fix(langgraph): resolve 4 bugs in channels, graph branching, and pregel subgraph traversal#6944
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Subham Singhania (Subhamsinghania18) wants to merge 4 commits intolangchain-ai:mainfrom
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…OperatorAggregate MISSING fast-path When a BinaryOperatorAggregate channel has no default constructor and self.value starts as MISSING, the update() method fast-pathed the first value directly into self.value without inspecting it. This had two observable bugs: 1. An Overwrite(x) wrapper was stored verbatim, so channel.get() returned Overwrite(value=x) instead of x. Graph output was silently wrong. 2. seen_overwrite was never set in the MISSING branch, so passing two Overwrite() values in the same super-step raised no error, violating the documented one-Overwrite-per-super-step contract. The fix moves seen_overwrite initialisation before the MISSING check and runs _get_overwrite() on the first value just like the loop does for all subsequent values. Closes: langchain-ai#6909
…and copy Topic.from_checkpoint() only guarded against MISSING. When a channel key exists in checkpoint['channel_values'] with an explicit None value (observed with the PostgreSQL checkpointer after partial writes), None was stored in self.values. The subsequent call to Topic.copy() then crashed with: AttributeError: 'NoneType' object has no attribute 'copy' This surfaced at runtime inside local_read(fresh=True), typically triggered by a conditional edge or middleware retry, making it a hard crash with no recovery path. Changes: - from_checkpoint: treat None identically to MISSING so self.values always defaults to the empty list defined by __init__. - copy: add a defensive guard (self.values is not None) so even if None leaks in from an unexpected path the method degrades gracefully to an empty list rather than crashing. Closes: langchain-ai#6791
…ath_map When add_conditional_edges() is called with an explicit path_map, the _finish() method blindly looked up every router return value in self.ends. If the router returned '__end__' (END) without a corresponding '__end__': '__end__' entry in the map it raised: KeyError: '__end__' This was inconsistent: when no path_map is supplied, returning END works fine. The fix special-cases r == END so that it passes through directly without a dict lookup, matching the no-path_map behaviour and user expectations. Returning END without an explicit mapping is intuitive — users should not have to enumerate the terminal condition as a dict entry just because they supplied other mappings. Closes: langchain-ai#6770
get_subgraphs() used str.startswith(name) to filter nodes by namespace.
If two sibling nodes share a common name prefix — e.g. 'agent' and
'agent_v2' — and are added in that order, the iteration could match the
shorter name ('agent') against a namespace meant for the longer one
('agent_v2|child'), then strip the wrong number of characters from the
namespace string, producing garbage like 'v2|child' for the recursive
call. The recursion found nothing and the method returned an empty
iterator instead of the expected subgraph.
The fix replaces the startswith check with an exact delimiter-aware test:
namespace == name or namespace.startswith(name + NS_SEP)
This ensures 'agent' only matches the namespace 'agent' or 'agent|...',
never 'agent_v2|...'. NS_SEP is the '|' constant already used
throughout the module.
aget_subgraphs() delegates entirely to get_subgraphs(), so a single
change is sufficient.
Closes: langchain-ai#6924
Joehunk
reviewed
Mar 1, 2026
| # incorrectly match the longer namespace. Fixes #6924. | ||
| if namespace is not None: | ||
| if not namespace.startswith(name): | ||
| if namespace != name and not namespace.startswith(name + NS_SEP): |
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Note, already fixed in #6366 (which also includes a repro test case).
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Four independent bug fixes in the core
langgraphpackage:BinaryOperatorAggregate:Overwritenot unwrapped when initial value isMISSINGWhen a channel has no default constructor (
self.value = MISSING), theupdate()fast-path assigned the first value raw without calling_get_overwrite(). This stored theOverwrite(x)wrapper object insteadof
x, producing silently wrong graph output. It also never setseen_overwrite, so passing twoOverwritevalues in the same super-stepraised no error, violating the one-Overwrite-per-super-step contract.
Closes #6909
Topic.copy()crashes withAttributeErrorwhen checkpoint holdsNoneTopic.from_checkpoint()only guarded againstMISSING. When a channelkey existed in
checkpoint["channel_values"]with an explicitNone(observedwith the PostgreSQL saver after partial writes),
Nonewas stored inself.values. The subsequent call toTopic.copy()inlocal_read()thencrashed:
AttributeError: 'NoneType' object has no attribute 'copy'
from_checkpointnow treatsNoneidentically toMISSING.copy()hasan added defensive guard for belt-and-suspenders safety.
Closes #6791
3.
KeyError('__end__')when conditional router returnsENDwithout explicitpath_mapentry_finish()inBranchSpeclooked up every router return value inself.endsunconditionally. If a router returnedENDwithout an explicit"__end__": "__end__"entry in the path map, it raisedKeyError.ENDis now treated as a pass-through, consistent with the no-path-mapbehaviour.
Closes #6770
4.
get_subgraphs()fails when node names share a common string prefixget_subgraphs()usednamespace.startswith(name)to filter nodes. Ifsibling nodes share a prefix (e.g.
"agent"and"agent_v2"), the shortername false-matched the longer namespace, stripped the wrong number of
characters, and passed a garbage string into the recursive call — returning
an empty iterator instead of the expected subgraph.
The fix uses an exact delimiter-aware check:
namespace == name or namespace.startswith(name + "|")Closes #6924
Issue: #6909, #6791, #6770, #6924
Dependencies: None