From 6288662d1b0e637b2d9054621d700f27baec4bec Mon Sep 17 00:00:00 2001
From: Anneshu Nag <nkworks777@gmail.com>
Date: Wed, 14 Oct 2026 19:07:08 +0530
Subject: [PATCH] fix(proxy): accept 2048-char OAuth state in MCP gateway DCR
 flow

Gemini on the web sends an 1112-character state which the 1024 cap
rejected with a 400. RFC 6749 sets no maximum for state. A 2048-char
state still seals into the per-flow cookie inside the ~4KB budget.
---
 .../mcp_server/gateway_dcr_flow.py            |  9 ++--
 .../mcp_server/test_gateway_dcr_flow.py       | 46 ++++++++++++++++++-
 2 files changed, 51 insertions(+), 4 deletions(-)

diff --git a/litellm/proxy/_experimental/mcp_server/gateway_dcr_flow.py b/litellm/proxy/_experimental/mcp_server/gateway_dcr_flow.py
index a895dfed162..07f03381a4c 100644
--- a/litellm/proxy/_experimental/mcp_server/gateway_dcr_flow.py
+++ b/litellm/proxy/_experimental/mcp_server/gateway_dcr_flow.py
@@ -141,10 +141,13 @@ every session-token claim set. Four 256-character ASCII URIs seal to roughly 1.5
 the encoded client_id is checked against its own cap before registration succeeds.
 VS Code registers four callbacks for its web and desktop environments."""
 
-MAX_STATE_LENGTH: Final = 1024
+MAX_STATE_LENGTH: Final = 2048
 """Bound on the client ``state`` sealed into the flow cookie and echoed on the auth-code
-redirect. An unbounded ``state`` can push the sealed cookie past the browser's ~4KB cap
-(silently dropped, breaking the flow); spec clients send a short opaque value."""
+redirect. RFC 6749 sets no maximum: ``state`` is opaque and must round-trip unchanged,
+and deployed clients send values past 1024 characters (Gemini on the web sends 1112).
+An unbounded ``state`` can still push the sealed cookie past the browser's ~4KB cap
+(silently dropped, breaking the flow); a 2048-character ``state`` alongside typical
+client_id and redirect_uri values seals to roughly 3.5KB, inside that budget."""
 
 MIN_CODE_VERIFIER_LENGTH: Final = 43
 MAX_CODE_VERIFIER_LENGTH: Final = 128
diff --git a/tests/unit/proxy/_experimental/mcp_server/test_gateway_dcr_flow.py b/tests/unit/proxy/_experimental/mcp_server/test_gateway_dcr_flow.py
index c79b2efde5e..d7ef2026da4 100644
--- a/tests/unit/proxy/_experimental/mcp_server/test_gateway_dcr_flow.py
+++ b/tests/unit/proxy/_experimental/mcp_server/test_gateway_dcr_flow.py
@@ -21,6 +21,7 @@ from litellm.proxy._experimental.mcp_server.gateway_dcr_flow import (
     GATEWAY_AUTH_CODE_TTL_SECONDS,
     MANUAL_DELIVERY_AUTH_CODE_TTL_SECONDS,
     MAX_CLIENT_ID_LENGTH,
+    MAX_STATE_LENGTH,
     SUBJECT_TOKEN_TYPES,
     TOKEN_EXCHANGE_GRANT_TYPE,
     ConsentTeam,
@@ -292,6 +293,49 @@ async def test_authorize_validation_failures_never_redirect_to_client():
         assert json.loads(response.body)["error"] == expected_error
 
 
+@pytest.mark.asyncio
+async def test_authorize_accepts_gemini_length_state_and_echoes_it_back():
+    """Gemini on the web sends an 1112-character state (issue #46414): authorize must
+    accept it and the code redirect must echo it back unchanged."""
+    client_id = (await _register([REDIRECT_URI]))["client_id"]
+    long_state: Final = "s" * 1112
+    response = aggregate_authorize(
+        request=_request(query=f"client_id={client_id}"),
+        client_id=client_id,
+        redirect_uri=REDIRECT_URI,
+        state=long_state,
+        code_challenge=CODE_CHALLENGE,
+        code_challenge_method="S256",
+        response_type="code",
+        session_user_id="u1",
+    )
+    assert response.status_code == 303
+    handle, cookies = _flow_cookie_from(response)
+    assert len(next(iter(cookies.values()))) < 4096
+    completed = await complete_connect_flow(
+        request=_request("/authorize/complete", cookies=cookies, method="POST"),
+        flow_handle=handle,
+        session_user_id="u1",
+        cache=DualCache(),
+    )
+    assert completed.status_code == 303
+    returned = parse_qs(urlparse(completed.headers["location"]).query)
+    assert returned["state"] == [long_state]
+
+    over_cap = aggregate_authorize(
+        request=_request(query=f"client_id={client_id}"),
+        client_id=client_id,
+        redirect_uri=REDIRECT_URI,
+        state="s" * (MAX_STATE_LENGTH + 1),
+        code_challenge=CODE_CHALLENGE,
+        code_challenge_method="S256",
+        response_type="code",
+        session_user_id="u1",
+    )
+    assert over_cap.status_code == 400
+    assert json.loads(over_cap.body)["error"] == "invalid_request"
+
+
 @pytest.mark.asyncio
 async def test_authorize_without_session_redirects_to_login_with_return_to():
     client_id = (await _register([REDIRECT_URI]))["client_id"]
@@ -616,7 +660,7 @@ async def test_authorize_rejects_over_long_state():
         request=_request(query=f"client_id={client_id}"),
         client_id=client_id,
         redirect_uri=REDIRECT_URI,
-        state="s" * 2000,
+        state="s" * (MAX_STATE_LENGTH + 1),
         code_challenge=CODE_CHALLENGE,
         code_challenge_method="S256",
         response_type="code",