diff --git a/packages/opencode/src/session/auto-reply/auto-reply.ts b/packages/opencode/src/session/auto-reply/auto-reply.ts
index 0f94dde5d617..a715385b83b3 100644
--- a/packages/opencode/src/session/auto-reply/auto-reply.ts
+++ b/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"
@@ -47,29 +46,18 @@ export const make = Effect.fn("SessionAutoReply.make")(function* (config: Config
     livelockWarned: false,
   }
 
-  const descendants = new Set<SessionID>([config.rootSessionID])
+  // Reused across calls so the parent walk is traversed at most once per
+  // descendant id. Session.isDescendantOf auto-seeds `rootSessionID` on every
+  // call, so an empty Set is the simplest seed. Wrapped in Effect.fn so
+  // AutoReply-specific lineage checks remain identifiable in traces alongside
+  // the inner Session.isDescendantOf span.
+  const descendants = new Set<SessionID>()
 
   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
+    return yield* session.isDescendantOf(sid, config.rootSessionID, {
+      maxDepth: MAX_LINEAGE_DEPTH,
+      cache: descendants,
+    })
   })
 
   const bump = (kind: "question" | "permission", sid: SessionID) => {
diff --git a/packages/opencode/src/session/session.ts b/packages/opencode/src/session/session.ts
index a7607798ba40..d3ac6aadc03d 100644
--- a/packages/opencode/src/session/session.ts
+++ b/packages/opencode/src/session/session.ts
@@ -375,6 +375,30 @@ 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. The cache is auto-seeded with `root` on every call, so callers
+   * can pass a fresh `new Set()` and reuse it across calls without seeding.
+   *
+   * Cache reuse is only safe within a single root. As a partial guard, if a
+   * non-empty cache is passed that does not already contain `root`, the call
+   * dies — that case can only arise from a cache leaked from another lineage.
+   * The guard does NOT catch caches that have been mixed (root present plus
+   * entries from a different root); detecting that would require tagging the
+   * cache. Each `SessionAutoReply` instance owns its own cache, which is the
+   * intended usage. If you have a use case that needs disjoint roots to
+   * share a cache, build a `Map<root, Set<SessionID>>` outside this helper
+   * and pass the per-root inner Set.
+   */
+  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 +695,54 @@ 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>()
+      // Defend against accidental cache sharing across different roots: if a
+      // caller passes a non-empty cache that lacks `root`, those entries were
+      // populated under a different lineage and `known.has(sid)` could
+      // short-circuit to true with the wrong answer. Fail loudly instead of
+      // returning a silently wrong result.
+      if (known.size > 0 && !known.has(root)) {
+        return yield* Effect.die(
+          new Error(
+            `Session.isDescendantOf: opts.cache appears to belong to a different root (size=${known.size}, missing root=${root}). Caches must not be shared across roots.`,
+          ),
+        )
+      }
+      // Always seed `root` into the working set so the parent walk has a
+      // termination anchor and the next cross-root reuse check still works.
+      known.add(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 +765,7 @@ export const layer: Layer.Layer<Service, never, Bus.Service | Storage.Service> =
       getPart,
       updatePartDelta,
       findMessage,
+      isDescendantOf,
     })
   }),
 )
diff --git a/packages/opencode/test/session/session.test.ts b/packages/opencode/test/session/session.test.ts
index f63ad9beed9f..8a4d76d5d4f7 100644
--- a/packages/opencode/test/session/session.test.ts
+++ b/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 +179,164 @@ 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)
+      },
+    })
+  })
+
+  test("auto-seeds root into cache so callers can pass an empty Set", 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 })
+        // Caller passes a fresh empty Set — root must still be reachable.
+        // Without the auto-seed in isDescendantOf this would walk past root
+        // (since `known.has(root)` would be false), bottom out at the
+        // not-found parent of root, and return false.
+        const cache = new Set<SessionID>()
+        expect(await isDescendantOf(child.id, root.id, { cache })).toBe(true)
+        expect(cache.has(root.id)).toBe(true)
+        await remove(root.id)
+      },
+    })
+  })
+
+  test("dies if cache is non-empty but missing root (cross-root reuse guard)", 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" })
+        const childA = await create({ title: "childA", parentID: rootA.id })
+        // Populate a cache under rootA, then incorrectly try to reuse it
+        // against rootB. The guard must reject this loudly.
+        const cache = new Set<SessionID>()
+        expect(await isDescendantOf(childA.id, rootA.id, { cache })).toBe(true)
+        // cache now contains {rootA, childA} but not rootB.
+        let died = false
+        await isDescendantOf(rootA.id, rootB.id, { cache }).catch(() => {
+          died = true
+        })
+        expect(died).toBe(true)
+        await remove(rootA.id)
+        await remove(rootB.id)
+      },
+    })
+  })
+})