diff --git a/ADR/ADR-008-Stack-Dependencies.md b/ADR/ADR-008-Stack-Dependencies.md
new file mode 100644
index 0000000..db047d3
--- /dev/null
+++ b/ADR/ADR-008-Stack-Dependencies.md
@@ -0,0 +1,342 @@
+# Architectural Decision Record 008: Stack Dependencies - Dependencies from Source
+
+## Status
+
+<s>Proposed</s> | **Accepted** | <s>Deprecated</s> | <s>Superseded by [ADR-XXX]</s>
+
+## Last Updated
+
+2026-04-07
+
+## Decision
+
+ECMWF adopts the `stack-dependencies` repository as the standard mechanism for
+provisioning approved third-party C++ dependencies across all build
+environments (CI on VMs, CI on HPC, and HPC production).
+
+Adoption is **transitionary**: dependencies move into `stack-dependencies`
+gradually, dependency-by-dependency, as each is incorporated and validated.
+Projects are not required to migrate overnight. During the transition,
+environments may continue to use system-installed versions of dependencies that
+have not yet been incorporated.
+
+Each managed dependency is pinned as a git submodule at a specific version tag.
+All builds — CI and production — use the from-source build rather than
+system-installed packages for dependencies that `stack-dependencies` manages.
+
+### Scope
+
+This ADR covers **third-party libraries** only — libraries developed and
+maintained outside ECMWF that our stack consumes (e.g. CLI11, libfmt, libaec,
+pybind11, qhull).
+
+**Out of scope — internal ECMWF libraries** (eckit, atlas, mir, fdb, etc.) are
+not managed by `stack-dependencies`. They have their own release and integration
+processes.
+
+### Permanent Exemptions
+
+The following system-level dependencies are **permanently exempt** from
+`stack-dependencies`. They are provided by the operating system, HPC
+environment, or site administrator and must not be built from source via this
+mechanism:
+
+- **Compilers**: GCC, Clang, Intel (oneAPI/icc), NVHPC, Cray compiler wrappers
+- **MPI implementations**: OpenMPI, MPICH, Cray MPICH, Intel MPI
+- **OpenMP**: part of the compiler runtime
+- **Python interpreter**: CPython
+- **OS-provided system libraries**: libc, libm, libpthread, etc.
+
+These dependencies are tightly coupled to the host environment — the scheduler,
+interconnect, filesystem, and hardware. Building them from source would
+introduce more problems than it solves.
+
+### Governance
+
+#### Adding a New Dependency
+
+1. **Approval via ADR**: the dependency must first be approved through its own
+   ADR, following the precedent of [ADR-002](./ADR-002-Approved-Dependency-CLI11.md),
+   [ADR-003](./ADR-003-PyBind11-For-CPP-Bindings.md), and
+   [ADR-007](./ADR-007-Approved-Dependency-libfmt.md). The individual ADR
+   evaluates *what* library to adopt; this ADR governs *how* it is provisioned.
+
+2. **Integration**: once approved, a build script is added to `build-scripts/`
+   and the library is added as a git submodule pinned to the approved version.
+   A corresponding `STACK_DEP_<NAME>` toggle is added.
+
+3. **Validation**: the new dependency must build successfully across all CI
+   environments (VM and HPC) before being merged.
+
+#### Version Updates
+
+- **Patch/minor updates**: submodule pointer changes go through standard PR
+  review.
+- **Major version upgrades**: must be discussed with the team before merging, as
+  they may introduce breaking API changes affecting downstream projects.
+
+#### Retirement
+
+If a dependency is deprecated (via its ADR being superseded), it is removed from
+`stack-dependencies` after all downstream projects have migrated off.
+
+## Context
+
+ECMWF's software stack is deployed across a wide range of environments:
+
+1. **CI on virtual machines** — GitHub Actions runners, internal VMs (typically
+   Ubuntu or Rocky Linux).
+2. **CI on HPC** — build and test against the production toolchain.
+3. **HPC production** — operational workloads on ECMWF's HPC systems.
+4. **Non-HPC production** — containers, VMs, and bare-metal machines that are
+   not HPC nodes.
+5. **Distribution artefacts** — RPM/DEB packages and tarballs with binaries
+   (e.g. installed to `/opt`).
+
+Third-party C++ dependencies are currently installed as system packages in each
+environment — via `apt` or `dnf` on VMs, via administrator-managed module
+installations on HPC, and via whatever mechanism is available in the target
+packaging or container image. This creates several problems:
+
+- **Version drift**: a CI VM running Ubuntu 24.04 may ship a different version
+  of a library than the HPC module system provides. Code that compiles and
+  passes tests in CI can fail or behave differently in production.
+- **Release lag**: when the team approves a new dependency version (e.g. via
+  an ADR), the delay until that version appears in distribution repositories
+  varies from weeks to months. On HPC systems with long module refresh cycles,
+  it can take even longer. This blocks adoption.
+- **ABI mismatches**: when an ECMWF library is compiled against dependency
+  version X in CI but linked against version Y in production, the result ranges
+  from link errors to silent ABI incompatibilities.
+- **Blocked testing**: evaluating a dependency upgrade currently requires
+  coordinating with system administrators in each environment to install the
+  new version, or using ad-hoc workarounds.
+
+## Options Considered
+
+### Option A: Status Quo — System-Installed Packages
+
+Continue relying on OS package managers on VMs and administrator-managed module
+installations on HPC.
+
+**Advantages:**
+- Familiar to all developers and system administrators.
+
+**Disadvantages:**
+- All problems described in Context persist.
+- High cost to introduce or upgrade a dependency
+
+### Option B: From-Source Builds via `stack-dependencies`
+
+A single git repository containing approved third-party libraries as git
+submodules, each pinned to a specific tag. A `build.sh` script orchestrates
+per-dependency build scripts. A `CMakeLists.txt` supports three integration
+modes: standalone build, ecbuild bundle inclusion, and plain
+`add_subdirectory`. Dependencies install to a prefix with headers, libraries,
+and CMake/pkg-config metadata.
+
+**Advantages:**
+- Version-pinned: every environment builds the same dependency versions from the
+  same source with the same flags.
+- Reproducible: same source + same compiler + same flags = same binaries.
+- Self-contained: no reliance on external package managers or administrator
+  intervention.
+- ecbuild-native: produces `find_package()`-compatible install trees and
+  integrates directly as an ecbuild bundle component.
+- Selective: environments can skip a dependency via `STACK_DEP_<NAME>=OFF`.
+- Auditable: submodule pins and build scripts are version-controlled, reviewable
+  in PRs, and traceable in `git log`.
+- Environment-agnostic: works on HPC, VMs, containers, and bare-metal. Equally
+  suited for producing RPM/DEB packages or tarballs installed to `/opt`.
+- Uses only tools already present everywhere: git, CMake, and a C/C++ compiler.
+
+**Disadvantages:**
+- Build time: from-source compilation adds minutes to CI pipelines. Mitigated by
+  CI caching and a sentinel file mechanism that skips redundant rebuilds.
+- Maintenance burden: each dependency needs a build script (~10–20 lines of
+  CMake invocation). Upstream CMake quirks must be handled.
+- Git submodule friction: developers must remember `--recurse-submodules` when
+  cloning. CI scripts must be configured accordingly.
+- Compiler matrix: must be tested across all target compilers, though this also
+  surfaces toolchain-specific issues early.
+
+### Option C: Spack / Conan / vcpkg / lmod
+
+Use a dedicated package manager — Spack for HPC, Conan or vcpkg for non-HPC —
+or lmod modules to provide dependencies.
+
+Spack is the most relevant candidate: it is purpose-built for HPC, supports
+building from source with configurable compilers and variants, and handles
+complex dependency graphs. Conan and vcpkg are widely adopted in the broader
+C++ ecosystem but have limited HPC presence.
+
+However, all of these tools solve a **different problem** than
+`stack-dependencies`. Package managers manage installed software across an
+environment — resolving versions, handling transitive dependencies, and
+providing environment modules or activation scripts. By contrast,
+`stack-dependencies` produces a **self-contained, version-pinned source bundle**
+that builds alongside the ECMWF stack using only git, CMake, and a compiler.
+It is orthogonal to package managers: the output of a `stack-dependencies` build
+could itself be packaged as a Spack spec, a Conan recipe, or an lmod module.
+
+**Advantages:**
+- Designed for dependency management.
+- Supports multiple concurrent installations with different configurations.
+- Spack specifically handles HPC toolchains and module generation.
+
+**Disadvantages:**
+- Requires different tooling for HPC (Spack/lmod) and non-HPC (Conan/vcpkg)
+  environments — no single solution spans all targets.
+- Introduces a tool dependency (Spack, Conan, or vcpkg) that is not universally
+  available or configured across all ECMWF environments today.
+- Still requires administrator intervention on HPC to install and maintain the
+  package manager itself and its configuration.
+- Need to maintain separate build recipes or package definitions per tool, in
+  addition to the upstream CMake build.
+- Does not directly integrate with ecbuild bundles or produce
+  `find_package()`-compatible install trees without additional wrapper work.
+
+### Option D: Docker / Container-Based Isolation
+
+Build and ship a Docker image containing all required dependencies.
+
+**Advantages:**
+- Complete environment reproducibility.
+- Already used for python wheel builds.
+
+**Disadvantages:**
+- **Singularity** available but no operational experience on HPC, not trusted yet.
+- Forces developers to work inside containers for local development, or maintain
+  a separate native build path.
+- Suitable as a complementary strategy (for VM CI and wheel builds) but not as
+  the primary strategy.
+
+### Option E: Git Submodules Directly in Each Project
+
+Each ECMWF project adds needed dependencies as git submodules and builds them
+via `add_subdirectory()`.
+
+**Advantages:**
+- Simple, self-contained per project.
+- No shared infrastructure.
+
+**Disadvantages:**
+- Non-trivial CMake integration due to multiple ECMWF libraries requiring the same dependency
+- Possibly multiple submodules of the same dependency
+
+## Analysis
+
+### Key Observations
+
+1. Options C and D introduce tool dependencies not universally available
+   across all target environments. Option C (Spack/Conan/vcpkg) solves a
+   different problem — environment-level package management — whereas
+   `stack-dependencies` produces self-contained source bundles that are
+   orthogonal to and composable with any package manager. The HPC production
+   constraint eliminates containers (D) as a primary strategy.
+
+2. Option B uses only tools already present everywhere: git, CMake, and a C/C++
+   compiler. It adds no new tool dependency.
+
+3. Option B's main weakness — build time — is addressable through CI caching.
+   The sentinel file mechanism already exists for this purpose.
+
+4. Option B and D are complementary, as the stack-dependencies build can be used
+   to pre-populate a Docker image.
+
+5. Option E would lead to multiple copies of the same dependency across projects,
+   with no centralised version control or build scripts. This would increase
+   maintenance burden and risk of version drift between projects.
+   In particular, the caching of build artifacts would have to be replicated everywhere
+   potentially deteriorating the build time issue.
+
+6. The python_wheels documentation already identifies this approach as the
+   intended path forward for compiled dependencies in wheel builds, indicating
+   organisational alignment.
+
+7. If the dependency count grows significantly (beyond ~30 libraries), adopting
+   a full package manager such as Spack may warrant re-evaluation. This should
+   be treated as a reassessment trigger, not a near-term concern.
+
+8. Fortran is currently out of scope for `stack-dependencies` as typical
+   Fortran libraries, such as BLAS and LAPACK, are tightly coupled to the
+   compiler and HPC environment. However, if a need arises to manage Fortran
+   dependencies in the future, the repository's build scripts and CMake
+   integration could be extended to support Fortran libraries as well.
+
+9. The libraries and versions provided by `stack-dependencies` are not to be
+   considered the only versions of those libraries compatible with the ECMWF
+   stack, but rather a curated set of commonly used libraries that are
+   guaranteed to be available across all environments.
+
+### Trade-Off Summary
+
+The core trade-off is between a small, bounded maintenance burden (build scripts
+for each dependency) and the larger, unpredictable burden of diagnosing
+environment-specific build failures. The former is version-controlled and
+reviewable; the latter is opaque and unbounded.
+
+## Related Decisions
+
+- [ADR-002 Approved Dependency CLI11](./ADR-002-Approved-Dependency-CLI11.md) —
+  CLI11 is managed by `stack-dependencies`. ADR-002 approves the dependency;
+  this ADR addresses how it is provisioned.
+- [ADR-003 PyBind11 for C++ Bindings](./ADR-003-PyBind11-For-CPP-Bindings.md) —
+  pybind11 is managed by `stack-dependencies`.
+- [ADR-007 Approved Dependency libfmt](./ADR-007-Approved-Dependency-libfmt.md) —
+  libfmt is managed by `stack-dependencies`. ADR-007 notes that libfmt is
+  "trivially built from source with CMake" — `stack-dependencies` provides the
+  `find_package(fmt)`-compatible install tree.
+- The python_wheels infrastructure (`Languages/python_wheels.md`) references
+  this repository as the intended mechanism for external compiled dependencies
+  in wheel builds.
+
+This ADR does not modify or supersede any existing ADR. It fills an orthogonal
+gap: existing ADRs approve *what* dependencies to use; this ADR decides *how* to
+provision them.
+
+## Consequences
+
+### Positive
+
+- **Reproducibility**: every environment builds the same dependency versions
+  from the same source with the same flags.
+- **CI/production parity**: integration issues surface in CI rather than
+  production.
+- **Version control**: dependency version changes are explicit git commits
+  (submodule pin updates), reviewable in PRs and attributable in `git log`.
+- **Unblocked adoption**: new dependency versions approved by ADR can be adopted
+  immediately, without waiting for distribution packaging or HPC module updates.
+- **ecbuild native**: three integration modes (shell, CMake, ecbuild bundle)
+  cover all existing build workflows.
+- **Auditability**: the exact source of every dependency is available in the
+  repository, satisfying compliance and licence audit requirements.
+- **Python wheel alignment**: unifies the dependency provisioning strategy
+  between the C++ build pipeline and the python_wheels pipeline.
+
+### Negative
+
+- **Build time increase**: from-source compilation adds time to CI pipelines.
+  Mitigated by CI caching, the sentinel file mechanism, and selective dependency
+  building.
+- **Maintenance burden**: each dependency requires a build script. When upstream
+  changes their CMake configuration, the script may need updating. Build scripts
+  are typically 10–20 lines, so per-dependency cost is low.
+- **Git submodule friction**: developers must remember `--recurse-submodules`.
+  CI templates should enforce this.
+- **Compiler matrix testing**: from-source builds must be validated across all
+  target compilers. This also catches issues that system packages would hide.
+
+## References
+
+- `stack-dependencies` repository: https://github.com/ecmwf/stack-dependencies
+- Python wheels documentation: `Languages/python_wheels.md`
+- ecbuild: https://github.com/ecmwf/ecbuild
+- Spack: https://spack.io/
+- Conan: https://conan.io/
+- vcpkg: https://vcpkg.io/
+
+## Authors
+
+- Kai Kratz
+- Oskar Weser
diff --git a/ADR/ADR-Index.md b/ADR/ADR-Index.md
index f8ad779..9e594fa 100644
--- a/ADR/ADR-Index.md
+++ b/ADR/ADR-Index.md
@@ -7,5 +7,6 @@
 | [ADR-003 PyBind11 for C++ Bindings](./ADR-003-PyBind11-For-CPP-Bindings.md) | Accepted |  
 | [ADR-004 Cascade Features](./ADR-004-Cascade-Features.md) | Accepted |
 | [ADR-005 Storing of ICON Grid Files](./ADR-005-Storing-Of-ICON-Grid-Files.md) | Accepted |
-| [ADR-006 MARS Model Keyword for DestinE ODEDT](./ADR-006-MARS-Model-Keyword-for-DestinE-ODEDT.md) | Accepted |
+| [ADR-006 MARS Model Keyword for DestinE ODEDT](./ADR-006-MARS-Model-Keyword-for-DestinE-ODEDT.md) | Withdrawn — moved to [MARS-001](../MARS%20language/MARS-001-MARS-Model-Keyword-for-DestinE-ODEDT.md) |
 | [ADR-007 Approved Dependency libfmt](./ADR-007-Approved-Dependency-libfmt.md) | Proposed |
+| [ADR-008 From-Source Builds for C++ Stack Dependencies](./ADR-008-Stack-Dependencies.md) | Accepted |