bug(core): RunnableBranch._invoke falls through to default on falsy branch output (0/""/false)

Checked other resources

• This is a bug, not a usage question. For questions, please use the LangChain Forum (https://forum.langchain.com/).
• I added a very descriptive title to this issue.
• I searched the LangChain.js documentation with the integrated search.
• I used the GitHub search to find a similar question and didn't find it.
• I am sure that this is a bug in LangChain.js rather than my code.
• The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package).

Example Code

import { RunnableBranch } from "@langchain/core/runnables";

const branch = RunnableBranch.from([ [(x: number) => x > 0, (x: number) => 0], (x: number) => -1, ]); console.log(await branch.invoke(5)); // expected: 0, actual: -1

Error Message and Stack Trace (if applicable)

N/A - no throw, silent wrong routing

Description

• I'm trying to use RunnableBranch where a branch returns 0 / "" / false.
• I expect the matched branch output to be returned.
• Instead libs/langchain-core/src/runnables/branch.ts:170 does if (!result) so falsy output is discarded and default runs. Same file :236 in _streamIterator correctly uses if (stream === undefined).

System Info

platform: win32 x64 (Windows) Node: v24.6.0 npm: 11.5.1 pnpm: 10.14.0 @langchain/core: 1.2.9 HEAD: 56130def8 chore: version packages (rc) (#11532) File: libs/langchain-core/src/runnables/branch.ts:170