diff --git a/Cargo.lock b/Cargo.lock
index e17bb71ffb4..60187483380 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2418,6 +2418,7 @@ dependencies = [
  "fs_extra",
  "gix-discover",
  "gix-fs",
+ "gix-hash",
  "gix-lock",
  "gix-tempfile",
  "gix-worktree",
diff --git a/gix-commitgraph/Cargo.toml b/gix-commitgraph/Cargo.toml
index 1bc5fd784a0..7257d14ff3c 100644
--- a/gix-commitgraph/Cargo.toml
+++ b/gix-commitgraph/Cargo.toml
@@ -33,6 +33,7 @@ document-features = { version = "0.2.0", optional = true }
 [dev-dependencies]
 gix-testtools = { path = "../tests/tools" }
 gix-date = { path = "../gix-date" }
+gix-hash = { path = "../gix-hash", features = ["sha256"] }
 
 [package.metadata.docs.rs]
 all-features = true
diff --git a/gix-commitgraph/src/access.rs b/gix-commitgraph/src/access.rs
index e09ff635047..85cd5daf5c1 100644
--- a/gix-commitgraph/src/access.rs
+++ b/gix-commitgraph/src/access.rs
@@ -11,6 +11,20 @@ impl Graph {
         r.file.commit_at(r.pos)
     }
 
+    /// The kind of hash used in this `Graph`.
+    ///
+    /// Note that it is always conforming to the hash used in the owning repository.
+    ///
+    /// # Panics
+    ///
+    /// If the graph does not contain any `File`.
+    pub fn object_hash(&self) -> gix_hash::Kind {
+        self.files
+            .first()
+            .map(super::File::object_hash)
+            .expect("graph to have at least one file")
+    }
+
     /// Returns the commit matching the given `id`.
     pub fn commit_by_id(&self, id: impl AsRef<gix_hash::oid>) -> Option<Commit<'_>> {
         let r = self.lookup_by_id(id.as_ref())?;
diff --git a/gix-commitgraph/tests/commitgraph.rs b/gix-commitgraph/tests/commitgraph.rs
index 8b2ae8776dc..1952b2be37e 100644
--- a/gix-commitgraph/tests/commitgraph.rs
+++ b/gix-commitgraph/tests/commitgraph.rs
@@ -81,6 +81,14 @@ pub fn graph_and_expected_named(
     let expected = inspect_refs(&repo_dir, refs);
     let cg =
         Graph::from_info_dir(&repo_dir.join(".git").join("objects").join("info")).expect("graph present and valid");
+    let object_hash = cg.object_hash();
+    let any_ref = expected.values().next().expect("at least one ref");
+    assert_eq!(
+        object_hash,
+        any_ref.id().kind(),
+        "graph hash kind should match fixture object IDs"
+    );
+
     (cg, expected)
 }
 
diff --git a/gix-commitgraph/tests/fixtures/generated-archives/generation_number_overflow_sha256.tar b/gix-commitgraph/tests/fixtures/generated-archives/generation_number_overflow_sha256.tar
new file mode 100644
index 00000000000..96db68e7a87
Binary files /dev/null and b/gix-commitgraph/tests/fixtures/generated-archives/generation_number_overflow_sha256.tar differ
diff --git a/gix-commitgraph/tests/fixtures/generated-archives/octopus_merges_sha256.tar b/gix-commitgraph/tests/fixtures/generated-archives/octopus_merges_sha256.tar
new file mode 100644
index 00000000000..757aa8ef786
Binary files /dev/null and b/gix-commitgraph/tests/fixtures/generated-archives/octopus_merges_sha256.tar differ
diff --git a/gix-commitgraph/tests/fixtures/generated-archives/single_commit_huge_dates_sha256.tar b/gix-commitgraph/tests/fixtures/generated-archives/single_commit_huge_dates_sha256.tar
new file mode 100644
index 00000000000..b68642249e3
Binary files /dev/null and b/gix-commitgraph/tests/fixtures/generated-archives/single_commit_huge_dates_sha256.tar differ
diff --git a/gix-commitgraph/tests/fixtures/generated-archives/single_commit_sha256.tar b/gix-commitgraph/tests/fixtures/generated-archives/single_commit_sha256.tar
new file mode 100644
index 00000000000..9016ac17286
Binary files /dev/null and b/gix-commitgraph/tests/fixtures/generated-archives/single_commit_sha256.tar differ
diff --git a/gix-commitgraph/tests/fixtures/generated-archives/single_parent_sha256.tar b/gix-commitgraph/tests/fixtures/generated-archives/single_parent_sha256.tar
new file mode 100644
index 00000000000..f0e15f61bbc
Binary files /dev/null and b/gix-commitgraph/tests/fixtures/generated-archives/single_parent_sha256.tar differ
diff --git a/gix-commitgraph/tests/fixtures/generated-archives/two_parents_sha256.tar b/gix-commitgraph/tests/fixtures/generated-archives/two_parents_sha256.tar
new file mode 100644
index 00000000000..06de56f9102
Binary files /dev/null and b/gix-commitgraph/tests/fixtures/generated-archives/two_parents_sha256.tar differ
diff --git a/gix-diff/src/tree/visit.rs b/gix-diff/src/tree/visit.rs
index b410fe89ee6..6bc05b75283 100644
--- a/gix-diff/src/tree/visit.rs
+++ b/gix-diff/src/tree/visit.rs
@@ -151,9 +151,12 @@ mod tests {
     #[test]
     fn size_of_change() {
         let actual = std::mem::size_of::<Change>();
+        let sha1 = 48;
+        let sha256_extra = 24;
+        let ceiling = sha1 + sha256_extra;
         assert!(
-            actual <= 48,
-            "{actual} <= 48: this type shouldn't grow without us knowing"
+            actual <= ceiling,
+            "{actual} <= {ceiling}: this type shouldn't grow without us knowing"
         );
     }
 }
diff --git a/gix-index/src/extension/tree/mod.rs b/gix-index/src/extension/tree/mod.rs
index 5e68ff7197f..6846c4a0016 100644
--- a/gix-index/src/extension/tree/mod.rs
+++ b/gix-index/src/extension/tree/mod.rs
@@ -18,7 +18,9 @@ mod tests {
     #[test]
     fn size_of_tree() {
         let actual = std::mem::size_of::<crate::extension::Tree>();
-        let expected = 88;
+        let sha1 = 88;
+        let sha256_extra = 16;
+        let expected = sha1 + sha256_extra;
         assert!(
             size_ok(actual, expected),
             "the size of this structure should not change unexpectedly: {actual} <~ {expected}"
diff --git a/gix-index/tests/Cargo.toml b/gix-index/tests/Cargo.toml
index faaa0438296..5ace21e1ba8 100644
--- a/gix-index/tests/Cargo.toml
+++ b/gix-index/tests/Cargo.toml
@@ -24,6 +24,6 @@ gix-features = { path = "../../gix-features", features = ["progress"] }
 gix-testtools = { path = "../../tests/tools" }
 gix-odb = { path = "../../gix-odb" }
 gix-object = { path = "../../gix-object" }
-gix-hash = { path = "../../gix-hash" }
+gix-hash = { path = "../../gix-hash", features = ["sha256"] }
 filetime = "0.2.27"
 bstr = { version = "1.12.0", default-features = false }
diff --git a/gix-index/tests/index/mod.rs b/gix-index/tests/index/mod.rs
index 1878edcaa1f..ccb5c4d61ba 100644
--- a/gix-index/tests/index/mod.rs
+++ b/gix-index/tests/index/mod.rs
@@ -27,7 +27,9 @@ pub fn loose_file_path(name: &str) -> PathBuf {
 #[test]
 fn size_of_entry() {
     let actual = std::mem::size_of::<gix_index::Entry>();
-    let expected = 80;
+    let sha1 = 80;
+    let sha256_extra = 16;
+    let expected = sha1 + sha256_extra;
     assert!(
         size_ok(actual, expected),
         "the size of this structure should not change unexpectedly: {actual} <~ {expected}"
diff --git a/gix-negotiate/tests/negotiate.rs b/gix-negotiate/tests/negotiate.rs
index 9410308e39f..ac4244093a5 100644
--- a/gix-negotiate/tests/negotiate.rs
+++ b/gix-negotiate/tests/negotiate.rs
@@ -39,7 +39,9 @@ mod baseline;
 #[test]
 fn size_of_entry() {
     let actual = std::mem::size_of::<gix_revwalk::graph::Commit<gix_negotiate::Metadata>>();
-    let expected = 56;
+    let sha1 = 56;
+    let sha256_extra = 16;
+    let expected = sha1 + sha256_extra;
     assert!(
         size_ok(actual, expected),
         "we may keep a lot of these, so let's not let them grow unnoticed: {actual} <~ {expected}"
diff --git a/gix-object/tests/object/main.rs b/gix-object/tests/object/main.rs
index bf7c8eeea55..7d5d3c005c0 100644
--- a/gix-object/tests/object/main.rs
+++ b/gix-object/tests/object/main.rs
@@ -80,9 +80,12 @@ fn fixture_name(kind: &str, path: &str) -> Vec<u8> {
 #[test]
 fn size_in_memory() {
     let actual = std::mem::size_of::<gix_object::Object>();
+    let sha1 = 272;
+    let sha256_extra = 16;
+    let expected = sha1 + sha256_extra;
     assert!(
-        actual <= 272,
-        "{actual} <= 272: Prevent unexpected growth of what should be lightweight objects"
+        actual <= expected,
+        "{actual} <= {expected}: Prevent unexpected growth of what should be lightweight objects"
     );
 }
 
diff --git a/gix-pack/Cargo.toml b/gix-pack/Cargo.toml
index 0dc5488b592..999f5bbdd4a 100644
--- a/gix-pack/Cargo.toml
+++ b/gix-pack/Cargo.toml
@@ -65,6 +65,7 @@ gix-tempfile = { version = "^21.0.0", default-features = false, path = "../gix-t
 
 [dev-dependencies]
 gix-testtools = { path = "../tests/tools" }
+gix-hash = { version = "^0.22.0", path = "../gix-hash", features = ["sha256"] }
 
 [package.metadata.docs.rs]
 all-features = true
diff --git a/gix-pack/src/cache/delta/tree.rs b/gix-pack/src/cache/delta/tree.rs
index f93608727f4..b122ada5f6a 100644
--- a/gix-pack/src/cache/delta/tree.rs
+++ b/gix-pack/src/cache/delta/tree.rs
@@ -255,7 +255,9 @@ mod tests {
             }
 
             let actual = std::mem::size_of::<[Item<EntryWithDefault>; 7_500_000]>();
-            let expected = 840_000_000;
+            let sha1 = 840_000_000;
+            let sha256_extra = 120_000_000;
+            let expected = sha1 + sha256_extra;
             assert!(
                 size_ok(actual, expected),
                 "we don't want these to grow unnoticed: {actual} <~ {expected}"
diff --git a/gix-pack/tests/Cargo.toml b/gix-pack/tests/Cargo.toml
index 82725f7ae79..1e401a21d66 100644
--- a/gix-pack/tests/Cargo.toml
+++ b/gix-pack/tests/Cargo.toml
@@ -6,7 +6,7 @@ version = "0.0.0"
 repository = "https://github.com/GitoxideLabs/gitoxide"
 authors = ["Sebastian Thiel <sebastian.thiel@icloud.com>"]
 license = "MIT OR Apache-2.0"
-description = "Please use `gix-<thiscrate>` instead ('git' -> 'gix')"
+description = "Tests for the gix-pack crate"
 edition = "2021"
 rust-version = "1.82"
 
@@ -27,5 +27,5 @@ bstr = { version = "1.12.0", default-features = false, features = ["std"] }
 maplit = "1.0.2"
 gix-object = { path = "../../gix-object" }
 gix-traverse = { path = "../../gix-traverse" }
-gix-hash = { path = "../../gix-hash" }
+gix-hash = { path = "../../gix-hash", features = ["sha256"] }
 memmap2 = "0.9.7"
diff --git a/gix-pack/tests/pack/data/output/mod.rs b/gix-pack/tests/pack/data/output/mod.rs
index 59e80d2b303..961ee1ae4b1 100644
--- a/gix-pack/tests/pack/data/output/mod.rs
+++ b/gix-pack/tests/pack/data/output/mod.rs
@@ -6,7 +6,9 @@ use gix_testtools::size_ok;
 #[test]
 fn size_of_entry() {
     let actual = std::mem::size_of::<output::Entry>();
-    let expected = 80;
+    let sha1 = 80;
+    let sha256_extra = 32;
+    let expected = sha1 + sha256_extra;
     assert!(
         size_ok(actual, expected),
         "The size of the structure shouldn't change unexpectedly: {actual} <~ {expected}"
@@ -16,7 +18,9 @@ fn size_of_entry() {
 #[test]
 fn size_of_count() {
     let actual = std::mem::size_of::<output::Count>();
-    let expected = 56;
+    let sha1 = 56;
+    let sha256_extra = 16;
+    let expected = sha1 + sha256_extra;
     assert!(
         size_ok(actual, expected),
         "The size of the structure shouldn't change unexpectedly: {actual} <~ {expected}"
diff --git a/gix-pack/tests/pack/iter.rs b/gix-pack/tests/pack/iter.rs
index 4b7b0cb32f3..b4c829a158e 100644
--- a/gix-pack/tests/pack/iter.rs
+++ b/gix-pack/tests/pack/iter.rs
@@ -4,7 +4,9 @@ use gix_testtools::size_ok;
 #[test]
 fn size_of_entry() {
     let actual = std::mem::size_of::<pack::data::input::Entry>();
-    let expected = 104;
+    let sha1 = 104;
+    let sha256_extra = 32;
+    let expected = sha1 + sha256_extra;
     assert!(
         size_ok(actual, expected),
         "let's keep the size in check as we have many of them: {actual} <~ {expected}"
diff --git a/gix-ref/src/raw.rs b/gix-ref/src/raw.rs
index d476bd17cf9..d5d6978048e 100644
--- a/gix-ref/src/raw.rs
+++ b/gix-ref/src/raw.rs
@@ -100,7 +100,9 @@ mod tests {
     #[test]
     fn size_of_reference() {
         let actual = std::mem::size_of::<Reference>();
-        let expected = 80;
+        let sha1 = 80;
+        let sha256_extra = 24;
+        let expected = sha1 + sha256_extra;
         assert!(
             size_ok(actual, expected),
             "let's not let it change size undetected: {actual} <~ {expected}"
diff --git a/gix-revwalk/tests/revwalk.rs b/gix-revwalk/tests/revwalk.rs
index 371eee9a781..3f532b43d7c 100644
--- a/gix-revwalk/tests/revwalk.rs
+++ b/gix-revwalk/tests/revwalk.rs
@@ -5,7 +5,9 @@ mod graph {
         #[test]
         fn size_of_commit() {
             let actual = std::mem::size_of::<gix_revwalk::graph::Commit<()>>();
-            let expected = 48;
+            let sha1 = 48;
+            let sha256_extra = 16;
+            let expected = sha1 + sha256_extra;
             assert!(
                 size_ok(actual, expected),
                 "We might see quite a lot of these, so they shouldn't grow unexpectedly: {actual} <~ {expected}"
diff --git a/gix-worktree-state/tests/Cargo.toml b/gix-worktree-state/tests/Cargo.toml
index b9790e00142..10fa4400731 100644
--- a/gix-worktree-state/tests/Cargo.toml
+++ b/gix-worktree-state/tests/Cargo.toml
@@ -24,7 +24,7 @@ gix-object = { path = "../../gix-object" }
 gix-discover = { path = "../../gix-discover" }
 gix-filter = { path = "../../gix-filter" }
 gix-index = { path = "../../gix-index" }
-gix-hash = { path = "../../gix-hash" }
+gix-hash = { path = "../../gix-hash", features = ["sha256"] }
 gix-fs = { path = "../../gix-fs" }
 gix-features = { path = "../../gix-features" }
 gix-testtools = { path = "../../tests/tools" }
diff --git a/gix-worktree/tests/Cargo.toml b/gix-worktree/tests/Cargo.toml
index fb8bd2a2920..827ba3e6fea 100644
--- a/gix-worktree/tests/Cargo.toml
+++ b/gix-worktree/tests/Cargo.toml
@@ -22,7 +22,7 @@ gix-features-parallel = ["gix-features/parallel"]
 gix-worktree = { path = "..", features = ["attributes"] }
 gix-index = { path = "../../gix-index" }
 gix-fs = { path = "../../gix-fs" }
-gix-hash = { path = "../../gix-hash" }
+gix-hash = { path = "../../gix-hash", features = ["sha256"] }
 gix-object = { path = "../../gix-object" }
 gix-glob = { path = "../../gix-glob" }
 gix-path = { path = "../../gix-path" }
diff --git a/gix/Cargo.toml b/gix/Cargo.toml
index 918fcfce1ca..8152a79ba6e 100644
--- a/gix/Cargo.toml
+++ b/gix/Cargo.toml
@@ -411,6 +411,7 @@ document-features = { version = "0.2.0", optional = true }
 gix = { path = ".", default-features = false, features = [
     "need-more-recent-msrv", "tree-error"
 ] }
+gix-hash = { version = "^0.22.0", path = "../gix-hash", features = ["sha256"] }
 pretty_assertions = "1.4.0"
 gix-testtools = { path = "../tests/tools" }
 is_ci = "1.1.1"
diff --git a/gix/src/id.rs b/gix/src/id.rs
index 317f9340b50..fa6c7e13ad5 100644
--- a/gix/src/id.rs
+++ b/gix/src/id.rs
@@ -198,7 +198,9 @@ mod tests {
     #[test]
     fn size_of_oid() {
         let actual = std::mem::size_of::<Id<'_>>();
-        let ceiling = 32;
+        let sha1 = 32;
+        let sha256_extra = 16;
+        let ceiling = sha1 + sha256_extra;
         assert!(
             actual <= ceiling,
             "size of oid shouldn't change without notice: {actual} <= {ceiling}"
diff --git a/gix/tests/gix/object/mod.rs b/gix/tests/gix/object/mod.rs
index 8dd1aedaae5..2bc05ba122f 100644
--- a/gix/tests/gix/object/mod.rs
+++ b/gix/tests/gix/object/mod.rs
@@ -7,7 +7,9 @@ use gix_testtools::size_ok;
 #[test]
 fn object_ref_size_in_memory() {
     let actual = std::mem::size_of::<gix::Object<'_>>();
-    let expected = 56;
+    let sha1 = 56;
+    let sha256_extra = 16;
+    let expected = sha1 + sha256_extra;
     assert!(
         size_ok(actual, expected),
         "the size of this structure should not change unexpectedly: {actual} <~ {expected}"
@@ -17,7 +19,9 @@ fn object_ref_size_in_memory() {
 #[test]
 fn oid_size_in_memory() {
     let actual = std::mem::size_of::<gix::Id<'_>>();
-    let expected = 32;
+    let sha1 = 32;
+    let sha256_extra = 16;
+    let expected = sha1 + sha256_extra;
     assert!(
         size_ok(actual, expected),
         "the size of this structure should not change unexpectedly: {actual} <~ {expected}"
diff --git a/gix/tests/gix/revision/spec/from_bytes/mod.rs b/gix/tests/gix/revision/spec/from_bytes/mod.rs
index 2755ff00f25..109b263369e 100644
--- a/gix/tests/gix/revision/spec/from_bytes/mod.rs
+++ b/gix/tests/gix/revision/spec/from_bytes/mod.rs
@@ -147,7 +147,7 @@ fn bad_objects_are_valid_until_they_are_actually_read_from_the_odb() {
         |
         └─ An error occurred while obtaining an object from the loose object store
         |
-        └─ decompression of loose object at 'tests/fixtures/generated-do-not-edit/make_rev_spec_parse_repos/2990428670-unix/blob.corrupt/objects/ca/fea31147e840161a1860c50af999917ae1536b' failed
+        └─ decompression of loose object at 'tests/fixtures/generated-do-not-edit/make_rev_spec_parse_repos/sha1/2990428670-unix/blob.corrupt/objects/ca/fea31147e840161a1860c50af999917ae1536b' failed
         |
         └─ Could not decode zip stream, status was 'Invalid input data'
         |
diff --git a/gix/tests/gix/status.rs b/gix/tests/gix/status.rs
index 8b832f5c2c8..eb7f69355ee 100644
--- a/gix/tests/gix/status.rs
+++ b/gix/tests/gix/status.rs
@@ -29,7 +29,9 @@ mod into_iter {
     #[test]
     fn item_size() {
         let actual = std::mem::size_of::<Item>();
-        let expected = 264;
+        let sha1 = 264;
+        let sha256_extra = 56;
+        let expected = sha1 + sha256_extra;
         assert!(
             size_ok(actual, expected),
             "The size is the same as the one for the index-worktree-item: {actual} <~ {expected}"
@@ -303,7 +305,9 @@ mod index_worktree {
         #[test]
         fn item_size() {
             let actual = std::mem::size_of::<Item>();
-            let expected = 264;
+            let sha1 = 264;
+            let sha256_extra = 56;
+            let expected = sha1 + sha256_extra;
             assert!(
                 size_ok(actual, expected),
                 "The size is pretty huge and goes down ideally: {actual} <~ {expected}"
diff --git a/justfile b/justfile
index 62fdbc85dab..1b0d568e566 100755
--- a/justfile
+++ b/justfile
@@ -162,6 +162,8 @@ unit-tests:
     cargo nextest run -p gix-hash --no-fail-fast
     cargo nextest run -p gix-hash --features sha256 --no-fail-fast
     cargo nextest run -p gix-hash --no-default-features --features sha256 --no-fail-fast # TODO: make this actually work by removing 'sha1' from default features.
+    env GIX_TEST_FIXTURE_HASH=sha1 cargo nextest run -p gix-commitgraph --no-fail-fast
+    env GIX_TEST_FIXTURE_HASH=sha256 cargo nextest run -p gix-commitgraph --no-fail-fast
     cargo nextest run -p gix-object --no-fail-fast
     cargo nextest run -p gix-object --features verbose-object-parsing-errors --no-fail-fast
     cargo nextest run -p gix-tempfile --features signals --no-fail-fast
@@ -169,7 +171,8 @@ unit-tests:
     cargo nextest run -p gix-ref-tests --all-features --no-fail-fast
     cargo nextest run -p gix-odb --all-features --no-fail-fast
     cargo nextest run -p gix-odb-tests --features gix-features-parallel --no-fail-fast
-    cargo nextest run -p gix-pack --all-features --no-fail-fast
+    env GIX_TEST_FIXTURE_HASH=sha1 cargo nextest run -p gix-pack --all-features --no-fail-fast
+    env GIX_TEST_FIXTURE_HASH=sha256 cargo nextest run -p gix-pack --all-features --no-fail-fast
     cargo nextest run -p gix-pack-tests --features all-features --no-fail-fast
     cargo nextest run -p gix-pack-tests --features gix-features-parallel --no-fail-fast
     cargo nextest run -p gix-index-tests --features gix-features-parallel --no-fail-fast
diff --git a/tests/tools/Cargo.toml b/tests/tools/Cargo.toml
index d65e8d92030..1a2625e171b 100644
--- a/tests/tools/Cargo.toml
+++ b/tests/tools/Cargo.toml
@@ -25,6 +25,7 @@ default = []
 xz = ["dep:xz2"]
 
 [dependencies]
+gix-hash = { version = "^0.22.0", path = "../../gix-hash" }
 gix-lock = { version = "^21.0.0", path = "../../gix-lock" }
 gix-discover = { version = "^0.46.0", path = "../../gix-discover" }
 gix-worktree = { version = "^0.47.0", path = "../../gix-worktree" }
diff --git a/tests/tools/src/lib.rs b/tests/tools/src/lib.rs
index 7f8001c24f2..f236f8ea005 100644
--- a/tests/tools/src/lib.rs
+++ b/tests/tools/src/lib.rs
@@ -1,5 +1,14 @@
 //! Utilities for testing `gitoxide` crates, many of which might be useful for testing programs that use `git` in general.
 //!
+//! ## Environment Variables
+//!
+//! ### `GIX_TEST_FIXTURE_HASH`
+//!
+//! Set this variable to control which hash function is used when creating or loading test fixtures.
+//! Valid values are the names of hash functions supported by `gix_hash::Kind` (e.g., `sha1`, `sha256`).
+//! If not set, the default hash function via `gix_hash::Kind::default()` is used.
+//!
+
 //! ## Feature Flags
 #![cfg_attr(
     all(doc, feature = "document-features"),
@@ -287,7 +296,7 @@ enum DirectoryRoot {
     StandaloneTest,
 }
 
-/// Don't add a suffix to the archive name as `args` are platform dependent, none-deterministic,
+/// Don't add a suffix to the archive name as `args` are platform dependent, non-deterministic,
 /// or otherwise don't influence the content of the archive.
 /// Note that this also means that `args` won't be used to control the hash of the archive itself.
 #[derive(Copy, Clone)]
@@ -508,7 +517,7 @@ pub fn copy_recursively_into_existing_dir(src_dir: impl AsRef<Path>, dst_dir: im
     Ok(())
 }
 
-/// Like `scripted_fixture_read_only()`], but passes `args` to `script_name`.
+/// Like [`scripted_fixture_read_only()`], but passes `args` to `script_name`.
 pub fn scripted_fixture_read_only_with_args(
     script_name: impl AsRef<Path>,
     args: impl IntoIterator<Item = impl Into<String>>,
@@ -874,7 +883,7 @@ pub fn rust_fixture_read_only<T, F>(name: &str, version: u32, make_fixture: F) -
 where
     F: FnOnce(FixtureState<'_>) -> PostResult<T>,
 {
-    rust_fixture_read_only_inner(name, version, make_fixture, None, DirectoryRoot::IntegrationTest)
+    rust_fixture_read_only_inner(name, version, None, make_fixture, None, DirectoryRoot::IntegrationTest)
 }
 
 /// Like [`rust_fixture_read_only()`], but does not prefix the fixture directory with `tests`.
@@ -882,7 +891,7 @@ pub fn rust_fixture_read_only_standalone<T, F>(name: &str, version: u32, make_fi
 where
     F: FnOnce(FixtureState<'_>) -> PostResult<T>,
 {
-    rust_fixture_read_only_inner(name, version, make_fixture, None, DirectoryRoot::StandaloneTest)
+    rust_fixture_read_only_inner(name, version, None, make_fixture, None, DirectoryRoot::StandaloneTest)
 }
 
 /// Execute a Rust closure in a directory, returning a writable temporary directory.
@@ -922,7 +931,7 @@ pub fn rust_fixture_writable<T, F>(
 where
     F: FnMut(FixtureState<'_>) -> PostResult<T>,
 {
-    rust_fixture_writable_inner(name, version, make_fixture, mode, DirectoryRoot::IntegrationTest)
+    rust_fixture_writable_inner(name, version, None, make_fixture, mode, DirectoryRoot::IntegrationTest)
 }
 
 /// Like [`rust_fixture_writable()`], but does not prefix the fixture directory with `tests`.
@@ -935,12 +944,13 @@ pub fn rust_fixture_writable_standalone<T, F>(
 where
     F: FnMut(FixtureState<'_>) -> PostResult<T>,
 {
-    rust_fixture_writable_inner(name, version, make_fixture, mode, DirectoryRoot::StandaloneTest)
+    rust_fixture_writable_inner(name, version, None, make_fixture, mode, DirectoryRoot::StandaloneTest)
 }
 
 fn rust_fixture_writable_inner<T, F>(
     name: &str,
     version: u32,
+    object_hash: Option<gix_hash::Kind>,
     mut make_fixture: F,
     mode: Creation,
     root: DirectoryRoot,
@@ -951,13 +961,15 @@ where
     let dst = tempfile::TempDir::new()?;
     let res = match mode {
         Creation::CopyFromReadOnly => {
-            let (ro_dir, _res_ignored) = rust_fixture_read_only_inner(name, version, &mut make_fixture, None, root)?;
+            let (ro_dir, _res_ignored) =
+                rust_fixture_read_only_inner(name, version, object_hash, &mut make_fixture, None, root)?;
             copy_recursively_into_existing_dir(ro_dir, dst.path())?;
             let res = make_fixture(FixtureState::Fresh(dst.path()))?;
             res
         }
         Creation::Execute => {
-            let (_, res) = rust_fixture_read_only_inner(name, version, make_fixture, Some(dst.path()), root)?;
+            let (_, res) =
+                rust_fixture_read_only_inner(name, version, object_hash, make_fixture, Some(dst.path()), root)?;
             res
         }
     };
@@ -967,6 +979,7 @@ where
 fn rust_fixture_read_only_inner<T, F>(
     name: &str,
     version: u32,
+    object_hash: Option<gix_hash::Kind>,
     make_fixture: F,
     destination_dir: Option<&Path>,
     root: DirectoryRoot,
@@ -988,7 +1001,8 @@ where
         Path::new(ARCHIVE_DIR_NAME).join(format!("{archive_name}.{}", tar_extension())),
         root,
     );
-    let (force_run, script_result_directory) = force_and_dir(destination_dir, root, &archive_name, &script_identity);
+    let (force_run, script_result_directory) =
+        force_and_dir(destination_dir, root, &archive_name, object_hash, &script_identity);
     let _marker = marker_if_needed(destination_dir, archive_name)?;
 
     run_fixture_generator_with_marker_handling(
@@ -1024,16 +1038,16 @@ fn force_and_dir(
     destination_dir: Option<&Path>,
     root: DirectoryRoot,
     archive_name: impl AsRef<Path>,
+    hash_kind: Option<gix_hash::Kind>,
     script_identity: &dyn std::fmt::Display,
 ) -> (bool, PathBuf) {
     destination_dir.map_or_else(
         || {
             let dir = fixture_path_inner(
-                Path::new("generated-do-not-edit").join(archive_name).join(format!(
-                    "{}-{}",
-                    script_identity,
-                    family_name()
-                )),
+                Path::new("generated-do-not-edit")
+                    .join(archive_name)
+                    .join(hash_kind.unwrap_or_else(hash_kind_from_env).to_string())
+                    .join(format!("{}-{}", script_identity, family_name())),
                 root,
             );
             (false, dir)
@@ -1127,6 +1141,9 @@ where
         gix_tempfile::signal::handler::Mode::DeleteTempfilesOnTerminationAndRestoreDefaultBehaviour,
     );
 
+    let hash_kind = hash_kind_from_env();
+    eprintln!("Using hash '{hash_kind}' when determining which fixture to use or recreate");
+
     let script_location = script_name.as_ref();
     let script_path = fixture_path_inner(script_location, root);
 
@@ -1135,7 +1152,12 @@ where
     let post_version = post_process.as_ref().map(|(v, _)| *v);
     let script_identity = {
         let mut map = SCRIPT_IDENTITY.lock();
-        let key = args.iter().fold(script_path.clone(), |p, a| p.join(a));
+        let init = if hash_kind == gix_hash::Kind::Sha1 {
+            script_path.clone()
+        } else {
+            script_path.clone().join(hash_kind.to_string())
+        };
+        let key = args.iter().fold(init, |p, a| p.join(a));
         // Include post_version in the key if present
         let key = if let Some(v) = post_version {
             key.join(format!("post-v{v}"))
@@ -1178,15 +1200,26 @@ where
                 }
                 ArgsInHash::No => "".into(),
             };
+            let potential_hash_suffix = if hash_kind == gix_hash::Kind::Sha1 {
+                "".into()
+            } else {
+                format!("_{hash_kind}")
+            };
             Path::new(ARCHIVE_DIR_NAME).join(format!(
-                "{}{suffix}.{}",
+                "{}{suffix}{potential_hash_suffix}.{}",
                 script_basename.to_str().expect("valid UTF-8"),
                 tar_extension()
             ))
         },
         root,
     );
-    let (force_run, script_result_directory) = force_and_dir(destination_dir, root, script_basename, &script_identity);
+    let (force_run, script_result_directory) = force_and_dir(
+        destination_dir,
+        root,
+        script_basename,
+        Some(hash_kind),
+        &script_identity,
+    );
     let _marker = marker_if_needed(destination_dir, script_basename)?;
 
     let script_identity_for_archive = match args_in_hash {
@@ -1205,14 +1238,14 @@ where
         |fixture_state| {
             if let FixtureState::Uninitialized(dir) = fixture_state {
                 let mut cmd = std::process::Command::new(&script_absolute_path);
-                let output = match configure_command(&mut cmd, &args, dir).output() {
+                let output = match configure_command(&mut cmd, hash_kind, &args, dir).output() {
                     Ok(out) => out,
                     Err(err)
                         if err.kind() == std::io::ErrorKind::PermissionDenied
                             || err.raw_os_error() == Some(193) /* windows */ =>
                     {
                         cmd = std::process::Command::new(bash_program());
-                        configure_command(cmd.arg(&script_absolute_path), &args, dir).output()?
+                        configure_command(cmd.arg(&script_absolute_path), hash_kind, &args, dir).output()?
                     }
                     Err(err) => return Err(err.into()),
                 };
@@ -1233,6 +1266,17 @@ where
     Ok((script_result_directory, res))
 }
 
+fn hash_kind_from_env() -> gix_hash::Kind {
+    env::var_os("GIX_TEST_FIXTURE_HASH").and_then(|value| value.into_string().ok()).map(|object_kind| {
+        gix_hash::Kind::from_str(&object_kind).unwrap_or_else(|_| {
+                    panic!(
+                        "GIX_TEST_FIXTURE_HASH was set to {object_kind} which is an invalid value. Valid values are {}. Exiting.",
+                        gix_hash::Kind::all().iter().map(std::string::ToString::to_string).collect::<Vec<_>>().join(", ")
+                    )
+                })
+    }).unwrap_or_default()
+}
+
 #[cfg(windows)]
 const NULL_DEVICE: &str = "nul"; // See `gix_path::env::git::NULL_DEVICE` on why this form is used.
 #[cfg(not(windows))]
@@ -1240,6 +1284,7 @@ const NULL_DEVICE: &str = "/dev/null";
 
 fn configure_command<'a, I: IntoIterator<Item = S>, S: AsRef<OsStr>>(
     cmd: &'a mut std::process::Command,
+    hash_kind: gix_hash::Kind,
     args: I,
     script_result_directory: &Path,
 ) -> &'a mut std::process::Command {
@@ -1279,6 +1324,7 @@ fn configure_command<'a, I: IntoIterator<Item = S>, S: AsRef<OsStr>>(
         .env("GIT_CONFIG_VALUE_2", "main")
         .env("GIT_CONFIG_KEY_3", "protocol.file.allow")
         .env("GIT_CONFIG_VALUE_3", "always")
+        .env("GIT_DEFAULT_HASH", hash_kind.to_string())
 }
 
 /// Get the path attempted as a `bash` interpreter, for fixture scripts having no `#!` we can use.
diff --git a/tests/tools/src/tests.rs b/tests/tools/src/tests.rs
index 049a379e127..e879cb2cc77 100644
--- a/tests/tools/src/tests.rs
+++ b/tests/tools/src/tests.rs
@@ -44,7 +44,12 @@ fn configure_command_clears_external_config() {
     let mut cmd = std::process::Command::new(GIT_PROGRAM);
     cmd.env("GIT_CONFIG_SYSTEM", SCOPE_ENV_VALUE);
     cmd.env("GIT_CONFIG_GLOBAL", SCOPE_ENV_VALUE);
-    configure_command(&mut cmd, ["config", "-l", "--show-origin"], temp.path());
+    configure_command(
+        &mut cmd,
+        gix_hash::Kind::default(),
+        ["config", "-l", "--show-origin"],
+        temp.path(),
+    );
 
     let output = cmd.output().expect("can run git");
     let lines: Vec<_> = output