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Sentry MCP server fails to initialize: cannot unmarshal JSON Schema array union type into string #14
Description
Summary
mcpx v1.5.0 fails to initialize the @sentry/mcp-server with the following error:
error: server "sentry": initialize: mcp: initialization failed: mcp: transport closed
Running with verbose output reveals the root cause:
json: cannot unmarshal array into Go struct field PropertySchema.tools.inputSchema.properties.type of type string
Root Cause
The Sentry MCP server uses JSON Schema draft-07 union types where type is an array of strings for optional/nullable fields:
{
"type": ["string", "null"],
"description": "..."
}mcpx's Go JSON unmarshaler maps inputSchema.properties.*.type to type string, which cannot hold an array value. The spec allows both "type": "string" and "type": ["string", "null"].
Expected behavior
mcpx should handle type as either a string or []string (union) per the JSON Schema spec. Unknown or unsupported types could fall back to "any" / untyped rather than hard-failing initialization.
Reproduction
- Add to
~/.mcpx/config.yml:sentry: command: npx args: ["-y", "@sentry/mcp-server"] transport: stdio
- Authenticate:
npx -y @sentry/mcp-server auth login - Run:
mcpx list sentry -v
Result: initialization error as above.
Environment
- mcpx version: 1.5.0 (brew, codestz/tap)
@sentry/mcp-serverversion: latest (npx -y)- OS: macOS 25.3.0 (Darwin)
Workaround
Use the Sentry REST API directly with the OAuth token cached at ~/.sentry/mcp.json by the MCP server's device code flow.
Activity
fix: dashboard banner fires only on real tool calls
Codestz commentedon May 3, 2026
Fixed in v1.6.0 (released today, just merged via #15).
The schema normalizer in
internal/mcp/schema.gonow handles JSON Schema union types correctly:type: ["string", "null"]is parsed asType: "string", Nullable: true, matching the JSON Schema draft-07 spec. The same pass also handlesoneOf,anyOf,allOf, and$ref, with unknown keywords preserved in anExtmap for round-trip integrity.Validated end-to-end against the live
@sentry/mcp-serveraftersentry-mcp auth login:All 22 Sentry tools list cleanly — no transport-closed, no unmarshal errors.
Auto-closed by the PR merge. Thanks for the precise reproducer with the verbose log — it made the diagnosis straightforward and the fix is now covered by unit tests (
TestSentryUnionType,TestSentryFullToolSchemaininternal/mcp/schema_test.go) so this can't regress silently.Upgrade:
brew upgrade mcpxonce the v1.6.0 release artifacts publish (Homebrew tap auto-updates from the goreleaser workflow), orgo install github.com/codestz/mcpx/cmd/mcpx@v1.6.0.