diff --git a/scarf/Packages/ScarfCore/Sources/ScarfCore/Services/HermesUpdaterCommandBuilder.swift b/scarf/Packages/ScarfCore/Sources/ScarfCore/Services/HermesUpdaterCommandBuilder.swift
new file mode 100644
index 0000000..fde28ee
--- /dev/null
+++ b/scarf/Packages/ScarfCore/Sources/ScarfCore/Services/HermesUpdaterCommandBuilder.swift
@@ -0,0 +1,34 @@
+import Foundation
+
+/// Pure helpers that build argv arrays for `hermes update` invocations.
+///
+/// Lives in ScarfCore so the eventual UI surface (Mac / iOS / remote)
+/// shares flag selection. There is no in-app "Update Hermes" affordance
+/// in v2.7.5 — Sparkle handles Scarf-self-update and `hermes update` is
+/// invoked by users in their terminal — but capability-gated flag logic
+/// is forward-compat plumbing that the future affordance will call. Each
+/// helper is a `nonisolated static` pure function: no transport, no
+/// MainActor, no mocking surface required.
+public enum HermesUpdaterCommandBuilder {
+    /// Argv for an `hermes update` invocation, capability-gated.
+    ///
+    /// Pre-v0.12 hosts only had `update` (no flags). v0.12+ accepts
+    /// `--check` for preflight. v0.13+ accepts `--yes` / `-y` for
+    /// unattended runs (skips the interactive confirmation prompt).
+    /// Flags are silently dropped when the connected host can't honor
+    /// them so callers don't need to branch on capabilities themselves.
+    public static func updateArgv(
+        capabilities: HermesCapabilities,
+        unattended: Bool,
+        checkOnly: Bool
+    ) -> [String] {
+        var args: [String] = ["update"]
+        if checkOnly && capabilities.hasUpdateCheck {
+            args.append("--check")
+        }
+        if unattended && capabilities.hasUpdateNonInteractive {
+            args.append("--yes")
+        }
+        return args
+    }
+}
diff --git a/scarf/Packages/ScarfCore/Tests/ScarfCoreTests/M0eUpdaterTests.swift b/scarf/Packages/ScarfCore/Tests/ScarfCoreTests/M0eUpdaterTests.swift
new file mode 100644
index 0000000..c063028
--- /dev/null
+++ b/scarf/Packages/ScarfCore/Tests/ScarfCoreTests/M0eUpdaterTests.swift
@@ -0,0 +1,87 @@
+import Testing
+import Foundation
+@testable import ScarfCore
+
+/// Pure-function matrix for `HermesUpdaterCommandBuilder.updateArgv`. The
+/// builder degrades flags silently when the connected host can't honor
+/// them, so the "is the right flag emitted on the right version?" matrix
+/// is the meaningful test surface.
+@Suite struct M0eUpdaterTests {
+
+    // MARK: - Helpers
+
+    private func caps(_ versionLine: String?) -> HermesCapabilities {
+        guard let line = versionLine else { return .empty }
+        return HermesCapabilities.parseLine(line)
+    }
+
+    // MARK: - Pre-v0.12 (no flags supported)
+
+    @Test func preV012_returnsBareUpdateRegardlessOfFlags() {
+        let pre = caps("Hermes Agent v0.11.0 (2026.4.23)")
+        #expect(HermesUpdaterCommandBuilder.updateArgv(
+            capabilities: pre, unattended: false, checkOnly: false
+        ) == ["update"])
+        #expect(HermesUpdaterCommandBuilder.updateArgv(
+            capabilities: pre, unattended: true, checkOnly: false
+        ) == ["update"])
+        #expect(HermesUpdaterCommandBuilder.updateArgv(
+            capabilities: pre, unattended: true, checkOnly: true
+        ) == ["update"])
+    }
+
+    @Test func unknownVersion_returnsBareUpdate() {
+        // No detected version means we can't guarantee any flag is
+        // honored; defensively emit the bare verb.
+        #expect(HermesUpdaterCommandBuilder.updateArgv(
+            capabilities: .empty, unattended: true, checkOnly: true
+        ) == ["update"])
+    }
+
+    // MARK: - v0.12 (--check supported, --yes is not)
+
+    @Test func v012_checkOnly_emitsCheckFlag() {
+        let v012 = caps("Hermes Agent v0.12.0 (2026.4.30)")
+        #expect(HermesUpdaterCommandBuilder.updateArgv(
+            capabilities: v012, unattended: false, checkOnly: true
+        ) == ["update", "--check"])
+    }
+
+    @Test func v012_unattended_dropsYesFlag() {
+        // v0.12 doesn't honor --yes; the helper degrades silently.
+        let v012 = caps("Hermes Agent v0.12.0 (2026.4.30)")
+        #expect(HermesUpdaterCommandBuilder.updateArgv(
+            capabilities: v012, unattended: true, checkOnly: false
+        ) == ["update"])
+    }
+
+    @Test func v012_checkOnlyAndUnattended_emitsOnlyCheck() {
+        let v012 = caps("Hermes Agent v0.12.0 (2026.4.30)")
+        #expect(HermesUpdaterCommandBuilder.updateArgv(
+            capabilities: v012, unattended: true, checkOnly: true
+        ) == ["update", "--check"])
+    }
+
+    // MARK: - v0.13 (full flag support)
+
+    @Test func v013_unattended_emitsYesFlag() {
+        let v013 = caps("Hermes Agent v0.13.0 (2026.5.7)")
+        #expect(HermesUpdaterCommandBuilder.updateArgv(
+            capabilities: v013, unattended: true, checkOnly: false
+        ) == ["update", "--yes"])
+    }
+
+    @Test func v013_checkOnlyAndUnattended_emitsBothFlags() {
+        let v013 = caps("Hermes Agent v0.13.0 (2026.5.7)")
+        #expect(HermesUpdaterCommandBuilder.updateArgv(
+            capabilities: v013, unattended: true, checkOnly: true
+        ) == ["update", "--check", "--yes"])
+    }
+
+    @Test func v013_neither_emitsBareUpdate() {
+        let v013 = caps("Hermes Agent v0.13.0 (2026.5.7)")
+        #expect(HermesUpdaterCommandBuilder.updateArgv(
+            capabilities: v013, unattended: false, checkOnly: false
+        ) == ["update"])
+    }
+}