### What happened?

[BUG] Claude.ai Web Client Fails to Execute tools/list for Custom MCP Connectors — Mobile App Works Correctly

Labels: bug area:mcp has repro platform:web

---

Summary

Custom MCP connectors registered in Settings > Connectors show as "Connected" in the web client, but expose zero tools in any conversation. The tools/list request is silently never sent by the web client. The exact same connectors work correctly on the Claude mobile app (iOS), confirming the servers are healthy and the issue is client-side.

This issue has been intermittent since approximately mid-February 2026 (coinciding with the Cowork/Customize menu UI changes) and has been permanently broken on web for approximately the last two weeks, with no resolution from any client-side remediation steps.

---

Environment

Field | Value
-- | --
Platform affected | Claude.ai web (Chrome, confirmed multiple browsers)
Platform working | Claude iOS mobile app
Plan | Pro
Issue onset | ~mid-February 2026 (intermittent); ~late February 2026 (permanent)
MCP server infrastructure | Custom servers on AWS EC2, proxied via Caddy
MCP transport | streamable-http
Protocol version | 2024-11-05
Number of custom connectors | 10 (Chronicler, Majeston, Indexer, Shell-MCP, Servital, Batch Dispatcher, GQW, Caliper, SoloniaScraper, Sanobim)

---

Key Observation: Mobile vs. Web Code Path Divergence

The mobile app and web client appear to use fundamentally different code paths for MCP tool discovery. The mobile client successfully executes the full initialize → tools/list sequence. The web client completes initialize but silently skips tools/list.

This is consistent with Issue #11146, which documents undocumented architectural divergence between web-containerized and native session modes, and with the November 2025 SSE incident where mobile continued to work while web/Desktop broke.

---

Impact

This issue has eliminated the ability to run parallel work branches simultaneously on web, which was a core part of the affected workflow. The user is now limited to single-branch sessions via mobile app. This represents a significant regression from functionality that was stable for several months prior.

---

Related Issues

• Issue #1675 — modelcontextprotocol/modelcontextprotocol: Tools not visible despite successful initialization


• Issue #11146 — anthropics/claude-code: Web session mode lacks MCP server access


• Issue #14807 — anthropics/claude-code: MCP tools stopped working, server connected, no errors


• Atlassian Community thread (Nov 2025): Integration broken on web/Desktop, mobile works

---

Request

1. Investigate why the web client is not issuing tools/list after a successful initialize handshake


2. Clarify the documented difference in MCP tool discovery behavior between the web client and mobile app


3. Provide a timeline for resolution or a supported workaround for web

[BUG] Claude.ai Web Client Fails to Execute tools/list for Custom MCP Connectors — Mobile App Works Correctly
Labels: bug area:mcp has repro platform:web

Summary
Custom MCP connectors registered in Settings > Connectors show as "Connected" in the web client, but expose zero tools in any conversation. The tools/list request is silently never sent by the web client. The exact same connectors work correctly on the Claude mobile app (iOS), confirming the servers are healthy and the issue is client-side.
This issue has been intermittent since approximately mid-February 2026 (coinciding with the Cowork/Customize menu UI changes) and has been permanently broken on web for approximately the last two weeks, with no resolution from any client-side remediation steps.

Environment

Field
Value

Platform affected
Claude.ai web (Chrome, confirmed multiple browsers)

Platform working
Claude iOS mobile app

Plan
Pro

Issue onset
~mid-February 2026 (intermittent); ~late February 2026 (permanent)

MCP server infrastructure
Custom servers on AWS EC2, proxied via Caddy

MCP transport
streamable-http

Protocol version
2024-11-05

Number of custom connectors
10 (Chronicler, Majeston, Indexer, Shell-MCP, Servital, Batch Dispatcher, GQW, Caliper, SoloniaScraper, Sanobim)

Reproduction Steps

Add a custom MCP connector via Settings > Connectors > Add custom connector with a valid streamable-http server URL
Confirm the connector shows as "Connected" (green indicator) in Settings
Open a new conversation on Claude.ai web
Observe: no tools from the connector are available; tool_search returns empty or stale results
Open the same conversation on the Claude iOS mobile app
Observe: all tools load and function correctly with no configuration changes

Technical Detail
Server-side handshake is valid. Direct curl test confirms the MCP initialize handshake succeeds and returns a well-formed response:
curl -s -H "Accept: application/json, text/event-stream" \
-H "Content-Type: application/json" \
-X POST https://mcp.example.com/observe/mcp \
-d '{"jsonrpc":"2.0","method":"initialize","params":{"protocolVersion":"2024-11-05","capabilities":{},"clientInfo":{"name":"test","version":"1.0"}},"id":1}'

# Response:
event: message
data: {"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2024-11-05","capabilities":{"tools":{"listChanged":false}},"serverInfo":{"name":"Sanobim","version":"1.26.0"}}}

The web client never sends tools/list. Server-side access logs show the web client completes the initialize handshake but does not follow up with a tools/list request. The mobile client does send tools/list and receives the full tool manifest.
Accept header fix applied but insufficient. In S092 (late February 2026), it was discovered that the web client was returning a -32600 JSON-RPC error due to missing Accept headers. A Caddy header_up Accept "application/json, text/event-stream" injection was applied to all routes. This resolved the -32600 error but did not restore tools/list behavior on web — suggesting the web client has a second, independent failure in its tool discovery flow.

Remediation Attempts (All Failed for Web)

Action
Result

Clear browser cache and cookies for claude.ai
Temporarily resolved in S070 (Feb 26); no longer effective

Sign out and sign back in
No effect

Remove all connectors, re-add fresh
Connectors reconnect successfully; tools still not visible on web

Toggle connectors off/on in Customize panel
Previously worked in S067; no longer effective

Try different browsers (Chrome, Brave, Firefox)
Issue persists across all browsers

Try mobile web (browser on iOS)
Does not work — issue is specific to the native mobile app

Add Accept header injection at Caddy proxy layer
Resolved -32600 handshake error; did not restore tool visibility

Start fresh conversation after each remediation
No effect

Key Observation: Mobile vs. Web Code Path Divergence
The mobile app and web client appear to use fundamentally different code paths for MCP tool discovery. The mobile client successfully executes the full initialize → tools/list sequence. The web client completes initialize but silently skips tools/list.
This is consistent with [Issue #11146](https://github.com/anthropics/claude-code/issues/11146), which documents undocumented architectural divergence between web-containerized and native session modes, and with the November 2025 SSE incident where mobile continued to work while web/Desktop broke.

Impact
This issue has eliminated the ability to run parallel work branches simultaneously on web, which was a core part of the affected workflow. The user is now limited to single-branch sessions via mobile app. This represents a significant regression from functionality that was stable for several months prior.

Related Issues

Issue #1675 — modelcontextprotocol/modelcontextprotocol: Tools not visible despite successful initialization
Issue #11146 — anthropics/claude-code: Web session mode lacks MCP server access
Issue #14807 — anthropics/claude-code: MCP tools stopped working, server connected, no errors
Atlassian Community thread (Nov 2025): Integration broken on web/Desktop, mobile works

Request

Investigate why the web client is not issuing tools/list after a successful initialize handshake
Clarify the documented difference in MCP tool discovery behavior between the web client and mobile app
Provide a timeline for resolution or a supported workaround for web

### What did you expect to happen?

MCP tools connect as normal in Claude web.

### Steps to reproduce

Attempt to use custom MCP connectors on Claude web.

### Area

MCP Connector (adding/managing servers)

### MCP Server (if applicable)

_No response_

### Error messages or logs

```shell

```

### Additional context

_No response_