feat(cache): add cache-mode client behavior (read-denied warning + ACTIONS_CACHE_MODE skip) (#2447)

* feat(cache): surface cache read-denied as a distinct restore warning

Mirror the existing cache write-denied handling on the restore path. When
the receiver refuses a download URL because the run's token has no readable
cache scopes, it returns a twirp PermissionDenied (HTTP 403). The twirp
client wraps that 403 in a generic Error, so the stable 'cache read denied:'
prefix is embedded in the message rather than at the start.

- Add CACHE_READ_DENIED_PREFIX and CacheReadDeniedError
- Dispatch on the prefix in the restoreCacheV2 catch block (V2 only), log a
  policy-specific warning, and report a cache miss so the run continues
- Add a test mirroring the write-denied coverage

* chore(cache): trim comments, bump to 6.2.0, add RELEASES entry

* refactor(cache): dispatch read-denied by error name to mirror write path

Re-throw CacheReadDeniedError from an inner try/catch around
GetCacheEntryDownloadURL and dispatch on typedError.name in the outer catch,
matching how saveCacheV2 handles CacheWriteDeniedError.

* feat(cache): handle read-denied on the v1 restore path

Extend the read-denied handling to Cache Service v1 so GHES (which forces v1
via _apis/artifactcache) is covered when read-scope enforcement ships there.

- Surface the receiver's error body message from getCacheEntry instead of a
  generic status-code error, so the cache read denied: prefix reaches callers
- Re-throw CacheReadDeniedError from restoreCacheV1 and dispatch on it in the
  outer catch, mirroring restoreCacheV2 and the write-denied v1 handling
- Add a v1 read-denied test

* refactor(cache): only surface receiver body for read-denied on v1

* test(cache): assert getCacheEntry only surfaces body for read-denied

* test(cache): cover non-read-denied getCacheEntry passthrough on v1

* refactor(cache): share read-denied prefix via constants to avoid drift

* feat(cache): skip restore/save per ACTIONS_CACHE_MODE

* test(cache): expand ACTIONS_CACHE_MODE skip coverage across v1/v2 and unknown modes

* fix copilot pr feedback

Co-authored-by: Copilot Autofix powered by AI <[email protected]>

* docs(cache): remove internal reference from cache-mode comment

* test(cache): merge redundant cache-mode skip tests and simplify read-denied handling

Address PR review feedback:
- Merge the duplicate restore/save skip test.each blocks into single blocks parametrized over ACTIONS_CACHE_SERVICE_V2.
- Drop the redundant CacheReadDeniedError catch arms; the typed error is not an HttpClientError so it already falls through to a non-fatal warning.
- Clarify why read-denied classification happens both in getCacheEntry and cache.ts (dependency-free internal module cannot import the typed error).

* refactor(cache): drop redundant CacheWriteDeniedError catch arms

Mirror the read-denied simplification on the save path. CacheWriteDeniedError
is not an HttpClientError and its name does not match the ReserveCacheError
arm, so it falls through to the same non-fatal warning. Logging behavior is
unchanged (warns, never fails the run) and the exported type is still thrown
internally for consumers and tests. Also refresh stale doc wording.

* test(cache): collapse redundant restore getCacheEntry-failure tests

The two restoreCache tests exercised the identical warning + cache-miss path
now that read-denied is no longer reclassified in the catch, so merge them into
one. The read-denied prefix detection that actually branches on the message is
covered by getCacheEntry tests in cacheHttpClient.test.ts.

---------

Co-authored-by: Copilot Autofix powered by AI <[email protected]>

packages/cache/RELEASES.md

@@ -1,5 +1,10 @@
 # @actions/cache Releases
 
+## 6.2.0
+
+- Handle cache read error due to read-only token: detect the `cache read denied:` prefix on cache download failures (both the v2 twirp path and the v1 `_apis/artifactcache` path) and surface it as a `core.warning` (without failing the run).
+- Honor the `ACTIONS_CACHE_MODE` environment variable: skip restore when the effective cache-mode does not permit reads (`none`, `write-only`) and skip save when it does not permit writes (`none`, `read`), logging a single non-fatal `core.info` line. When `ACTIONS_CACHE_MODE` is unset or unrecognized, behavior is unchanged.
+
 ## 6.1.0
 
 - Handle cache write error due to read-only token: detect the `cache write denied:` prefix on cache reservation failures and surface it as a `core.warning` (without failing the run).

packages/cache/__tests__/cacheHttpClient.test.ts

@@ -1,8 +1,10 @@
-import {downloadCache} from '../src/internal/cacheHttpClient'
+import {downloadCache, getCacheEntry} from '../src/internal/cacheHttpClient'
 import {getCacheVersion} from '../src/internal/cacheUtils'
 import {CompressionMethod} from '../src/internal/constants'
 import * as downloadUtils from '../src/internal/downloadUtils'
+import * as requestUtils from '../src/internal/requestUtils'
 import {DownloadOptions, getDownloadOptions} from '../src/options'
+import {HttpClientError} from '@actions/http-client'
 
 jest.mock('../src/internal/downloadUtils')
 
@@ -57,6 +59,37 @@ test('getCacheVersion with enableCrossOsArchive as false returns version on wind
   }
 })
 
+test('getCacheEntry throws a generic status-code error for non-read-denied failures', async () => {
+  // Regression: a non read-denied failure must NOT leak the server's body
+  // message; it should surface the generic status-code error.
+  jest.spyOn(requestUtils, 'retryTypedResponse').mockResolvedValue({
+    statusCode: 403,
+    result: null,
+    headers: {},
+    error: new HttpClientError('some other server detail', 403)
+  })
+
+  await expect(getCacheEntry(['key'], ['node_modules'])).rejects.toThrow(
+    'Cache service responded with 403'
+  )
+})
+
+test('getCacheEntry surfaces the body message for a cache read denial', async () => {
+  jest.spyOn(requestUtils, 'retryTypedResponse').mockResolvedValue({
+    statusCode: 403,
+    result: null,
+    headers: {},
+    error: new HttpClientError(
+      'cache read denied: token has no readable scopes',
+      403
+    )
+  })
+
+  await expect(getCacheEntry(['key'], ['node_modules'])).rejects.toThrow(
+    'cache read denied: token has no readable scopes'
+  )
+})
+
 test('downloadCache uses http-client for non-Azure URLs', async () => {
   const downloadCacheHttpClientMock = jest.spyOn(
     downloadUtils,

packages/cache/__tests__/config.test.ts

@@ -23,3 +23,37 @@ test('isGhes returns false for ghe.localhost', () => {
   process.env.GITHUB_SERVER_URL = 'https://my.domain.ghe.localhost'
   expect(config.isGhes()).toBe(false)
 })
+
+describe('cache-mode helpers', () => {
+  const original = process.env.ACTIONS_CACHE_MODE
+
+  afterEach(() => {
+    if (original === undefined) {
+      delete process.env.ACTIONS_CACHE_MODE
+    } else {
+      process.env.ACTIONS_CACHE_MODE = original
+    }
+  })
+
+  test('getCacheMode normalizes whitespace and case', () => {
+    process.env.ACTIONS_CACHE_MODE = '  Write-Only  '
+    expect(config.getCacheMode()).toBe('write-only')
+  })
+
+  test('getCacheMode returns empty string when unset', () => {
+    delete process.env.ACTIONS_CACHE_MODE
+    expect(config.getCacheMode()).toBe('')
+  })
+
+  test.each([
+    ['', true, true],
+    ['read', true, false],
+    ['write', true, true],
+    ['write-only', false, true],
+    ['none', false, false],
+    ['garbage', true, true]
+  ])("mode '%s' -> readable=%s writable=%s", (mode, readable, writable) => {
+    expect(config.isCacheReadable(mode)).toBe(readable)
+    expect(config.isCacheWritable(mode)).toBe(writable)
+  })
+})

packages/cache/__tests__/restoreCache.test.ts

@@ -99,6 +99,93 @@ test('restore with server error should fail', async () => {
   delete process.env['ACTIONS_RESULTS_URL']
 })
 
+test('restore surfaces a getCacheEntry failure as a warning and reports a cache miss', async () => {
+  // restoreCache treats any getCacheEntry failure (read-denied or otherwise)
+  // as a non-fatal warning and a cache miss so the workflow continues. The
+  // read-denied prefix detection itself is covered in cacheHttpClient.test.ts.
+  const paths = ['node_modules']
+  const key = 'node-test'
+  const logErrorMock = jest.spyOn(core, 'error')
+  const logWarningMock = jest.spyOn(core, 'warning')
+  const message = 'cache read denied: token has no readable scopes'
+
+  jest.spyOn(cacheHttpClient, 'getCacheEntry').mockImplementation(async () => {
+    throw new Error(message)
+  })
+
+  const cacheKey = await restoreCache(paths, key)
+  expect(cacheKey).toBe(undefined)
+  expect(logErrorMock).not.toHaveBeenCalled()
+  expect(logWarningMock).toHaveBeenCalledWith(`Failed to restore: ${message}`)
+  expect(logWarningMock).toHaveBeenCalledTimes(1)
+})
+
+describe('restore cache-mode gating', () => {
+  const originalMode = process.env.ACTIONS_CACHE_MODE
+  const originalV2 = process.env.ACTIONS_CACHE_SERVICE_V2
+
+  const restoreEnv = (key: string, value: string | undefined): void => {
+    if (value === undefined) {
+      delete process.env[key]
+    } else {
+      process.env[key] = value
+    }
+  }
+
+  afterEach(() => {
+    restoreEnv('ACTIONS_CACHE_MODE', originalMode)
+    restoreEnv('ACTIONS_CACHE_SERVICE_V2', originalV2)
+  })
+
+  // The skip short-circuits before v1/v2 dispatch, so it applies regardless of
+  // the ACTIONS_CACHE_SERVICE_V2 feature flag.
+  test.each([
+    ['none', undefined],
+    ['none', 'true'],
+    ['write-only', undefined],
+    ['write-only', 'true']
+  ])(
+    "mode '%s' skips restore with ACTIONS_CACHE_SERVICE_V2=%s",
+    async (mode, v2) => {
+      process.env.ACTIONS_CACHE_MODE = mode
+      restoreEnv('ACTIONS_CACHE_SERVICE_V2', v2)
+      const logInfoMock = jest.spyOn(core, 'info')
+      const getCacheEntryMock = jest.spyOn(cacheHttpClient, 'getCacheEntry')
+
+      const cacheKey = await restoreCache(['node_modules'], 'node-test')
+
+      expect(cacheKey).toBe(undefined)
+      expect(getCacheEntryMock).not.toHaveBeenCalled()
+      expect(logInfoMock).toHaveBeenCalledTimes(1)
+      expect(logInfoMock).toHaveBeenCalledWith(
+        `Cache restore skipped: the effective cache-mode '${mode}' does not permit reads.`
+      )
+    }
+  )
+
+  test.each(['read', 'write', '', 'garbage'])(
+    "mode '%s' does not skip restore",
+    async mode => {
+      if (mode === '') {
+        delete process.env.ACTIONS_CACHE_MODE
+      } else {
+        process.env.ACTIONS_CACHE_MODE = mode
+      }
+      const logInfoMock = jest.spyOn(core, 'info')
+      const getCacheEntryMock = jest
+        .spyOn(cacheHttpClient, 'getCacheEntry')
+        .mockResolvedValue(null as never)
+
+      await restoreCache(['node_modules'], 'node-test')
+
+      expect(getCacheEntryMock).toHaveBeenCalledTimes(1)
+      expect(logInfoMock).not.toHaveBeenCalledWith(
+        expect.stringContaining('Cache restore skipped')
+      )
+    }
+  )
+})
+
 test('restore with restore keys and no cache found', async () => {
   const paths = ['node_modules']
   const key = 'node-test'

packages/cache/__tests__/restoreCacheV2.test.ts

@@ -33,6 +33,19 @@ beforeAll(() => {
   // Ensure that we're using v2 for these tests
   jest.spyOn(config, 'getCacheServiceVersion').mockReturnValue('v2')
 
+  // config is auto-mocked; use the real cache-mode helpers so gating reflects
+  // ACTIONS_CACHE_MODE and unset stays permissive.
+  const actualConfig = jest.requireActual('../src/internal/config')
+  jest
+    .spyOn(config, 'getCacheMode')
+    .mockImplementation(actualConfig.getCacheMode)
+  jest
+    .spyOn(config, 'isCacheReadable')
+    .mockImplementation(actualConfig.isCacheReadable)
+  jest
+    .spyOn(config, 'isCacheWritable')
+    .mockImplementation(actualConfig.isCacheWritable)
+
   logDebugMock = jest.spyOn(core, 'debug')
   logInfoMock = jest.spyOn(core, 'info')
 })
@@ -112,6 +125,33 @@ test('restore with server error should fail', async () => {
   )
 })
 
+test('restore denied by read-only token logs warning and reports cache miss', async () => {
+  // The receiver returns twirp PermissionDenied (403) when the run's token has
+  // no readable cache scopes; the client wraps it so the `cache read denied:`
+  // prefix arrives embedded. Expect a single warning (not error) and a miss.
+  const paths = ['node_modules']
+  const key = 'node-test'
+  const logErrorMock = jest.spyOn(core, 'error')
+  const logWarningMock = jest.spyOn(core, 'warning')
+  const wrappedDeniedMessage =
+    'Failed to GetCacheEntryDownloadURL: Received non-retryable error: ' +
+    'Failed request: (403) Forbidden: cache read denied: token has no readable scopes'
+
+  jest
+    .spyOn(CacheServiceClientJSON.prototype, 'GetCacheEntryDownloadURL')
+    .mockImplementation(() => {
+      throw new Error(wrappedDeniedMessage)
+    })
+
+  const cacheKey = await restoreCache(paths, key)
+  expect(cacheKey).toBe(undefined)
+  expect(logErrorMock).not.toHaveBeenCalled()
+  expect(logWarningMock).toHaveBeenCalledWith(
+    `Failed to restore: ${wrappedDeniedMessage}`
+  )
+  expect(logWarningMock).toHaveBeenCalledTimes(1)
+})
+
 test('restore with restore keys and no cache found', async () => {
   const paths = ['node_modules']
   const key = 'node-test'

packages/cache/__tests__/saveCache.test.ts

@@ -35,6 +35,18 @@ beforeAll(() => {
   jest.spyOn(cacheUtils, 'createTempDirectory').mockImplementation(async () => {
     return Promise.resolve('/foo/bar')
   })
+  // config is auto-mocked; use the real cache-mode helpers so gating reflects
+  // ACTIONS_CACHE_MODE and unset stays permissive.
+  const actualConfig = jest.requireActual('../src/internal/config')
+  jest
+    .spyOn(config, 'getCacheMode')
+    .mockImplementation(actualConfig.getCacheMode)
+  jest
+    .spyOn(config, 'isCacheReadable')
+    .mockImplementation(actualConfig.isCacheReadable)
+  jest
+    .spyOn(config, 'isCacheWritable')
+    .mockImplementation(actualConfig.isCacheWritable)
 })
 
 test('save with missing input should fail', async () => {
@@ -45,6 +57,75 @@ test('save with missing input should fail', async () => {
   )
 })
 
+describe('save cache-mode gating', () => {
+  const originalMode = process.env.ACTIONS_CACHE_MODE
+  const originalV2 = process.env.ACTIONS_CACHE_SERVICE_V2
+
+  const restoreEnv = (key: string, value: string | undefined): void => {
+    if (value === undefined) {
+      delete process.env[key]
+    } else {
+      process.env[key] = value
+    }
+  }
+
+  afterEach(() => {
+    restoreEnv('ACTIONS_CACHE_MODE', originalMode)
+    restoreEnv('ACTIONS_CACHE_SERVICE_V2', originalV2)
+  })
+
+  // The skip short-circuits before v1/v2 dispatch, so it applies regardless of
+  // the ACTIONS_CACHE_SERVICE_V2 feature flag.
+  test.each([
+    ['read', undefined],
+    ['read', 'true'],
+    ['none', undefined],
+    ['none', 'true']
+  ])(
+    "mode '%s' skips save with ACTIONS_CACHE_SERVICE_V2=%s",
+    async (mode, v2) => {
+      process.env.ACTIONS_CACHE_MODE = mode
+      restoreEnv('ACTIONS_CACHE_SERVICE_V2', v2)
+      const logInfoMock = jest.spyOn(core, 'info')
+      const resolvePathsMock = jest.spyOn(cacheUtils, 'resolvePaths')
+
+      const cacheId = await saveCache(['node_modules'], 'node-test')
+
+      expect(cacheId).toBe(-1)
+      expect(resolvePathsMock).not.toHaveBeenCalled()
+      expect(logInfoMock).toHaveBeenCalledTimes(1)
+      expect(logInfoMock).toHaveBeenCalledWith(
+        `Cache save skipped: the effective cache-mode '${mode}' does not permit writes.`
+      )
+    }
+  )
+
+  test.each(['write', 'write-only', '', 'garbage'])(
+    "mode '%s' does not skip save",
+    async mode => {
+      if (mode === '') {
+        delete process.env.ACTIONS_CACHE_MODE
+      } else {
+        process.env.ACTIONS_CACHE_MODE = mode
+      }
+      const logInfoMock = jest.spyOn(core, 'info')
+      const resolvePathsMock = jest.spyOn(cacheUtils, 'resolvePaths')
+
+      try {
+        await saveCache(['node_modules'], 'node-test')
+      } catch {
+        // Downstream client is not fully mocked here; we only assert the guard
+        // let execution proceed past it.
+      }
+
+      expect(resolvePathsMock).toHaveBeenCalled()
+      expect(logInfoMock).not.toHaveBeenCalledWith(
+        expect.stringContaining('Cache save skipped')
+      )
+    }
+  )
+})
+
 test('save with large cache outputs should fail', async () => {
   const filePath = 'node_modules'
   const primaryKey = 'Linux-node-bb828da54c148048dd17899ba9fda624811cfb43'

packages/cache/package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "@actions/cache",
-  "version": "6.1.0",
+  "version": "6.2.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "@actions/cache",
-      "version": "6.1.0",
+      "version": "6.2.0",
       "license": "MIT",
       "dependencies": {
         "@actions/core": "^3.0.1",

packages/cache/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@actions/cache",
-  "version": "6.1.0",
+  "version": "6.2.0",
   "description": "Actions cache lib",
   "keywords": [
     "github",

packages/cache/src/cache.ts

@@ -3,7 +3,13 @@ import * as path from 'path'
 import * as utils from './internal/cacheUtils.js'
 import * as cacheHttpClient from './internal/cacheHttpClient.js'
 import * as cacheTwirpClient from './internal/shared/cacheTwirpClient.js'
-import {getCacheServiceVersion, isGhes} from './internal/config.js'
+import {
+  getCacheServiceVersion,
+  isGhes,
+  getCacheMode,
+  isCacheReadable,
+  isCacheWritable
+} from './internal/config.js'
 import {DownloadOptions, UploadOptions} from './options.js'
 import {createTar, extractTar, listTar} from './internal/tar.js'
 import {
@@ -13,6 +19,7 @@ import {
   GetCacheEntryDownloadURLRequest
 } from './generated/results/api/v1/cache.js'
 import {HttpClientError} from '@actions/http-client'
+import {CacheReadDeniedMessagePrefix} from './internal/constants.js'
 
 export type {DownloadOptions, UploadOptions}
 export class ValidationError extends Error {
@@ -32,12 +39,13 @@ export class ReserveCacheError extends Error {
 }
 
 /**
- * Stable prefix the receiver writes into the cache reservation response when
- * the issuer downgraded the cache token to read-only (for example, because
+ * Stable prefix the cache service writes into the cache reservation response
+ * when the issuer downgraded the cache token to read-only (for example, because
  * the run was triggered by an untrusted event). saveCacheV1 / saveCacheV2
- * dispatch on this prefix to re-classify the failure as a
- * CacheWriteDeniedError so consumers (and the outer catch arm) can
- * distinguish a policy denial from other reservation failures.
+ * dispatch on this prefix to re-classify the failure as a CacheWriteDeniedError
+ * so consumers and tests can distinguish a policy denial from other reservation
+ * failures. Internally it is logged as a non-fatal warning like other
+ * best-effort save failures.
  */
 export const CACHE_WRITE_DENIED_PREFIX = 'cache write denied:'
 
@@ -45,7 +53,7 @@ export const CACHE_WRITE_DENIED_PREFIX = 'cache write denied:'
  * Raised when the cache backend refuses to reserve a writable cache entry
  * because the JWT issued for this run was scoped read-only (for example, the
  * run was triggered by an event the repository administrator classified as
- * untrusted). The receiver-supplied detail message always begins with
+ * untrusted). The service-supplied detail message always begins with
  * `cache write denied:` (the full error message includes additional context
  * like the cache key).
  *
@@ -62,6 +70,21 @@ export class CacheWriteDeniedError extends ReserveCacheError {
   }
 }
 
+// Re-exported from constants so consumers keep referencing it here; the shared
+// value also drives detection in cacheHttpClient without duplicating the string.
+export const CACHE_READ_DENIED_PREFIX = CacheReadDeniedMessagePrefix
+
+// Raised when the cache backend denies a download URL because the run's token
+// has no readable cache scopes. Caching is best-effort, so restoreCache logs a
+// warning and reports a cache miss rather than rethrowing this.
+export class CacheReadDeniedError extends Error {
+  constructor(message: string) {
+    super(message)
+    this.name = 'CacheReadDeniedError'
+    Object.setPrototypeOf(this, CacheReadDeniedError.prototype)
+  }
+}
+
 export class FinalizeCacheError extends Error {
   constructor(message: string) {
     super(message)
@@ -134,6 +157,17 @@ export async function restoreCache(
 
   checkPaths(paths)
 
+  const cacheMode = getCacheMode()
+  if (!isCacheReadable(cacheMode)) {
+    core.info(
+      `Cache restore skipped: the effective cache-mode '${cacheMode}' does not permit reads.`
+    )
+    core.debug(
+      `Skipped restore for paths [${paths.join(', ')}] with primary key '${primaryKey}'.`
+    )
+    return undefined
+  }
+
   switch (cacheServiceVersion) {
     case 'v2':
       return await restoreCacheV2(
@@ -191,10 +225,25 @@ async function restoreCacheV1(
   let archivePath = ''
   try {
     // path are needed to compute version
-    const cacheEntry = await cacheHttpClient.getCacheEntry(keys, paths, {
-      compressionMethod,
-      enableCrossOsArchive
-    })
+    let cacheEntry
+    try {
+      cacheEntry = await cacheHttpClient.getCacheEntry(keys, paths, {
+        compressionMethod,
+        enableCrossOsArchive
+      })
+    } catch (error) {
+      // The v1 artifact cache service returns HTTP 403 with a
+      // `cache read denied:` body when the run's token has no readable cache
+      // scopes. getCacheEntry lives in a dependency-free internal module and
+      // cannot import CacheReadDeniedError without a circular dependency, so it
+      // only surfaces the raw denial message; we classify it into the typed
+      // error here so the outer catch and consumers can dispatch on it.
+      const errorMessage = (error as Error)?.message ?? ''
+      if (errorMessage.includes(CACHE_READ_DENIED_PREFIX)) {
+        throw new CacheReadDeniedError(errorMessage)
+      }
+      throw error
+    }
     if (!cacheEntry?.archiveLocation) {
       // Cache not found
       return undefined
@@ -239,7 +288,9 @@ async function restoreCacheV1(
       throw error
     } else {
       // warn on cache restore failure and continue build
-      // Log server errors (5xx) as errors, all other errors as warnings
+      // Log server errors (5xx) as errors, all other errors as warnings.
+      // A read denied by policy (CacheReadDeniedError) is not an HttpClientError
+      // so it falls here and is warned, treated as a cache miss.
       if (
         typedError instanceof HttpClientError &&
         typeof typedError.statusCode === 'number' &&
@@ -314,7 +365,19 @@ async function restoreCacheV2(
       )
     }
 
-    const response = await twirpClient.GetCacheEntryDownloadURL(request)
+    let response
+    try {
+      response = await twirpClient.GetCacheEntryDownloadURL(request)
+    } catch (error) {
+      // The receiver returns twirp PermissionDenied (403) when the run's token
+      // has no readable cache scopes. The client wraps that 403, so the stable
+      // prefix is embedded in the message rather than leading it.
+      const errorMessage = (error as Error)?.message ?? ''
+      if (errorMessage.includes(CACHE_READ_DENIED_PREFIX)) {
+        throw new CacheReadDeniedError(errorMessage)
+      }
+      throw error
+    }
 
     if (!response.ok) {
       core.debug(
@@ -370,8 +433,10 @@ async function restoreCacheV2(
     if (typedError.name === ValidationError.name) {
       throw error
     } else {
-      // Supress all non-validation cache related errors because caching should be optional
-      // Log server errors (5xx) as errors, all other errors as warnings
+      // Suppress all non-validation cache related errors because caching should be optional
+      // Log server errors (5xx) as errors, all other errors as warnings.
+      // A read denied by policy (CacheReadDeniedError) is not an HttpClientError
+      // so it falls here and is warned, treated as a cache miss.
       if (
         typedError instanceof HttpClientError &&
         typeof typedError.statusCode === 'number' &&
@@ -414,6 +479,18 @@ export async function saveCache(
   core.debug(`Cache service version: ${cacheServiceVersion}`)
   checkPaths(paths)
   checkKey(key)
+
+  const cacheMode = getCacheMode()
+  if (!isCacheWritable(cacheMode)) {
+    core.info(
+      `Cache save skipped: the effective cache-mode '${cacheMode}' does not permit writes.`
+    )
+    core.debug(
+      `Skipped save for paths [${paths.join(', ')}] with key '${key}'.`
+    )
+    return -1
+  }
+
   switch (cacheServiceVersion) {
     case 'v2':
       return await saveCacheV2(paths, key, options, enableCrossOsArchive)
@@ -521,15 +598,13 @@ async function saveCacheV1(
     const typedError = error as Error
     if (typedError.name === ValidationError.name) {
       throw error
-    } else if (typedError.name === CacheWriteDeniedError.name) {
-      // Cache write was denied by policy (read-only token). Surface to the
-      // customer at warning level so it is visible in the workflow log
-      // without failing the run.
-      core.warning(`Failed to save: ${typedError.message}`)
     } else if (typedError.name === ReserveCacheError.name) {
       core.info(`Failed to save: ${typedError.message}`)
     } else {
-      // Log server errors (5xx) as errors, all other errors as warnings
+      // Log server errors (5xx) as errors, all other errors as warnings.
+      // A write denied by policy (CacheWriteDeniedError) is not an
+      // HttpClientError and its name does not match the ReserveCacheError arm,
+      // so it falls here and is warned without failing the run.
       if (
         typedError instanceof HttpClientError &&
         typeof typedError.statusCode === 'number' &&
@@ -683,17 +758,15 @@ async function saveCacheV2(
     const typedError = error as Error
     if (typedError.name === ValidationError.name) {
       throw error
-    } else if (typedError.name === CacheWriteDeniedError.name) {
-      // Cache write was denied by policy (read-only token). Surface to the
-      // customer at warning level so it is visible in the workflow log
-      // without failing the run.
-      core.warning(`Failed to save: ${typedError.message}`)
     } else if (typedError.name === ReserveCacheError.name) {
       core.info(`Failed to save: ${typedError.message}`)
     } else if (typedError.name === FinalizeCacheError.name) {
       core.warning(typedError.message)
     } else {
-      // Log server errors (5xx) as errors, all other errors as warnings
+      // Log server errors (5xx) as errors, all other errors as warnings.
+      // A write denied by policy (CacheWriteDeniedError) is not an
+      // HttpClientError and its name does not match the ReserveCacheError arm,
+      // so it falls here and is warned without failing the run.
       if (
         typedError instanceof HttpClientError &&
         typeof typedError.statusCode === 'number' &&

packages/cache/src/internal/cacheHttpClient.ts

@@ -35,6 +35,7 @@ import {
   retryTypedResponse
 } from './requestUtils.js'
 import {getCacheServiceURL} from './config.js'
+import {CacheReadDeniedMessagePrefix} from './constants.js'
 import {getUserAgentString} from './shared/user-agent.js'
 
 function getCacheApiUrl(resource: string): string {
@@ -101,6 +102,12 @@ export async function getCacheEntry(
     return null
   }
   if (!isSuccessStatusCode(response.statusCode)) {
+    // Only surface the receiver's body for a `cache read denied:` policy denial
+    // so callers can dispatch on it; keep the generic message otherwise.
+    const errorMessage = response.error?.message
+    if (errorMessage?.includes(CacheReadDeniedMessagePrefix)) {
+      throw new Error(errorMessage)
+    }
     throw new Error(`Cache service responded with ${response.statusCode}`)
   }
 

packages/cache/src/internal/config.ts

@@ -19,6 +19,26 @@ export function getCacheServiceVersion(): string {
   return process.env['ACTIONS_CACHE_SERVICE_V2'] ? 'v2' : 'v1'
 }
 
+// The cache-mode lattice: readable = {read, write}, writable = {write,
+// write-only}, none = neither.
+const KNOWN_CACHE_MODES = ['none', 'read', 'write', 'write-only']
+
+// The effective cache-mode exported by the runner, or '' when not set.
+export function getCacheMode(): string {
+  return (process.env['ACTIONS_CACHE_MODE'] || '').trim().toLowerCase()
+}
+
+// Unset or unrecognized modes are permissive so behavior matches today.
+export function isCacheReadable(mode: string): boolean {
+  if (!KNOWN_CACHE_MODES.includes(mode)) return true
+  return mode === 'read' || mode === 'write'
+}
+
+export function isCacheWritable(mode: string): boolean {
+  if (!KNOWN_CACHE_MODES.includes(mode)) return true
+  return mode === 'write' || mode === 'write-only'
+}
+
 export function getCacheServiceURL(): string {
   const version = getCacheServiceVersion()
 

packages/cache/src/internal/constants.ts

@@ -38,3 +38,8 @@ export const TarFilename = 'cache.tar'
 export const ManifestFilename = 'manifest.txt'
 
 export const CacheFileSizeLimit = 10 * Math.pow(1024, 3) // 10GiB per repository
+
+// Prefix the cache backend embeds in a read-denial message (v2 twirp
+// GetCacheEntryDownloadURL error or the GHES v1 `_apis/artifactcache` 403 body).
+// Shared so cache.ts and cacheHttpClient.ts match the same contract value.
+export const CacheReadDeniedMessagePrefix = 'cache read denied:'