feat: correlate provider requests with the root session

docs/acceptance/README.md

@@ -132,7 +132,7 @@ PR after verification. There is no cleanup or status-only PR.
 - [Provider-scoped model discovery](provider-model-discovery.md) — one composition-owned provider lifecycle for shared discovery, ordered publication, context-safe admission, and truthful retry recovery. Status: draft.
 
 - [Purpose-attributed auxiliary token usage](auxiliary-token-usage.md) — records every reachable session-associated auxiliary LLM call in the canonical ledger under a purpose-specific provider/model-attributed kind, without changing main-run accounting or budgets. Status: proposed.
-- [Root-conversation provider correlation](root-session-provider-correlation.md) — adds an outbound-only root-session header for grouping delegated and routed model requests while preserving the active session ID and its ingress-affinity contract. Status: proposed; awaiting Plan / Interface review.
+- [Root-conversation provider correlation](root-session-provider-correlation.md) — adds an outbound-only root-session header for grouping delegated and routed model requests while preserving the active session ID and its ingress-affinity contract. Status: landed in this implementation candidate; authoritative on merge.
 - [Acceptance-proof resolution efficiency](acceptance-proof-resolution-efficiency.md) — keeps plan-proof resolution fail closed while avoiding duplicate task execution within the existing Doc validation runner; Vitest batching is deferred. Status: proposed.
 - [Workspace enrollment preserves session authority](workspace-enrollment-authority.md) — preserves all still-authorized composed tools while atomically replacing the authenticated broker bundle; already-corrupted snapshots remain narrowed by explicit compatibility decision. Status: proposed.
 - [Contextual investigative guardrails](contextual-guardrails.md) — proposed Architectural contract for one contextual action/inbound reviewer, exact post-mutation permission re-evaluation, session-only exact repeat grants, held-result release-once, main/worker trajectory, session-provider-bound guardrail-slot routing, and separate durable-machine/transient-human status. Product choices are approved; implementation builds capacity calibration and quality-measurement deliverables, while separately authorized real-model release validation is required before any production-readiness claim. Status: proposed.

docs/acceptance/root-session-provider-correlation.md

@@ -4,12 +4,12 @@
 **Work classification:** Architectural — adds a durable outbound HTTP correlation contract and an exported engine context API across delegation, model routing, independently versioned providers, and the Jev adapter while preserving the existing affinity contract.
 **Decision record:** [ADR 0360](../adr/0360-root-session-provider-correlation.md)
 **Phase:** delegated model-request correlation
-**Status:** proposed, 2026-09-24. Exact interface contract ready for human Plan / Interface review; it remains proposed until this plan PR merges.
-**Delivery:** Split docs → implementation. On 2026-09-24 the directing human asked for the design and implementation branches to be pushed and stacked, and confirmed "yes, keep on going", so the implementation proceeds as a stacked PR on this proposed-docs commit before the plan merges. This does not mark the plan approved, authorize any merge, relax contract-drift stops, or remove either human merge gate.
-**Expected tasks:** 1 (implemented on the stacked implementation branch).
+**Status:** landed in this implementation candidate; authoritative on merge.
+**Delivery:** Split docs → implementation. Plan / Interface PR #1882 merged as approved baseline `8f049d8a4`; this implementation candidate carries the verified contract and its `landed` transition.
+**Expected tasks:** 1 (implemented in this candidate).
 **Issue:** None — directed operator request.
-**Plan PR:** [#1882](https://github.com/stacklok/mecatl/pull/1882).
-**Approved baseline:** absent until contract approval by merge.
+**Plan PR:** [#1882](https://github.com/stacklok/mecatl/pull/1882), merged.
+**Approved baseline:** `8f049d8a4`.
 
 Add an outbound-only `X-Mecatl-Root-Session-ID` alongside the existing active-session
 `X-Mecatl-Session-ID`. Provider calls made by a main run and its nested engines, plus both

docs/adr/0360-root-session-provider-correlation.md

@@ -1,6 +1,6 @@
 # ADR 0360 — Separate root-conversation correlation from active-session identity
 
-- Status: Proposed
+- Status: Accepted
 - Date: 2026-09-24
 - Scope: agent run context, provider calls made inside main/delegated run trees, direct teams, official LLM-provider composition, and the Jev delegated-model router.
 - Supersedes: ADR 0216 only where one outbound header was the complete correlation surface; retains its active-session header semantics.

engine/CHANGELOG.md

@@ -13,6 +13,11 @@ The covered surface is the eight core packages (`session`, `governance`, `learni
 
 ### Added
 
+- **Root-session run correlation context** — adds `port.WithRootSessionID` and
+  `port.RootSessionIDFromContext` so trusted engine composition can preserve one
+  causal session identity across nested runs without changing active session
+  identity. Added (minor).
+
 - **Delegated-model routing decision evidence** — adds `agent.ModelRouteResult`,
   `agent.SubagentModelRouter`, and `session.RoutingDecision`, with optional decision
   snapshots on Subagent, Parallel, and Team-member start payloads. Added (minor).

engine/agent/childask_adjudicator_test.go

@@ -11,6 +11,7 @@ import (
 	"github.com/stacklok/mecatl/engine/adapter/permpolicy"
 	"github.com/stacklok/mecatl/engine/agent"
 	"github.com/stacklok/mecatl/engine/governance"
+	"github.com/stacklok/mecatl/engine/port"
 	"github.com/stacklok/mecatl/engine/session"
 	"github.com/stacklok/mecatl/engine/team"
 	"github.com/stacklok/mecatl/engine/tool"
@@ -31,14 +32,17 @@ type scriptedAdjudicator struct {
 	script   []adjOutcome
 	calls    int
 	isolated []bool
+	roots    []session.SessionID
 }
 
-func (s *scriptedAdjudicator) Review(_ context.Context, req agent.ChildAskReviewRequest) (agent.ChildAskReview, error) {
+func (s *scriptedAdjudicator) Review(ctx context.Context, req agent.ChildAskReviewRequest) (agent.ChildAskReview, error) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 	i := s.calls
 	s.calls++
 	s.isolated = append(s.isolated, req.Isolated)
+	root, _ := port.RootSessionIDFromContext(ctx)
+	s.roots = append(s.roots, root)
 	if i < len(s.script) {
 		return s.script[i].review, s.script[i].err
 	}
@@ -51,6 +55,12 @@ func (s *scriptedAdjudicator) count() int {
 	return s.calls
 }
 
+func (s *scriptedAdjudicator) capturedRoots() []session.SessionID {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return append([]session.SessionID(nil), s.roots...)
+}
+
 func allow(reason string) adjOutcome {
 	return adjOutcome{review: agent.ChildAskReview{Allowed: true, Reason: reason}}
 }

engine/agent/dispatch.go

@@ -1945,10 +1945,9 @@ func (e *Engine) parentCaps(r *Run, sess *session.Session, turnIdx int) parentCa
 				return askReviewOutcome{}
 			default:
 			}
-			// A fresh background context + timeout: resolveChildAsk runs on the drain
-			// path with no ctx of its own, and the parked child must not hang on a
-			// wedged reviewer. A timed-out review is a failure → fall-through deny.
-			ctx, cancel := context.WithTimeout(context.Background(), askReviewTimeout)
+			// Detach cancellation while preserving only the causal root; the review
+			// remains bounded by its own timeout and the hard-abort gate above.
+			ctx, cancel := context.WithTimeout(detachedRunContext(r.ctx), askReviewTimeout)
 			defer cancel()
 			review, err := reviewer.Review(ctx, ChildAskReviewRequest{Ask: ask, Isolated: isolated})
 			switch {

engine/agent/loop.go

@@ -1561,8 +1561,27 @@ func (e *Engine) startRun(ctx context.Context, sess *session.Session, req RunReq
 	return run
 }
 
+// detachedRunContext returns a fresh context that carries only parent's causal
+// root session. Use it instead of context.Background() when nested engine work
+// must outlive parent cancellation but still belongs to parent's run tree; the
+// caller bounds it with its own deadline.
+func detachedRunContext(parent context.Context) context.Context {
+	ctx := context.Background()
+	if rootID, ok := port.RootSessionIDFromContext(parent); ok {
+		ctx = port.WithRootSessionID(ctx, rootID)
+	}
+	return ctx
+}
+
+// prepareRun seeds the causal root from sess only when ctx carries none, so every
+// nested Engine.Run whose ctx derives from its parent run (or from
+// detachedRunContext) inherits the tree's root without extra wiring.
 func (e *Engine) prepareRun(ctx context.Context, sess *session.Session, req RunRequest, budgetBaseline session.Usage, body func(context.Context, *Run)) *PreparedRun {
 	ctx, cancel := context.WithCancel(ctx)
+	rootSessionID, hasRootSessionID := port.RootSessionIDFromContext(ctx)
+	if !hasRootSessionID || rootSessionID == "" {
+		ctx = port.WithRootSessionID(ctx, sess.ID)
+	}
 	serial := runSerial.Add(1)
 	ctx = port.WithRunAttemptContext(ctx, sess.ID, serial)
 	ctx = withSessionOrigin(ctx, sess.ID)