diff --git a/gitnexus-shared/src/scope-resolution/symbol-definition.ts b/gitnexus-shared/src/scope-resolution/symbol-definition.ts
index e27605ac4..d1b30abcc 100644
--- a/gitnexus-shared/src/scope-resolution/symbol-definition.ts
+++ b/gitnexus-shared/src/scope-resolution/symbol-definition.ts
@@ -53,6 +53,10 @@ export interface SymbolDefinition {
    *  `ScopeResolver.constraintCompatibility` hook during overload narrowing.
    *  Absent for symbols that have no constraints (the common case). */
   templateConstraints?: unknown;
+  /** True when the producing language marked this callable as explicit.
+   *  Currently used by C++ overload ranking to exclude explicit constructors
+   *  from implicit user-defined conversion candidates. */
+  isExplicit?: boolean;
   /** Links Method/Constructor/Property to owning Class/Struct/Trait nodeId */
   ownerId?: string;
 }
diff --git a/gitnexus/src/core/ingestion/languages/cpp/captures.ts b/gitnexus/src/core/ingestion/languages/cpp/captures.ts
index 1c669738d..354f6ce4c 100644
--- a/gitnexus/src/core/ingestion/languages/cpp/captures.ts
+++ b/gitnexus/src/core/ingestion/languages/cpp/captures.ts
@@ -126,6 +126,13 @@ export function emitCppScopeCaptures(
             JSON.stringify(arity.parameterTypeClasses),
           );
         }
+        if (hasExplicitSpecifier(fnNode)) {
+          grouped['@declaration.is-explicit'] = syntheticCapture(
+            '@declaration.is-explicit',
+            fnNode,
+            'true',
+          );
+        }
 
         // Detect static storage class (file-local linkage)
         if (hasStaticStorageClass(fnNode)) {
@@ -1542,6 +1549,20 @@ function extractDeclaratorLeafName(node: SyntaxNode): string | null {
   return null;
 }
 
+/**
+ * Check if a C++ declaration has an `explicit` specifier. Tree-sitter-cpp
+ * exposes `explicit` as a direct keyword child on constructor declarations in
+ * current grammar builds; the bounded text prefix keeps this resilient across
+ * small grammar shape differences without scanning whole function bodies.
+ */
+function hasExplicitSpecifier(node: SyntaxNode): boolean {
+  for (let i = 0; i < node.childCount; i++) {
+    const child = node.child(i);
+    if (child !== null && child.text === 'explicit') return true;
+  }
+  return /\bexplicit\b/.test(node.text.slice(0, 128));
+}
+
 /**
  * Check if a C++ function_definition or declaration has `static` storage class.
  */
diff --git a/gitnexus/src/core/ingestion/languages/cpp/query.ts b/gitnexus/src/core/ingestion/languages/cpp/query.ts
index d42b586ce..af15e81e6 100644
--- a/gitnexus/src/core/ingestion/languages/cpp/query.ts
+++ b/gitnexus/src/core/ingestion/languages/cpp/query.ts
@@ -225,6 +225,12 @@ const CPP_SCOPE_QUERY = `
     declarator: (function_declarator
       declarator: (field_identifier) @declaration.name))) @declaration.method
 
+;; Constructor prototype in class body: User(int id);
+(field_declaration_list
+  (declaration
+    declarator: (function_declarator
+      declarator: (identifier) @declaration.name)) @declaration.method)
+
 ;; Method prototype with reference return: User& getRef();
 (field_declaration
   declarator: (reference_declarator
diff --git a/gitnexus/src/core/ingestion/languages/cpp/scope-resolver.ts b/gitnexus/src/core/ingestion/languages/cpp/scope-resolver.ts
index de0ff6d21..40ff978d6 100644
--- a/gitnexus/src/core/ingestion/languages/cpp/scope-resolver.ts
+++ b/gitnexus/src/core/ingestion/languages/cpp/scope-resolver.ts
@@ -116,8 +116,8 @@ export const cppScopeResolver: ScopeResolver = {
     // associated namespace for Koenig lookup.
     populateCppAssociatedNamespaces(parsed);
     // Build conservative one-step user-defined conversion facts for
-    // overload ranking (#1631): converting constructors and conversion
-    // operators only, with no chaining or explicit-constructor handling.
+    // overload ranking (#1631): implicit converting constructors only,
+    // with no chaining or conversion-operator handling.
     populateCppUserDefinedConversions(parsed);
   },
 
diff --git a/gitnexus/src/core/ingestion/languages/cpp/user-defined-conversions.ts b/gitnexus/src/core/ingestion/languages/cpp/user-defined-conversions.ts
index 20d6eff29..85fc951df 100644
--- a/gitnexus/src/core/ingestion/languages/cpp/user-defined-conversions.ts
+++ b/gitnexus/src/core/ingestion/languages/cpp/user-defined-conversions.ts
@@ -3,9 +3,19 @@ import type { ScopeId } from 'gitnexus-shared';
 import { normalizeCppParamType } from './arity-metadata.js';
 
 const userDefinedConversions = new Set<string>();
+const pendingUserDefinedConversions: PendingUserDefinedConversion[] = [];
+const classIdentitiesBySimpleName = new Map<string, Set<string>>();
+
+interface PendingUserDefinedConversion {
+  readonly argType: string;
+  readonly paramType: string;
+  readonly ownerClassName: string;
+}
 
 export function clearCppUserDefinedConversions(): void {
   userDefinedConversions.clear();
+  pendingUserDefinedConversions.length = 0;
+  classIdentitiesBySimpleName.clear();
 }
 
 export function hasCppUserDefinedConversion(argType: string, paramType: string): boolean {
@@ -16,6 +26,12 @@ export function populateCppUserDefinedConversions(parsed: ParsedFile): void {
   const scopesById = new Map<ScopeId, (typeof parsed.scopes)[number]>();
   for (const scope of parsed.scopes) scopesById.set(scope.id, scope);
 
+  for (const classScope of parsed.scopes) {
+    if (classScope.kind !== 'Class') continue;
+    const classDef = classScope.ownedDefs.find(isClassLike);
+    if (classDef !== undefined) recordClassIdentity(classDef);
+  }
+
   for (const classScope of parsed.scopes) {
     if (classScope.kind !== 'Class') continue;
     const classDef = classScope.ownedDefs.find(isClassLike);
@@ -27,16 +43,13 @@ export function populateCppUserDefinedConversions(parsed: ParsedFile): void {
     for (const def of methodDefs) {
       const simpleName = simpleNameOf(def);
       if (simpleName === className && def.parameterTypes?.length === 1) {
-        registerCppUserDefinedConversion(def.parameterTypes[0], className);
-        continue;
-      }
-
-      const operatorTarget = conversionOperatorTarget(simpleName);
-      if (operatorTarget !== undefined && def.parameterTypes?.length === 0) {
-        registerCppUserDefinedConversion(className, operatorTarget);
+        if (def.isExplicit === true) continue;
+        registerPendingCppUserDefinedConversion(def.parameterTypes[0], className, className);
       }
     }
   }
+
+  rebuildCppUserDefinedConversions();
 }
 
 export function registerCppUserDefinedConversion(argType: string, paramType: string): void {
@@ -67,17 +80,49 @@ function collectClassMethodDefs(
   return methods;
 }
 
-function conversionOperatorTarget(simpleName: string): string | undefined {
-  const match = /^operator\s+(.+)$/.exec(simpleName);
-  if (match === null) return undefined;
-  const target = normalizeCppParamType(match[1]);
-  return target.length > 0 ? target : undefined;
-}
-
 function conversionKey(argType: string, paramType: string): string {
   return `${argType}\0${paramType}`;
 }
 
+function registerPendingCppUserDefinedConversion(
+  argType: string,
+  paramType: string,
+  ownerClassName: string,
+): void {
+  if (argType === '' || paramType === '') return;
+  if (argType === paramType) return;
+  pendingUserDefinedConversions.push({ argType, paramType, ownerClassName });
+}
+
+function rebuildCppUserDefinedConversions(): void {
+  userDefinedConversions.clear();
+  for (const conversion of pendingUserDefinedConversions) {
+    if (isAmbiguousClassName(conversion.ownerClassName)) continue;
+    userDefinedConversions.add(conversionKey(conversion.argType, conversion.paramType));
+  }
+}
+
+function recordClassIdentity(def: SymbolDefinition): void {
+  const simpleName = normalizedSimpleName(def);
+  if (simpleName === '') return;
+  const identities = classIdentitiesBySimpleName.get(simpleName) ?? new Set<string>();
+  identities.add(normalizedQualifiedClassName(def));
+  classIdentitiesBySimpleName.set(simpleName, identities);
+}
+
+function isAmbiguousClassName(simpleName: string): boolean {
+  return (classIdentitiesBySimpleName.get(simpleName)?.size ?? 0) > 1;
+}
+
+function normalizedQualifiedClassName(def: SymbolDefinition): string {
+  const qualifiedName = def.qualifiedName ?? simpleNameOf(def);
+  if (qualifiedName === '' || !qualifiedName.includes('.')) return `${def.filePath}:${def.nodeId}`;
+  return qualifiedName
+    .split('.')
+    .map((part) => normalizeCppParamType(part))
+    .join('.');
+}
+
 function normalizedSimpleName(def: SymbolDefinition): string {
   return normalizeCppParamType(simpleNameOf(def));
 }
diff --git a/gitnexus/src/core/ingestion/scope-extractor.ts b/gitnexus/src/core/ingestion/scope-extractor.ts
index 7a57e704a..963cf4862 100644
--- a/gitnexus/src/core/ingestion/scope-extractor.ts
+++ b/gitnexus/src/core/ingestion/scope-extractor.ts
@@ -574,6 +574,7 @@ function buildDefFromDeclarationMatch(
   const declaredType = match['@declaration.field-type']?.text;
   const returnType = match['@declaration.return-type']?.text;
   const templateConstraints = parseJsonCapture(match['@declaration.template-constraints']);
+  const isExplicit = parseBooleanCapture(match['@declaration.is-explicit']);
 
   return {
     nodeId: makeDefId(filePath, anchor.range, type, nameCap.text),
@@ -588,6 +589,7 @@ function buildDefFromDeclarationMatch(
     ...(returnType !== undefined ? { returnType } : {}),
     ...(templateArguments !== undefined ? { templateArguments } : {}),
     ...(templateConstraints !== undefined ? { templateConstraints } : {}),
+    ...(isExplicit === true ? { isExplicit: true } : {}),
   };
 }
 
@@ -610,6 +612,13 @@ function parseIntCapture(cap: { readonly text: string } | undefined): number | u
   return Number.isFinite(n) ? n : undefined;
 }
 
+function parseBooleanCapture(cap: { readonly text: string } | undefined): boolean | undefined {
+  if (cap === undefined) return undefined;
+  if (cap.text === 'true') return true;
+  if (cap.text === 'false') return false;
+  return undefined;
+}
+
 function parseJsonParameterTypeClassesCapture(
   cap: { readonly text: string } | undefined,
 ): ParameterTypeClass[] | undefined {
@@ -1079,6 +1088,7 @@ const KNOWN_SUB_TAGS: ReadonlySet<string> = new Set<string>([
   '@declaration.parameter-types',
   '@declaration.parameter-type-classes',
   '@declaration.template-constraints',
+  '@declaration.is-explicit',
 ]);
 
 /**
diff --git a/gitnexus/test/fixtures/lang-resolution/cpp-overload-udc-namespace-collision/lib.cpp b/gitnexus/test/fixtures/lang-resolution/cpp-overload-udc-namespace-collision/lib.cpp
new file mode 100644
index 000000000..380679ba2
--- /dev/null
+++ b/gitnexus/test/fixtures/lang-resolution/cpp-overload-udc-namespace-collision/lib.cpp
@@ -0,0 +1,15 @@
+#include "lib.h"
+
+namespace alpha {
+
+Other::Other(int value) {}
+void Service::f(Token value) {}
+void Service::f(Other value) {}
+
+} // namespace alpha
+
+namespace beta {
+
+Token::Token(int value) {}
+
+} // namespace beta
diff --git a/gitnexus/test/fixtures/lang-resolution/cpp-overload-udc-namespace-collision/lib.h b/gitnexus/test/fixtures/lang-resolution/cpp-overload-udc-namespace-collision/lib.h
new file mode 100644
index 000000000..10517d6d7
--- /dev/null
+++ b/gitnexus/test/fixtures/lang-resolution/cpp-overload-udc-namespace-collision/lib.h
@@ -0,0 +1,31 @@
+#pragma once
+
+namespace alpha {
+
+class Token {};
+
+class Other {
+public:
+  Other(int value);
+};
+
+class Service {
+public:
+  void f(Token value);
+  void f(Other value);
+
+  void run() {
+    f(42);
+  }
+};
+
+} // namespace alpha
+
+namespace beta {
+
+class Token {
+public:
+  Token(int value);
+};
+
+} // namespace beta
diff --git a/gitnexus/test/fixtures/lang-resolution/cpp-overload-user-defined-conversion/lib.cpp b/gitnexus/test/fixtures/lang-resolution/cpp-overload-user-defined-conversion/lib.cpp
index 79ad0b84d..30111c7f3 100644
--- a/gitnexus/test/fixtures/lang-resolution/cpp-overload-user-defined-conversion/lib.cpp
+++ b/gitnexus/test/fixtures/lang-resolution/cpp-overload-user-defined-conversion/lib.cpp
@@ -3,9 +3,12 @@
 Wrap::Wrap(int value) {}
 WrapA::WrapA(int value) {}
 WrapB::WrapB(int value) {}
+ExplicitWrap::ExplicitWrap(int value) {}
 
 void Service::f(Wrap value) {}
 void Service::f(double value) {}
 void Service::g(Wrap value) {}
 void Service::h(WrapA value) {}
 void Service::h(WrapB value) {}
+void Service::e(Wrap value) {}
+void Service::e(ExplicitWrap value) {}
diff --git a/gitnexus/test/fixtures/lang-resolution/cpp-overload-user-defined-conversion/lib.h b/gitnexus/test/fixtures/lang-resolution/cpp-overload-user-defined-conversion/lib.h
index 054891c6b..f6e117241 100644
--- a/gitnexus/test/fixtures/lang-resolution/cpp-overload-user-defined-conversion/lib.h
+++ b/gitnexus/test/fixtures/lang-resolution/cpp-overload-user-defined-conversion/lib.h
@@ -15,6 +15,11 @@ public:
   WrapB(int value);
 };
 
+class ExplicitWrap {
+public:
+  explicit ExplicitWrap(int value);
+};
+
 class Service {
 public:
   void f(Wrap value);
@@ -22,10 +27,13 @@ public:
   void g(Wrap value);
   void h(WrapA value);
   void h(WrapB value);
+  void e(Wrap value);
+  void e(ExplicitWrap value);
 
   void run() {
     f(42);
     g(42);
     h(42);
+    e(42);
   }
 };
diff --git a/gitnexus/test/integration/resolvers/cpp.test.ts b/gitnexus/test/integration/resolvers/cpp.test.ts
index d80734ec8..babdaf569 100644
--- a/gitnexus/test/integration/resolvers/cpp.test.ts
+++ b/gitnexus/test/integration/resolvers/cpp.test.ts
@@ -2017,6 +2017,35 @@ describe('C++ overload resolution — user-defined conversion rank (#1631)', ()
 
     expect(hCalls.length).toBe(0);
   });
+
+  it('e(42) ignores the explicit-constructor overload and keeps the implicit UDC viable', () => {
+    const calls = getRelationships(result, 'CALLS');
+    const eCalls = calls.filter((c) => c.source === 'run' && c.target === 'e');
+
+    expect(eCalls.length).toBe(1);
+    const target = result.graph.getNode(eCalls[0].rel.targetId);
+    expect(target?.properties.parameterTypes).toEqual(['Wrap']);
+  });
+});
+
+describe('C++ overload resolution — UDC namespace collision guard (#1631)', () => {
+  let result: PipelineResult;
+
+  beforeAll(async () => {
+    result = await runPipelineFromRepo(
+      path.join(FIXTURES, 'cpp-overload-udc-namespace-collision'),
+      () => {},
+    );
+  }, 60000);
+
+  it('does not let beta::Token(int) tie the valid alpha::Other(int) conversion', () => {
+    const calls = getRelationships(result, 'CALLS');
+    const fCalls = calls.filter((c) => c.source === 'run' && c.target === 'f');
+
+    expect(fCalls.length).toBe(1);
+    const target = result.graph.getNode(fCalls[0].rel.targetId);
+    expect(target?.properties.parameterTypes).toEqual(['Other']);
+  });
 });
 
 // ---------------------------------------------------------------------------
diff --git a/gitnexus/test/unit/scope-resolution/cpp/cpp-overload-ranking.test.ts b/gitnexus/test/unit/scope-resolution/cpp/cpp-overload-ranking.test.ts
index 5d35cefe1..93cf86dc2 100644
--- a/gitnexus/test/unit/scope-resolution/cpp/cpp-overload-ranking.test.ts
+++ b/gitnexus/test/unit/scope-resolution/cpp/cpp-overload-ranking.test.ts
@@ -1,4 +1,4 @@
-import { describe, expect, it } from 'vitest';
+import { afterEach, describe, expect, it } from 'vitest';
 import type { ParameterTypeClass, SymbolDefinition } from 'gitnexus-shared';
 import { cppConversionRank } from '../../../../src/core/ingestion/languages/cpp/conversion-rank.js';
 import {
@@ -44,6 +44,10 @@ const mkDef = (
   parameterTypeClasses: [...parameterTypeClasses],
 });
 
+afterEach(() => {
+  clearCppUserDefinedConversions();
+});
+
 describe('cppConversionRank pointer/nullptr/ellipsis ranks (#1637)', () => {
   it('ranks nullptr -> T* ahead of nullptr -> bool', () => {
     expect(cppConversionRank('null', 'int', value('null'), pointer('int'))).toBe(2);
@@ -72,8 +76,6 @@ describe('cppConversionRank user-defined conversion ranks (#1631)', () => {
 
     expect(cppConversionRank('int', 'Wrap', value('int'), value('Wrap'))).toBe(4);
     expect(cppConversionRank('int', 'double', value('int'), value('double'))).toBe(2);
-
-    clearCppUserDefinedConversions();
   });
 
   it('keeps tied user-defined conversion candidates ambiguous', () => {
@@ -90,7 +92,6 @@ describe('cppConversionRank user-defined conversion ranks (#1631)', () => {
     });
 
     expect(result.map((d) => d.nodeId)).toEqual(['h:WrapA', 'h:WrapB']);
-    clearCppUserDefinedConversions();
   });
 });