fix(sandbox): anchor fs-bridge mkdirp

src/agents/sandbox/fs-bridge.test.ts

@@ -135,10 +135,12 @@ async function expectMkdirpAllowsExistingDirectory(params?: { forceBoundaryIoFal
 
     await expect(bridge.mkdirp({ filePath: "memory/kemik" })).resolves.toBeUndefined();
 
-    const mkdirCall = findCallByScriptFragment('mkdir -p -- "$1"');
+    const mkdirCall = findCallByScriptFragment('mkdir -p -- "$2"');
     expect(mkdirCall).toBeDefined();
-    const mkdirPath = mkdirCall ? getDockerPathArg(mkdirCall[0]) : "";
-    expect(mkdirPath).toBe("/workspace/memory/kemik");
+    const mkdirParent = mkdirCall ? getDockerArg(mkdirCall[0], 1) : "";
+    const mkdirBase = mkdirCall ? getDockerArg(mkdirCall[0], 2) : "";
+    expect(mkdirParent).toBe("/workspace/memory");
+    expect(mkdirBase).toBe("kemik");
   });
 }
 
@@ -250,6 +252,24 @@ describe("sandbox fs bridge shell compatibility", () => {
     expect(scripts.some((script) => script.includes('mv -f -- "$1" "$2"'))).toBe(true);
   });
 
+  it("anchors mkdirp operations on canonical parent + basename", async () => {
+    const bridge = createSandboxFsBridge({ sandbox: createSandbox() });
+
+    await bridge.mkdirp({ filePath: "nested/leaf" });
+
+    const mkdirCall = findCallByScriptFragment('mkdir -p -- "$2"');
+    expect(mkdirCall).toBeDefined();
+    const args = mkdirCall?.[0] ?? [];
+    expect(getDockerArg(args, 1)).toBe("/workspace/nested");
+    expect(getDockerArg(args, 2)).toBe("leaf");
+    expect(args).not.toContain("/workspace/nested/leaf");
+
+    const canonicalCalls = findCallsByScriptFragment('readlink -f -- "$cursor"');
+    expect(
+      canonicalCalls.some(([callArgs]) => getDockerArg(callArgs, 1) === "/workspace/nested"),
+    ).toBe(true);
+  });
+
   it("anchors remove operations on canonical parent + basename", async () => {
     const bridge = createSandboxFsBridge({ sandbox: createSandbox() });
 
@@ -329,7 +349,8 @@ describe("sandbox fs bridge shell compatibility", () => {
       await expect(bridge.mkdirp({ filePath: "memory/kemik" })).rejects.toThrow(
         /cannot create directories/i,
       );
-      expect(mockedExecDockerRaw).not.toHaveBeenCalled();
+      const scripts = getScriptsFromCalls();
+      expect(scripts.some((script) => script.includes('mkdir -p -- "$2"'))).toBe(false);
     });
   });
 

src/agents/sandbox/fs-bridge.ts

@@ -155,6 +155,7 @@ class SandboxFsBridgeImpl implements SandboxFsBridge {
   async mkdirp(params: { filePath: string; cwd?: string; signal?: AbortSignal }): Promise<void> {
     const target = this.resolveResolvedPath(params);
     this.ensureWriteAccess(target, "create directories");
+    const anchoredTarget = await this.resolveAnchoredSandboxEntry(target);
     await this.runCheckedCommand({
       checks: [
         {
@@ -166,8 +167,8 @@ class SandboxFsBridgeImpl implements SandboxFsBridge {
           },
         },
       ],
-      script: 'set -eu; mkdir -p -- "$1"',
-      args: [target.containerPath],
+      script: 'set -eu\ncd -- "$1"\nmkdir -p -- "$2"',
+      args: [anchoredTarget.canonicalParentPath, anchoredTarget.basename],
       signal: params.signal,
     });
   }