feat(client): send MCP standard POST headers (#894)
## Problem SEP-2243 requires Streamable HTTP clients to mirror applicable JSON-RPC request metadata into standard HTTP headers. The Kotlin client currently sends `Mcp-Protocol-Version` and session metadata but does not emit `Mcp-Method` or `Mcp-Name`. ## Solution - Add `Mcp-Method` for JSON-RPC requests and notifications. - Add `Mcp-Name` from a string `params.name`, falling back to a string `params.uri`. - Omit both headers for JSON-RPC responses. - Add focused transport tests for name-, URI-, notification-, and response-based behavior. ## Validation - `.\gradlew.bat :kotlin-sdk-client:jvmTest --tests "io.modelcontextprotocol.kotlin.sdk.client.streamable.http.StreamableHttpClientTransportTest"` - `.\gradlew.bat :kotlin-sdk-client:ktlintCheck :kotlin-sdk-client:detekt :kotlin-sdk-client:apiCheck` - `git diff --check` Closes #787 AI assistance was used while preparing this PR. I reviewed the final change, ran the validation listed above, and take responsibility for it. --------- Co-authored-by: Pavel Gorgulov <devcrocod@gmail.com>
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Andre.Kalberer
GitHub
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@ -43,6 +43,10 @@ import kotlinx.coroutines.cancelAndJoin
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import kotlinx.coroutines.delay
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import kotlinx.coroutines.isActive
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import kotlinx.coroutines.launch
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import kotlinx.serialization.json.JsonObject
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import kotlinx.serialization.json.JsonPrimitive
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import kotlin.io.encoding.Base64
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import kotlin.io.encoding.ExperimentalEncodingApi
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import kotlin.math.pow
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import kotlin.time.Duration
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import kotlin.time.Duration.Companion.milliseconds
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@ -51,6 +55,10 @@ import kotlin.time.Duration.Companion.seconds
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private const val MCP_SESSION_ID_HEADER = "mcp-session-id"
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private const val MCP_PROTOCOL_VERSION_HEADER = "mcp-protocol-version"
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private const val MCP_RESUMPTION_TOKEN_HEADER = "Last-Event-ID"
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private const val MCP_METHOD_HEADER = "Mcp-Method"
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private const val MCP_NAME_HEADER = "Mcp-Name"
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private const val MCP_BASE64_PREFIX = "=?base64?"
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private const val MCP_BASE64_SUFFIX = "?="
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/**
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* Default maximum size, in characters, of a single inline SSE event assembled from a POST response.
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@ -167,6 +175,7 @@ public class StreamableHttpClientTransport(
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val jsonBody = McpJson.encodeToString(message)
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val response = client.post(url) {
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applyCommonHeaders(this)
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applyStandardPostHeaders(this, message)
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headers.append(HttpHeaders.Accept, "${ContentType.Application.Json}, ${ContentType.Text.EventStream}")
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contentType(ContentType.Application.Json)
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setBody(jsonBody)
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@ -390,6 +399,36 @@ public class StreamableHttpClientTransport(
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}
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}
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private fun applyStandardPostHeaders(builder: HttpRequestBuilder, message: JSONRPCMessage) {
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val (method, params) = when (message) {
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is JSONRPCRequest -> message.method to message.params
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is JSONRPCNotification -> message.method to message.params
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else -> return
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}
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builder.headers {
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append(MCP_METHOD_HEADER, method)
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val paramsObject = params as? JsonObject ?: return@headers
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val mcpName = paramsObject.stringValue("name") ?: paramsObject.stringValue("uri")
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mcpName?.let { append(MCP_NAME_HEADER, it.encodeMcpHeaderValue()) }
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}
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}
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private fun JsonObject.stringValue(key: String): String? =
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(get(key) as? JsonPrimitive)?.takeIf { it.isString }?.content
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@OptIn(ExperimentalEncodingApi::class)
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private fun String.encodeMcpHeaderValue(): String {
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val containsUnsafeCharacters = any { it != '\t' && it.code !in 0x20..0x7e }
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val hasEdgeWhitespace = firstOrNull()?.isWhitespace() == true || lastOrNull()?.isWhitespace() == true
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val matchesBase64Sentinel = startsWith(MCP_BASE64_PREFIX) && endsWith(MCP_BASE64_SUFFIX)
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if (!containsUnsafeCharacters && !hasEdgeWhitespace && !matchesBase64Sentinel) return this
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return "$MCP_BASE64_PREFIX${Base64.Default.encode(encodeToByteArray())}$MCP_BASE64_SUFFIX"
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}
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private suspend fun collectSse(
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session: ClientSSESession,
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replayMessageId: RequestId?,
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@ -246,6 +246,100 @@ class StreamableHttpClientTransportTest {
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transport.close()
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}
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@Test
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fun `should send MCP method and name headers from name`() = runTest {
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val transport = createTransport { request ->
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assertEquals("tools/call", request.headers["Mcp-Method"])
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assertEquals("read_file", request.headers["Mcp-Name"])
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respond(content = "", status = HttpStatusCode.Accepted)
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}
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transport.start()
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transport.send(
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JSONRPCRequest(
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id = "test-id",
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method = "tools/call",
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params = buildJsonObject {
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put("name", JsonPrimitive("read_file"))
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put("arguments", buildJsonObject { put("path", JsonPrimitive("README.md")) })
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},
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),
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)
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transport.close()
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}
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@Test
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fun `should send MCP name header from URI`() = runTest {
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val transport = createTransport { request ->
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assertEquals("resources/read", request.headers["Mcp-Method"])
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assertEquals("file:///README.md", request.headers["Mcp-Name"])
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respond(content = "", status = HttpStatusCode.Accepted)
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}
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transport.start()
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transport.send(
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JSONRPCRequest(
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id = "test-id",
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method = "resources/read",
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params = buildJsonObject { put("uri", JsonPrimitive("file:///README.md")) },
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),
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)
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transport.close()
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}
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@Test
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fun `should encode MCP name header values when required`() = runTest {
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val cases = listOf(
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"Hello, 世界" to "=?base64?SGVsbG8sIOS4lueVjA==?=",
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" padded " to "=?base64?IHBhZGRlZCA=?=",
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"line1\nline2" to "=?base64?bGluZTEKbGluZTI=?=",
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"=?base64?literal?=" to "=?base64?PT9iYXNlNjQ/bGl0ZXJhbD89?=",
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)
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cases.forEachIndexed { index, (name, expectedHeader) ->
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val transport = createTransport { request ->
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assertEquals(expectedHeader, request.headers["Mcp-Name"])
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respond(content = "", status = HttpStatusCode.Accepted)
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}
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transport.start()
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transport.send(
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JSONRPCRequest(
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id = "test-id-$index",
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method = "tools/call",
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params = buildJsonObject { put("name", JsonPrimitive(name)) },
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),
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)
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transport.close()
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}
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}
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@Test
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fun `should send MCP method header for notifications`() = runTest {
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val transport = createTransport { request ->
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assertEquals("notifications/tools/list_changed", request.headers["Mcp-Method"])
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assertNull(request.headers["Mcp-Name"])
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respond(content = "", status = HttpStatusCode.Accepted)
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}
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transport.start()
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transport.send(JSONRPCNotification(method = "notifications/tools/list_changed"))
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transport.close()
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}
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@Test
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fun `should omit MCP standard headers for responses`() = runTest {
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val transport = createTransport { request ->
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assertNull(request.headers["Mcp-Method"])
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assertNull(request.headers["Mcp-Name"])
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respond(content = "", status = HttpStatusCode.Accepted)
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}
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transport.start()
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transport.send(JSONRPCResponse(id = RequestId.StringId("test-id")))
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transport.close()
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}
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// Engine doesn't support SSECapability: https://youtrack.jetbrains.com/issue/KTOR-8177/MockEngine-Add-SSE-support
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@Ignore
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@Test
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