diff --git a/src/main/ssh/ssh-relay-handshake-mismatch.test.ts b/src/main/ssh/ssh-relay-handshake-mismatch.test.ts
new file mode 100644
index 00000000000..2fae7cc5716
--- /dev/null
+++ b/src/main/ssh/ssh-relay-handshake-mismatch.test.ts
@@ -0,0 +1,48 @@
+import { describe, expect, it } from 'vitest'
+import { buildRelayHandshakeRefusalError } from './ssh-relay-handshake-mismatch'
+import {
+  RelayVersionMismatchError,
+  RELAY_EXIT_CODE_VERSION_MISMATCH
+} from './ssh-relay-version-mismatch-error'
+import {
+  RelayCredentialMismatchError,
+  RELAY_EXIT_CODE_CREDENTIAL_MISMATCH
+} from './ssh-relay-credential-mismatch-error'
+
+describe('buildRelayHandshakeRefusalError', () => {
+  // The bridge's mismatch line now carries `daemonProtocol=` and `ours=` after the version, so a
+  // `daemon=` capture that merely stops at `;` swallows the separating comma into the version.
+  it('names the daemon version without the comma that follows it', () => {
+    const error = buildRelayHandshakeRefusalError(
+      RELAY_EXIT_CODE_VERSION_MISMATCH,
+      '[relay-connect] Handshake mismatch: expected=0.1.0+aaa, daemon=0.1.0+bbb, ' +
+        'daemonProtocol=6, ours=1; exiting 42\n'
+    )
+    expect(error).toBeInstanceOf(RelayVersionMismatchError)
+    expect(error).toMatchObject({ expected: '0.1.0+aaa', got: '0.1.0+bbb' })
+  })
+
+  // A relay from before the protocol fields still emits the short form.
+  it('still reads the pre-negotiation form of the line', () => {
+    const error = buildRelayHandshakeRefusalError(
+      RELAY_EXIT_CODE_VERSION_MISMATCH,
+      '[relay-connect] Handshake mismatch: expected=0.1.0+aaa, daemon=0.1.0+bbb; exiting 42\n'
+    )
+    expect(error).toMatchObject({ expected: '0.1.0+aaa', got: '0.1.0+bbb' })
+  })
+
+  it('reports an unparseable mismatch line without inventing versions', () => {
+    const error = buildRelayHandshakeRefusalError(RELAY_EXIT_CODE_VERSION_MISMATCH, 'nothing here')
+    expect(error).toMatchObject({ expected: undefined, got: undefined })
+  })
+
+  it('keeps a refused credential distinct from a version skew', () => {
+    expect(
+      buildRelayHandshakeRefusalError(RELAY_EXIT_CODE_CREDENTIAL_MISMATCH, 'refused')
+    ).toBeInstanceOf(RelayCredentialMismatchError)
+  })
+
+  it('returns nothing for an exit code that encodes no refusal', () => {
+    expect(buildRelayHandshakeRefusalError(1, 'crashed')).toBeNull()
+  })
+})
diff --git a/src/main/ssh/ssh-relay-handshake-mismatch.ts b/src/main/ssh/ssh-relay-handshake-mismatch.ts
index 90841675e3f..646a14ff9c4 100644
--- a/src/main/ssh/ssh-relay-handshake-mismatch.ts
+++ b/src/main/ssh/ssh-relay-handshake-mismatch.ts
@@ -24,11 +24,13 @@ export function buildRelayHandshakeRefusalError(
 
 // Why: extract the expected/got version pair from --connect's stderr line
 // "Handshake mismatch: expected=<x>, daemon=<y>" so diagnostics name both versions.
+// Why a comma is excluded from `daemon`: the line grew trailing `daemonProtocol=`/`ours=`
+// fields, so the version is no longer the last thing before the `;`.
 function parseHandshakeMismatchStderr(stderr: string): {
   expected: string | undefined
   got: string | undefined
 } {
-  const match = /expected=([^,\s]+),\s*daemon=([^\s;]+)/.exec(stderr)
+  const match = /expected=([^,\s]+),\s*daemon=([^\s;,]+)/.exec(stderr)
   if (!match) {
     return { expected: undefined, got: undefined }
   }
diff --git a/src/relay/protocol.ts b/src/relay/protocol.ts
index 0f31b448f55..3f5db21e57d 100644
--- a/src/relay/protocol.ts
+++ b/src/relay/protocol.ts
@@ -24,6 +24,27 @@ export {
 }
 export type { DecodedFrame, FrameDecoderOptions } from './relay-frame-decoder'
 
+import {
+  MIN_RELAY_PROTOCOL_VERSION,
+  RELAY_PROTOCOL_VERSION,
+  describeRelayProtocolVersion,
+  relayProtocolOffer,
+  relayProtocolOfferAdmits,
+  type RelayHandshakeCapabilities,
+  type RelayProtocolOffer
+} from '../shared/relay-protocol-version'
+
+export {
+  MIN_RELAY_PROTOCOL_VERSION,
+  RELAY_PROTOCOL_VERSION,
+  describeRelayProtocolVersion,
+  relayProtocolOffer,
+  relayProtocolOfferAdmits
+}
+export type { RelayHandshakeCapabilities, RelayProtocolOffer }
+
+// Why frozen at 0.1.0: the deploy path carries a content hash of the bundle instead, so nothing
+// ever needed this to move. `RELAY_PROTOCOL_VERSION` is the number that describes the wire.
 export const RELAY_VERSION = '0.1.0'
 export const RELAY_SENTINEL = `ORCA-RELAY v${RELAY_VERSION} READY\n`
 
@@ -37,10 +58,24 @@ export const MessageType = {
 // reads exactly one Handshake frame before attaching the JSON-RPC dispatcher,
 // to refuse mismatched-version --connect bridges that would otherwise drive a
 // stale daemon.
+// Why optional on every arm: `parseHandshakeMessage` throws on an unknown `type` and the throw
+// closes the socket, so this envelope can only ever grow by optional fields on the arms that
+// already ship. `endpointCredential` is the precedent. See
+// docs/reference/remote-wire-compatibility.md Rule 1.
 export type HandshakeMessage =
-  | { type: 'orca-relay-handshake'; version: string; endpointCredential?: string }
-  | { type: 'orca-relay-handshake-ok'; version: string }
-  | { type: 'orca-relay-handshake-mismatch'; expected: string; got: string }
+  | ({
+      type: 'orca-relay-handshake'
+      version: string
+      endpointCredential?: string
+    } & RelayProtocolOffer)
+  | ({
+      type: 'orca-relay-handshake-ok'
+      version: string
+      capabilities?: RelayHandshakeCapabilities
+    } & RelayProtocolOffer)
+  // `protocolVersion` here tells a client whose offered range missed *what* it missed, which a
+  // content hash cannot: the two builds may be wire-compatible and merely differ in bytes.
+  | ({ type: 'orca-relay-handshake-mismatch'; expected: string; got: string } & RelayProtocolOffer)
   // Why a distinct reply: the bridge exits with its own code so the client can tell a refused
   // credential from a crashed relay. Old bridges reject the unknown type and exit 1 pre-sentinel.
   | { type: 'orca-relay-handshake-credential-mismatch' }
diff --git a/src/relay/relay-handshake-roundtrip.test.ts b/src/relay/relay-handshake-roundtrip.test.ts
index 0713d62295e..68148f1ef8b 100644
--- a/src/relay/relay-handshake-roundtrip.test.ts
+++ b/src/relay/relay-handshake-roundtrip.test.ts
@@ -13,9 +13,14 @@ import {
   encodeHandshakeFrame,
   encodeJsonRpcFrame,
   FrameDecoder,
+  parseHandshakeMessage,
+  MIN_RELAY_PROTOCOL_VERSION,
+  RELAY_PROTOCOL_VERSION,
   type DecodedFrame,
+  type HandshakeMessage,
   MessageType
 } from './protocol'
+import { PTY_CONSUMER_SESSION_PROTOCOL_VERSION } from '../shared/pty-consumer-session-contract'
 import { relayTestSocketPath } from './relay-test-socket-path'
 
 // Why: --connect normally calls process.exit on mismatch / fatal handshake
@@ -210,9 +215,86 @@ describe('handshake round-trip over a real Socket pair', () => {
     bridgeSock.destroy()
   })
 
-  it('exits with EXIT_CODE_VERSION_MISMATCH when the daemon reports a mismatch', async () => {
+  // Reads the daemon's single handshake reply off a raw socket, so a test can assert what the
+  // envelope carries rather than only whether the bridge accepted it.
+  function readDaemonReply(sock: Socket): Promise<HandshakeMessage> {
+    return new Promise((resolve) => {
+      const decoder = new FrameDecoder((frame) => {
+        if (frame.type === MessageType.Handshake) {
+          resolve(parseHandshakeMessage(frame.payload))
+        }
+      })
+      sock.on('data', (chunk: Buffer) => decoder.feed(chunk))
+    })
+  }
+
+  it('refuses a cross-build bridge that offers no protocol range at all', async () => {
+    // Why raw bytes: runConnectHandshake now always offers a range. Every relay already deployed
+    // sends none, and for those the build hash stays the only gate — this is that fallback.
     await startDaemon('0.1.0+server-version')
 
+    const bridgeSock = connect(sockPath)
+    await new Promise<void>((r) => bridgeSock.once('connect', () => r()))
+    const reply = readDaemonReply(bridgeSock)
+
+    bridgeSock.write(
+      encodeHandshakeFrame({ type: 'orca-relay-handshake', version: '0.1.0+different' })
+    )
+
+    // The refusal now names the daemon's protocol version, which a content hash cannot express.
+    await expect(reply).resolves.toMatchObject({
+      type: 'orca-relay-handshake-mismatch',
+      expected: '0.1.0+server-version',
+      got: '0.1.0+different',
+      protocolVersion: RELAY_PROTOCOL_VERSION,
+      minProtocolVersion: MIN_RELAY_PROTOCOL_VERSION
+    })
+
+    bridgeSock.destroy()
+  })
+
+  it('refuses a cross-build bridge whose offered range excludes this daemon', async () => {
+    await startDaemon('0.1.0+server-version')
+
+    const bridgeSock = connect(sockPath)
+    await new Promise<void>((r) => bridgeSock.once('connect', () => r()))
+    const reply = readDaemonReply(bridgeSock)
+
+    bridgeSock.write(
+      encodeHandshakeFrame({
+        type: 'orca-relay-handshake',
+        version: '0.1.0+different',
+        protocolVersion: RELAY_PROTOCOL_VERSION + 5,
+        minProtocolVersion: RELAY_PROTOCOL_VERSION + 5
+      })
+    )
+
+    await expect(reply).resolves.toMatchObject({ type: 'orca-relay-handshake-mismatch' })
+
+    bridgeSock.destroy()
+  })
+
+  it('exits with EXIT_CODE_VERSION_MISMATCH when the daemon reports a mismatch', async () => {
+    server = createServer((sock) => {
+      trackServerSocket(sock)
+      const decoder = new FrameDecoder((frame) => {
+        if (frame.type !== MessageType.Handshake) {
+          return
+        }
+        sock.write(
+          encodeHandshakeFrame({
+            type: 'orca-relay-handshake-mismatch',
+            expected: '0.1.0+server-version',
+            got: '0.1.0+different',
+            protocolVersion: RELAY_PROTOCOL_VERSION + 5,
+            minProtocolVersion: RELAY_PROTOCOL_VERSION + 5
+          })
+        )
+      })
+      sock.on('data', (chunk: Buffer) => decoder.feed(chunk))
+    })
+    await new Promise<void>((r) => server.listen(sockPath, () => r()))
+
     const bridgeSock = connect(sockPath)
     await new Promise<void>((r) => bridgeSock.once('connect', () => r()))
 
@@ -226,6 +308,107 @@ describe('handshake round-trip over a real Socket pair', () => {
     bridgeSock.destroy()
   })
 
+  // #13852: after an app update the client's bundle hashes differently, so the incumbent relay
+  // holding every live PTY refused it and that work became permanently unreachable.
+  it('admits a bridge from a different build once it offers a range this daemon falls in', async () => {
+    const { accepted } = await startDaemon('0.1.0+incumbent-holding-live-ptys')
+
+    const bridgeSock = connect(sockPath)
+    await new Promise<void>((r) => bridgeSock.once('connect', () => r()))
+
+    const acceptedCb = vi.fn<(leftover: Buffer) => void>()
+    runConnectHandshake(bridgeSock, '0.1.0+freshly-updated-client', { onAccepted: acceptedCb })
+
+    await accepted
+    await vi.waitFor(() => expect(acceptedCb).toHaveBeenCalledTimes(1))
+    expect(exitSpy).not.toHaveBeenCalled()
+
+    bridgeSock.destroy()
+  })
+
+  it('answers a cross-build bridge with the protocol version and capabilities it must speak', async () => {
+    await startDaemon('0.1.0+incumbent-holding-live-ptys')
+
+    const bridgeSock = connect(sockPath)
+    await new Promise<void>((r) => bridgeSock.once('connect', () => r()))
+    const reply = readDaemonReply(bridgeSock)
+
+    bridgeSock.write(
+      encodeHandshakeFrame({
+        type: 'orca-relay-handshake',
+        version: '0.1.0+freshly-updated-client',
+        protocolVersion: RELAY_PROTOCOL_VERSION + 3,
+        minProtocolVersion: MIN_RELAY_PROTOCOL_VERSION
+      })
+    )
+
+    await expect(reply).resolves.toMatchObject({
+      type: 'orca-relay-handshake-ok',
+      // The build the client actually reached, which is no longer its own.
+      version: '0.1.0+incumbent-holding-live-ptys',
+      protocolVersion: RELAY_PROTOCOL_VERSION,
+      minProtocolVersion: MIN_RELAY_PROTOCOL_VERSION,
+      capabilities: { ptyConsumerSession: PTY_CONSUMER_SESSION_PROTOCOL_VERSION }
+    })
+
+    bridgeSock.destroy()
+  })
+
+  // The refusal path logs the peer's claim, and `JSON.parse` yields objects a template literal
+  // cannot stringify. A throw there is inside the frame-decoder callback, so it would kill the
+  // daemon — and every PTY it still holds — on an unauthenticated frame.
+  it('refuses a hostile protocolVersion claim without taking the daemon down', async () => {
+    const { accepted } = await startDaemon('0.1.0+server-version')
+
+    const bridgeSock = connect(sockPath)
+    await new Promise<void>((r) => bridgeSock.once('connect', () => r()))
+    const reply = readDaemonReply(bridgeSock)
+
+    bridgeSock.write(
+      encodeHandshakeFrame(
+        JSON.parse(
+          '{"type":"orca-relay-handshake","version":"0.1.0+different","protocolVersion":{"toString":1}}'
+        ) as HandshakeMessage
+      )
+    )
+
+    await expect(reply).resolves.toMatchObject({ type: 'orca-relay-handshake-mismatch' })
+
+    // Still serving: a second, well-formed client is admitted after the hostile one.
+    const good = connect(sockPath)
+    await new Promise<void>((r) => good.once('connect', () => r()))
+    runConnectHandshake(good, '0.1.0+server-version', { onAccepted: vi.fn() })
+    await accepted
+
+    bridgeSock.destroy()
+    good.destroy()
+  })
+
+  // Tolerance replaces a compatibility gate, never the auth gate: the credential file lives inside
+  // the incumbent's own install dir, so presenting it is what proves the caller may reach it.
+  it('still refuses a cross-build bridge that cannot present the endpoint credential', async () => {
+    const { accepted } = await startDaemon('0.1.0+incumbent', 'secret-credential')
+
+    const bridgeSock = connect(sockPath)
+    await new Promise<void>((r) => bridgeSock.once('connect', () => r()))
+    const closed = new Promise<void>((r) => bridgeSock.once('close', () => r()))
+
+    runConnectHandshake(
+      bridgeSock,
+      '0.1.0+freshly-updated-client',
+      { onAccepted: vi.fn() },
+      'wrong-credential'
+    )
+
+    await closed
+    await expect(
+      Promise.race([
+        accepted.then(() => 'accepted'),
+        new Promise<string>((r) => setTimeout(() => r('closed'), 20))
+      ])
+    ).resolves.toBe('closed')
+  })
+
   it('does not call onAccepted before any handshake-ok frame arrives', async () => {
     // Why: silent server that never replies. acceptedCb must stay
     // un-invoked even though the bridge has flushed its handshake frame.
diff --git a/src/relay/relay-handshake.ts b/src/relay/relay-handshake.ts
index ed76baf024d..27505a6fa06 100644
--- a/src/relay/relay-handshake.ts
+++ b/src/relay/relay-handshake.ts
@@ -5,14 +5,26 @@ import { existsSync, readFileSync, realpathSync } from 'node:fs'
 import type { Socket } from 'node:net'
 import {
   RELAY_VERSION,
+  RELAY_PROTOCOL_VERSION,
   MessageType,
   FrameDecoder,
   encodeHandshakeFrame,
   parseHandshakeMessage,
-  type DecodedFrame
+  describeRelayProtocolVersion,
+  relayProtocolOffer,
+  relayProtocolOfferAdmits,
+  type DecodedFrame,
+  type RelayHandshakeCapabilities
 } from './protocol'
+import { PTY_CONSUMER_SESSION_PROTOCOL_VERSION } from '../shared/pty-consumer-session-contract'
 import { relayLogLine } from './relay-diagnostic-log'
 
+// Contracts that turn over independently of the handshake integer, so a client cannot infer them
+// from `protocolVersion` and must not have to infer them from the build hash either.
+function relayHandshakeCapabilities(): RelayHandshakeCapabilities {
+  return { ptyConsumerSession: PTY_CONSUMER_SESSION_PROTOCOL_VERSION }
+}
+
 // Why: clients treat this exit code as non-retryable; other non-zero exits are transient.
 export const EXIT_CODE_VERSION_MISMATCH = 42
 // Why distinct from 42: a refused credential is a live daemon saying no, which the client must
@@ -121,16 +133,23 @@ function handleDaemonHandshakeFrame(
     sock.destroy()
     return false
   }
-  if (msg.version !== launchVersion) {
+  // Why the build hash is still first: it is the only gate every already-deployed relay has, and
+  // it stays the fallback for peers that offer no protocol range at all. Negotiation is what lets
+  // a relay stranded by an app update keep serving the PTYs it already owns (#13852) — the hash
+  // differs by construction there, while the wire is unchanged.
+  const negotiated = msg.version === launchVersion || relayProtocolOfferAdmits(msg)
+  if (!negotiated) {
     relayLogLine(
-      `[relay] Handshake mismatch: own=${launchVersion}, client=${msg.version}; closing socket`
+      `[relay] Handshake mismatch: own=${launchVersion}/p${RELAY_PROTOCOL_VERSION}, ` +
+        `client=${msg.version}/p${describeRelayProtocolVersion(msg.protocolVersion)}; closing socket`
     )
     try {
       sock.write(
         encodeHandshakeFrame({
           type: 'orca-relay-handshake-mismatch',
           expected: launchVersion,
-          got: msg.version
+          got: msg.version,
+          ...relayProtocolOffer()
         })
       )
     } catch {
@@ -150,8 +169,17 @@ function handleDaemonHandshakeFrame(
     sock.end()
     return false
   }
-  process.stderr.write(`[relay] Handshake OK from version=${msg.version}\n`)
-  sock.write(encodeHandshakeFrame({ type: 'orca-relay-handshake-ok', version: launchVersion }))
+  process.stderr.write(
+    `[relay] Handshake OK from version=${msg.version} (${msg.version === launchVersion ? 'same build' : `cross-build, protocol ${RELAY_PROTOCOL_VERSION}`})\n`
+  )
+  sock.write(
+    encodeHandshakeFrame({
+      type: 'orca-relay-handshake-ok',
+      version: launchVersion,
+      ...relayProtocolOffer(),
+      capabilities: relayHandshakeCapabilities()
+    })
+  )
   return true
 }
 
@@ -194,7 +222,11 @@ export function runConnectHandshake(
         process.exit(1)
       }
       if (msg.type === 'orca-relay-handshake-ok') {
-        process.stderr.write(`[relay-connect] Handshake OK at version=${msg.version}\n`)
+        // Why both numbers: with negotiation the daemon's build can legitimately differ from this
+        // bridge's, so the version alone no longer says which relay answered.
+        process.stderr.write(
+          `[relay-connect] Handshake OK at version=${msg.version} protocol=${describeRelayProtocolVersion(msg.protocolVersion)}\n`
+        )
         handshakeDone = true
         const leftover = decoder.drain()
         sock.removeAllListeners('data')
@@ -204,7 +236,9 @@ export function runConnectHandshake(
       if (msg.type === 'orca-relay-handshake-mismatch') {
         // Why: exit inside the write callback; stderr is async on pipe transports, so exiting early drops the version detail.
         process.stderr.write(
-          `[relay-connect] Handshake mismatch: expected=${msg.expected}, daemon=${msg.got}; exiting ${EXIT_CODE_VERSION_MISMATCH}\n`,
+          `[relay-connect] Handshake mismatch: expected=${msg.expected}, daemon=${msg.got}, ` +
+            `daemonProtocol=${describeRelayProtocolVersion(msg.protocolVersion)}, ours=${RELAY_PROTOCOL_VERSION}; ` +
+            `exiting ${EXIT_CODE_VERSION_MISMATCH}\n`,
           () => {
             sock.destroy()
             process.exit(EXIT_CODE_VERSION_MISMATCH)
@@ -243,7 +277,9 @@ export function runConnectHandshake(
     encodeHandshakeFrame({
       type: 'orca-relay-handshake',
       version: myVersion,
-      ...(endpointCredential ? { endpointCredential } : {})
+      ...(endpointCredential ? { endpointCredential } : {}),
+      // A relay that predates the offer ignores these keys and compares the hash as before.
+      ...relayProtocolOffer()
     })
   )
 }
diff --git a/src/shared/relay-protocol-version.test.ts b/src/shared/relay-protocol-version.test.ts
new file mode 100644
index 00000000000..0fcfeba228d
--- /dev/null
+++ b/src/shared/relay-protocol-version.test.ts
@@ -0,0 +1,91 @@
+import { describe, expect, it } from 'vitest'
+import {
+  MIN_RELAY_PROTOCOL_VERSION,
+  RELAY_PROTOCOL_VERSION,
+  describeRelayProtocolVersion,
+  relayProtocolOffer,
+  relayProtocolOfferAdmits
+} from './relay-protocol-version'
+
+describe('relay protocol version', () => {
+  it('is a whole number at or above its own floor', () => {
+    expect(Number.isSafeInteger(RELAY_PROTOCOL_VERSION)).toBe(true)
+    expect(Number.isSafeInteger(MIN_RELAY_PROTOCOL_VERSION)).toBe(true)
+    expect(RELAY_PROTOCOL_VERSION).toBeGreaterThanOrEqual(MIN_RELAY_PROTOCOL_VERSION)
+  })
+
+  it('offers its own range, so a same-build peer is admitted by negotiation as well as by hash', () => {
+    expect(relayProtocolOfferAdmits(relayProtocolOffer())).toBe(true)
+  })
+
+  // The whole point of Release N: the stranded relay is always the OLDER side, and it must be
+  // able to say yes on the first frame with no earlier round trip to downgrade in.
+  it('admits a newer client whose range reaches back to this relay', () => {
+    expect(relayProtocolOfferAdmits({ protocolVersion: 9, minProtocolVersion: 1 }, 1)).toBe(true)
+    expect(relayProtocolOfferAdmits({ protocolVersion: 9, minProtocolVersion: 5 }, 1)).toBe(false)
+  })
+
+  it('admits a client older than this relay when the relay still falls in its range', () => {
+    expect(relayProtocolOfferAdmits({ protocolVersion: 4, minProtocolVersion: 2 }, 3)).toBe(true)
+    expect(relayProtocolOfferAdmits({ protocolVersion: 2, minProtocolVersion: 1 }, 3)).toBe(false)
+  })
+
+  it('treats a lone protocolVersion as a range of exactly one', () => {
+    expect(relayProtocolOfferAdmits({ protocolVersion: 3 }, 3)).toBe(true)
+    expect(relayProtocolOfferAdmits({ protocolVersion: 3 }, 2)).toBe(false)
+    expect(relayProtocolOfferAdmits({ protocolVersion: 3 }, 4)).toBe(false)
+  })
+
+  // A peer that sends no offer predates negotiation; admitting it would hand every unversioned
+  // caller the cross-build path that the build hash is supposed to gate.
+  it('never admits an absent or empty offer', () => {
+    expect(relayProtocolOfferAdmits(undefined, 1)).toBe(false)
+    expect(relayProtocolOfferAdmits({}, 1)).toBe(false)
+    expect(relayProtocolOfferAdmits({ minProtocolVersion: 1 }, 1)).toBe(false)
+  })
+
+  // Zero is the one bad value the band check cannot catch on its own: a peer claiming protocol 0
+  // would be admitted by a relay whose own version was also read as 0.
+  it('rejects a zero protocol version rather than treating it as a real version', () => {
+    expect(relayProtocolOfferAdmits({ protocolVersion: 0, minProtocolVersion: 0 }, 0)).toBe(false)
+  })
+
+  it('rejects offers that are not plausible integers', () => {
+    for (const bad of [0, -1, 1.5, Number.NaN, Number.POSITIVE_INFINITY, 2_000_000]) {
+      expect(
+        relayProtocolOfferAdmits({ protocolVersion: bad, minProtocolVersion: 1 }, 1),
+        `protocolVersion=${bad}`
+      ).toBe(false)
+    }
+    for (const bad of ['1', null, {}, []]) {
+      expect(
+        relayProtocolOfferAdmits(
+          { protocolVersion: 1, minProtocolVersion: bad as unknown as number },
+          1
+        ),
+        `minProtocolVersion=${JSON.stringify(bad)}`
+      ).toBe(false)
+    }
+  })
+
+  it('rejects an inverted range instead of silently reordering it', () => {
+    expect(relayProtocolOfferAdmits({ protocolVersion: 1, minProtocolVersion: 5 }, 1)).toBe(false)
+    expect(relayProtocolOfferAdmits({ protocolVersion: 1, minProtocolVersion: 5 }, 5)).toBe(false)
+  })
+
+  // A relay daemon logs the peer's claim on the refusal path, and `JSON.parse` can produce an
+  // object a template literal cannot stringify. Throwing there is inside the frame-decoder
+  // callback, which would take the daemon and every PTY it still holds down with it.
+  it('renders a peer version claim that a template literal would throw on', () => {
+    const hostile = JSON.parse('{"protocolVersion": {"toString": 1}}').protocolVersion
+    expect(() => `${hostile}`).toThrow()
+    expect(describeRelayProtocolVersion(hostile)).toBe('none')
+  })
+
+  it('renders real numbers and refuses every other shape', () => {
+    expect(describeRelayProtocolVersion(7)).toBe('7')
+    for (const bad of [undefined, null, '3', {}, [], Number.NaN, Number.POSITIVE_INFINITY]) {
+      expect(describeRelayProtocolVersion(bad), JSON.stringify(bad ?? null)).toBe('none')
+    }
+  })
+})
diff --git a/src/shared/relay-protocol-version.ts b/src/shared/relay-protocol-version.ts
new file mode 100644
index 00000000000..0a4c6303d9e
--- /dev/null
+++ b/src/shared/relay-protocol-version.ts
@@ -0,0 +1,143 @@
+/**
+ * Semantic wire version for the relay handshake.
+ *
+ * `RELAY_VERSION` ('0.1.0') has never moved, so the deploy path is namespaced by a content hash
+ * of the bundle instead (`config/scripts/build-relay.mjs`). That hash changes on nearly every
+ * release, which makes every install directory — and the socket inside it — unreachable to the
+ * next build, stranding the PTYs and agents the incumbent still owns (#13852).
+ *
+ * This integer is the escape: it names what the *wire* can do, independent of what the bytes
+ * hash to, exactly as `PROTOCOL_VERSION` does for the local terminal daemon
+ * (`src/main/daemon/daemon-protocol-version.ts`). Bump it only when a client and a relay of
+ * adjacent versions genuinely cannot drive each other; a rebuild is not a reason.
+ */
+export const RELAY_PROTOCOL_VERSION = 1
+
+/**
+ * ## This negotiation is not reachable yet. Do not read it as fixing #13852.
+ *
+ * Measured, not inferred: the deploy path namespaces the relay directory by the CLIENT'S OWN build
+ * hash — `remoteInstallDirName` is `relay-<fullVersion>` — and the daemon socket lives inside that
+ * directory (`ssh-relay-deploy.ts`, `--connect --sock-path ~/.orca-remote/relay-<ownVersion>/…`;
+ * the short-socket fallback derives its segment from the same version dir). Every
+ * `runConnectHandshake` caller takes its path from that one deploy result. So a bridge can only
+ * ever meet a daemon of its own build, `msg.version === launchVersion` short-circuits first, and
+ * `relayProtocolOfferAdmits` never decides anything on any live path. The stranded incumbent this
+ * exists for sits in `relay-<otherVersion>/`, which nothing dials.
+ *
+ * What ships today is therefore three log lines: `describeRelayProtocolVersion` in
+ * `relay-handshake.ts`. `MIN_RELAY_PROTOCOL_VERSION` and the `minProtocolVersion` wire field have
+ * no live reader at all.
+ *
+ * Keep it — the mechanism is correct and hostile-input-safe, and it is the part that has to exist
+ * first. Making it live needs THREE more changes, all of which must land together. Landing only
+ * the first two ships a negotiation that still refuses, later and less legibly:
+ *   1. route the bridge to the incumbent's socket rather than to its own version directory;
+ *   2. stop `validateGrant` (`ssh-pty-consumer-session.ts`) refusing on `serverBuildId`. Its
+ *      premise, "client and relay ship in one build", is still TRUE today and becomes false the
+ *      moment (1) lands — it is a second gate that would refuse what the handshake just admitted.
+ *   3. give that same refusal a way to survive a protocol-version difference. It is ONE `if` with
+ *      two disjuncts, and deleting the `serverBuildId` clause leaves the other one standing:
+ *      `grant.protocolVersion !== PTY_CONSUMER_SESSION_PROTOCOL_VERSION` — a DIFFERENT constant
+ *      from this file's `RELAY_PROTOCOL_VERSION`, compared for exact equality, with no range, no
+ *      floor and no fallback. Two peers that just negotiated a compatible relay protocol are still
+ *      refused if their PTY-session constants differ by one. And there is nothing to negotiate it
+ *      with: `capabilities` on `orca-relay-handshake-ok` is written at exactly one site
+ *      (`relay-handshake.ts`) and read by nothing outside tests, so (3) means giving that field a
+ *      reader before it can carry the peer's PTY-session protocol.
+ *
+ * Budget a debugging session for the error text too: the refusal interpolates only the build ids,
+ * so a pure protocol-version mismatch reports as "expected build X, got X" with two IDENTICAL ids,
+ * which points at the gate that is not the problem.
+ *
+ * Until all three land, a change here cannot be validated by any end-to-end test, only by the
+ * handshake's own unit suite.
+ */
+
+/**
+ * Oldest peer protocol this build can still speak. Raising it strands every relay below the new
+ * floor for good, so it moves only when serving a version is actually impossible.
+ *
+ * No live reader — see the note above. It is serialized onto every handshake and reply so that the
+ * field exists on the wire before any peer needs it (Rule 1, docs/reference/remote-wire-
+ * compatibility.md); an older peer ignoring it today is the point.
+ */
+export const MIN_RELAY_PROTOCOL_VERSION = 1
+
+/** Rejects a peer's absurd or non-integer claim before it reaches a comparison. */
+const MAX_PLAUSIBLE_RELAY_PROTOCOL_VERSION = 1_000_000
+
+/**
+ * Contract versions a peer cannot derive from `protocolVersion` alone, because they turn over
+ * independently of the handshake.
+ */
+export type RelayHandshakeCapabilities = {
+  /** `pty.openClient` grant contract this relay mints (`PTY_CONSUMER_SESSION_PROTOCOL_VERSION`). */
+  readonly ptyConsumerSession?: number
+}
+
+/**
+ * The protocol range a peer says it can speak. Both fields are optional on the wire: a relay or
+ * bridge that predates this negotiation sends neither, which is the signal to fall back to
+ * comparing build hashes.
+ */
+export type RelayProtocolOffer = {
+  readonly protocolVersion?: number
+  readonly minProtocolVersion?: number
+}
+
+function readVersion(value: unknown): number | null {
+  return typeof value === 'number' &&
+    Number.isSafeInteger(value) &&
+    value >= 1 &&
+    value <= MAX_PLAUSIBLE_RELAY_PROTOCOL_VERSION
+    ? value
+    : null
+}
+
+/**
+ * Renders a peer's claimed version for a log line without interpolating it.
+ *
+ * `JSON.parse` can hand back `{ "toString": 1 }`, and a template literal on that throws
+ * `Cannot convert object to primitive value` — inside the frame-decoder callback, which would
+ * take the daemon and every PTY it holds down with it.
+ */
+export function describeRelayProtocolVersion(value: unknown): string {
+  return typeof value === 'number' && Number.isFinite(value) ? String(value) : 'none'
+}
+
+/** This build's own offer, sent on every handshake and reply. */
+export function relayProtocolOffer(): Required<RelayProtocolOffer> {
+  return {
+    protocolVersion: RELAY_PROTOCOL_VERSION,
+    minProtocolVersion: MIN_RELAY_PROTOCOL_VERSION
+  }
+}
+
+/**
+ * Whether a peer offering `offer` can be served by a relay speaking `ownVersion`.
+ *
+ * The peer states a range rather than a single number so an *older* relay can admit a *newer*
+ * client on the first frame — there is no earlier round trip in which the client could learn
+ * what to downgrade to, and the stranded relay is by construction the older side.
+ *
+ * An offer with no `protocolVersion` is not a negotiation at all and never admits: that is a
+ * pre-negotiation peer, and the build-hash comparison stays its only gate.
+ */
+export function relayProtocolOfferAdmits(
+  offer: RelayProtocolOffer | undefined,
+  ownVersion: number = RELAY_PROTOCOL_VERSION
+): boolean {
+  const max = readVersion(offer?.protocolVersion)
+  if (max === null) {
+    return false
+  }
+  // Why default to `max`: a peer that names one version speaks exactly that one.
+  const min = offer?.minProtocolVersion === undefined ? max : readVersion(offer.minProtocolVersion)
+  if (min === null) {
+    return false
+  }
+  // An inverted range admits nothing rather than being reordered into something the peer never
+  // offered: `min > max` simply fails this band.
+  return ownVersion >= min && ownVersion <= max
+}