diff --git a/tools/tailor/crates/tailor-config/src/fragment.rs b/tools/tailor/crates/tailor-config/src/fragment.rs
index e899e91254..01fca4c6e5 100644
--- a/tools/tailor/crates/tailor-config/src/fragment.rs
+++ b/tools/tailor/crates/tailor-config/src/fragment.rs
@@ -41,6 +41,12 @@ pub(crate) struct Fragment {
     pub(crate) params: IndexMap<String, ParamValue>,
     #[serde(default)]
     pub(crate) rpm_sources: Vec<PathBuf>,
+    /// Extra local paths (scripts, injected assets referenced only from `config:` text, etc.) that
+    /// invalidate this cell's incremental fingerprint when their content changes; concatenated across
+    /// matched fragments (base -> most-specific), mirroring `rpmSources`. Resolved the same way as the
+    /// image-level `extraDependencies` (relative to the image directory, not this fragment's own).
+    #[serde(default)]
+    pub(crate) extra_dependencies: Vec<PathBuf>,
     /// Extra IC command-line flags this fragment contributes; concatenated across matched fragments
     /// (base → most-specific), mirroring `rpmSources`.
     #[serde(default)]
diff --git a/tools/tailor/crates/tailor-config/src/render.rs b/tools/tailor/crates/tailor-config/src/render.rs
index 45ac3b6cfa..d8acd91d00 100644
--- a/tools/tailor/crates/tailor-config/src/render.rs
+++ b/tools/tailor/crates/tailor-config/src/render.rs
@@ -63,6 +63,9 @@ pub struct RenderedCell {
     pub outputs: Vec<OutputSpec>,
     /// Local RPM sources (directories or `.repo` files) passed to IC as `--rpm-source`.
     pub rpm_sources: Vec<PathBuf>,
+    /// Extra local dependency paths, concatenated across matched fragments; content-hashed into the
+    /// incremental fingerprint alongside the image-level `extraDependencies`.
+    pub extra_dependencies: Vec<PathBuf>,
     /// Extra IC command-line flags appended verbatim, concatenated across matched fragments.
     pub extra_params: Vec<ExtraParam>,
     /// The resolved per-cell `signing:` opt-in after fragment merging.
@@ -124,7 +127,14 @@ pub fn merge_plan(
         .map(|f| MergeStep {
             label: f.label.clone(),
             reason: f.reason(),
-            includes: collect_includes(f.doc.config.as_ref()),
+            // `base:` fragments can `$include` a shared snippet too (`resolve_base` below), so the
+            // explain plan must report those targets alongside `config:`'s, or it silently omits
+            // whichever base the fragment actually spliced in.
+            includes: {
+                let mut includes = collect_includes(f.doc.config.as_ref());
+                includes.extend(collect_includes(f.doc.base.as_ref()));
+                includes
+            },
         })
         .collect())
 }
@@ -192,7 +202,7 @@ fn render_cell(
     let mut ic_config = Value::Mapping(config);
     interpolate::interpolate_tree(&mut ic_config, &context)?;
 
-    let base = resolve_base(image, &tuple, &matched, &context)?;
+    let base = resolve_base(image, image_dir, &tuple, &matched, &context)?;
     let outputs = resolve_outputs(image, &tuple, &matched)?;
     for output in &outputs {
         validate_output_compression(image, &tuple, output)?;
@@ -201,6 +211,10 @@ fn render_cell(
         .iter()
         .flat_map(|f| f.doc.rpm_sources.clone())
         .collect();
+    let extra_dependencies = matched
+        .iter()
+        .flat_map(|f| f.doc.extra_dependencies.clone())
+        .collect();
     let extra_params = matched
         .iter()
         .flat_map(|f| f.doc.extra_params.clone())
@@ -225,6 +239,7 @@ fn render_cell(
         base,
         outputs,
         rpm_sources,
+        extra_dependencies,
         extra_params,
         signing,
         skip,
@@ -256,13 +271,17 @@ fn merge_params(matched: &[&LoadedFragment]) -> Result<IndexMap<String, String>,
 
 fn resolve_base(
     image: &ImageDefinition,
+    image_dir: &Path,
     tuple: &AxisTuple,
     matched: &[&LoadedFragment],
     context: &interpolate::Context,
 ) -> Result<BaseSource, ConfigError> {
     let mut base: Option<Value> = None;
     for fragment in matched {
-        if let Some(value) = fragment.doc.base.clone() {
+        if let Some(mut value) = fragment.doc.base.clone() {
+            // Mirror config:'s $include handling (resolved before merge, see render_cell above) so a
+            // base: fragment can splice a shared snippet too, instead of only config: being able to.
+            include::resolve_includes(&mut value, image_dir)?;
             base = Some(merge::merge_field(
                 base,
                 value,
@@ -500,6 +519,41 @@ mod tests {
             .collect()
     }
 
+    #[test]
+    fn merge_plan_reports_includes_from_base_as_well_as_config() {
+        // `resolve_base` splices `$include` under `base:` just like `config:` does, so the explain
+        // plan must surface it too -- otherwise `tailor explain` silently omits a library an image
+        // actually depends on.
+        let tmp = TempDir::new().unwrap();
+        write(
+            tmp.path(),
+            "image.yaml",
+            indoc! {"
+                name: based
+                matrix:
+                  arch: [amd64]
+                outputs:
+                  - format: cosi
+                config: {}
+            "},
+        );
+        write(
+            tmp.path(),
+            "by-arch/amd64.yaml",
+            "base:\n  $include: common/base-amd64.yaml\n",
+        );
+        write(tmp.path(), "common/base-amd64.yaml", "path: ./amd64.img\n");
+        let image = load_image(tmp.path().join("image.yaml")).unwrap();
+        let mut axes = BTreeMap::new();
+        axes.insert("arch".to_owned(), "amd64".to_owned());
+        let plan = merge_plan(&image, tmp.path(), &axes).unwrap();
+        let step = plan
+            .iter()
+            .find(|step| step.label == "by-arch/amd64.yaml")
+            .expect("by-arch step");
+        assert_eq!(step.includes, vec!["common/base-amd64.yaml".to_owned()]);
+    }
+
     #[test]
     fn compression_parses_and_is_validated_against_the_format() {
         // A raw disk-image format accepts compression; the parsed OutputSpec carries it.
@@ -677,6 +731,160 @@ mod tests {
         );
     }
 
+    #[test]
+    fn a_fragment_signs_only_its_own_matched_cells() {
+        // Regression: signing is resolved per cell from the matched fragment chain
+        // (resolve_signing_ref), not image-wide -- a by-<axis> fragment's signing: opt-in must
+        // apply only to the cells it matches, leaving sibling cells unsigned.
+        let tmp = TempDir::new().unwrap();
+        write(
+            tmp.path(),
+            "image.yaml",
+            indoc! {"
+                name: xs
+                matrix:
+                  arch: [amd64, arm64]
+                base:
+                  path: ./b.img
+                config:
+                  os: { hostname: xs }
+            "},
+        );
+        write(
+            tmp.path(),
+            "by-arch/arm64.yaml",
+            indoc! {"
+                signing: release
+            "},
+        );
+        let image = load_image(tmp.path().join("image.yaml")).unwrap();
+        let cells = render_image(&image, tmp.path()).unwrap();
+
+        let amd64 = cells
+            .iter()
+            .find(|c| c.tuple.get("arch") == Some("amd64"))
+            .unwrap();
+        let arm64 = cells
+            .iter()
+            .find(|c| c.tuple.get("arch") == Some("arm64"))
+            .unwrap();
+
+        assert_eq!(
+            amd64.signing, None,
+            "unmatched sibling cell must stay unsigned"
+        );
+        assert_eq!(
+            arm64.signing,
+            Some(SigningRef::Profile("release".to_owned())),
+            "the matched fragment's signing opt-in must apply to its own cell"
+        );
+    }
+
+    #[test]
+    fn a_more_specific_fragment_can_override_signing_to_false() {
+        // Regression: resolve_signing_ref must be last-wins across the matched chain (base ->
+        // most-specific), so a more-specific `signing: false` can override a less-specific
+        // `signing: true` -- not get silently ignored in favor of the image-wide opt-in.
+        let tmp = TempDir::new().unwrap();
+        write(
+            tmp.path(),
+            "image.yaml",
+            indoc! {"
+                name: xf
+                matrix:
+                  arch: [amd64, arm64]
+                signing: true
+                base:
+                  path: ./b.img
+                config:
+                  os: { hostname: xf }
+            "},
+        );
+        write(
+            tmp.path(),
+            "by-arch/arm64.yaml",
+            indoc! {"
+                signing: false
+            "},
+        );
+        let image = load_image(tmp.path().join("image.yaml")).unwrap();
+        let cells = render_image(&image, tmp.path()).unwrap();
+
+        let amd64 = cells
+            .iter()
+            .find(|c| c.tuple.get("arch") == Some("amd64"))
+            .unwrap();
+        let arm64 = cells
+            .iter()
+            .find(|c| c.tuple.get("arch") == Some("arm64"))
+            .unwrap();
+
+        assert_eq!(
+            amd64.signing,
+            Some(SigningRef::Enabled(true)),
+            "the base document's image-wide opt-in applies where no fragment overrides it"
+        );
+        assert_eq!(
+            arm64.signing,
+            Some(SigningRef::Enabled(false)),
+            "a more-specific fragment's signing: false must win over the less-specific opt-in"
+        );
+    }
+
+    #[test]
+    fn extra_dependencies_concatenate_base_then_fragment() {
+        // Regression: a by-variant/by-scenario fragment's own `extraDependencies:` was silently
+        // dropped (the schema had no such field), so edits to a fragment-declared local asset never
+        // busted the incremental fingerprint for that variant. `extraDependencies` must merge across
+        // matched fragments exactly like `extraParams`/`rpmSources` (base -> most-specific).
+        let tmp = TempDir::new().unwrap();
+        write(
+            tmp.path(),
+            "image.yaml",
+            indoc! {"
+                name: xd
+                matrix:
+                  variant: [plain, extra]
+                extraDependencies:
+                  - ./base-dep.txt
+                base:
+                  path: ./b.img
+                config:
+                  os: { hostname: xd }
+            "},
+        );
+        write(
+            tmp.path(),
+            "by-variant/extra.yaml",
+            indoc! {"
+                extraDependencies:
+                  - ./variant-only-dep.txt
+            "},
+        );
+        let image = load_image(tmp.path().join("image.yaml")).unwrap();
+        let cells = render_image(&image, tmp.path()).unwrap();
+
+        let plain = cells
+            .iter()
+            .find(|c| c.tuple.get("variant") == Some("plain"))
+            .unwrap();
+        let extra = cells
+            .iter()
+            .find(|c| c.tuple.get("variant") == Some("extra"))
+            .unwrap();
+
+        let deps = |c: &RenderedCell| -> Vec<String> {
+            c.extra_dependencies
+                .iter()
+                .map(|p| p.to_string_lossy().into_owned())
+                .collect()
+        };
+        assert_eq!(deps(plain), ["./base-dep.txt"]);
+        // base -> most-specific: the fragment's own extraDependencies is appended after the base
+        // document's, not replacing it.
+        assert_eq!(deps(extra), ["./base-dep.txt", "./variant-only-dep.txt"]);
+    }
+
     #[test]
     fn lite_cell_keeps_path_base_appends_packages_and_resolves_include_and_nested_params() {
         let tmp = TempDir::new().unwrap();
@@ -713,6 +921,70 @@ mod tests {
         assert_eq!(install(pro), ["boot-pro"]);
     }
 
+    #[test]
+    fn base_include_splices_a_shared_snippet_before_merge() {
+        // A by-arch fragment can $include a shared base snippet instead of inlining it, exactly
+        // like config: already could - regression test for the "must be resolved before merge"
+        // bug where base: never ran $include resolution.
+        let tmp = TempDir::new().unwrap();
+        write(
+            tmp.path(),
+            "image.yaml",
+            indoc! {"
+                name: shared-base
+                matrix:
+                  arch: [amd64, arm64]
+                outputs:
+                  - format: cosi
+                config: {}
+            "},
+        );
+        write(
+            tmp.path(),
+            "by-arch/amd64.yaml",
+            "base:
+  $include: common/base-amd64.yaml
+",
+        );
+        write(
+            tmp.path(),
+            "by-arch/arm64.yaml",
+            "base:
+  $include: common/base-arm64.yaml
+",
+        );
+        write(
+            tmp.path(),
+            "common/base-amd64.yaml",
+            "path: ./amd64.img
+",
+        );
+        write(
+            tmp.path(),
+            "common/base-arm64.yaml",
+            "path: ./arm64.img
+",
+        );
+
+        let image = load_image(tmp.path().join("image.yaml")).unwrap();
+        let cells = render_image(&image, tmp.path()).unwrap();
+        let by_arch = |arch: &str| -> &RenderedCell {
+            cells
+                .iter()
+                .find(|c| c.tuple.get("arch") == Some(arch))
+                .unwrap_or_else(|| panic!("no {arch} cell"))
+        };
+
+        match &by_arch("amd64").base {
+            BaseSource::Path { path, .. } => assert_eq!(path, Path::new("./amd64.img")),
+            other => panic!("expected a path base, got {other:?}"),
+        }
+        match &by_arch("arm64").base {
+            BaseSource::Path { path, .. } => assert_eq!(path, Path::new("./arm64.img")),
+            other => panic!("expected a path base, got {other:?}"),
+        }
+    }
+
     #[test]
     fn an_image_with_no_base_is_an_error() {
         let tmp = TempDir::new().unwrap();
diff --git a/tools/tailor/crates/tailor-core/src/domain.rs b/tools/tailor/crates/tailor-core/src/domain.rs
index f80a015432..271ebd439b 100644
--- a/tools/tailor/crates/tailor-core/src/domain.rs
+++ b/tools/tailor/crates/tailor-core/src/domain.rs
@@ -60,6 +60,12 @@ pub struct Cell {
     pub base_image: Option<String>,
     /// Local RPM sources passed to IC as `--rpm-source`.
     pub rpm_sources: Vec<PathBuf>,
+    /// The complete, merged `extraDependencies:` dependency list for this cell -- `image.yaml`
+    /// is itself parsed as the first matched fragment, so this already includes the image-level
+    /// paths concatenated with every matched fragment's own (base -> most-specific); it's the sole
+    /// input hashed into the fingerprint, so an edit to any of those declared assets invalidates
+    /// this cell.
+    pub extra_dependencies: Vec<PathBuf>,
     /// Extra IC command-line flags appended verbatim after every tailor-managed flag.
     pub extra_params: Vec<ExtraParam>,
     /// The resolved per-cell `signing:` opt-in after fragment merging.
diff --git a/tools/tailor/crates/tailor-core/src/orchestrator.rs b/tools/tailor/crates/tailor-core/src/orchestrator.rs
index fcc1a31cbf..be7a3ec1e6 100644
--- a/tools/tailor/crates/tailor-core/src/orchestrator.rs
+++ b/tools/tailor/crates/tailor-core/src/orchestrator.rs
@@ -144,9 +144,13 @@ impl<E: Executor, R: BaseResolver> Orchestrator<E, R> {
                 let resolved_tools_dir = resolved_tools_dir(tools_dir_sources, &cell)?;
                 // Hash the cell's declared local dependencies so an edit to an IC-config-referenced
                 // asset (an `additionalFiles` script, a local RPM) invalidates the incremental
-                // fingerprint — the config text only *names* these files (`deps.rs`).
+                // fingerprint — the config text only *names* these files (`deps.rs`). `image.yaml`
+                // is itself parsed as the first matched fragment, so `cell.extra_dependencies`
+                // already is the full base + matched-fragment merge (base -> most-specific) -- no
+                // separate image-level set needs to be chained in here, that would just hash the
+                // base document's own paths a second time.
                 let extra_dependency_hashes = deps::hash_dependencies(
-                    &cell.target.definition.extra_dependencies,
+                    &cell.extra_dependencies,
                     &cell.target.dir,
                     None,
                     hash_cache_dir,
@@ -555,6 +559,7 @@ pub fn cells(target: &Arc<Target>) -> Result<Vec<Cell>, CoreError> {
                     base: base.clone(),
                     base_image: base_image.clone(),
                     rpm_sources: rc.rpm_sources.clone(),
+                    extra_dependencies: rc.extra_dependencies.clone(),
                     extra_params: rc.extra_params.clone(),
                     signing: rc.signing.clone(),
                     input_deps: Vec::new(),
diff --git a/tools/tailor/crates/tailor-core/src/selector.rs b/tools/tailor/crates/tailor-core/src/selector.rs
index 469a6c123a..551e10176c 100644
--- a/tools/tailor/crates/tailor-core/src/selector.rs
+++ b/tools/tailor/crates/tailor-core/src/selector.rs
@@ -169,6 +169,7 @@ mod tests {
             },
             base_image: None,
             rpm_sources: vec![],
+            extra_dependencies: vec![],
             extra_params: vec![],
             input_deps: vec![],
             tools_dir: None,
diff --git a/tools/tailor/crates/tailor-exec/src/arg_builder.rs b/tools/tailor/crates/tailor-exec/src/arg_builder.rs
index 8840c8b284..3c6c58a595 100644
--- a/tools/tailor/crates/tailor-exec/src/arg_builder.rs
+++ b/tools/tailor/crates/tailor-exec/src/arg_builder.rs
@@ -1291,6 +1291,7 @@ mod tests {
             base,
             base_image: None,
             rpm_sources: vec![PathBuf::from("/rpms/one")],
+            extra_dependencies: Vec::new(),
             extra_params: Vec::new(),
             input_deps: Vec::new(),
             tools_dir: None,
diff --git a/tools/tailor/crates/tailor-exec/src/executor.rs b/tools/tailor/crates/tailor-exec/src/executor.rs
index 7855a9ef0d..66ee73bb14 100644
--- a/tools/tailor/crates/tailor-exec/src/executor.rs
+++ b/tools/tailor/crates/tailor-exec/src/executor.rs
@@ -799,6 +799,7 @@ mod tests {
             },
             base_image: None,
             rpm_sources: Vec::new(),
+            extra_dependencies: Vec::new(),
             extra_params: Vec::new(),
             input_deps: Vec::new(),
             tools_dir: None,
diff --git a/tools/tailor/crates/tailor/src/run.rs b/tools/tailor/crates/tailor/src/run.rs
index 32624ee409..fb930d85fe 100644
--- a/tools/tailor/crates/tailor/src/run.rs
+++ b/tools/tailor/crates/tailor/src/run.rs
@@ -1445,6 +1445,7 @@ fn convert_cell(input: &Path, dir: &Path, arch: Arch, format: OutputFormat, slug
         },
         base_image: None,
         rpm_sources: Vec::new(),
+        extra_dependencies: Vec::new(),
         extra_params: Vec::new(),
         signing: None,
         input_deps: Vec::new(),
diff --git a/tools/tailor/docs/reference/image-yaml.md b/tools/tailor/docs/reference/image-yaml.md
index f15b7c95a7..8b2c4e91a4 100644
--- a/tools/tailor/docs/reference/image-yaml.md
+++ b/tools/tailor/docs/reference/image-yaml.md
@@ -17,7 +17,7 @@ An image definition lives in an `image.yaml`. The top level belongs to tailor. T
 | `rpmSources` | path list | no | Each path is a directory of RPMs or a `.repo` file; passed as IC `--rpm-source`. |
 | `operation` | `customize` or `convert` | no | Default: `customize`. |
 | `signing` | `true` or profile id | no | Opt in to the signed-image pipeline. `true` ⇒ the workspace `signing.default` profile; a string ⇒ that named profile; omitted ⇒ unsigned. See [Sign an image](../how-to/sign-an-image.md). |
-| `extraDependencies` | path list | no | Extra files/directories to hash for incremental checks; use for IC-config-referenced assets. |
+| `extraDependencies` | path list | no | Extra files/directories to hash for incremental checks; use for IC-config-referenced assets. Mergeable like `rpmSources`/`extraParams` -- see below. |
 | `dependsOn` | string list | no | Order-only build dependencies on other workspace images. See [Inter-image dependencies](#inter-image-dependencies). |
 | `inputs` | list of `{name, image, output?, cell?}` | no | Named producer artifacts embedded in `config:` via `${inputs.<name>}`. See [Embedding an artifact with `inputs:`](#embedding-an-artifact-with-inputs). |
 | `extraParams` | list of `{param, value?}` | no | Extra Image Customizer command-line flags, appended verbatim after every flag tailor manages. For experimental/non-standard IC builds. See [Extra params](#extra-params). |
@@ -81,6 +81,11 @@ so changing a `param` or `value` rebuilds the affected cells.
 every matched fragment, base → most-specific, so a `by-<axis>/<value>.yaml` fragment can add a flag
 for just its cells.
 
+`extraDependencies` is mergeable the same way: a fragment's own `extraDependencies:` concatenates
+with the base document's (and every other matched fragment's), base → most-specific, and resolves
+relative to the image directory just like the base document's paths do -- so a `by-<axis>/<value>.yaml`
+fragment can declare extra local dependencies that only its own cells need hashed.
+
 A flag tailor already emits itself (`--config-file`, `--build-dir`, `--output-image-file`,
 `--output-image-format`, `--rpm-source`, `--tools-dir`, `--image`, `--image-file`,
 `--image-cache-dir`, `--cosi-compression-level`, and the `--log-*` flags) is **rejected** — tailor