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claude-code - 💡(How to fix) Fix MCP connections return 503 DNS cache overflow from sandbox egress proxy (web/mobile) [1 participants]

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anthropics/claude-code#52914•Fetched 2026-04-25 06:17:33
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MCP tool connections from Claude Code web/mobile sessions fail with HTTP 503. The TLS certificate on the connection reveals traffic is being intercepted by Anthropic's own egress proxy (sandbox-egress-production TLS Inspection CA), and the HTTP response body is DNS cache overflow. The upstream MCP server (AWS Lambda function URL) is confirmed healthy and returns correct responses when accessed directly.

Root Cause

MCP tool connections from Claude Code web/mobile sessions fail with HTTP 503. The TLS certificate on the connection reveals traffic is being intercepted by Anthropic's own egress proxy (sandbox-egress-production TLS Inspection CA), and the HTTP response body is DNS cache overflow. The upstream MCP server (AWS Lambda function URL) is confirmed healthy and returns correct responses when accessed directly.

Code Example

< HTTP/2 503
< content-type: text/plain
DNS cache overflow

* SSL cert issuer: O=Anthropic; CN=sandbox-egress-production TLS Inspection CA
RAW_BUFFERClick to expand / collapse

Description

MCP tool connections from Claude Code web/mobile sessions fail with HTTP 503. The TLS certificate on the connection reveals traffic is being intercepted by Anthropic's own egress proxy (sandbox-egress-production TLS Inspection CA), and the HTTP response body is DNS cache overflow. The upstream MCP server (AWS Lambda function URL) is confirmed healthy and returns correct responses when accessed directly.

Steps to reproduce

  1. Configure an MCP server using a Lambda function URL (or any external HTTPS endpoint)
  2. Connect via Claude Code web or mobile
  3. Observe 503 responses from MCP tool calls

Expected behavior

MCP calls reach the upstream server and return valid responses.

Actual behavior

HTTP 503 with body DNS cache overflow returned by the sandbox egress proxy before the request reaches the upstream server.

Evidence

< HTTP/2 503
< content-type: text/plain
DNS cache overflow

* SSL cert issuer: O=Anthropic; CN=sandbox-egress-production TLS Inspection CA

Impact

All MCP servers requiring outbound HTTPS are broken from web/mobile Claude Code. Claude Code desktop CLI is unaffected (bypasses the sandbox proxy and connects directly).

Environment

  • Claude Code web/mobile (sandbox-hosted)
  • MCP transport: Streamable HTTP
  • Upstream: AWS Lambda function URL (us-east-2)
  • Source IP: 160.79.106.35

extent analysis

TL;DR

The issue can likely be resolved by adjusting the DNS cache settings or implementing a workaround to bypass the sandbox egress proxy for MCP tool connections.

Guidance

  • Investigate the DNS cache configuration on the sandbox egress proxy to determine if there are any limits or restrictions that could be causing the overflow.
  • Consider implementing a temporary workaround, such as using a different DNS resolver or caching mechanism, to bypass the issue until a permanent fix can be applied.
  • Verify that the upstream MCP server (AWS Lambda function URL) is correctly configured and responding as expected when accessed directly.
  • Review the Claude Code web and mobile session configurations to ensure that they are properly set up to use the MCP server and not relying on any cached DNS entries.

Example

No code snippet is provided as the issue appears to be related to network configuration and DNS caching rather than code.

Notes

The issue seems to be specific to the sandbox egress proxy and its DNS caching mechanism. The fact that the Claude Code desktop CLI is unaffected suggests that the issue is related to the proxy and not the MCP server or Lambda function URL.

Recommendation

Apply a workaround, such as using a different DNS resolver or caching mechanism, to bypass the sandbox egress proxy for MCP tool connections. This is recommended because the issue appears to be related to the proxy's DNS caching mechanism, and adjusting or bypassing this mechanism may resolve the issue.

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FAQ

Expected behavior

MCP calls reach the upstream server and return valid responses.

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claude-code - 💡(How to fix) Fix MCP connections return 503 DNS cache overflow from sandbox egress proxy (web/mobile) [1 participants]