LiteLLM / LiteLLM

LiteLLM MCP Token Forwarding Silently Swallows Upstream 401 Unauthorized Errors

Fix LiteLLM MCP gateway returning empty tools instead of propagating upstream 401 authentication errors Includes evidence for LiteLLM troubleshooting demand.

Source-backed Last updated May 13, 2026 1 source Needs local verification

Category: LiteLLM
Error signature: LiteLLM returns 200 {"tools":[]} instead of upstream 401 for token-forwarding MCP servers
Quick fix: Verify the account session, API key, provider settings, and environment where the failing tool is running.
Updated: May 13, 2026
Verification status: Source-backed
Evidence: 1 public source URL

Before you change production

This page includes public source URLs in the imported troubleshooting record. Compare those references with your version and environment before applying changes.

Reproduce the smallest failing action and save non-secret logs before changing configuration.
Check versions for LiteLLM, related SDKs, package managers, CI runners, and hosting providers.
Change one setting or dependency at a time, then rerun the same failing command or request.
Avoid destructive commands, credential rotation, billing changes, or security relaxations without a rollback plan.

What this error means

LiteLLM returns 200 {"tools":[]} instead of upstream 401 for token-forwarding MCP servers is a LiteLLM failure pattern reported for developers trying to fix litellm mcp gateway returning empty tools instead of propagating upstream 401 authentication errors. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

LiteLLM PR #27847 fixes MCP SDK StreamableHTTPSessionManager silently swallowing 401 errors. For MCP servers configured with extra_headers: [Authorization], the gateway was returning 200 {tools:[]} instead of propagating upstream 401. This is a critical debugging blocker for OAuth passthrough setups.

Common causes

When LiteLLM forwards client Bearer tokens to upstream OAuth2 MCP servers, expired or invalid tokens should return 401 — but LiteLLM silently swallows the error and returns 200 with empty tools. This makes debugging authentication failures extremely difficult in production MCP deployments.
LiteLLM PR #27847 fixes MCP SDK StreamableHTTPSessionManager silently swallowing 401 errors. For MCP servers configured with extra_headers: [Authorization], the gateway was returning 200 {tools:[]} instead of propagating upstream 401. This is a critical debugging blocker for OAuth passthrough setups.

Quick fixes

Confirm the exact error signature matches LiteLLM returns 200 {"tools":[]} instead of upstream 401 for token-forwarding MCP servers.
Check the LiteLLM account, local tool state, and provider configuration involved in the failing workflow.
Verify the account session, API key, provider settings, and environment where the failing tool is running.

Platform/tool-specific checks

Verify the command, editor, extension, or API client that produced the error.
Compare local settings with CI, deployment, or editor-level settings when the error appears in only one environment.
Avoid deleting credentials, local model data, or project settings until the failing scope is clear.

Step-by-step troubleshooting

Capture the exact error message and the command, editor action, or request that triggered it.
Check whether the failure is account/auth, quota/rate, model/provider, local runtime, or deployment configuration.
Review the source evidence below and compare it with your environment.
Apply one change at a time and rerun the smallest failing action.
Keep the working fix documented for the team or deployment environment.

How to prevent it

Keep provider/tool configuration documented.
Record non-secret diagnostics such as tool version, provider name, model name, and command path.
Add a lightweight check before CI or production workflows depend on the tool.

Diagnostic flow for this page

Match LiteLLM returns 200 {"tools":[]} instead of upstream 401 for token-forwarding MCP servers exactly before applying the quick fix.
Compare the failing environment with LiteLLM versions, account scope, provider settings, and deployment context.
Check the listed common causes in order, starting with the cause that best matches your logs.
Use the evidence status below to decide whether to confirm against public sources or official documentation.
Apply one reversible change, rerun the smallest failing action, and keep rollback notes.

FAQ

What does LiteLLM returns 200 {"tools":[]} instead of upstream 401 for token-forwarding MCP servers mean in LiteLLM?

LiteLLM returns 200 {"tools":[]} instead of upstream 401 for token-forwarding MCP servers indicates the workflow described by "LiteLLM MCP Token Forwarding Silently Swallows Upstream 401 Unauthorized Errors" failed in a LiteLLM context. Start by confirming the exact signature and the component that emitted it.

What should I check first for LiteLLM returns 200 {"tools":[]} instead of upstream 401 for token-forwarding MCP servers?

Check whether this cause matches your environment: When LiteLLM forwards client Bearer tokens to upstream OAuth2 MCP servers, expired or invalid tokens should return 401 — but LiteLLM silently swallows the error and returns 200 with empty tools. This makes debugging authentication failures extremely difficult in production MCP deployments.

Is the LiteLLM returns 200 {"tools":[]} instead of upstream 401 for token-forwarding MCP servers guidance source-backed?

The "LiteLLM MCP Token Forwarding Silently Swallows Upstream 401 Unauthorized Errors" page is labeled source-backed. It has 1 public source URL from github.com, so use that evidence status when deciding how much additional verification you need.

When should I avoid the quick fix for LiteLLM returns 200 {"tools":[]} instead of upstream 401 for token-forwarding MCP servers?

For "LiteLLM MCP Token Forwarding Silently Swallows Upstream 401 Unauthorized Errors", avoid applying "Verify the account session, API key, provider settings, and environment where the failing tool is running." when the error text differs, the affected version or provider is different, or the change would touch credentials, billing, DNS, production deploy settings, or irreversible data.

What should I collect before debugging LiteLLM returns 200 {"tools":[]} instead of upstream 401 for token-forwarding MCP servers?

For "LiteLLM MCP Token Forwarding Silently Swallows Upstream 401 Unauthorized Errors", collect the failing LiteLLM command or request, non-secret configuration, versions, timestamps, request IDs when available, and the smallest reproduction that still triggers the signature.