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feat: generate JSON Schema for tool inputs from proto descriptors (protojson-compatible) #88
Description
Part of #44. The official SDK validates tool arguments against the tool's input schema (mark3labs did not), so the schemas must accurately describe what protojson.Unmarshal accepts. Today's schemas don't: arguments are nested under the request-message name, "required": true is put on properties (invalid JSON Schema), and repeated, enum and most numeric fields are dropped.
Do
Add a self-contained schema builder to the plugin (e.g. cmd/protoc-gen-go-mcp/schema.go) that turns a protogen.Message into a JSON Schema (draft 2020-12) map[string]any or JSON. Don't wire it into the generated code yet (that's #89). The tool's input schema is the request message itself (top-level fields, not nested under the message name).
| proto | schema |
|---|---|
| message | {"type":"object","properties":{...},"additionalProperties":false}; property names = field.Desc.JSONName() (lowerCamel, what protojson emits) |
| string | string |
| bytes | string, "contentEncoding":"base64" |
| bool | boolean |
| int32/sint32/sfixed32 | integer |
| uint32/fixed32 | integer, minimum: 0 |
| int64/sint64/sfixed64/uint64/fixed64 | ["integer","string"] (protojson writes 64-bit ints as strings, accepts both) |
| float/double | number |
| enum | string with enum: the value names |
| repeated X | array of X |
| map<K,V> | object with additionalProperties: V |
| nested message | inline; recursive messages via $defs + $ref |
oneof / proto3 optional |
normal properties, never required; mention the oneof group in the description |
| well-known types | Timestamp → string format: date-time; Duration → string; wrappers → the wrapped scalar; Struct → object; Value → {}; ListValue → array; FieldMask → string; Empty → object |
Descriptions come from the proto leading comments (field and message). Proto3 has no required fields, so emit no required.
Acceptance criteria
- A test proto under
testdata/(orcmd/protoc-gen-go-mcp/testdata/) covers every row above, including a recursive message, a map, an enum, a oneof and well-known types. - Unit tests check the schema for each kind.
- Property test: for populated messages of that test proto,
protojson.Marshal(msg)validates against the generated schema (usegithub.com/google/jsonschema-go, the library the official SDK uses), and so does an empty message. - Generated output of
examples/is unchanged (not wired yet).
Activity
erikist commentedon Oct 8, 2026
🤖 Factory: picked up by dispatcher; working on branch factory/88-feat-generate-json-schema-for-tool.
test(e2e): drop low-value deadline test, dedupe known-bug comment (#87)
test(e2e): isolate MCP client, stop asserting known-buggy values (#87)
fix(schema): handle enum-valued maps and fix UInt64Value minimum (#88)
erikist commentedon Oct 8, 2026
🤖 Factory: opened #98 with a self-contained JSON Schema (draft 2020-12) builder in
cmd/protoc-gen-go-mcp/schema.go(MessageInputSchema), covering every row of the table above plus a property test validatingprotojson.Marshaloutput against the generated schema. Not wired into the generated code yet, per the issue — that's left for a follow-up (#89). See the PR for test evidence.docs(schema): correct caching comment and test message description (#88)
fix(schema): fix recursive root panic, NaN/Inf floats, and Any (#88)
docs(schema): clarify anySchema's additionalProperties and lock it in (…
test(schema): compile schema.proto in-process, drop binary fixture (#88)
feat: generate JSON Schema for tool inputs from proto descriptors (#88)…