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fix(sdk): recover from a server-side thread stream drop instead of freezing - #2812

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Sep 9, 2026
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fix(sdk): recover from a server-side thread stream drop instead of freezing#2812
Hunter Lovell (hntrl) merged 1 commit into
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fix/sdk-reconnect-on-clean-close

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@eliornl

@eliornl Elior Nataf Lackritz (eliornl) commented Sep 9, 2026 •
edited by hntrl
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Summary

When the server closed the protocol SSE stream cleanly, the transport treated it as the end of the thread: no reconnect, no error, and a useStream UI froze mid-run with isLoading stuck and nothing in stream.error. The thread stream is open-ended by contract, so a clean close only ever means the server's upstream consumer died or the server is restarting. The transport now reconnects on it, and stream-level failures reach stream.error.

Details

  • ProtocolSseTransportAdapter: a clean close of the event stream takes the same path as a dropped connection (backoff, onReconnect, maxReconnectAttempts). maxReconnectAttempts: 0 keeps the old end-on-close behavior. A connection that delivered at least one event resets the attempt counter, so a page that lives through several deploys does not die on the fifth one.
  • Both reference servers in this repo (libs/langgraph-api/src/api/protocol.mts, experimental/embed/protocol.mts) hold the SSE open until the client aborts, and the Python server ends it only when its thread consumer dies, so no server uses a clean close to mean "done".
  • ThreadStream.onError(listener): unsolicited error frames (id: null) were dropped because command routing found no pending request for them. They are now delivered to onError, as is a shared stream that exhausts its reconnects. Subscriptions stay open on an error frame so the reconnected stream keeps feeding the run.
  • StreamController: on a thread error it sets error, clears isLoading, and settles the in-flight submit() as failed.

Tests

  • transport/http.test.ts: reconnects on a clean close; resets the reconnect budget after a connection delivered events; still ends on a clean close with maxReconnectAttempts: 0.
  • shared-stream.test.ts: an unsolicited error frame reaches onError as a ProtocolError and leaves the stream open; a failed shared stream reaches onError.
  • controller.test.ts: a thread error lands on rootStore.error, clears isLoading, and settles submit().
  • Full libs/sdk unit suite (797) and the libs/sdk-react browser suite (244) pass.

Validated on the real stack (langgraph-api postgres runtime, core server killed with kill -9 mid-run): without the fix the client got two events and then silence; with the fix it got ProtocolError unknown_error "Thread event stream failed: ...", reconnected, and received the rest of the run through lifecycle: completed.

Changeset: @langchain/langgraph-sdk patch.

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🦋 Changeset detected

Latest commit: 1f07fe5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 5 packages
Name Type
@langchain/langgraph-sdk Patch
@langchain/angular Patch
@langchain/react Patch
@langchain/svelte Patch
@langchain/vue Patch

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✅ Open SWE Review: No issues found

Open SWE reviewed this PR and found no potential bugs to report.

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@langchain/langgraph-checkpoint

npm i https://pkg.pr.new/langchain-ai/langgraphjs/@langchain/langgraph-checkpoint@2812

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commit: 1f07fe5

@hntrl
Hunter Lovell (hntrl) force-pushed the fix/sdk-reconnect-on-clean-close branch from 9dbb437 to dfe8b4a Compare September 9, 2026 17:37
@eliornl @hntrl
fix(sdk): recover from a server-side thread stream drop instead of fr…
1f07fe5 
…eezing

The protocol SSE transport treated a clean close of the thread event
stream as the end of the thread. The stream is open-ended, so a clean
close only happens when the server's upstream consumer died or it is
restarting; the client stayed silent, never reconnected, and the UI froze
mid-run. The transport now reconnects on a clean close, with the same
backoff and attempt budget as a dropped connection, and a connection that
delivered events resets that budget so long-lived pages survive repeated
deploys. maxReconnectAttempts: 0 keeps the end-on-close behavior.

Unsolicited server error frames (no command id) were dropped because
command routing found no pending request. ThreadStream now exposes them,
and a shared stream that exhausts its reconnects, through onError; the
StreamController sets stream.error, clears isLoading, and settles the
in-flight submit() as failed.
@hntrl
Hunter Lovell (hntrl) force-pushed the fix/sdk-reconnect-on-clean-close branch from dfe8b4a to 1f07fe5 Compare September 9, 2026 17:46
@hntrl
Hunter Lovell (hntrl) merged commit db4bdad into main Sep 9, 2026
30 checks passed
@hntrl
Hunter Lovell (hntrl) deleted the fix/sdk-reconnect-on-clean-close branch September 9, 2026 17:58
@github-actions github-actions Bot mentioned this pull request Sep 9, 2026
Merged
Hunter Lovell (hntrl) pushed a commit that referenced this pull request Sep 9, 2026
@github-actions
chore: version packages (rc) (#2816)
5b2736f 
This PR was opened by the [Changesets
release](https://github.com/changesets/action) GitHub action. When
you're ready to do a release, you can merge this and the packages will
be published to npm automatically. If you're not ready to do a release
yet, that's fine, whenever you add more changesets to main, this PR will
be updated.

⚠️⚠️⚠️⚠️⚠️⚠️

`main` is currently in **pre mode** so this branch has prereleases
rather than normal releases. If you want to exit prereleases, run
`changeset pre exit` on `main`.

⚠️⚠️⚠️⚠️⚠️⚠️

# Releases
## @langchain/langgraph-sdk@1.10.3-rc.2

### Patch Changes

- [#2813](#2813)
[`4fc118f`](4fc118f)
Thanks [@eliornl](https://github.com/eliornl)! - fix(sdk): show
interrupts raised after a passive thread rejoin
  
After a page refresh mid-run, `useStream` filtered every interrupt it
did not
already know from the hydrated thread state as replayed history, and
waited
for a `checkpoints` event to lift that filter. Current runtimes never
emit
that event and the replay buffer trims it on long runs, so interrupts
raised
after the refresh never appeared until the next reload. Unknown
interrupts are
now settled against the server's thread state when the run reaches a
terminal
lifecycle: the ones the server lists as pending are shown, the rest are
  dropped as history.

- [#2812](#2812)
[`db4bdad`](db4bdad)
Thanks [@eliornl](https://github.com/eliornl)! - fix(sdk): recover from
a server-side thread stream drop instead of freezing
  
The protocol SSE transport now reconnects when the server closes the
event
stream cleanly. The thread stream is open-ended, so a clean close only
happens
when the server's own upstream consumer died or it is restarting;
before, the
client treated it as the end of the thread and the UI froze mid-run with
no
error. A connection that delivered events also resets the reconnect
budget, so
long-lived pages survive repeated deploys. `maxReconnectAttempts: 0`
keeps the
  old end-on-close behavior.
  
Unsolicited server error frames (no command id) and a shared stream that
gives
up reconnecting now reach `stream.error`: `ThreadStream.onError` exposes
them,
`useStream` sets `error`, clears `isLoading`, and settles the in-flight
  `submit()` as failed.
## @langchain/angular@1.0.36-rc.2

### Patch Changes

- Updated dependencies
[[`4fc118f`](4fc118f),
[`db4bdad`](db4bdad)]:
  - @langchain/langgraph-sdk@1.10.3-rc.2
## @langchain/react@1.0.36-rc.2

### Patch Changes

- Updated dependencies
[[`4fc118f`](4fc118f),
[`db4bdad`](db4bdad)]:
  - @langchain/langgraph-sdk@1.10.3-rc.2
## @langchain/svelte@1.0.36-rc.2

### Patch Changes

- Updated dependencies
[[`4fc118f`](4fc118f),
[`db4bdad`](db4bdad)]:
  - @langchain/langgraph-sdk@1.10.3-rc.2
## @langchain/vue@1.0.36-rc.2

### Patch Changes

- Updated dependencies
[[`4fc118f`](4fc118f),
[`db4bdad`](db4bdad)]:
  - @langchain/langgraph-sdk@1.10.3-rc.2

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
This was referenced Sep 11, 2026
Merged
Open
Hunter Lovell (hntrl) pushed a commit that referenced this pull request Sep 12, 2026
@github-actions
chore: version packages (#2832)
592fd0c 
This PR was opened by the [Changesets
release](https://github.com/changesets/action) GitHub action. When
you're ready to do a release, you can merge this and the packages will
be published to npm automatically. If you're not ready to do a release
yet, that's fine, whenever you add more changesets to main, this PR will
be updated.


# Releases
## @langchain/langgraph-sdk@1.11.0

### Minor Changes

- [#2831](#2831)
[`3234c69`](3234c69)
Thanks [@ramon-langchain](https://github.com/ramon-langchain)! - Expose
connection lifecycle callbacks for built-in streaming transports.
  
`onConnected` runs after the initial SSE or WebSocket connection becomes
usable and after every successful reconnect. Its payload distinguishes
an `initial` connection from a `reconnected` connection and includes the
reconnect attempt number.
  
`onReconnect` now also receives the scheduled `delayMs`, allowing
applications to display accurate retry state before the next connection
attempt. React, Vue, Svelte, and Angular stream hooks forward both
callbacks.

### Patch Changes

- [#2831](#2831)
[`3234c69`](3234c69)
Thanks [@ramon-langchain](https://github.com/ramon-langchain)! - Recover
cleanly when a thread's root event stream terminates unexpectedly.
  
If the stream fails or closes while a run is active, the controller now
records the transport error and clears `isLoading` rather than leaving
the UI in a permanently running state. Once the failed pump settles, a
later submission can start a fresh root subscription without recreating
the thread stream or replaying the failed command.

- [#2809](#2809)
[`11a4535`](11a4535)
Thanks [@hntrl](https://github.com/hntrl)! - fix(sdk): appropriately
track persisted seq for stream replay
  
Sequences weren't being appropriately attributed when rehydrating the
page (e.g. on refresh). This meant we'd lose stream information on
`useStream` on reloads. This has been fixed by adding a lookup step to
determine what the most appropriate sequence index is to track in the
event stream.:x

- [#2762](#2762)
[`2fab6fd`](2fab6fd)
Thanks [@JessYanCoding](https://github.com/JessYanCoding)! - Send
`checkpoint_id` in the `runs.stream()` request body, so a `checkpointId`
passed to `client.runs.stream()` forks from the requested checkpoint
instead of being silently dropped. Matches `runs.create()` and
`runs.wait()`, which already send it.

- [#2813](#2813)
[`4fc118f`](4fc118f)
Thanks [@eliornl](https://github.com/eliornl)! - fix(sdk): show
interrupts raised after a passive thread rejoin
  
After a page refresh mid-run, `useStream` filtered every interrupt it
did not
already know from the hydrated thread state as replayed history, and
waited
for a `checkpoints` event to lift that filter. Current runtimes never
emit
that event and the replay buffer trims it on long runs, so interrupts
raised
after the refresh never appeared until the next reload. Unknown
interrupts are
now settled against the server's thread state when the run reaches a
terminal
lifecycle: the ones the server lists as pending are shown, the rest are
  dropped as history.

- [#2812](#2812)
[`db4bdad`](db4bdad)
Thanks [@eliornl](https://github.com/eliornl)! - fix(sdk): recover from
a server-side thread stream drop instead of freezing
  
The protocol SSE transport now reconnects when the server closes the
event
stream cleanly. The thread stream is open-ended, so a clean close only
happens
when the server's own upstream consumer died or it is restarting;
before, the
client treated it as the end of the thread and the UI froze mid-run with
no
error. A connection that delivered events also resets the reconnect
budget, so
long-lived pages survive repeated deploys. `maxReconnectAttempts: 0`
keeps the
  old end-on-close behavior.
  
Unsolicited server error frames (no command id) and a shared stream that
gives
up reconnecting now reach `stream.error`: `ThreadStream.onError` exposes
them,
`useStream` sets `error`, clears `isLoading`, and settles the in-flight
  `submit()` as failed.

- [#2808](#2808)
[`55fa26b`](55fa26b)
Thanks [@hntrl](https://github.com/hntrl)! - fix(sdk): coalesce locally
resolved interrupts
  
when resolving interrupts using `useStream`, there was a case where we
prioritized the remote state values (which we lookup in React Strict
mode on every page transition) over the local interrupt responses we
know we've responded with.
  
This has since been fixed to first prioritize the local cache of
interrupts, resolved against the remote state values when a run hits a
terminal event
## @langchain/angular@1.1.0

### Minor Changes

- [#2831](#2831)
[`3234c69`](3234c69)
Thanks [@ramon-langchain](https://github.com/ramon-langchain)! - Expose
connection lifecycle callbacks for built-in streaming transports.
  
`onConnected` runs after the initial SSE or WebSocket connection becomes
usable and after every successful reconnect. Its payload distinguishes
an `initial` connection from a `reconnected` connection and includes the
reconnect attempt number.
  
`onReconnect` now also receives the scheduled `delayMs`, allowing
applications to display accurate retry state before the next connection
attempt. React, Vue, Svelte, and Angular stream hooks forward both
callbacks.

### Patch Changes

- Updated dependencies
[[`3234c69`](3234c69),
[`3234c69`](3234c69),
[`11a4535`](11a4535),
[`2fab6fd`](2fab6fd),
[`4fc118f`](4fc118f),
[`db4bdad`](db4bdad),
[`55fa26b`](55fa26b)]:
  - @langchain/langgraph-sdk@1.11.0
## @langchain/react@1.1.0

### Minor Changes

- [#2831](#2831)
[`3234c69`](3234c69)
Thanks [@ramon-langchain](https://github.com/ramon-langchain)! - Expose
connection lifecycle callbacks for built-in streaming transports.
  
`onConnected` runs after the initial SSE or WebSocket connection becomes
usable and after every successful reconnect. Its payload distinguishes
an `initial` connection from a `reconnected` connection and includes the
reconnect attempt number.
  
`onReconnect` now also receives the scheduled `delayMs`, allowing
applications to display accurate retry state before the next connection
attempt. React, Vue, Svelte, and Angular stream hooks forward both
callbacks.

### Patch Changes

- Updated dependencies
[[`3234c69`](3234c69),
[`3234c69`](3234c69),
[`11a4535`](11a4535),
[`2fab6fd`](2fab6fd),
[`4fc118f`](4fc118f),
[`db4bdad`](db4bdad),
[`55fa26b`](55fa26b)]:
  - @langchain/langgraph-sdk@1.11.0
## @langchain/svelte@1.1.0

### Minor Changes

- [#2831](#2831)
[`3234c69`](3234c69)
Thanks [@ramon-langchain](https://github.com/ramon-langchain)! - Expose
connection lifecycle callbacks for built-in streaming transports.
  
`onConnected` runs after the initial SSE or WebSocket connection becomes
usable and after every successful reconnect. Its payload distinguishes
an `initial` connection from a `reconnected` connection and includes the
reconnect attempt number.
  
`onReconnect` now also receives the scheduled `delayMs`, allowing
applications to display accurate retry state before the next connection
attempt. React, Vue, Svelte, and Angular stream hooks forward both
callbacks.

### Patch Changes

- Updated dependencies
[[`3234c69`](3234c69),
[`3234c69`](3234c69),
[`11a4535`](11a4535),
[`2fab6fd`](2fab6fd),
[`4fc118f`](4fc118f),
[`db4bdad`](db4bdad),
[`55fa26b`](55fa26b)]:
  - @langchain/langgraph-sdk@1.11.0
## @langchain/vue@1.1.0

### Minor Changes

- [#2831](#2831)
[`3234c69`](3234c69)
Thanks [@ramon-langchain](https://github.com/ramon-langchain)! - Expose
connection lifecycle callbacks for built-in streaming transports.
  
`onConnected` runs after the initial SSE or WebSocket connection becomes
usable and after every successful reconnect. Its payload distinguishes
an `initial` connection from a `reconnected` connection and includes the
reconnect attempt number.
  
`onReconnect` now also receives the scheduled `delayMs`, allowing
applications to display accurate retry state before the next connection
attempt. React, Vue, Svelte, and Angular stream hooks forward both
callbacks.

### Patch Changes

- Updated dependencies
[[`3234c69`](3234c69),
[`3234c69`](3234c69),
[`11a4535`](11a4535),
[`2fab6fd`](2fab6fd),
[`4fc118f`](4fc118f),
[`db4bdad`](db4bdad),
[`55fa26b`](55fa26b)]:
  - @langchain/langgraph-sdk@1.11.0
## @langchain/langgraph-api@1.4.6

### Patch Changes

- [#2751](#2751)
[`0a9611a`](0a9611a)
Thanks [@t3s7r](https://github.com/t3s7r)! - fix(api): `run.start` with
input on a cancelled thread no longer folds the input into
`Command(resume)`. A cancelled run shares the "interrupted" status with
a genuine `interrupt()` pause, but has no pending interrupt to consume
the resume value, so the submitted message was silently dropped. The
input-vs-resume decision now keys on whether the thread actually has
pending interrupts (both in the protocol service and the embed
protocol).
- Updated dependencies []:
  - @langchain/langgraph-ui@1.4.6
## @langchain/langgraph-cli@1.4.6

### Patch Changes

- Updated dependencies
[[`0a9611a`](0a9611a)]:
  - @langchain/langgraph-api@1.4.6
## @langchain/langgraph@1.4.15

### Patch Changes

- [#2794](#2794)
[`83a4b62`](83a4b62)
Thanks [@hntrl](https://github.com/hntrl)! - feat(langgraph): add
per-node `tracePolicy` input/output processors and `omitPayload`
  
Transform the payloads recorded on a node's own trace run while
retaining its span and timing. Processors receive raw values and fall
back to the original payload if they throw. Graph state, root runs, and
child runs remain unchanged when processors do not mutate their
arguments.
  
Matches Python's callback-level behavior: transforms also affect chain
events and message streaming, so omitting outputs can suppress messages
returned directly by nodes and omitting inputs can affect message
deduplication.
- Updated dependencies
[[`3234c69`](3234c69),
[`3234c69`](3234c69),
[`11a4535`](11a4535),
[`2fab6fd`](2fab6fd),
[`4fc118f`](4fc118f),
[`db4bdad`](db4bdad),
[`55fa26b`](55fa26b)]:
  - @langchain/langgraph-sdk@1.11.0
## @langchain/langgraph-cua@1.0.3

No changes in this release.
## @langchain/langgraph-supervisor@1.1.2

No changes in this release.
## @langchain/langgraph-swarm@1.0.3

No changes in this release.
## @langchain/langgraph-ui@1.4.6

No changes in this release.

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
@github-actions github-actions Bot mentioned this pull request Sep 12, 2026
Open
This was referenced Sep 16, 2026
Closed
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@eliornl @hntrl