diff --git a/src/agents/sandbox-paths.test.ts b/src/agents/sandbox-paths.test.ts
index 328366860..0969c8550 100644
--- a/src/agents/sandbox-paths.test.ts
+++ b/src/agents/sandbox-paths.test.ts
@@ -1,10 +1,54 @@
 import fs from "node:fs/promises";
 import os from "node:os";
 import path from "node:path";
+import { pathToFileURL } from "node:url";
 import { describe, expect, it } from "vitest";
 import { resolveSandboxedMediaSource } from "./sandbox-paths.js";
 
 describe("resolveSandboxedMediaSource", () => {
+  // Group 1: /tmp paths (the bug fix)
+  it("allows absolute paths under os.tmpdir()", async () => {
+    const sandboxDir = await fs.mkdtemp(path.join(os.tmpdir(), "sandbox-media-"));
+    try {
+      const result = await resolveSandboxedMediaSource({
+        media: path.join(os.tmpdir(), "image.png"),
+        sandboxRoot: sandboxDir,
+      });
+      expect(result).toBe(path.join(os.tmpdir(), "image.png"));
+    } finally {
+      await fs.rm(sandboxDir, { recursive: true, force: true });
+    }
+  });
+
+  it("allows file:// URLs pointing to os.tmpdir()", async () => {
+    const sandboxDir = await fs.mkdtemp(path.join(os.tmpdir(), "sandbox-media-"));
+    try {
+      const tmpFile = path.join(os.tmpdir(), "photo.png");
+      const fileUrl = pathToFileURL(tmpFile).href;
+      const result = await resolveSandboxedMediaSource({
+        media: fileUrl,
+        sandboxRoot: sandboxDir,
+      });
+      expect(result).toBe(tmpFile);
+    } finally {
+      await fs.rm(sandboxDir, { recursive: true, force: true });
+    }
+  });
+
+  it("allows nested paths under os.tmpdir()", async () => {
+    const sandboxDir = await fs.mkdtemp(path.join(os.tmpdir(), "sandbox-media-"));
+    try {
+      const result = await resolveSandboxedMediaSource({
+        media: path.join(os.tmpdir(), "subdir", "deep", "file.png"),
+        sandboxRoot: sandboxDir,
+      });
+      expect(result).toBe(path.join(os.tmpdir(), "subdir", "deep", "file.png"));
+    } finally {
+      await fs.rm(sandboxDir, { recursive: true, force: true });
+    }
+  });
+
+  // Group 2: Sandbox-relative paths (existing behavior)
   it("resolves sandbox-relative paths", async () => {
     const sandboxDir = await fs.mkdtemp(path.join(os.tmpdir(), "sandbox-media-"));
     try {
@@ -18,7 +62,8 @@ describe("resolveSandboxedMediaSource", () => {
     }
   });
 
-  it("rejects paths outside sandbox root", async () => {
+  // Group 3: Rejections (security)
+  it("rejects paths outside sandbox root and tmpdir", async () => {
     const sandboxDir = await fs.mkdtemp(path.join(os.tmpdir(), "sandbox-media-"));
     try {
       await expect(
@@ -71,6 +116,7 @@ describe("resolveSandboxedMediaSource", () => {
     }
   });
 
+  // Group 4: Passthrough
   it("passes HTTP URLs through unchanged", async () => {
     const result = await resolveSandboxedMediaSource({
       media: "https://example.com/image.png",
diff --git a/src/agents/sandbox-paths.ts b/src/agents/sandbox-paths.ts
index c5547291c..8dbe822d3 100644
--- a/src/agents/sandbox-paths.ts
+++ b/src/agents/sandbox-paths.ts
@@ -90,12 +90,18 @@ export async function resolveSandboxedMediaSource(params: {
       throw new Error(`Invalid file:// URL for sandboxed media: ${raw}`);
     }
   }
-  const resolved = await assertSandboxPath({
+  // Allow files under os.tmpdir() — consistent with buildMediaLocalRoots() defaults.
+  const resolved = path.resolve(params.sandboxRoot, candidate);
+  const tmpDir = os.tmpdir();
+  if (resolved === tmpDir || resolved.startsWith(tmpDir + path.sep)) {
+    return resolved;
+  }
+  const sandboxResult = await assertSandboxPath({
     filePath: candidate,
     cwd: params.sandboxRoot,
     root: params.sandboxRoot,
   });
-  return resolved.resolved;
+  return sandboxResult.resolved;
 }
 
 async function assertNoSymlinkEscape(