diff --git a/packages/session-query/session-query/src/index.ts b/packages/session-query/session-query/src/index.ts
index c468f31696..e5659c554a 100644
--- a/packages/session-query/session-query/src/index.ts
+++ b/packages/session-query/session-query/src/index.ts
@@ -78,6 +78,7 @@ export class SessionQueryService extends Service {
    * Trace known ancestry and descendants from one corpus observation.
    * @param sessionId - logical session id to trace.
    * @returns a complete lineage or an explicit unresolved parent boundary.
+   * @throws when corpus resolution fails, the target is absent, or its known ancestry cycles.
    */
   async traceSession(sessionId: SessionId): Promise<SessionLineageTrace> {
     const records = await this._corpus.listSessions()
@@ -88,6 +89,7 @@ export class SessionQueryService extends Service {
    * Trace one event's direct positional and provenance relationships.
    * @param request - target session id and event seq.
    * @returns direct links plus the target's positional replacement chain.
+   * @throws when source resolution fails, the target is absent, or surface/provenance validation fails.
    */
   async traceEvent(request: SessionEventTraceRequest): Promise<SessionEventTrace> {
     const loaded = await this._corpus.load(request.sessionId)
diff --git a/packages/session-query/session-query/src/tracing.ts b/packages/session-query/session-query/src/tracing.ts
index d0c7f200cb..c7cfd89b9a 100644
--- a/packages/session-query/session-query/src/tracing.ts
+++ b/packages/session-query/session-query/src/tracing.ts
@@ -105,7 +105,12 @@ export function traceLineage(
   let unresolvedParentId: SessionId | undefined
   let parentId = target.header.parentSession
   while (parentId !== undefined) {
-    if (ancestrySeen.has(parentId)) lineageCycle(parentId)
+    if (ancestrySeen.has(parentId)) {
+      throw new SessionQueryError(
+        `session lineage contains a cycle at "${parentId}"`,
+        'SESSION_QUERY_INVALID_LINEAGE',
+      )
+    }
     ancestrySeen.add(parentId)
     const parent = byId.get(parentId)
     if (parent === undefined) {
@@ -146,7 +151,17 @@ function analyzeEventLog(
   sessionId: SessionId,
   events: readonly SessionEvent[],
 ): EventLogAnalysis {
-  const folded = safeFold(events)
+  let folded: ReturnType<typeof foldSurface>
+  try {
+    folded = foldSurface(events)
+  } catch (error: unknown) {
+    throw new SessionQueryError(
+      /* v8 ignore next -- foldSurface throws Error instances */
+      `invalid session surface: ${error instanceof Error ? error.message : 'unknown error'}`,
+      'SESSION_QUERY_INVALID_SURFACE',
+      { cause: error },
+    )
+  }
   const current = new Set(folded.nodes.map(node => node.seq))
   const shadowed = new Set<number>()
   const replacedBy = new Map<number, number>()
@@ -182,15 +197,24 @@ function validateProvenance(
     const sources = rawEventSources(event)
     if (sources === undefined) continue
     if (!isSurfaceEligibleType(event.type)) {
-      invalidProvenance(`non-surface event at seq ${event.seq} carries sourceEventSeqs`)
+      throw new SessionQueryError(
+        `invalid session provenance: non-surface event at seq ${event.seq} carries sourceEventSeqs`,
+        'SESSION_QUERY_INVALID_PROVENANCE',
+      )
     }
     if (!Array.isArray(sources) || sources.length === 0) {
-      invalidProvenance(`event at seq ${event.seq} has an empty or invalid sourceEventSeqs`)
+      throw new SessionQueryError(
+        `invalid session provenance: event at seq ${event.seq} has an empty or invalid sourceEventSeqs`,
+        'SESSION_QUERY_INVALID_PROVENANCE',
+      )
     }
     const unique = new Set<unknown>()
     for (const source of sources as unknown[]) {
       if (unique.has(source)) {
-        invalidProvenance(`event at seq ${event.seq} repeats source seq ${String(source)}`)
+        throw new SessionQueryError(
+          `invalid session provenance: event at seq ${event.seq} repeats source seq ${String(source)}`,
+          'SESSION_QUERY_INVALID_PROVENANCE',
+        )
       }
       unique.add(source)
       if (
@@ -200,7 +224,10 @@ function validateProvenance(
         || source >= event.seq
         || events[source]?.seq !== source
       ) {
-        invalidProvenance(`event at seq ${event.seq} references unknown or non-earlier source seq ${String(source)}`)
+        throw new SessionQueryError(
+          `invalid session provenance: event at seq ${event.seq} references unknown or non-earlier source seq ${String(source)}`,
+          'SESSION_QUERY_INVALID_PROVENANCE',
+        )
       }
     }
   }
@@ -212,12 +239,18 @@ function validateProvenance(
     const replacement = events[replacementSeq]!
     const sources = rawEventSources(replacement)
     if (!Array.isArray(sources)) {
-      invalidProvenance(`replacement at seq ${replacementSeq} omits its shadowed surface sources`)
+      throw new SessionQueryError(
+        `invalid session provenance: replacement at seq ${replacementSeq} omits its shadowed surface sources`,
+        'SESSION_QUERY_INVALID_PROVENANCE',
+      )
     }
     const sourceSet = new Set(sources as unknown[])
     for (const removedSeq of removedSeqs) {
       if (!sourceSet.has(removedSeq)) {
-        invalidProvenance(`replacement at seq ${replacementSeq} omits shadowed surface seq ${removedSeq}`)
+        throw new SessionQueryError(
+          `invalid session provenance: replacement at seq ${replacementSeq} omits shadowed surface seq ${removedSeq}`,
+          'SESSION_QUERY_INVALID_PROVENANCE',
+        )
       }
     }
   }
@@ -232,19 +265,6 @@ function eventSources(event: SessionEvent): number[] {
   return Array.isArray(sources) ? sources as number[] : []
 }
 
-function safeFold(events: readonly SessionEvent[]): ReturnType<typeof foldSurface> {
-  try {
-    return foldSurface(events)
-  } catch (error: unknown) {
-    throw new SessionQueryError(
-      /* v8 ignore next -- foldSurface throws Error instances */
-      `invalid session surface: ${error instanceof Error ? error.message : 'unknown error'}`,
-      'SESSION_QUERY_INVALID_SURFACE',
-      { cause: error },
-    )
-  }
-}
-
 function buildDescendants(
   childrenByParent: ReadonlyMap<SessionId, readonly SessionRecord[]>,
   sessionId: SessionId,
@@ -278,17 +298,3 @@ function compareSessionsAscending(a: SessionRecord, b: SessionRecord): number {
 function cloneRecord(record: SessionRecord): SessionRecord {
   return { ...record, header: structuredClone(record.header) }
 }
-
-function lineageCycle(id: SessionId): never {
-  throw new SessionQueryError(
-    `session lineage contains a cycle at "${id}"`,
-    'SESSION_QUERY_INVALID_LINEAGE',
-  )
-}
-
-function invalidProvenance(message: string): never {
-  throw new SessionQueryError(
-    `invalid session provenance: ${message}`,
-    'SESSION_QUERY_INVALID_PROVENANCE',
-  )
-}