diff --git a/native/macos/MCPProxy/MCPProxy/MCPProxyApp.swift b/native/macos/MCPProxy/MCPProxy/MCPProxyApp.swift
index 14013aed0..8569f7b07 100644
--- a/native/macos/MCPProxy/MCPProxy/MCPProxyApp.swift
+++ b/native/macos/MCPProxy/MCPProxy/MCPProxyApp.swift
@@ -27,6 +27,15 @@ final class AppController: NSObject, NSApplicationDelegate, NSWindowDelegate, NS
     func applicationWillFinishLaunching(_ notification: Notification) {
         // Prevent focus steal on launch — no Dock icon, no Cmd+Tab entry
         NSApp.setActivationPolicy(.prohibited)
+
+        // Disable macOS automatic text substitutions app-wide (issue #538).
+        // Smart-dash substitution rewrites "--" as an em-dash "—", which
+        // silently corrupts CLI flags typed into server Command/Arguments/Env
+        // fields (e.g. "--flag" → "—flag"), producing broken configs. Done
+        // before any window (and thus any NSTextView field editor) is created
+        // so every text field inherits the disabled state. See
+        // TextSubstitution.disableAutomaticTextSubstitutions.
+        TextSubstitution.disableAutomaticTextSubstitutions()
     }
 
     func applicationDidFinishLaunching(_ notification: Notification) {
diff --git a/native/macos/MCPProxy/MCPProxy/TextSubstitution.swift b/native/macos/MCPProxy/MCPProxy/TextSubstitution.swift
new file mode 100644
index 000000000..7f67e10f7
--- /dev/null
+++ b/native/macos/MCPProxy/MCPProxy/TextSubstitution.swift
@@ -0,0 +1,43 @@
+// TextSubstitution.swift
+// MCPProxy
+//
+// Disables macOS automatic text substitutions (smart dashes, smart quotes,
+// text replacement, spelling autocorrection, autocapitalization) for the whole
+// app. See issue #538: smart-dash substitution rewrites "--" as an em-dash
+// "—", silently corrupting CLI flags typed into server Command/Arguments/Env
+// fields (e.g. "--flag" → "—flag"), producing broken configs.
+//
+// None of this app's text fields hold prose — they hold commands, flags,
+// paths, URLs, KEY=VALUE pairs, names, and tokens — so every automatic
+// substitution is unwanted everywhere. SwiftUI exposes no modifier for smart
+// dashes (`.autocorrectionDisabled()` does not affect dash substitution), so
+// the reliable lever is the AppKit defaults the field editor (NSTextView)
+// reads at creation time.
+
+import AppKit
+
+enum TextSubstitution {
+    /// The automatic-substitution default keys NSTextView consults when a field
+    /// editor is created. Writing them to the standard (application) defaults
+    /// domain overrides the user's system-wide NSGlobalDomain setting for this
+    /// app only.
+    static let substitutionDefaultsKeys = [
+        "NSAutomaticDashSubstitutionEnabled",
+        "NSAutomaticQuoteSubstitutionEnabled",
+        "NSAutomaticTextReplacementEnabled",
+        "NSAutomaticSpellingCorrectionEnabled",
+        "NSAutomaticCapitalizationEnabled",
+    ]
+
+    /// Disable all automatic text substitutions app-wide. MUST be called before
+    /// any window / text field editor is created (e.g. from
+    /// `applicationWillFinishLaunching`) so every NSTextView inherits the
+    /// disabled state.
+    static func disableAutomaticTextSubstitutions(
+        defaults: UserDefaults = .standard
+    ) {
+        for key in substitutionDefaultsKeys {
+            defaults.set(false, forKey: key)
+        }
+    }
+}
diff --git a/native/macos/MCPProxy/MCPProxyTests/ModelsTests.swift b/native/macos/MCPProxy/MCPProxyTests/ModelsTests.swift
index 44e7b83ae..6510c9fee 100644
--- a/native/macos/MCPProxy/MCPProxyTests/ModelsTests.swift
+++ b/native/macos/MCPProxy/MCPProxyTests/ModelsTests.swift
@@ -643,7 +643,7 @@ final class ModelsTests: XCTestCase {
         XCTAssertEqual(stats.tokenMetrics?.totalServerToolListSize, 120000)
         XCTAssertEqual(stats.tokenMetrics?.averageQueryResultSize, 5000)
         XCTAssertEqual(stats.tokenMetrics?.savedTokens, 115000)
-        XCTAssertEqual(stats.tokenMetrics?.savedTokensPercentage, 95.83, accuracy: 0.01)
+        XCTAssertEqual(try XCTUnwrap(stats.tokenMetrics?.savedTokensPercentage), 95.83, accuracy: 0.01)
         XCTAssertEqual(stats.tokenMetrics?.perServerToolListSizes?["github"], 80000)
         XCTAssertEqual(stats.tokenMetrics?.perServerToolListSizes?["gitlab"], 40000)
     }
diff --git a/native/macos/MCPProxy/MCPProxyTests/TextSubstitutionTests.swift b/native/macos/MCPProxy/MCPProxyTests/TextSubstitutionTests.swift
new file mode 100644
index 000000000..6beca8be7
--- /dev/null
+++ b/native/macos/MCPProxy/MCPProxyTests/TextSubstitutionTests.swift
@@ -0,0 +1,72 @@
+import XCTest
+import AppKit
+@testable import MCPProxy
+
+/// Regression tests for issue #538 — "Editor autocorrect creates broken
+/// configs". macOS smart-dash substitution turns "--" into "—" in the tray's
+/// Add Server / config text fields. `TextSubstitution.disableAutomatic...`
+/// must turn every automatic substitution off so a freshly created NSTextView
+/// field editor (the type SwiftUI TextField/TextEditor use on macOS) has dash
+/// substitution disabled.
+final class TextSubstitutionTests: XCTestCase {
+
+    /// After calling the helper, the substitution defaults are all false.
+    @MainActor
+    func test_disableAutomaticTextSubstitutions_setsAllDefaultsFalse() {
+        let suiteName = "TextSubstitutionTests.\(UUID().uuidString)"
+        let defaults = UserDefaults(suiteName: suiteName)!
+        defer { defaults.removePersistentDomain(forName: suiteName) }
+
+        // Pre-condition: simulate a machine where the user has smart dashes ON.
+        for key in TextSubstitution.substitutionDefaultsKeys {
+            defaults.set(true, forKey: key)
+        }
+
+        TextSubstitution.disableAutomaticTextSubstitutions(defaults: defaults)
+
+        for key in TextSubstitution.substitutionDefaultsKeys {
+            XCTAssertFalse(
+                defaults.bool(forKey: key),
+                "\(key) must be disabled after disableAutomaticTextSubstitutions() (issue #538)"
+            )
+        }
+    }
+
+    /// The dash-substitution key (the one that produces the em-dash bug) is
+    /// among the keys the helper disables.
+    func test_substitutionKeys_includeDashSubstitution() {
+        XCTAssertTrue(
+            TextSubstitution.substitutionDefaultsKeys.contains("NSAutomaticDashSubstitutionEnabled"),
+            "Dash substitution is the key that causes '--' → '—' (issue #538)"
+        )
+    }
+
+    /// End-to-end mechanism check: with the substitution defaults written to
+    /// the STANDARD domain (what the app does at launch) BEFORE the field
+    /// editor exists, a freshly created NSTextView reports dash substitution
+    /// disabled. This is the property AppKit consults before rewriting "--".
+    @MainActor
+    func test_freshNSTextView_hasDashSubstitutionDisabled_afterHelper() {
+        // Capture and restore the real standard-domain values so the test is
+        // side-effect free regardless of the host machine's settings.
+        let keys = TextSubstitution.substitutionDefaultsKeys
+        let saved = keys.map { ($0, UserDefaults.standard.object(forKey: $0)) }
+        defer {
+            for (key, value) in saved {
+                if let value { UserDefaults.standard.set(value, forKey: key) }
+                else { UserDefaults.standard.removeObject(forKey: key) }
+            }
+        }
+
+        // Model the buggy condition, then apply the fix to the standard domain.
+        UserDefaults.standard.set(true, forKey: "NSAutomaticDashSubstitutionEnabled")
+        TextSubstitution.disableAutomaticTextSubstitutions()
+
+        // A field editor created now must inherit the disabled state.
+        let textView = NSTextView(frame: .zero)
+        XCTAssertFalse(
+            textView.isAutomaticDashSubstitutionEnabled,
+            "A fresh NSTextView must have smart-dash substitution OFF so '--' is preserved (issue #538)"
+        )
+    }
+}