diff --git a/docs/arc-dind.md b/docs/arc-dind.md
index a40ae1a34..5b8c5082b 100644
--- a/docs/arc-dind.md
+++ b/docs/arc-dind.md
@@ -52,7 +52,7 @@ services:
     depends_on:
       sysroot-stage: { condition: service_completed_successfully }
     volumes:
-      - sysroot:/host:ro
+      - sysroot:/host:rw
       - /tmp/gh-aw/tool-cache:/host/tmp/gh-aw/tool-cache:ro
 
 volumes:
@@ -120,6 +120,10 @@ For fine-grained control (or when not using `runner.topology`):
       "path": "/usr/local/bin/copilot",
       "targetPath": "/usr/local/bin/copilot"
     }
+  },
+  "runner": {
+    "topology": "arc-dind",
+    "sysrootImage": "ghcr.io/github/gh-aw-firewall/build-tools:latest"
   }
 }
 ```
@@ -132,6 +136,24 @@ For fine-grained control (or when not using `runner.topology`):
 - `dind.stageEngineBinary`: copies an engine binary from the runner path into daemon-visible filesystem before compose startup.
 - `dind.stagingImage`: image used for short-lived staging containers.
 - `dind.workDir`: target root for DinD pre-staged directory tree (`/tmp/gh-aw` default).
+- `runner.topology: "arc-dind"`: enables sysroot staging (`sysroot-stage` init service + `sysroot` volume mounted on agent at `/host:rw`).
+- `runner.sysrootImage`: optional override for the sysroot image used by `runner.topology=arc-dind`.
+
+## Build-tools sysroot image
+
+When `runner.topology` is `arc-dind`, AWF starts a one-shot `sysroot-stage` service that copies
+the filesystem from a build-tools image derived from the same `--image-registry` and `--image-tag`
+settings as the other AWF containers (unless `runner.sysrootImage` overrides it) into a named
+`sysroot` volume. The agent mounts that volume at `/host:rw`.
+
+This image pre-installs root-required system build dependencies (for example gcc/make/cmake,
+libssl-dev/libc6-dev/libicu-dev, capsh/gosu/gh) so ARC workflow steps can stay non-root.
+
+## Tool cache path guidance for ARC
+
+If `RUNNER_TOOL_CACHE` points under `/opt` (for example `/opt/hostedtoolcache`) AWF logs a warning
+in `runner.topology=arc-dind` mode because `/opt` is commonly not visible from the DinD daemon
+filesystem. Prefer a shared runner/daemon path under `/tmp/gh-aw` when possible.
 
 ## Auto-detection of split filesystem setups
 
diff --git a/src/commands/validators/network-options.test.ts b/src/commands/validators/network-options.test.ts
index 723606b5d..f9ef9ef3b 100644
--- a/src/commands/validators/network-options.test.ts
+++ b/src/commands/validators/network-options.test.ts
@@ -60,9 +60,17 @@ function makeDefaultMocks() {
 }
 
 describe('validateNetworkOptions', () => {
+  const savedRunnerToolCache = process.env.RUNNER_TOOL_CACHE;
+
   beforeEach(() => {
     jest.clearAllMocks();
     makeDefaultMocks();
+    delete process.env.RUNNER_TOOL_CACHE;
+  });
+
+  afterAll(() => {
+    if (savedRunnerToolCache === undefined) delete process.env.RUNNER_TOOL_CACHE;
+    else process.env.RUNNER_TOOL_CACHE = savedRunnerToolCache;
   });
 
   describe('happy path', () => {
@@ -261,4 +269,22 @@ describe('validateNetworkOptions', () => {
       expect(result.dnsOverHttps).toBe('https://1.1.1.1/dns-query');
     });
   });
+
+  describe('arc-dind RUNNER_TOOL_CACHE warnings', () => {
+    it('warns when RUNNER_TOOL_CACHE is under /opt in arc-dind topology', () => {
+      process.env.RUNNER_TOOL_CACHE = '/opt/hostedtoolcache';
+      validateNetworkOptions({ runnerTopology: 'arc-dind' });
+
+      const warnCalls = (logger.warn as jest.Mock).mock.calls.map((c: string[]) => c[0]);
+      expect(warnCalls.some((m: string) => m.includes('RUNNER_TOOL_CACHE is under /opt'))).toBe(true);
+    });
+
+    it('does not warn when topology is not arc-dind', () => {
+      process.env.RUNNER_TOOL_CACHE = '/opt/hostedtoolcache';
+      validateNetworkOptions({});
+
+      const warnCalls = (logger.warn as jest.Mock).mock.calls.map((c: string[]) => c[0]);
+      expect(warnCalls.some((m: string) => m.includes('RUNNER_TOOL_CACHE is under /opt'))).toBe(false);
+    });
+  });
 });
diff --git a/src/commands/validators/network-options.ts b/src/commands/validators/network-options.ts
index 0f97e23a2..55d30430a 100644
--- a/src/commands/validators/network-options.ts
+++ b/src/commands/validators/network-options.ts
@@ -78,6 +78,18 @@ export function validateNetworkOptions(options: Record<string, unknown>): Networ
     );
   }
 
+  if (options.runnerTopology === 'arc-dind') {
+    const runnerToolCache = process.env.RUNNER_TOOL_CACHE?.trim();
+    if (runnerToolCache === '/opt' || runnerToolCache?.startsWith('/opt/')) {
+      logger.warn(
+        '⚠️  RUNNER_TOOL_CACHE is under /opt, which is typically invisible to DinD daemons in ARC.',
+      );
+      logger.warn(
+        '   Prefer a runner-visible shared path (for example under /tmp/gh-aw) for tool-cache mounts.',
+      );
+    }
+  }
+
   // --- Domain resolution --------------------------------------------------
 
   // Resolve allowed and blocked domains (parse, merge, validate)
diff --git a/src/compose-generator.test.ts b/src/compose-generator.test.ts
index 3cf5ef403..ff8a54eee 100644
--- a/src/compose-generator.test.ts
+++ b/src/compose-generator.test.ts
@@ -354,11 +354,11 @@ describe('generateDockerCompose', () => {
         expect(result.volumes!.sysroot).toEqual({});
       });
 
-      it('adds sysroot:/host:ro to agent volumes', () => {
+      it('adds sysroot:/host:rw to agent volumes', () => {
         const config = { ...mockConfig, runnerTopology: 'arc-dind' as const };
         const result = generateDockerCompose(config, mockNetworkConfig);
 
-        expect(result.services.agent.volumes).toContain('sysroot:/host:ro');
+        expect(result.services.agent.volumes).toContain('sysroot:/host:rw');
       });
 
       it('does not retain base-system bind mounts that shadow sysroot', () => {
@@ -375,16 +375,16 @@ describe('generateDockerCompose', () => {
         expect(volumes).not.toContain('/lib:/host/lib:ro');
         expect(volumes).not.toContain('/lib64:/host/lib64:ro');
         expect(volumes).not.toContain('/opt:/host/opt:ro');
-        expect(volumes).not.toContain('/sys:/host/sys:ro');
-        expect(volumes).not.toContain('/dev:/host/dev:ro');
+        expect(volumes).toContain('/sys:/host/sys:ro');
+        expect(volumes).toContain('/dev:/host/dev:ro');
         expect(volumes.some(v => v.includes(':/host/usr:ro'))).toBe(false);
         expect(volumes.some(v => v.includes(':/host/bin:ro'))).toBe(false);
         expect(volumes.some(v => v.includes(':/host/sbin:ro'))).toBe(false);
         expect(volumes.some(v => v.includes(':/host/lib:ro'))).toBe(false);
         expect(volumes.some(v => v.includes(':/host/lib64:ro'))).toBe(false);
         expect(volumes.some(v => v.includes(':/host/opt:ro'))).toBe(false);
-        expect(volumes.some(v => v.includes(':/host/sys:ro'))).toBe(false);
-        expect(volumes.some(v => v.includes(':/host/dev:ro'))).toBe(false);
+        expect(volumes.filter(v => v.endsWith(':/host/sys:ro'))).toEqual(['/sys:/host/sys:ro']);
+        expect(volumes.filter(v => v.endsWith(':/host/dev:ro'))).toEqual(['/dev:/host/dev:ro']);
       });
 
       it('does not declare sysroot volume when topology is standard', () => {
diff --git a/src/compose-generator.ts b/src/compose-generator.ts
index 870ef2dc9..a3a8ab864 100644
--- a/src/compose-generator.ts
+++ b/src/compose-generator.ts
@@ -124,8 +124,6 @@ export function generateDockerCompose(
       '/host/lib',
       '/host/lib64',
       '/host/opt',
-      '/host/sys',
-      '/host/dev',
     ]);
     const filteredVolumes = agentVolumes.filter(volume => {
       const target = volume.split(':')[1];
@@ -263,15 +261,17 @@ export function generateDockerCompose(
   // ── Final compose result ───────────────────────────────────────────────────
 
   // When sysroot staging is active, declare the named volume and mount it
-  // on the agent at /host (replacing the per-directory system bind mounts).
+  // on the agent at /host (replacing the per-directory userspace bind mounts,
+  // while /sys and /dev remain live host mounts).
   const namedVolumes: Record<string, any> | undefined = sysrootActive
     ? { sysroot: {} }
     : undefined;
 
   if (sysrootActive) {
-    // The sysroot named volume provides /host content (system binaries, libs, etc.)
-    // via the sysroot-stage init container instead of per-directory bind mounts.
-    agentVolumes.push('sysroot:/host:ro');
+    // The sysroot named volume provides most /host content (system binaries,
+    // libs, etc.) via the sysroot-stage init container instead of per-directory
+    // userspace bind mounts.
+    agentVolumes.push('sysroot:/host:rw');
   }
 
   if (networkIsolation) {
diff --git a/src/config-file-validation.test.ts b/src/config-file-validation.test.ts
index 55f0e5623..fc6d9e4a6 100644
--- a/src/config-file-validation.test.ts
+++ b/src/config-file-validation.test.ts
@@ -399,6 +399,21 @@ describe('validateAwfFileConfig', () => {
     expect(errors).toContain('config.container.unknown is not supported');
   });
 
+  it('rejects non-object runner', () => {
+    const errors = validateAwfFileConfig({ runner: 'invalid' });
+    expect(errors).toContain('config.runner must be an object');
+  });
+
+  it('rejects invalid runner field types', () => {
+    expect(validateAwfFileConfig({ runner: { topology: 'invalid' } })).toContain('config.runner.topology must be one of: standard, arc-dind');
+    expect(validateAwfFileConfig({ runner: { sysrootImage: 123 } })).toContain('config.runner.sysrootImage must be a string');
+  });
+
+  it('rejects unknown runner keys', () => {
+    const errors = validateAwfFileConfig({ runner: { unknown: true } });
+    expect(errors).toContain('config.runner.unknown is not supported');
+  });
+
   it('rejects non-object chroot', () => {
     const errors = validateAwfFileConfig({ chroot: 'invalid' });
     expect(errors).toContain('config.chroot must be an object');
diff --git a/src/etc-mounts-branches.test.ts b/src/etc-mounts-branches.test.ts
index 5cc81fbde..dfd917850 100644
--- a/src/etc-mounts-branches.test.ts
+++ b/src/etc-mounts-branches.test.ts
@@ -286,6 +286,14 @@ describe('system-mounts branch coverage', () => {
     expect(mounts).toContain('/custom/tools:/host/tmp/awf-runner-bin:ro');
   });
 
+  it('keeps live /sys and /dev mounts when sysroot mode is enabled', () => {
+    const mounts = buildSystemMounts('/workspace', undefined, true);
+    expect(mounts).toContain('/sys:/host/sys:ro');
+    expect(mounts).toContain('/dev:/host/dev:ro');
+    expect(mounts).not.toContain('/usr:/host/usr:ro');
+    expect(mounts).not.toContain('/bin:/host/bin:ro');
+  });
+
   it('excludes runner-bin mount when chrootBinariesSourcePath is whitespace-only', () => {
     const mounts = buildSystemMounts('/workspace', '   ');
     expect(mounts.every(m => !m.includes('awf-runner-bin'))).toBe(true);
diff --git a/src/schema.test.ts b/src/schema.test.ts
index 21ebda851..f9d866f10 100644
--- a/src/schema.test.ts
+++ b/src/schema.test.ts
@@ -244,6 +244,13 @@ describe('awf-config.schema.json', () => {
     expect(validate({ container: { runnerToolCachePath: 123 } })).toBe(false);
   });
 
+  it('accepts runner.topology and runner.sysrootImage', () => {
+    expect(validate({ runner: { topology: 'arc-dind' } })).toBe(true);
+    expect(validate({ runner: { topology: 'invalid' } })).toBe(false);
+    expect(validate({ runner: { sysrootImage: 'ghcr.io/github/gh-aw-firewall/build-tools:latest' } })).toBe(true);
+    expect(validate({ runner: { sysrootImage: 123 } })).toBe(false);
+  });
+
   it('validates chroot.identity fields', () => {
     expect(validate({ chroot: { identity: { home: '/tmp/gh-aw/home', user: 'runner', uid: 1001, gid: 1001 } } })).toBe(true);
     expect(validate({ chroot: { identity: { uid: 1.2 } } })).toBe(false);
diff --git a/src/services/agent-volumes/system-mounts.ts b/src/services/agent-volumes/system-mounts.ts
index 8c3198c49..c13a6abe4 100644
--- a/src/services/agent-volumes/system-mounts.ts
+++ b/src/services/agent-volumes/system-mounts.ts
@@ -10,16 +10,27 @@ function normalizeChrootBinariesSourcePath(chrootBinariesSourcePath?: string): s
   return normalized === '/' ? undefined : normalized;
 }
 
-export function buildSystemMounts(workspaceDir: string, chrootBinariesSourcePath?: string): string[] {
+export function buildSystemMounts(
+  workspaceDir: string,
+  chrootBinariesSourcePath?: string,
+  useSysroot = false
+): string[] {
   const mounts = [
-    '/usr:/host/usr:ro',
-    '/bin:/host/bin:ro',
-    '/sbin:/host/sbin:ro',
-    '/lib:/host/lib:ro',
-    '/lib64:/host/lib64:ro',
-    '/opt:/host/opt:ro',
-    '/sys:/host/sys:ro',
-    '/dev:/host/dev:ro',
+    ...(useSysroot
+      ? [
+        '/sys:/host/sys:ro',
+        '/dev:/host/dev:ro',
+      ]
+      : [
+        '/usr:/host/usr:ro',
+        '/bin:/host/bin:ro',
+        '/sbin:/host/sbin:ro',
+        '/lib:/host/lib:ro',
+        '/lib64:/host/lib64:ro',
+        '/opt:/host/opt:ro',
+        '/sys:/host/sys:ro',
+        '/dev:/host/dev:ro',
+      ]),
     `${workspaceDir}:/host${workspaceDir}:rw`,
     '/tmp:/host/tmp:rw',
   ];
diff --git a/src/services/agent-volumes/volume-builder.ts b/src/services/agent-volumes/volume-builder.ts
index 696e9105f..e74bea9be 100644
--- a/src/services/agent-volumes/volume-builder.ts
+++ b/src/services/agent-volumes/volume-builder.ts
@@ -38,7 +38,8 @@ export function buildAgentVolumes(params: AgentVolumesParams): string[] {
 
   logger.debug('Using selective path mounts for security');
 
-  agentVolumes.push(...buildSystemMounts(workspaceDir, config.chrootBinariesSourcePath));
+  const useSysroot = config.runnerTopology === 'arc-dind';
+  agentVolumes.push(...buildSystemMounts(workspaceDir, config.chrootBinariesSourcePath, useSysroot));
   agentVolumes.push(...buildHomeMounts({ config, effectiveHome, agentLogsPath, sessionStatePath }));
   agentVolumes.push(...buildEtcMounts(config));
   agentVolumes.push(generateHostsFileMount(config));
diff --git a/src/types/platform-options.ts b/src/types/platform-options.ts
index fb6ba6f1c..b0344af8a 100644
--- a/src/types/platform-options.ts
+++ b/src/types/platform-options.ts
@@ -1,9 +1,7 @@
 /**
- * GitHub platform deployment type and runner topology options.
+ * GitHub platform deployment type options.
  */
 
-export type RunnerTopology = 'standard' | 'arc-dind';
-
 export interface PlatformOptions {
   /**
    * The GitHub deployment type. Explicitly declares the environment so AWF can
@@ -19,30 +17,4 @@ export interface PlatformOptions {
    * regardless of the resolved API target hostname.
    */
   platformType?: 'github.com' | 'ghes' | 'ghec' | 'ghec-self-hosted';
-
-  /**
-   * Runner deployment topology.
-   *
-   * - 'standard' (default) — GitHub-hosted VM or self-hosted runner with local Docker.
-   * - 'arc-dind' — ARC (Actions Runner Controller) with Docker-in-Docker sidecar,
-   *   where the runner and Docker daemon have separate filesystems.
-   *
-   * When set to 'arc-dind', AWF applies overridable defaults:
-   *   - network.isolation = true (ARC k8s lacks NET_ADMIN)
-   *   - dind.preStageDirs = true
-   *   - Sysroot image activation (build-tools init container)
-   *   - Tool cache validation (warns if under /opt)
-   */
-  runnerTopology?: RunnerTopology;
-
-  /**
-   * Container image providing system-level build tools (gcc, make, libraries)
-   * for the agent's chroot base on ARC/DinD.
-   *
-   * Used as an init container that copies its filesystem into a named volume
-   * mounted at /host. Only used when runnerTopology is 'arc-dind'.
-   *
-   * Defaults to 'ghcr.io/github/gh-aw-firewall/build-tools:<imageTag>'.
-   */
-  sysrootImage?: string;
 }
diff --git a/src/types/runner-options.ts b/src/types/runner-options.ts
new file mode 100644
index 000000000..fc9970d90
--- /dev/null
+++ b/src/types/runner-options.ts
@@ -0,0 +1,20 @@
+/**
+ * Runner topology configuration options.
+ */
+export interface RunnerOptions {
+  /**
+   * Runner topology mode for AWF compose generation.
+   *
+   * - 'standard' (default) - GitHub-hosted VM or self-hosted runner with local Docker.
+   * - 'arc-dind' - ARC with Docker-in-Docker sidecar, enables sysroot staging
+   *   for split runner/daemon filesystems.
+   */
+  runnerTopology?: 'standard' | 'arc-dind';
+
+  /**
+   * Sysroot image used by arc-dind topology to stage build tools into /host.
+   *
+   * @default 'ghcr.io/github/gh-aw-firewall/build-tools:latest'
+   */
+  sysrootImage?: string;
+}
diff --git a/src/types/wrapper-config.ts b/src/types/wrapper-config.ts
index ad7ce952b..b5ef3fd06 100644
--- a/src/types/wrapper-config.ts
+++ b/src/types/wrapper-config.ts
@@ -14,6 +14,7 @@ import type { CliProxyOptions } from './cli-proxy-options';
 import type { RateLimitOptions } from './rate-limit-options';
 import type { RuntimeOptions } from './runtime-options';
 import type { PlatformOptions } from './platform-options';
+import type { RunnerOptions } from './runner-options';
 
 export type WrapperConfig =
   ContainerImageOptions
@@ -24,4 +25,5 @@ export type WrapperConfig =
   & CliProxyOptions
   & RateLimitOptions
   & RuntimeOptions
-  & PlatformOptions;
+  & PlatformOptions
+  & RunnerOptions;