diff --git a/.changeset/tall-cats-flash.md b/.changeset/tall-cats-flash.md
new file mode 100644
index 000000000..62c3b8e5c
--- /dev/null
+++ b/.changeset/tall-cats-flash.md
@@ -0,0 +1,7 @@
+---
+"@apollo/composition": patch
+"@apollo/federation-internals": patch
+---
+
+Fixing issue where redeclaration of custom scalars in a fed1 schema may cause upgrade errors
+  
\ No newline at end of file
diff --git a/.cspell/cspell-dict.txt b/.cspell/cspell-dict.txt
index 396cfd18d..3de941c55 100644
--- a/.cspell/cspell-dict.txt
+++ b/.cspell/cspell-dict.txt
@@ -175,6 +175,7 @@ Queryf
 reacheable
 reasonse
 recusive
+redeclaration
 refered
 Referencer
 referencer
diff --git a/composition-js/src/__tests__/composeFed1Subgraphs.test.ts b/composition-js/src/__tests__/composeFed1Subgraphs.test.ts
index e22bede09..8e158fb41 100644
--- a/composition-js/src/__tests__/composeFed1Subgraphs.test.ts
+++ b/composition-js/src/__tests__/composeFed1Subgraphs.test.ts
@@ -727,4 +727,22 @@ describe('override', () => {
       '[subgraphB] Invalid definition for directive \"@override\": missing required argument "from"',
     ]]);
   });
+
+  it('repro redefined built-in scalar breaks @key directive', () => {
+    const subgraphA = {
+      typeDefs: gql`
+        scalar Boolean
+        type Query {
+          q: String
+        }
+        type A @key(fields: "k") {
+          k: ID!
+        }
+      `,
+      name: 'subgraphA',
+    };
+
+    const result = composeServices([subgraphA,]);
+    assertCompositionSuccess(result);
+  });
 });
diff --git a/internals-js/src/validate.ts b/internals-js/src/validate.ts
index e4a02bf36..755f79a2d 100644
--- a/internals-js/src/validate.ts
+++ b/internals-js/src/validate.ts
@@ -8,6 +8,7 @@ import {
   InterfaceType,
   isInputObjectType,
   isNonNullType,
+  isScalarType,
   NamedSchemaElement,
   ObjectType,
   Schema,
@@ -17,7 +18,7 @@ import {
   VariableDefinitions
 } from "./definitions";
 import { assertName, ASTNode, GraphQLError, GraphQLErrorOptions } from "graphql";
-import { isValidValue, valueToString } from "./values";
+import { isValidValue, valueToString, isValidValueApplication } from "./values";
 import { introspectionTypeNames, isIntrospectionName } from "./introspection";
 import { isSubtype, sameType } from "./types";
 import { ERRORS } from "./error";
@@ -290,10 +291,14 @@ class Validator {
       );
     }
     if (arg.defaultValue !== undefined && !isValidValue(arg.defaultValue, arg, new VariableDefinitions())) {
-      this.addError(
-        `Invalid default value (got: ${valueToString(arg.defaultValue)}) provided for argument ${arg.coordinate} of type ${arg.type}.`,
-        { nodes: sourceASTs(arg) },
-      );
+      // don't error if custom scalar is shadowing a builtin scalar
+      const builtInScalar = this.schema.builtInScalarTypes().find((t) => arg.type && isScalarType(arg.type) && t.name === arg.type.name);
+      if (!builtInScalar || !isValidValueApplication(arg.defaultValue, builtInScalar, arg.defaultValue, new VariableDefinitions())) {
+        this.addError(
+          `Invalid default value (got: ${valueToString(arg.defaultValue)}) provided for argument ${arg.coordinate} of type ${arg.type}.`,
+          { nodes: sourceASTs(arg) },
+        );
+      }
     }
   }
 
diff --git a/internals-js/src/values.ts b/internals-js/src/values.ts
index 7dd7ddd2d..11b2968e5 100644
--- a/internals-js/src/values.ts
+++ b/internals-js/src/values.ts
@@ -492,7 +492,7 @@ export function isValidValue(value: any, argument: ArgumentDefinition<any> | Inp
   return isValidValueApplication(value, argument.type!, argument.defaultValue, variableDefinitions);
 }
 
-function isValidValueApplication(value: any, locationType: InputType, locationDefault: any, variableDefinitions: VariableDefinitions): boolean {
+export function isValidValueApplication(value: any, locationType: InputType, locationDefault: any, variableDefinitions: VariableDefinitions): boolean {
   // Note that this needs to be first, or the recursive call within 'isNonNullType' would break for variables
   if (isVariable(value)) {
     const definition = variableDefinitions.definition(value);