diff --git a/.repository-projection.json b/.repository-projection.json
index 31f2a4c..ac02063 100644
--- a/.repository-projection.json
+++ b/.repository-projection.json
@@ -3,11 +3,11 @@
   "projection": "endpoint",
   "projectionSchemaVersion": 1,
   "sourceRepository": "dx-corp/mono",
-  "sourceSha": "56d36fc674bad5b86ba169de0e49905f44c8dcc2",
+  "sourceSha": "b6466c724d17bf35841cefb93c8babd3e3153a70",
   "destinationRepository": "dx-corp/endpoint",
-  "priorProjectedBase": "c0338dbec4311ab2e8f0f455986aa5ec54257f50",
+  "priorProjectedBase": "1e0bf9b19a9df23c7c563ea581576dea99d4ec08",
   "definitionDigest": "8068fb5528eff3a9256419584bb34a9722ea322c288ee4cfda088ff93fb60ec6",
   "toolDigest": "898e8657d9153a2a51d7c283bf83bb3350b5d1e6",
-  "contentDigest": "379aa13d48e400a1ca4bca32d082610dc9d5c6c89694a5670bacd39d12232dde",
+  "contentDigest": "f841d481955629cc52b5488664f2354cdc6fe144e024b5b867e2a0748e71d38f",
   "publicationEligible": true
 }
diff --git a/macos/Sources/MerlinMacOS/CLI.swift b/macos/Sources/MerlinMacOS/CLI.swift
index cc626c5..7a57525 100644
--- a/macos/Sources/MerlinMacOS/CLI.swift
+++ b/macos/Sources/MerlinMacOS/CLI.swift
@@ -16,7 +16,8 @@ struct Merlin: ParsableCommand {
     static let configuration = CommandConfiguration(
         commandName: "merlin-macos",
         abstract: "Deixic Endpoint: endpoint telemetry and policy enforcement for macOS",
-        subcommands: [RunCommand.self, CheckCommand.self, PostureCommand.self, GenHashCommand.self]
+        subcommands: [RunCommand.self, CheckCommand.self, PostureCommand.self, GenHashCommand.self,
+                      MCPHookCommand.self]
     )
 }
 
diff --git a/macos/Sources/MerlinMacOS/MCPHook.swift b/macos/Sources/MerlinMacOS/MCPHook.swift
new file mode 100644
index 0000000..9edaaf6
--- /dev/null
+++ b/macos/Sources/MerlinMacOS/MCPHook.swift
@@ -0,0 +1,177 @@
+// Managed MCP tool-call hook for Cursor, Claude Code, and Codex.
+// The hook reads only server and tool names. Arguments never leave the client.
+import ArgumentParser
+import Darwin
+import Foundation
+
+private let mcpHookPolicyPath = "/Library/Application Support/Merlin/mcp-hook-policy.json"
+private let mcpHookMaximumBytes = 64 * 1024
+
+private enum MCPHookClient: String, ExpressibleByArgument {
+    case cursor, claude, codex
+}
+
+struct MCPHookCommand: ParsableCommand {
+    static let configuration = CommandConfiguration(
+        commandName: "mcp-hook",
+        abstract: "Apply an administrator-installed MCP server allowlist to a client tool call."
+    )
+
+    @Option(help: "Client emitting the hook: cursor, claude, or codex.")
+    private var client: MCPHookClient
+
+    func run() throws {
+        let output: [String: Any]
+        do {
+            let input = try readMCPHookInput()
+            let policy = try readMCPHookPolicy(path: mcpHookPolicyPath)
+            let verdict = try policy.verdict(client: client.rawValue, input: input)
+            output = mcpHookOutput(client: client.rawValue, verdict: verdict)
+        } catch {
+            // Endpoint enforcement points allow on internal errors. Client
+            // hooks must never turn a missing or malformed policy into a deny.
+            fputs("deixic endpoint mcp hook: \(error)\n", stderr)
+            output = mcpHookOutput(client: client.rawValue, verdict: .allow)
+        }
+        let data = try JSONSerialization.data(withJSONObject: output, options: [.sortedKeys])
+        FileHandle.standardOutput.write(data)
+        FileHandle.standardOutput.write(Data([0x0a]))
+    }
+}
+
+private enum MCPHookError: Error {
+    case invalidInput, invalidPolicy, unsafePolicyFile, oversizedInput
+}
+
+enum MCPHookVerdict: Equatable {
+    case allow
+    case deny(String)
+}
+
+struct MCPHookPolicy {
+    let enforced: Bool
+    let approvedServers: [String: Set<String>]
+    let approvedName: String?
+    let approvedURL: String?
+
+    static func parse(_ data: Data) throws -> MCPHookPolicy {
+        guard data.count <= mcpHookMaximumBytes,
+              let root = try JSONSerialization.jsonObject(with: data) as? [String: Any],
+              Set(root.keys).isSubset(of: ["schema_version", "mode", "approved_servers", "approved_alternative"]),
+              root["schema_version"] as? Int == 1,
+              let mode = root["mode"] as? String, ["audit", "enforce"].contains(mode),
+              let entries = root["approved_servers"] as? [[String: Any]], entries.count <= 128 else {
+            throw MCPHookError.invalidPolicy
+        }
+        var approved: [String: Set<String>] = [:]
+        for entry in entries {
+            guard Set(entry.keys) == Set(["client", "server"]),
+                  let client = entry["client"] as? String,
+                  ["cursor", "claude", "codex"].contains(client),
+                  let server = entry["server"] as? String,
+                  validMCPHookName(server) else {
+                throw MCPHookError.invalidPolicy
+            }
+            approved[client, default: []].insert(server)
+        }
+        var name: String?
+        var url: String?
+        if let alternative = root["approved_alternative"] {
+            guard let value = alternative as? [String: String],
+                  Set(value.keys) == Set(["name", "url"]),
+                  let candidateName = value["name"],
+                  !candidateName.isEmpty, candidateName.utf8.count <= 80,
+                  candidateName == candidateName.trimmingCharacters(in: .whitespacesAndNewlines),
+                  !candidateName.unicodeScalars.contains(where: { CharacterSet.controlCharacters.contains($0) }),
+                  let candidateURL = value["url"], candidateURL.utf8.count <= 2048,
+                  let parsed = URLComponents(string: candidateURL), parsed.scheme == "https",
+                  parsed.host != nil, parsed.user == nil, parsed.password == nil,
+                  parsed.query == nil, parsed.fragment == nil else {
+                throw MCPHookError.invalidPolicy
+            }
+            name = candidateName
+            url = candidateURL
+        }
+        return MCPHookPolicy(enforced: mode == "enforce", approvedServers: approved,
+                             approvedName: name, approvedURL: url)
+    }
+
+    func verdict(client: String, input: [String: Any]) throws -> MCPHookVerdict {
+        guard let server = mcpHookServer(client: client, input: input) else {
+            // Unrecognized hook events and malformed names do not become an
+            // implicit block. Managed client matchers limit calls to MCP.
+            return .allow
+        }
+        guard enforced, !approvedServers[client, default: []].contains(server) else {
+            return .allow
+        }
+        var message = "Deixic Endpoint blocked an unapproved MCP server (\(server))."
+        if let approvedName, let approvedURL {
+            message += " Use the administrator-approved tool \(approvedName): \(approvedURL)"
+        } else {
+            message += " Contact your administrator for an approved tool."
+        }
+        return .deny(message)
+    }
+}
+
+private func validMCPHookName(_ value: String) -> Bool {
+    !value.isEmpty && value.utf8.count <= 128 && value.unicodeScalars.allSatisfy {
+        CharacterSet(charactersIn: "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789._-").contains($0)
+    }
+}
+
+private func mcpHookServer(client: String, input: [String: Any]) -> String? {
+    if client == "cursor" {
+        guard let server = input["mcp_server_name"] as? String, validMCPHookName(server),
+              let tool = input["tool_name"] as? String, validMCPHookName(tool) else { return nil }
+        return server
+    }
+    guard let tool = input["tool_name"] as? String, tool.hasPrefix("mcp__") else { return nil }
+    let parts = tool.dropFirst(5).components(separatedBy: "__")
+    guard parts.count == 2, validMCPHookName(parts[0]), validMCPHookName(parts[1]) else { return nil }
+    return parts[0]
+}
+
+func mcpHookOutput(client: String, verdict: MCPHookVerdict) -> [String: Any] {
+    switch (client, verdict) {
+    case ("cursor", .allow):
+        return ["permission": "allow"]
+    case ("cursor", .deny(let message)):
+        return ["permission": "deny", "user_message": message, "agent_message": message]
+    case (_, .allow):
+        return [:]
+    case (_, .deny(let message)):
+        return ["hookSpecificOutput": [
+            "hookEventName": "PreToolUse", "permissionDecision": "deny",
+            "permissionDecisionReason": message,
+        ]]
+    }
+}
+
+private func readMCPHookInput() throws -> [String: Any] {
+    let data = FileHandle.standardInput.readData(ofLength: mcpHookMaximumBytes + 1)
+    guard data.count <= mcpHookMaximumBytes else { throw MCPHookError.oversizedInput }
+    guard let input = try JSONSerialization.jsonObject(with: data) as? [String: Any] else {
+        throw MCPHookError.invalidInput
+    }
+    return input
+}
+
+func readMCPHookPolicy(path: String) throws -> MCPHookPolicy {
+    let fd = open(path, O_RDONLY | O_NOFOLLOW | O_CLOEXEC | O_NONBLOCK)
+    guard fd >= 0 else { throw MCPHookError.unsafePolicyFile }
+    defer { close(fd) }
+    var metadata = stat()
+    guard fstat(fd, &metadata) == 0,
+          metadata.st_mode & S_IFMT == S_IFREG,
+          metadata.st_uid == 0,
+          metadata.st_mode & 0o022 == 0,
+          metadata.st_size >= 0,
+          metadata.st_size <= mcpHookMaximumBytes else {
+        throw MCPHookError.unsafePolicyFile
+    }
+    let data = FileHandle(fileDescriptor: fd, closeOnDealloc: false).readData(ofLength: mcpHookMaximumBytes + 1)
+    guard data.count <= mcpHookMaximumBytes else { throw MCPHookError.unsafePolicyFile }
+    return try MCPHookPolicy.parse(data)
+}
diff --git a/macos/Tests/MerlinMacOSTests/MCPHookTests.swift b/macos/Tests/MerlinMacOSTests/MCPHookTests.swift
new file mode 100644
index 0000000..88eb00b
--- /dev/null
+++ b/macos/Tests/MerlinMacOSTests/MCPHookTests.swift
@@ -0,0 +1,82 @@
+import Foundation
+import Testing
+@testable import MerlinMacOS
+
+@Suite("managed MCP hooks")
+struct MCPHookTests {
+    private let policy = """
+    {"schema_version":1,"mode":"enforce",
+     "approved_servers":[{"client":"cursor","server":"deixic-gateway"},
+                         {"client":"claude","server":"deixic-gateway"},
+                         {"client":"codex","server":"deixic-gateway"}],
+     "approved_alternative":{"name":"Approved agent","url":"https://tools.example.com/agent"}}
+    """
+
+    @Test("blocks unapproved Cursor, Claude, and Codex MCP calls")
+    func blockedClients() throws {
+        let parsed = try MCPHookPolicy.parse(Data(policy.utf8))
+        let inputs: [(String, [String: Any])] = [
+            ("cursor", ["mcp_server_name": "shadow", "tool_name": "search", "tool_input": ["secret": "do not emit"]]),
+            ("claude", ["tool_name": "mcp__shadow__search", "tool_input": ["secret": "do not emit"]]),
+            ("codex", ["tool_name": "mcp__shadow__search", "tool_input": ["secret": "do not emit"]]),
+        ]
+        for (client, input) in inputs {
+            let verdict = try parsed.verdict(client: client, input: input)
+            guard case .deny(let reason) = verdict else {
+                Issue.record("\(client) did not deny an unapproved server")
+                continue
+            }
+            #expect(reason.contains("Approved agent"))
+            #expect(reason.contains("https://tools.example.com/agent"))
+            let output = mcpHookOutput(client: client, verdict: verdict)
+            let encoded = String(data: try JSONSerialization.data(withJSONObject: output), encoding: .utf8) ?? ""
+            #expect(!encoded.contains("do not emit"))
+            if client == "cursor" {
+                #expect(output["permission"] as? String == "deny")
+            } else {
+                let specific = output["hookSpecificOutput"] as? [String: String]
+                #expect(specific?["permissionDecision"] == "deny")
+            }
+        }
+    }
+
+    @Test("allows approved servers and unrelated tools")
+    func approvedAndUnrelated() throws {
+        let parsed = try MCPHookPolicy.parse(Data(policy.utf8))
+        #expect(try parsed.verdict(client: "cursor", input: ["mcp_server_name": "deixic-gateway", "tool_name": "search"]) == .allow)
+        #expect(try parsed.verdict(client: "claude", input: ["tool_name": "mcp__deixic-gateway__search"]) == .allow)
+        #expect(try parsed.verdict(client: "codex", input: ["tool_name": "Bash"]) == .allow)
+        #expect(try parsed.verdict(client: "codex", input: ["tool_name": "mcp__plugin_my-plugin_db__search"]) != .allow)
+    }
+
+    @Test("rejects unknown policy fields, unsafe links, and oversized policy")
+    func invalidPolicy() throws {
+        for invalid in [
+            policy.replacingOccurrences(of: "\"mode\":\"enforce\"", with: "\"mode\":\"enforce\",\"surprise\":true"),
+            policy.replacingOccurrences(of: "https://tools.example.com/agent", with: "http://tools.example.com/agent"),
+            policy.replacingOccurrences(of: "\"server\":\"deixic-gateway\"", with: "\"server\":\"*\""),
+            String(repeating: "x", count: 65_537),
+        ] {
+            #expect(throws: Error.self) { try MCPHookPolicy.parse(Data(invalid.utf8)) }
+        }
+    }
+
+    @Test("audit policy never denies")
+    func audit() throws {
+        let parsed = try MCPHookPolicy.parse(Data(policy.replacingOccurrences(of: "enforce", with: "audit").utf8))
+        #expect(try parsed.verdict(client: "cursor", input: ["mcp_server_name": "shadow", "tool_name": "search"]) == .allow)
+    }
+
+    @Test("refuses user-owned and symlinked policy files")
+    func unsafeFiles() throws {
+        let directory = FileManager.default.temporaryDirectory.appendingPathComponent(UUID().uuidString)
+        try FileManager.default.createDirectory(at: directory, withIntermediateDirectories: true)
+        defer { try? FileManager.default.removeItem(at: directory) }
+        let plain = directory.appendingPathComponent("policy.json")
+        try Data(policy.utf8).write(to: plain)
+        #expect(throws: Error.self) { try readMCPHookPolicy(path: plain.path) }
+        let link = directory.appendingPathComponent("link.json")
+        try FileManager.default.createSymbolicLink(at: link, withDestinationURL: plain)
+        #expect(throws: Error.self) { try readMCPHookPolicy(path: link.path) }
+    }
+}