.call_tool("charge", {"amount": 100}, context=context)

# Isolated handler side: this is task-local and never merged into arguments.
async def charge(arguments):
    context = get_tool_execution_context()
    if context is not None:
        await effect_boundary.deduplicate(context.idempotency_key)

The context schema has no free-form metadata field: it accepts only bounded invocation, idempotency, retry, trigger identifiers, and timezone-aware trigger timestamps. The service clears it after every successful, failed, or cancelled invocation; asyncio.to_thread sync handlers receive the same active context.

Channels, Delivery, And Output Contracts

Channel and delivery packages use SDK contracts rather than importing from the full framework:

from kestrel_sdk.channels import ChannelAdapter, ChannelMessage
from kestrel_sdk.delivery import DeliveryProvider, DeliveryTask, DeliveryResult
from kestrel_sdk.outputs import OutputEvent, OutputKind

Feature packages register concrete channel adapters through:

[project.entry-points."kestrel_sovereign.channel_adapters"]
telegram = "kestrel_channel_telegram:TelegramAdapter"

Delivery providers register through:

[project.entry-points."kestrel_sovereign.delivery_providers"]
sendgrid = "kestrel_delivery_sendgrid:SendGridDeliveryProvider"

The SDK owns only the public contracts. The framework owns runtime privacy checks, signal dispatch, durable queues, and server composition.

Timeline Protocols

Timeline implementations (e.g., story archive, health timelines) use SDK protocols for cross-package interoperability. The SDK provides three core protocols: TimelineProtocol defines the minimal shape any timeline must conform to, TimelineSharingProtocol enables pluggable serialization formats (JSON, FHIR, IPFS), and VectorSearchBackend abstracts semantic search across different vector stores (pgvector, pure-Python cosine).

Implementing TimelineProtocol

Any class with the required attributes can serve as a timeline:

from datetime import datetime

class StoryTimeline:
    def __init__(self):
        self.id = "timeline-123"
        self.agent_did = "did:key:abc"
        self.subject_name = "Jane Doe"
        self.title = "Jane's Life Story"
        self.coherence_score = 0.95
        self.created_at = datetime.now()

Sharing and Serialization

Use JSONTimelineSerializer for default JSON output, or implement TimelineSharingProtocol for custom formats:

from kestrel_sdk.timeline import JSONTimelineSerializer, TimelineSharingProtocol
import json

# Default JSON sharing
serializer = JSONTimelineSerializer()
data = serializer.serialize(timeline, events, people)

# Custom FHIR serializer
class FHIRTimelineSerializer:
    content_type = "application/fhir+json"

    def serialize(self, timeline, events, people) -> bytes:
        # Convert to FHIR Bundle format
        bundle = {"resourceType": "Bundle", "entry": [...]}
        return json.dumps(bundle).encode("utf-8")

Vector Search

Implement VectorSearchBackend for semantic timeline search. The SDK ships two reference implementations in kestrel-feature-story-archive: PgVectorBackend (PostgreSQL with pgvector extension) and PurePythonBackend (SQLite with cosine similarity).

from kestrel_sdk.timeline import VectorSearchBackend

class MyVectorBackend:
    async def knn(self, query_embedding: bytes, k: int, filter: dict | None = None):
        # Return k-nearest neighbors by cosine similarity
        return [("event-5", 0.95), ("event-12", 0.89)]

    @property
    def supports_filters(self) -> bool:
        return True  # Can filter by timeline_id at query time

For a full timeline implementation with persistence, embeddings, and IPFS export, see kestrel-feature-story-archive.

Configuration

No environment variables required. This is a development-time dependency only.

Development

uv pip install kestrel-sovereign-sdk && uv pip install -e .
uv run pytest