Linear MCP: "Anthropic Proxy: Invalid content from server" when calling get_issue with includeRelations: true

Summary

When a Claude Code Routine (cloud session) calls the Anthropic-managed
Linear connector's mcp__Linear__get_issue tool with
includeRelations: true, the managed proxy returns:

Anthropic Proxy: Invalid content from server

Omitting the flag (or setting it to false, the default) succeeds.
The Linear MCP server's direct endpoint returns valid content for the
includeRelations: true call — the failure is introduced by
Anthropic's proxy layer between the Claude Code cloud container and
the Linear MCP.

Reproduction

• Session ID: session_014q149nJQSe5ARevSoBkLmk
• Exact call: mcp__Linear__get_issue(id: "ALP-549", includeRelations: true)
• Exact response: "Anthropic Proxy: Invalid content from server"

Reproducing with includeRelations omitted (default false):

• Call: mcp__Linear__get_issue(id: "ALP-549")
• Response: valid issue payload (description, status, labels,

attachments, etc. — all fields except relations)

The failure is consistent on retry with the same flag; it is not
transient.

Expected

The proxy should forward the includeRelations: true call to the
upstream Linear MCP and return whatever the upstream returns
(including the relations field).

Impact

Routine sessions (and any interactive cloud Claude Code session using
the managed Linear connector) cannot fetch blockedBy / blocks /
relatedTo in a single call. The workaround — omit the flag, then
follow each relation via separate get_issue calls — adds round-trips
and extra context pressure for what should be a single call.

Diagnosis hypothesis

Most likely a response-size or schema-validation mismatch in the proxy
layer when the relations payload is included. The upstream
mcp.linear.app/mcp endpoint responds correctly when called directly.

Suggested fix

Investigate the proxy's response validation for the Linear
get_issue tool's includeRelations: true variant. If a schema
validator is rejecting the larger payload, relax or update it to
match the upstream Linear MCP's current response shape.