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Accept omitted params per JSON-RPC 2.0 §4 - #106
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Per JSON-RPC 2.0 §4 the `params` member MAY be omitted. The dispatcher now normalizes an absent or null `params` to an empty object before running the endpoint codec, so parameterless methods are reachable from spec-compliant clients (e.g. MCP TypeScript SDK calling `tools/list`).
Push the empty-params default down from `MessageDispatcher` into
`RawMessage.toMessage`, so absent/null `params` is normalized to `{}`
as the JSON message is decoded. The dispatcher keeps its original
pattern match; the `None` branches are now only reachable via direct
programmatic construction.
Tests parse messages from raw JSON to exercise the wire-layer fix.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Replace `params: Option[Payload]` on `RequestMessage` and
`NotificationMessage` with `params: Payload`, introducing a
`Payload.Empty` (= `{}`) sentinel. The wire decoder normalizes
absent/null params to `Payload.Empty`; the encoder elides the field
when it equals `Payload.Empty` (per spec, omitted and empty-object
params are equivalent).
This removes the dead "Missing payload" branches from the dispatcher
and the wire-vs-programmatic construction inconsistency.
Binary-breaking: case class signatures changed.
ghostbuster91
marked this pull request as ready for review
May 27, 2026 12:04
kubukoz
approved these changes
May 27, 2026
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Fixes #104.
Problem
Per JSON-RPC 2.0 §4 the
paramsmember MAY be omitted. The dispatcherrejected any request whose
paramsfield was absent (ornull) with-32600 InvalidRequest "Missing payload", making spec-compliant clientsunable to call parameterless methods (e.g. the official MCP TypeScript
SDK calling
tools/list).Approach
Payload.Emptysentinel =Payload(Json.obj()).RequestMessage.params/NotificationMessage.paramschange fromOption[Payload]toPayload—Noneis no longer expressible.RawMessage.toMessagenormalizes an absent/null wireparamsto
Payload.Empty.RawMessage.fromelides theparamsfield on the wire when itequals
Payload.Empty(spec treats omitted and{}as equivalent).MessageDispatcherdrops the now-impossibleNonebranches —the pattern match shrinks from 6 cases to 4.
The earlier commits in this PR are intermediate iterations of the same
fix; happy to squash before merging.
Binary compatibility
This is a binary-breaking change to public case-class signatures
(
InputMessage.RequestMessage/NotificationMessage). MIMA isn'tconfigured with previous artifacts so CI won't catch it — worth calling
out in release notes / version bump.
Test plan
core/testandcore3/test—RawMessageSpeccovers wire-layerdecoding (omitted / null / notification) and serialization elision.
fs2/testandfs23/test— end-to-end tests viaFS2Channelparsing raw JSON for the three relevant shapes.
smithy4sTests/testandsmithy4sTests3/test— existing suitestill green.
scalafmtCheckAll.🤖 Generated with Claude Code