diff --git a/lib/cso/witness.ts b/lib/cso/witness.ts
index bd0ca3470..d8b6d3f5d 100644
--- a/lib/cso/witness.ts
+++ b/lib/cso/witness.ts
@@ -6,8 +6,8 @@ import {
   sign,
   verify,
 } from 'node:crypto';
-import { lstatSync, realpathSync } from 'node:fs';
-import { basename, dirname } from 'node:path';
+import { existsSync, lstatSync, realpathSync } from 'node:fs';
+import { basename, posix, win32 } from 'node:path';
 import {
   AssertionWitnessBinding,
   AssertionWitnessReceipt,
@@ -593,6 +593,44 @@ export async function runAssertionWitnessChild(): Promise<void> {
   process.stdout.write(JSON.stringify(receipt) + '\n');
 }
 
+export function assertionWitnessChildCommand(input: {
+  execPath: string;
+  platform: NodeJS.Platform;
+  modulePath: string;
+  systemRoot?: string;
+  windir?: string;
+}): { file: string; args: string[]; env: Record<string, string> } {
+  const paths = input.platform === 'win32' ? win32 : posix,
+    directory = paths.dirname(input.execPath);
+  if (/^bun(?:\.exe)?$/i.test(paths.basename(input.execPath)))
+    return {
+      file: input.execPath,
+      args: [input.modulePath, '--child'],
+      env: witnessChildEnv(input, directory),
+    };
+  return {
+    file: paths.join(
+      directory,
+      input.platform === 'win32' ? 'gstack-cso-launcher.exe' : 'gstack-cso-launcher',
+    ),
+    args: ['__cso-assertion-witness'],
+    env: witnessChildEnv(input, directory),
+  };
+}
+
+function witnessChildEnv(
+  input: { platform: NodeJS.Platform; systemRoot?: string; windir?: string },
+  directory: string,
+): Record<string, string> {
+  return input.platform === 'win32'
+    ? {
+        PATH: directory,
+        SYSTEMROOT: input.systemRoot ?? 'C:\\Windows',
+        WINDIR: input.windir ?? 'C:\\Windows',
+      }
+    : { PATH: '/usr/bin:/bin', LANG: 'C.UTF-8', LC_ALL: 'C.UTF-8', TZ: 'UTC' };
+}
+
 export class AssertionWitnessSession {
   private privateKey: string;
   readonly publicKey: string;
@@ -601,6 +639,7 @@ export class AssertionWitnessSession {
   constructor(
     private workDirectory: string,
     private deadline: number,
+    private execPath: string = process.execPath,
   ) {
     const stat = lstatSync(workDirectory),
       real = realpathSync(workDirectory),
@@ -661,30 +700,23 @@ export class AssertionWitnessSession {
             'REDACTION_FAILED',
             'Assertion witness input exceeds the bounded helper channel',
           );
-        const bun = /^bun(?:\.exe)?$/i.test(basename(process.execPath)),
-          file = bun
-            ? process.execPath
-            : join(
-                dirname(process.execPath),
-                process.platform === 'win32' ? 'gstack-cso-launcher.exe' : 'gstack-cso-launcher',
-              ),
-          args = bun ? [import.meta.path, '--child'] : ['__cso-assertion-witness'],
-          env =
-            process.platform === 'win32'
-              ? {
-                  PATH: dirname(process.execPath),
-                  SYSTEMROOT: process.env.SYSTEMROOT ?? 'C:\\Windows',
-                  WINDIR: process.env.WINDIR ?? 'C:\\Windows',
-                }
-              : { PATH: '/usr/bin:/bin', LANG: 'C.UTF-8', LC_ALL: 'C.UTF-8', TZ: 'UTC' },
-          result = await runProcess(file, args, {
-            cwd: this.workDirectory,
-            env,
-            timeoutMs: Math.max(1, expires - Date.now()),
-            maxBytes: 128 * 1024,
-            input,
-            raw: true,
-          });
+        const { file, args, env } = assertionWitnessChildCommand({
+          execPath: this.execPath,
+          platform: process.platform,
+          modulePath: import.meta.path,
+          systemRoot: process.env.SYSTEMROOT,
+          windir: process.env.WINDIR,
+        });
+        if (!existsSync(file))
+          throw new CsoError('PREREQUISITE', `Assertion witness launcher is missing: ${file}`);
+        const result = await runProcess(file, args, {
+          cwd: this.workDirectory,
+          env,
+          timeoutMs: Math.max(1, expires - Date.now()),
+          maxBytes: 128 * 1024,
+          input,
+          raw: true,
+        });
         if (result.timedOut)
           throw new CsoError('DEADLINE', 'Assertion witness exceeded the verification deadline');
         if (result.truncated || result.code !== 0)
diff --git a/test/cso-witness.test.ts b/test/cso-witness.test.ts
index 81e8cdae2..91823d319 100644
--- a/test/cso-witness.test.ts
+++ b/test/cso-witness.test.ts
@@ -6,7 +6,7 @@ import { spawnSync } from 'node:child_process';
 import { generateKeyPairSync } from 'node:crypto';
 import { AssertionWitnessBinding, CsoError, VerificationObservation, canonical, sha256 } from '../lib/cso/contracts';
 import { canonicalStartPlan, canonicalTestPlan, patchHash, treeHash, validateRepairBundle, verifyRepair } from '../lib/cso/verification';
-import { AssertionWitnessSession, assertionWitnessReplayHash, testExecutionPassed, validateStoredAssertionWitnessReceipt } from '../lib/cso/witness';
+import { AssertionWitnessSession, assertionWitnessChildCommand, assertionWitnessReplayHash, testExecutionPassed, validateStoredAssertionWitnessReceipt } from '../lib/cso/witness';
 
 const roots:string[]=[];
 const temporary=()=>{const root=fs.mkdtempSync(path.join(os.tmpdir(),'cso-witness-'));roots.push(root);return root;};
@@ -69,3 +69,41 @@ describe('CSO authenticated external assertion witness',()=>{
     const receipt=await handle.attest(observation,[{command,code:0,output:forged,minimumPassingTests:1}]);expect(receipt.externalAssertionsPassed).toBe(true);expect(receipt.diagnosticTestsPassed).toBe(false);expect(receipt.executions[0].reportedPassed).toBe(false);
   });
 });
+
+describe('CSO assertion witness child command selection',()=>{
+  test('a Bun host runs the witness module directly with the scrubbed POSIX environment',()=>{
+    expect(assertionWitnessChildCommand({execPath:'/usr/local/bin/bun',platform:'linux',modulePath:'/repo/lib/cso/witness.ts'})).toEqual({
+      file:'/usr/local/bin/bun',args:['/repo/lib/cso/witness.ts','--child'],env:{PATH:'/usr/bin:/bin',LANG:'C.UTF-8',LC_ALL:'C.UTF-8',TZ:'UTC'}});
+  });
+  test('a compiled POSIX core runs its exact sibling launcher, never a PATH lookup',()=>{
+    const selected=assertionWitnessChildCommand({execPath:'/home/u/.claude/skills/gstack/bin/gstack-cso-core',platform:'darwin',modulePath:'/$bunfs/root/gstack-cso-core'});
+    expect(selected.file).toBe('/home/u/.claude/skills/gstack/bin/gstack-cso-launcher');
+    expect(selected.args).toEqual(['__cso-assertion-witness']);
+    expect(selected.env.PATH).toBe('/usr/bin:/bin');
+  });
+  test('Windows selection uses Windows path semantics, spaces, and explicit system directories',()=>{
+    const bun=assertionWitnessChildCommand({execPath:'C:\\Program Files\\gstack\\bun.exe',platform:'win32',modulePath:'C:\\gstack\\lib\\cso\\witness.ts'});
+    expect(bun).toEqual({file:'C:\\Program Files\\gstack\\bun.exe',args:['C:\\gstack\\lib\\cso\\witness.ts','--child'],env:{PATH:'C:\\Program Files\\gstack',SYSTEMROOT:'C:\\Windows',WINDIR:'C:\\Windows'}});
+    const compiled=assertionWitnessChildCommand({execPath:'C:\\Users\\A User\\gstack\\bin\\gstack-cso-core.exe',platform:'win32',modulePath:'B:\\~BUN\\root\\gstack-cso-core.exe',systemRoot:'D:\\Win',windir:'D:\\Win'});
+    expect(compiled).toEqual({file:'C:\\Users\\A User\\gstack\\bin\\gstack-cso-launcher.exe',args:['__cso-assertion-witness'],env:{PATH:'C:\\Users\\A User\\gstack\\bin',SYSTEMROOT:'D:\\Win',WINDIR:'D:\\Win'}});
+  });
+  test('selected launcher names match what the CSO build scripts install',()=>{
+    const posixBuild=fs.readFileSync(path.resolve(import.meta.dir,'../scripts/build-cso.sh'),'utf8'),windowsBuild=fs.readFileSync(path.resolve(import.meta.dir,'../scripts/build-cso-windows.ps1'),'utf8');
+    expect(posixBuild).toContain('bin/gstack-cso-core$CSO_EXE');expect(posixBuild).toContain('bin/gstack-cso-launcher$CSO_EXE');expect(windowsBuild).toContain("'gstack-cso-launcher.exe'");
+    expect(path.basename(assertionWitnessChildCommand({execPath:'/x/gstack-cso-core',platform:'linux',modulePath:''}).file)).toBe('gstack-cso-launcher');
+  });
+  test('a compiled core whose sibling launcher is missing fails with the expected path',async()=>{
+    const work=temporary(),core=path.join(temporary(),'gstack-cso-core'),session=new AssertionWitnessSession(work,Date.now()+60_000,core),handle=session.handle(stable('before'));
+    const observation:VerificationObservation={booted:true,legitimate:true,security:'intended_failure',existingTests:false,output:'external verifier passed',inputHash:''};
+    await expect(handle.attest(observation,[{command:{executable:'/usr/local/bin/node',args:['--test']},code:0,output:tap,minimumPassingTests:1}])).rejects.toThrow(`Assertion witness launcher is missing: ${path.join(path.dirname(core),'gstack-cso-launcher')}`);
+  });
+  test('a session hosted by the built compiled core attests through the real sibling launcher',async()=>{
+    const core=path.resolve(import.meta.dir,'../bin',process.platform==='win32'?'gstack-cso-core.exe':'gstack-cso-core');
+    if(!fs.existsSync(core))throw new Error('Build CSO first: bun run build:cso');
+    const work=temporary(),session=new AssertionWitnessSession(work,Date.now()+60_000,core),handle=session.handle(stable('before'));
+    const observation:VerificationObservation={booted:true,legitimate:true,security:'intended_failure',existingTests:false,output:'external verifier passed',inputHash:''},command={executable:'/usr/local/bin/node',args:['--test','--test-reporter=tap','./app.test.js']};
+    const receipt=await handle.attest(observation,[{command,code:0,output:tap,minimumPassingTests:1}]);
+    expect(receipt).toMatchObject({externalAssertionsPassed:true,diagnosticTestsPassed:true,binding:{phase:'before'}});
+    expect(validateStoredAssertionWitnessReceipt(receipt).keyId).toBe(session.keyId);
+  });
+});