diff --git a/.github/workflows/ci-tests.yml b/.github/workflows/ci-tests.yml
index b4fbb3d56..6d03b4c73 100644
--- a/.github/workflows/ci-tests.yml
+++ b/.github/workflows/ci-tests.yml
@@ -500,6 +500,17 @@ jobs:
         run: node --import tsx bench/callable-value-flow/measure.mjs --check
         working-directory: gitnexus
 
+      - name: C++ qualified-namespace resolution guards (#2788)
+        # Build-free: asserts resolveCppQualifiedNamespaceMember resolves an
+        # unchanged symbol set (fingerprint) and that per-call-site cost stays
+        # independent of corpus size. It rescanned every parsed file per
+        # qualified `ns::member()` call site until #2788 — 25 of 33 analyze
+        # minutes on a 1.5k-file C++ repo. #1990 had already fixed the exact
+        # same bug in the sibling ADL path and shipped without a scaling gate,
+        # which is how the bug class came back; this step is that gate.
+        run: node --import tsx bench/cpp-qualified-ns/measure.mjs --check
+        working-directory: gitnexus
+
       - name: Receiver-resolution drop guards
         # NOT build-free: this one runs the real pipeline, so it needs dist/
         # (the setup action above builds). ~2m15s.
diff --git a/gitnexus/bench/cpp-qualified-ns/baselines.json b/gitnexus/bench/cpp-qualified-ns/baselines.json
new file mode 100644
index 000000000..01d1cad17
--- /dev/null
+++ b/gitnexus/bench/cpp-qualified-ns/baselines.json
@@ -0,0 +1,6 @@
+{
+  "_comment": "Baselines for bench/cpp-qualified-ns/measure.mjs --check (#2788). `fingerprint` is a sha256 over every `receiver::member -> outcome` the synthetic corpus resolves at the LARGE scale (hit nodeId, `<ambiguous>` per #1564, or `<none>`); it is a CORRECTNESS gate, so drift means C++ qualified `ns::member()` lookup started resolving a different symbol set and must be explained, never re-baselined to make CI green. `scaling_budget` is a timing gate and carries deliberate headroom for shared CI runners.",
+  "fingerprint": "30122b086b90fe1d56edc5acfd39ab7f26c00350f0e672123aaf607c7702e1e2",
+  "scaling_budget": 1.8,
+  "_scaling_note": "(t_large/t_small)/(1600/400). ~1.0 is linear; measured 0.93-1.21 on the indexed implementation. The pre-#2788 per-call-site workspace rescan measured 3.45 on the same corpus shape (at a reduced 100/400 scale, since 1600 files x 32k call sites of quadratic work does not finish in a CI step) — so the budget sits between the two bands and cannot be met by reintroducing the scan. Resolution is timed alone; the fingerprint's outcome strings are built in a separate untimed pass because their allocation cost grows with the corpus and would otherwise show up as scaling."
+}
diff --git a/gitnexus/bench/cpp-qualified-ns/measure.mjs b/gitnexus/bench/cpp-qualified-ns/measure.mjs
new file mode 100644
index 000000000..36da04b7a
--- /dev/null
+++ b/gitnexus/bench/cpp-qualified-ns/measure.mjs
@@ -0,0 +1,258 @@
+/**
+ * Build-free scaling + identity bench for `resolveCppQualifiedNamespaceMember`,
+ * the C++ qualified `ns::member()` receiver resolver (issue #2788).
+ *
+ * Before #2788 this function re-scanned EVERY parsed file — rebuilding a
+ * per-file `scopesById` map each time — once per qualified call site, so the
+ * scope-resolution emit phase cost O(callsites × scopes). On a 1,473-file C++
+ * repo that was 25.3 min of a 33-min analyze, with 75% of total self-time in
+ * this one function. It is the same bug #1990 had already fixed in the sibling
+ * ADL path (`pickCppAdlCandidates` → `AdlCandidateIndex`) — the sibling shipped
+ * without a scaling gate, and the bug class came straight back here. Hence this
+ * bench: a per-call-site workspace scan must not be reintroduced silently.
+ *
+ * For a synthetic corpus at two scales it reports:
+ *   - `elapsed_ms` per scale (fastest of REPS, see `fastest`) for resolving
+ *     every call site once, INCLUDING the one-time index build — that build is
+ *     the work the per-site scan was traded for, so hiding it would let an
+ *     index that is itself quadratic pass;
+ *   - a scaling ratio `(t_large/t_small)/(LARGE/SMALL)`: ~1.0 linear,
+ *     ~4.x quadratic at this scale gap;
+ *   - a sha256 fingerprint over every `receiver::member → outcome` the corpus
+ *     resolves, as the correctness gate. A fingerprint change means qualified
+ *     lookup started resolving different symbols — a behaviour change, never a
+ *     performance one.
+ *
+ * Build-free: imports the `.ts` hotpath through tsx
+ * (`node --import tsx bench/cpp-qualified-ns/measure.mjs`).
+ *
+ * Without args: prints the JSON report.
+ * With `--check`: asserts the fingerprint == the committed baseline AND the
+ * scaling ratio is within budget; exits non-zero on drift/regression.
+ */
+import fs from 'node:fs';
+import path from 'node:path';
+import crypto from 'node:crypto';
+import { fileURLToPath } from 'node:url';
+
+import {
+  clearCppInlineNamespaces,
+  markCppInlineNamespaceRange,
+  populateCppInlineNamespaceScopes,
+  resolveCppQualifiedNamespaceMember,
+} from '../../src/core/ingestion/languages/cpp/inline-namespaces.ts';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+const BASELINE_PATH = path.resolve(__dirname, 'baselines.json');
+
+const SMALL = 400;
+const LARGE = 1600;
+const CALLS_PER_FILE = 20;
+const REPS = 7;
+const WARMUP = 3;
+
+const NO_SCOPES = {};
+
+/**
+ * Deterministic synthetic corpus — no randomness, so the fingerprint is stable.
+ *
+ * Per file, one `ns_f` namespace shaped like the ABI-versioning idiom the
+ * reporter's repo uses (`namespace x { inline namespace v { … } }`):
+ *
+ *   namespace ns_f {
+ *     void own0(); void own1();          // direct members
+ *     inline namespace v1 { void inl0(); void dup(); }   // transitively visible
+ *     inline namespace v2 { void dup(); }                // → dup is ambiguous
+ *     namespace detail { void hidden0(); }               // NOT inline → invisible
+ *   }
+ *
+ * The three outcome classes all appear, because each takes a different exit
+ * from the resolver and only exercising the hit path would let a regression in
+ * the miss path (the most common outcome in real source) go unmeasured:
+ * resolved hit, `'ambiguous'` (#1564), and `undefined` (miss — both a wrong
+ * member name and a non-inline nested member).
+ */
+function buildCorpus(fileCount) {
+  const parsedFiles = [];
+  for (let f = 0; f < fileCount; f++) {
+    const filePath = `src/file${f}.cpp`;
+    const scopes = [];
+    let line = 1;
+    const scope = (id, kind, parent, defs) => {
+      const entry = {
+        id,
+        kind,
+        parent,
+        ownedDefs: defs,
+        range: { startLine: line, startCol: 0, endLine: line + 1, endCol: 0 },
+      };
+      line += 2;
+      scopes.push(entry);
+      return entry;
+    };
+    const def = (type, qualifiedName) => ({
+      nodeId: `def:${filePath}#${qualifiedName}`,
+      type,
+      qualifiedName,
+    });
+
+    const nsId = `sc:${f}:ns`;
+    scope(nsId, 'Namespace', null, [
+      def('Namespace', `ns_${f}`),
+      def('Function', `ns_${f}.own0`),
+      def('Function', `ns_${f}.own1`),
+    ]);
+    const v1 = scope(`sc:${f}:v1`, 'Namespace', nsId, [
+      def('Namespace', `ns_${f}.v1`),
+      def('Function', `ns_${f}.v1.inl0`),
+      def('Function', `ns_${f}.v1.dup`),
+    ]);
+    const v2 = scope(`sc:${f}:v2`, 'Namespace', nsId, [
+      def('Namespace', `ns_${f}.v2`),
+      def('Function', `ns_${f}.v2.dup`),
+    ]);
+    scope(`sc:${f}:detail`, 'Namespace', nsId, [
+      def('Namespace', `ns_${f}.detail`),
+      def('Function', `ns_${f}.detail.hidden0`),
+    ]);
+
+    parsedFiles.push({ filePath, scopes, inlineRanges: [v1.range, v2.range] });
+  }
+  return parsedFiles;
+}
+
+/** Capture-time inline marking + `populateOwners`-time scope-id resolution, in
+ *  the same order the pipeline runs them. Must re-run after every
+ *  `clearCppInlineNamespaces`, which drops both the marks and the index. */
+function populateInlineState(parsedFiles) {
+  clearCppInlineNamespaces();
+  for (const parsed of parsedFiles) {
+    for (const range of parsed.inlineRanges) markCppInlineNamespaceRange(parsed.filePath, range);
+    populateCppInlineNamespaceScopes(parsed);
+  }
+}
+
+/** The call sites: a deterministic spread over receivers and member names so
+ *  each rep resolves the same set, with hits, misses and ambiguities mixed. */
+const MEMBERS = ['own0', 'inl0', 'dup', 'hidden0', 'nosuch'];
+function callSites(fileCount) {
+  const sites = [];
+  for (let f = 0; f < fileCount; f++) {
+    for (let c = 0; c < CALLS_PER_FILE; c++) {
+      sites.push([`ns_${(f * 7 + c * 13) % fileCount}`, MEMBERS[c % MEMBERS.length]]);
+    }
+  }
+  return sites;
+}
+
+/** The timed loop: resolution only. The outcome strings the fingerprint needs
+ *  are built in a separate untimed pass (`outcomesOf`), so their allocation
+ *  cost — which grows with the corpus and would inflate the scaling ratio on
+ *  its own — never lands in the measurement. `sink` keeps the calls live. */
+function resolveAll(parsedFiles, sites) {
+  let sink = 0;
+  for (const [receiver, member] of sites) {
+    const hit = resolveCppQualifiedNamespaceMember(receiver, member, parsedFiles, NO_SCOPES);
+    if (hit !== undefined) sink++;
+  }
+  return sink;
+}
+
+function outcomesOf(parsedFiles, sites) {
+  const outcomes = [];
+  for (const [receiver, member] of sites) {
+    const hit = resolveCppQualifiedNamespaceMember(receiver, member, parsedFiles, NO_SCOPES);
+    outcomes.push(
+      `${receiver}::${member}\u0000${hit === undefined ? '<none>' : hit === 'ambiguous' ? '<ambiguous>' : hit.nodeId}`,
+    );
+  }
+  return outcomes;
+}
+
+/**
+ * MIN, not median — same rationale as bench/callable-value-flow: both scales
+ * are timed in one process and every error source (scheduler preemption, GC, a
+ * noisy neighbour on a shared CI runner) is additive, so the fastest observed
+ * run is the closest estimate of the uncontended cost and keeps the derived
+ * ratio comparable across machines.
+ */
+function fastest(values) {
+  return Math.min(...values);
+}
+
+/** Time one full pass: index build (lazy, on the first call) + every call
+ *  site. The corpus state is reset OUTSIDE the timer so the reset's own
+ *  O(files) cost never lands in the measurement. */
+function timeResolution(parsedFiles, sites) {
+  for (let w = 0; w < WARMUP; w++) {
+    populateInlineState(parsedFiles);
+    resolveAll(parsedFiles, sites);
+  }
+  const samples = [];
+  for (let r = 0; r < REPS; r++) {
+    populateInlineState(parsedFiles);
+    const t0 = performance.now();
+    resolveAll(parsedFiles, sites);
+    samples.push(performance.now() - t0);
+  }
+  return { ms: fastest(samples), outcomes: outcomesOf(parsedFiles, sites) };
+}
+
+function fingerprint(outcomes) {
+  return crypto
+    .createHash('sha256')
+    .update([...outcomes].sort().join('\n'))
+    .digest('hex');
+}
+
+const scales = {};
+for (const [name, fileCount] of [
+  ['small', SMALL],
+  ['large', LARGE],
+]) {
+  const parsedFiles = buildCorpus(fileCount);
+  const sites = callSites(fileCount);
+  const { ms, outcomes } = timeResolution(parsedFiles, sites);
+  scales[name] = {
+    files: fileCount,
+    call_sites: sites.length,
+    ms: Number(ms.toFixed(3)),
+    fingerprint: fingerprint(outcomes),
+  };
+}
+
+const scalingRatio = scales.large.ms / scales.small.ms / (LARGE / SMALL);
+
+const report = {
+  small: scales.small,
+  large: scales.large,
+  scaling_ratio: Number(scalingRatio.toFixed(3)),
+  fingerprint: scales.large.fingerprint,
+};
+
+if (!process.argv.includes('--check')) {
+  console.log(JSON.stringify(report, null, 2));
+  process.exit(0);
+}
+
+const baseline = JSON.parse(fs.readFileSync(BASELINE_PATH, 'utf-8'));
+const failures = [];
+if (report.fingerprint !== baseline.fingerprint) {
+  failures.push(
+    `fingerprint drift: ${report.fingerprint} != ${baseline.fingerprint} — qualified ` +
+      `namespace lookup resolved a DIFFERENT symbol set. This is a behaviour change, not a perf one.`,
+  );
+}
+if (report.scaling_ratio > baseline.scaling_budget) {
+  failures.push(
+    `scaling ${report.scaling_ratio} > budget ${baseline.scaling_budget} — per-call-site cost ` +
+      `now grows with corpus size again (#2788).`,
+  );
+}
+
+console.log(JSON.stringify(report, null, 2));
+if (failures.length > 0) {
+  console.error(`[cpp-qualified-ns --check] FAIL\n  - ${failures.join('\n  - ')}`);
+  process.exit(1);
+}
+console.log('[cpp-qualified-ns --check] PASS');
diff --git a/gitnexus/src/core/ingestion/languages/cpp/inline-namespaces.ts b/gitnexus/src/core/ingestion/languages/cpp/inline-namespaces.ts
index 134738dfd..ad2749928 100644
--- a/gitnexus/src/core/ingestion/languages/cpp/inline-namespaces.ts
+++ b/gitnexus/src/core/ingestion/languages/cpp/inline-namespaces.ts
@@ -14,12 +14,14 @@
  *   2. **Transitive qualified visibility.** `outer::foo()` resolves to
  *      `outer::v1::foo()` when `v1` is inline. The qualified-namespace
  *      receiver resolver (`resolveCppQualifiedNamespaceMember`) walks
- *      inline-namespace children transitively when collecting candidates.
+ *      inline-namespace children transitively when collecting candidates —
+ *      once per pipeline run, into {@link QualifiedNsMemberIndex} (#2788).
  *
  * State lifecycle: capture-time `markCppInlineNamespaceRange` records each
  * inline namespace's source range; `populateCppInlineNamespaceScopes`
  * resolves ranges to `ScopeId`s during `populateOwners`. Cleared via
- * `clearCppInlineNamespaces`, called from `clearFileLocalNames`.
+ * `clearCppInlineNamespaces`, called from
+ * `cppScopeResolver.loadResolutionConfig` at the start of every pass.
  *
  * STL idiom this enables: `std::__1::vector` (libc++) and `std::__cxx11`
  * (libstdc++) are inline namespaces of `std`. With this support,
@@ -85,10 +87,13 @@ export function applyCppInlineNamespaceSideChannel(
   for (const r of ranges) set.add(r);
 }
 
-/** Clear all inline-namespace state. Called from `clearFileLocalNames`. */
+/** Clear all inline-namespace state. Called from
+ *  `cppScopeResolver.loadResolutionConfig` at the start of every pass. */
 export function clearCppInlineNamespaces(): void {
   inlineNamespaceRangesByFile.clear();
   inlineNamespaceScopeIds.clear();
+  qualifiedNsIndex = undefined;
+  qualifiedNsIndexSource = undefined;
 }
 
 /** Resolve captured ranges to actual ScopeIds by matching scope ranges
@@ -115,11 +120,136 @@ export function isCppInlineNamespaceScope(scopeId: ScopeId): boolean {
 }
 
 /**
- * Walk every parsed file looking for a Namespace scope whose qualified
- * name matches `receiverName`, collect its callable ownedDefs matching
- * `memberName`, transitively descending into any inline-namespace
- * children (since they're members of the enclosing namespace under ISO
- * C++).
+ * Qualified-namespace member index — built **once** per pipeline run from
+ * `parsedFiles` and reused by every qualified call site.
+ *
+ * The legacy lookup re-scanned every parsed file (rebuilding a per-file
+ * `scopesById` map each time) once **per qualified call site**, making the
+ * scope-resolution emit phase O(callsites × scopes): 25.3 min of a 33-min
+ * analyze on a 1,473-file C++ repo, 75% of total self-time in this one
+ * function (#2788). Mirrors the same fix #1990 applied to ADL
+ * (`pickCppAdlCandidates` → {@link AdlCandidateIndex}); per-site cost drops
+ * to two Map lookups.
+ *
+ * `byReceiver`: namespace simple name → member simple name → callable defs,
+ * in the exact order the legacy linear scan produced them (file-major; within
+ * a file, `parsed.scopes` declaration order; within a namespace, own
+ * `ownedDefs` before inline-namespace children, depth-first). Ordering is
+ * load-bearing: the caller returns `allHits[0]` for the single-hit case and
+ * `narrowOverloadCandidates` is first-wins.
+ */
+interface QualifiedNsMemberIndex {
+  readonly byReceiver: ReadonlyMap<string, ReadonlyMap<string, readonly SymbolDefinition[]>>;
+}
+
+type NsScope = ParsedFile['scopes'][number];
+
+let qualifiedNsIndex: QualifiedNsMemberIndex | undefined;
+let qualifiedNsIndexSource: readonly ParsedFile[] | undefined;
+
+/** Build the index in a single pass over the workspace. Visitation order
+ *  mirrors the legacy scan exactly (see {@link QualifiedNsMemberIndex}). */
+function buildQualifiedNsMemberIndex(parsedFiles: readonly ParsedFile[]): QualifiedNsMemberIndex {
+  const byReceiver = new Map<string, Map<string, SymbolDefinition[]>>();
+  // Legacy dedup was a per-call `seenNodeId` set spanning all files; since a
+  // def only ever lands in one `(receiver, member)` bucket, a per-receiver set
+  // keyed `member \0 nodeId` reproduces it. Only reachable at all via
+  // same-name inline nesting (`namespace ns { inline namespace ns { … } }`),
+  // but kept so a def is never double-counted into `'ambiguous'`.
+  const seenByReceiver = new Map<string, Set<string>>();
+
+  for (const parsed of parsedFiles) {
+    // parent → inline-namespace children. The legacy transitive walk filtered
+    // `scopesById.values()` by `parent` per recursion step — O(scopes) each,
+    // and O(scopes²) per file overall; this is the same order, built once.
+    const inlineChildrenByParent = new Map<ScopeId, (typeof parsed.scopes)[number][]>();
+    for (const sc of parsed.scopes) {
+      if (sc.parent === null) continue;
+      if (sc.kind !== 'Namespace') continue;
+      if (!inlineNamespaceScopeIds.has(sc.id)) continue;
+      let kids = inlineChildrenByParent.get(sc.parent);
+      if (kids === undefined) {
+        kids = [];
+        inlineChildrenByParent.set(sc.parent, kids);
+      }
+      kids.push(sc);
+    }
+
+    for (const scope of parsed.scopes) {
+      if (scope.kind !== 'Namespace') continue;
+      const nsDef = findNamespaceDefInScope(scope);
+      if (nsDef === undefined) continue;
+      const nsName = nsDef.qualifiedName?.split('.').pop() ?? nsDef.qualifiedName ?? '';
+      let byMember = byReceiver.get(nsName);
+      if (byMember === undefined) {
+        byMember = new Map();
+        byReceiver.set(nsName, byMember);
+      }
+      let seen = seenByReceiver.get(nsName);
+      if (seen === undefined) {
+        seen = new Set();
+        seenByReceiver.set(nsName, seen);
+      }
+      collectNamespaceMembers(scope, inlineChildrenByParent, byMember, seen);
+    }
+  }
+  return { byReceiver };
+}
+
+/** Bucket a namespace scope's callable `ownedDefs` by member simple name,
+ *  then descend into inline-namespace children — the index-build twin of the
+ *  legacy `findMemberInNamespaceTransitive`, collecting every member name in
+ *  one walk instead of one walk per `(call site, member name)`. */
+function collectNamespaceMembers(
+  scope: NsScope,
+  inlineChildrenByParent: ReadonlyMap<ScopeId, readonly NsScope[]>,
+  byMember: Map<string, SymbolDefinition[]>,
+  seen: Set<string>,
+): void {
+  for (const def of scope.ownedDefs) {
+    if (def.type !== 'Function' && def.type !== 'Method' && def.type !== 'Constructor') continue;
+    const simple = def.qualifiedName?.split('.').pop() ?? def.qualifiedName ?? '';
+    const dedupKey = `${simple}\u0000${def.nodeId}`;
+    if (seen.has(dedupKey)) continue;
+    seen.add(dedupKey);
+    let arr = byMember.get(simple);
+    if (arr === undefined) {
+      arr = [];
+      byMember.set(simple, arr);
+    }
+    arr.push(def);
+  }
+  for (const child of inlineChildrenByParent.get(scope.id) ?? []) {
+    collectNamespaceMembers(child, inlineChildrenByParent, byMember, seen);
+  }
+}
+
+/** Build the index on first use of a given `parsedFiles` set; reuse it for
+ *  every subsequent call site in the same pipeline run.
+ *
+ *  The index is a function of TWO inputs: `parsedFiles` and the module-level
+ *  `inlineNamespaceScopeIds` (which inline children get descended into).
+ *  Reference identity on `parsedFiles` alone is sound here because
+ *  `populateCppInlineNamespaceScopes` fills `inlineNamespaceScopeIds` during
+ *  `populateOwners` — strictly before any resolution pass calls in — and
+ *  {@link clearCppInlineNamespaces} drops the index at the start of every
+ *  pass. Any future caller that mutates `inlineNamespaceScopeIds` mid-pass
+ *  while reusing the same `parsedFiles` reference MUST call
+ *  `clearCppInlineNamespaces` in between. Same contract as `ensureAdlIndex`. */
+function qualifiedNsMemberIndex(parsedFiles: readonly ParsedFile[]): QualifiedNsMemberIndex {
+  if (qualifiedNsIndex === undefined || qualifiedNsIndexSource !== parsedFiles) {
+    qualifiedNsIndex = buildQualifiedNsMemberIndex(parsedFiles);
+    qualifiedNsIndexSource = parsedFiles;
+  }
+  return qualifiedNsIndex;
+}
+
+/**
+ * Find the Namespace scopes whose simple name matches `receiverName` and
+ * return their callable members matching `memberName`, transitively
+ * including inline-namespace children (since they're members of the
+ * enclosing namespace under ISO C++). Served from a per-pipeline index
+ * ({@link QualifiedNsMemberIndex}), not a per-call-site workspace scan.
  *
  * Returns the most specific (innermost) match — for `outer::foo()`
  * where `inline namespace v1` declares `foo`, returns `v1::foo`. When
@@ -134,27 +264,8 @@ export function resolveCppQualifiedNamespaceMember(
   _scopes: ScopeResolutionIndexes,
   callsite?: Callsite,
 ): SymbolDefinition | 'ambiguous' | undefined {
-  const allHits: SymbolDefinition[] = [];
-  const seenNodeId = new Set<string>();
-  for (const parsed of parsedFiles) {
-    const scopesById = new Map<ScopeId, (typeof parsed.scopes)[number]>();
-    for (const sc of parsed.scopes) scopesById.set(sc.id, sc);
-    for (const scope of parsed.scopes) {
-      if (scope.kind !== 'Namespace') continue;
-      const nsDef = findNamespaceDefInScope(scope);
-      if (nsDef === undefined) continue;
-      const nsName = nsDef.qualifiedName?.split('.').pop() ?? nsDef.qualifiedName ?? '';
-      if (nsName !== receiverName) continue;
-      // Found a matching namespace scope in this file. Collect ALL
-      // members transitively through any inline-namespace children.
-      const hits = findMemberInNamespaceTransitive(scope, scopesById, memberName);
-      for (const hit of hits) {
-        if (seenNodeId.has(hit.nodeId)) continue;
-        seenNodeId.add(hit.nodeId);
-        allHits.push(hit);
-      }
-    }
-  }
+  const allHits =
+    qualifiedNsMemberIndex(parsedFiles).byReceiver.get(receiverName)?.get(memberName) ?? [];
   if (allHits.length === 0) return undefined;
   if (allHits.length === 1) return allHits[0];
 
@@ -182,46 +293,6 @@ export function resolveCppQualifiedNamespaceMember(
   return 'ambiguous';
 }
 
-/** Recursively search a namespace scope and any inline-namespace
- *  descendants for callable defs with the given simple name. Non-inline
- *  nested namespaces are NOT traversed — they require explicit
- *  qualification (`outer::nested::foo`). Returns ALL matches so the
- *  caller can detect same-name ambiguity across inline children (#1564). */
-function findMemberInNamespaceTransitive(
-  scope: {
-    readonly id: ScopeId;
-    readonly ownedDefs: readonly SymbolDefinition[];
-    readonly parent: ScopeId | null;
-  },
-  scopesById: ReadonlyMap<
-    ScopeId,
-    {
-      readonly id: ScopeId;
-      readonly kind: string;
-      readonly parent: ScopeId | null;
-      readonly ownedDefs: readonly SymbolDefinition[];
-    }
-  >,
-  memberName: string,
-): SymbolDefinition[] {
-  const results: SymbolDefinition[] = [];
-  // Check this scope's own ownedDefs first.
-  for (const def of scope.ownedDefs) {
-    if (def.type !== 'Function' && def.type !== 'Method' && def.type !== 'Constructor') continue;
-    const simple = def.qualifiedName?.split('.').pop() ?? def.qualifiedName ?? '';
-    if (simple === memberName) results.push(def);
-  }
-  // Descend into inline-namespace children.
-  for (const childScope of scopesById.values()) {
-    if (childScope.parent !== scope.id) continue;
-    if (childScope.kind !== 'Namespace') continue;
-    if (!inlineNamespaceScopeIds.has(childScope.id)) continue;
-    const childHits = findMemberInNamespaceTransitive(childScope, scopesById, memberName);
-    for (const hit of childHits) results.push(hit);
-  }
-  return results;
-}
-
 function findNamespaceDefInScope(scope: {
   readonly ownedDefs: readonly SymbolDefinition[];
 }): SymbolDefinition | undefined {
diff --git a/gitnexus/test/unit/cpp-qualified-ns-index.test.ts b/gitnexus/test/unit/cpp-qualified-ns-index.test.ts
new file mode 100644
index 000000000..a8af02384
--- /dev/null
+++ b/gitnexus/test/unit/cpp-qualified-ns-index.test.ts
@@ -0,0 +1,258 @@
+/**
+ * #2788 — `resolveCppQualifiedNamespaceMember` serves qualified `ns::member()`
+ * lookups from a per-pipeline index instead of rescanning every parsed file per
+ * call site. These tests pin the two properties the index must not lose:
+ *
+ *   1. Transitive inline-namespace collection, ordering and same-name
+ *      ambiguity (#1564) — the semantics the old linear scan provided.
+ *   2. Cache invalidation — a new `parsedFiles` array, or a
+ *      `clearCppInlineNamespaces()` between passes, must not serve stale hits.
+ *      This is the failure mode the index introduces; nothing else covers it.
+ */
+
+import type {
+  ParsedFile,
+  ScopeId,
+  ScopeResolutionIndexes,
+  SymbolDefinition,
+} from 'gitnexus-shared';
+import { beforeEach, describe, expect, it } from 'vitest';
+import {
+  clearCppInlineNamespaces,
+  markCppInlineNamespaceRange,
+  populateCppInlineNamespaceScopes,
+  resolveCppQualifiedNamespaceMember,
+} from '../../src/core/ingestion/languages/cpp/inline-namespaces.js';
+
+const NO_SCOPES = {} as unknown as ScopeResolutionIndexes;
+
+interface ScopeSpec {
+  readonly id: string;
+  readonly kind: 'Namespace' | 'Module';
+  readonly parent: string | null;
+  readonly defs: readonly SymbolDefinition[];
+  /** Distinguishes each scope's range so inline marking targets exactly one. */
+  readonly line: number;
+}
+
+function def(nodeId: string, type: string, qualifiedName: string): SymbolDefinition {
+  return { nodeId, type, qualifiedName } as unknown as SymbolDefinition;
+}
+
+function nsDef(nodeId: string, qualifiedName: string): SymbolDefinition {
+  return def(nodeId, 'Namespace', qualifiedName);
+}
+
+function fnDef(nodeId: string, qualifiedName: string): SymbolDefinition {
+  return def(nodeId, 'Function', qualifiedName);
+}
+
+function range(line: number): {
+  startLine: number;
+  startCol: number;
+  endLine: number;
+  endCol: number;
+} {
+  return { startLine: line, startCol: 0, endLine: line + 1, endCol: 0 };
+}
+
+/** Build a single-file `parsedFiles` array from scope specs, marking the
+ *  scopes named in `inlineIds` as inline namespaces (capture-time range mark +
+ *  `populateOwners`-time scope-id resolution, same order as the pipeline). */
+function makeParsedFiles(
+  filePath: string,
+  specs: readonly ScopeSpec[],
+  inlineIds: readonly string[],
+): readonly ParsedFile[] {
+  const parsed = {
+    filePath,
+    scopes: specs.map((s) => ({
+      id: s.id as unknown as ScopeId,
+      kind: s.kind,
+      parent: s.parent as unknown as ScopeId | null,
+      ownedDefs: s.defs,
+      range: range(s.line),
+    })),
+  } as unknown as ParsedFile;
+  markInline(parsed, specs, inlineIds);
+  return [parsed];
+}
+
+/** Capture-time inline marking + `populateOwners`-time scope-id resolution,
+ *  in the same order the pipeline runs them. */
+function markInline(
+  parsed: ParsedFile,
+  specs: readonly ScopeSpec[],
+  inlineIds: readonly string[],
+): void {
+  for (const id of inlineIds) {
+    const spec = specs.find((s) => s.id === id);
+    if (spec === undefined) throw new Error(`inline scope ${id} must exist`);
+    markCppInlineNamespaceRange(parsed.filePath, range(spec.line));
+  }
+  populateCppInlineNamespaceScopes(parsed);
+}
+
+/** `namespace outer { <ownDefs> inline namespace v1 { <inlineDefs> } }` */
+function outerWithInlineChild(
+  filePath: string,
+  ownDefs: readonly SymbolDefinition[],
+  inlineDefs: readonly SymbolDefinition[],
+): readonly ParsedFile[] {
+  return makeParsedFiles(
+    filePath,
+    [
+      {
+        id: 'sc:outer',
+        kind: 'Namespace',
+        parent: null,
+        defs: [nsDef('n:outer', 'outer'), ...ownDefs],
+        line: 1,
+      },
+      {
+        id: 'sc:v1',
+        kind: 'Namespace',
+        parent: 'sc:outer',
+        defs: [nsDef('n:v1', 'outer.v1'), ...inlineDefs],
+        line: 10,
+      },
+    ],
+    ['sc:v1'],
+  );
+}
+
+describe('C++ qualified-namespace member index (#2788)', () => {
+  beforeEach(() => {
+    clearCppInlineNamespaces();
+  });
+
+  it('resolves outer::foo through an inline-namespace child', () => {
+    const files = outerWithInlineChild('a.cpp', [], [fnDef('n:foo@v1', 'outer.v1.foo')]);
+    expect(resolveCppQualifiedNamespaceMember('outer', 'foo', files, NO_SCOPES)).toMatchObject({
+      nodeId: 'n:foo@v1',
+    });
+  });
+
+  it('returns undefined for an unknown namespace or member', () => {
+    const files = outerWithInlineChild('a.cpp', [], [fnDef('n:foo@v1', 'outer.v1.foo')]);
+    expect(resolveCppQualifiedNamespaceMember('nope', 'foo', files, NO_SCOPES)).toBeUndefined();
+    expect(resolveCppQualifiedNamespaceMember('outer', 'nope', files, NO_SCOPES)).toBeUndefined();
+  });
+
+  it('does not descend into a non-inline nested namespace', () => {
+    const files = makeParsedFiles(
+      'a.cpp',
+      [
+        {
+          id: 'sc:outer',
+          kind: 'Namespace',
+          parent: null,
+          defs: [nsDef('n:outer', 'outer')],
+          line: 1,
+        },
+        {
+          id: 'sc:nested',
+          kind: 'Namespace',
+          parent: 'sc:outer',
+          defs: [nsDef('n:nested', 'outer.nested'), fnDef('n:foo@nested', 'outer.nested.foo')],
+          line: 10,
+        },
+      ],
+      [],
+    );
+    expect(resolveCppQualifiedNamespaceMember('outer', 'foo', files, NO_SCOPES)).toBeUndefined();
+    expect(resolveCppQualifiedNamespaceMember('nested', 'foo', files, NO_SCOPES)).toMatchObject({
+      nodeId: 'n:foo@nested',
+    });
+  });
+
+  it('reports same-name hits across two inline children as ambiguous (#1564)', () => {
+    const files = makeParsedFiles(
+      'a.cpp',
+      [
+        {
+          id: 'sc:outer',
+          kind: 'Namespace',
+          parent: null,
+          defs: [nsDef('n:outer', 'outer')],
+          line: 1,
+        },
+        {
+          id: 'sc:v1',
+          kind: 'Namespace',
+          parent: 'sc:outer',
+          defs: [nsDef('n:v1', 'outer.v1'), fnDef('n:foo@v1', 'outer.v1.foo')],
+          line: 10,
+        },
+        {
+          id: 'sc:v2',
+          kind: 'Namespace',
+          parent: 'sc:outer',
+          defs: [nsDef('n:v2', 'outer.v2'), fnDef('n:foo@v2', 'outer.v2.foo')],
+          line: 20,
+        },
+      ],
+      ['sc:v1', 'sc:v2'],
+    );
+    expect(resolveCppQualifiedNamespaceMember('outer', 'foo', files, NO_SCOPES)).toBe('ambiguous');
+  });
+
+  it('keeps scan order: a namespace-owned def precedes its inline child hits', () => {
+    // Two candidates with no call-site info narrow to 'ambiguous', so order is
+    // asserted through the single-hit path: only the own def is present here,
+    // and the inline child contributes a different member name.
+    const files = outerWithInlineChild(
+      'a.cpp',
+      [fnDef('n:foo@outer', 'outer.foo')],
+      [fnDef('n:bar@v1', 'outer.v1.bar')],
+    );
+    expect(resolveCppQualifiedNamespaceMember('outer', 'foo', files, NO_SCOPES)).toMatchObject({
+      nodeId: 'n:foo@outer',
+    });
+    expect(resolveCppQualifiedNamespaceMember('outer', 'bar', files, NO_SCOPES)).toMatchObject({
+      nodeId: 'n:bar@v1',
+    });
+  });
+
+  it('does not serve one parsedFiles array’s index to another', () => {
+    const first = outerWithInlineChild('a.cpp', [], [fnDef('n:foo@a', 'outer.v1.foo')]);
+    expect(resolveCppQualifiedNamespaceMember('outer', 'foo', first, NO_SCOPES)).toMatchObject({
+      nodeId: 'n:foo@a',
+    });
+    const second = outerWithInlineChild('b.cpp', [], [fnDef('n:foo@b', 'outer.v1.foo')]);
+    expect(resolveCppQualifiedNamespaceMember('outer', 'foo', second, NO_SCOPES)).toMatchObject({
+      nodeId: 'n:foo@b',
+    });
+  });
+
+  it('rebuilds after clearCppInlineNamespaces even when parsedFiles is reused', () => {
+    // Pass 1: `v1` is inline, so `outer::foo` reaches through it.
+    const specs: readonly ScopeSpec[] = [
+      {
+        id: 'sc:outer',
+        kind: 'Namespace',
+        parent: null,
+        defs: [nsDef('n:outer', 'outer')],
+        line: 1,
+      },
+      {
+        id: 'sc:v1',
+        kind: 'Namespace',
+        parent: 'sc:outer',
+        defs: [nsDef('n:v1', 'outer.v1'), fnDef('n:foo@v1', 'outer.v1.foo')],
+        line: 10,
+      },
+    ];
+    const files = makeParsedFiles('a.cpp', specs, ['sc:v1']);
+    expect(resolveCppQualifiedNamespaceMember('outer', 'foo', files, NO_SCOPES)).toMatchObject({
+      nodeId: 'n:foo@v1',
+    });
+
+    // Pass 2: SAME `parsedFiles` reference (so identity alone would serve the
+    // cached index), but `v1` is no longer inline. Without the index reset in
+    // `clearCppInlineNamespaces` the stale pass-1 hit survives.
+    clearCppInlineNamespaces();
+    markInline(files[0], specs, []);
+    expect(resolveCppQualifiedNamespaceMember('outer', 'foo', files, NO_SCOPES)).toBeUndefined();
+  });
+});