diff --git a/src/agents/sandbox/fs-paths.test.ts b/src/agents/sandbox/fs-paths.test.ts
index a796df22a74..40d59395f01 100644
--- a/src/agents/sandbox/fs-paths.test.ts
+++ b/src/agents/sandbox/fs-paths.test.ts
@@ -1,12 +1,12 @@
 import path from "node:path";
 import { describe, expect, it } from "vitest";
+import type { SandboxContext } from "./types.js";
 import {
   buildSandboxFsMounts,
   parseSandboxBindMount,
   resolveSandboxFsPathWithMounts,
 } from "./fs-paths.js";
 import { createSandboxTestContext } from "./test-fixtures.js";
-import type { SandboxContext } from "./types.js";
 
 function createSandbox(overrides?: Partial<SandboxContext>): SandboxContext {
   return createSandboxTestContext({ overrides });
@@ -38,6 +38,14 @@ describe("parseSandboxBindMount", () => {
       writable: true,
     });
   });
+
+  it("parses UNC-style host paths", () => {
+    expect(parseSandboxBindMount("//server/share:/workspace:ro")).toEqual({
+      hostRoot: path.resolve("//server/share"),
+      containerRoot: "/workspace",
+      writable: false,
+    });
+  });
 });
 
 describe("resolveSandboxFsPathWithMounts", () => {
diff --git a/src/agents/sandbox/fs-paths.ts b/src/agents/sandbox/fs-paths.ts
index ce0f0932079..446d5266843 100644
--- a/src/agents/sandbox/fs-paths.ts
+++ b/src/agents/sandbox/fs-paths.ts
@@ -1,7 +1,7 @@
 import path from "node:path";
+import type { SandboxContext } from "./types.js";
 import { resolveSandboxInputPath, resolveSandboxPath } from "../sandbox-paths.js";
 import { SANDBOX_AGENT_WORKSPACE_MOUNT } from "./constants.js";
-import type { SandboxContext } from "./types.js";
 
 export type SandboxFsMount = {
   hostRoot: string;
@@ -61,36 +61,34 @@ export function parseSandboxBindMount(spec: string): ParsedBindMount | null {
 }
 
 function splitBindSpec(spec: string): SplitBindSpec | null {
-  // Windows drive-letter host path: C:\\path:/container[:opts] or C:/path:/container[:opts]
-  if (/^[A-Za-z]:[\\/]/.test(spec)) {
-    const hostEnd = spec.indexOf(":", 2);
-    if (hostEnd === -1) {
-      return null;
-    }
-    const host = spec.slice(0, hostEnd);
-    const rest = spec.slice(hostEnd + 1);
-    const optionsStart = rest.indexOf(":");
-    if (optionsStart === -1) {
-      return { host, container: rest, options: "" };
-    }
-    return {
-      host,
-      container: rest.slice(0, optionsStart),
-      options: rest.slice(optionsStart + 1),
-    };
-  }
-
-  const parts = spec.split(":");
-  if (parts.length < 2) {
+  const separator = getHostContainerSeparatorIndex(spec);
+  if (separator === -1) {
     return null;
   }
+
+  const host = spec.slice(0, separator);
+  const rest = spec.slice(separator + 1);
+  const optionsStart = rest.indexOf(":");
+  if (optionsStart === -1) {
+    return { host, container: rest, options: "" };
+  }
   return {
-    host: parts[0] ?? "",
-    container: parts[1] ?? "",
-    options: parts.slice(2).join(":"),
+    host,
+    container: rest.slice(0, optionsStart),
+    options: rest.slice(optionsStart + 1),
   };
 }
 
+function getHostContainerSeparatorIndex(spec: string): number {
+  const hasDriveLetterPrefix = /^[A-Za-z]:[\\/]/.test(spec);
+  for (let i = hasDriveLetterPrefix ? 2 : 0; i < spec.length; i += 1) {
+    if (spec[i] === ":") {
+      return i;
+    }
+  }
+  return -1;
+}
+
 export function buildSandboxFsMounts(sandbox: SandboxContext): SandboxFsMount[] {
   const mounts: SandboxFsMount[] = [
     {