Research: Starfield (Creation Engine 2, 2023) — engine, animation, file formats, modding

Date: 2026-08-18 Status: research notes for a reference guide at reference/bethesda/ Confidence: HIGH for facts sourced from primary code (CommonLibSF, xEdit/BSArch) and official sites (SFSE, Steam); MEDIUM where flagged.

Scope: Starfield (2023) keeps the Bethesda Havok-based .hkx animation lineage from Fallout 4 (unlike Oblivion Remastered, which is a UE5 hybrid). CE2 drives animation through the Animation Graph (AnimGraph) framework — the same wrapper around Havok behavior that Skyrim SE / FO4 use. It is 64-bit throughout.

1. Engine — Creation Engine 2

Fact	Detail	Source (URL)	Confidence
Game	Starfield, released 2023-09-05 (Steam AppID 1716740); expansion Shattered Space 2024-09-30 (AppID 2721670)	Steam Store AppDetails API	HIGH
Engine	Creation Engine 2 (CE2), successor to the Creation Engine (FO4/Skyrim). 64-bit (x64).	CommonLibSF (x64, `SF_RTTI_VTABLE` etc.)	HIGH
Official CK	Starfield Creation Kit released 2024-06-09 (Steam AppID 2722710, free)	Steam Store AppDetails	HIGH
Script extender	SFSE (Starfield Script Extender) by ianpatt/behippo; Steam only (Game Pass NOT supported); build 0.2.21 / game 1.16.244	sfse.silverlock.org; github.com/ianpatt/sfse; nexusmods.com/starfield/mods/106	HIGH

2. Animation — Havok `.hkx` + AnimGraph (CONFIRMED lineage)

The single most important finding: Starfield retains the Havok behavior/animation .hkx lineage, exposed through the Animation Graph (AnimGraph) framework.

Primary-source evidence from CommonLibSF (include/RE/):

BSAnimationGraph — base class (derives BSIntrusiveRefCounted + several BSTEventSources). Subclass AnimationManager : public BSAnimationGraph.
BSAnimationGraphManager (sizeof == 0x80) — the graph manager object.
BGSAnimationGraphComponent (sizeof == 0x40) — Form component carrying the graph project/behavior names as BSFixedString members (unk10..unk28) + a smart-ptr.
IAnimationGraphManagerHolder — interface with NotifyAnimationGraphImpl, GetAnimationGraphManagerImpl, GetGraphVariableImpl{Float,Int,Bool}, and Pre/PostUpdateAnimationGraphManager. This is identical in role to the Skyrim SE BGSAnimationGraphManager / IAnimationGraphManagerHolder AnimGraph system.
No hkb* or hkx* Havok types are exposed in CommonLibSF — Havok behavior is handled opaquely behind the AnimGraph interface (same as Skyrim SE).

Implication (MEDIUM-HIGH confidence, inference from architecture):

Starfield .hkx files (Havok packfile, same magic 0x57e0e057 0x10c0c010) still carry behavior graphs (hkbBehaviorGraph), skeletons (hkaSkeleton), and animation bindings (hkaAnimationBinding), organized into project / character / behavior file sets.
The AnimGraph framework (graph variables, NotifyAnimationGraph events) is the high-level API over those .hkx files — the same design Bethesda used since Skyrim SE.
Behavior project authoring tooling for CE2 (official Havok Behavior Tool or a community equivalent) is not confirmed — see Open Questions.

Fact	Detail	Source	Confidence
Animation API	CE2 exposes `BSAnimationGraph`/`BSAnimationGraphManager`/`BGSAnimationGraphComponent`/`IAnimationGraphManagerHolder`	CommonLibSF include/RE (fetched 2026-08-18)	HIGH (primary code)
Behavior events	`NotifyAnimationGraph(const BSFixedString&)`, graph variables (float/int/bool) via `GetGraphVariable*`	CommonLibSF IAnimationGraphManagerHolder.h	HIGH
`.hkx` files	`.hkx` is a first-class `meshes` asset-folder extension in BSArch (see §3)	xEdit wbBSArchive.pas (`atMesh` ext list)	HIGH
Havok version string	NOT confirmed (e.g. does CE2 use `hk_2019.2`/`hk_2023.x`? no public contents-version fact found)	—	LOW / gap
`.hkx` class layout vs FO4	Whether `hkb`/`hka` class layouts changed vs FO4's 2014 SDK — unverified; FO4-era tools (hkxcmd/hkxpack) may not round-trip	—	gap

3. File formats — BA2 archives & assets (CONFIRMED from xEdit/BSArch source)

Authoritative from TES5Edit/TES5Edit Core/wbBSArchive.pas (dev-4.1.6), fetched 2026-08-18. This is the same BSArch engine that packs/unpacks the game archives.

BA2 / BSA header versions (magic 4 bytes)

MAGIC_BSA = 'BSA\0' (classic TES4/FO3/SSE .bsa)
MAGIC_BTDX = 'BTDX' (FO4 / Starfield .ba2)
MAGIC_GNRL = 'GNRL', MAGIC_DX10 (BA2 subtype: general vs texture/DDS)
Version values:
0x67 TES4 (Oblivion), 0x68 FO3/FNV/TES5, 0x69 SSE
0x01 FO4, 0x07/0x08 FO4 NG/AE
0x02 Starfield v2, 0x03 Starfield v3 (SF)

Starfield BA2 header layout (after `BTDX` magic)

Version — 0x02 (SFv2) or 0x03 (SFv3)
Magic2 — GNRL (general) or DX10 (textures/DDS); DX10 → the archive is a DDS archive
FileCount (uint32)
FileTableOffset (int64)
SF v2+: ReadUInt64 — "always set to 1, immediately discarded on load" (padding/hash placeholder)
SF v3+: CompressionMethod (uint32):

COMPRESSION_METHOD_ZLIB = 0 (default)
COMPRESSION_METHOD_LZ4 = 3

Compression support (per archive type; first = default)

baSF (Starfield general): [ctZLib, ctLZ4] → default ZLib
baSFdds (Starfield DDS/texture): [ctLZ4, ctZLib] → default LZ4
(FO4 general baFO4: [ctZLib]; SSE baSSE: [ctLZ4F])
Note (line 1608): "official Archive2 tool creates v3 archives for lz4 compression only" → Starfield v3 (0x03) header + CompressionMethod is used specifically when LZ4 is chosen.

Asset folders (root + extensions) — from `cBSAssets` table

meshes (atMesh): .nif .kf .kfm .egm .egt .tri .psa .hkt .hkx .ssf .btr .bto .btt .dtl → .hkx animation files live in meshes/; NIF .nif is still a mesh extension (FO4-era).
geometries (atGeometry): .mesh → Starfield's new geometry format, root geometries/
textures (atTexture): .dds .tga .png
materials (atMaterial): .bgsm .bgem
scripts (atScript): .pex .psc; source\scripts (atSourceSSE): .psc
strings (atStrings): .strings .ilstrings .dlstrings
sound\voice, sound, music, menus, interface, seq, vis, grass, trees, video etc.

Important distinction: `.nif` vs `.mesh`

xEdit whatsnew (dev-4.1.6): "Add a Geometries root asset folder for Starfield .mesh files."
Confirmed in cBSAssets: geometry uses .mesh under geometries/, while the meshes/ root still lists .nif (legacy/FO4-era). So Starfield ships geometry as .mesh but the archive system still understands NIF .nif + .hkx in meshes/.
Whether CE2 runtime reads .nif directly is not fully confirmed; the xEdit model treats .mesh as the current geometry and .nif as legacy. MEDIUM confidence.

4. Plugin (.esm/.esp) format — CONFIRMED from xEdit whatsnew (dev-4.1.6)

Starfield .esp editing enabled with small/medium master support.
Master limits: up to 252 full, 4095 light, 254 medium masters in one file (vs FO4's 254 full / 4095 light).
"small or medium masters" + blueprint master concept: modules with a blueprint master can't be saved; blueprints can't be masters.
An .esp may be loaded as a master for editing but cannot be saved as a master; small/medium flags cannot be set on .esps.
Adding a master also adds its required up-chain masters (engine requires the full master chain).
Starfield record updates: SCEN changes (v1.8.83.0), Freelanes, Reflection organization, new DLC records; navmesh cover flag addition; wbDOBJ etc. moved to Common.
xEdit ships Core/Hardcoded/Starfield.esm + Core/wbDefinitionsSF1.pas (Starfield record definitions, ~19k lines).
.mesh geometry handled in xEdit's SNIFF via Sniff/Proc/ProcAnalyzeMesh.pas (uses zeux/meshoptimizer for vertex-cache/fetch efficiency metrics; also checks NIF NifVersion >= nfFO4).

5. Modding tools (Starfield)

Tool	Role	URL	Status
SFSE	Starfield Script Extender (ianpatt/behippo)	https://sfse.silverlock.org/ ; https://github.com/ianpatt/sfse ; https://www.nexusmods.com/starfield/mods/106	Active; Steam-only
Address Library for SFSE Plugins	SFSE plugin address database	https://www.nexusmods.com/starfield/mods/3256	Required by many plugins
CommonLibSF	Collaborative RE C++ library	https://github.com/Starfield-Reverse-Engineering/CommonLibSF → moved to https://github.com/libxse/commonlibsf	Unmaintained repo redirects to libxse fork
CLibSFPluginTemplate / commonlibsf-template-xmake	Plugin templates (CMake / XMake)	https://github.com/Starfield-Reverse-Engineering/CLibSFPluginTemplate	—
Creation Kit (Starfield)	Official CK, AppID 2722710	https://www.creationkit.com/starfield/ (wiki down for maintenance)	Wiki down
xEdit (TES5Edit)	Plugin editor w/ Starfield mode ("SF1")	https://github.com/TES5Edit/TES5Edit (dev-4.1.6+)	Active; `.esp` editing + small/medium master support
BSArch / Archive2	BA2/BSA packer/unpacker (source of format facts above)	bundled in TES5Edit repo (`BSArch/`, `Core/wbBSArchive.pas`)	Active
NifSkope	NIF editor	https://github.com/niftools/nifskope	NIF still relevant for meshes; `.mesh` (SF) not a NifSkope format
hkxcmd	Havok cmd tools (Skyrim/FO4 era)	https://github.com/figment/hkxcmd	CE2 `.hkx` compat unverified
hkxpack	HKX ↔ XML (hk_2014.1.0-r1 target)	https://github.com/Dexesttp/hkxpack	CE2 class changes unverified

6. Community resources

Resource	URL	Notes
SFSE official	https://sfse.silverlock.org/	build 0.2.21 / game 1.16.244
SFSE Nexus	https://www.nexusmods.com/starfield/mods/106	download page
CommonLibSF (new home)	https://github.com/libxse/commonlibsf	unmaintained repo redirects here
Starfield Wiki	https://starfieldwiki.net/	HTTP 403 / Cloudflare — not scrapable
UESP Starfield	https://en.uesp.net/wiki/Starfield:Starfield (via MediaWiki API)	UESP hosts Starfield namespace; use API form
Creation Kit wiki	https://www.creationkit.com/starfield/	down for backend maintenance
Bethesda Game Studios Discord	https://discord.com/invite/BethesdaStudios	official; `#modding-news` channel posts CK status

7. URLs that could NOT be accessed (user may paste content)

https://www.creationkit.com/starfield/ and .../starfield/index.php?title=Main_Page — XWiki returns "This wiki is currently down for backend maintenance" (as of 2026-08-18).
https://starfieldwiki.net/wiki/... and its api.php — Cloudflare "Just a moment..." challenge (HTTP 403).
https://www.nexusmods.com/starfield/mods/106 — page shell fetched (7 lines) but description not rendered.
DuckDuckGo / Bing HTML search — served anomaly/captcha pages; no organic results usable.
No public source found stating the exact Havok contents-version string for Starfield .hkx files (e.g. hk_2014.1.0-r1 vs a newer SDK). This is the biggest remaining gap.

8. Guide-writing pointers (for later)

Emphasize the Animation Graph (AnimGraph) framework as the bridge: CE2 keeps Havok .hkx behavior/animation but exposes it via BSAnimationGraph/BSAnimationGraphManager (graph variables + NotifyAnimationGraph events) — same architecture as Skyrim SE.
.hkx files remain in the meshes/ asset folder inside .ba2 archives.
BA2 format: BTDX magic, version 0x02/0x03, GNRL/DX10 subtype, ZLib default (LZ4 for DDS, or for v3 archives packed by Archive2). This is a concrete, verifiable spec.
.esm/.esp: 252 full / 4095 light / 254 medium masters; small/medium/blueprint master concepts.
Geometry moved to .mesh under geometries/; NIF .nif retained as legacy mesh.
Honest gaps: exact Havok SDK version, .hkx class layout vs FO4, behavior-authoring tooling.

1. Engine — Creation Engine 2​

2. Animation — Havok .hkx + AnimGraph (CONFIRMED lineage)​

3. File formats — BA2 archives & assets (CONFIRMED from xEdit/BSArch source)​

BA2 / BSA header versions (magic 4 bytes)​

Starfield BA2 header layout (after BTDX magic)​

Compression support (per archive type; first = default)​

Asset folders (root + extensions) — from cBSAssets table​

Important distinction: .nif vs .mesh​

4. Plugin (.esm/.esp) format — CONFIRMED from xEdit whatsnew (dev-4.1.6)​

5. Modding tools (Starfield)​

6. Community resources​

7. URLs that could NOT be accessed (user may paste content)​

8. Guide-writing pointers (for later)​