diff --git a/src/main/orcad/native-host-abi.test.ts b/src/main/orcad/native-host-abi.test.ts
index 44f469f0faa..dfa2f7d6a7e 100644
--- a/src/main/orcad/native-host-abi.test.ts
+++ b/src/main/orcad/native-host-abi.test.ts
@@ -6,6 +6,8 @@ import {
   GLIBC_FLOOR,
   isBelowGlibcFloor,
   nativeSlotName,
+  parseIncompatibleArchitecture,
+  parseMissingSharedLibrary,
   parseNodeAbiMismatch,
   parseUnmetGlibcVersion
 } from './native-host-abi'
@@ -97,6 +99,37 @@ describe('loader error parsing', () => {
       )
     ).toEqual({ built: '115', host: '127' })
   })
+
+  it('names both architectures on mach-o, and admits ELF names none', () => {
+    expect(
+      parseIncompatibleArchitecture(
+        "dlopen(/opt/pty.node, 0x0001): tried: '/opt/pty.node' (mach-o file, but is an incompatible architecture (have 'arm64', need 'x86_64'))"
+      )
+    ).toEqual({ built: 'arm64', host: 'x86_64' })
+    // The ELF loader refuses without saying what it found, so the verdict stands but the
+    // numbers do not exist to report.
+    expect(parseIncompatibleArchitecture('invalid ELF header')).toEqual({
+      built: null,
+      host: null
+    })
+    expect(parseIncompatibleArchitecture('wrong ELF class: ELFCLASS32')).not.toBeNull()
+    // A wrong-libc binary is not a wrong-arch binary; conflating them sends the operator
+    // to rebuild for an architecture that was never wrong.
+    expect(parseIncompatibleArchitecture("version `GLIBC_2.34' not found")).toBeNull()
+  })
+
+  it('names the shared object the loader could not open, on either loader', () => {
+    expect(
+      parseMissingSharedLibrary(
+        'libstdc++.so.6: cannot open shared object file: No such file or directory'
+      )
+    ).toBe('libstdc++.so.6')
+    expect(parseMissingSharedLibrary('Library not loaded: /usr/local/lib/libfoo.dylib')).toBe(
+      '/usr/local/lib/libfoo.dylib'
+    )
+    // A symbol version that is absent is a rebuild, not an install: must not match here.
+    expect(parseMissingSharedLibrary("/lib/libc.so.6: version `GLIBC_2.34' not found")).toBeNull()
+  })
 })
 
 describe('detectNativeHostAbi', () => {
diff --git a/src/main/orcad/native-host-abi.ts b/src/main/orcad/native-host-abi.ts
index 2890bb07a15..2335ad3274b 100644
--- a/src/main/orcad/native-host-abi.ts
+++ b/src/main/orcad/native-host-abi.ts
@@ -121,3 +121,40 @@ export function parseNodeAbiMismatch(loaderError: string): { built: string; host
   const match = loaderError.match(/NODE_MODULE_VERSION\s+(\d+)\D+NODE_MODULE_VERSION\s+(\d+)/)
   return match ? { built: match[1], host: match[2] } : null
 }
+
+/**
+ * The architecture the loader refused, e.g. `incompatible architecture (have 'arm64',
+ * need 'x86_64')` -> { built: 'arm64', host: 'x86_64' }. ELF hosts name no architecture
+ * (`invalid ELF header`, `wrong ELF class: ELFCLASS32`), so both sides read null there —
+ * the verdict still holds, only the numbers are missing.
+ */
+export function parseIncompatibleArchitecture(
+  loaderError: string
+): { built: string | null; host: string | null } | null {
+  const machO = loaderError.match(
+    /incompatible architecture \(have '?([\w.]+)'?,?\s*need '?([\w.]+)'?/
+  )
+  if (machO) {
+    return { built: machO[1], host: machO[2] }
+  }
+  if (/invalid ELF header|wrong ELF class|Exec format error|ELFCLASS(?:32|64)/.test(loaderError)) {
+    return { built: null, host: null }
+  }
+  return null
+}
+
+/**
+ * The shared object the loader could not find, e.g.
+ * `libstdc++.so.6: cannot open shared object file` -> 'libstdc++.so.6'.
+ *
+ * Kept apart from the glibc floor deliberately: a missing library can be installed,
+ * whereas a symbol version that does not exist can only be fixed by rebuilding.
+ */
+export function parseMissingSharedLibrary(loaderError: string): string | null {
+  const elf = loaderError.match(/([\w.+-]+\.so[\w.]*): cannot open shared object file/)
+  if (elf) {
+    return elf[1]
+  }
+  const machO = loaderError.match(/Library not loaded:\s*(\S+)/)
+  return machO ? machO[1] : null
+}
diff --git a/src/main/orcad/node-pty-loader-diagnosis.ts b/src/main/orcad/node-pty-loader-diagnosis.ts
new file mode 100644
index 00000000000..8024a8e0ecb
--- /dev/null
+++ b/src/main/orcad/node-pty-loader-diagnosis.ts
@@ -0,0 +1,85 @@
+/**
+ * Read a dynamic-loader message and name the one thing that has to change.
+ *
+ * Pure on purpose: every shape that matters here belongs to a host we are not — Alpine,
+ * Ubuntu 20.04, an arm64 box handed an x64 binary — so the classification has to be
+ * testable from a machine that cannot reproduce any of them.
+ *
+ * Shared by the two places a node-pty load can fail: `orcad`'s boot precondition
+ * (out of process, before anything requires node-pty) and the SSH relay's spawn path.
+ */
+import type { RuntimeTerminalUnavailableReason } from '../../shared/runtime-types'
+import {
+  parseIncompatibleArchitecture,
+  parseMissingSharedLibrary,
+  parseNodeAbiMismatch,
+  parseUnmetGlibcVersion
+} from './native-host-abi'
+
+export type NodePtyLoadCause = {
+  reason: RuntimeTerminalUnavailableReason
+  /** Short human phrase naming the actual values found, not a remedy. */
+  detail: string
+}
+
+/**
+ * node-pty's own loader walks several directories and rethrows only the LAST failure,
+ * wrapped in this sentence. The tail is therefore the `prebuilds/<platform>-<arch>`
+ * miss — `Cannot find module` — even when the real failure was the dynamic loader
+ * refusing `build/Release/pty.node`. Anything acting on that tail sends the operator to
+ * install a module that is already installed.
+ */
+export function isFlattenedNodePtyLoaderMessage(message: string): boolean {
+  return /Failed to load native module: (?:conpty|pty)\.node(?:,|:|$)/.test(message)
+}
+
+/** The real cause a flattened message still carries, when the last attempt was the telling one. */
+export function classifyNodePtyLoaderMessage(message: string): NodePtyLoadCause {
+  const abiMismatch = parseNodeAbiMismatch(message)
+  if (abiMismatch) {
+    return {
+      reason: 'abi_mismatch',
+      detail: `built for Node ABI ${abiMismatch.built}, this host runs ABI ${abiMismatch.host}`
+    }
+  }
+  const unmetGlibc = parseUnmetGlibcVersion(message)
+  if (unmetGlibc) {
+    return { reason: 'libc_floor', detail: `the binary requires GLIBC_${unmetGlibc}` }
+  }
+  const unmetCxx = message.match(/((?:GLIBCXX_|CXXABI_)[0-9.]+)'? not found/)
+  if (unmetCxx) {
+    return { reason: 'libc_floor', detail: `the binary requires ${unmetCxx[1]}` }
+  }
+  const arch = parseIncompatibleArchitecture(message)
+  if (arch) {
+    return {
+      reason: 'arch_mismatch',
+      detail:
+        arch.built && arch.host
+          ? `built for ${arch.built}, this host needs ${arch.host}`
+          : `the loader rejected the binary's format (${firstErrorLine(message)})`
+    }
+  }
+  const missingLibrary = parseMissingSharedLibrary(message)
+  if (missingLibrary) {
+    return {
+      reason: 'shared_library_missing',
+      detail: `${missingLibrary} is not installed on this host`
+    }
+  }
+  if (/MODULE_NOT_FOUND|Cannot find module/.test(message)) {
+    return { reason: 'dependency_missing', detail: firstErrorLine(message) }
+  }
+  return { reason: 'load_failed', detail: firstErrorLine(message) }
+}
+
+/**
+ * Why not simply the first non-empty line: when a child dies without catching, node
+ * prints the offending source line and a caret before the error, so line one is the
+ * script rather than the diagnosis. Prefer the first line that reads as an error.
+ */
+export function firstErrorLine(text: string): string {
+  const lines = text.split('\n').filter((candidate) => candidate.trim().length > 0)
+  const errorLine = lines.find((candidate) => /^[A-Za-z]*(Error|Exception):/.test(candidate.trim()))
+  return (errorLine ?? lines[0] ?? text).trim().slice(0, 400)
+}
diff --git a/src/main/orcad/node-pty-precondition.ts b/src/main/orcad/node-pty-precondition.ts
index 2857449c736..7d633bd0f9a 100644
--- a/src/main/orcad/node-pty-precondition.ts
+++ b/src/main/orcad/node-pty-precondition.ts
@@ -25,13 +25,8 @@ import {
   parseBuildToolchainProbe,
   toolchainInstallHintLines
 } from '../ssh/build-toolchain-diagnosis'
-import {
-  detectNativeHostAbi,
-  nativeSlotName,
-  parseNodeAbiMismatch,
-  parseUnmetGlibcVersion,
-  type NativeHostAbi
-} from './native-host-abi'
+import { detectNativeHostAbi, nativeSlotName, type NativeHostAbi } from './native-host-abi'
+import { classifyNodePtyLoaderMessage, firstErrorLine } from './node-pty-loader-diagnosis'
 import { installPrebuiltSlot, type PrebuiltSlotOutcome } from './node-pty-prebuilt-slot'
 
 // Why every verdict travels on STDOUT: node echoes the whole `-e` source into stderr
@@ -72,21 +67,35 @@ export type NodePtyProbeFailure = {
 }
 
 /**
- * Read the child's exit into a cause. Pure, so every failure shape is testable from a
- * host that cannot reproduce it — the whole point, since the shapes that matter belong
- * to Alpine and Ubuntu 20.04.
+ * What the child actually reported, before any judgement is made about it.
+ *
+ * Split from the classification so callers that need the loader's own words — the relay,
+ * which quotes them back when nothing recognizes the shape — do not have to re-derive
+ * them from a formatted verdict.
  */
-export function classifyNodePtyProbeResult(
+export type NodePtyProbeOutcome =
+  | { kind: 'loaded'; loadedDir: string | null }
+  | { kind: 'noBinary' }
+  | { kind: 'loaderError'; message: string }
+  | { kind: 'signalled'; signal: NodeJS.Signals }
+  /** The probe never answered. Not evidence about node-pty either way. */
+  | { kind: 'unanswered'; detail: string }
+  /** It answered, but with nothing that names a cause. */
+  | { kind: 'unexplained'; detail: string }
+
+export function readNodePtyProbeOutcome(
   result: Pick<ProcessResult, 'code' | 'signal' | 'stdout' | 'stderr' | 'timedOut'>
-): NodePtyProbeFailure | null {
+): NodePtyProbeOutcome {
   const stdout = result.stdout
   if (result.code === 0 && stdout.includes(PROBE_OK_TOKEN)) {
-    return null
+    return {
+      kind: 'loaded',
+      loadedDir: stdout.split(PROBE_OK_TOKEN)[1]?.trim().split('\n')[0]?.trim() || null
+    }
   }
   if (result.timedOut) {
     return {
-      status: 'unverifiable',
-      reason: 'unknown',
+      kind: 'unanswered',
       detail: 'the node-pty load probe did not finish in time, so nothing was established'
     }
   }
@@ -94,27 +103,51 @@ export function classifyNodePtyProbeResult(
   // the loader never reaches the catch, and often prints nothing at all. That silence is
   // exactly the uncatchable case this probe is a separate process for.
   if (result.signal) {
-    return {
-      status: 'blocked',
-      reason: 'load_crashed',
-      detail: `the load probe was killed by ${result.signal}`
-    }
+    return { kind: 'signalled', signal: result.signal }
   }
   if (stdout.includes(NO_BINARY_TOKEN)) {
-    return {
-      status: 'blocked',
-      reason: 'dependency_missing',
-      detail: 'node-pty is installed but has no compiled binary for this platform'
-    }
+    return { kind: 'noBinary' }
   }
   const reported = readReportedLoadError(stdout)
   if (reported !== null) {
-    return classifyLoaderMessage(reported)
+    return { kind: 'loaderError', message: reported }
   }
   return {
-    status: 'blocked',
-    reason: 'load_failed',
-    detail: firstLine(result.stderr) || `the load probe exited with code ${result.code}`
+    kind: 'unexplained',
+    detail: firstErrorLine(result.stderr) || `the load probe exited with code ${result.code}`
+  }
+}
+
+/**
+ * Read the child's exit into a cause. Pure, so every failure shape is testable from a
+ * host that cannot reproduce it — the whole point, since the shapes that matter belong
+ * to Alpine and Ubuntu 20.04.
+ */
+export function classifyNodePtyProbeResult(
+  result: Pick<ProcessResult, 'code' | 'signal' | 'stdout' | 'stderr' | 'timedOut'>
+): NodePtyProbeFailure | null {
+  const outcome = readNodePtyProbeOutcome(result)
+  switch (outcome.kind) {
+    case 'loaded':
+      return null
+    case 'unanswered':
+      return { status: 'unverifiable', reason: 'unknown', detail: outcome.detail }
+    case 'signalled':
+      return {
+        status: 'blocked',
+        reason: 'load_crashed',
+        detail: `the load probe was killed by ${outcome.signal}`
+      }
+    case 'noBinary':
+      return {
+        status: 'blocked',
+        reason: 'dependency_missing',
+        detail: 'node-pty is installed but has no compiled binary for this platform'
+      }
+    case 'loaderError':
+      return classifyLoaderMessage(outcome.message)
+    case 'unexplained':
+      return { status: 'blocked', reason: 'load_failed', detail: outcome.detail }
   }
 }
 
@@ -133,40 +166,7 @@ function readReportedLoadError(stdout: string): string | null {
 
 /** Read a dynamic-loader message. Pure, so shapes this host cannot reproduce are testable. */
 export function classifyLoaderMessage(message: string): NodePtyProbeFailure {
-  const abiMismatch = parseNodeAbiMismatch(message)
-  if (abiMismatch) {
-    return {
-      status: 'blocked',
-      reason: 'abi_mismatch',
-      detail: `built for Node ABI ${abiMismatch.built}, this host runs ABI ${abiMismatch.host}`
-    }
-  }
-  const unmetGlibc = parseUnmetGlibcVersion(message)
-  if (unmetGlibc) {
-    return {
-      status: 'blocked',
-      reason: 'libc_floor',
-      detail: `the binary requires GLIBC_${unmetGlibc}`
-    }
-  }
-  if (/(GLIBCXX_|CXXABI_)[0-9.]+'? not found/.test(message)) {
-    return { status: 'blocked', reason: 'libc_floor', detail: firstLine(message) }
-  }
-  if (/MODULE_NOT_FOUND|Cannot find module/.test(message)) {
-    return { status: 'blocked', reason: 'dependency_missing', detail: firstLine(message) }
-  }
-  return { status: 'blocked', reason: 'load_failed', detail: firstLine(message) }
-}
-
-/**
- * Why not simply the first non-empty line: when the child dies without catching, node
- * prints the offending source line and a caret before the error, so line one is the
- * script rather than the diagnosis. Prefer the first line that reads as an error.
- */
-function firstLine(text: string): string {
-  const lines = text.split('\n').filter((candidate) => candidate.trim().length > 0)
-  const errorLine = lines.find((candidate) => /^[A-Za-z]*(Error|Exception):/.test(candidate.trim()))
-  return (errorLine ?? lines[0] ?? text).trim().slice(0, 400)
+  return { status: 'blocked', ...classifyNodePtyLoaderMessage(message) }
 }
 
 /**
@@ -305,7 +305,8 @@ export function checkNodePtyPrecondition(
   // Loaded. The remaining way terminals fail is spawn-time: node-pty posix_spawns
   // build/Release/spawn-helper, and a missing one turns every terminal.create into ENOENT
   // on a host that otherwise looks healthy. That is a degradation, not a boot blocker.
-  const loadedDir = result.stdout.split(PROBE_OK_TOKEN)[1]?.trim().split('\n')[0]?.trim()
+  const outcome = readNodePtyProbeOutcome(result)
+  const loadedDir = outcome.kind === 'loaded' ? outcome.loadedDir : null
   if (abi.platform !== 'win32') {
     const helper = join(loadedDir || join(nodePtyDir, 'build', 'Release'), 'spawn-helper')
     if (!isExecutableFile(helper)) {
diff --git a/src/relay/dispatcher-rpc-routing.ts b/src/relay/dispatcher-rpc-routing.ts
index c30d222ff3d..164a375e1c6 100644
--- a/src/relay/dispatcher-rpc-routing.ts
+++ b/src/relay/dispatcher-rpc-routing.ts
@@ -3,6 +3,10 @@ import {
   SKILL_INSTALL_RPC_ERROR_CODE,
   SkillInstallFailureSchema
 } from '../shared/skill-install-failure'
+import {
+  TERMINAL_UNAVAILABLE_RPC_ERROR_CODE,
+  TerminalUnavailableCauseSchema
+} from '../shared/terminal-unavailable-cause'
 import {
   RelayErrorCode,
   type JsonRpcNotification,
@@ -135,11 +139,16 @@ export abstract class RelayDispatcherRpcRouting extends RelayDispatcherFrameCode
       const message = err instanceof Error ? err.message : String(err)
       const errorCode = (err as { code?: unknown }).code
       const code = typeof errorCode === 'number' ? errorCode : -32000
-      const skillFailure =
+      // Why an allowlist keyed on the error code: error `data` is otherwise dropped, so a
+      // handler cannot leak internals by attaching them. Each published shape is validated
+      // against its own schema before it crosses.
+      const structured =
         errorCode === SKILL_INSTALL_RPC_ERROR_CODE
           ? SkillInstallFailureSchema.safeParse((err as { data?: unknown }).data)
-          : null
-      const data = skillFailure?.success === true ? skillFailure.data : undefined
+          : errorCode === TERMINAL_UNAVAILABLE_RPC_ERROR_CODE
+            ? TerminalUnavailableCauseSchema.safeParse((err as { data?: unknown }).data)
+            : null
+      const data = structured?.success === true ? structured.data : undefined
       const accepted = this.sendResponse(
         client,
         req.id,
diff --git a/src/relay/dispatcher-structured-error.test.ts b/src/relay/dispatcher-structured-error.test.ts
index 0ba8e164216..3ce0a79f19f 100644
--- a/src/relay/dispatcher-structured-error.test.ts
+++ b/src/relay/dispatcher-structured-error.test.ts
@@ -1,6 +1,10 @@
 import { afterEach, describe, expect, it, vi } from 'vitest'
 import { RelayDispatcher } from './dispatcher'
 import { encodeJsonRpcFrame, MessageType, type JsonRpcResponse } from './protocol'
+import {
+  TERMINAL_UNAVAILABLE_RPC_ERROR_CODE,
+  type TerminalUnavailableCause
+} from '../shared/terminal-unavailable-cause'
 
 function decodeResponse(frame: Buffer): JsonRpcResponse | null {
   if (frame[0] !== MessageType.Regular) {
@@ -52,4 +56,54 @@ describe('RelayDispatcher structured errors', () => {
       message: 'boom'
     })
   })
+
+  it('carries a terminal-unavailable cause across the wire, and rejects a malformed one', async () => {
+    // Why this must cross: the fault is proved on the relay at spawn time, and the only
+    // machinery that can repair it runs on the client. Prose cannot be acted on.
+    vi.useFakeTimers()
+    const written: Buffer[] = []
+    const dispatcher = new RelayDispatcher((data) => {
+      written.push(Buffer.from(data))
+    })
+    dispatchers.push(dispatcher)
+    const cause: TerminalUnavailableCause = {
+      status: 'blocked',
+      reason: 'abi_mismatch',
+      detail: 'built for Node ABI 127, this host runs ABI 115',
+      repairable: true,
+      host: {
+        platform: 'linux',
+        arch: 'x64',
+        libc: 'glibc',
+        glibcVersion: '2.31',
+        nodeAbi: '115',
+        nodeVersion: 'v20.11.0'
+      }
+    }
+    dispatcher.onRequest('pty.spawn', async () => {
+      throw Object.assign(new Error('Remote terminals are unavailable'), {
+        code: TERMINAL_UNAVAILABLE_RPC_ERROR_CODE,
+        data: cause
+      })
+    })
+    dispatcher.onRequest('pty.spawnBogus', async () => {
+      throw Object.assign(new Error('Remote terminals are unavailable'), {
+        code: TERMINAL_UNAVAILABLE_RPC_ERROR_CODE,
+        data: { status: 'blocked', repairable: true }
+      })
+    })
+
+    dispatcher.feed(encodeJsonRpcFrame({ jsonrpc: '2.0', id: 8, method: 'pty.spawn' }, 1, 0))
+    dispatcher.feed(encodeJsonRpcFrame({ jsonrpc: '2.0', id: 9, method: 'pty.spawnBogus' }, 2, 0))
+    await vi.advanceTimersByTimeAsync(0)
+
+    const responses = written.map(decodeResponse)
+    expect(responses.find((message) => message?.id === 8)?.error?.data).toEqual(cause)
+    // A cause that does not validate is dropped entirely; a half-read cause must never
+    // authorize a repair.
+    expect(responses.find((message) => message?.id === 9)?.error).toEqual({
+      code: -32000,
+      message: 'Remote terminals are unavailable'
+    })
+  })
 })
diff --git a/src/relay/node-pty-binding-survey.test.ts b/src/relay/node-pty-binding-survey.test.ts
new file mode 100644
index 00000000000..b51a57fd4a8
--- /dev/null
+++ b/src/relay/node-pty-binding-survey.test.ts
@@ -0,0 +1,138 @@
+import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import process from 'node:process'
+import { afterEach, describe, expect, it } from 'vitest'
+import { isFlattenedNodePtyLoaderMessage } from '../main/orcad/node-pty-loader-diagnosis'
+import {
+  collectNodePtyUnavailableDiagnosis,
+  readNodeGypBuildRecord,
+  surveyNodePtyBinding
+} from './node-pty-binding-survey'
+import { formatNodePtyUnavailableMessage } from './node-pty-unavailable-diagnosis'
+
+const HOST = { platform: process.platform, arch: process.arch }
+/** What node-pty throws once its loader has replaced the real cause with its last miss. */
+const FLATTENED =
+  'Failed to load native module: pty.node, checked: build/Release, build/Debug, ' +
+  `prebuilds/${process.platform}-${process.arch}: Error: Cannot find module './pty.node'`
+
+const roots: string[] = []
+
+function fixture(options: { binding?: boolean; configGypi?: string } = {}): string {
+  const root = mkdtempSync(join(tmpdir(), 'orca-node-pty-'))
+  roots.push(root)
+  const dir = join(root, 'node-pty')
+  mkdirSync(join(dir, 'lib'), { recursive: true })
+  writeFileSync(join(dir, 'lib', 'index.js'), 'module.exports = {}\n')
+  writeFileSync(join(dir, 'lib', 'utils.js'), 'exports.loadNativeModule = () => ({})\n')
+  if (options.binding) {
+    mkdirSync(join(dir, 'build', 'Release'), { recursive: true })
+    // Deliberately not a valid addon: the point is to make the dynamic loader talk.
+    for (const name of ['pty.node', 'conpty.node']) {
+      writeFileSync(join(dir, 'build', 'Release', name), 'not an addon\n')
+    }
+  }
+  if (options.configGypi !== undefined) {
+    mkdirSync(join(dir, 'build'), { recursive: true })
+    writeFileSync(join(dir, 'build', 'config.gypi'), options.configGypi)
+  }
+  return dir
+}
+
+afterEach(() => {
+  while (roots.length > 0) {
+    rmSync(roots.pop()!, { recursive: true, force: true })
+  }
+})
+
+describe('surveyNodePtyBinding', () => {
+  it('finds the file node-pty itself would open, and lists where it looked when there is none', () => {
+    const withBinding = surveyNodePtyBinding(fixture({ binding: true }), HOST)
+    expect(withBinding?.bindingPath).toMatch(/build[/\\]Release[/\\](con)?pty\.node$/)
+
+    const without = surveyNodePtyBinding(fixture(), HOST)
+    expect(without?.bindingPath).toBeNull()
+    expect(without?.searched).toHaveLength(3)
+    expect(without?.searched.join(' ')).toContain(`prebuilds/${process.platform}-${process.arch}`)
+  })
+})
+
+describe('readNodeGypBuildRecord', () => {
+  it('reads what node-gyp configured for, past its leading comment lines', () => {
+    const dir = fixture({
+      configGypi:
+        '# Do not edit. File was generated by node-gyp\'s "configure" step\n' +
+        '{ "variables": { "node_module_version": 127, "target_arch": "arm64" } }\n'
+    })
+    expect(readNodeGypBuildRecord(dir)).toEqual({ nodeAbi: '127', arch: 'arm64' })
+  })
+
+  it('answers nothing rather than guessing when there is no build record', () => {
+    expect(readNodeGypBuildRecord(fixture())).toEqual({ nodeAbi: null, arch: null })
+  })
+})
+
+describe('collectNodePtyUnavailableDiagnosis', () => {
+  it("recovers the dynamic loader's own words that node-pty threw away", async () => {
+    // The whole defect in one assertion: what reaches the relay is FLATTENED, which names
+    // no cause; the diagnosis must carry what the loader actually said about the file.
+    const diagnosis = await collectNodePtyUnavailableDiagnosis({
+      nodePtyDir: fixture({ binding: true }),
+      error: new Error(FLATTENED)
+    })
+    expect(diagnosis.status).toBe('blocked')
+    expect(diagnosis.rawError).toBeTruthy()
+    expect(isFlattenedNodePtyLoaderMessage(diagnosis.rawError!)).toBe(false)
+    expect(diagnosis.rawError).not.toBe(FLATTENED)
+    expect(diagnosis.rawError).toMatch(/pty\.node/)
+  }, 20_000)
+
+  it("prefers node-gyp's build record over a loader message that named no fault", async () => {
+    const diagnosis = await collectNodePtyUnavailableDiagnosis({
+      nodePtyDir: fixture({
+        binding: true,
+        configGypi: '{ "variables": { "node_module_version": 4242, "target_arch": "x64" } }'
+      }),
+      error: new Error(FLATTENED)
+    })
+    // A garbage binary reads differently per platform (mach-o vs ELF), so only assert the
+    // build record is consulted when the loader message did not name the fault itself.
+    if (diagnosis.reason === 'abi_mismatch') {
+      expect(formatNodePtyUnavailableMessage(diagnosis)).toContain(
+        `built for Node ABI 4242, this host runs ABI ${process.versions.modules}`
+      )
+    } else {
+      expect(['arch_mismatch', 'load_failed', 'load_crashed']).toContain(diagnosis.reason)
+    }
+  }, 20_000)
+
+  it('reports an unlocatable install as unverifiable, not as a diagnosis', async () => {
+    const diagnosis = await collectNodePtyUnavailableDiagnosis({
+      nodePtyDir: null,
+      error: new Error(FLATTENED)
+    })
+    expect(diagnosis.status).toBe('unverifiable')
+    const text = formatNodePtyUnavailableMessage(diagnosis)
+    expect(text).toContain('could not establish why')
+    // It still has to be reportable: the raw error is the only thing an issue can quote.
+    expect(text).toContain(FLATTENED)
+  })
+
+  it('probes the host toolchain only when nothing was compiled', async () => {
+    const diagnosis = await collectNodePtyUnavailableDiagnosis({
+      nodePtyDir: fixture(),
+      error: new Error(FLATTENED)
+    })
+    expect(diagnosis.survey?.bindingPath).toBeNull()
+    expect(['toolchain_missing', 'dependency_missing']).toContain(diagnosis.reason)
+    // Non-Linux hosts ship a node-pty prebuild, so a toolchain answer there would be noise.
+    expect(diagnosis.toolchain === null).toBe(process.platform !== 'linux')
+
+    const compiled = await collectNodePtyUnavailableDiagnosis({
+      nodePtyDir: fixture({ binding: true }),
+      error: new Error(FLATTENED)
+    })
+    expect(compiled.toolchain).toBeNull()
+  }, 20_000)
+})
diff --git a/src/relay/node-pty-binding-survey.ts b/src/relay/node-pty-binding-survey.ts
new file mode 100644
index 00000000000..805527f1aad
--- /dev/null
+++ b/src/relay/node-pty-binding-survey.ts
@@ -0,0 +1,215 @@
+/**
+ * Gather the evidence a node-pty spawn failure needs, on the host that failed.
+ *
+ * Three sources, because no single one is sufficient:
+ *
+ *  1. What is on disk where node-pty's loader looks, and what node-gyp recorded it was
+ *     configured for (`build/config.gypi`). This answers "wrong ABI / wrong arch" even
+ *     when the loader said nothing useful, and it is the only source available when the
+ *     binding is absent entirely.
+ *  2. The dynamic loader's own words, recovered by dlopen'ing the file node-pty would
+ *     have opened. node-pty's loader rethrows only its LAST attempt, so the real message
+ *     is otherwise destroyed before the relay sees it. This runs in a CHILD process: a
+ *     binding that aborts inside the loader would take the relay down with it, and a
+ *     relay that dies is a reconnect loop rather than an error message.
+ *  3. The host's C/C++ toolchain, but only when nothing was compiled — "install
+ *     build-essential" is the right answer for a compile that never ran, and noise for a
+ *     binary that exists and is simply wrong.
+ *
+ * Every step is best-effort and failure-tolerant: whatever cannot be established is
+ * reported as unestablished rather than guessed (docs/reference/ssh-execution-boundary.md).
+ */
+import { existsSync, readFileSync } from 'node:fs'
+import { join } from 'node:path'
+import { release } from 'node:os'
+import process from 'node:process'
+import { runProcess } from '../shared/child-process/run-process'
+import {
+  buildToolchainProbeCommand,
+  parseBuildToolchainProbe,
+  type BuildToolchainStatus
+} from '../main/ssh/build-toolchain-diagnosis'
+import { detectNativeHostAbi } from '../main/orcad/native-host-abi'
+import {
+  buildNodePtyLoadProbeScript,
+  readNodePtyProbeOutcome
+} from '../main/orcad/node-pty-precondition'
+import {
+  diagnoseNodePtyUnavailable,
+  type NodePtyBindingSurvey,
+  type NodePtyDiagnosisInput,
+  type NodePtyUnavailableDiagnosis,
+  type NodePtyUnavailableHost
+} from './node-pty-unavailable-diagnosis'
+
+/** Bounded so a wedged loader delays one spawn rejection, not the relay. */
+const LOAD_PROBE_TIMEOUT_MS = 10_000
+const TOOLCHAIN_PROBE_TIMEOUT_MS = 5_000
+
+/** node-pty's own search order, so the file surveyed is the file it would have opened. */
+function bindingSearchDirs(platform: NodeJS.Platform, arch: string): string[] {
+  return ['build/Release', 'build/Debug', `prebuilds/${platform}-${arch}`]
+}
+
+/** Windows defers to conpty.node on builds that have ConPTY, exactly as node-pty picks it. */
+function bindingBaseName(platform: NodeJS.Platform): string {
+  if (platform !== 'win32') {
+    return 'pty'
+  }
+  return Number(release().split('.')[2]) >= 18309 ? 'conpty' : 'pty'
+}
+
+export function surveyNodePtyBinding(
+  nodePtyDir: string,
+  host: Pick<NodePtyUnavailableHost, 'platform' | 'arch'>
+): NodePtyBindingSurvey | null {
+  const name = bindingBaseName(host.platform)
+  const searched = bindingSearchDirs(host.platform, host.arch)
+  let bindingPath: string | null = null
+  try {
+    for (const dir of searched) {
+      for (const root of [nodePtyDir, join(nodePtyDir, 'lib')]) {
+        const candidate = join(root, dir, `${name}.node`)
+        if (existsSync(candidate)) {
+          bindingPath = candidate
+          break
+        }
+      }
+      if (bindingPath) {
+        break
+      }
+    }
+  } catch {
+    return null
+  }
+  const built = readNodeGypBuildRecord(nodePtyDir)
+  return {
+    moduleDir: nodePtyDir,
+    bindingPath,
+    searched,
+    builtNodeAbi: built.nodeAbi,
+    builtArch: built.arch
+  }
+}
+
+/**
+ * What node-gyp configured this build for.
+ *
+ * Why this file and not the binary: `build/config.gypi` is written by `node-gyp
+ * configure` from the headers it downloaded, so it names the ABI and architecture the
+ * `.node` was compiled against without parsing ELF. It survives a build that later
+ * failed, which is the case where the loader has nothing to say.
+ */
+export function readNodeGypBuildRecord(nodePtyDir: string): {
+  nodeAbi: string | null
+  arch: string | null
+} {
+  try {
+    const raw = readFileSync(join(nodePtyDir, 'build', 'config.gypi'), 'utf8')
+    // node-gyp prefixes the JSON with `# Do not edit…` comment lines.
+    const body = raw
+      .split('\n')
+      .filter((line) => !line.trim().startsWith('#'))
+      .join('\n')
+    const variables = (JSON.parse(body) as { variables?: Record<string, unknown> }).variables
+    const nodeAbi = variables?.node_module_version
+    const arch = variables?.target_arch
+    return {
+      nodeAbi: nodeAbi === undefined || nodeAbi === null ? null : String(nodeAbi),
+      arch: typeof arch === 'string' && arch.length > 0 ? arch : null
+    }
+  } catch {
+    return { nodeAbi: null, arch: null }
+  }
+}
+
+/**
+ * The loader's verdict on the binding, recovered out of process.
+ *
+ * Returns the pieces `diagnoseNodePtyUnavailable` reads; a probe that could not run
+ * answers `unverifiableBecause` rather than a cause, because it established nothing.
+ */
+async function probeNodePtyLoader(
+  nodePtyDir: string
+): Promise<Pick<NodePtyDiagnosisInput, 'loaderError' | 'probeSignal' | 'unverifiableBecause'>> {
+  let result
+  try {
+    result = await runProcess({
+      program: process.execPath,
+      args: ['-e', buildNodePtyLoadProbeScript(nodePtyDir)],
+      timeoutMs: LOAD_PROBE_TIMEOUT_MS
+    })
+  } catch (error) {
+    return {
+      unverifiableBecause: `the node-pty load probe could not be started (${(error as Error).message})`
+    }
+  }
+  const outcome = readNodePtyProbeOutcome(result)
+  switch (outcome.kind) {
+    case 'loaderError':
+      return { loaderError: outcome.message }
+    case 'signalled':
+      return { probeSignal: outcome.signal }
+    case 'unanswered':
+      return { unverifiableBecause: outcome.detail }
+    // `loaded` here means the binding is fine under plain Node while the relay's own
+    // require failed — real, and not something the loader can explain. `noBinary` and
+    // `unexplained` are both better answered by the on-disk survey than by the probe.
+    case 'loaded':
+    case 'noBinary':
+    case 'unexplained':
+      return {}
+  }
+}
+
+/** node-pty has no Linux prebuild, so only there does a missing toolchain explain anything. */
+async function probeRelayBuildToolchain(
+  platform: NodeJS.Platform
+): Promise<BuildToolchainStatus | null> {
+  if (platform !== 'linux') {
+    return null
+  }
+  try {
+    const result = await runProcess({
+      program: '/bin/sh',
+      args: ['-c', buildToolchainProbeCommand()],
+      timeoutMs: TOOLCHAIN_PROBE_TIMEOUT_MS
+    })
+    return result.timedOut ? null : parseBuildToolchainProbe(result.stdout)
+  } catch {
+    return null
+  }
+}
+
+function readErrorMessage(error: unknown): string | null {
+  if (error instanceof Error) {
+    return error.message
+  }
+  return typeof error === 'string' && error.length > 0 ? error : null
+}
+
+/**
+ * Everything above, in the order that makes each step's cost conditional on the previous
+ * one's answer. Called only on the failure path, so a spawn that works pays nothing.
+ */
+export async function collectNodePtyUnavailableDiagnosis(options: {
+  nodePtyDir: string | null
+  error?: unknown
+}): Promise<NodePtyUnavailableDiagnosis> {
+  const abi = detectNativeHostAbi()
+  const host: NodePtyUnavailableHost = { ...abi, nodeVersion: process.version }
+  const requireError = readErrorMessage(options.error)
+  if (!options.nodePtyDir) {
+    return diagnoseNodePtyUnavailable({
+      host,
+      survey: null,
+      requireError,
+      unverifiableBecause: 'the relay could not locate its node-pty install directory'
+    })
+  }
+  const survey = surveyNodePtyBinding(options.nodePtyDir, host)
+  const probed = survey?.bindingPath ? await probeNodePtyLoader(options.nodePtyDir) : {}
+  const toolchain =
+    survey && !survey.bindingPath ? await probeRelayBuildToolchain(host.platform) : null
+  return diagnoseNodePtyUnavailable({ ...probed, host, survey, requireError, toolchain })
+}
diff --git a/src/relay/node-pty-unavailable-diagnosis.test.ts b/src/relay/node-pty-unavailable-diagnosis.test.ts
new file mode 100644
index 00000000000..7bed6ef256e
--- /dev/null
+++ b/src/relay/node-pty-unavailable-diagnosis.test.ts
@@ -0,0 +1,223 @@
+import { describe, expect, it } from 'vitest'
+import {
+  diagnoseNodePtyUnavailable,
+  formatNodePtyUnavailableMessage,
+  type NodePtyBindingSurvey,
+  type NodePtyDiagnosisInput,
+  toTerminalUnavailableCause,
+  type NodePtyUnavailableHost
+} from './node-pty-unavailable-diagnosis'
+import { parseBuildToolchainProbe } from '../main/ssh/build-toolchain-diagnosis'
+import {
+  mayRepairFromCause,
+  parseTerminalUnavailableCause
+} from '../shared/terminal-unavailable-cause'
+
+const UBUNTU_2004: NodePtyUnavailableHost = {
+  platform: 'linux',
+  arch: 'x64',
+  libc: 'glibc',
+  glibcVersion: '2.31',
+  nodeAbi: '115',
+  nodeVersion: 'v20.11.0'
+}
+
+const MODULE_DIR = '/opt/orca/relay/node_modules/node-pty'
+const SEARCHED = ['build/Release', 'build/Debug', 'prebuilds/linux-x64']
+
+const INSTALLED: NodePtyBindingSurvey = {
+  moduleDir: MODULE_DIR,
+  bindingPath: `${MODULE_DIR}/build/Release/pty.node`,
+  searched: SEARCHED,
+  builtNodeAbi: null,
+  builtArch: null
+}
+
+const NOTHING_INSTALLED: NodePtyBindingSurvey = { ...INSTALLED, bindingPath: null }
+
+/** What node-pty itself throws: the real cause replaced by its LAST directory miss. */
+const FLATTENED =
+  'Failed to load native module: pty.node, checked: build/Release, build/Debug, ' +
+  "prebuilds/linux-x64: Error: Cannot find module '../prebuilds/linux-x64//pty.node'"
+
+const diagnose = (overrides: Partial<NodePtyDiagnosisInput> = {}) =>
+  diagnoseNodePtyUnavailable({
+    host: UBUNTU_2004,
+    survey: INSTALLED,
+    requireError: FLATTENED,
+    ...overrides
+  })
+
+const message = (overrides: Partial<NodePtyDiagnosisInput> = {}) =>
+  formatNodePtyUnavailableMessage(diagnose(overrides))
+
+const toolchain = (present: readonly string[]) =>
+  parseBuildToolchainProbe([...present.map((tool) => `HAVE ${tool}`), 'PKG apt-get'].join('\n'))
+
+describe('diagnoseNodePtyUnavailable', () => {
+  it("never treats node-pty's flattened wrapper as the cause", () => {
+    // node-pty rethrows only its last directory miss, so acting on that text sends the
+    // user to install a module that is already installed.
+    expect(
+      diagnose({ survey: NOTHING_INSTALLED, toolchain: toolchain(['make', 'g++', 'python3']) })
+    ).toMatchObject({ reason: 'dependency_missing' })
+    expect(diagnose().reason).not.toBe('dependency_missing')
+  })
+
+  it('names the glibc the host actually has next to the one the binary needs', () => {
+    const verdict = diagnose({
+      loaderError:
+        "/lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by /opt/orca/node_modules/node-pty/build/Release/pty.node)"
+    })
+    expect(verdict).toMatchObject({ status: 'blocked', reason: 'libc_floor' })
+    const text = formatNodePtyUnavailableMessage(verdict)
+    expect(text).toContain('GLIBC_2.34')
+    expect(text).toContain('glibc 2.31')
+    // The remedy is a rebuild; offering "install build-essential" here is a wrong answer.
+    expect(text).not.toContain('build tools')
+  })
+
+  it('names both ABI numbers from the loader message', () => {
+    const text = message({
+      loaderError:
+        'The module was compiled against a different Node.js version using NODE_MODULE_VERSION 115. ' +
+        'This version of Node.js requires NODE_MODULE_VERSION 127.'
+    })
+    expect(text).toContain('built for Node ABI 115, this host runs ABI 127')
+    expect(text).toContain('v20.11.0')
+  })
+
+  it("reads the ABI mismatch off node-gyp's build record when the loader said nothing", () => {
+    // The case the old message could only hedge about: the binding is present, node-pty
+    // destroyed the loader error, and the only evidence left is what node-gyp configured.
+    const text = message({
+      survey: { ...INSTALLED, builtNodeAbi: '127' }
+    })
+    expect(text).toContain('built for Node ABI 127, this host runs ABI 115')
+  })
+
+  it('separates an architecture mismatch from an ABI mismatch', () => {
+    expect(diagnose({ loaderError: 'invalid ELF header' }).reason).toBe('arch_mismatch')
+    expect(message({ survey: { ...INSTALLED, builtArch: 'arm64' } })).toContain(
+      'built for arm64, this host runs x64'
+    )
+    expect(
+      message({
+        loaderError:
+          "dlopen(/opt/pty.node, 0x0001): tried: '/opt/pty.node' (mach-o file, but is an " +
+          "incompatible architecture (have 'arm64', need 'x86_64'))"
+      })
+    ).toContain('built for arm64, this host needs x86_64')
+  })
+
+  it('separates a missing shared library from a libc floor break', () => {
+    // Different remedies: install a package, versus rebuild against an older toolchain.
+    const verdict = diagnose({
+      loaderError: 'libstdc++.so.6: cannot open shared object file: No such file or directory'
+    })
+    expect(verdict.reason).toBe('shared_library_missing')
+    const text = formatNodePtyUnavailableMessage(verdict)
+    expect(text).toContain('libstdc++.so.6 is not installed on this host')
+    expect(text).toContain('Install that library')
+  })
+
+  it('offers the build-tools remedy only when it probed the toolchain and found it missing', () => {
+    const missing = diagnose({
+      survey: NOTHING_INSTALLED,
+      toolchain: toolchain(['python3'])
+    })
+    expect(missing.reason).toBe('toolchain_missing')
+    const text = formatNodePtyUnavailableMessage(missing)
+    expect(text).toContain('make and a C++ compiler are not installed')
+    expect(text).toContain(`checked ${SEARCHED.join(', ')} under ${MODULE_DIR}`)
+    expect(text).toContain('sudo apt-get install -y build-essential python3')
+
+    // Toolchain present and nothing compiled: the install failed for another reason, and
+    // "install make/g++/python3" would send the user chasing tools they already have.
+    const present = diagnose({
+      survey: NOTHING_INSTALLED,
+      toolchain: toolchain(['make', 'g++', 'python3'])
+    })
+    expect(present.reason).toBe('dependency_missing')
+    expect(formatNodePtyUnavailableMessage(present)).not.toContain('apt-get')
+  })
+
+  it('reports a binding that killed the probe as a crash rather than a miss', () => {
+    const text = message({ probeSignal: 'SIGSEGV' })
+    expect(text).toContain('SIGSEGV')
+    expect(text).toContain('incompatible with this host rather than missing')
+  })
+
+  it('quotes the loader verbatim when nothing recognizes it', () => {
+    const raw = 'dlopen(/opt/pty.node): unexpected relocation kind 0x9f'
+    const verdict = diagnose({ loaderError: raw })
+    expect(verdict).toMatchObject({ status: 'blocked', reason: 'load_failed', rawError: raw })
+    const text = formatNodePtyUnavailableMessage(verdict)
+    expect(text).toContain(`Loader error: ${raw}`)
+    expect(text).toContain('file an issue')
+  })
+
+  it('reports a probe that never answered as unverifiable and diagnoses nothing', () => {
+    // docs/reference/ssh-execution-boundary.md: loss of contact is not a verdict.
+    const verdict = diagnose({
+      unverifiableBecause: 'the node-pty load probe did not finish in time'
+    })
+    expect(verdict.status).toBe('unverifiable')
+    const text = formatNodePtyUnavailableMessage(verdict)
+    expect(text).toContain('could not establish why')
+    expect(text).toContain('not evidence node-pty is broken')
+    expect(text).not.toContain('Reconnect to rebuild')
+    expect(text).not.toContain('apt-get')
+  })
+
+  it('marks rebuildable faults repairable and everything else not', () => {
+    // The relay's node-pty is compiled ON the remote, so a binding that no longer matches
+    // the machine is fixed by recompiling there. A missing compiler or a missing library
+    // is not: the rebuild would need the very thing that is absent.
+    const repairable = (overrides: Partial<NodePtyDiagnosisInput>) =>
+      toTerminalUnavailableCause(diagnose(overrides)).repairable
+
+    expect(repairable({ survey: { ...INSTALLED, builtNodeAbi: '127' } })).toBe(true)
+    expect(repairable({ survey: { ...INSTALLED, builtArch: 'arm64' } })).toBe(true)
+    expect(repairable({ loaderError: "version `GLIBC_2.34' not found" })).toBe(true)
+    expect(repairable({ probeSignal: 'SIGSEGV' })).toBe(true)
+    expect(
+      repairable({ survey: NOTHING_INSTALLED, toolchain: toolchain(['make', 'g++', 'python3']) })
+    ).toBe(true)
+
+    expect(repairable({ survey: NOTHING_INSTALLED, toolchain: toolchain(['python3']) })).toBe(false)
+    expect(repairable({ loaderError: 'libstdc++.so.6: cannot open shared object file' })).toBe(
+      false
+    )
+    // Nothing was established, so nothing may be rewritten on the host (#14830).
+    expect(repairable({ unverifiableBecause: 'probe timed out' })).toBe(false)
+  })
+
+  it('publishes a cause that survives its own wire schema', () => {
+    const cause = toTerminalUnavailableCause(
+      diagnose({ loaderError: "version `GLIBC_2.34' not found" })
+    )
+    expect(parseTerminalUnavailableCause(cause)).toEqual(cause)
+    expect(mayRepairFromCause(cause)).toBe(true)
+    expect(cause.host).toMatchObject({ arch: 'x64', nodeAbi: '115', glibcVersion: '2.31' })
+
+    // A peer claiming repairable on an unverifiable status must not be believed.
+    expect(mayRepairFromCause({ ...cause, status: 'unverifiable' })).toBe(false)
+    expect(parseTerminalUnavailableCause({ ...cause, host: undefined })).toBeNull()
+    // A reason this client has never heard of must not discard the whole cause; the
+    // relay may name faults added after the client shipped.
+    expect(parseTerminalUnavailableCause({ ...cause, reason: 'invented_later' })).not.toBeNull()
+  })
+
+  it('puts the host on every message so a bug report needs no follow-up question', () => {
+    for (const overrides of [
+      {},
+      { loaderError: 'invalid ELF header' },
+      { unverifiableBecause: 'probe timed out' }
+    ]) {
+      expect(message(overrides)).toContain(
+        'linux/x64, glibc 2.31, Node v20.11.0 (ABI 115), prebuild slot linux-x64-glibc'
+      )
+    }
+  })
+})
diff --git a/src/relay/node-pty-unavailable-diagnosis.ts b/src/relay/node-pty-unavailable-diagnosis.ts
new file mode 100644
index 00000000000..590eedcc375
--- /dev/null
+++ b/src/relay/node-pty-unavailable-diagnosis.ts
@@ -0,0 +1,362 @@
+/**
+ * Why the remote host cannot spawn terminals, in terms the user can act on and check.
+ *
+ * The relay used to answer this with one hedged paragraph — "install build tools, or
+ * else reconnect, or else check your Node version" — because the only thing it looked at
+ * was that `require('node-pty')` threw. That paragraph names three different remedies for
+ * four different faults and lets the user verify none of them.
+ *
+ * node-pty's own loader is why the raw cause went missing: it walks build/Release,
+ * build/Debug and prebuilds/<platform>-<arch>, then rethrows only the LAST error. So a
+ * `pty.node` the dynamic loader refused arrives as `Cannot find module '../prebuilds/…'`,
+ * and the GLIBC/ABI/arch sentence that actually says what is wrong is discarded before
+ * the relay ever sees it. Recovering it needs a separate dlopen of the file the loader
+ * would have opened — see node-pty-binding-survey.ts.
+ *
+ * Everything here is pure so every verdict is testable from a host that is none of the
+ * hosts that break. `unverifiable` is a first-class outcome: a probe that did not answer
+ * is not a diagnosis (docs/reference/ssh-execution-boundary.md).
+ */
+import { GLIBC_FLOOR, nativeSlotName, type NativeHostAbi } from '../main/orcad/native-host-abi'
+import {
+  classifyNodePtyLoaderMessage,
+  isFlattenedNodePtyLoaderMessage
+} from '../main/orcad/node-pty-loader-diagnosis'
+import {
+  toolchainInstallHintLines,
+  type BuildToolchainStatus
+} from '../main/ssh/build-toolchain-diagnosis'
+import type { RuntimeTerminalUnavailableReason } from '../shared/runtime-types'
+import type { TerminalUnavailableCause } from '../shared/terminal-unavailable-cause'
+
+/** What is actually on disk where node-pty's loader looks, and what it was built for. */
+export type NodePtyBindingSurvey = {
+  /** The node-pty install the relay would load from. */
+  moduleDir: string
+  /** The compiled binding the loader would open, or null when no directory holds one. */
+  bindingPath: string | null
+  /** Directories checked, so "nothing is installed" is a statement with evidence. */
+  searched: string[]
+  /** `node_module_version` from node-gyp's build/config.gypi, when it is readable. */
+  builtNodeAbi: string | null
+  /** `target_arch` from node-gyp's build/config.gypi, when it is readable. */
+  builtArch: string | null
+}
+
+export type NodePtyUnavailableHost = NativeHostAbi & { nodeVersion: string }
+
+export type NodePtyUnavailableDiagnosis = {
+  /** `blocked` — proved. `unverifiable` — nothing answered, which is not evidence. */
+  status: 'blocked' | 'unverifiable'
+  reason: RuntimeTerminalUnavailableReason
+  host: NodePtyUnavailableHost
+  /** Short phrase naming the values found. */
+  detail: string
+  /** The loader's own words, kept verbatim so an unclassified verdict is still reportable. */
+  rawError: string | null
+  survey: NodePtyBindingSurvey | null
+  toolchain: BuildToolchainStatus | null
+}
+
+export type NodePtyDiagnosisInput = {
+  /** What the recovered dlopen said, when one ran. Preferred over `requireError`. */
+  loaderError?: string | null
+  /** What `require('node-pty')`/`pty.spawn` threw. Usually flattened by node-pty. */
+  requireError?: string | null
+  /** A load probe that was killed rather than answering. */
+  probeSignal?: NodeJS.Signals | null
+  /** Set when the load probe never answered at all; forces `unverifiable`. */
+  unverifiableBecause?: string | null
+  host: NodePtyUnavailableHost
+  survey: NodePtyBindingSurvey | null
+  toolchain?: BuildToolchainStatus | null
+}
+
+/** Reasons a loader message can establish on its own, and which nothing else outranks. */
+const LOADER_NAMED_FAULTS: ReadonlySet<RuntimeTerminalUnavailableReason> = new Set([
+  'abi_mismatch',
+  'arch_mismatch',
+  'libc_floor',
+  'shared_library_missing'
+])
+
+export function diagnoseNodePtyUnavailable(
+  input: NodePtyDiagnosisInput
+): NodePtyUnavailableDiagnosis {
+  const { host, survey } = input
+  const toolchain = input.toolchain ?? null
+  // Why the require error is only a fallback: node-pty flattens the real cause away, so
+  // its text is evidence of "did not load", never of why.
+  const usableRequireError =
+    input.requireError && !isFlattenedNodePtyLoaderMessage(input.requireError)
+      ? input.requireError
+      : null
+  // Capped because a macOS dlopen error lists every path it tried; the message quotes this
+  // verbatim when nothing classifies it, and a toast is not a log file.
+  const rawError = truncate(input.loaderError ?? usableRequireError ?? input.requireError ?? null)
+  const base = { host, rawError, survey, toolchain } as const
+
+  if (input.unverifiableBecause) {
+    return {
+      ...base,
+      status: 'unverifiable',
+      reason: 'unknown',
+      detail: input.unverifiableBecause
+    }
+  }
+  // Before anything the loader said: a binary that aborts inside the loader never reaches
+  // a catch and often prints nothing, so the signal is the only evidence there is.
+  if (input.probeSignal) {
+    return {
+      ...base,
+      status: 'blocked',
+      reason: 'load_crashed',
+      detail: `loading the binding killed the probe with ${input.probeSignal}`
+    }
+  }
+
+  const classifiable = input.loaderError ?? usableRequireError
+  const classified = classifiable ? classifyNodePtyLoaderMessage(classifiable) : null
+  // Only a loader message that named the fault outranks the build record. `load_failed`
+  // and `dependency_missing` do not: the first named nothing, and the second is what
+  // node-pty says about a binding it never reached.
+  if (classified && LOADER_NAMED_FAULTS.has(classified.reason)) {
+    return { ...base, status: 'blocked', ...classified }
+  }
+
+  // The loader said nothing usable. The binding's own build record still can: node-gyp
+  // records the ABI and arch it configured for, and either differing from this runtime is
+  // a fault the user can check without reproducing the load.
+  if (survey?.bindingPath) {
+    if (survey.builtNodeAbi && survey.builtNodeAbi !== host.nodeAbi) {
+      return {
+        ...base,
+        status: 'blocked',
+        reason: 'abi_mismatch',
+        detail: `built for Node ABI ${survey.builtNodeAbi}, this host runs ABI ${host.nodeAbi}`
+      }
+    }
+    if (survey.builtArch && survey.builtArch !== host.arch) {
+      return {
+        ...base,
+        status: 'blocked',
+        reason: 'arch_mismatch',
+        detail: `built for ${survey.builtArch}, this host runs ${host.arch}`
+      }
+    }
+    return {
+      ...base,
+      status: 'blocked',
+      reason: classified?.reason ?? 'load_failed',
+      detail: classified?.detail ?? 'the binding is present but the loader refused it'
+    }
+  }
+
+  if (!survey) {
+    return {
+      ...base,
+      status: 'unverifiable',
+      reason: 'unknown',
+      detail: "the relay could not read node-pty's install directory"
+    }
+  }
+  // Nothing compiled anywhere. On Linux that is either a compile that never ran for want
+  // of a toolchain, or an install that failed for some other reason — different remedies.
+  if (toolchain?.toolchainMissing) {
+    return {
+      ...base,
+      status: 'blocked',
+      reason: 'toolchain_missing',
+      detail: `no compiled binding exists and ${missingToolSummary(toolchain)} missing`
+    }
+  }
+  return {
+    ...base,
+    status: 'blocked',
+    reason: 'dependency_missing',
+    detail: 'no compiled node-pty binding exists on this host'
+  }
+}
+
+const RAW_ERROR_MAX = 600
+
+function truncate(message: string | null): string | null {
+  if (message === null || message.length <= RAW_ERROR_MAX) {
+    return message
+  }
+  return `${message.slice(0, RAW_ERROR_MAX)}…`
+}
+
+function missingToolSummary(toolchain: BuildToolchainStatus): string {
+  const present = new Set(toolchain.present)
+  const missing: string[] = []
+  if (!present.has('make')) {
+    missing.push('make')
+  }
+  if (!present.has('g++') && !present.has('c++') && !present.has('clang++')) {
+    missing.push('a C++ compiler')
+  }
+  if (!present.has('python3') && !present.has('python')) {
+    missing.push('python3')
+  }
+  if (missing.length <= 1) {
+    return `${missing[0] ?? 'the build tools'} is`
+  }
+  return `${missing.slice(0, -1).join(', ')} and ${missing.at(-1)} are`
+}
+
+/**
+ * Faults a rebuild on the host actually fixes.
+ *
+ * The relay's node-pty is compiled ON the remote by `npm install`, so a binding that is
+ * absent, built for another Node ABI, built for another architecture, or linked against a
+ * newer libc than the host provides is all one thing: the compiled artifact no longer
+ * matches the machine, and recompiling here produces one that does. That is different
+ * from the packaged desktop app, where the binary is built elsewhere and the glibc floor
+ * in docs/reference/linux-glibc-compatibility.md is the binding constraint.
+ *
+ * Excluded on purpose: `toolchain_missing` (no compiler to rebuild with) and
+ * `shared_library_missing` (the compile would need the same absent library).
+ */
+const REBUILD_FIXES: ReadonlySet<RuntimeTerminalUnavailableReason> = new Set([
+  'abi_mismatch',
+  'arch_mismatch',
+  'libc_floor',
+  'load_crashed',
+  'dependency_missing'
+])
+
+/**
+ * The machine-readable cause, for a client that can act instead of printing.
+ *
+ * `repairable` requires a proved status AND a toolchain that is not known-missing: a
+ * rebuild the host cannot perform is not a repair, it is a wasted `npm install` — which
+ * is the shape of #14830.
+ */
+export function toTerminalUnavailableCause(
+  diagnosis: NodePtyUnavailableDiagnosis
+): TerminalUnavailableCause {
+  const { host } = diagnosis
+  return {
+    status: diagnosis.status,
+    reason: diagnosis.reason,
+    detail: diagnosis.detail.slice(0, 400),
+    repairable:
+      diagnosis.status === 'blocked' &&
+      REBUILD_FIXES.has(diagnosis.reason) &&
+      diagnosis.toolchain?.toolchainMissing !== true,
+    host: {
+      platform: host.platform,
+      arch: host.arch,
+      libc: host.libc,
+      ...(host.glibcVersion ? { glibcVersion: host.glibcVersion } : {}),
+      nodeAbi: host.nodeAbi,
+      nodeVersion: host.nodeVersion
+    },
+    ...(diagnosis.rawError ? { rawError: diagnosis.rawError.slice(0, 1000) } : {})
+  }
+}
+
+/** `linux/x64, glibc 2.31, Node v20.11.0 (ABI 115), prebuild slot linux-x64-glibc`. */
+function formatNodePtyHostLine(host: NodePtyUnavailableHost): string {
+  const libc =
+    host.libc === 'none' ? null : `${host.libc}${host.glibcVersion ? ` ${host.glibcVersion}` : ''}`
+  return [
+    `${host.platform}/${host.arch}`,
+    libc,
+    `Node ${host.nodeVersion} (ABI ${host.nodeAbi})`,
+    `prebuild slot ${nativeSlotName(host)}`
+  ]
+    .filter((part): part is string => part !== null)
+    .join(', ')
+}
+
+/**
+ * One remedy per fault, each naming a value the user can go and check.
+ *
+ * `unverifiable` deliberately prescribes nothing: the relay proved only that it could not
+ * establish a cause, and dressing that up as a diagnosis is the bug this replaces.
+ */
+export function formatNodePtyUnavailableMessage(diagnosis: NodePtyUnavailableDiagnosis): string {
+  const { host } = diagnosis
+  // Unverifiable deliberately prescribes nothing beyond a retry: nothing was established,
+  // and dressing that up as a diagnosis is the bug this replaces.
+  const opening =
+    diagnosis.status === 'unverifiable'
+      ? `Remote terminals are unavailable, and the relay could not establish why: ${diagnosis.detail}. ` +
+        `That is not evidence node-pty is broken — reconnect to retry.`
+      : `Remote terminals are unavailable: ${remedyFor(diagnosis)}`
+  const lines = [opening, `Host: ${formatNodePtyHostLine(host)}.`]
+  // Quoted only where nothing else named the fault: elsewhere the remedy already carries
+  // the numbers, and a dlopen dump would bury them.
+  const quoteRaw =
+    diagnosis.status === 'unverifiable' ||
+    diagnosis.reason === 'load_failed' ||
+    diagnosis.reason === 'unknown'
+  if (diagnosis.rawError && quoteRaw) {
+    lines.push(`Loader error: ${diagnosis.rawError}`)
+  }
+  return lines.join('\n')
+}
+
+function remedyFor(diagnosis: NodePtyUnavailableDiagnosis): string {
+  const { host, survey, toolchain } = diagnosis
+  switch (diagnosis.reason) {
+    case 'toolchain_missing':
+      return (
+        `node-pty ships no prebuilt binary for Linux and this host has no compiled one ` +
+        `(${searchedPhrase(survey)}), because ${toolchain ? missingToolSummary(toolchain) : 'the build tools are'} not installed. ` +
+        `Install them on the remote host, then reconnect:\n` +
+        `${(toolchain ? toolchainInstallHintLines(toolchain) : []).join('\n')}`
+      )
+    case 'dependency_missing':
+      return (
+        `node-pty has no compiled binary on this host (${searchedPhrase(survey)}). ` +
+        `The C/C++ build tools needed to compile it are present, so reconnect to reinstall ` +
+        `the relay's native modules.`
+      )
+    case 'abi_mismatch':
+      return (
+        `the installed node-pty binding was built for a different Node ABI than the remote's ` +
+        `Node — ${diagnosis.detail}. Reconnect to rebuild node-pty against ${host.nodeVersion}, ` +
+        `or run the relay on the Node version the binding was built for.`
+      )
+    case 'arch_mismatch':
+      return (
+        `the installed node-pty binding does not match this host's CPU architecture — ` +
+        `${diagnosis.detail}. Reconnect to rebuild node-pty on the remote host; a binding ` +
+        `copied from a machine of another architecture can never load here.`
+      )
+    case 'libc_floor':
+      return (
+        `${diagnosis.detail}, which this host's C library does not provide ` +
+        `(${host.glibcVersion ? `glibc ${host.glibcVersion}` : 'this host reports no glibc version'}). ` +
+        `The binding was compiled on a newer system than this one. Reconnect to rebuild ` +
+        `node-pty here; Orca's own Linux floor is glibc ${GLIBC_FLOOR}.`
+      )
+    case 'shared_library_missing':
+      return (
+        `node-pty's native binding cannot be opened because ${diagnosis.detail}. ` +
+        `Install that library on the remote host, then reconnect.`
+      )
+    case 'load_crashed':
+      return (
+        `${diagnosis.detail}, which means the binding is incompatible with this host rather ` +
+        `than missing. Reconnect to rebuild the relay's native modules.`
+      )
+    case 'load_failed':
+    case 'spawn_helper_missing':
+    case 'unknown':
+      return (
+        `this host refused to load node-pty's native binding and the cause was not recognized. ` +
+        `Reconnect to rebuild the relay's native modules; if that does not help, please file an ` +
+        `issue quoting the loader error below.`
+      )
+  }
+}
+
+function searchedPhrase(survey: NodePtyBindingSurvey | null): string {
+  return survey && survey.searched.length > 0
+    ? `checked ${survey.searched.join(', ')} under ${survey.moduleDir}`
+    : 'nothing was found where node-pty looks'
+}
diff --git a/src/relay/pty-handler-spawn-admission.test.ts b/src/relay/pty-handler-spawn-admission.test.ts
index 2df29b95420..728f883d3b1 100644
--- a/src/relay/pty-handler-spawn-admission.test.ts
+++ b/src/relay/pty-handler-spawn-admission.test.ts
@@ -32,7 +32,7 @@ vi.mock('../main/shell-prompt-readiness-probe', () => ({
   createShellPromptReadinessProbe: mockCreateShellPromptReadinessProbe
 }))
 
-import { MAX_RELAY_PTY_SESSIONS, PtyHandler, formatNodePtyUnavailableMessage } from './pty-handler'
+import { MAX_RELAY_PTY_SESSIONS, PtyHandler } from './pty-handler'
 import type { RelayDispatcher } from './dispatcher'
 import {
   beginPtyHandlerTest,
@@ -222,24 +222,6 @@ describe('PtyHandler', () => {
     expect(mockPtySpawn).toHaveBeenCalledOnce()
   })
 
-  it('hedges both causes on Linux and offers the build-tools remedy nowhere else', () => {
-    const linux = formatNodePtyUnavailableMessage('linux')
-    expect(linux).toContain('Remote terminals are unavailable')
-    // Conditional, not asserted: a host with build-essential can still hit an ABI/Node-version flip.
-    expect(linux).toMatch(/If it is missing the C\/C\+\+ build tools/)
-    expect(linux).toContain('python3')
-    expect(linux).toContain('version and architecture match the installed binding')
-
-    // Windows/macOS ship node-pty prebuilds, so "install make/g++/python3" sends the user chasing nothing.
-    for (const platform of ['win32', 'darwin'] as const) {
-      const message = formatNodePtyUnavailableMessage(platform)
-      expect(message).toContain('Remote terminals are unavailable')
-      expect(message).not.toContain('build tools')
-      expect(message).not.toContain('python3')
-      expect(message).toMatch(/reconnect/i)
-    }
-  })
-
   it('normalizes a missing native binding as degraded node-pty availability', async () => {
     mockPtySpawn.mockImplementationOnce(() => {
       throw new Error(
@@ -253,6 +235,30 @@ describe('PtyHandler', () => {
     expect(handler.activePtyCount).toBe(0)
   })
 
+  it('keeps the load error it was handed instead of replacing it with guesses', async () => {
+    // #17830: the user got three remedies for four possible faults and could verify none.
+    // The relay must carry what it was actually told, and must not prescribe a toolchain
+    // install it never probed for.
+    const thrown =
+      'Failed to load native module: conpty.node, checked: build/Release, prebuilds/win32-x64'
+    mockPtySpawn.mockImplementationOnce(() => {
+      throw new Error(thrown)
+    })
+
+    const message = await dispatcher.callRequest('pty.spawn', {}).then(
+      () => '',
+      (error: Error) => error.message
+    )
+
+    expect(message).toContain(thrown)
+    expect(message).not.toContain('install make, a C++ compiler, and python3')
+    // Nothing here established a cause — the relay's node-pty directory is not on disk in
+    // this harness — so per docs/reference/ssh-execution-boundary.md it must say so rather
+    // than pick a diagnosis. Every message still names the host, for the bug report.
+    expect(message).toContain('could not establish why')
+    expect(message).toMatch(/Host: linux\/\w+, .*Node v[\d.]+ \(ABI \d+\)/)
+  })
+
   it('preserves unrelated node-pty spawn failures', async () => {
     mockPtySpawn.mockImplementationOnce(() => {
       throw new Error('File not found: missing-shell.exe')
diff --git a/src/relay/pty-handler.ts b/src/relay/pty-handler.ts
index 82f0b19b9ff..f8bd2351cf2 100644
--- a/src/relay/pty-handler.ts
+++ b/src/relay/pty-handler.ts
@@ -102,23 +102,16 @@ import {
   injectRelayFishHistoryEnv,
   injectRelayHistoryEnv
 } from './terminal-history'
-
-// Why: only Linux compiles node-pty (no prebuilt), so the build-tools remedy is a closable setup gap
-// there and wrong advice anywhere node-pty ships one. The relay only sees an unloadable binding, never
-// why — a skipped compile and a later Node/ABI flip look identical here — so Linux hedges both causes.
-export function formatNodePtyUnavailableMessage(platform: NodeJS.Platform): string {
-  const remedy =
-    platform === 'linux'
-      ? "node-pty's native binding is not loadable on this host. If it is missing the C/C++ build tools needed to compile node-pty, install make, a C++ compiler, and python3 on the remote host, then reconnect. Otherwise reconnect to reinstall the relay's native modules, and check that the remote Node.js version and architecture match the installed binding."
-      : "node-pty's native binding failed to load on this host. Reconnect to reinstall the relay's native modules; if it persists, check that the remote Node.js version and architecture match the installed binding."
-  return `Remote terminals are unavailable: ${remedy}`
-}
+import { isFlattenedNodePtyLoaderMessage } from '../main/orcad/node-pty-loader-diagnosis'
+import { collectNodePtyUnavailableDiagnosis } from './node-pty-binding-survey'
+import {
+  formatNodePtyUnavailableMessage,
+  toTerminalUnavailableCause
+} from './node-pty-unavailable-diagnosis'
+import { TERMINAL_UNAVAILABLE_RPC_ERROR_CODE } from '../shared/terminal-unavailable-cause'
 
 function isMissingNodePtyNativeBinding(error: unknown): boolean {
-  return (
-    error instanceof Error &&
-    /Failed to load native module: (?:conpty|pty)\.node(?:,|$)/.test(error.message)
-  )
+  return error instanceof Error && isFlattenedNodePtyLoaderMessage(error.message)
 }
 
 function parseSourceRecoveryRequest(value: unknown): PtySourceRecoveryRequest | undefined {
@@ -474,6 +467,8 @@ export class PtyHandler {
   private ptyModule: typeof NodePty | null = null
   private ptyModuleLoadPromise: Promise<typeof NodePty | null> | null = null
   private reloadPtyModuleFromDisk = false
+  /** The last thing `require('node-pty')` threw, kept because it is the only cause anyone has. */
+  private lastPtyLoadError: unknown = null
   // Why: single optional slot is intentional — callers compose externally; a throw is swallowed so it can't block cleanup.
   private exitListener: PtyExitListener | null = null
   private surfaceRetiredListener: PtySurfaceRetiredListener | null = null
@@ -547,27 +542,56 @@ export class PtyHandler {
       try {
         this.ptyModule = await import('node-pty')
         return this.ptyModule
-      } catch {
+      } catch (error) {
+        // Why keep it: this is the only place the load error exists. Discarding it here is
+        // what left the relay able to say "unavailable" and never why.
+        this.lastPtyLoadError = error
         this.reloadPtyModuleFromDisk = true
       }
     }
     // Why: tie module resolution to the deployed bundle dir, not cwd.
-    const moduleEntry = join(__dirname, 'node_modules', 'node-pty', 'lib', 'index.js')
+    const moduleEntry = join(this.relayNodePtyDir(), 'lib', 'index.js')
     if (!existsSync(moduleEntry)) {
+      this.lastPtyLoadError = this.lastPtyLoadError ?? new Error(`no node-pty at ${moduleEntry}`)
       return null
     }
     try {
       this.ptyModule = require(moduleEntry) as typeof NodePty
       return this.ptyModule
-    } catch {
+    } catch (error) {
+      this.lastPtyLoadError = error
       return null
     }
   }
 
+  /** Where the relay's own node-pty lives — the deployed bundle dir, never cwd. */
+  private relayNodePtyDir(): string {
+    return join(__dirname, 'node_modules', 'node-pty')
+  }
+
+  /**
+   * The rejection for a spawn that cannot happen: prose for a human, and the structured
+   * cause for a client that can repair the host instead of printing a paragraph.
+   *
+   * Runs the survey and out-of-process load probe only here, on the failure path, so a
+   * healthy relay never pays for them.
+   */
+  private async nodePtyUnavailableError(spawnError?: unknown): Promise<Error> {
+    const nodePtyDir = this.relayNodePtyDir()
+    const diagnosis = await collectNodePtyUnavailableDiagnosis({
+      nodePtyDir: existsSync(nodePtyDir) ? nodePtyDir : null,
+      error: spawnError ?? this.lastPtyLoadError
+    })
+    return Object.assign(new Error(formatNodePtyUnavailableMessage(diagnosis)), {
+      code: TERMINAL_UNAVAILABLE_RPC_ERROR_CODE,
+      data: toTerminalUnavailableCause(diagnosis)
+    })
+  }
+
   private invalidatePtyModuleAfterBindingFailure(): void {
     this.ptyModule = null
     this.reloadPtyModuleFromDisk = true
-    const moduleRoot = join(__dirname, 'node_modules', 'node-pty')
+    const moduleRoot = this.relayNodePtyDir()
     for (const cachedPath of Object.keys(require.cache)) {
       if (isPathInsideOrEqual(moduleRoot, cachedPath)) {
         delete require.cache[cachedPath]
@@ -1718,7 +1742,7 @@ export class PtyHandler {
   }> {
     const pty = await this.loadPty()
     if (!pty) {
-      throw new Error(formatNodePtyUnavailableMessage(process.platform))
+      throw await this.nodePtyUnavailableError()
     }
 
     const cols = (params.cols as number) || 80
@@ -1831,7 +1855,7 @@ export class PtyHandler {
       // Why: Windows loads conpty.node only on first spawn, so handle that late binding failure here.
       if (isMissingNodePtyNativeBinding(error)) {
         this.invalidatePtyModuleAfterBindingFailure()
-        throw new Error(formatNodePtyUnavailableMessage(process.platform))
+        throw await this.nodePtyUnavailableError(error)
       }
       throw error
     }
diff --git a/src/shared/runtime-capability-degradation.ts b/src/shared/runtime-capability-degradation.ts
index 05620ac9942..977cd68737f 100644
--- a/src/shared/runtime-capability-degradation.ts
+++ b/src/shared/runtime-capability-degradation.ts
@@ -20,8 +20,11 @@ export type RuntimeBrowserUnavailableReason =
  */
 export type RuntimeTerminalUnavailableReason =
   | 'dependency_missing'
+  | 'toolchain_missing'
   | 'libc_floor'
+  | 'shared_library_missing'
   | 'abi_mismatch'
+  | 'arch_mismatch'
   | 'load_failed'
   | 'load_crashed'
   | 'spawn_helper_missing'
@@ -43,10 +46,16 @@ export type RuntimeDegradation = {
 const TERMINAL_UNAVAILABLE_MESSAGES: Record<RuntimeTerminalUnavailableReason, string> = {
   dependency_missing:
     'Terminals are unavailable on this host: node-pty has no native binary for this platform. Install or rebuild it, or deploy a build that ships a prebuilt binary for this platform.',
+  toolchain_missing:
+    'Terminals are unavailable on this host: node-pty has no prebuilt binary for Linux and this host is missing the C/C++ build tools needed to compile one. Install them, then reconnect.',
   libc_floor:
     "This host's node-pty binary was built against a newer C library than the host provides, so the dynamic loader refuses it. Rebuild node-pty on this host, or deploy a build whose prebuilt binary matches this platform's libc.",
+  shared_library_missing:
+    'Terminals are unavailable on this host: a shared library that node-pty links against is not installed, so the dynamic loader cannot open the binary. Install the named library, then reconnect.',
   abi_mismatch:
     "This host's node-pty binary was built for a different Node ABI than the running Node, so it cannot be loaded. Rebuild node-pty against this Node version.",
+  arch_mismatch:
+    "This host's node-pty binary was built for a different CPU architecture than the running Node, so the dynamic loader refuses it. Rebuild node-pty on this host, or deploy a build for this architecture.",
   load_failed: 'Terminals are unavailable on this host: node-pty failed to load.',
   load_crashed:
     'Terminals are unavailable on this host: loading node-pty terminated the probe process, which means the binary is incompatible with this host rather than merely missing.',
diff --git a/src/shared/terminal-unavailable-cause.ts b/src/shared/terminal-unavailable-cause.ts
new file mode 100644
index 00000000000..2671941da30
--- /dev/null
+++ b/src/shared/terminal-unavailable-cause.ts
@@ -0,0 +1,73 @@
+/**
+ * The machine-readable half of "remote terminals are unavailable".
+ *
+ * Why this exists: the fault is proved on the relay, at spawn time, and the machinery
+ * that can repair it (`repairInstalledNativeDeps`) lives on the client, at connect time.
+ * Until now the only thing that crossed the wire was prose, so the client could not tell
+ * a rebuildable ABI flip from a host whose glibc will never satisfy the binary — and the
+ * message had to hedge across all of them.
+ *
+ * Wire compatibility (docs/reference/remote-wire-compatibility.md): this rides as the
+ * optional `data` of an existing JSON-RPC error, so it is Rule 1 — additive. A client
+ * that does not read it still renders `error.message`, which is exactly today's
+ * behaviour, so no capability negotiation is needed.
+ *
+ * `repairable` is the field with teeth: it is true only for a fault that was PROVED and
+ * that rebuilding node-pty on the host actually fixes. An `unverifiable` cause is never
+ * repairable — a probe that did not answer must not trigger a destructive repair, which
+ * is the #14830 lesson recorded in docs/reference/ssh-execution-boundary.md.
+ */
+import { z } from 'zod'
+import { TERMINAL_UNAVAILABLE_ERROR_CODE } from './runtime-capability-degradation'
+
+export const TERMINAL_UNAVAILABLE_RPC_ERROR_CODE = TERMINAL_UNAVAILABLE_ERROR_CODE
+
+const TerminalUnavailableHostSchema = z
+  .object({
+    platform: z.string().min(1).max(32),
+    arch: z.string().min(1).max(32),
+    libc: z.enum(['glibc', 'musl', 'none']),
+    /** Absent, not null, when the host reports no version — see native-host-abi.ts. */
+    glibcVersion: z.string().min(1).max(32).optional(),
+    /** `NODE_MODULE_VERSION` the remote runtime accepts. */
+    nodeAbi: z.string().min(1).max(16),
+    nodeVersion: z.string().min(1).max(32)
+  })
+  .strict()
+
+export const TerminalUnavailableCauseSchema = z
+  .object({
+    /** `blocked` — proved. `unverifiable` — nothing answered; never act on it. */
+    status: z.enum(['blocked', 'unverifiable']),
+    /**
+     * Open vocabulary, deliberately `string` rather than an enum: a newer relay may name a
+     * reason this client has never heard of, and a strict enum would drop the whole cause
+     * (including `repairable`) rather than the one field it cannot interpret.
+     */
+    reason: z.string().min(1).max(64),
+    detail: z.string().max(400),
+    /** Proved, and rebuilding node-pty on the host is the fix. */
+    repairable: z.boolean(),
+    host: TerminalUnavailableHostSchema,
+    /** The dynamic loader's own words, when they were recovered. */
+    rawError: z.string().max(1000).optional()
+  })
+  .strict()
+
+export type TerminalUnavailableCause = z.infer<typeof TerminalUnavailableCauseSchema>
+
+/** Null for anything that does not validate; a malformed cause must never be acted on. */
+export function parseTerminalUnavailableCause(value: unknown): TerminalUnavailableCause | null {
+  const parsed = TerminalUnavailableCauseSchema.safeParse(value)
+  return parsed.success ? parsed.data : null
+}
+
+/**
+ * Whether the client may rewrite the host's `node_modules` on the strength of this cause.
+ *
+ * Deliberately re-derived here rather than trusting `repairable` alone: the flag arrives
+ * from a peer, and only a `blocked` status is evidence of anything.
+ */
+export function mayRepairFromCause(cause: TerminalUnavailableCause | null): boolean {
+  return cause !== null && cause.status === 'blocked' && cause.repairable
+}