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feat: add streamable http client
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‎src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StreamableHttpClientTransport.kt‎

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package io.modelcontextprotocol.kotlin.sdk.client
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import io.github.oshai.kotlinlogging.KotlinLogging
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import io.ktor.client.HttpClient
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import io.ktor.client.plugins.sse.ClientSSESession
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import io.ktor.client.plugins.sse.sseSession
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import io.ktor.client.request.HttpRequestBuilder
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import io.ktor.client.request.post
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import io.ktor.client.request.setBody
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import io.ktor.client.statement.bodyAsText
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import io.ktor.http.ContentType
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import io.ktor.http.HttpHeaders
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import io.ktor.http.append
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import io.ktor.http.contentType
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import io.ktor.http.isSuccess
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import io.modelcontextprotocol.kotlin.sdk.JSONRPCMessage
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import io.modelcontextprotocol.kotlin.sdk.shared.AbstractTransport
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import io.modelcontextprotocol.kotlin.sdk.shared.McpJson
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import kotlinx.coroutines.CoroutineName
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import kotlinx.coroutines.CoroutineScope
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import kotlinx.coroutines.Job
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import kotlinx.coroutines.SupervisorJob
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import kotlinx.coroutines.cancel
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import kotlinx.coroutines.cancelAndJoin
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import kotlinx.coroutines.launch
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import kotlin.concurrent.atomics.AtomicBoolean
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import kotlin.concurrent.atomics.ExperimentalAtomicApi
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private val logger = KotlinLogging.logger {}
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/**
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* Client transport for Streamable HTTP: this will send messages via HTTP POST requests
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* and optionally receive streaming responses via SSE.
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*
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* This implements the Streamable HTTP transport as specified in MCP 2024-11-05.
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*/
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@OptIn(ExperimentalAtomicApi::class)
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public class StreamableHttpClientTransport(
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private val client: HttpClient,
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private val url: String,
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private val requestBuilder: HttpRequestBuilder.() -> Unit = {},
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) : AbstractTransport() {
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private val initialized: AtomicBoolean = AtomicBoolean(false)
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private var sseSession: ClientSSESession? = null
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private val scope by lazy { CoroutineScope(SupervisorJob()) }
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private var sseJob: Job? = null
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private var sessionId: String? = null
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override suspend fun start() {
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if (!initialized.compareAndSet(expectedValue = false, newValue = true)) {
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error("StreamableHttpClientTransport already started!")
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}
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logger.debug { "Client transport starting..." }
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startSseSession()
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}
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private suspend fun startSseSession() {
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logger.debug { "Client attempting to start SSE session at url: $url" }
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try {
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sseSession = client.sseSession(
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urlString = url,
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block = requestBuilder,
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)
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logger.debug { "Client SSE session started successfully." }
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sseJob = scope.launch(CoroutineName("StreamableHttpTransport.collect#${hashCode()}")) {
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sseSession?.incoming?.collect { event ->
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logger.trace { "Client received SSE event: event=${event.event}, data=${event.data}" }
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when (event.event) {
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"error" -> {
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val e = IllegalStateException("SSE error: ${event.data}")
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logger.error(e) { "SSE stream reported an error event." }
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_onError(e)
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}
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else -> {
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// All non-error events are treated as JSON-RPC messages
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try {
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val eventData = event.data
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if (!eventData.isNullOrEmpty()) {
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val message = McpJson.decodeFromString<JSONRPCMessage>(eventData)
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_onMessage(message)
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}
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} catch (e: Exception) {
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logger.error(e) { "Error processing SSE message" }
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_onError(e)
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}
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}
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}
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}
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}
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} catch (e: Exception) {
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// SSE session is optional, don't fail if it can't be established
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// The server might not support GET requests for SSE
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logger.warn(e) { "Client failed to start SSE session. This may be expected if the server does not support GET." }
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_onError(e)
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}
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}
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override suspend fun send(message: JSONRPCMessage) {
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logger.debug { "Client sending message via POST to $url: ${McpJson.encodeToString(message)}" }
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try {
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val response = client.post(url) {
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requestBuilder()
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contentType(ContentType.Application.Json)
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headers.append(HttpHeaders.Accept, "${ContentType.Application.Json}, ${ContentType.Text.EventStream}")
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// Add session ID if we have one
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sessionId?.let {
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headers.append("Mcp-Session-Id", it)
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}
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setBody(McpJson.encodeToString(message))
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}
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logger.debug { "Client received POST response: ${response.status}" }
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if (!response.status.isSuccess()) {
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val text = response.bodyAsText()
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val error = Exception("HTTP ${response.status}: $text")
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logger.error(error) { "Client POST request failed." }
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_onError(error)
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throw error
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}
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// Extract session ID from response headers if present
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response.headers["Mcp-Session-Id"]?.let {
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sessionId = it
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}
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// Handle response based on content type
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when (response.contentType()?.contentType) {
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ContentType.Application.Json.contentType -> {
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// Single JSON response
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val responseBody = response.bodyAsText()
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logger.trace { "Client processing JSON response: $responseBody" }
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if (responseBody.isNotEmpty()) {
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try {
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val responseMessage = McpJson.decodeFromString<JSONRPCMessage>(responseBody)
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_onMessage(responseMessage)
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} catch (e: Exception) {
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logger.error(e) { "Error processing JSON response" }
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_onError(e)
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}
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}
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}
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ContentType.Text.EventStream.contentType -> {
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logger.trace { "Client received SSE stream in POST response. Messages will be handled by the main SSE session." }
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}
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else -> {
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logger.trace { "Client received response with unexpected or no content type: ${response.contentType()}" }
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}
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}
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} catch (e: Exception) {
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logger.error(e) { "Client send failed." }
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_onError(e)
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throw e
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}
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}
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override suspend fun close() {
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if (!initialized.load()) {
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return // Already closed or never started
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}
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logger.debug { "Client transport closing." }
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try {
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sseSession?.cancel()
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sseJob?.cancelAndJoin()
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scope.cancel()
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} catch (e: Exception) {
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// Ignore errors during cleanup
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} finally {
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_onClose()
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}
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}
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}

‎src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StreamableHttpMcpKtorClientExtensions.kt‎

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package io.modelcontextprotocol.kotlin.sdk.client
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import io.ktor.client.HttpClient
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import io.ktor.client.request.HttpRequestBuilder
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import io.modelcontextprotocol.kotlin.sdk.Implementation
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import io.modelcontextprotocol.kotlin.sdk.LIB_VERSION
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import io.modelcontextprotocol.kotlin.sdk.shared.IMPLEMENTATION_NAME
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/**
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* Returns a new Streamable HTTP transport for the Model Context Protocol using the provided HttpClient.
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*
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* @param url URL of the MCP server.
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* @param requestBuilder Optional lambda to configure the HTTP request.
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* @return A [StreamableHttpClientTransport] configured for MCP communication.
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*/
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public fun HttpClient.mcpStreamableHttpTransport(
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url: String,
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requestBuilder: HttpRequestBuilder.() -> Unit = {},
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): StreamableHttpClientTransport = StreamableHttpClientTransport(this, url, requestBuilder)
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/**
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* Creates and connects an MCP client over Streamable HTTP using the provided HttpClient.
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*
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* @param url URL of the MCP server.
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* @param requestBuilder Optional lambda to configure the HTTP request.
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* @return A connected [Client] ready for MCP communication.
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*/
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public suspend fun HttpClient.mcpStreamableHttp(
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url: String,
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requestBuilder: HttpRequestBuilder.() -> Unit = {},
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): Client {
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val transport = mcpStreamableHttpTransport(url, requestBuilder)
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val client = Client(
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Implementation(
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name = IMPLEMENTATION_NAME,
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version = LIB_VERSION
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)
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)
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client.connect(transport)
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return client
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}

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