docs: add customSchemas documentation and clarify domain uppercase constraint

[Copilot]

Two undocumented behaviors in the JSON stdin config format that would leave users confused or unable to discover features.

Changes

• customSchemas field: Added a new subsection under Server Configuration Fields documenting the top-level customSchemas map, which allows defining custom server types beyond stdio/http. Includes JSON example and validation rules (reserved type names blocked, HTTPS schema URLs required).

{
  "customSchemas": {
    "myCustomType": "https://example.com/schemas/my-custom-type.json"
  },
  "mcpServers": {
    "myServer": { "type": "myCustomType" }
  }
}

• domain uppercase constraint: Added a note to the domain field description clarifying that variable expressions only accept uppercase names — ${MY_VAR} is valid, ${my_var} is silently rejected with a validation error. Reflects the actual pattern ^\$\{[A-Z_][A-Z0-9_]*\}$.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• example.com
  • Triggering command: /tmp/go-build1462136783/b298/launcher.test /tmp/go-build1462136783/b298/launcher.test -test.testlogfile=/tmp/go-build1462136783/b298/testlog.txt -test.paniconexit0 -test.timeout=10m0s conf�� 64/src/runtime/cgo transform/transform.go x_amd64/compile (dns block)
• invalid-host-that-does-not-exist-12345.com
  • Triggering command: /tmp/go-build1462136783/b280/config.test /tmp/go-build1462136783/b280/config.test -test.testlogfile=/tmp/go-build1462136783/b280/testlog.txt -test.paniconexit0 -test.timeout=10m0s ortc�� 64/src/runtime/cgo lkem/cast.go x_amd64/compile user.email (dns block)
• nonexistent.local
  • Triggering command: /tmp/go-build1462136783/b298/launcher.test /tmp/go-build1462136783/b298/launcher.test -test.testlogfile=/tmp/go-build1462136783/b298/testlog.txt -test.paniconexit0 -test.timeout=10m0s conf�� 64/src/runtime/cgo transform/transform.go x_amd64/compile (dns block)
• slow.example.com
  • Triggering command: /tmp/go-build1462136783/b298/launcher.test /tmp/go-build1462136783/b298/launcher.test -test.testlogfile=/tmp/go-build1462136783/b298/testlog.txt -test.paniconexit0 -test.timeout=10m0s conf�� 64/src/runtime/cgo transform/transform.go x_amd64/compile (dns block)
• this-host-does-not-exist-12345.com
  • Triggering command: /tmp/go-build1462136783/b307/mcp.test /tmp/go-build1462136783/b307/mcp.test -test.testlogfile=/tmp/go-build1462136783/b307/testlog.txt -test.paniconexit0 -test.timeout=10m0s conf�� 64/src/runtime/cgo dna/go118.go x_amd64/compile (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

---

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[Copilot]

Pull request overview

This documentation-only PR adds two previously undocumented configuration behaviors to the README: the customSchemas top-level field for defining custom server types, and the uppercase-only constraint on variable expressions in the domain gateway field.

Changes:

• Added a new #### Custom Schemas subsection documenting the customSchemas top-level config field, including a JSON example and validation rules (reserved type blocking, HTTPS-only schema URLs, helpful error for unregistered types).
• Added a clarifying note to the domain field that variable expressions only accept uppercase names (${MY_VAR}), reflecting the regex pattern ^\$\{[A-Z_][A-Z0-9_]*\}$.

---

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[Copilot]

The new "Validation Rules for customSchemas:" at line 234 sits immediately above the pre-existing "Validation Rules:" at line 239, which can be confusing to readers since both are bold-text headers about validation rules. Consider renaming one of them to make the distinction clearer — for example, changing line 234 to something like "customSchemas constraints:" or moving the customSchemas validation rules into the existing "Validation Rules" section as a sub-bullet.

Suggested change

	**Validation Rules for `customSchemas`:**
	**`customSchemas` constraints:**