diff --git a/src/main/ssh/connection/ssh-connection-host-key-pin-race.test.ts b/src/main/ssh/connection/ssh-connection-host-key-pin-race.test.ts
new file mode 100644
index 00000000000..54fac618167
--- /dev/null
+++ b/src/main/ssh/connection/ssh-connection-host-key-pin-race.test.ts
@@ -0,0 +1,57 @@
+import { expect, it, vi } from 'vitest'
+import { clientInstances, ssh2Mock } from '../__tests__/ssh-connection-test-client'
+import { resetSshConnectionMocks, spawnSystemSshCommandMock } from './ssh-connection-test-harness'
+import { createCallbacks, createTarget } from './ssh-connection-test-fixtures'
+import { SshConnection } from './ssh-connection'
+
+vi.mock('ssh2', async () =>
+  (await import('../__tests__/ssh-connection-test-client')).createSsh2Module()
+)
+vi.mock('../system-transport/system-ssh-binary', async () =>
+  (await import('./ssh-connection-test-harness')).createSystemSshBinaryModule()
+)
+vi.mock('../system-transport/system-ssh-command', async (importOriginal) =>
+  (await import('./ssh-connection-test-harness')).withSystemSshCommandMocks(await importOriginal())
+)
+vi.mock('../ssh-control-socket', async () =>
+  (await import('./ssh-connection-test-harness')).createControlSocketModule()
+)
+vi.mock('../config/ssh-g-config-resolution', async (importOriginal) =>
+  (await import('./ssh-connection-test-harness')).withSshGConfigResolutionMocks(
+    await importOriginal()
+  )
+)
+
+const { installMock } = vi.hoisted(() => ({ installMock: vi.fn() }))
+vi.mock('./ssh2-host-key-verification', () => ({
+  installSsh2HostKeyVerification: installMock,
+  readHostKeyTrustSources: vi.fn()
+}))
+
+it('keeps the current attempt pinned when a superseded attempt finishes its trust read later', async () => {
+  resetSshConnectionMocks()
+  ssh2Mock.connectBehavior = 'pending'
+  let finishStaleRead: (value: unknown) => void = () => {}
+  installMock.mockImplementationOnce(
+    () =>
+      new Promise((resolve) => {
+        finishStaleRead = resolve
+      })
+  )
+  installMock.mockResolvedValue({ rejection: null, pinnedByOrcaStore: true })
+  const conn = new SshConnection(createTarget(), createCallbacks())
+  const stale = conn.connect().catch(() => {})
+  await vi.waitFor(() => expect(installMock).toHaveBeenCalledTimes(1))
+  const current = conn.connect()
+  await vi.waitFor(() => expect(clientInstances).toHaveLength(1))
+  const currentClient = clientInstances[0]
+
+  finishStaleRead({ rejection: null, pinnedByOrcaStore: true })
+  await vi.waitFor(() => expect(clientInstances).toHaveLength(2))
+  clientInstances[1].emit('error', new Error('stale attempt failed'))
+  await stale
+  currentClient.emit('error', new Error('Handshake failed: no matching host key format'))
+
+  await expect(current).rejects.toThrow('no matching host key format')
+  expect(spawnSystemSshCommandMock).not.toHaveBeenCalled()
+})
diff --git a/src/main/ssh/connection/ssh-connection-manager-sign-in-reuse.test.ts b/src/main/ssh/connection/ssh-connection-manager-sign-in-reuse.test.ts
index 7057b5015a3..46df112082e 100644
--- a/src/main/ssh/connection/ssh-connection-manager-sign-in-reuse.test.ts
+++ b/src/main/ssh/connection/ssh-connection-manager-sign-in-reuse.test.ts
@@ -101,8 +101,8 @@ describe('SshConnectionManager on a password host', () => {
 
   it('keeps the sign-in across a replacement that failed for a reason other than sign-in', async () => {
     await manager.disconnectConnection('target-1', manager.getConnection('target-1')!)
-    ssh2Mock.connectSequence = [new Error('Handshake failed: no matching cipher')]
-    await expect(manager.connect(createTarget())).rejects.toThrow('no matching cipher')
+    ssh2Mock.connectSequence = [new Error('Handshake failed: signature verification failed')]
+    await expect(manager.connect(createTarget())).rejects.toThrow('signature verification failed')
 
     ssh2Mock.connectSequence = ['ready']
     await manager.connect(createTarget())
@@ -152,8 +152,11 @@ describe('SshConnectionManager on a password host', () => {
   it('does not hand on a typed password the host never accepted', async () => {
     await manager.disconnect('target-1')
     onCredentialRequest.mockResolvedValueOnce('typo')
-    ssh2Mock.connectSequence = [authFailure(), new Error('Handshake failed: no matching cipher')]
-    await expect(manager.connect(createTarget())).rejects.toThrow('no matching cipher')
+    ssh2Mock.connectSequence = [
+      authFailure(),
+      new Error('Handshake failed: signature verification failed')
+    ]
+    await expect(manager.connect(createTarget())).rejects.toThrow('signature verification failed')
     expect(manager.hasCachedCredential(createTarget())).toBe(false)
 
     ssh2Mock.connectSequence = [authFailure(), 'ready']
diff --git a/src/main/ssh/connection/ssh-connection-system-transport.test.ts b/src/main/ssh/connection/ssh-connection-system-transport.test.ts
index 2d47ed8e459..de4f07e039b 100644
--- a/src/main/ssh/connection/ssh-connection-system-transport.test.ts
+++ b/src/main/ssh/connection/ssh-connection-system-transport.test.ts
@@ -23,6 +23,7 @@ import { shouldUseSystemSshTransport } from '../ssh-transport-selection'
 import { resolveWithSshG } from '../config/ssh-g-config-resolution'
 import { writeFileViaSystemSsh } from '../system-transport/system-ssh-file-transfer'
 import { CONNECT_TIMEOUT_MS } from './ssh-connection-timing'
+import { initSshHostKeyStoreFile, trustHostKey } from '../ssh-host-key-store'
 import type { SshTarget } from '../../../shared/ssh-types'
 import {
   createOpenSshPrivateKeyFixture,
@@ -332,6 +333,69 @@ describe('SshConnection', () => {
     )
     expect(conn.usesSystemSshTransport()).toBe(false)
   })
+
+  it.each([
+    'Handshake failed: no matching host key format',
+    'Handshake failed: no matching key exchange algorithm',
+    'Handshake failed: no matching C->S cipher'
+  ])(
+    'falls back to system SSH when ssh2 shares no algorithm with the host (%s)',
+    async (message) => {
+      ssh2Mock.connectBehavior = 'error'
+      ssh2Mock.connectErrorMessage = message
+      const conn = new SshConnection(
+        createTarget({ host: 'cert.example.com', label: 'Cert host' }),
+        createCallbacks()
+      )
+
+      await conn.connect()
+
+      expect(conn.getState().status).toBe('connected')
+      expect(conn.usesSystemSshTransport()).toBe(true)
+    }
+  )
+
+  it('reports the system SSH failure when ssh2 cannot negotiate with the host', async () => {
+    ssh2Mock.connectBehavior = 'error'
+    ssh2Mock.connectErrorMessage = 'Handshake failed: no matching host key format'
+    spawnSystemSshCommandMock.mockImplementation(() => {
+      throw new Error('No system ssh binary found. Install OpenSSH to use system SSH transport.')
+    })
+    const conn = new SshConnection(
+      createTarget({ host: 'cert.example.com', label: 'Cert host' }),
+      createCallbacks()
+    )
+
+    await expect(conn.connect()).rejects.toThrow('No system ssh binary found')
+    expect(conn.usesSystemSshTransport()).toBe(false)
+  })
+
+  it('keeps a host pinned in the Orca key store on ssh2 when negotiation fails', async () => {
+    const storeDir = mkdtempSync(join(tmpdir(), 'orca-ssh-pinned-store-'))
+    try {
+      initSshHostKeyStoreFile(join(storeDir, 'orca.json'))
+      await trustHostKey({
+        host: 'cert.example.com',
+        port: 22,
+        keyType: 'ssh-ed25519',
+        key: Buffer.from(
+          'AAAAC3NzaC1lZDI1NTE5AAAAIKqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq',
+          'base64'
+        )
+      })
+      ssh2Mock.connectBehavior = 'error'
+      ssh2Mock.connectErrorMessage = 'Handshake failed: no matching host key format'
+      const conn = new SshConnection(
+        createTarget({ host: 'cert.example.com', label: 'Cert host' }),
+        createCallbacks()
+      )
+
+      await expect(conn.connect()).rejects.toThrow('no matching host key format')
+      expect(spawnSystemSshCommandMock).not.toHaveBeenCalled()
+    } finally {
+      rmSync(storeDir, { recursive: true, force: true })
+    }
+  })
 })
 
 describe('shouldUseSystemSshTransport', () => {
diff --git a/src/main/ssh/connection/ssh-connection.ts b/src/main/ssh/connection/ssh-connection.ts
index d2f80faef26..1297f605894 100644
--- a/src/main/ssh/connection/ssh-connection.ts
+++ b/src/main/ssh/connection/ssh-connection.ts
@@ -95,6 +95,8 @@ export class SshConnection {
     generation: number
     sources: Promise<HostKeyTrustSources>
   } | null = null
+  /** Generation whose ssh2 attempt found an Orca-stored key for this endpoint. */
+  private orcaStorePinnedGeneration: number | null = null
   private systemSshControlMasterDisabledForSession = false
   private systemSshGssapiOnlyForSession = false
   private systemSshLoopbackRoute: SystemSshLoopbackRoute | null = null
@@ -510,6 +512,7 @@ export class SshConnection {
       connect: (config) => this.doSsh2Connect(config, connectGeneration),
       respawnProxy: (config, proxy) => this.respawnProxy(config, proxy, connectGeneration),
       killProxyIfCurrent: () => this.killProxyIfCurrent(connectGeneration),
+      isPinnedByOrcaHostKeyStore: () => this.orcaStorePinnedGeneration === connectGeneration,
       probeSystemSsh: (resolved, gssapiOnly) =>
         this.doSystemSshProbeWithControlMasterRetry(connectGeneration, resolved, gssapiOnly),
       resetSystemTransportIfCurrent: () => this.resetSystemTransportIfCurrent(connectGeneration),
@@ -742,6 +745,9 @@ export class SshConnection {
         this.hostKeyFingerprint = fingerprint
       }
     })
+    if (verdict.pinnedByOrcaStore && isCurrentAttempt()) {
+      this.orcaStorePinnedGeneration = connectGeneration
+    }
     return runSsh2Handshake(config, {
       label: this.target.label,
       pendingClients: this.pendingSsh2Clients,
diff --git a/src/main/ssh/connection/ssh2-credential-fallback.ts b/src/main/ssh/connection/ssh2-credential-fallback.ts
index 4e154e571bb..732317a5fa7 100644
--- a/src/main/ssh/connection/ssh2-credential-fallback.ts
+++ b/src/main/ssh/connection/ssh2-credential-fallback.ts
@@ -9,6 +9,7 @@ import {
   isAgentFallbackError,
   isGssapiSystemSshFallbackCandidate,
   isPassphraseError,
+  isSsh2AlgorithmNegotiationError,
   isSystemSshFallbackError
 } from '../ssh-reconnect-error-classification'
 import { isHostKeyVerificationError } from '../ssh-host-key-decision'
@@ -44,6 +45,8 @@ export type Ssh2CredentialFallbackHost = {
   respawnProxy: (config: ConnectConfig, proxy: EffectiveProxy) => void
   killProxyIfCurrent: () => void
   probeSystemSsh: (resolved: SshResolvedConfig | null, gssapiOnly?: boolean) => Promise<void>
+  /** System OpenSSH ignores Orca's store, so a host pinned only there must stay on ssh2. */
+  isPinnedByOrcaHostKeyStore: () => boolean
   resetSystemTransportIfCurrent: () => void
   requestCredential: (kind: SshCredentialKind, detail: string) => Promise<string | null | undefined>
 }
@@ -111,6 +114,19 @@ export async function recoverFailedSsh2Attempt(
     }
   }
 
+  // A pinned host that suddenly shares no algorithm could be an impostor steering Orca away from its pin.
+  if (isSsh2AlgorithmNegotiationError(err) && !host.isPinnedByOrcaHostKeyStore()) {
+    host.killProxyIfCurrent()
+    try {
+      await host.probeSystemSsh(resolved)
+      return
+    } catch (probeErr) {
+      host.resetSystemTransportIfCurrent()
+      // ssh2 can never reach this host, so the system SSH failure is the one the user can act on.
+      throw probeErr
+    }
+  }
+
   let authError = err
   let passphrasePromptHandled = false
   let credentialRetryConfig = config
diff --git a/src/main/ssh/connection/ssh2-host-key-verification.ts b/src/main/ssh/connection/ssh2-host-key-verification.ts
index bafffa84902..ca692155776 100644
--- a/src/main/ssh/connection/ssh2-host-key-verification.ts
+++ b/src/main/ssh/connection/ssh2-host-key-verification.ts
@@ -76,7 +76,11 @@ type Ssh2HostKeyVerificationHost = {
 }
 
 /** The denial this attempt's verifier recorded; local to one attempt so a stale one cannot leak. */
-type Ssh2HostKeyVerdict = { rejection: HostKeyVerificationError | null }
+type Ssh2HostKeyVerdict = {
+  rejection: HostKeyVerificationError | null
+  /** Orca's own store holds a key for this endpoint, which system OpenSSH never consults. */
+  pinnedByOrcaStore: boolean
+}
 
 /** Installs the host verifier and key-type order on `config` before ssh2 dials. */
 export async function installSsh2HostKeyVerification(
@@ -97,6 +101,11 @@ export async function installSsh2HostKeyVerification(
     await host.loadTrustSources(sshGSuppressedSiteConfig)
   const siteConfigSuppressed = siteMayRestrict
   const knownHostsEntries = knownHostsEvidence.entries
+  const storedKeyTypes = storedKeyTypesForEndpoint(
+    trustedHostKeys,
+    hostKeyLookupHost,
+    config.port ?? 22
+  )
   // If ssh2's own algorithm list is unreadable, keep its defaults rather than guessing a reorder.
   const serverHostKeyOrder = DEFAULT_SERVER_HOST_KEY_ALGORITHMS
     ? orderServerHostKeyAlgorithms(
@@ -104,12 +113,15 @@ export async function installSsh2HostKeyVerification(
         hostKeyLookupHost,
         config.port ?? 22,
         DEFAULT_SERVER_HOST_KEY_ALGORITHMS,
-        storedKeyTypesForEndpoint(trustedHostKeys, hostKeyLookupHost, config.port ?? 22),
+        storedKeyTypes,
         isHostKeyAlias
       )
     : undefined
   // Keep denial attempt-local and retain original ssh2 error codes so stale attempts and transient failures stay distinguishable.
-  const verdict: Ssh2HostKeyVerdict = { rejection: null }
+  const verdict: Ssh2HostKeyVerdict = {
+    rejection: null,
+    pinnedByOrcaStore: storedKeyTypes.length > 0
+  }
 
   // Accepted server fingerprints isolate shared-home relay locks across hosts; preserve mixed-version format and leave hostHash unset.
   config.hostVerifier = createHostKeyVerifier({
diff --git a/src/main/ssh/ssh-reconnect-error-classification.ts b/src/main/ssh/ssh-reconnect-error-classification.ts
index fce31189dd0..f7cf0b76950 100644
--- a/src/main/ssh/ssh-reconnect-error-classification.ts
+++ b/src/main/ssh/ssh-reconnect-error-classification.ts
@@ -70,6 +70,11 @@ export function isSystemSshFallbackError(err: Error): boolean {
   return err.message.includes('EHOSTUNREACH') || err.message.includes('ENETUNREACH')
 }
 
+/** ssh2 supports fewer algorithms than OpenSSH (no host certificates, for one), so no shared algorithm is a client gap. */
+export function isSsh2AlgorithmNegotiationError(err: Error): boolean {
+  return err.message.startsWith('Handshake failed: no matching ')
+}
+
 // ssh2 lacks GSSAPI; use system SSH for config-enabled Kerberos only after auth-shaped failures, preserving network retry semantics.
 export function isGssapiSystemSshFallbackCandidate(
   err: Error,