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Fix MCP tool contracts #116

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Fix MCP tool contracts #116

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22 changes: 19 additions & 3 deletions src/main/kotlin/net/portswigger/mcp/schema/JsonSchema.kt
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Original file line number Diff line number Diff line change
Expand Up @@ -2,10 +2,13 @@ package net.portswigger.mcp.schema

import io.modelcontextprotocol.kotlin.sdk.types.ToolSchema
import kotlinx.serialization.json.JsonElement
import kotlinx.serialization.json.JsonArray
import kotlinx.serialization.json.JsonObject
import kotlinx.serialization.json.JsonPrimitive
import kotlinx.serialization.json.jsonObject
import kotlin.reflect.KClass
import kotlin.reflect.full.memberProperties
import kotlin.reflect.full.primaryConstructor

fun getJsonSchemaForProperty(kType: kotlin.reflect.KType): JsonElement {
return when (kType.classifier) {
Expand Down Expand Up @@ -47,11 +50,24 @@ fun getJsonSchemaForProperty(kType: kotlin.reflect.KType): JsonElement {
fun KClass<*>.asInputSchema(): ToolSchema {
val properties = mutableMapOf<String, JsonElement>()
val required = mutableListOf<String>()
val parameters = primaryConstructor?.parameters?.associateBy { it.name }.orEmpty()

for (prop in memberProperties) {
properties[prop.name] = getJsonSchemaForProperty(prop.returnType)
val schema = getJsonSchemaForProperty(prop.returnType).jsonObject.toMutableMap()
if (prop.returnType.isMarkedNullable) {
schema["type"] = JsonArray(listOf(schema.getValue("type"), JsonPrimitive("null")))
}
when (prop.name) {
"targetPort" -> {
schema["minimum"] = JsonPrimitive(1)
schema["maximum"] = JsonPrimitive(65535)
}
"count" -> schema["minimum"] = JsonPrimitive(1)
"offset", "length" -> schema["minimum"] = JsonPrimitive(0)
}
properties[prop.name] = JsonObject(schema)

if (!prop.returnType.isMarkedNullable) {
if (parameters[prop.name]?.isOptional == false) {
required.add(prop.name)
}
}
Expand All @@ -60,4 +76,4 @@ fun KClass<*>.asInputSchema(): ToolSchema {
properties = JsonObject(properties),
required = required
)
}
}
76 changes: 67 additions & 9 deletions src/main/kotlin/net/portswigger/mcp/tools/McpTool.kt
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Expand Up @@ -8,11 +8,61 @@ import io.modelcontextprotocol.kotlin.sdk.types.ContentBlock
import io.modelcontextprotocol.kotlin.sdk.types.TextContent
import io.modelcontextprotocol.kotlin.sdk.types.ToolSchema
import kotlinx.serialization.InternalSerializationApi
import kotlinx.serialization.SerializationException
import kotlinx.serialization.json.Json
import kotlinx.serialization.json.JsonObject
import kotlinx.serialization.json.JsonPrimitive
import kotlinx.serialization.serializer
import net.portswigger.mcp.schema.asInputSchema
import kotlin.experimental.ExperimentalTypeInference
import kotlin.reflect.KClass
import kotlin.reflect.full.memberProperties

@PublishedApi
internal class McpToolException(message: String) : IllegalArgumentException(message)

fun mcpError(message: String): Nothing = throw McpToolException(message)

@PublishedApi
internal val ToolJson = Json { ignoreUnknownKeys = true }

@PublishedApi
internal fun <I : Any> coerceWholeNumberArguments(arguments: JsonObject, inputClass: KClass<I>): JsonObject {
val integerProperties = inputClass.memberProperties
.filter { it.returnType.classifier == Int::class || it.returnType.classifier == Long::class }
.associate { it.name to it.returnType.classifier }

return JsonObject(arguments.mapValues { (name, value) ->
val primitive = value as? JsonPrimitive
val classifier = integerProperties[name]
if (primitive == null || primitive.isString || classifier == null) return@mapValues value

val integer = primitive.content.toBigDecimalOrNull()?.let {
try { it.toBigIntegerExact() } catch (_: ArithmeticException) { null }
} ?: return@mapValues value

when (classifier) {
Int::class -> integer.toIntExactOrNull()?.let(::JsonPrimitive) ?: value
Long::class -> integer.toLongExactOrNull()?.let(::JsonPrimitive) ?: value
else -> value
}
})
}

private fun java.math.BigInteger.toIntExactOrNull() = try { intValueExact() } catch (_: ArithmeticException) { null }
private fun java.math.BigInteger.toLongExactOrNull() = try { longValueExact() } catch (_: ArithmeticException) { null }

@PublishedApi
internal fun toolErrorResult(toolName: String, error: Exception): CallToolResult {
val detail = error.message?.substringBefore(" at path:") ?: error::class.simpleName ?: "unknown error"
val message = when (error) {
is McpToolException -> detail
is SerializationException, is IllegalArgumentException ->
"Invalid arguments for '$toolName': $detail. Check the tool schema from tools/list and retry."
else -> "Tool '$toolName' failed: $detail. Check Burp's extension output for details."
}
return CallToolResult(content = listOf(TextContent(message)), isError = true)
}

@OptIn(InternalSerializationApi::class)
inline fun <reified I : Any> Server.mcpTool(
Expand All @@ -27,18 +77,18 @@ inline fun <reified I : Any> Server.mcpTool(
try {
CallToolResult(
content = execute(
Json.decodeFromJsonElement(
ToolJson.decodeFromJsonElement(
serializer,
request.params.arguments ?: JsonObject(emptyMap())
coerceWholeNumberArguments(
request.params.arguments ?: JsonObject(emptyMap()),
I::class
)
)
),
isError = false
)
} catch (e: Exception) {
CallToolResult(
content = listOf(TextContent("Error: ${e.message}")),
isError = true
)
toolErrorResult(toolName, e)
}
}

Expand Down Expand Up @@ -83,7 +133,7 @@ inline fun <reified I : Paginated, J : Any> Server.mcpPaginatedTool(
}

else -> {
val upperLimit = (offset + count).coerceAtMost(items.size)
val upperLimit = (offset.toLong() + count).coerceAtMost(items.size.toLong()).toInt()

items.subList(offset, upperLimit)
.joinToString(separator = "\n\n", transform = mapper)
Expand Down Expand Up @@ -117,7 +167,11 @@ inline fun Server.mcpTool(
crossinline execute: () -> List<ContentBlock>
) {
val handler: suspend (ClientConnection, CallToolRequest) -> CallToolResult = { _, _ ->
CallToolResult(content = execute(), isError = false)
try {
CallToolResult(content = execute(), isError = false)
} catch (e: Exception) {
toolErrorResult(name, e)
}
}
addTool(name = name, description = description, inputSchema = ToolSchema(), handler = handler)
}
Expand All @@ -128,7 +182,11 @@ inline fun Server.mcpTool(
crossinline execute: () -> String
) {
val handler: suspend (ClientConnection, CallToolRequest) -> CallToolResult = { _, _ ->
CallToolResult(content = listOf(TextContent(execute())), isError = false)
try {
CallToolResult(content = listOf(TextContent(execute())), isError = false)
} catch (e: Exception) {
toolErrorResult(name, e)
}
}
addTool(name = name, description = description, inputSchema = ToolSchema(), handler = handler)
}
Expand Down
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