diff --git a/xtask/Cargo.toml b/xtask/Cargo.toml
new file mode 100644
index 0000000..209bbaf
--- /dev/null
+++ b/xtask/Cargo.toml
@@ -0,0 +1,21 @@
+[package]
+name = "xtask"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+# Standalone workspace so xtask compiles even when the parent rustelo workspace
+# is mid-edit (the tool's whole purpose is fixing that state).
+[workspace]
+
+# xtask intentionally uses inline dep versions (not workspace = true) so it can
+# build even when [workspace.dependencies] is mid-edit. It is the tool that
+# manages that section, so it must not depend on it being well-formed.
+
+[dependencies]
+clap = { version = "4.6", features = ["derive"] }
+toml = "1.1"
+serde = { version = "1.0", features = ["derive"] }
+anyhow = "1.0"
+pathdiff = "0.2"
+indexmap = { version = "2.7", features = ["serde"] }
diff --git a/xtask/src/main.rs b/xtask/src/main.rs
new file mode 100644
index 0000000..7d8c543
--- /dev/null
+++ b/xtask/src/main.rs
@@ -0,0 +1,71 @@
+use std::path::PathBuf;
+use std::process::ExitCode;
+
+use anyhow::Result;
+use clap::{Parser, Subcommand};
+
+mod overlay;
+mod registry;
+mod render;
+mod sync;
+
+#[derive(Parser)]
+#[command(
+    name = "xtask",
+    about = "Rustelo workspace meta-tools (deps sync, integrity checks)"
+)]
+struct Cli {
+    #[command(subcommand)]
+    cmd: Cmd,
+}
+
+#[derive(Subcommand)]
+enum Cmd {
+    /// Sync [workspace.dependencies] from registry/Cargo.toml into a Cargo.toml.
+    SyncDeps(SyncDepsArgs),
+}
+
+#[derive(Parser)]
+pub struct SyncDepsArgs {
+    /// Verify only — exit non-zero if any target Cargo.toml differs from the rendered registry.
+    #[arg(long)]
+    pub check: bool,
+
+    /// Target Cargo.toml directory. Defaults to the rustelo workspace root.
+    /// For implementations, point this at the impl workspace dir (e.g. ../jpl-website).
+    /// Mutually exclusive with --all.
+    #[arg(long)]
+    pub target: Option<PathBuf>,
+
+    /// Path to the rustelo workspace root (where registry/Cargo.toml lives).
+    /// Used to compute relative path rebasing for impl targets.
+    #[arg(long, default_value = ".")]
+    pub rustelo_root: PathBuf,
+
+    /// Explicit path to an overlay file. Default: `{target}/rustelo-deps-overlay.toml`
+    /// (auto-detected; absent = no overlay). Cannot be combined with --all
+    /// (each impl uses its own auto-detected overlay).
+    #[arg(long)]
+    pub overlay: Option<PathBuf>,
+
+    /// Sync the rustelo workspace plus every implementation listed in
+    /// `registry/sync.toml` `[impls.known]`. Drift in any target sets a
+    /// non-zero exit status; all targets are visited regardless of intermediate
+    /// drift (no fail-fast). Mutually exclusive with --target and --overlay.
+    #[arg(long)]
+    pub all: bool,
+}
+
+fn main() -> ExitCode {
+    let cli = Cli::parse();
+    let result: Result<ExitCode> = match cli.cmd {
+        Cmd::SyncDeps(args) => sync::run(args),
+    };
+    match result {
+        Ok(code) => code,
+        Err(e) => {
+            eprintln!("xtask: {e:#}");
+            ExitCode::from(2)
+        }
+    }
+}
diff --git a/xtask/src/overlay.rs b/xtask/src/overlay.rs
new file mode 100644
index 0000000..df8a5d4
--- /dev/null
+++ b/xtask/src/overlay.rs
@@ -0,0 +1,229 @@
+use std::collections::HashSet;
+use std::fs;
+use std::path::{Path, PathBuf};
+
+use anyhow::{Context, Result, bail};
+use indexmap::IndexMap;
+use serde::Deserialize;
+
+use crate::registry::{Dep, Registry};
+
+pub const OVERLAY_FILENAME: &str = "rustelo-deps-overlay.toml";
+
+#[derive(Deserialize, Debug, Default)]
+pub struct Overlay {
+    /// Parent layer this overlay extends, as a path to the directory containing
+    /// the parent `rustelo-deps-overlay.toml`, relative to THIS overlay's own
+    /// directory. Resolution walks the chain parent→child; feature additions and
+    /// extras accumulate down it under the same additive-only rule a single
+    /// overlay obeys over the registry. Absent = a one-link chain (the historical
+    /// single-overlay behavior, byte-for-byte).
+    #[serde(default)]
+    pub extends: Option<String>,
+    /// Feature additions to registry deps (or to extras introduced upstream in
+    /// the chain). Cannot override version or path.
+    #[serde(default)]
+    pub overlay: IndexMap<String, FeatureOverlay>,
+    /// Deps the layer needs but that are not in the registry (impl-specific
+    /// feature crates, sibling-project paths, the layer's own crates). Paths are
+    /// rustelo-root-relative, like registry path-deps, and rebased per target.
+    #[serde(default)]
+    pub extra: IndexMap<String, Dep>,
+}
+
+#[derive(Deserialize, Debug, Default)]
+pub struct FeatureOverlay {
+    #[serde(default)]
+    pub features: Vec<String>,
+}
+
+/// One resolved overlay in a chain, paired with the canonical directory its file
+/// lives in. The directory is needed only to resolve the next `extends` hop;
+/// rendering rebases extra paths via the rustelo root, not this directory.
+#[derive(Debug)]
+pub struct Layer {
+    pub overlay: Overlay,
+    pub dir: PathBuf,
+}
+
+/// Loads `{target_dir}/rustelo-deps-overlay.toml` if it exists. Absent file is
+/// not an error — most layers do not need an overlay.
+pub fn load(target_dir: &Path) -> Result<Option<Overlay>> {
+    let path = target_dir.join(OVERLAY_FILENAME);
+    if !path.exists() {
+        return Ok(None);
+    }
+    Ok(Some(load_file(&path)?))
+}
+
+/// Loads an overlay from an explicit file path (used by the `--overlay` flag).
+pub fn load_file(path: &Path) -> Result<Overlay> {
+    let text =
+        fs::read_to_string(path).with_context(|| format!("reading overlay {}", path.display()))?;
+    toml::from_str(&text).with_context(|| format!("parsing overlay {}", path.display()))
+}
+
+/// Resolves the overlay chain for a target, ordered parent→child (the target's
+/// own overlay is last, so its feature additions win the rendering order). Each
+/// overlay's `extends` names the directory of its parent layer, relative to the
+/// overlay's own location. A target with no overlay file yields an empty chain
+/// (equivalent to "no overlay"); an `extends` that points at a directory with no
+/// overlay file, or that forms a cycle, is an error.
+pub fn load_chain(target_dir: &Path) -> Result<Vec<Layer>> {
+    let mut chain: Vec<Layer> = Vec::new();
+    let mut seen: HashSet<PathBuf> = HashSet::new();
+    let mut dir = target_dir.to_path_buf();
+
+    loop {
+        let canon = dir
+            .canonicalize()
+            .with_context(|| format!("canonicalize layer dir {}", dir.display()))?;
+        if !seen.insert(canon.clone()) {
+            bail!(
+                "overlay `extends` cycle detected: {} is reached twice",
+                canon.display()
+            );
+        }
+
+        let overlay = match load(&canon)? {
+            Some(o) => o,
+            None => {
+                if chain.is_empty() {
+                    // Target itself has no overlay → no chain at all.
+                    return Ok(Vec::new());
+                }
+                bail!(
+                    "{} is referenced via `extends` but has no {}",
+                    canon.display(),
+                    OVERLAY_FILENAME
+                );
+            }
+        };
+
+        let next = overlay
+            .extends
+            .as_ref()
+            .map(|rel| canon.join(rel));
+        chain.push(Layer {
+            overlay,
+            dir: canon,
+        });
+        match next {
+            Some(parent) => dir = parent,
+            None => break,
+        }
+    }
+
+    chain.reverse();
+    Ok(chain)
+}
+
+/// Validates an ordered overlay chain against the registry. The additive-only
+/// rule recurses down the chain: a `[overlay.X]` may target a registry dep or an
+/// extra introduced by an *upstream* layer; a `[extra.X]` may not collide with a
+/// registry dep nor redefine an upstream extra (add features via `[overlay.X]`
+/// instead).
+pub fn validate_chain(layers: &[Layer], registry: &Registry) -> Result<()> {
+    let mut upstream_extras: HashSet<String> = HashSet::new();
+
+    for layer in layers {
+        let o = &layer.overlay;
+        for name in o.overlay.keys() {
+            if !registry.deps.contains_key(name) && !upstream_extras.contains(name) {
+                bail!(
+                    "overlay references unknown dep `{name}` at {} — add it to registry/Cargo.toml, introduce it via an upstream [extra], or declare it in [extra]",
+                    layer.dir.display()
+                );
+            }
+        }
+        for (name, dep) in &o.extra {
+            if registry.deps.contains_key(name) {
+                bail!(
+                    "[extra.{name}] at {} conflicts with a registry dep — use [overlay.{name}] for feature additions",
+                    layer.dir.display()
+                );
+            }
+            if upstream_extras.contains(name) {
+                bail!(
+                    "[extra.{name}] at {} redefines an extra from an upstream layer — use [overlay.{name}] to add features instead",
+                    layer.dir.display()
+                );
+            }
+            dep.validate(name)?;
+            upstream_extras.insert(name.clone());
+        }
+    }
+    Ok(())
+}
+
+/// Merges overlay feature additions onto a dep. Order preserved, duplicates
+/// dropped.
+pub fn apply_features(dep: &Dep, fo: Option<&FeatureOverlay>) -> Dep {
+    let mut merged = dep.clone();
+    if let Some(fo) = fo {
+        for f in &fo.features {
+            if !merged.features.contains(f) {
+                merged.features.push(f.clone());
+            }
+        }
+    }
+    merged
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::registry::Registry;
+
+    fn registry_with(names: &[&str]) -> Registry {
+        let mut deps = IndexMap::new();
+        for n in names {
+            deps.insert(
+                (*n).to_string(),
+                Dep {
+                    version: Some("1.0".into()),
+                    rebase: true,
+                    ..Dep::default()
+                },
+            );
+        }
+        Registry { deps }
+    }
+
+    fn layer(toml_src: &str, dir: &str) -> Layer {
+        Layer {
+            overlay: toml::from_str(toml_src).expect("overlay parses"),
+            dir: PathBuf::from(dir),
+        }
+    }
+
+    #[test]
+    fn overlay_may_feature_an_upstream_extra() {
+        let reg = registry_with(&["rustelo_server"]);
+        let l2 = layer("[extra.content-graph]\nversion = \"0.1\"\n", "/l2");
+        let l3 = layer("[overlay.content-graph]\nfeatures = [\"ssr\"]\n", "/l3");
+        validate_chain(&[l2, l3], &reg).expect("upstream extra is a valid overlay target");
+    }
+
+    #[test]
+    fn overlay_on_unknown_dep_is_rejected() {
+        let reg = registry_with(&["rustelo_server"]);
+        let l3 = layer("[overlay.nope]\nfeatures = [\"x\"]\n", "/l3");
+        assert!(validate_chain(&[l3], &reg).is_err());
+    }
+
+    #[test]
+    fn redefining_an_upstream_extra_is_rejected() {
+        let reg = registry_with(&[]);
+        let l2 = layer("[extra.shared]\npath = \"../a/shared\"\n", "/l2");
+        let l3 = layer("[extra.shared]\npath = \"../b/shared\"\n", "/l3");
+        assert!(validate_chain(&[l2, l3], &reg).is_err());
+    }
+
+    #[test]
+    fn extra_colliding_with_registry_is_rejected() {
+        let reg = registry_with(&["rustelo_server"]);
+        let l3 = layer("[extra.rustelo_server]\nversion = \"9.9\"\n", "/l3");
+        assert!(validate_chain(&[l3], &reg).is_err());
+    }
+}
diff --git a/xtask/src/registry.rs b/xtask/src/registry.rs
new file mode 100644
index 0000000..b0c2c1f
--- /dev/null
+++ b/xtask/src/registry.rs
@@ -0,0 +1,201 @@
+use std::collections::HashSet;
+use std::fs;
+use std::path::{Path, PathBuf};
+
+use anyhow::{Context, Result, bail};
+use indexmap::IndexMap;
+use serde::Deserialize;
+
+#[derive(Debug)]
+pub struct Registry {
+    pub deps: IndexMap<String, Dep>,
+}
+
+#[derive(Debug, Clone, Default)]
+pub struct Dep {
+    pub version: Option<String>,
+    pub path: Option<String>,
+    pub features: Vec<String>,
+    pub default_features: Option<bool>,
+    /// When true (default), `path` is rebased relative to the target's Cargo.toml
+    /// during sync. When false, `path` is rebased relative to the rustelo
+    /// workspace root and the same string ships to every target — meant for
+    /// paths that point outside the rustelo workspace.
+    pub rebase: bool,
+}
+
+impl Dep {
+    pub fn validate(&self, name: &str) -> Result<()> {
+        if self.version.is_none() && self.path.is_none() {
+            bail!("dep `{name}` declares neither `version` nor `path`");
+        }
+        Ok(())
+    }
+
+    pub fn is_simple_inline(&self) -> bool {
+        self.path.is_none()
+            && self.features.is_empty()
+            && self.default_features.is_none()
+            && self.version.is_some()
+    }
+}
+
+/// Accepts both Cargo's compact form (`name = "x.y"`) and table form
+/// (`name = { version = "x.y", features = [...] }`). The `rebase` field is
+/// non-Cargo metadata used only by overlays' `[extra]` table; the registry
+/// loader sources it from `registry/sync.toml` instead.
+impl<'de> Deserialize<'de> for Dep {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        #[derive(Deserialize)]
+        #[serde(untagged)]
+        enum Value {
+            Version(String),
+            Table(Inner),
+        }
+
+        #[derive(Deserialize, Default)]
+        #[serde(rename_all = "kebab-case")]
+        struct Inner {
+            #[serde(default)]
+            version: Option<String>,
+            #[serde(default)]
+            path: Option<String>,
+            #[serde(default)]
+            features: Vec<String>,
+            #[serde(default)]
+            default_features: Option<bool>,
+            #[serde(default = "default_rebase")]
+            rebase: bool,
+        }
+
+        match Value::deserialize(deserializer)? {
+            Value::Version(v) => Ok(Dep {
+                version: Some(v),
+                rebase: true,
+                ..Dep::default()
+            }),
+            Value::Table(t) => Ok(Dep {
+                version: t.version,
+                path: t.path,
+                features: t.features,
+                default_features: t.default_features,
+                rebase: t.rebase,
+            }),
+        }
+    }
+}
+
+fn default_rebase() -> bool {
+    true
+}
+
+#[derive(Deserialize)]
+struct CargoManifest {
+    workspace: WorkspaceSection,
+}
+
+#[derive(Deserialize)]
+struct WorkspaceSection {
+    #[serde(default)]
+    dependencies: IndexMap<String, Dep>,
+}
+
+#[derive(Deserialize, Default)]
+struct SyncSidecar {
+    #[serde(default)]
+    no_rebase: Vec<String>,
+    #[serde(default)]
+    impls: ImplsSection,
+}
+
+#[derive(Deserialize, Default)]
+struct ImplsSection {
+    #[serde(default)]
+    known: Vec<String>,
+}
+
+/// Loaded view of `registry/sync.toml` — non-Cargo metadata used by the xtask.
+#[derive(Debug, Default)]
+pub struct Sidecar {
+    /// Names of registry deps whose `path` is NOT rebased per target.
+    pub no_rebase: HashSet<String>,
+    /// Paths (relative to the rustelo workspace root) of implementations that
+    /// consume this registry. Used by `sync-deps --all` to fan out.
+    pub known_impls: Vec<String>,
+}
+
+/// Strips the leading `../` from a path stored in `registry/Cargo.toml`,
+/// yielding a path relative to the rustelo workspace root. The leading `../`
+/// is mandatory: `registry/Cargo.toml` lives one level inside the workspace,
+/// so Cargo expects every path entry to begin with `../`. This normalization
+/// lets the renderer treat `Dep.path` as rustelo-root-relative just like the
+/// previous custom-schema format did, keeping the path-rebase logic stable
+/// across the schema change.
+fn normalize_registry_path(name: &str, raw: &str) -> Result<String> {
+    let stripped = Path::new(raw).strip_prefix("..").map_err(|_| {
+        anyhow::anyhow!(
+            "registry path `{raw}` for `{name}` must start with `../` (registry/Cargo.toml is one level inside the workspace)"
+        )
+    })?;
+    if stripped == Path::new("") {
+        bail!("registry path `{raw}` for `{name}` resolves to the workspace root itself");
+    }
+    Ok(PathBuf::from(stripped).to_string_lossy().into_owned())
+}
+
+/// Loads `registry/sync.toml` once. Returns an empty Sidecar if the file is
+/// absent (no_rebase + no impls).
+pub fn load_sidecar(rustelo_root: &Path) -> Result<Sidecar> {
+    let path = rustelo_root.join("registry/sync.toml");
+    if !path.exists() {
+        return Ok(Sidecar::default());
+    }
+    let text = fs::read_to_string(&path)
+        .with_context(|| format!("reading {}", path.display()))?;
+    let raw: SyncSidecar = toml::from_str(&text)
+        .with_context(|| format!("parsing {}", path.display()))?;
+    Ok(Sidecar {
+        no_rebase: raw.no_rebase.into_iter().collect(),
+        known_impls: raw.impls.known,
+    })
+}
+
+pub fn load(rustelo_root: &Path) -> Result<Registry> {
+    let manifest_path = rustelo_root.join("registry/Cargo.toml");
+    let text = fs::read_to_string(&manifest_path)
+        .with_context(|| format!("reading {}", manifest_path.display()))?;
+    let manifest: CargoManifest = toml::from_str(&text)
+        .with_context(|| format!("parsing {}", manifest_path.display()))?;
+
+    let sidecar = load_sidecar(rustelo_root)?;
+
+    let mut deps: IndexMap<String, Dep> =
+        IndexMap::with_capacity(manifest.workspace.dependencies.len());
+    for (name, mut dep) in manifest.workspace.dependencies {
+        if sidecar.no_rebase.contains(&name) {
+            dep.rebase = false;
+        }
+        if let Some(p) = &dep.path {
+            dep.path = Some(normalize_registry_path(&name, p)?);
+        }
+        dep.validate(&name)?;
+        deps.insert(name, dep);
+    }
+
+    for n in &sidecar.no_rebase {
+        match deps.get(n) {
+            Some(d) if d.path.is_some() => {}
+            Some(_) => bail!(
+                "registry/sync.toml lists `{n}` in no_rebase but it has no `path` in registry/Cargo.toml"
+            ),
+            None => bail!(
+                "registry/sync.toml lists `{n}` in no_rebase but it is not in registry/Cargo.toml"
+            ),
+        }
+    }
+
+    Ok(Registry { deps })
+}
diff --git a/xtask/src/render.rs b/xtask/src/render.rs
new file mode 100644
index 0000000..798bf28
--- /dev/null
+++ b/xtask/src/render.rs
@@ -0,0 +1,129 @@
+use std::path::{Path, PathBuf};
+
+use anyhow::{Context, Result};
+
+use crate::overlay::{self, Layer};
+use crate::registry::{Dep, Registry};
+
+pub const REGION_START: &str =
+    "# >>> rustelo-sync:workspace-deps (managed by `cargo xtask sync-deps` — DO NOT EDIT)";
+pub const REGION_END: &str = "# <<< rustelo-sync:workspace-deps";
+
+/// Translates path-deps (stored as rustelo-root-relative strings on `Dep`)
+/// into the form needed by a target Cargo.toml.
+///
+/// - `is_self` (target == rustelo root): emit the stored path as-is.
+/// - `rebase = false`: emit the stored path as-is on every target. Used for
+///   external paths whose location relative to siblings is identical from
+///   each workspace (e.g. `../stratumiops/...`).
+/// - Otherwise: resolve to absolute via the rustelo root, then diff against
+///   the target's Cargo.toml location.
+pub struct Rebase {
+    pub rustelo_root: PathBuf,
+    pub target_dir: PathBuf,
+    pub is_self: bool,
+}
+
+impl Rebase {
+    pub fn new(rustelo_root: &Path, target_dir: &Path) -> Result<Self> {
+        let rustelo_root = rustelo_root
+            .canonicalize()
+            .with_context(|| format!("canonicalize rustelo_root {}", rustelo_root.display()))?;
+        let target_dir = target_dir
+            .canonicalize()
+            .with_context(|| format!("canonicalize target {}", target_dir.display()))?;
+        let is_self = rustelo_root == target_dir;
+        Ok(Self {
+            rustelo_root,
+            target_dir,
+            is_self,
+        })
+    }
+
+    fn translate_path(&self, path: &str, rebase: bool) -> String {
+        if self.is_self || !rebase {
+            return path.to_string();
+        }
+        let absolute = self.rustelo_root.join(path);
+        match pathdiff::diff_paths(&absolute, &self.target_dir) {
+            Some(rel) => rel.to_string_lossy().into_owned(),
+            None => path.to_string(),
+        }
+    }
+}
+
+/// Renders the entire managed region (markers included) for a target from an
+/// ordered overlay chain (parent→child). Registry-dep features are folded across
+/// every layer; extras accumulate down the chain and receive feature additions
+/// from any layer that targets them. `validate_chain` has already guaranteed no
+/// extra redefinition and no overlay of an unknown name, so accumulation here is
+/// insert-once. A one-link chain (or empty chain) reproduces the historical
+/// single-overlay (or no-overlay) rendering byte-for-byte.
+pub fn render_region(registry: &Registry, layers: &[Layer], rebase: &Rebase) -> String {
+    let mut out = String::new();
+    out.push_str(REGION_START);
+    out.push('\n');
+
+    for (name, dep) in &registry.deps {
+        let mut merged = dep.clone();
+        for layer in layers {
+            if let Some(fo) = layer.overlay.overlay.get(name) {
+                merged = overlay::apply_features(&merged, Some(fo));
+            }
+        }
+        out.push_str(&render_entry(name, &merged, rebase));
+        out.push('\n');
+    }
+
+    let mut extras: indexmap::IndexMap<String, Dep> = indexmap::IndexMap::new();
+    for layer in layers {
+        for (name, dep) in &layer.overlay.extra {
+            extras.insert(name.clone(), dep.clone());
+        }
+    }
+    // Feature additions onto extras (a later layer featuring an upstream extra).
+    // Registry-targeting overlays are skipped here — they are not in `extras`.
+    for layer in layers {
+        for (name, fo) in &layer.overlay.overlay {
+            if let Some(dep) = extras.get_mut(name) {
+                *dep = overlay::apply_features(dep, Some(fo));
+            }
+        }
+    }
+    for (name, dep) in &extras {
+        out.push_str(&render_entry(name, dep, rebase));
+        out.push('\n');
+    }
+
+    out.push_str(REGION_END);
+    out
+}
+
+fn render_entry(name: &str, dep: &Dep, rebase: &Rebase) -> String {
+    if dep.is_simple_inline() {
+        let v = dep.version.as_deref().unwrap_or_default();
+        return format!("{name} = \"{v}\"");
+    }
+
+    let mut parts: Vec<String> = Vec::new();
+    if let Some(v) = &dep.version {
+        parts.push(format!("version = \"{v}\""));
+    }
+    if let Some(p) = &dep.path {
+        let translated = rebase.translate_path(p, dep.rebase);
+        parts.push(format!("path = \"{translated}\""));
+    }
+    if let Some(df) = dep.default_features {
+        parts.push(format!("default-features = {df}"));
+    }
+    if !dep.features.is_empty() {
+        let feats = dep
+            .features
+            .iter()
+            .map(|f| format!("\"{f}\""))
+            .collect::<Vec<_>>()
+            .join(", ");
+        parts.push(format!("features = [{feats}]"));
+    }
+    format!("{name} = {{ {} }}", parts.join(", "))
+}
diff --git a/xtask/src/sync.rs b/xtask/src/sync.rs
new file mode 100644
index 0000000..1eee3fa
--- /dev/null
+++ b/xtask/src/sync.rs
@@ -0,0 +1,222 @@
+use std::fs;
+use std::path::{Path, PathBuf};
+use std::process::ExitCode;
+
+use anyhow::{Context, Result, bail};
+
+use crate::SyncDepsArgs;
+use crate::overlay::{self, Layer};
+use crate::registry::{self, Registry};
+use crate::render::{REGION_END, REGION_START, Rebase, render_region};
+
+/// Outcome of a single-target sync.
+enum SyncOutcome {
+    UpToDate,
+    Synced,
+    Drift,
+}
+
+pub fn run(args: SyncDepsArgs) -> Result<ExitCode> {
+    if args.all && args.overlay.is_some() {
+        bail!(
+            "--overlay cannot be combined with --all (each impl uses its own auto-detected overlay)"
+        );
+    }
+    if args.all && args.target.is_some() {
+        bail!("--target cannot be combined with --all");
+    }
+
+    let registry = registry::load(&args.rustelo_root)?;
+
+    let targets = resolve_targets(&args, &registry)?;
+
+    let mut drifted = false;
+    for target in targets {
+        let outcome = sync_one(
+            &args.rustelo_root,
+            &target,
+            args.overlay.as_deref(),
+            &registry,
+            args.check,
+        )?;
+        let cargo_path = target.join("Cargo.toml");
+        match outcome {
+            SyncOutcome::UpToDate => {
+                println!("sync-deps: {} is up to date", cargo_path.display());
+            }
+            SyncOutcome::Synced => {
+                println!("sync-deps: wrote {}", cargo_path.display());
+            }
+            SyncOutcome::Drift => {
+                eprintln!(
+                    "sync-deps: drift detected in {} — run `cargo xtask sync-deps` to fix",
+                    cargo_path.display()
+                );
+                drifted = true;
+            }
+        }
+    }
+
+    Ok(if drifted {
+        ExitCode::from(1)
+    } else {
+        ExitCode::SUCCESS
+    })
+}
+
+/// Builds the ordered list of target directories to sync.
+///
+/// - `--all`: rustelo workspace root + every path listed in
+///   `registry/sync.toml` `[impls.known]` (resolved relative to the
+///   rustelo root). Each impl path is validated to contain a `Cargo.toml`.
+/// - Otherwise: just `--target` (defaults to ".").
+fn resolve_targets(args: &SyncDepsArgs, _registry: &Registry) -> Result<Vec<PathBuf>> {
+    if args.all {
+        let sidecar = registry::load_sidecar(&args.rustelo_root)?;
+        let mut targets = Vec::with_capacity(1 + sidecar.known_impls.len());
+        targets.push(args.rustelo_root.clone());
+        for rel in &sidecar.known_impls {
+            let abs = args.rustelo_root.join(rel);
+            let cargo = abs.join("Cargo.toml");
+            if !cargo.exists() {
+                bail!(
+                    "registry/sync.toml [impls.known] points at `{}` but {} does not exist",
+                    rel,
+                    cargo.display()
+                );
+            }
+            targets.push(abs);
+        }
+        Ok(targets)
+    } else {
+        let target = args.target.clone().unwrap_or_else(|| PathBuf::from("."));
+        Ok(vec![target])
+    }
+}
+
+/// Syncs (or checks) one target. The overlay path can override auto-detection
+/// of `{target}/rustelo-deps-overlay.toml`.
+fn sync_one(
+    rustelo_root: &Path,
+    target_dir: &Path,
+    overlay_path: Option<&Path>,
+    registry: &Registry,
+    check: bool,
+) -> Result<SyncOutcome> {
+    let rebase = Rebase::new(rustelo_root, target_dir)?;
+
+    let layers = load_chain(target_dir, overlay_path)?;
+    overlay::validate_chain(&layers, registry)?;
+
+    let cargo_path = target_dir.join("Cargo.toml");
+    let original = fs::read_to_string(&cargo_path)
+        .with_context(|| format!("reading {}", cargo_path.display()))?;
+
+    let rendered_region = render_region(registry, &layers, &rebase);
+    let new_contents = splice_region(&original, &rendered_region, &cargo_path)?;
+
+    if new_contents == original {
+        return Ok(SyncOutcome::UpToDate);
+    }
+
+    if check {
+        return Ok(SyncOutcome::Drift);
+    }
+
+    fs::write(&cargo_path, &new_contents)
+        .with_context(|| format!("writing {}", cargo_path.display()))?;
+    Ok(SyncOutcome::Synced)
+}
+
+/// Builds the ordered overlay chain (parent→child) for a target. With an
+/// explicit `--overlay` path the chain is that single file — the escape hatch
+/// stays deliberately flat and does not follow `extends`. Otherwise the chain is
+/// auto-resolved from `{target}/rustelo-deps-overlay.toml` and its `extends`
+/// links.
+fn load_chain(target_dir: &Path, explicit: Option<&Path>) -> Result<Vec<Layer>> {
+    match explicit {
+        Some(path) => {
+            let overlay = overlay::load_file(path)?;
+            let dir = path
+                .parent()
+                .map(Path::to_path_buf)
+                .unwrap_or_else(|| PathBuf::from("."));
+            Ok(vec![Layer { overlay, dir }])
+        }
+        None => overlay::load_chain(target_dir),
+    }
+}
+
+/// Replaces the managed region inside [workspace.dependencies] with the rendered text.
+///
+/// If markers are absent, the region is inserted at the top of the
+/// [workspace.dependencies] table (preserving any manually-curated lines below
+/// the end marker — those will live outside the managed region).
+fn splice_region(original: &str, region: &str, cargo_path: &Path) -> Result<String> {
+    if let (Some(start), Some(end)) = (original.find(REGION_START), original.find(REGION_END)) {
+        if end < start {
+            bail!(
+                "{}: end marker appears before start marker",
+                cargo_path.display()
+            );
+        }
+        let end_full = end + REGION_END.len();
+        let mut out = String::with_capacity(original.len());
+        out.push_str(&original[..start]);
+        out.push_str(region);
+        out.push_str(&original[end_full..]);
+        return Ok(out);
+    }
+
+    let Some(hdr_idx) = find_table_header(original, "[workspace.dependencies]") else {
+        bail!(
+            "{}: no [workspace.dependencies] section found and no existing sync markers",
+            cargo_path.display()
+        );
+    };
+    let body_start = original[hdr_idx..]
+        .find('\n')
+        .map(|n| hdr_idx + n + 1)
+        .unwrap_or(original.len());
+
+    let body_end = next_table_header(&original[body_start..])
+        .map(|off| body_start + off)
+        .unwrap_or(original.len());
+
+    let mut out = String::with_capacity(original.len() + region.len());
+    out.push_str(&original[..body_start]);
+    out.push_str(region);
+    out.push('\n');
+    out.push('\n');
+    out.push_str(&original[body_end..]);
+    Ok(out)
+}
+
+/// Finds the offset of a specific table header (e.g. `[workspace.dependencies]`),
+/// requiring it to start at column 0 of a line. Avoids false matches inside
+/// comments or strings that mention the bracketed phrase.
+fn find_table_header(text: &str, header: &str) -> Option<usize> {
+    let mut offset = 0;
+    for line in text.split_inclusive('\n') {
+        let body = line.trim_end_matches('\n').trim_end_matches('\r');
+        if body == header {
+            return Some(offset);
+        }
+        offset += line.len();
+    }
+    None
+}
+
+/// Finds the offset of the next top-level table header line (a line starting with `[`
+/// at column 0) within `text`. Returns None if no such header exists.
+fn next_table_header(text: &str) -> Option<usize> {
+    let mut offset = 0;
+    for line in text.split_inclusive('\n') {
+        let trimmed_start = line.trim_start_matches([' ', '\t']);
+        if trimmed_start.starts_with('[') && trimmed_start == line {
+            return Some(offset);
+        }
+        offset += line.len();
+    }
+    None
+}