Add streamable http client transport

[devcrocod]

#79

Motivation and Context

How Has This Been Tested?

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[Copilot]

Pull Request Overview

Adds a new Streamable HTTP transport implementation for the Model Context Protocol, complete with client extensions and unit tests.

• Introduces StreamableHttpClientTransport to send JSON-RPC messages over POST and receive via SSE
• Provides HttpClient extension functions for easy transport/client creation
• Adds tests and updates dependencies to support mock HTTP interactions

Reviewed Changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StreamableHttpClientTransport.kt	Implements the new streamable HTTP transport with SSE and inline-SSE handling
src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StreamableHttpMcpKtorClientExtensions.kt	Adds mcpStreamableHttpTransport and mcpStreamableHttp extension functions on HttpClient
src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types.kt	Adds a copy method to JSONRPCResponse
src/jvmTest/kotlin/client/StreamableHttpClientTransportTest.kt	Unit tests for the new transport, including error handling and header behaviors
gradle/libs.versions.toml	Adds ktor-client-mock dependency for testing
api/kotlin-sdk.api	Exposes the new transport and extension APIs in the public SDK

Comments suppressed due to low confidence (2)

src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types.kt:249

• [nitpick] The newly added copy function for JSONRPCResponse lacks KDoc. Consider adding a brief description explaining its purpose and usage.

    public fun copy(

src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StreamableHttpMcpKtorClientExtensions.kt:18

• [nitpick] Consider adding unit tests for the new extension functions mcpStreamableHttpTransport and mcpStreamableHttp to ensure they configure the transport and client correctly.

public fun HttpClient.mcpStreamableHttpTransport(

[Copilot]

This appends the Accept header after applyCommonHeaders, which itself appends an Accept header for JSON. Consider consolidating these into a single header call or using headers.set to avoid duplicate Accept entries.

Suggested change

	headers.append(HttpHeaders.Accept, "${ContentType.Application.Json}, ${ContentType.Text.EventStream}")
	headers.set(HttpHeaders.Accept, "${ContentType.Application.Json}, ${ContentType.Text.EventStream}")

[Copilot]

The string template uses $$ct which escapes to a literal $ct. To include the actual content type, use $ct instead of $$ct.

Suggested change

	val error = StreamableHttpError(-1, "Unexpected content type: $$ct")
	val error = StreamableHttpError(-1, "Unexpected content type: $ct")

[Copilot]

The test for terminateSession skips setting sessionId, so the method returns early and never sends the DELETE request. Uncomment or set transport.sessionId before calling terminateSession().

Suggested change

	// transport.sessionId = "test-session-id"
	transport.sessionId = "test-session-id"

[Copilot]

[nitpick] There's a commented-out test (testStoreSessionId). If it's not needed, consider removing it, or if it's a work in progress, add a TODO so it's clear why it's disabled.

Suggested change

	// fun testStoreSessionId() = runTest {
	// fun testStoreSessionId() = runTest {
	// // TODO: This test is currently disabled. It needs further development or debugging
	// // to verify the handling of session IDs. Uncomment and complete the test when ready.

[e5l]

lgtm, could you add end2end test on notification schema as well?

[joreilly]

Are there any examples of using this yet?

Ah, just realised this is for client only? Is there way yet to standup server yet that uses streamable HTTP transport?

[devcrocod]

Are there any examples of using this yet?

Yeap, we don’t have an example yet, our current focus is on the upcoming release.
Updating the documentation and adding more examples is planned for the future: #160, #163

Ah, just realised this is for client only? Is there way yet to standup server yet that uses streamable HTTP transport?

No, you can use the SSE transport