diff --git a/CHANGELOG.md b/CHANGELOG.md
index 30677d7..7415e69 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -85,6 +85,10 @@ Cutting that release is tracked in
 
 ### Changed
 
+- Machine-readable capabilities now derive their command and engine inventories
+  exhaustively from the protocol enums, preventing new commands or engines from
+  being silently omitted.
+
 - macOS agent startup now opens browser windows without activating Headless or
   covering the user's current app. The startup presentation can be configured
   persistently or overridden for one launch.
diff --git a/apps/headless/Sources/HeadlessProtocol/Capabilities.swift b/apps/headless/Sources/HeadlessProtocol/Capabilities.swift
index 736f5ad..ada309e 100644
--- a/apps/headless/Sources/HeadlessProtocol/Capabilities.swift
+++ b/apps/headless/Sources/HeadlessProtocol/Capabilities.swift
@@ -106,7 +106,17 @@ public struct BrowserEngineCapabilities: Sendable {
         screenshotClipboard: false
     )
 
-    public static let all: [BrowserEngineCapabilities] = [.webkit, .chromium]
+    public static func profile(for engine: BrowserEngineName) -> BrowserEngineCapabilities {
+        switch engine {
+        case .webkit: webkit
+        case .chromium: chromium
+        }
+    }
+
+    /// Generated from the engine enum so adding an engine cannot silently
+    /// leave it out of the machine-readable capability document.
+    public static let all: [BrowserEngineCapabilities] =
+        BrowserEngineName.allCases.map { BrowserEngineCapabilities.profile(for: $0) }
 }
 
 public var currentBrowserEngineCapabilities: BrowserEngineCapabilities {
diff --git a/apps/headless/Tests/HeadlessProtocolTests/ProtocolTests.swift b/apps/headless/Tests/HeadlessProtocolTests/ProtocolTests.swift
index 304b5cd..7af5aaa 100644
--- a/apps/headless/Tests/HeadlessProtocolTests/ProtocolTests.swift
+++ b/apps/headless/Tests/HeadlessProtocolTests/ProtocolTests.swift
@@ -1213,7 +1213,12 @@ struct ProtocolTests {
             engines.count == BrowserEngineName.allCases.count,
             "capabilities should contain exactly one profile for every engine"
         )
-        for profile in BrowserEngineCapabilities.all {
+        try expect(
+            BrowserEngineCapabilities.all.map(\.engine) == BrowserEngineName.allCases,
+            "capability profiles should be generated in engine-enum order"
+        )
+        for engineName in BrowserEngineName.allCases {
+            let profile = BrowserEngineCapabilities.profile(for: engineName)
             guard case .object(let engine)? = engines[profile.engine.rawValue],
                   case .array(let rawSupported)? = engine["commands"],
                   case .array(let rawUnsupported)? = engine["unsupportedCommands"],
diff --git a/docs/roadmap/improvements-backlog.md b/docs/roadmap/improvements-backlog.md
index 9686050..13a1494 100644
--- a/docs/roadmap/improvements-backlog.md
+++ b/docs/roadmap/improvements-backlog.md
@@ -269,8 +269,12 @@ oversized line / local-command rejection (`MCP/main.swift:64-66`).~~ **Done:**
 `HeadlessMCPTests` drives the built stdio process through every named case and
 uses a private local socket server to verify a real browser-command round trip.
 
-**C4. Machine-accurate `capabilities`** ([#32](https://github.com/LockInTime/headless/issues/32)) — generate from `CommandName.allCases`
-and the engine matrix (see B6) so agents can trust it.
+**C4. Machine-accurate `capabilities`** ([#32](https://github.com/LockInTime/headless/issues/32)) — ~~generate from `CommandName.allCases`
+and the engine matrix (see B6) so agents can trust it.~~ **Done:** the document
+derives commands from `CommandName.allCases`, engine profiles from
+`BrowserEngineName.allCases`, and capture formats and bounds from their runtime
+definitions. The protocol suite proves both command partitions are exhaustive
+and every declared engine has exactly one profile.
 
 **C5. Harness onboarding [exists: skill content].** ([#33](https://github.com/LockInTime/headless/issues/33)) ~~Root `AGENTS.md` +
 `CLAUDE.md`; mirror the skill into `.claude/skills/` so Claude Code