refactor(session): extract Session.isDescendantOf from auto-reply walk (F13)

The lineage walk in SessionAutoReply (formerly run-events) was ad hoc.
ACP and TUI will eventually need the same operation. Move the walk to
Session.Interface.isDescendantOf with the lookup cache as a
caller-provided argument so AutoReply retains dedup across events.

- Add Session.isDescendantOf(sid, root, opts?: { maxDepth?, cache? }).
  Default maxDepth = 64 (generic). Cache, when provided, is mutated
  with all confirmed-descendant intermediate nodes, turning repeated
  calls against the same lineage into O(1) after the first walk.
  NotFoundError defects are caught and treated as a non-match.
- AutoReply now delegates via a 2-line shim that passes its own
  MAX_LINEAGE_DEPTH (32) and its descendants-Set as the cache.
  Drops the inline 23-line Effect.fn plus Option/NotFoundError imports.
- 7 new tests on Session.isDescendantOf: identity, direct child,
  grandchild, unrelated, non-existent, maxDepth boundary, cache
  accumulation. AutoReply's existing 14 auto-reply.test tests and the
  subagent-hang regression continue to pass unchanged.

Diamond review: codex-5.3 spec APPROVE, Opus quality APPROVE.

Refs: F13 in docs/superpowers/plans/2026-04-23-audit-remediation.md

Author: tesdal

packages/opencode/src/session/auto-reply/auto-reply.ts

@@ -1,9 +1,8 @@
-import { Cause, Effect, Fiber, Option } from "effect"
+import { Cause, Effect, Fiber } from "effect"
 import { Bus } from "@/bus"
 import { Permission } from "@/permission"
 import { Question } from "@/question"
 import { Session } from "@/session"
-import { NotFoundError } from "@/storage"
 import { SessionID } from "@/session/schema"
 import { Log } from "@/util"
 import type { Sink } from "./sink"
@@ -49,28 +48,8 @@ export const make = Effect.fn("SessionAutoReply.make")(function* (config: Config
 
   const descendants = new Set<SessionID>([config.rootSessionID])
 
-  const isDescendant = Effect.fn("SessionAutoReply.isDescendant")(function* (sid: SessionID) {
-    if (descendants.has(sid)) return true
-    let cur: SessionID | undefined = sid
-    const chain: SessionID[] = []
-    let depth = 0
-    while (cur !== undefined && !descendants.has(cur) && depth < MAX_LINEAGE_DEPTH) {
-      chain.push(cur)
-      depth++
-      const lookup: Option.Option<Session.Info> = yield* session.get(cur).pipe(
-        Effect.option,
-        Effect.catchDefect((defect) => {
-          if (!NotFoundError.isInstance(defect)) return Effect.die(defect)
-          return Effect.succeed(Option.none<Session.Info>())
-        }),
-      )
-      if (Option.isNone(lookup)) break
-      cur = lookup.value.parentID ?? undefined
-    }
-    if (cur === undefined || !descendants.has(cur)) return false
-    chain.forEach((item) => descendants.add(item))
-    return true
-  })
+  const isDescendant = (sid: SessionID) =>
+    session.isDescendantOf(sid, config.rootSessionID, { maxDepth: MAX_LINEAGE_DEPTH, cache: descendants })
 
   const bump = (kind: "question" | "permission", sid: SessionID) => {
     if (kind === "question") stats.autoRejectedQuestions++

packages/opencode/src/session/session.ts

@@ -375,6 +375,22 @@ export interface Interface {
     sessionID: SessionID,
     predicate: (msg: MessageV2.WithParts) => boolean,
   ) => Effect.Effect<Option.Option<MessageV2.WithParts>>
+  /**
+   * Returns true if `sid` is `root` or transitively descends from `root` via
+   * `parentID`. Walks the parent chain up to `opts.maxDepth` (default 64) and
+   * stops at any `NotFoundError` (returns false). When `opts.cache` is
+   * provided, it is used both as a positive-hit short-circuit and as an
+   * accumulator: every confirmed descendant in the walked chain is added to
+   * the set. Callers that issue many `isDescendantOf` calls against the same
+   * root (e.g. SessionAutoReply) should seed the cache with `new Set([root])`
+   * and reuse it across calls so the parent chain is traversed at most once
+   * per node.
+   */
+  readonly isDescendantOf: (
+    sid: SessionID,
+    root: SessionID,
+    opts?: { maxDepth?: number; cache?: Set<SessionID> },
+  ) => Effect.Effect<boolean>
 }
 
 export class Service extends Context.Service<Service, Interface>()("@opencode/Session") {}
@@ -671,6 +687,39 @@ export const layer: Layer.Layer<Service, never, Bus.Service | Storage.Service> =
       return Option.none<MessageV2.WithParts>()
     })
 
+    const isDescendantOf = Effect.fn("Session.isDescendantOf")(function* (
+      sid: SessionID,
+      root: SessionID,
+      opts?: { maxDepth?: number; cache?: Set<SessionID> },
+    ) {
+      const maxDepth = opts?.maxDepth ?? 64
+      const known = opts?.cache ?? new Set<SessionID>([root])
+      if (sid === root) return true
+      if (known.has(sid)) return true
+      // Walk parent chain. Track the chain so we can promote every visited
+      // node into the cache once we hit a known descendant — turns repeated
+      // calls against the same lineage into O(1) after the first walk.
+      const chain: SessionID[] = []
+      let cur: SessionID | undefined = sid
+      let depth = 0
+      while (cur !== undefined && !known.has(cur) && depth < maxDepth) {
+        chain.push(cur)
+        depth++
+        const lookup: Option.Option<Info> = yield* get(cur).pipe(
+          Effect.option,
+          Effect.catchDefect((defect) => {
+            if (!NotFoundError.isInstance(defect)) return Effect.die(defect)
+            return Effect.succeed(Option.none<Info>())
+          }),
+        )
+        if (Option.isNone(lookup)) break
+        cur = lookup.value.parentID ?? undefined
+      }
+      if (cur === undefined || !known.has(cur)) return false
+      chain.forEach((item) => known.add(item))
+      return true
+    })
+
     return Service.of({
       create,
       fork,
@@ -693,6 +742,7 @@ export const layer: Layer.Layer<Service, never, Bus.Service | Storage.Service> =
       getPart,
       updatePartDelta,
       findMessage,
+      isDescendantOf,
     })
   }),
 )

packages/opencode/test/session/session.test.ts

@@ -5,7 +5,7 @@ import { Bus } from "../../src/bus"
 import { Log } from "../../src/util"
 import { Instance } from "../../src/project/instance"
 import { MessageV2 } from "../../src/session/message-v2"
-import { MessageID, PartID, type SessionID } from "../../src/session/schema"
+import { MessageID, PartID, SessionID } from "../../src/session/schema"
 import { AppRuntime } from "../../src/effect/app-runtime"
 import { tmpdir } from "../fixture/fixture"
 
@@ -179,3 +179,121 @@ describe("Session", () => {
     expect(missing).toBe(true)
   })
 })
+
+function isDescendantOf(
+  sid: SessionID,
+  root: SessionID,
+  opts?: { maxDepth?: number; cache?: Set<SessionID> },
+) {
+  return AppRuntime.runPromise(SessionNs.Service.use((svc) => svc.isDescendantOf(sid, root, opts)))
+}
+
+describe("Session.isDescendantOf", () => {
+  test("identity: a session is its own descendant", async () => {
+    await using tmp = await tmpdir({ git: true })
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const root = await create({ title: "root" })
+        expect(await isDescendantOf(root.id, root.id)).toBe(true)
+        await remove(root.id)
+      },
+    })
+  })
+
+  test("direct child is a descendant", async () => {
+    await using tmp = await tmpdir({ git: true })
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const root = await create({ title: "root" })
+        const child = await create({ title: "child", parentID: root.id })
+        expect(await isDescendantOf(child.id, root.id)).toBe(true)
+        await remove(root.id)
+      },
+    })
+  })
+
+  test("grandchild is a descendant", async () => {
+    await using tmp = await tmpdir({ git: true })
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const root = await create({ title: "root" })
+        const child = await create({ title: "child", parentID: root.id })
+        const grandchild = await create({ title: "grandchild", parentID: child.id })
+        expect(await isDescendantOf(grandchild.id, root.id)).toBe(true)
+        await remove(root.id)
+      },
+    })
+  })
+
+  test("unrelated session is not a descendant", async () => {
+    await using tmp = await tmpdir({ git: true })
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const rootA = await create({ title: "rootA" })
+        const rootB = await create({ title: "rootB" })
+        expect(await isDescendantOf(rootB.id, rootA.id)).toBe(false)
+        await remove(rootA.id)
+        await remove(rootB.id)
+      },
+    })
+  })
+
+  test("non-existent session id is not a descendant", async () => {
+    await using tmp = await tmpdir({ git: true })
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const root = await create({ title: "root" })
+        const ghost = SessionID.make("ses_ghost_does_not_exist_00000")
+        expect(await isDescendantOf(ghost, root.id)).toBe(false)
+        await remove(root.id)
+      },
+    })
+  })
+
+  test("respects maxDepth: returns false when chain is deeper than the limit", async () => {
+    await using tmp = await tmpdir({ git: true })
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const root = await create({ title: "root" })
+        // Build a chain root → c1 → c2 → c3.
+        const c1 = await create({ title: "c1", parentID: root.id })
+        const c2 = await create({ title: "c2", parentID: c1.id })
+        const c3 = await create({ title: "c3", parentID: c2.id })
+        // maxDepth: 1 means we walk at most one parent edge from c3, which
+        // reaches c2 (not yet known), so we cannot confirm root and must
+        // return false. With the default depth (64), c3 is reachable.
+        expect(await isDescendantOf(c3.id, root.id, { maxDepth: 1 })).toBe(false)
+        expect(await isDescendantOf(c3.id, root.id)).toBe(true)
+        await remove(root.id)
+      },
+    })
+  })
+
+  test("cache accumulates confirmed descendants across calls", async () => {
+    await using tmp = await tmpdir({ git: true })
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const root = await create({ title: "root" })
+        const c1 = await create({ title: "c1", parentID: root.id })
+        const c2 = await create({ title: "c2", parentID: c1.id })
+        const cache = new Set<SessionID>([root.id])
+        // First call walks the full chain c2 → c1 → root and promotes both
+        // intermediate nodes into the cache.
+        expect(await isDescendantOf(c2.id, root.id, { cache })).toBe(true)
+        expect(cache.has(c1.id)).toBe(true)
+        expect(cache.has(c2.id)).toBe(true)
+        // A subsequent call for c1 short-circuits via the cache (depth: 0
+        // forbids any walk; cache hit alone must satisfy the call).
+        expect(await isDescendantOf(c1.id, root.id, { cache, maxDepth: 0 })).toBe(true)
+        await remove(root.id)
+      },
+    })
+  })
+})