diff --git a/src/mcp/client/auth/oauth2.py b/src/mcp/client/auth/oauth2.py
index 7f5af5186..5e49c526c 100644
--- a/src/mcp/client/auth/oauth2.py
+++ b/src/mcp/client/auth/oauth2.py
@@ -34,6 +34,7 @@
     handle_registration_response,
     handle_token_response_scopes,
     is_valid_client_metadata_url,
+    merge_scopes,
     should_use_client_metadata_url,
 )
 from mcp.client.streamable_http import MCP_PROTOCOL_VERSION
@@ -570,12 +571,13 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
                         else:
                             logger.debug(f"OAuth metadata discovery failed: {url}")
 
-                    # Step 3: Apply scope selection strategy
-                    self.context.client_metadata.scope = get_client_metadata_scopes(
+                    # Step 3: Apply scope selection strategy, merging with existing scopes
+                    new_scope = get_client_metadata_scopes(
                         extract_scope_from_www_auth(response),
                         self.context.protected_resource_metadata,
                         self.context.oauth_metadata,
                     )
+                    self.context.client_metadata.scope = merge_scopes(self.context.client_metadata.scope, new_scope)
 
                     # Step 4: Register client or use URL-based client ID (CIMD)
                     if not self.context.client_info:
@@ -619,10 +621,11 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
                 # Step 2: Check if we need to step-up authorization
                 if error == "insufficient_scope":  # pragma: no branch
                     try:
-                        # Step 2a: Update the required scopes
-                        self.context.client_metadata.scope = get_client_metadata_scopes(
+                        # Step 2a: Update the required scopes, merging with existing
+                        new_scope = get_client_metadata_scopes(
                             extract_scope_from_www_auth(response), self.context.protected_resource_metadata
                         )
+                        self.context.client_metadata.scope = merge_scopes(self.context.client_metadata.scope, new_scope)
 
                         # Step 2b: Perform (re-)authorization and token exchange
                         token_response = yield await self._perform_authorization()
diff --git a/src/mcp/client/auth/utils.py b/src/mcp/client/auth/utils.py
index 0ca36b98d..3d7a517ec 100644
--- a/src/mcp/client/auth/utils.py
+++ b/src/mcp/client/auth/utils.py
@@ -119,6 +119,23 @@ def get_client_metadata_scopes(
         return None
 
 
+def merge_scopes(existing: str | None, incoming: str | None) -> str | None:
+    """Merge OAuth scopes by computing the union of space-delimited scope strings.
+
+    Per RFC 6749 §3.3, scopes are space-delimited, case-sensitive strings.
+    This prevents the infinite re-authorization loop that occurs when a server
+    uses per-operation scopes and the client overwrites previously-granted scopes.
+    """
+    if not incoming:
+        return existing
+    if not existing:
+        return incoming
+
+    existing_set = set(existing.split())
+    existing_set.update(incoming.split())
+    return " ".join(sorted(existing_set))
+
+
 def build_oauth_authorization_server_metadata_discovery_urls(auth_server_url: str | None, server_url: str) -> list[str]:
     """Generate an ordered list of URLs for authorization server metadata discovery.
 
diff --git a/tests/client/test_auth.py b/tests/client/test_auth.py
index 5aa985e36..da8c9435b 100644
--- a/tests/client/test_auth.py
+++ b/tests/client/test_auth.py
@@ -1327,7 +1327,7 @@ async def test_403_insufficient_scope_updates_scope_from_header(
         mock_storage: MockTokenStorage,
         valid_tokens: OAuthToken,
     ):
-        """Test that 403 response correctly updates scope from WWW-Authenticate header."""
+        """Test that 403 response correctly accumulates scope from WWW-Authenticate header."""
         # Pre-store valid tokens and client info
         client_info = OAuthClientInformationFull(
             client_id="test_client_id",
@@ -1350,10 +1350,10 @@ async def test_403_insufficient_scope_updates_scope_from_header(
         async def capture_redirect(url: str) -> None:
             nonlocal redirect_captured, captured_state
             redirect_captured = True
-            # Verify the new scope is included in authorization URL
-            assert "scope=admin%3Awrite+admin%3Adelete" in url or "scope=admin:write+admin:delete" in url.replace(
-                "%3A", ":"
-            ).replace("+", " ")
+            # Verify the accumulated scopes are included (original + new)
+            decoded = url.replace("%3A", ":").replace("+", " ")
+            for s in ["admin:write", "admin:delete", "read", "write"]:
+                assert s in decoded, f"Expected scope '{s}' in URL"
             # Extract state from redirect URL
             parsed = urlparse(url)
             params = parse_qs(parsed.query)
@@ -1383,8 +1383,9 @@ async def mock_callback() -> tuple[str, str | None]:
         # Trigger step-up - should get token exchange request
         token_exchange_request = await auth_flow.asend(response_403)
 
-        # Verify scope was updated
-        assert oauth_provider.context.client_metadata.scope == "admin:write admin:delete"
+        # Verify scope was accumulated (original "read write" + new "admin:write admin:delete")
+        accumulated = set(oauth_provider.context.client_metadata.scope.split())
+        assert accumulated == {"admin:delete", "admin:write", "read", "write"}
         assert redirect_captured
 
         # Complete the flow with successful token response
@@ -2264,3 +2265,50 @@ async def callback_handler() -> tuple[str, str | None]:
             await auth_flow.asend(final_response)
         except StopAsyncIteration:
             pass
+
+
+class TestMergeScopes:
+    """Tests for merge_scopes utility function."""
+
+    def test_merge_none_existing_returns_incoming(self):
+        from mcp.client.auth.utils import merge_scopes
+
+        assert merge_scopes(None, "mcp:tools:read") == "mcp:tools:read"
+
+    def test_merge_none_incoming_returns_existing(self):
+        from mcp.client.auth.utils import merge_scopes
+
+        assert merge_scopes("init", None) == "init"
+
+    def test_merge_both_none_returns_none(self):
+        from mcp.client.auth.utils import merge_scopes
+
+        assert merge_scopes(None, None) is None
+
+    def test_merge_disjoint_scopes(self):
+        from mcp.client.auth.utils import merge_scopes
+
+        result = merge_scopes("init", "mcp:tools:read")
+        assert result is not None
+        scopes = set(result.split())
+        assert scopes == {"init", "mcp:tools:read"}
+
+    def test_merge_overlapping_scopes_deduplicates(self):
+        from mcp.client.auth.utils import merge_scopes
+
+        result = merge_scopes("init mcp:tools:read", "mcp:tools:read mcp:tools:write")
+        assert result is not None
+        scopes = set(result.split())
+        assert scopes == {"init", "mcp:tools:read", "mcp:tools:write"}
+
+    def test_merge_identical_scopes(self):
+        from mcp.client.auth.utils import merge_scopes
+
+        result = merge_scopes("init", "init")
+        assert result == "init"
+
+    def test_merge_empty_strings(self):
+        from mcp.client.auth.utils import merge_scopes
+
+        assert merge_scopes("init", "") == "init"
+        assert merge_scopes("", "init") == "init"