diff --git a/src/domain/app.sx b/src/domain/app.sx
new file mode 100644
index 0000000..bc8e0f6
--- /dev/null
+++ b/src/domain/app.sx
@@ -0,0 +1,31 @@
+#import "modules/std.sx";
+#import "platform.sx";
+
+// Who can see an app in listings: `private` = owner only, `unlisted` =
+// reachable by direct link, `public` = listed.
+Visibility :: enum u8 {
+    private;
+    unlisted;
+    public;
+}
+
+// A reverse-DNS bundle identifier (e.g. "co.acme.app") for one platform.
+// An app carries at most one per platform; kept as a flat list scanned
+// linearly (no HashMap yet).
+BundleId :: struct {
+    platform: Platform;
+    value: string;
+}
+
+// A distributable application. `slug` is the URL-safe handle; `id` is the
+// opaque primary key. Timestamps are unix epoch seconds.
+App :: struct {
+    id: string;
+    slug: string;
+    display_name: string;
+    bundle_ids: List(BundleId);
+    owner: string;
+    visibility: Visibility = .private;
+    created_at: s64;
+    updated_at: s64;
+}
diff --git a/src/domain/artifact.sx b/src/domain/artifact.sx
new file mode 100644
index 0000000..c79f257
--- /dev/null
+++ b/src/domain/artifact.sx
@@ -0,0 +1,26 @@
+#import "modules/std.sx";
+#import "platform.sx";
+
+// Where an artifact sits in the validation pipeline: artifacts enter
+// `pending`, then validation moves them to `valid` or `invalid`.
+ValidationStatus :: enum u8 {
+    pending;
+    valid;
+    invalid;
+}
+
+// A single uploaded binary belonging to a release, for one platform.
+// `sha256` is the lowercase hex digest; `storage_key` locates the bytes
+// in the blob store.
+Artifact :: struct {
+    id: string;
+    app_id: string;
+    release_id: string;
+    platform: Platform;
+    filename: string;
+    content_type: string;
+    size_bytes: s64;
+    sha256: string;
+    storage_key: string;
+    validation_status: ValidationStatus = .pending;
+}
diff --git a/src/domain/audit.sx b/src/domain/audit.sx
new file mode 100644
index 0000000..89cfaec
--- /dev/null
+++ b/src/domain/audit.sx
@@ -0,0 +1,15 @@
+#import "modules/std.sx";
+
+// An append-only record of a mutation. `actor` is the user or service that
+// performed `action` on the target identified by `target_type` +
+// `target_id`. `metadata` is an opaque string for now (structured JSON
+// arrives with std/json in a later step). `created_at` is unix epoch seconds.
+AuditEvent :: struct {
+    id: string;
+    actor: string;
+    action: string;
+    target_type: string;
+    target_id: string;
+    metadata: string;
+    created_at: s64;
+}
diff --git a/src/domain/channel.sx b/src/domain/channel.sx
new file mode 100644
index 0000000..cbe1e44
--- /dev/null
+++ b/src/domain/channel.sx
@@ -0,0 +1,19 @@
+#import "modules/std.sx";
+
+// How a channel rolls a release out: `manual` is all-or-nothing once
+// published; `percentage` stages exposure by `rollout_percent` (0..100).
+RolloutPolicy :: enum u8 {
+    manual;
+    percentage;
+}
+
+// A named publishing track for an app (e.g. "stable", "beta"), pointing at
+// the release currently serving traffic. `rollout_percent` is meaningful
+// only under the `percentage` policy.
+Channel :: struct {
+    app_id: string;
+    name: string;
+    current_release_id: string;
+    policy: RolloutPolicy = .manual;
+    rollout_percent: s64 = 100;
+}
diff --git a/src/domain/platform.sx b/src/domain/platform.sx
new file mode 100644
index 0000000..33e51df
--- /dev/null
+++ b/src/domain/platform.sx
@@ -0,0 +1,11 @@
+#import "modules/std.sx";
+
+// Target platforms a release can ship to. `android_apk` names the APK
+// artifact form specifically; the remaining variants are one per OS.
+Platform :: enum u8 {
+    ios;
+    android_apk;
+    macos;
+    linux;
+    windows;
+}
diff --git a/src/domain/release.sx b/src/domain/release.sx
new file mode 100644
index 0000000..4942401
--- /dev/null
+++ b/src/domain/release.sx
@@ -0,0 +1,16 @@
+#import "modules/std.sx";
+
+// A versioned build of an app, published to one channel. `version` is the
+// human semver string; `build` is the monotonic build number. `channel`
+// names the Channel it targets. Timestamps are unix epoch seconds.
+Release :: struct {
+    id: string;
+    app_id: string;
+    version: string;
+    build: s64;
+    channel: string;
+    notes: string;
+    created_by: string;
+    created_at: s64;
+    updated_at: s64;
+}
diff --git a/src/domain/validate.sx b/src/domain/validate.sx
new file mode 100644
index 0000000..3a1f5d7
--- /dev/null
+++ b/src/domain/validate.sx
@@ -0,0 +1,153 @@
+#import "modules/std.sx";
+#import "platform.sx";
+#import "app.sx";
+#import "release.sx";
+#import "artifact.sx";
+#import "channel.sx";
+
+// Typed failures from boundary validation. One distinct tag per failure
+// class so callers (and, later, the CLI) can surface a precise message
+// instead of a bare bool that drops the reason.
+ValidationErr :: error {
+    MissingField,     // a required string field was empty
+    BadSlug,          // slug broke the [a-z0-9-] / hyphen-position rules
+    EmptyVersion,     // version string was empty
+    BadVersion,       // version non-empty but not MAJOR.MINOR.PATCH(-suffix)
+    BadChannelName,   // channel name empty or outside [a-z0-9-]
+    UnknownPlatform,  // platform id did not name a Platform variant
+    BadRollout,       // rollout_percent outside 0..100
+}
+
+// ── Character classes (ASCII byte codes) ────────────────────────────────
+is_lower :: (c: u8) -> bool { return c >= 97 and c <= 122; }   // 'a'..'z'
+is_upper :: (c: u8) -> bool { return c >= 65 and c <= 90; }    // 'A'..'Z'
+is_digit :: (c: u8) -> bool { return c >= 48 and c <= 57; }    // '0'..'9'
+
+// ── Field validators ────────────────────────────────────────────────────
+
+// slug: non-empty; every byte in [a-z0-9-]; no leading or trailing hyphen;
+// no two consecutive hyphens. So "acme-app" passes; "", "-x", "x-", "a--b",
+// and "App" are all rejected.
+validate_slug :: (s: string) -> !ValidationErr {
+    if s.len == 0 { raise error.BadSlug; }
+    if s[0] == 45 { raise error.BadSlug; }            // 45='-' leading
+    if s[s.len - 1] == 45 { raise error.BadSlug; }    // trailing
+    i := 0;
+    while i < s.len {
+        c := s[i];
+        if !(is_lower(c) or is_digit(c) or c == 45) { raise error.BadSlug; }
+        if c == 45 and i > 0 {
+            if s[i - 1] == 45 { raise error.BadSlug; } // double hyphen
+        }
+        i += 1;
+    }
+    return;
+}
+
+// version: MAJOR.MINOR.PATCH, each component a run of >=1 digit, optionally
+// followed by "-<suffix>" where suffix is a non-empty run of [A-Za-z0-9.-]
+// (e.g. "1.0.0", "2.10.3-beta.1"). Empty -> EmptyVersion; any other
+// deviation -> BadVersion.
+validate_version :: (s: string) -> !ValidationErr {
+    if s.len == 0 { raise error.EmptyVersion; }
+    i := 0;
+    comp := 0;
+    while comp < 3 {
+        start := i;
+        while i < s.len {
+            if !is_digit(s[i]) { break; }
+            i += 1;
+        }
+        if i == start { raise error.BadVersion; }      // no digits in component
+        if comp < 2 {
+            if i >= s.len { raise error.BadVersion; }   // missing '.' separator
+            if s[i] != 46 { raise error.BadVersion; }   // 46='.'
+            i += 1;
+        }
+        comp += 1;
+    }
+    if i < s.len {
+        if s[i] != 45 { raise error.BadVersion; }       // only a '-' suffix allowed
+        i += 1;
+        if i >= s.len { raise error.BadVersion; }        // empty suffix
+        while i < s.len {
+            c := s[i];
+            if !(is_lower(c) or is_upper(c) or is_digit(c) or c == 45 or c == 46) {
+                raise error.BadVersion;
+            }
+            i += 1;
+        }
+    }
+    return;
+}
+
+// channel name: non-empty; every byte in [a-z0-9-]. Looser than slug —
+// hyphen position is unconstrained (e.g. "stable", "beta-2").
+validate_channel_name :: (s: string) -> !ValidationErr {
+    if s.len == 0 { raise error.BadChannelName; }
+    i := 0;
+    while i < s.len {
+        c := s[i];
+        if !(is_lower(c) or is_digit(c) or c == 45) { raise error.BadChannelName; }
+        i += 1;
+    }
+    return;
+}
+
+// Parse a platform id into the Platform enum. Accepted ids are exactly the
+// variant names; anything else -> UnknownPlatform.
+parse_platform :: (s: string) -> (Platform, !ValidationErr) {
+    if s == "ios"         { return .ios; }
+    if s == "android_apk" { return .android_apk; }
+    if s == "macos"       { return .macos; }
+    if s == "linux"       { return .linux; }
+    if s == "windows"     { return .windows; }
+    raise error.UnknownPlatform;
+}
+
+// ── Aggregate validators (required-field presence + field rules) ─────────
+
+// App requires id, slug, display_name, owner; slug must also pass
+// validate_slug. Presence is checked first, so an empty slug surfaces as
+// MissingField and a present-but-malformed slug as BadSlug.
+validate_app :: (a: App) -> !ValidationErr {
+    if a.id.len == 0 { raise error.MissingField; }
+    if a.slug.len == 0 { raise error.MissingField; }
+    if a.display_name.len == 0 { raise error.MissingField; }
+    if a.owner.len == 0 { raise error.MissingField; }
+    try validate_slug(a.slug);
+    return;
+}
+
+// Release requires id, app_id, created_by; version goes through
+// validate_version (empty -> EmptyVersion) and channel through
+// validate_channel_name.
+validate_release :: (r: Release) -> !ValidationErr {
+    if r.id.len == 0 { raise error.MissingField; }
+    if r.app_id.len == 0 { raise error.MissingField; }
+    if r.created_by.len == 0 { raise error.MissingField; }
+    try validate_version(r.version);
+    try validate_channel_name(r.channel);
+    return;
+}
+
+// Artifact requires id, app_id, release_id, filename, sha256, storage_key.
+// platform and validation_status are enums, hence always well-formed.
+validate_artifact :: (a: Artifact) -> !ValidationErr {
+    if a.id.len == 0 { raise error.MissingField; }
+    if a.app_id.len == 0 { raise error.MissingField; }
+    if a.release_id.len == 0 { raise error.MissingField; }
+    if a.filename.len == 0 { raise error.MissingField; }
+    if a.sha256.len == 0 { raise error.MissingField; }
+    if a.storage_key.len == 0 { raise error.MissingField; }
+    return;
+}
+
+// Channel requires app_id; name goes through validate_channel_name and
+// rollout_percent must be in 0..100.
+validate_channel :: (c: Channel) -> !ValidationErr {
+    if c.app_id.len == 0 { raise error.MissingField; }
+    try validate_channel_name(c.name);
+    if c.rollout_percent < 0 or c.rollout_percent > 100 { raise error.BadRollout; }
+    return;
+}
diff --git a/tests/domain_validate.sx b/tests/domain_validate.sx
new file mode 100644
index 0000000..f444c7c
--- /dev/null
+++ b/tests/domain_validate.sx
@@ -0,0 +1,178 @@
+// Acceptance for P2.1 — the domain boundary validator. Constructs valid
+// App/Release/Artifact/Channel and asserts they are accepted, then mutates
+// each into one invalid form and asserts it is rejected with the SPECIFIC
+// typed ValidationErr (not merely "some failure"). Prints a pass/fail line
+// per case and exits non-zero if any case behaves unexpectedly.
+#import "modules/std.sx";
+#import "../src/domain/platform.sx";
+#import "../src/domain/app.sx";
+#import "../src/domain/release.sx";
+#import "../src/domain/artifact.sx";
+#import "../src/domain/channel.sx";
+#import "../src/domain/validate.sx";
+
+// ── Valid fixtures ──────────────────────────────────────────────────────
+
+valid_app :: () -> App {
+    a : App = .{
+        id = "app_01",
+        slug = "acme-app",
+        display_name = "Acme App",
+        owner = "user_01",
+        visibility = .public,
+        created_at = 1700000000,
+        updated_at = 1700000000,
+    };
+    a.bundle_ids.append(BundleId.{ platform = .ios, value = "co.acme.app" });
+    return a;
+}
+
+valid_release :: () -> Release {
+    return Release.{
+        id = "rel_01",
+        app_id = "app_01",
+        version = "1.2.3-beta.1",
+        build = 42,
+        channel = "stable",
+        notes = "first cut",
+        created_by = "user_01",
+        created_at = 1700000000,
+        updated_at = 1700000000,
+    };
+}
+
+valid_artifact :: () -> Artifact {
+    return Artifact.{
+        id = "art_01",
+        app_id = "app_01",
+        release_id = "rel_01",
+        platform = .android_apk,
+        filename = "acme.apk",
+        content_type = "application/vnd.android.package-archive",
+        size_bytes = 10485760,
+        sha256 = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
+        storage_key = "apps/app_01/rel_01/acme.apk",
+        validation_status = .valid,
+    };
+}
+
+valid_channel :: () -> Channel {
+    return Channel.{
+        app_id = "app_01",
+        name = "stable",
+        current_release_id = "rel_01",
+        policy = .percentage,
+        rollout_percent = 25,
+    };
+}
+
+// ── Cases ───────────────────────────────────────────────────────────────
+
+check_accepts_app :: () -> bool {
+    a := valid_app();
+    ok := true;
+    validate_app(a) catch { ok = false; };
+    return ok;
+}
+
+check_accepts_release :: () -> bool {
+    r := valid_release();
+    ok := true;
+    validate_release(r) catch { ok = false; };
+    return ok;
+}
+
+check_accepts_artifact :: () -> bool {
+    a := valid_artifact();
+    ok := true;
+    validate_artifact(a) catch { ok = false; };
+    return ok;
+}
+
+check_accepts_channel :: () -> bool {
+    c := valid_channel();
+    ok := true;
+    validate_channel(c) catch { ok = false; };
+    return ok;
+}
+
+check_accepts_platform :: () -> bool {
+    p, e := parse_platform("android_apk");
+    ok := false;
+    if !e { ok = (p == .android_apk); }
+    return ok;
+}
+
+check_rejects_bad_slug :: () -> bool {
+    a := valid_app();
+    a.slug = "Bad_Slug";              // uppercase + underscore
+    matched := false;
+    validate_app(a) catch e { matched = (e == error.BadSlug); };
+    return matched;
+}
+
+check_rejects_empty_version :: () -> bool {
+    r := valid_release();
+    r.version = "";
+    matched := false;
+    validate_release(r) catch e { matched = (e == error.EmptyVersion); };
+    return matched;
+}
+
+check_rejects_bad_version :: () -> bool {
+    r := valid_release();
+    r.version = "1.2";                // missing PATCH component
+    matched := false;
+    validate_release(r) catch e { matched = (e == error.BadVersion); };
+    return matched;
+}
+
+check_rejects_unknown_platform :: () -> bool {
+    _, e := parse_platform("plan9");
+    return e == error.UnknownPlatform;
+}
+
+check_rejects_bad_channel :: () -> bool {
+    c := valid_channel();
+    c.name = "Bad Channel";           // space + uppercase
+    matched := false;
+    validate_channel(c) catch e { matched = (e == error.BadChannelName); };
+    return matched;
+}
+
+check_rejects_missing_field :: () -> bool {
+    a := valid_artifact();
+    a.sha256 = "";                    // required field cleared
+    matched := false;
+    validate_artifact(a) catch e { matched = (e == error.MissingField); };
+    return matched;
+}
+
+run_case :: (label: string, ok: bool) -> s32 {
+    if ok { print("  PASS  {}\n", label); return 0; }
+    print("  FAIL  {}\n", label);
+    return 1;
+}
+
+main :: () -> s32 {
+    failures : s32 = 0;
+    failures += run_case("accept: valid app",                       check_accepts_app());
+    failures += run_case("accept: valid release",                   check_accepts_release());
+    failures += run_case("accept: valid artifact",                  check_accepts_artifact());
+    failures += run_case("accept: valid channel",                   check_accepts_channel());
+    failures += run_case("accept: platform 'android_apk'",          check_accepts_platform());
+    failures += run_case("reject: bad slug -> BadSlug",             check_rejects_bad_slug());
+    failures += run_case("reject: empty version -> EmptyVersion",   check_rejects_empty_version());
+    failures += run_case("reject: bad version -> BadVersion",       check_rejects_bad_version());
+    failures += run_case("reject: unknown platform -> UnknownPlatform", check_rejects_unknown_platform());
+    failures += run_case("reject: bad channel -> BadChannelName",   check_rejects_bad_channel());
+    failures += run_case("reject: missing field -> MissingField",   check_rejects_missing_field());
+
+    print("------------------------------------------------\n");
+    if failures == 0 {
+        print("domain_validate: ALL CASES PASS\n");
+        return 0;
+    }
+    print("domain_validate: {} CASE(S) FAILED\n", failures);
+    return 1;
+}