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Fix JSON Schema type arrays for nullable fields #96

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Merged
adamw merged 1 commit into from
Feb 5, 2026
+46 −1
Merged

Fix JSON Schema type arrays for nullable fields #96

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2 changes: 1 addition & 1 deletion core/src/main/scala/chimp/McpHandler.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -55,7 +55,7 @@ class McpHandler[F[_]](
/** Converts a ServerTool to its protocol definition. */
private def toolToDefinition(tool: ServerTool[?, F]): ToolDefinition =
val jsonSchema =
val base = TapirSchemaToJsonSchema(tool.inputSchema, markOptionsAsNullable = true)
val base = TapirSchemaToJsonSchema(tool.inputSchema, markOptionsAsNullable = false)
if showJsonSchemaMetadata then base
else base.copy($schema = None)

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45 changes: 45 additions & 0 deletions core/src/test/scala/chimp/McpHandlerSpec.scala
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Expand Up @@ -416,6 +416,51 @@ class McpHandlerSpec extends AnyFlatSpec with Matchers:
resultObj.content.head shouldBe ToolContent.Text("text", "no header")
case _ => fail("Expected Response")

it should "not use type arrays for optional fields in JSON schema" in:
// Given - a tool with optional fields
case class OptionalFieldInput(requiredField: String, optionalField: Option[Long]) derives Schema, Codec
val optionalTool = tool("optionalTest")
.description("Test tool with optional fields.")
.input[OptionalFieldInput]
.handle(_ => Right("ok"))

val handlerWithOptional = McpHandler(List(optionalTool), "Test", "1.0.0", true)

val req: JSONRPCMessage = Request(method = "tools/list", id = RequestId("opt1"))
val json = req.asJson
// When
val response = handlerWithOptional.handleJsonRpc(json, Seq.empty)
val respJson = extractJsonFromResponse(response)
val resp = respJson.as[JSONRPCMessage].getOrElse(fail("Failed to decode response"))
// Then
resp match
case Response(_, _, result) =>
val resultObj = result.as[ListToolsResponse].getOrElse(fail("Failed to decode result"))
val toolDef = resultObj.tools.find(_.name == "optionalTest").get
val inputSchema = toolDef.inputSchema

// Check that optionalField does NOT use ["integer", "null"] type array
// Claude API rejects this format - it should just be "integer" with the field not in required
val optionalFieldType = inputSchema.hcursor
.downField("properties")
.downField("optionalField")
.downField("type")
.focus

optionalFieldType match
case Some(typeValue) =>
// Should be a simple string "integer", not an array ["integer", "null"]
typeValue.isString shouldBe true
typeValue.asString.get shouldBe "integer"
case None =>
fail("optionalField type not found in schema")

// Verify requiredField is in required array but optionalField is not
val requiredFields = inputSchema.hcursor.downField("required").as[List[String]].getOrElse(Nil)
requiredFields should contain("requiredField")
requiredFields should not contain "optionalField"
case _ => fail("Expected Response")

it should "handle batch requests with mixed headers" in:
// Given
val req1 = Request(
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