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Research: Skyrim Havok / Animation Reference

Date: 2026-08-18 Purpose: fact base for a Skyrim Havok/animation reference guide at reference/bethesda/ (alongside fallout4-havok-guide.md and reference/havok/hkx_format.md). Scope: animation/Havok focus; BSA/BA2, .pex, and plugin formats covered briefly.

1. Havok version(s) — LE vs SE vs AE​

Skyrim Legendary Edition (LE, 2011)​

  • Uses the Havok 2010.2.0 SDK; contents version string hk_2010.2.0-r1.
  • Source: figment/hkxcmd README (only converts "native HKX files built with Havok 2010.2.0 SDK") — https://github.com/figment/hkxcmd
  • Source: Dexesttp/hkxpack README ("support for hk_2010.2.0-r1 or other hkx formats may be added"; target format is hk_2014.1.0-r1 = FO4) — https://github.com/Dexesttp/hkxpack
  • LE packfiles are 32-bit (x86), little-endian.
  • Tooling for LE (hkxcmd) required the Intel "Havok PcXs Content Tools 2010.2.0" / Preview Tool.

Skyrim Special Edition (SE, 2016) and Anniversary Edition (AE, 2021)​

  • SE packfile version 8 (per existing repo doc reference/havok/hkx_format.md, line 33: version = 11 for FO4, 8 for Skyrim SE), 64-bit pointers, little-endian. This is the pre-2015 "old" packfile layout shared with FO4 (which uses version 11), NOT the newer FourCC-chunked container.
  • SE/AE behavior/animation .hkx are 64-bit; LE ones are 32-bit — a key LE→SE porting difference. (Implied by hkx_format.md header pointer-size note: bytes_in_pointer = 8 for 64-bit, 4 for x86 packfiles.)
  • SE introduced BSA v105 (LZ4 compression) vs LE's BSA v104 (zlib) — see section 3.
  • AE is the 2021 re-release (update 1.6+, current 1.6.1170); it keeps SE's Havok/packfile layout. ESL plugins introduced in SE update 1.5 (see §3).

Comparison: Fallout 4 (for reference)​

  • FO4 uses Havok 2014.1.0, contents hk_2014.1.0-r1, packfile version 11, 64-bit — see reference/havok/hkx_format.md.
  • So Skyrim (both LE and SE) predates FO4's Havok version; class layouts and packfile version differ (8 vs 11).

Gotcha from hkxcmd README​

  • The standard Havok SDK (2010.2) ships Physics + Animation support only. Bethesda also uses Havok Behavior classes, which the stock SDK cannot read — those files must be handled by the Behavior Tool / community reflection tools, not the plain SDK converters.

2. Animation files & workflow​

File types (all are Havok .hkx packfiles unless noted)​

  • Skeleton: meshes\actors\<race>\character assets\skeleton.nif (NetImmerse) plus a matching skeleton.hkx (hkaSkeleton). The .nif holds the bone nodes; the .hkx specifies which nodes are bones and the bind pose.
  • Animation: individual .hkx (hkaAnimationBinding + hkaInterleavedUncompressedAnimation / spline-compressed variants).
  • Behavior: behavior-project files (hkbBehaviorGraph / hkbBehaviorProjectData), one per actor/race in meshes\actors\<race>\ (e.g. 0_master.hkx).
  • Project file: the top-level behavior project .hkx referencing the skeleton, behavior file, and animations (hkbBehaviorProjectData) — the FO4 guide documents the same structure (project/behavior/skeleton/animation).

Behavior project workflow (Havok Content Tools / Behavior Tool)​

  • Authored in the Havok Behavior Tool (Intel "Havok Content Tools 2010.2"), the same tool family as FO4's (which uses Havok 2014's Behavior Tool).
  • Workflow (shared with FO4, per fallout4-havok-guide.md): create a project file that references a rig (skeleton, rigName → .hkt tagfile), a behavior file (behaviorFilename → .hkx), and animation folders; pack loose files into .hkx.
  • The FO4 guide's structure (GenericBehaviors / Behaviors / CharacterAssets folders; .hkt intermediate tags; #NN object references) mirrors Skyrim's.
  • Pack/unpack via hkxcmd (convert HKX↔XML, KF↔HKX) and the Havok Preview Tool for previewing animations on a skeleton.

hkxcmd workflow (LE; from figment/hkxcmd README)​

  • hkxcmd convert <dir> — packed HKX → XML HKX (loadable in Preview Tool).
  • hkxcmd exportkf <dir> — HKX → Gamebryo .kf (for 3ds Max via niftools importer).
  • hkxcmd convertkf <skeleton.hkx> <anim.kf> <out.hkx> — KF → HKX (-v:XML for XML output).
  • hkxcmd DumpText — dumps skeleton bone list.
  • Known limits: no non-uniform scale in KF; no visibility/camera tracks; 1-to-1 bone→track assumption.

Ragdoll​

  • Ragdolls/physics (hkbRagdollDriver, constraint setups) live in the behavior project; physics packfiles use Havok's Physics module (2010.2-era classes). (Note: ragdoll specifics not deeply verified this pass — needs a dedicated research round if the guide covers ragdoll internals.)

3. File containers (brief)​

BSA / BA2​

  • BSA is Skyrim's archive (BSA, ESM/ESP, and ESS are IFF variants).
  • LE v104 (0x68), compression zlib; SE v105 (0x69), compression LZ4. Source: UESP "Skyrim Mod:Archive File Format".
  • Header: "BSA\0", version, offset, archiveFlags (bit 1 include dir names, bit 2 include file names, bit 3 compressed archive, bit 7 Xbox big-endian, bit 9 embed file names...), folderCount, fileCount, total name lengths, fileFlags (mesh/texture/.../misc type bits), padding.
  • Folder record: nameHash, count, offset (+v105-only padding fields).
  • File record: nameHash, size (bit 30 = per-file compression toggle), offset.
  • Names sorted by 64-bit hash.
  • Tools: BSAOpt, B.A.E (Bethesda Archive Extractor) (SE Nexus mod 974, also handles BA2/Starfield), FO3 Archive Utility, FOMM/OBMM unpackers.
  • BA2 is FO4's format (not Skyrim); SE uses BSA.

.pex (compiled Papyrus scripts)​

  • Magic 0xFA57C0DE, big-endian regardless of platform, versions 3.0–3.1 (Dawnguard/Hearthfire/Dragonborn scripts = 3.1).
  • Header: magic, majorVersion, minorVersion, gameId, compilationTime, sourceFileName, username, machinename, then string table, debug info, user flags, objects.
  • Objects contain variables/properties/states/functions; bytecode opcodes (00 nop … 23 array_rfindelement). Source: UESP "Skyrim Mod:Compiled Script File Format".
  • .psc (source) is compiled to .pex by the Papyrus compiler (in the Creation Kit); decompilers exist (Caprica/Champollion, TESVTranslator, xTranslator).

Plugin format (ESM / ESP / ESL)​

  • Plugins are record/group/field files: a TES4 header record + top-level GRUP groups containing records (form IDs), each record has fields.
  • ESM master (pinned top of load order), ESP plugin, ESL light master (up to 4096 plugins in load order; up to 4096 form-ID records — 2048 before SE patch 1.6.1130, which reserved 0x0000–0x07FF).
  • ESL introduced with Creation Club/"Creations" in SE update 1.5; works in AE, not in LE/pre-1.5 SE.
  • Plain ESP references are treated as permanent (all loaded, count toward the reference cap) — prefer ESL/ESM.
  • Group types 0–9 (top/type, world children, interior/exterior cell blocks, cell children, topic children, persistent/temporary children).
  • Source: UESP "Skyrim Mod:Mod File Format".

4. Modding tools​

ToolWhat it doesURL
hkxcmd (figment)Havok CLI: HKX↔XML, HKX↔KF, skeleton dump; LE 2010.2https://github.com/figment/hkxcmd
NifSkope (niftools).nif (NetImmerse) viewer/editor; Skyrim/FOhttps://github.com/niftools/nifskope
hkxpack (Dexesttp)pack/unpack HKX↔TagXML (FO4 2014.1 target; 2010.2 noted)https://github.com/Dexesttp/hkxpack
hkxanim (Dexesttp)FBX→HKX animation creator (FO4 focus)https://github.com/Dexesttp/hkxanim
Havok Content Tools / Behavior Tool / Preview Tool (Intel 2010.2)author/preview behaviors & animations(Intel Havok site; 2010.2.0 build)
FNIS (Fore)"Fores New Idles" animation/idle patcherSkyrim Nexus mod 11811
Nemesis (Project New Reign)SE/AE behavior/animation engine patcherSE Nexus mod 60033
SKSE (Skyrim Script Extender)DLL script extender; needed by most modshttps://skse.silverlock.org/
B.A.E (Bethesda Archive Extractor)unpack BSA/BA2SE Nexus mod 974
TES5Edit / xEditplugin editing/cleaningNexus / github (zilav)
Wrye Bashmod install/management, bashed patchSkyrim Nexus mod 1840
PyNifly (BadDogSkyrim)Blender addon: Skyrim/FO models+animationshttps://github.com/BadDogSkyrim/PyNifly
hkxparse (exyorha)C++ Havok packfile/tagfile parser; 2010 onlyhttps://github.com/exyorha/hkxparse

SKSE builds (from skse.silverlock.org, verified 2026-08-14):

  • Classic (LE): 1.7.3
  • SE: 2.0.20 (game 1.5.97)
  • AE: 2.2.6 (game 1.6.1170); GOG AE 2.2.6 (1.6.1179)
  • VR: 2.0.12 (1.4.15)
  • No support for Windows Store/Game Pass or Epic releases.

5. Community documentation URLs (collected)​

Verified fetched (content confirmed):

Could NOT access (blocked/empty — user may want to paste content):

  • ck.uesp.net (Creation Kit wiki, UESP-hosted) — HTTP 403 Forbidden on multiple pages (Animation_Files, Havok_Behavior_Tool, BSA, Mod_File_Format). This is the main lost source; it has the Skyrim behavior-project/animation docs.
  • www.creationkit.com — currently "down for backend maintenance" (the Havok_Behavior_Tool page returns a maintenance page). Try again later or use a web-archive snapshot: https://web.archive.org/web/*/www.creationkit.com/index.php?title=Havok_Behavior_Tool
  • UESP raw pages return empty body via the normal .wiki/ URL (worked via /w/index.php?title=...&action=raw). Some guessed page names 404 (Animation_Files, Animation_File_Format, Papyrus, Plugin_File_Format, BSA_File_Format).

Notes / confidence​

  • HIGH (verified official/primary): SKSE builds; hkxcmd README (Havok 2010.2.0 for LE); hkxpack/hkxanim; niftools/nifskope; UESP archive/plugin/pex formats; Nemesis/FNIS/B.A.E URLs; existing repo hkx_format.md (SE packfile version 8).
  • MEDIUM (inference from adjacent sources): SE 64-bit vs LE 32-bit behavior .hkx; AE keeping SE's packfile layout (implied by SE→AE continuity, not a single authoritative statement).
  • The Skyrim behavior-project internals (hkbBehaviorProjectData structure, Behavior Tool authoring specifics) are NOT confirmed this pass — ck.uesp.net was blocked and creationkit.com is down. Recommend a follow-up fetch of a web-archive snapshot of ck.uesp.net/wiki/Havok_Behavior_Tool and ck.uesp.net/wiki/Animation_Files before writing that section of the guide.