diff --git a/src/client/subagent-detect.ts b/src/client/subagent-detect.ts
index f45b3ad..a8668ef 100644
--- a/src/client/subagent-detect.ts
+++ b/src/client/subagent-detect.ts
@@ -9,23 +9,20 @@
  * - {@link countSubagentDescendants}: uninterrupted subagent-origin lineage
  *   totals (mirror of the official `indexSubagentDescendants` over the
  *   plugin's own summary rows).
+ *
+ * The lineage walks themselves ({@link isSideThreadSummary}, {@link
+ * rootAncestor}, {@link countSubagentDescendants}) live in
+ * ./subagent-lineage.ts — the single shared walk implementation — and are
+ * re-exported below for their established import sites.
  */
 import type {
   SidebarSessionList,
-  SidebarSessionSummary,
   SidebarSubagentCatalog,
 } from '../context-types.ts'
-import { SIDE_LABEL_PREFIX } from '../sidechat-core.ts'
+import { countSubagentDescendants, isSideThreadSummary, rootAncestor } from './subagent-lineage.ts'
 
-/**
- * Side Chat threads ride the subagent origin (main-list hiding + the RPC
- * ownership fence) but they are NOT subagent topology: they carry the
- * durable 'Side: ' label and live as sidebar tabs. Excluding them here
- * keeps the auto-open trigger and the Subagent page counts clean.
- */
-export function isSideThreadSummary(summary: SidebarSessionSummary): boolean {
-  return summary.origin === 'subagent' && summary.displayTitle.startsWith(SIDE_LABEL_PREFIX)
-}
+export { countSubagentDescendants, isSideThreadSummary, rootAncestor }
+export type { SubagentDescendantTotals } from './subagent-lineage.ts'
 
 /** Count the direct subagent children of one session (durable `origin` rows). */
 export function directSubagentCount(
@@ -40,28 +37,6 @@ export function directSubagentCount(
   return count
 }
 
-/**
- * The main agent of the current session's tree: walk the durable parent
- * chain upward until the first non-subagent session. The Subagent page shows
- * THIS root's full topology regardless of how deep the current selection is
- * (a session whose row is still hydrating, or a broken chain, degrades to
- * the session itself).
- */
-export function rootAncestor(
-  byId: SidebarSessionList['byId'],
-  sessionId: string | undefined,
-): string | undefined {
-  if (sessionId === undefined) return undefined
-  const seen = new Set<string>()
-  let current: SidebarSessionSummary | undefined = byId[sessionId]
-  while (current !== undefined && current.origin === 'subagent'
-    && current.parentId !== undefined && !seen.has(current.id)) {
-    seen.add(current.id)
-    current = byId[current.parentId]
-  }
-  return current?.id ?? sessionId
-}
-
 /**
  * Collect every catalog branch (an entry with `hasChildren`) reachable from
  * the root — the set of catalogs the always-expanded topology consumes.
@@ -101,37 +76,3 @@ export function detectNewDirectSubagent(
   return directSubagentCount(prev.byId, sessionId) === 0
     && directSubagentCount(next.byId, sessionId) > 0
 }
-
-/** Descendant totals of one session through an uninterrupted subagent-origin chain. */
-export interface SubagentDescendantTotals {
-  count: number
-  runningCount: number
-}
-
-/**
- * Index every subagent descendant under each ancestor it reaches through an
- * uninterrupted subagent-origin chain (same semantics as the official
- * `indexSubagentDescendants`; cycles fail soft).
- */
-export function countSubagentDescendants(
-  byId: SidebarSessionList['byId'],
-  sessionId: string,
-): SubagentDescendantTotals {
-  const totals: SubagentDescendantTotals = { count: 0, runningCount: 0 }
-  for (const descendant of Object.values(byId)) {
-    if (descendant.origin !== 'subagent' || isSideThreadSummary(descendant)) continue
-    const seen = new Set<string>()
-    let current: SidebarSessionSummary | undefined = descendant
-    while (current?.origin === 'subagent' && current.parentId !== undefined
-      && !seen.has(current.id)) {
-      seen.add(current.id)
-      if (current.parentId === sessionId) {
-        totals.count += 1
-        if (descendant.running === true) totals.runningCount += 1
-        break
-      }
-      current = byId[current.parentId]
-    }
-  }
-  return totals
-}
diff --git a/src/client/subagent-jobs.ts b/src/client/subagent-jobs.ts
index 4962392..76af4a6 100644
--- a/src/client/subagent-jobs.ts
+++ b/src/client/subagent-jobs.ts
@@ -8,11 +8,11 @@
  */
 import type {
   SidebarSessionList,
-  SidebarSessionSummary,
   SidebarJobStatus,
   SidebarJobView,
 } from '../context-types.ts'
 import type { CopyKey } from './locales.ts'
+import { treeSessionIds } from './subagent-lineage.ts'
 
 /** One row of the jobs section: the job plus its owning session's title. */
 export interface TreeJob {
@@ -26,36 +26,10 @@ export function isJobLive(job: SidebarJobView): boolean {
   return job.status === 'running' || job.status === 'stopping'
 }
 
-/**
- * Every session id of the topology tree rooted at `rootId` (the root plus
- * each session whose uninterrupted subagent-origin chain reaches it — same
- * lineage semantics as {@link countSubagentDescendants}; cycles fail soft).
- * Sessions outside the tree (orphans, other trees) are excluded, so the
- * jobs section never shows foreign work.
- */
-export function treeSessionIds(
-  byId: SidebarSessionList['byId'],
-  rootId: string | undefined,
-): Set<string> {
-  const ids = new Set<string>()
-  if (rootId === undefined) return ids
-  for (const summary of Object.values(byId)) {
-    const seen = new Set<string>()
-    let current: SidebarSessionSummary | undefined = summary
-    let reachesRoot = false
-    while (current !== undefined && !seen.has(current.id)) {
-      seen.add(current.id)
-      if (current.id === rootId) {
-        reachesRoot = true
-        break
-      }
-      if (current.origin !== 'subagent' || current.parentId === undefined) break
-      current = byId[current.parentId]
-    }
-    if (reachesRoot) ids.add(summary.id)
-  }
-  return ids
-}
+// The lineage walk itself lives in ./subagent-lineage.ts (the single shared
+// subagent-origin chain implementation); re-exported for the established
+// import sites of this module.
+export { treeSessionIds }
 
 /**
  * Whether a NEW background job appeared for one session between two
diff --git a/src/client/subagent-lineage.ts b/src/client/subagent-lineage.ts
new file mode 100644
index 0000000..d4cce66
--- /dev/null
+++ b/src/client/subagent-lineage.ts
@@ -0,0 +1,131 @@
+/**
+ * The shared subagent-lineage walks over the sessions list feed (structural
+ * mirror world — no runtime imports). Every public walk here answers a
+ * topology question by following the DURABLE subagent-origin chain upward
+ * (child → parent → …); one private generator ({@link subagentOriginChain})
+ * is the single implementation of that walk, seen-set guarded so cycles fail
+ * soft (the walk just stops; nothing throws).
+ *
+ * subagent-detect.ts and subagent-jobs.ts re-export these for their
+ * established import sites — the auto-activation triggers and the Subagent
+ * page both consume them through the original module paths.
+ */
+import type { SidebarSessionList, SidebarSessionSummary } from '../context-types.ts'
+import { SIDE_LABEL_PREFIX } from '../sidechat-core.ts'
+
+/**
+ * Side Chat threads ride the subagent origin (main-list hiding + the RPC
+ * ownership fence) but they are NOT subagent topology: they carry the
+ * durable 'Side: ' label and live as sidebar tabs. Excluding them here
+ * keeps the auto-open trigger and the Subagent page counts clean.
+ */
+export function isSideThreadSummary(summary: SidebarSessionSummary): boolean {
+  return summary.origin === 'subagent' && summary.displayTitle.startsWith(SIDE_LABEL_PREFIX)
+}
+
+/**
+ * Yield `start`'s uninterrupted subagent-origin chain upward: each summary
+ * while it is a subagent with a known parent, then its parent row, and so
+ * on. A revisited id ends the walk (cycles fail soft); the row that BREAKS
+ * the chain — the first non-subagent ancestor, or a parent id with no row —
+ * is deliberately NOT yielded: callers observe it through where the walk
+ * stopped (the row after the last yielded one is `byId[last.parentId]`).
+ */
+function* subagentOriginChain(
+  byId: SidebarSessionList['byId'],
+  start: SidebarSessionSummary,
+): Generator<SidebarSessionSummary> {
+  const seen = new Set<string>()
+  let current: SidebarSessionSummary | undefined = start
+  while (current?.origin === 'subagent' && current.parentId !== undefined && !seen.has(current.id)) {
+    seen.add(current.id)
+    yield current
+    current = byId[current.parentId]
+  }
+}
+
+/**
+ * The main agent of the current session's tree: walk the durable parent
+ * chain upward until the first non-subagent session. The Subagent page shows
+ * THIS root's full topology regardless of how deep the current selection is
+ * (a session whose row is still hydrating, or a broken chain, degrades to
+ * the session itself).
+ */
+export function rootAncestor(
+  byId: SidebarSessionList['byId'],
+  sessionId: string | undefined,
+): string | undefined {
+  if (sessionId === undefined) return undefined
+  const start = byId[sessionId]
+  // A hydrating (or unknown) row degrades to the session itself.
+  if (start === undefined) return sessionId
+  let last: SidebarSessionSummary | undefined
+  for (const node of subagentOriginChain(byId, start)) last = node
+  if (last === undefined) return start.id
+  // The walk stopped one row past the last chain node: that parent row is
+  // the root (still missing from the feed → the session itself). Every
+  // yielded node carries a parent id (the generator's walk condition).
+  return byId[last.parentId!]?.id ?? sessionId
+}
+
+/** Descendant totals of one session through an uninterrupted subagent-origin chain. */
+export interface SubagentDescendantTotals {
+  count: number
+  runningCount: number
+}
+
+/**
+ * Index every subagent descendant under each ancestor it reaches through an
+ * uninterrupted subagent-origin chain (same semantics as the official
+ * `indexSubagentDescendants`; cycles fail soft).
+ */
+export function countSubagentDescendants(
+  byId: SidebarSessionList['byId'],
+  sessionId: string,
+): SubagentDescendantTotals {
+  const totals: SubagentDescendantTotals = { count: 0, runningCount: 0 }
+  for (const descendant of Object.values(byId)) {
+    if (descendant.origin !== 'subagent' || isSideThreadSummary(descendant)) continue
+    for (const node of subagentOriginChain(byId, descendant)) {
+      if (node.parentId === sessionId) {
+        totals.count += 1
+        if (descendant.running === true) totals.runningCount += 1
+        break
+      }
+    }
+  }
+  return totals
+}
+
+/**
+ * Every session id of the topology tree rooted at `rootId` (the root plus
+ * each session whose uninterrupted subagent-origin chain reaches it — same
+ * lineage semantics as {@link countSubagentDescendants}; cycles fail soft).
+ * Sessions outside the tree (orphans, other trees) are excluded, so the
+ * jobs section never shows foreign work.
+ */
+export function treeSessionIds(
+  byId: SidebarSessionList['byId'],
+  rootId: string | undefined,
+): Set<string> {
+  const ids = new Set<string>()
+  // A root with no row in the feed cannot be reached by walking: only real
+  // rows anchor tree membership (a parent pointer naming a hydrating root
+  // does not pull the orphaned branch into the tree).
+  if (rootId === undefined || byId[rootId] === undefined) return ids
+  for (const summary of Object.values(byId)) {
+    // The root row itself belongs to its own tree even though it is not a
+    // subagent (it is usually the main agent).
+    if (summary.id === rootId) {
+      ids.add(summary.id)
+      continue
+    }
+    for (const node of subagentOriginChain(byId, summary)) {
+      if (node.parentId === rootId) {
+        ids.add(summary.id)
+        break
+      }
+    }
+  }
+  return ids
+}