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Portable Harness Engine Abstractions

Integration design: Deep Agents Adapter and Pluggable Agent Builder describes runtime reuse and the capability-driven editor. Consult the Engine README for supported APIs and rollout instructions.

Decision

In delivery Phase 1, Harness Engine is an independent control plane beside Dynamic Agents. The opt-in Deep Agents integration adds DeepAgentsAdapter; external sandbox execution and cross-harness delegation have separate qualification gates.

Provider-only installations remain independent of dynamic_agents. The optional Deep Agents integration imports and reuses the existing runtime.
It owns complete portable agent blueprints and immutable versions.
AgentCore and Claude Agent SDK implement one adapter contract.
The Next.js BFF stays horizontally stateless.
Harness Engine owns provider runs and durable session bindings.
Local SDK harnesses must move from in-process execution to one exclusive sandbox pod per binding before production certification (Phase 3). SandboxManager remains inside the engine container and calls a pluggable external backend.

Separate lifecycle concepts

Do not overload “session.” The engine uses five distinct resources:

Resource	Lifetime	Purpose
`AgentRecord`	Until deleted	Mutable pointer to the current immutable version
`AgentVersion`	Immutable	Exact portable blueprint and validated catalog fingerprint
`SessionBinding`	Conversation epoch	Pins owner, conversation, agent version, harness profile, and native session
`RunRecord`	One turn	Disconnect-independent execution and status
`SandboxLease`	Active/idle binding	Exclusive isolated worker and workspace; target design

This prevents an agent edit from changing an active thread. A new conversation uses the current version; an existing binding continues with the version it captured when it was created.

Portable agent blueprint

AgentBlueprint is user-owned intent. It contains no provider resource ARN, Kubernetes object, credential, command, image, environment map, or SDK client.

id: agent-example
name: Example agent
description: Neutral example
harness:
  id: claude_agent_sdk
  profile_id: safe
  options:
    max_turns: 20
prompt:
  system: Be helpful to {{audience}}.
  variables:
    audience: users
  context_sources: []
model:
  policy: harness_default
tools:
  bindings: []
  approval_policy: sensitive_only
thread:
  persistence: durable
  retention_profile: standard
memory:
  enabled: false
workspace:
  persistence: none
streaming:
  protocol: canonical
  replay: required
delegation:
  agents: []
  max_depth: 1
  max_parallel: 1
limits:
  max_run_seconds: 900
  max_tool_calls: 50

Portable policy belongs here even when a provider offers a native version. The adapter may implement a policy natively or the engine may emulate it, but the meaning presented to the user remains stable.

Harness descriptor and operator profiles

Every adapter returns a sanitized HarnessDescriptor:

stable harness, adapter, and contract versions;
provider_managed, sandbox_pod, or in_process execution placement;
availability and experimental/certified/blocked status;
operator-owned profile aliases;
a bounded JSON Schema and presentation hints for safe options;
a capability map.

Capability levels have precise meanings:

Level	Meaning
`native`	The harness supplies the portable behavior directly
`emulated`	Harness Engine supplies equivalent behavior outside the harness
`unsupported`	The harness fundamentally cannot provide the behavior
`unavailable`	The contract permits it, but this deployment/adapter has not connected it

Profiles separate operator policy from agent-author intent. An AgentCore profile either resolves to an operator-owned shared ARN or authorizes Harness Engine to create one AWS Harness per CAIPE agent using an approved execution role, model, and Region. A Claude profile resolves to an approved model, workspace root, and permission mode. The API and browser receive the alias and description only; provider identifiers and execution-role ARNs stay private.

Adapter contract

An adapter has four responsibilities:

class HarnessAdapter(Protocol):
    @property
    def descriptor(self) -> HarnessDescriptor: ...

    @property
    def session_manager(self) -> ProviderSessionManager: ...

    def evaluate(self, blueprint: AgentBlueprint) -> AdapterEvaluation: ...

    def stream(self, context: RunContext) -> AsyncIterator[CanonicalEventDraft]: ...

evaluate validates and normalizes only safe harness options.
session_manager handles only native provider session mechanics. It cannot choose the owner, agent version, conversation identity, or epoch.
stream receives the validated blueprint, pinned binding, compiled prompt, message, and trace context. It emits canonical events only.
Provider errors are translated to sanitized terminal events by the coordinator.

The AgentCore adapter sends the stable binding-derived runtimeSessionId. The Claude SDK adapter stores ResultMessage.session_id, supplies it with resume on the next turn, maps TextBlock to content.delta, and maps usage to usage.updated. The SDK profile, not the draft, controls model, working directory, and permission mode.

CAIPE and provider session managers

Session ownership is two-layered:

class ProviderSessionManager(Protocol):
    async def open(self, context: ProviderSessionContext) -> ProviderSessionState: ...
    async def observe(self, binding: SessionBinding, event: CanonicalEventDraft) -> str | None: ...
    async def close(self, binding: SessionBinding) -> None: ...

CAIPEAgentSessionManager owns:

keyed opaque binding identity;
authenticated owner + agent + conversation lookup;
immutable agent-version pinning for an active epoch;
durable binding creation and optimistic revision;
routing to the selected adapter's provider session manager;
persistence of validated native session updates;
close and monotonic epoch rotation.

Provider session managers own only provider mechanics:

Provider style	Open	Observe	Close
AgentCore	Derive a stable `runtimeSessionId` from the opaque binding	Ignore unrelated events	Hook for provider cleanup
Claude Agent SDK	Start without a native ID or resume the stored one	Capture validated `ResultMessage.session_id`	Hook for SDK session cleanup
Future sandbox SDK	Allocate/restore native checkpoint handle	Capture native checkpoint/session changes	Flush and release provider state

Clearing a session cancels active runs owned by the receiving coordinator, invokes provider close, marks the binding closed, and returns next_epoch. Another replica fences a stale run at its next canonical event or terminal commit because every event path rechecks the durable binding status. A future distributed cancellation signal can reduce that bounded delay. The next turn creates a new opaque binding on the current agent version. A clear by another subject finds no binding because owner identity is part of the lookup and keyed binding digest.

Platform broker boundaries

SDK code must not directly own authorization, secrets, shared memory, or Kubernetes lifecycle. These narrow interfaces make policy reusable:

Interface	Responsibility
`ThreadStateStore`	Harness-native checkpoint blobs, compare-and-set heads, restore
`MemoryBroker`	Authorized long-term retrieval/write with scope and provenance
`CAIPEAgentSessionManager`	Binding identity, ownership, epochs, persistence, version pinning
`ProviderSessionManager`	Harness-native session open/update/close mechanics
`ToolBroker`	Authorization, approval, credential injection, invocation, audit
`SandboxManager`	Exclusive claim/lease, fencing, health, replacement, release
`PromptCompiler`	Portable variables/context into a rendered system prompt
`CanonicalEventSink`	Ordered durable events and replay cursors
`DelegationBroker`	Subagent invocation through the same authorization/run path
`TelemetrySink`	Low-cardinality spans/metrics without protected content

Only PromptCompiler, session/event persistence, and the canonical run coordinator are connected in the current slice. Descriptor capabilities mark memory, portable tools, workspace leasing, and delegation as unavailable and server validation blocks agents that request them. This is deliberate: an experimental adapter must not claim a feature merely because its SDK has an API.

Thread persistence, memory, and sessions

Thread persistence

Short-term conversation state is binding-scoped and harness-native:

AgentCore uses a stable runtimeSessionId.
Claude uses the persisted SDK session ID through resume.
Future local adapters write native checkpoint blobs through ThreadStateStore; pod files are only a cache.
Clear creates a new binding epoch; old workers and state heads cannot attach to the new epoch.

Long-term memory

Long-term memory is not conversation history. MemoryPolicy controls:

user, agent, and organization read scopes;
user or agent write scope;
semantic, episodic-reference, and procedural kinds;
bounded startup or on-demand retrieval;
automatic/approval/disabled writes;
operator-owned retention profiles.

MemoryBroker must authorize before retrieval/ranking and store provenance, revision, content hash, source binding/run, and approval state. Memory cannot grant tools, credentials, egress, sandbox policy, or broader memory scope.

BFF versus Harness Engine state

The BFF remains stateless and replaceable. It authenticates/authorizes, exchanges the request for an internal service call, starts a run, and proxies replay/live events. Harness Engine stores sessions and owns provider tasks. A browser or BFF disconnect removes only a subscriber; the run continues.

The current design survives client/BFF disconnects and supports cross-replica event replay/session lookup with MongoDB. It does not yet take over an in-flight SDK/provider call after the Harness Engine process itself dies.

Canonical streaming and tracing

Adapters emit the canonical vocabulary:

run start/completion/failure/cancellation;
session update;
content and reasoning deltas;
tool start/completion and interrupts;
subagent start/event;
usage update.

The repository assigns a monotonic sequence. Clients reconnect with after=N or Last-Event-ID; event storage, not an HTTP socket, is the source of truth.

The BFF accepts W3C traceparent; Harness Engine validates its exact shape, stores it with the run, and passes it through the run context. The target TelemetrySink spans auth, binding, sandbox acquisition, worker, provider, state, memory, tool, subagent, event persistence, and stream delivery. Prompts, messages, memory bodies, tool payloads, credentials, raw subjects, and provider secrets must not be span attributes.

Sandbox pod target architecture

Provider-managed AgentCore remains remote. Local harnesses—Claude SDK, Strands, Deep Agents—should use sandbox_pod, not the API container.

Lease rules:

one binding owns one claim-exclusive pod at a time;
opaque binding labels contain no user/conversation text;
reuse is allowed only for the same binding and healthy generation;
each replacement increments a fencing generation;
stale worker events, state writes, and tool requests are rejected;
the worker receives scoped capabilities, never the user bearer token or raw provider/tool credentials;
workspace persistence is explicit (none, run, or thread) and durable thread state remains outside the pod;
warm-pool pods become exclusive when claimed and are destroyed or verifiably reset before any reassignment;
RuntimeClass, image digest, service account, network policy, volumes, limits, TTL, and pool are operator profile settings—not agent form fields.

This follows the Kubernetes Agent Sandbox resource model: templates define operator policy, claims allocate sandboxes, and warm pools reduce cold start. Deep Agents' backend abstraction similarly separates ephemeral state from a persistent store and supports composite routing by path.

Agent creation UI contract

The browser never contains a provider switch statement for every field. The flow is descriptor-driven:

GET /api/v1/harnesses returns descriptors and catalog_revision.
The editor shows Dynamic Agents unchanged plus available Harness Engine descriptors.
Selection exposes sanitized profiles.
Primitive safe options render from JSON Schema; complex resource pickers use first-party typed panels whose values remain schema-validated.
Common prompt/model/tool/thread/memory/workspace/stream/delegation values form one portable blueprint.
POST /api/v1/agent-drafts/validate normalizes the exact blueprint and returns field-addressable issues, effective capabilities, fingerprint, and catalog revision.
Save supplies the normalized blueprint and fingerprint. The server validates again against its current catalog and creates a new immutable version.

Server descriptors may supply data and JSON Schema only—never HTML, JavaScript, module paths, commands, environment maps, images, secrets, or raw Kubernetes/ cloud resource identifiers.

The current UI implements catalog selection, profile selection, primitive schema fields, server validation, and version save. Follow-up UI work must move validation before the legacy Dynamic Agents write to prevent a partial dual-save, add capability badges/diffs, memory controls, per-harness draft parking, and typed complex panels.

Persistence collections

Collection	Content
`harness_agents`	Mutable current-version pointer and optimistic revision
`harness_agent_versions`	Immutable normalized blueprint, fingerprint, catalog revision
`harness_sessions`	Owner/conversation identity, pinned version, profile, provider session
`harness_runs`	Turn status, binding, trace correlation, event cursor
`harness_events`	Ordered canonical event log with TTL

The initial Mongo save uses separate writes and is not transactionally atomic. Production hardening should use a transaction or a recoverable publish protocol so an immutable version and current pointer cannot diverge after a crash.

Implemented and deferred

Implemented:

independent service and collections;
portable blueprint, descriptor, capability, validation, and adapter contracts;
AgentCore and Claude Agent SDK adapters;
immutable versions and session pinning;
provider-native thread IDs and Claude resume;
disconnect-independent execution, replay, ownership checks, sanitized errors;
descriptor-driven UI options and BFF validation/save routes;
Python adapter/API/E2E tests and UI component/BFF tests.

Deferred and reported as unavailable:

sandbox pod worker protocol and SandboxManager implementation;
portable ToolBroker, MemoryBroker, and DelegationBroker implementations;
process-failure takeover of an in-flight call;
interrupts and reasoning/tool translation breadth;
full OpenTelemetry sink and provider trace links;
atomic coordination with the existing Dynamic Agents authoring write;
Strands/Deep Agents adapters and conformance certification.

Decision​

Separate lifecycle concepts​

Portable agent blueprint​

Harness descriptor and operator profiles​

Adapter contract​

CAIPE and provider session managers​

Platform broker boundaries​

Thread persistence, memory, and sessions​

Thread persistence​

Long-term memory​

BFF versus Harness Engine state​

Canonical streaming and tracing​

Sandbox pod target architecture​

Agent creation UI contract​

Persistence collections​

Implemented and deferred​

Research references​