diff --git a/.changeset/fix-scope-accumulation.md b/.changeset/fix-scope-accumulation.md
new file mode 100644
index 000000000..afe502273
--- /dev/null
+++ b/.changeset/fix-scope-accumulation.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/client': patch
+---
+
+Accumulate OAuth scopes across 401/403 responses for progressive authorization. When an MCP server returns a 401 or 403 with a new required scope, the client now merges the new scope into the existing set rather than replacing it, preventing authorization loops where gaining one scope loses another.
diff --git a/packages/client/src/client/streamableHttp.ts b/packages/client/src/client/streamableHttp.ts
index 79a20adfc..826aed97e 100644
--- a/packages/client/src/client/streamableHttp.ts
+++ b/packages/client/src/client/streamableHttp.ts
@@ -16,6 +16,21 @@ import { EventSourceParserStream } from 'eventsource-parser/stream';
 import type { AuthResult, OAuthClientProvider } from './auth.js';
 import { auth, extractWWWAuthenticateParams, UnauthorizedError } from './auth.js';
 
+/**
+ * Merges OAuth scopes by computing the union of existing and new space-delimited scope strings.
+ * Per RFC 6749 §3.3, scopes are space-delimited, case-sensitive strings.
+ */
+function mergeScopes(existing: string | undefined, incoming: string | undefined): string | undefined {
+    if (!incoming) return existing;
+    if (!existing) return incoming;
+
+    const existingSet = new Set(existing.split(' '));
+    for (const s of incoming.split(' ')) {
+        existingSet.add(s);
+    }
+    return [...existingSet].join(' ');
+}
+
 // Default reconnection options for StreamableHTTP connections
 const DEFAULT_STREAMABLE_HTTP_RECONNECTION_OPTIONS: StreamableHTTPReconnectionOptions = {
     initialReconnectionDelay: 1000,
@@ -504,7 +519,7 @@ export class StreamableHTTPClientTransport implements Transport {
 
                     const { resourceMetadataUrl, scope } = extractWWWAuthenticateParams(response);
                     this._resourceMetadataUrl = resourceMetadataUrl;
-                    this._scope = scope;
+                    this._scope = mergeScopes(this._scope, scope);
 
                     const result = await auth(this._authProvider, {
                         serverUrl: this._url,
@@ -537,7 +552,7 @@ export class StreamableHTTPClientTransport implements Transport {
                         }
 
                         if (scope) {
-                            this._scope = scope;
+                            this._scope = mergeScopes(this._scope, scope);
                         }
 
                         if (resourceMetadataUrl) {
diff --git a/packages/client/test/client/streamableHttp.test.ts b/packages/client/test/client/streamableHttp.test.ts
index 8a550feae..c12301faf 100644
--- a/packages/client/test/client/streamableHttp.test.ts
+++ b/packages/client/test/client/streamableHttp.test.ts
@@ -727,6 +727,88 @@ describe('StreamableHTTPClientTransport', () => {
         authSpy.mockRestore();
     });
 
+    it('accumulates scopes across multiple 403 responses for progressive authorization', async () => {
+        const message1: JSONRPCMessage = {
+            jsonrpc: '2.0',
+            method: 'tools/list',
+            params: {},
+            id: 'test-1'
+        };
+        const message2: JSONRPCMessage = {
+            jsonrpc: '2.0',
+            method: 'tools/call',
+            params: {},
+            id: 'test-2'
+        };
+
+        const fetchMock = globalThis.fetch as Mock;
+
+        // Spy on the imported auth function and mock successful authorization
+        const authModule = await import('../../src/client/auth.js');
+        const authSpy = vi.spyOn(authModule, 'auth');
+        authSpy.mockResolvedValue('AUTHORIZED');
+
+        // First request: 403 requesting "mcp:tools:read", then success
+        fetchMock
+            .mockResolvedValueOnce({
+                ok: false,
+                status: 403,
+                statusText: 'Forbidden',
+                headers: new Headers({
+                    'WWW-Authenticate': 'Bearer error="insufficient_scope", scope="mcp:tools:read"'
+                }),
+                text: () => Promise.resolve('Insufficient scope')
+            })
+            .mockResolvedValueOnce({
+                ok: true,
+                status: 202,
+                headers: new Headers()
+            });
+
+        await transport.send(message1);
+
+        // Verify auth was called with the first scope
+        expect(authSpy).toHaveBeenCalledWith(
+            mockAuthProvider,
+            expect.objectContaining({
+                scope: 'mcp:tools:read'
+            })
+        );
+
+        // Reset upscoping circuit breaker for next send
+        fetchMock.mockReset();
+        authSpy.mockClear();
+
+        // Second request: 403 requesting "mcp:tools:write", then success
+        fetchMock
+            .mockResolvedValueOnce({
+                ok: false,
+                status: 403,
+                statusText: 'Forbidden',
+                headers: new Headers({
+                    'WWW-Authenticate': 'Bearer error="insufficient_scope", scope="mcp:tools:write"'
+                }),
+                text: () => Promise.resolve('Insufficient scope')
+            })
+            .mockResolvedValueOnce({
+                ok: true,
+                status: 202,
+                headers: new Headers()
+            });
+
+        await transport.send(message2);
+
+        // Verify auth was called with ACCUMULATED scopes (both read and write)
+        expect(authSpy).toHaveBeenCalledWith(
+            mockAuthProvider,
+            expect.objectContaining({
+                scope: 'mcp:tools:read mcp:tools:write'
+            })
+        );
+
+        authSpy.mockRestore();
+    });
+
     describe('Reconnection Logic', () => {
         let transport: StreamableHTTPClientTransport;