diff --git a/packages/Bus/Package.swift b/packages/Bus/Package.swift
new file mode 100644
index 0000000..0e3464c
--- /dev/null
+++ b/packages/Bus/Package.swift
@@ -0,0 +1,31 @@
+// swift-tools-version:6.0
+import PackageDescription
+
+let package = Package(
+    name: "Bus",
+    platforms: [.macOS(.v13)],
+    products: [
+        .library(name: "Bus", targets: ["Bus"]),
+    ],
+    dependencies: [
+        .package(path: "../Logging"),
+    ],
+    targets: [
+        .target(
+            name: "Bus",
+            dependencies: [
+                .product(name: "JarvisLogging", package: "Logging"),
+            ],
+            swiftSettings: [.swiftLanguageMode(.v6)],
+            linkerSettings: [
+                .linkedFramework("WebKit"),
+            ]
+        ),
+        .testTarget(
+            name: "BusTests",
+            dependencies: ["Bus"],
+            resources: [.process("Fixtures")],
+            swiftSettings: [.swiftLanguageMode(.v6)]
+        ),
+    ]
+)
diff --git a/packages/Bus/Sources/Bus/BusError.swift b/packages/Bus/Sources/Bus/BusError.swift
new file mode 100644
index 0000000..358c56d
--- /dev/null
+++ b/packages/Bus/Sources/Bus/BusError.swift
@@ -0,0 +1,17 @@
+import Foundation
+
+/// Errors the bus raises across decode, send, and handshake paths.
+///
+/// Surfaces:
+/// - `.decodeFailed` — inbound JSON failed to decode into a `BusInbound`.
+/// - `.bridgeNotReady` — `send(_:)` called before the handshake armed.
+/// - `.handshakeMismatch` — `helloAck` returned a version the Swift side
+///   does not recognize; both sides recorded for alerting.
+/// - `.handshakeTimeout` — the webview failed to send `helloAck` within the
+///   2-second window defined by `HandshakeTiming.timeout`.
+public enum BusError: Error, Sendable, Equatable {
+    case decodeFailed(String)
+    case bridgeNotReady
+    case handshakeMismatch(swift: String, js: String)
+    case handshakeTimeout
+}
diff --git a/packages/Bus/Sources/Bus/BusInbound.swift b/packages/Bus/Sources/Bus/BusInbound.swift
new file mode 100644
index 0000000..c1774f3
--- /dev/null
+++ b/packages/Bus/Sources/Bus/BusInbound.swift
@@ -0,0 +1,49 @@
+import Foundation
+
+/// Messages the webview sends to the Swift side.
+///
+/// Minimal P2 scope (per RESEARCH §Deferred Ideas) — only the handshake ack
+/// and a UI-ready ping. P3+ adds user-input, tool-confirm, and other cases.
+///
+/// Hand-written `Codable` mirrors `BusOutbound` — same `type` discriminator,
+/// same exhaustive-switch drift preventer.
+public enum BusInbound: Equatable, Sendable {
+    case helloAck(version: String)
+    case uiReady
+}
+
+extension BusInbound: Codable {
+    private enum Discriminator: String, Codable {
+        case helloAck
+        case uiReady
+    }
+
+    private enum CodingKeys: String, CodingKey {
+        case type
+        case version
+    }
+
+    public init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        let tag = try container.decode(Discriminator.self, forKey: .type)
+        switch tag {
+        case .helloAck:
+            let version = try container.decode(String.self, forKey: .version)
+            self = .helloAck(version: version)
+        case .uiReady:
+            self = .uiReady
+        }
+        // NO default — adding a case without a matching arm is a compile error.
+    }
+
+    public func encode(to encoder: Encoder) throws {
+        var container = encoder.container(keyedBy: CodingKeys.self)
+        switch self {
+        case .helloAck(let version):
+            try container.encode(Discriminator.helloAck, forKey: .type)
+            try container.encode(version, forKey: .version)
+        case .uiReady:
+            try container.encode(Discriminator.uiReady, forKey: .type)
+        }
+    }
+}
diff --git a/packages/Bus/Sources/Bus/BusOutbound.swift b/packages/Bus/Sources/Bus/BusOutbound.swift
new file mode 100644
index 0000000..47c2f96
--- /dev/null
+++ b/packages/Bus/Sources/Bus/BusOutbound.swift
@@ -0,0 +1,151 @@
+import Foundation
+
+/// Messages the Swift side sends to the webview.
+///
+/// IMPORTANT: `Codable` conformance is declared in an **extension** (below),
+/// not on the enum declaration. This is load-bearing: Swift's synthesized
+/// `Codable` (SE-0295) would encode associated-value enums as
+/// `{"hudState": {"_0": "idle"}}`, which does NOT match the TS mirror's
+/// `{"type": "hudState", "state": "idle"}` shape. Hand-writing the
+/// `init(from:)` and `encode(to:)` methods with exhaustive switches is the
+/// schema-drift preventer at the language level — adding a case without
+/// updating both methods is a compile error.
+public enum BusOutbound: Equatable, Sendable {
+    case hello(version: String)
+    case hudState(HudState)
+    case tokenDelta(text: String)
+    case audioLevel(rms: Float)
+    case toolCallStart(id: UUID, name: String, argsPreview: String)
+    case toolCallEnd(id: UUID, ok: Bool, previewOrError: String)
+    case turnStarted(id: UUID)
+    case turnEnded(id: UUID, terminator: TurnTerminator)
+}
+
+extension BusOutbound: Codable {
+    /// Discriminator tag used as the `type` field in the wire format.
+    /// Declaring this as a `String`-raw-value enum means an unknown tag in
+    /// inbound JSON surfaces as `DecodingError.dataCorrupted` from the
+    /// `try c.decode(Discriminator.self, forKey: .type)` call below.
+    private enum Discriminator: String, Codable {
+        case hello
+        case hudState
+        case tokenDelta
+        case audioLevel
+        case toolCallStart
+        case toolCallEnd
+        case turnStarted
+        case turnEnded
+    }
+
+    /// All `CodingKeys` across every case. Swift's keyed container does not
+    /// care about unused keys per-case; the per-case `encode(to:)` only
+    /// writes the fields relevant to the variant.
+    private enum CodingKeys: String, CodingKey {
+        case type
+        case version
+        case state
+        case text
+        case rms
+        case id
+        case name
+        case argsPreview
+        case ok
+        case previewOrError
+        case terminator
+    }
+
+    public init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        let tag = try container.decode(Discriminator.self, forKey: .type)
+        switch tag {
+        case .hello:
+            let version = try container.decode(String.self, forKey: .version)
+            self = .hello(version: version)
+        case .hudState:
+            let state = try container.decode(HudState.self, forKey: .state)
+            self = .hudState(state)
+        case .tokenDelta:
+            let text = try container.decode(String.self, forKey: .text)
+            self = .tokenDelta(text: text)
+        case .audioLevel:
+            let rms = try container.decode(Float.self, forKey: .rms)
+            self = .audioLevel(rms: rms)
+        case .toolCallStart:
+            let id = try Self.decodeUUID(container: container, forKey: .id)
+            let name = try container.decode(String.self, forKey: .name)
+            let argsPreview = try container.decode(String.self, forKey: .argsPreview)
+            self = .toolCallStart(id: id, name: name, argsPreview: argsPreview)
+        case .toolCallEnd:
+            let id = try Self.decodeUUID(container: container, forKey: .id)
+            let ok = try container.decode(Bool.self, forKey: .ok)
+            let previewOrError = try container.decode(String.self, forKey: .previewOrError)
+            self = .toolCallEnd(id: id, ok: ok, previewOrError: previewOrError)
+        case .turnStarted:
+            let id = try Self.decodeUUID(container: container, forKey: .id)
+            self = .turnStarted(id: id)
+        case .turnEnded:
+            let id = try Self.decodeUUID(container: container, forKey: .id)
+            let terminator = try container.decode(TurnTerminator.self, forKey: .terminator)
+            self = .turnEnded(id: id, terminator: terminator)
+        }
+        // NO default branch — adding a Discriminator case without also adding
+        // a matching switch arm here is a compile error. That is the entire
+        // reason this Codable is hand-written.
+    }
+
+    public func encode(to encoder: Encoder) throws {
+        var container = encoder.container(keyedBy: CodingKeys.self)
+        switch self {
+        case .hello(let version):
+            try container.encode(Discriminator.hello, forKey: .type)
+            try container.encode(version, forKey: .version)
+        case .hudState(let state):
+            try container.encode(Discriminator.hudState, forKey: .type)
+            try container.encode(state, forKey: .state)
+        case .tokenDelta(let text):
+            try container.encode(Discriminator.tokenDelta, forKey: .type)
+            try container.encode(text, forKey: .text)
+        case .audioLevel(let rms):
+            try container.encode(Discriminator.audioLevel, forKey: .type)
+            try container.encode(rms, forKey: .rms)
+        case .toolCallStart(let id, let name, let argsPreview):
+            try container.encode(Discriminator.toolCallStart, forKey: .type)
+            try container.encode(id.uuidString.lowercased(), forKey: .id)
+            try container.encode(name, forKey: .name)
+            try container.encode(argsPreview, forKey: .argsPreview)
+        case .toolCallEnd(let id, let ok, let previewOrError):
+            try container.encode(Discriminator.toolCallEnd, forKey: .type)
+            try container.encode(id.uuidString.lowercased(), forKey: .id)
+            try container.encode(ok, forKey: .ok)
+            try container.encode(previewOrError, forKey: .previewOrError)
+        case .turnStarted(let id):
+            try container.encode(Discriminator.turnStarted, forKey: .type)
+            try container.encode(id.uuidString.lowercased(), forKey: .id)
+        case .turnEnded(let id, let terminator):
+            try container.encode(Discriminator.turnEnded, forKey: .type)
+            try container.encode(id.uuidString.lowercased(), forKey: .id)
+            try container.encode(terminator, forKey: .terminator)
+        }
+        // Exhaustive at encode site — adding a case without encoding it is a
+        // compile error, mirroring the init(from:) drift preventer.
+    }
+
+    /// `UUID` is encoded as a lowercase string (Pitfall 6). Decoder accepts
+    /// either case since `UUID(uuidString:)` is case-insensitive, but we
+    /// route the parse through an explicit helper so bad payloads surface
+    /// as structured `DecodingError`s instead of silent `fatalError`s.
+    private static func decodeUUID(
+        container: KeyedDecodingContainer<CodingKeys>,
+        forKey key: CodingKeys
+    ) throws -> UUID {
+        let raw = try container.decode(String.self, forKey: key)
+        guard let uuid = UUID(uuidString: raw) else {
+            throw DecodingError.dataCorruptedError(
+                forKey: key,
+                in: container,
+                debugDescription: "invalid UUID string: \(raw)"
+            )
+        }
+        return uuid
+    }
+}
diff --git a/packages/Bus/Sources/Bus/BusReply.swift b/packages/Bus/Sources/Bus/BusReply.swift
new file mode 100644
index 0000000..818cf79
--- /dev/null
+++ b/packages/Bus/Sources/Bus/BusReply.swift
@@ -0,0 +1,49 @@
+import Foundation
+
+/// Reply shape returned via `WKScriptMessageHandlerWithReply`'s `replyHandler`.
+///
+/// WebKit requires replies to be plist-serializable (`NSNumber`, `NSString`,
+/// `NSArray`, `NSDictionary`, `NSNull`, nested) — arbitrary `Codable` types
+/// are rejected at runtime. `asPlist()` produces the accepted dictionary form.
+///
+/// JS semantics (per HUD-03):
+/// - `(value, nil)` → JS-side `Promise.resolve(value)`
+/// - `(nil, errorString)` → JS-side `Promise.reject(new Error(errorString))`
+public struct BusReply: Sendable, Equatable {
+    public let ok: Bool
+    /// JSON-encoded payload as a string, or `nil` for success with no body.
+    public let value: String?
+    public let error: String?
+
+    public init(ok: Bool, value: String? = nil, error: String? = nil) {
+        self.ok = ok
+        self.value = value
+        self.error = error
+    }
+
+    /// Canonical empty-success reply.
+    public static let success = BusReply(ok: true)
+
+    /// Canonical failure reply. `message` is what the JS `Error.message`
+    /// will surface after the promise rejects.
+    public static func failure(_ message: String) -> BusReply {
+        BusReply(ok: false, error: message)
+    }
+
+    /// Plist-serializable dictionary form accepted by
+    /// `WKScriptMessageHandlerWithReply.replyHandler`. Strings and booleans
+    /// bridge to `NSString` / `NSNumber`; nil slots become `NSNull`.
+    public func asPlist() -> [String: Any] {
+        if ok {
+            return [
+                "ok": true,
+                "value": (value as Any?) ?? NSNull(),
+            ]
+        } else {
+            return [
+                "ok": false,
+                "error": error ?? "unknown",
+            ]
+        }
+    }
+}
diff --git a/packages/Bus/Sources/Bus/Protocol.swift b/packages/Bus/Sources/Bus/Protocol.swift
new file mode 100644
index 0000000..cb34ad8
--- /dev/null
+++ b/packages/Bus/Sources/Bus/Protocol.swift
@@ -0,0 +1,61 @@
+import Foundation
+
+/// The wire-format version for the Swift ↔ JS bus. Bumping this rejects any
+/// webview bundle that was built against a different constant (HUD-05).
+///
+/// Versioning policy (per RESEARCH §HUD-05):
+/// - `MAJOR` — structural schema change (field renamed, removed, re-typed).
+/// - `MINOR` — additive case or field (webview still decodes older shape).
+/// - `PATCH` — bugfix / fixture correction with no behavioural change.
+///
+/// Strict equality is enforced at runtime by the handshake. Additive-only
+/// relaxation (match on `MAJOR` alone) is deferred to a later phase.
+public let BUS_PROTOCOL_VERSION: String = "2.0.0"
+
+/// UI states the HUD can occupy. Encoded as its `rawValue` inside
+/// `BusOutbound.hudState(_:)` — see `BusOutbound.encode(to:)`.
+public enum HudState: String, Codable, Sendable, CaseIterable {
+    case idle
+    case listening
+    case thinking
+    case speaking
+    case awaitingConfirmation
+    case reconfiguring
+    case booting
+}
+
+/// Terminal reason for a conversation turn. Encoded as its `rawValue` inside
+/// `BusOutbound.turnEnded(id:terminator:)`.
+public enum TurnTerminator: String, Codable, Sendable, CaseIterable {
+    case completed
+    case cancelled
+    case errored
+    case superseded
+}
+
+/// Shared JSON coder configuration. Single source of truth for the wire
+/// format so `WebviewBridge`, `CodableRoundTripTests`, and any downstream
+/// consumers all agree on encoding.
+///
+/// Notes:
+/// - `outputFormatting` is intentionally empty — the wire form is minimal,
+///   not pretty-printed or key-sorted. Round-trip tests normalize via
+///   `JSONSerialization` to tolerate key-order differences.
+/// - `UUID` values are encoded as *lowercase* strings at each call site
+///   inside `BusOutbound.encode(to:)` because `JSONEncoder` encodes `UUID`
+///   as uppercase by default, which the TS mirror does not emit
+///   (Pitfall 6 in RESEARCH §pitfalls).
+public enum BusCoder {
+    public static func makeEncoder() -> JSONEncoder {
+        let encoder = JSONEncoder()
+        encoder.outputFormatting = []
+        encoder.dateEncodingStrategy = .iso8601
+        return encoder
+    }
+
+    public static func makeDecoder() -> JSONDecoder {
+        let decoder = JSONDecoder()
+        decoder.dateDecodingStrategy = .iso8601
+        return decoder
+    }
+}
diff --git a/packages/Bus/Tests/BusTests/CodableRoundTripTests.swift b/packages/Bus/Tests/BusTests/CodableRoundTripTests.swift
new file mode 100644
index 0000000..6f5dd2b
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/CodableRoundTripTests.swift
@@ -0,0 +1,258 @@
+import XCTest
+@testable import Bus
+
+/// Round-trip parity: decode fixture → re-encode → normalize → compare.
+///
+/// Plan 02 (TS mirror) reads the same fixture files and Plan 04's parity
+/// script asserts the two fixture directories stay in sync. Together with
+/// the exhaustive switches in `BusOutbound` / `BusInbound`, these tests are
+/// the build-time line of defense against schema drift.
+final class CodableRoundTripTests: XCTestCase {
+    // MARK: - Helpers
+
+    private func loadFixture(_ name: String) throws -> Data {
+        guard let url = Bundle.module.url(forResource: name, withExtension: "json") else {
+            XCTFail("missing fixture: \(name).json")
+            throw CocoaError(.fileNoSuchFile)
+        }
+        return try Data(contentsOf: url)
+    }
+
+    /// Normalize JSON so key-order differences don't fail comparisons.
+    /// `NSDictionary.isEqual` is order-independent, so we decode both sides
+    /// to a generic JSON object graph first.
+    private func normalize(_ data: Data) throws -> NSObject {
+        let value = try JSONSerialization.jsonObject(with: data, options: [])
+        return value as! NSObject
+    }
+
+    private func assertOutboundRoundTrips(
+        fixture: String,
+        expect: BusOutbound,
+        file: StaticString = #filePath,
+        line: UInt = #line
+    ) throws {
+        let data = try loadFixture(fixture)
+        let decoded = try BusCoder.makeDecoder().decode(BusOutbound.self, from: data)
+        XCTAssertEqual(decoded, expect, "decoded value mismatch for \(fixture)", file: file, line: line)
+
+        let reencoded = try BusCoder.makeEncoder().encode(decoded)
+        let originalNorm = try normalize(data)
+        let reencodedNorm = try normalize(reencoded)
+        XCTAssertEqual(
+            originalNorm, reencodedNorm,
+            "round-trip mismatch for \(fixture).\nexpected: \(String(data: data, encoding: .utf8) ?? "<?>")\ngot:      \(String(data: reencoded, encoding: .utf8) ?? "<?>")",
+            file: file, line: line
+        )
+    }
+
+    private func assertInboundRoundTrips(
+        fixture: String,
+        expect: BusInbound,
+        file: StaticString = #filePath,
+        line: UInt = #line
+    ) throws {
+        let data = try loadFixture(fixture)
+        let decoded = try BusCoder.makeDecoder().decode(BusInbound.self, from: data)
+        XCTAssertEqual(decoded, expect, "decoded value mismatch for \(fixture)", file: file, line: line)
+
+        let reencoded = try BusCoder.makeEncoder().encode(decoded)
+        let originalNorm = try normalize(data)
+        let reencodedNorm = try normalize(reencoded)
+        XCTAssertEqual(
+            originalNorm, reencodedNorm,
+            "round-trip mismatch for \(fixture)",
+            file: file, line: line
+        )
+    }
+
+    // MARK: - Outbound round-trips (8 variants; HudState expanded × 7)
+
+    func test_roundTrip_hello() throws {
+        try assertOutboundRoundTrips(
+            fixture: "hello",
+            expect: .hello(version: "2.0.0")
+        )
+    }
+
+    func test_roundTrip_hudState_idle() throws {
+        try assertOutboundRoundTrips(fixture: "hudState.idle", expect: .hudState(.idle))
+    }
+
+    func test_roundTrip_hudState_listening() throws {
+        try assertOutboundRoundTrips(fixture: "hudState.listening", expect: .hudState(.listening))
+    }
+
+    func test_roundTrip_hudState_thinking() throws {
+        try assertOutboundRoundTrips(fixture: "hudState.thinking", expect: .hudState(.thinking))
+    }
+
+    func test_roundTrip_hudState_speaking() throws {
+        try assertOutboundRoundTrips(fixture: "hudState.speaking", expect: .hudState(.speaking))
+    }
+
+    func test_roundTrip_hudState_awaitingConfirmation() throws {
+        try assertOutboundRoundTrips(
+            fixture: "hudState.awaitingConfirmation",
+            expect: .hudState(.awaitingConfirmation)
+        )
+    }
+
+    func test_roundTrip_hudState_reconfiguring() throws {
+        try assertOutboundRoundTrips(
+            fixture: "hudState.reconfiguring",
+            expect: .hudState(.reconfiguring)
+        )
+    }
+
+    func test_roundTrip_hudState_booting() throws {
+        try assertOutboundRoundTrips(fixture: "hudState.booting", expect: .hudState(.booting))
+    }
+
+    func test_roundTrip_tokenDelta() throws {
+        try assertOutboundRoundTrips(
+            fixture: "tokenDelta",
+            expect: .tokenDelta(text: "Hello, world!")
+        )
+    }
+
+    func test_roundTrip_audioLevel() throws {
+        try assertOutboundRoundTrips(
+            fixture: "audioLevel",
+            expect: .audioLevel(rms: 0.42)
+        )
+    }
+
+    func test_roundTrip_toolCallStart() throws {
+        let id = UUID(uuidString: "550e8400-e29b-41d4-a716-446655440000")!
+        try assertOutboundRoundTrips(
+            fixture: "toolCallStart",
+            expect: .toolCallStart(id: id, name: "get_time", argsPreview: "{}")
+        )
+    }
+
+    func test_roundTrip_toolCallEnd() throws {
+        let id = UUID(uuidString: "550e8400-e29b-41d4-a716-446655440000")!
+        try assertOutboundRoundTrips(
+            fixture: "toolCallEnd",
+            expect: .toolCallEnd(id: id, ok: true, previewOrError: "\"2026-04-23T16:00:00Z\"")
+        )
+    }
+
+    func test_roundTrip_turnStarted() throws {
+        let id = UUID(uuidString: "6ba7b810-9dad-11d1-80b4-00c04fd430c8")!
+        try assertOutboundRoundTrips(fixture: "turnStarted", expect: .turnStarted(id: id))
+    }
+
+    func test_roundTrip_turnEnded() throws {
+        let id = UUID(uuidString: "6ba7b810-9dad-11d1-80b4-00c04fd430c8")!
+        try assertOutboundRoundTrips(
+            fixture: "turnEnded",
+            expect: .turnEnded(id: id, terminator: .completed)
+        )
+    }
+
+    // MARK: - Inbound round-trips
+
+    func test_roundTrip_helloAck() throws {
+        try assertInboundRoundTrips(
+            fixture: "helloAck",
+            expect: .helloAck(version: "2.0.0")
+        )
+    }
+
+    // MARK: - Shape invariants (these prove hand-written Codable is load-bearing)
+
+    /// If SE-0295 synthesis leaked, we'd see `{"hudState":{"state":"idle"}}`.
+    /// The required wire shape is `{"type":"hudState","state":"idle"}`.
+    func test_hudStateEncodesWithTypeDiscriminatorNotSynthesizedShape() throws {
+        let encoded = try BusCoder.makeEncoder().encode(BusOutbound.hudState(.idle))
+        let obj = try JSONSerialization.jsonObject(with: encoded, options: []) as! [String: Any]
+
+        XCTAssertEqual(obj["type"] as? String, "hudState")
+        XCTAssertEqual(obj["state"] as? String, "idle")
+        XCTAssertNil(obj["hudState"], "synthesized SE-0295 Codable leaked: nested payload under case name")
+    }
+
+    /// Same invariant for `tokenDelta` — required wire shape is
+    /// `{"type":"tokenDelta","text":"hi"}`.
+    func test_tokenDeltaEncodesWithTypeDiscriminator() throws {
+        let encoded = try BusCoder.makeEncoder().encode(BusOutbound.tokenDelta(text: "hi"))
+        let obj = try JSONSerialization.jsonObject(with: encoded, options: []) as! [String: Any]
+
+        XCTAssertEqual(obj["type"] as? String, "tokenDelta")
+        XCTAssertEqual(obj["text"] as? String, "hi")
+        XCTAssertNil(obj["tokenDelta"])
+    }
+
+    /// The `helloAck` shape is `{"type":"helloAck","version":"2.0.0"}`.
+    func test_helloAckEncodesWithTypeDiscriminator() throws {
+        let encoded = try BusCoder.makeEncoder().encode(BusInbound.helloAck(version: "2.0.0"))
+        let obj = try JSONSerialization.jsonObject(with: encoded, options: []) as! [String: Any]
+
+        XCTAssertEqual(obj["type"] as? String, "helloAck")
+        XCTAssertEqual(obj["version"] as? String, "2.0.0")
+    }
+
+    // MARK: - UUID formatting (Pitfall 6: JSONEncoder defaults to uppercase)
+
+    func test_uuidIsEncodedAsLowercaseString() throws {
+        let id = UUID(uuidString: "550E8400-E29B-41D4-A716-446655440000")!
+        let encoded = try BusCoder.makeEncoder().encode(BusOutbound.turnStarted(id: id))
+        let obj = try JSONSerialization.jsonObject(with: encoded, options: []) as! [String: Any]
+
+        let raw = obj["id"] as? String
+        XCTAssertEqual(raw, "550e8400-e29b-41d4-a716-446655440000")
+        XCTAssertFalse(raw?.contains(where: { $0.isUppercase }) ?? true, "UUID must encode lowercase")
+    }
+
+    // MARK: - Negative tests (drift detection)
+
+    func test_decodeUnknownDiscriminatorFails() {
+        let payload = #"{"type":"unknown","state":"idle"}"#.data(using: .utf8)!
+        XCTAssertThrowsError(
+            try BusCoder.makeDecoder().decode(BusOutbound.self, from: payload)
+        ) { error in
+            XCTAssertTrue(error is DecodingError, "expected DecodingError for unknown discriminator")
+        }
+    }
+
+    func test_decodeMissingDiscriminatorFails() {
+        let payload = #"{"version":"2.0.0"}"#.data(using: .utf8)!
+        XCTAssertThrowsError(
+            try BusCoder.makeDecoder().decode(BusOutbound.self, from: payload)
+        ) { error in
+            XCTAssertTrue(error is DecodingError, "expected DecodingError for missing type key")
+        }
+    }
+
+    func test_decodeInvalidUUIDFails() {
+        let payload = #"{"type":"turnStarted","id":"not-a-uuid"}"#.data(using: .utf8)!
+        XCTAssertThrowsError(
+            try BusCoder.makeDecoder().decode(BusOutbound.self, from: payload)
+        ) { error in
+            XCTAssertTrue(error is DecodingError, "expected DecodingError for malformed UUID")
+        }
+    }
+
+    func test_inboundDecodeUnknownDiscriminatorFails() {
+        let payload = #"{"type":"unknown"}"#.data(using: .utf8)!
+        XCTAssertThrowsError(
+            try BusCoder.makeDecoder().decode(BusInbound.self, from: payload)
+        ) { error in
+            XCTAssertTrue(error is DecodingError, "expected DecodingError for unknown inbound discriminator")
+        }
+    }
+
+    // MARK: - HudState case coverage guard
+
+    func test_allHudStateCasesHaveFixtures() throws {
+        // Every HudState case must have a dedicated fixture. If this test
+        // fails after an enum case is added, add the fixture and update the
+        // switch below.
+        for state in HudState.allCases {
+            let fixtureName = "hudState.\(state.rawValue)"
+            try assertOutboundRoundTrips(fixture: fixtureName, expect: .hudState(state))
+        }
+    }
+}
diff --git a/packages/Bus/Tests/BusTests/Fixtures/audioLevel.json b/packages/Bus/Tests/BusTests/Fixtures/audioLevel.json
new file mode 100644
index 0000000..ce9372a
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/Fixtures/audioLevel.json
@@ -0,0 +1 @@
+{"type":"audioLevel","rms":0.42}
diff --git a/packages/Bus/Tests/BusTests/Fixtures/hello.json b/packages/Bus/Tests/BusTests/Fixtures/hello.json
new file mode 100644
index 0000000..b7ba1b9
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/Fixtures/hello.json
@@ -0,0 +1 @@
+{"type":"hello","version":"2.0.0"}
diff --git a/packages/Bus/Tests/BusTests/Fixtures/helloAck.json b/packages/Bus/Tests/BusTests/Fixtures/helloAck.json
new file mode 100644
index 0000000..201fee2
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/Fixtures/helloAck.json
@@ -0,0 +1 @@
+{"type":"helloAck","version":"2.0.0"}
diff --git a/packages/Bus/Tests/BusTests/Fixtures/hudState.awaitingConfirmation.json b/packages/Bus/Tests/BusTests/Fixtures/hudState.awaitingConfirmation.json
new file mode 100644
index 0000000..96b4714
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/Fixtures/hudState.awaitingConfirmation.json
@@ -0,0 +1 @@
+{"type":"hudState","state":"awaitingConfirmation"}
diff --git a/packages/Bus/Tests/BusTests/Fixtures/hudState.booting.json b/packages/Bus/Tests/BusTests/Fixtures/hudState.booting.json
new file mode 100644
index 0000000..b42bbe9
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/Fixtures/hudState.booting.json
@@ -0,0 +1 @@
+{"type":"hudState","state":"booting"}
diff --git a/packages/Bus/Tests/BusTests/Fixtures/hudState.idle.json b/packages/Bus/Tests/BusTests/Fixtures/hudState.idle.json
new file mode 100644
index 0000000..7fa2f3d
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/Fixtures/hudState.idle.json
@@ -0,0 +1 @@
+{"type":"hudState","state":"idle"}
diff --git a/packages/Bus/Tests/BusTests/Fixtures/hudState.listening.json b/packages/Bus/Tests/BusTests/Fixtures/hudState.listening.json
new file mode 100644
index 0000000..d6fa780
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/Fixtures/hudState.listening.json
@@ -0,0 +1 @@
+{"type":"hudState","state":"listening"}
diff --git a/packages/Bus/Tests/BusTests/Fixtures/hudState.reconfiguring.json b/packages/Bus/Tests/BusTests/Fixtures/hudState.reconfiguring.json
new file mode 100644
index 0000000..a6ce9a1
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/Fixtures/hudState.reconfiguring.json
@@ -0,0 +1 @@
+{"type":"hudState","state":"reconfiguring"}
diff --git a/packages/Bus/Tests/BusTests/Fixtures/hudState.speaking.json b/packages/Bus/Tests/BusTests/Fixtures/hudState.speaking.json
new file mode 100644
index 0000000..c99e087
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/Fixtures/hudState.speaking.json
@@ -0,0 +1 @@
+{"type":"hudState","state":"speaking"}
diff --git a/packages/Bus/Tests/BusTests/Fixtures/hudState.thinking.json b/packages/Bus/Tests/BusTests/Fixtures/hudState.thinking.json
new file mode 100644
index 0000000..915fe68
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/Fixtures/hudState.thinking.json
@@ -0,0 +1 @@
+{"type":"hudState","state":"thinking"}
diff --git a/packages/Bus/Tests/BusTests/Fixtures/tokenDelta.json b/packages/Bus/Tests/BusTests/Fixtures/tokenDelta.json
new file mode 100644
index 0000000..ffac94e
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/Fixtures/tokenDelta.json
@@ -0,0 +1 @@
+{"type":"tokenDelta","text":"Hello, world!"}
diff --git a/packages/Bus/Tests/BusTests/Fixtures/toolCallEnd.json b/packages/Bus/Tests/BusTests/Fixtures/toolCallEnd.json
new file mode 100644
index 0000000..b5f1b45
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/Fixtures/toolCallEnd.json
@@ -0,0 +1 @@
+{"type":"toolCallEnd","id":"550e8400-e29b-41d4-a716-446655440000","ok":true,"previewOrError":"\"2026-04-23T16:00:00Z\""}
diff --git a/packages/Bus/Tests/BusTests/Fixtures/toolCallStart.json b/packages/Bus/Tests/BusTests/Fixtures/toolCallStart.json
new file mode 100644
index 0000000..de1cf9e
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/Fixtures/toolCallStart.json
@@ -0,0 +1 @@
+{"type":"toolCallStart","id":"550e8400-e29b-41d4-a716-446655440000","name":"get_time","argsPreview":"{}"}
diff --git a/packages/Bus/Tests/BusTests/Fixtures/turnEnded.json b/packages/Bus/Tests/BusTests/Fixtures/turnEnded.json
new file mode 100644
index 0000000..3e3c027
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/Fixtures/turnEnded.json
@@ -0,0 +1 @@
+{"type":"turnEnded","id":"6ba7b810-9dad-11d1-80b4-00c04fd430c8","terminator":"completed"}
diff --git a/packages/Bus/Tests/BusTests/Fixtures/turnStarted.json b/packages/Bus/Tests/BusTests/Fixtures/turnStarted.json
new file mode 100644
index 0000000..e2f74a0
--- /dev/null
+++ b/packages/Bus/Tests/BusTests/Fixtures/turnStarted.json
@@ -0,0 +1 @@
+{"type":"turnStarted","id":"6ba7b810-9dad-11d1-80b4-00c04fd430c8"}
diff --git a/packages/Logging/Sources/JarvisLogging/JarvisLogChannel.swift b/packages/Logging/Sources/JarvisLogging/JarvisLogChannel.swift
index c6e2beb..bb123cb 100644
--- a/packages/Logging/Sources/JarvisLogging/JarvisLogChannel.swift
+++ b/packages/Logging/Sources/JarvisLogging/JarvisLogChannel.swift
@@ -5,4 +5,5 @@ public enum JarvisLogChannel: String, Sendable, CaseIterable {
     case tools
     case ui
     case system
+    case bus
 }
diff --git a/packages/Logging/Tests/JarvisLoggingTests/LoggingTests.swift b/packages/Logging/Tests/JarvisLoggingTests/LoggingTests.swift
index db4d399..11c6610 100644
--- a/packages/Logging/Tests/JarvisLoggingTests/LoggingTests.swift
+++ b/packages/Logging/Tests/JarvisLoggingTests/LoggingTests.swift
@@ -3,8 +3,10 @@ import Logging
 @testable import JarvisLogging
 
 final class LoggingTests: XCTestCase {
-    func test_fourChannelsAreDefined() {
-        let expected: Set<String> = ["agent", "tools", "ui", "system"]
+    func test_fiveChannelsAreDefined() {
+        // `bus` was added in Phase 2 for bus-specific logging (RESEARCH §OBS-06
+        // open question #7). Keep this set in sync with `JarvisLogChannel`.
+        let expected: Set<String> = ["agent", "tools", "ui", "system", "bus"]
         let actual = Set(JarvisLogChannel.allCases.map(\.rawValue))
         XCTAssertEqual(actual, expected)
     }