diff --git a/.changeset/calm-codex-flags.md b/.changeset/calm-codex-flags.md
new file mode 100644
index 00000000..ea350353
--- /dev/null
+++ b/.changeset/calm-codex-flags.md
@@ -0,0 +1,5 @@
+---
+"@stoneforge/smithy": patch
+---
+
+Update Codex interactive sessions to use the documented workspace-write sandbox flag.
diff --git a/packages/smithy/src/providers/codex/interactive.ts b/packages/smithy/src/providers/codex/interactive.ts
index f2d28c9a..f022117e 100644
--- a/packages/smithy/src/providers/codex/interactive.ts
+++ b/packages/smithy/src/providers/codex/interactive.ts
@@ -21,6 +21,31 @@ import { shellQuote } from '../shell-quote.js';
 // Helpers
 // ============================================================================
 
+type CodexInteractiveArgOptions = Pick<
+  InteractiveSpawnOptions,
+  'resumeSessionId' | 'workingDirectory' | 'model'
+>;
+
+export function buildCodexInteractiveArgs(
+  options: CodexInteractiveArgOptions,
+  platform: NodeJS.Platform = process.platform,
+): string[] {
+  const quote = (value: string) => shellQuote(value, platform);
+  const args: string[] = [];
+
+  if (options.resumeSessionId) {
+    args.push('resume', quote(options.resumeSessionId), '--sandbox', 'workspace-write');
+  } else {
+    args.push('--sandbox', 'workspace-write', '--cd', quote(options.workingDirectory));
+  }
+
+  if (options.model) {
+    args.push('--model', quote(options.model));
+  }
+
+  return args;
+}
+
 // ============================================================================
 // Codex Interactive Session
 // ============================================================================
@@ -95,7 +120,7 @@ export class CodexInteractiveProvider implements InteractiveProvider {
   }
 
   async spawn(options: InteractiveSpawnOptions): Promise<InteractiveSession> {
-    const args = this.buildArgs(options);
+    const args = buildCodexInteractiveArgs(options);
 
     const env: Record<string, string> = {
       ...(process.env as Record<string, string>),
@@ -156,20 +181,4 @@ export class CodexInteractiveProvider implements InteractiveProvider {
     }
   }
 
-  private buildArgs(options: InteractiveSpawnOptions): string[] {
-    const args: string[] = [];
-
-    if (options.resumeSessionId) {
-      args.push('resume', shellQuote(options.resumeSessionId), '--full-auto');
-    } else {
-      args.push('--full-auto', '--cd', shellQuote(options.workingDirectory));
-    }
-
-    // Add model flag if provided
-    if (options.model) {
-      args.push('--model', shellQuote(options.model));
-    }
-
-    return args;
-  }
 }
diff --git a/packages/smithy/src/providers/codex/provider.bun.test.ts b/packages/smithy/src/providers/codex/provider.bun.test.ts
index 02ccf979..e2344d8e 100644
--- a/packages/smithy/src/providers/codex/provider.bun.test.ts
+++ b/packages/smithy/src/providers/codex/provider.bun.test.ts
@@ -6,6 +6,7 @@
 
 import { describe, it, expect, mock, beforeEach, afterEach } from 'bun:test';
 import type { CodexClient, CodexModelInfo } from './server-manager.js';
+import { buildCodexInteractiveArgs } from './interactive.js';
 import { posixShellQuote, shellQuote } from '../shell-quote.js';
 
 // ---------------------------------------------------------------------------
@@ -204,68 +205,53 @@ describe('CodexHeadlessProvider model passthrough', () => {
 // ---------------------------------------------------------------------------
 
 describe('CodexInteractiveProvider model flag', () => {
-  // Use the POSIX form directly so assertions are deterministic regardless of
-  // the host OS the tests run on. A separate suite below covers the Windows
-  // quoter.
-  function buildArgs(options: {
-    resumeSessionId?: string;
-    workingDirectory: string;
-    model?: string;
-  }): string[] {
-    const shellQuote = posixShellQuote;
-    const args: string[] = [];
-
-    if (options.resumeSessionId) {
-      args.push('resume', shellQuote(options.resumeSessionId), '--full-auto');
-    } else {
-      args.push('--full-auto', '--cd', shellQuote(options.workingDirectory));
-    }
-
-    // Add model flag if provided
-    if (options.model) {
-      args.push('--model', shellQuote(options.model));
-    }
-
-    return args;
-  }
+  it('builds interactive spawn args with the documented workspace-write sandbox', () => {
+    const args = buildCodexInteractiveArgs({
+      workingDirectory: '/workspace',
+      model: 'gpt-4o',
+    }, 'linux');
+
+    expect(args).toEqual(['--sandbox', 'workspace-write', '--cd', "'/workspace'", '--model', "'gpt-4o'"]);
+  });
 
   it('should include --model flag when model is provided', () => {
-    const args = buildArgs({
+    const args = buildCodexInteractiveArgs({
       workingDirectory: '/workspace',
       model: 'gpt-4o',
-    });
+    }, 'linux');
 
     expect(args).toContain('--model');
     expect(args).toContain("'gpt-4o'");
-    expect(args).toEqual(['--full-auto', '--cd', "'/workspace'", '--model', "'gpt-4o'"]);
+    expect(args).toEqual(['--sandbox', 'workspace-write', '--cd', "'/workspace'", '--model', "'gpt-4o'"]);
   });
 
   it('should not include --model flag when model is undefined', () => {
-    const args = buildArgs({
+    const args = buildCodexInteractiveArgs({
       workingDirectory: '/workspace',
-    });
+    }, 'linux');
 
     expect(args).not.toContain('--model');
-    expect(args).toEqual(['--full-auto', '--cd', "'/workspace'"]);
+    expect(args).toEqual(['--sandbox', 'workspace-write', '--cd', "'/workspace'"]);
   });
 
   it('should include --model flag when resuming with model', () => {
-    const args = buildArgs({
+    const args = buildCodexInteractiveArgs({
       resumeSessionId: 'thr_abc123',
       workingDirectory: '/workspace',
       model: 'o3-mini',
-    });
+    }, 'linux');
 
     expect(args).toContain('resume');
     expect(args).toContain('--model');
     expect(args).toContain("'o3-mini'");
+    expect(args).toEqual(['resume', "'thr_abc123'", '--sandbox', 'workspace-write', '--model', "'o3-mini'"]);
   });
 
   it('should properly quote model names with special characters', () => {
-    const args = buildArgs({
+    const args = buildCodexInteractiveArgs({
       workingDirectory: '/workspace',
       model: "model's-name",
-    });
+    }, 'linux');
 
     expect(args).toContain("--model");
     expect(args).toContain("'model'\\''s-name'");