{"number":149,"title":"Segregated Witness (second deployment)","authors":["Shaolin Fry"],"status":"Closed","type":"Specification","layer":"Consensus (soft fork)","created":"2017-04-14","license":"BSD-3-Clause OR CC0-1.0","requires":[],"replaces":[],"replacedBy":[],"discussion":[],"abstract":"This document specifies a user activated soft fork for BIP141 , BIP143 and BIP147 using versionbits with guaranteed lock-in.","eli5":"Proposed a second attempt at activating SegWit using version-bits signaling with guaranteed lock-in after a deadline. It was offered as an alternative path in case the original SegWit deployment failed to gain enough miner support.","categories":["consensus"],"filename":"bip-0149.mediawiki","githubUrl":"https://github.com/bitcoin/bips/blob/master/bip-0149.mediawiki","format":"mediawiki","readingMinutes":1,"inbound":1,"revisions":9,"impact":33,"core":false,"references":[8,9,141,143,147],"lastModified":"2025-10-22","html":"

Abstract

\n

This document specifies a user activated soft fork for BIP141, BIP143 and BIP147 using versionbits with guaranteed lock-in.

\n

Motivation

\n

Miners have been reluctant to signal the BIP9 segwit deployment despite a large portion of the Bitcoin ecosystem who want the soft fork activated. This BIP specifies a user activated soft fork (UASF) that deploys segwit again using versionbits with guaranteed lock-in on timeout if the BIP is not already locked-in or activated by the timeout. This ensures users have sufficient time to prepare and no longer require a miner supermajority, while still allowing for an earlier miner activated soft fork (MASF).

\n

Reference implementation

\n

The reference implementation will refuse to run on Bitcoin mainnet before 7 November 2017, and can only be run on testnet and regtest until then.

\n

https://github.com/bitcoin/bitcoin/compare/master...shaolinfry:uasegwit-flagday

\n

Specification

\n

This deployment will set service bit (1<<5) as NODE_UAWITNESS.

\n

Deployment

\n

This BIP should only be deployed if BIP9-segwit fails to lock-in or activate before timeout on 15 November 2017. This BIP cannot be deployed before 15 November 2017.

\n

This BIP will be deployed by BIP8 with the name \"segwit\" and using bit 1.

\n

For Bitcoin mainnet, the BIP8 starttime will be midnight 16 November 2017 UTC (Epoch timestamp 1510790400) and BIP8 timeout will be 4 July 2018 UTC (Epoch timestamp 1530662400).

\n

For Bitcoin testnet, segwit is already activated so no deployment is specified.

\n

Backwards Compatibility

\n

This deployment reuses the GBT deployment name \"segwit\" to maintain compatibility with existing mining software.

\n

This deployment is incompatible with the BIP9-segwit deployment and should not be run concurrently with it.

\n

Rationale

\n

The starttime of this BIP is after the BIP9-segwit timeout to remove compatibility issues with old nodes.

\n

References

\n

Mailing list discussion

\n

BIP8

\n

BIP9

\n

BIP141

\n

BIP143

\n

BIP147

\n

Copyright

\n

This document is dual licensed as BSD 3-clause, and Creative Commons CC0 1.0 Universal.

","toc":[{"id":"abstract","title":"Abstract","level":2},{"id":"motivation","title":"Motivation","level":2},{"id":"reference-implementation","title":"Reference implementation","level":2},{"id":"specification","title":"Specification","level":2},{"id":"deployment","title":"Deployment","level":2},{"id":"backwards-compatibility","title":"Backwards Compatibility","level":2},{"id":"rationale","title":"Rationale","level":2},{"id":"references","title":"References","level":2},{"id":"copyright","title":"Copyright","level":2}],"history":{"totalCommits":9,"firstCommit":"2017-05-01T16:09:18+00:00","lastUpdated":"2025-10-22T20:02:57+10:00","events":[{"sha":"2885f13d3f37890e328683166dbcbc60b488d13a","date":"2025-10-22T20:02:57+10:00","author":"Anthony Towns","message":"Convert licenses to SPDX codes","url":"https://github.com/bitcoin/bips/commit/2885f13d3f37890e328683166dbcbc60b488d13a","kind":"rename"},{"sha":"24e96e870fffaa257b465ce1f0370c14aac588e8","date":"2025-10-08T14:59:36-07:00","author":"Murch","message":"process: Created ↦ Assigned","url":"https://github.com/bitcoin/bips/commit/24e96e870fffaa257b465ce1f0370c14aac588e8","kind":"edit"},{"sha":"5207ef92a52d0b8902895b0dbb40823a4d698ec9","date":"2025-04-11T20:08:17-07:00","author":"Murch","message":"process: Author ↦ Authors","url":"https://github.com/bitcoin/bips/commit/5207ef92a52d0b8902895b0dbb40823a4d698ec9","kind":"edit"},{"sha":"59730dec4fb641a41589aa1b91d3c452f13e0e36","date":"2025-04-11T20:05:13-07:00","author":"Murch","message":"process: Remove Comments-URI and -Summary","url":"https://github.com/bitcoin/bips/commit/59730dec4fb641a41589aa1b91d3c452f13e0e36","kind":"edit"},{"sha":"a233bde4af52c6a11269e2e6dd9707abce441ffe","date":"2025-04-11T18:19:21-07:00","author":"Murch","message":"process: Standards Track ↦ Specification","url":"https://github.com/bitcoin/bips/commit/a233bde4af52c6a11269e2e6dd9707abce441ffe","kind":"edit"},{"sha":"66defbdc03ad64ccddfd373e1c59af9a41ab990f","date":"2025-04-11T18:11:55-07:00","author":"Murch","message":"process: Deferred/Obsolete/Rejected/Replaced/Withdrawn ↦ Closed","url":"https://github.com/bitcoin/bips/commit/66defbdc03ad64ccddfd373e1c59af9a41ab990f","kind":"status"},{"sha":"d98333302f2a41114c6f006a77a51edc3d1e743b","date":"2017-08-11T05:36:07+00:00","author":"shaolinfry","message":"Update BIP status","url":"https://github.com/bitcoin/bips/commit/d98333302f2a41114c6f006a77a51edc3d1e743b","kind":"status"},{"sha":"37d7e90eb70d7ad3b2b8a2b793721ddc94db181b","date":"2017-05-06T18:11:34+00:00","author":"shaolinfry","message":"BIP8: clarify deployment terms","url":"https://github.com/bitcoin/bips/commit/37d7e90eb70d7ad3b2b8a2b793721ddc94db181b","kind":"edit"},{"sha":"54f63e56be6f3a7c52ded255b98bcbcee1fef604","date":"2017-05-01T16:09:18+00:00","author":"shaolinfry","message":"Assign BIP149","url":"https://github.com/bitcoin/bips/commit/54f63e56be6f3a7c52ded255b98bcbcee1fef604","kind":"edit"}]}}