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@lucarliglucarlig commented Aug 19, 2026

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Allow modern Streamable HTTP requests to be cancelled before their POST returns the first response event. The client worker stays responsive while request POSTs are in flight, and the same per-request cancellation token continues to own any returned SSE stream.

Motivation and Context

Fixes#1193.

RequestHandle::cancel() currently waits forever when a 2026-07-28 request POST is still waiting for its response stream. The worker awaits the POST inline, so it cannot process the cancellation that should close that same request.

This is required by the modern MCP transport rules:

How Has This Been Tested?

The new integration test uses an RMCP client and server over a real Axum HTTP listener. Before the fix, it fails after five seconds because RequestHandle::cancel() remains pending. With the fix, cancellation returns and the server's RequestContext::ct fires.

  • cargo +nightly fmt --all -- --check
  • cargo clippy --all-targets --all-features -- -D warnings
  • cargo test --all-features — 783 passed, 24 ignored
  • Full RMCP suite with all non-local features — 1,093 passed, 13 ignored
  • Focused disconnect suite — 3 passed

Breaking Changes

None. This fixes modern Streamable HTTP cancellation behavior without changing the public API. Legacy lifecycle, session, and re-initialization paths are unchanged.

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

The implementation uses RMCP's existing Tokio JoinSet and CancellationToken; it adds no dependency. A modern request's token is registered before its POST starts. Cancelling the request drops the pending HTTP future, and if the POST returns an SSE stream first, that same token controls the stream as before. Response-versus-cancellation races keep the existing behavior of ignoring a response after cancellation.

Issues #857 and PR #967 added the matching server-side disconnect handling. This change makes RMCP's public client cancellation API trigger that path even before response headers or the first SSE event arrive.

@lucarlig
fix(transport): cancel pending modern HTTP requests
e64ec5a
Signed-off-by: lucarlig <luca.carlig@ibm.com>
@github-actionsgithub-actionsBot added T-dependencies Dependencies related changes T-test Testing related changes T-config Configuration file changes T-core Core library changes T-transport Transport layer changes labels Aug 19, 2026
@lucarliglucarlig mentioned this pull request Aug 19, 2026
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streamable-http-client: RequestHandle::cancel hangs before response stream starts

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fix(transport): cancel pending modern HTTP requests by lucarlig · Pull Request #1194 · modelcontextprotocol/rust-sdk · GitHub
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fix(transport): cancel pending modern HTTP requests - #1194
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Aug 19, 2026

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Allow modern Streamable HTTP requests to be cancelled before their POST returns the first response event. The client worker stays responsive while request POSTs are in flight, and the same per-request cancellation token continues to own any returned SSE stream.
Motivation and Context
Fixes#1193.
RequestHandle::cancel() currently waits forever when a 2026-07-28 request POST is still waiting for its response stream. The worker awaits the POST inline, so it cannot process the cancellation that should close that same request.
This is required by the modern MCP transport rules:
How Has This Been Tested?
The new integration test uses an RMCP client and server over a real Axum HTTP listener. Before the fix, it fails after five seconds because RequestHandle::cancel() remains pending. With the fix, cancellation returns and the server's RequestContext::ct fires.
  • cargo +nightly fmt --all -- --check
  • cargo clippy --all-targets --all-features -- -D warnings
  • cargo test --all-features — 783 passed, 24 ignored
  • Full RMCP suite with all non-local features — 1,093 passed, 13 ignored
  • Focused disconnect suite — 3 passed
Breaking Changes
None. This fixes modern Streamable HTTP cancellation behavior without changing the public API. Legacy lifecycle, session, and re-initialization paths are unchanged.
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
The implementation uses RMCP's existing Tokio JoinSet and CancellationToken; it adds no dependency. A modern request's token is registered before its POST starts. Cancelling the request drops the pending HTTP future, and if the POST returns an SSE stream first, that same token controls the stream as before. Response-versus-cancellation races keep the existing behavior of ignoring a response after cancellation.
Issues #857 and PR #967 added the matching server-side disconnect handling. This change makes RMCP's public client cancellation API trigger that path even before response headers or the first SSE event arrive.
@lucarlig
fix(transport): cancel pending modern HTTP requests
e64ec5a
Signed-off-by: lucarlig <luca.carlig@ibm.com>
@github-actionsgithub-actionsBot
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Aug 19, 2026
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streamable-http-client: RequestHandle::cancel hangs before response stream starts


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, 'i');
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fix(transport): cancel pending modern HTTP requests by lucarlig · Pull Request #1194 · modelcontextprotocol/rust-sdk · GitHub
Skip to content
fix(transport): cancel pending modern HTTP requests - #1194
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lucarlig:agent/fix-modern-http-cancellation
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fix(transport): cancel pending modern HTTP requests#1194
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lucarlig:agent/fix-modern-http-cancellation
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@lucarlig
@lucarliglucarlig
commented
Aug 19, 2026

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Allow modern Streamable HTTP requests to be cancelled before their POST returns the first response event. The client worker stays responsive while request POSTs are in flight, and the same per-request cancellation token continues to own any returned SSE stream.
Motivation and Context
Fixes#1193.
RequestHandle::cancel() currently waits forever when a 2026-07-28 request POST is still waiting for its response stream. The worker awaits the POST inline, so it cannot process the cancellation that should close that same request.
This is required by the modern MCP transport rules:
How Has This Been Tested?
The new integration test uses an RMCP client and server over a real Axum HTTP listener. Before the fix, it fails after five seconds because RequestHandle::cancel() remains pending. With the fix, cancellation returns and the server's RequestContext::ct fires.
  • cargo +nightly fmt --all -- --check
  • cargo clippy --all-targets --all-features -- -D warnings
  • cargo test --all-features — 783 passed, 24 ignored
  • Full RMCP suite with all non-local features — 1,093 passed, 13 ignored
  • Focused disconnect suite — 3 passed
Breaking Changes
None. This fixes modern Streamable HTTP cancellation behavior without changing the public API. Legacy lifecycle, session, and re-initialization paths are unchanged.
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
The implementation uses RMCP's existing Tokio JoinSet and CancellationToken; it adds no dependency. A modern request's token is registered before its POST starts. Cancelling the request drops the pending HTTP future, and if the POST returns an SSE stream first, that same token controls the stream as before. Response-versus-cancellation races keep the existing behavior of ignoring a response after cancellation.
Issues #857 and PR #967 added the matching server-side disconnect handling. This change makes RMCP's public client cancellation API trigger that path even before response headers or the first SSE event arrive.
@lucarlig
fix(transport): cancel pending modern HTTP requests
e64ec5a
Signed-off-by: lucarlig <luca.carlig@ibm.com>
@github-actionsgithub-actionsBot
added

T-dependencies
Dependencies related changes
T-test
Testing related changes
T-config
Configuration file changes
T-core
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T-transport
Transport layer changes
labels
Aug 19, 2026
@lucarliglucarlig
mentioned this pull request
Aug 19, 2026
 Merged

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streamable-http-client: RequestHandle::cancel hangs before response stream starts


1 participant

@lucarlig

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Highlight search terms from Google/DuckDuckGo/Bing referrer
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} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
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try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
fix(transport): cancel pending modern HTTP requests by lucarlig · Pull Request #1194 · modelcontextprotocol/rust-sdk · GitHub
Skip to content
fix(transport): cancel pending modern HTTP requests - #1194
Draft
lucarlig wants to merge 1 commit into
modelcontextprotocol:mainfrom 
lucarlig:agent/fix-modern-http-cancellation
Draft
fix(transport): cancel pending modern HTTP requests#1194
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lucarlig:agent/fix-modern-http-cancellation
Conversation
@lucarlig
@lucarliglucarlig
commented
Aug 19, 2026

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Allow modern Streamable HTTP requests to be cancelled before their POST returns the first response event. The client worker stays responsive while request POSTs are in flight, and the same per-request cancellation token continues to own any returned SSE stream.
Motivation and Context
Fixes#1193.
RequestHandle::cancel() currently waits forever when a 2026-07-28 request POST is still waiting for its response stream. The worker awaits the POST inline, so it cannot process the cancellation that should close that same request.
This is required by the modern MCP transport rules:
How Has This Been Tested?
The new integration test uses an RMCP client and server over a real Axum HTTP listener. Before the fix, it fails after five seconds because RequestHandle::cancel() remains pending. With the fix, cancellation returns and the server's RequestContext::ct fires.
  • cargo +nightly fmt --all -- --check
  • cargo clippy --all-targets --all-features -- -D warnings
  • cargo test --all-features — 783 passed, 24 ignored
  • Full RMCP suite with all non-local features — 1,093 passed, 13 ignored
  • Focused disconnect suite — 3 passed
Breaking Changes
None. This fixes modern Streamable HTTP cancellation behavior without changing the public API. Legacy lifecycle, session, and re-initialization paths are unchanged.
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
The implementation uses RMCP's existing Tokio JoinSet and CancellationToken; it adds no dependency. A modern request's token is registered before its POST starts. Cancelling the request drops the pending HTTP future, and if the POST returns an SSE stream first, that same token controls the stream as before. Response-versus-cancellation races keep the existing behavior of ignoring a response after cancellation.
Issues #857 and PR #967 added the matching server-side disconnect handling. This change makes RMCP's public client cancellation API trigger that path even before response headers or the first SSE event arrive.
@lucarlig
fix(transport): cancel pending modern HTTP requests
e64ec5a
Signed-off-by: lucarlig <luca.carlig@ibm.com>
@github-actionsgithub-actionsBot
added

T-dependencies
Dependencies related changes
T-test
Testing related changes
T-config
Configuration file changes
T-core
Core library changes
T-transport
Transport layer changes
labels
Aug 19, 2026
@lucarliglucarlig
mentioned this pull request
Aug 19, 2026
 Merged

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streamable-http-client: RequestHandle::cancel hangs before response stream starts


1 participant

@lucarlig

, 'i');
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});
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(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '
fix(transport): cancel pending modern HTTP requests by lucarlig · Pull Request #1194 · modelcontextprotocol/rust-sdk · GitHub
Skip to content
fix(transport): cancel pending modern HTTP requests - #1194
Draft
lucarlig wants to merge 1 commit into
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lucarlig:agent/fix-modern-http-cancellation
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fix(transport): cancel pending modern HTTP requests#1194
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@lucarlig
@lucarliglucarlig
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Aug 19, 2026

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Allow modern Streamable HTTP requests to be cancelled before their POST returns the first response event. The client worker stays responsive while request POSTs are in flight, and the same per-request cancellation token continues to own any returned SSE stream.
Motivation and Context
Fixes#1193.
RequestHandle::cancel() currently waits forever when a 2026-07-28 request POST is still waiting for its response stream. The worker awaits the POST inline, so it cannot process the cancellation that should close that same request.
This is required by the modern MCP transport rules:
How Has This Been Tested?
The new integration test uses an RMCP client and server over a real Axum HTTP listener. Before the fix, it fails after five seconds because RequestHandle::cancel() remains pending. With the fix, cancellation returns and the server's RequestContext::ct fires.
  • cargo +nightly fmt --all -- --check
  • cargo clippy --all-targets --all-features -- -D warnings
  • cargo test --all-features — 783 passed, 24 ignored
  • Full RMCP suite with all non-local features — 1,093 passed, 13 ignored
  • Focused disconnect suite — 3 passed
Breaking Changes
None. This fixes modern Streamable HTTP cancellation behavior without changing the public API. Legacy lifecycle, session, and re-initialization paths are unchanged.
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
The implementation uses RMCP's existing Tokio JoinSet and CancellationToken; it adds no dependency. A modern request's token is registered before its POST starts. Cancelling the request drops the pending HTTP future, and if the POST returns an SSE stream first, that same token controls the stream as before. Response-versus-cancellation races keep the existing behavior of ignoring a response after cancellation.
Issues #857 and PR #967 added the matching server-side disconnect handling. This change makes RMCP's public client cancellation API trigger that path even before response headers or the first SSE event arrive.
@lucarlig
fix(transport): cancel pending modern HTTP requests
e64ec5a
Signed-off-by: lucarlig <luca.carlig@ibm.com>
@github-actionsgithub-actionsBot
added

T-dependencies
Dependencies related changes
T-test
Testing related changes
T-config
Configuration file changes
T-core
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T-transport
Transport layer changes
labels
Aug 19, 2026
@lucarliglucarlig
mentioned this pull request
Aug 19, 2026
 Merged

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streamable-http-client: RequestHandle::cancel hangs before response stream starts


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, 'i');
if (__m === '*' || __re.test(location.href)) {
// Auto-enable theater mode on YouTube
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})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
fix(transport): cancel pending modern HTTP requests by lucarlig · Pull Request #1194 · modelcontextprotocol/rust-sdk · GitHub
Skip to content
fix(transport): cancel pending modern HTTP requests - #1194
Draft
lucarlig wants to merge 1 commit into
modelcontextprotocol:mainfrom 
lucarlig:agent/fix-modern-http-cancellation
Draft
fix(transport): cancel pending modern HTTP requests#1194
lucarlig wants to merge 1 commit into
modelcontextprotocol:mainfrom 
lucarlig:agent/fix-modern-http-cancellation
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@lucarlig
@lucarliglucarlig
commented
Aug 19, 2026

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Allow modern Streamable HTTP requests to be cancelled before their POST returns the first response event. The client worker stays responsive while request POSTs are in flight, and the same per-request cancellation token continues to own any returned SSE stream.
Motivation and Context
Fixes#1193.
RequestHandle::cancel() currently waits forever when a 2026-07-28 request POST is still waiting for its response stream. The worker awaits the POST inline, so it cannot process the cancellation that should close that same request.
This is required by the modern MCP transport rules:
How Has This Been Tested?
The new integration test uses an RMCP client and server over a real Axum HTTP listener. Before the fix, it fails after five seconds because RequestHandle::cancel() remains pending. With the fix, cancellation returns and the server's RequestContext::ct fires.
  • cargo +nightly fmt --all -- --check
  • cargo clippy --all-targets --all-features -- -D warnings
  • cargo test --all-features — 783 passed, 24 ignored
  • Full RMCP suite with all non-local features — 1,093 passed, 13 ignored
  • Focused disconnect suite — 3 passed
Breaking Changes
None. This fixes modern Streamable HTTP cancellation behavior without changing the public API. Legacy lifecycle, session, and re-initialization paths are unchanged.
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
The implementation uses RMCP's existing Tokio JoinSet and CancellationToken; it adds no dependency. A modern request's token is registered before its POST starts. Cancelling the request drops the pending HTTP future, and if the POST returns an SSE stream first, that same token controls the stream as before. Response-versus-cancellation races keep the existing behavior of ignoring a response after cancellation.
Issues #857 and PR #967 added the matching server-side disconnect handling. This change makes RMCP's public client cancellation API trigger that path even before response headers or the first SSE event arrive.
@lucarlig
fix(transport): cancel pending modern HTTP requests
e64ec5a
Signed-off-by: lucarlig <luca.carlig@ibm.com>
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streamable-http-client: RequestHandle::cancel hangs before response stream starts


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fix(transport): cancel pending modern HTTP requests by lucarlig · Pull Request #1194 · modelcontextprotocol/rust-sdk · GitHub
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fix(transport): cancel pending modern HTTP requests - #1194
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lucarlig:agent/fix-modern-http-cancellation
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fix(transport): cancel pending modern HTTP requests#1194
lucarlig wants to merge 1 commit into
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lucarlig:agent/fix-modern-http-cancellation
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@lucarlig
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Aug 19, 2026

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Allow modern Streamable HTTP requests to be cancelled before their POST returns the first response event. The client worker stays responsive while request POSTs are in flight, and the same per-request cancellation token continues to own any returned SSE stream.
Motivation and Context
Fixes#1193.
RequestHandle::cancel() currently waits forever when a 2026-07-28 request POST is still waiting for its response stream. The worker awaits the POST inline, so it cannot process the cancellation that should close that same request.
This is required by the modern MCP transport rules:
How Has This Been Tested?
The new integration test uses an RMCP client and server over a real Axum HTTP listener. Before the fix, it fails after five seconds because RequestHandle::cancel() remains pending. With the fix, cancellation returns and the server's RequestContext::ct fires.
  • cargo +nightly fmt --all -- --check
  • cargo clippy --all-targets --all-features -- -D warnings
  • cargo test --all-features — 783 passed, 24 ignored
  • Full RMCP suite with all non-local features — 1,093 passed, 13 ignored
  • Focused disconnect suite — 3 passed
Breaking Changes
None. This fixes modern Streamable HTTP cancellation behavior without changing the public API. Legacy lifecycle, session, and re-initialization paths are unchanged.
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
The implementation uses RMCP's existing Tokio JoinSet and CancellationToken; it adds no dependency. A modern request's token is registered before its POST starts. Cancelling the request drops the pending HTTP future, and if the POST returns an SSE stream first, that same token controls the stream as before. Response-versus-cancellation races keep the existing behavior of ignoring a response after cancellation.
Issues #857 and PR #967 added the matching server-side disconnect handling. This change makes RMCP's public client cancellation API trigger that path even before response headers or the first SSE event arrive.
@lucarlig
fix(transport): cancel pending modern HTTP requests
e64ec5a
Signed-off-by: lucarlig <luca.carlig@ibm.com>
@github-actionsgithub-actionsBot
added

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T-test
Testing related changes
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Aug 19, 2026
@lucarliglucarlig
mentioned this pull request
Aug 19, 2026
 Merged

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streamable-http-client: RequestHandle::cancel hangs before response stream starts


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fix(transport): cancel pending modern HTTP requests by lucarlig · Pull Request #1194 · modelcontextprotocol/rust-sdk · GitHub
Skip to content
fix(transport): cancel pending modern HTTP requests - #1194
Draft
lucarlig wants to merge 1 commit into
modelcontextprotocol:mainfrom 
lucarlig:agent/fix-modern-http-cancellation
Draft
fix(transport): cancel pending modern HTTP requests#1194
lucarlig wants to merge 1 commit into
modelcontextprotocol:mainfrom 
lucarlig:agent/fix-modern-http-cancellation
Conversation
@lucarlig
@lucarliglucarlig
commented
Aug 19, 2026

Copy link


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Allow modern Streamable HTTP requests to be cancelled before their POST returns the first response event. The client worker stays responsive while request POSTs are in flight, and the same per-request cancellation token continues to own any returned SSE stream.
Motivation and Context
Fixes#1193.
RequestHandle::cancel() currently waits forever when a 2026-07-28 request POST is still waiting for its response stream. The worker awaits the POST inline, so it cannot process the cancellation that should close that same request.
This is required by the modern MCP transport rules:
How Has This Been Tested?
The new integration test uses an RMCP client and server over a real Axum HTTP listener. Before the fix, it fails after five seconds because RequestHandle::cancel() remains pending. With the fix, cancellation returns and the server's RequestContext::ct fires.
  • cargo +nightly fmt --all -- --check
  • cargo clippy --all-targets --all-features -- -D warnings
  • cargo test --all-features — 783 passed, 24 ignored
  • Full RMCP suite with all non-local features — 1,093 passed, 13 ignored
  • Focused disconnect suite — 3 passed
Breaking Changes
None. This fixes modern Streamable HTTP cancellation behavior without changing the public API. Legacy lifecycle, session, and re-initialization paths are unchanged.
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
The implementation uses RMCP's existing Tokio JoinSet and CancellationToken; it adds no dependency. A modern request's token is registered before its POST starts. Cancelling the request drops the pending HTTP future, and if the POST returns an SSE stream first, that same token controls the stream as before. Response-versus-cancellation races keep the existing behavior of ignoring a response after cancellation.
Issues #857 and PR #967 added the matching server-side disconnect handling. This change makes RMCP's public client cancellation API trigger that path even before response headers or the first SSE event arrive.
@lucarlig
fix(transport): cancel pending modern HTTP requests
e64ec5a
Signed-off-by: lucarlig <luca.carlig@ibm.com>
@github-actionsgithub-actionsBot
added

T-dependencies
Dependencies related changes
T-test
Testing related changes
T-config
Configuration file changes
T-core
Core library changes
T-transport
Transport layer changes
labels
Aug 19, 2026
@lucarliglucarlig
mentioned this pull request
Aug 19, 2026
 Merged

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Successfully merging this pull request may close these issues.

streamable-http-client: RequestHandle::cancel hangs before response stream starts


1 participant

@lucarlig