diff --git a/packages/app/src/attachments/utils.test.ts b/packages/app/src/attachments/utils.test.ts
index 6c50ce17c..319f247d2 100644
--- a/packages/app/src/attachments/utils.test.ts
+++ b/packages/app/src/attachments/utils.test.ts
@@ -34,6 +34,10 @@ describe("fileUriToPath", () => {
   it("converts Windows drive-letter file URIs back to paths", () => {
     expect(fileUriToPath("file:///C:/Users/file.txt")).toBe("C:/Users/file.txt");
   });
+
+  it("converts host-based file URIs back to UNC paths", () => {
+    expect(fileUriToPath("file://server/share/shot%231.png")).toBe("\\\\server\\share\\shot#1.png");
+  });
 });
 
 describe("localFileSourceToPath", () => {
diff --git a/packages/app/src/attachments/utils.ts b/packages/app/src/attachments/utils.ts
index e1422a7f4..bb1903f9f 100644
--- a/packages/app/src/attachments/utils.ts
+++ b/packages/app/src/attachments/utils.ts
@@ -162,7 +162,11 @@ export function fileUriToPath(uri: string): string {
   if (!uri.startsWith("file://")) {
     return uri;
   }
-  const decodedPath = decodeFilePathSource(uri.replace(/^file:\/\//, ""));
+  const fileSource = uri.slice("file://".length);
+  const decodedPath = decodeFilePathSource(fileSource);
+  if (!fileSource.startsWith("/")) {
+    return `\\\\${decodedPath.replace(/\//g, "\\")}`;
+  }
   return normalizeWindowsDrivePath(decodedPath.replace(/^\/([A-Za-z]:[\\/])/, "$1"));
 }
 
diff --git a/packages/server/src/server/agent/providers/codex-app-server-agent.test.ts b/packages/server/src/server/agent/providers/codex-app-server-agent.test.ts
index 4db6c748c..228f67124 100644
--- a/packages/server/src/server/agent/providers/codex-app-server-agent.test.ts
+++ b/packages/server/src/server/agent/providers/codex-app-server-agent.test.ts
@@ -3668,7 +3668,7 @@ describe("Codex app-server provider", () => {
     });
   });
 
-  test("emits imageView thread items as assistant markdown images using the path", () => {
+  test("emits imageView paths with spaces as valid assistant markdown images", () => {
     const session = createSession();
     const events: AgentStreamEvent[] = [];
     session.subscribe((event) => events.push(event));
@@ -3688,15 +3688,15 @@ describe("Codex app-server provider", () => {
         turnId: "test-turn",
         item: {
           type: "assistant_message",
-          text: "![Image](/tmp/paseo image.png)",
+          text: "![Image](file:///tmp/paseo%20image.png)",
         },
       },
     ]);
   });
 
   test.each([
-    ["savedPath", { savedPath: "/tmp/generated-camel.png" }, "/tmp/generated-camel.png"],
-    ["saved_path", { saved_path: "/tmp/generated-snake.png" }, "/tmp/generated-snake.png"],
+    ["savedPath", { savedPath: "/tmp/generated-camel.png" }, "file:///tmp/generated-camel.png"],
+    ["saved_path", { saved_path: "/tmp/generated-snake.png" }, "file:///tmp/generated-snake.png"],
   ])(
     "emits imageGeneration thread items with %s as assistant markdown images",
     (_fieldName, imageFields, expectedPath) => {
diff --git a/packages/server/src/server/agent/providers/provider-image-output.test.ts b/packages/server/src/server/agent/providers/provider-image-output.test.ts
index 49ffcf94b..0c32af04f 100644
--- a/packages/server/src/server/agent/providers/provider-image-output.test.ts
+++ b/packages/server/src/server/agent/providers/provider-image-output.test.ts
@@ -49,6 +49,37 @@ describe("isProviderImageMarkdown", () => {
     expect(isProviderImageMarkdown(markdown)).toBe(true);
   });
 
+  test("emits POSIX file paths with spaces as valid file URI markdown", () => {
+    const markdown = renderImageMarkdown("/home/user/Projects/Project With Spaces/screenshot.png");
+
+    expect(markdown).toBe(
+      "![Image](file:///home/user/Projects/Project%20With%20Spaces/screenshot.png)",
+    );
+  });
+
+  test("encodes URI-significant characters in POSIX file paths", () => {
+    const markdown = renderImageMarkdown("/tmp/screenshot#1?draft.png");
+
+    expect(markdown).toBe("![Image](file:///tmp/screenshot%231%3Fdraft.png)");
+  });
+
+  test("preserves double-leading slashes in POSIX file paths", () => {
+    const markdown = renderImageMarkdown("//tmp/screenshot#1.png");
+
+    expect(markdown).toBe("![Image](file:////tmp/screenshot%231.png)");
+  });
+
+  test.each([
+    ["UNC", "\\\\server\\share\\shot#1.png", "file://server/share/shot%231.png"],
+    [
+      "extended UNC",
+      "\\\\?\\UNC\\server\\share\\shot?draft.png",
+      "file://server/share/shot%3Fdraft.png",
+    ],
+  ])("encodes %s image paths as file URIs", (_label, imagePath, expectedSource) => {
+    expect(renderImageMarkdown(imagePath)).toBe(`![Image](${expectedSource})`);
+  });
+
   test("rejects user-authored markdown that is not a materialized attachment", () => {
     // No content hash — a hand-written path, not something the writer produced.
     expect(isProviderImageMarkdown("![diagram](./paseo-attachments/notes.png)")).toBe(false);
diff --git a/packages/server/src/server/agent/providers/provider-image-output.ts b/packages/server/src/server/agent/providers/provider-image-output.ts
index 649e5bdfe..387158b32 100644
--- a/packages/server/src/server/agent/providers/provider-image-output.ts
+++ b/packages/server/src/server/agent/providers/provider-image-output.ts
@@ -128,9 +128,39 @@ function escapeMarkdownImageAlt(value: string): string {
   return value.replace(/\\/g, "\\\\").replace(/\]/g, "\\]");
 }
 
+function encodeFilePath(value: string): string {
+  return value
+    .split("/")
+    .map((segment) => encodeURIComponent(segment))
+    .join("/");
+}
+
+function windowsFileUri(value: string): string | null {
+  const isWindowsNetworkPath = value.startsWith("\\\\");
+  let normalizedPath = value.replace(/\\/g, "/");
+  if (/^\/\/\?\/UNC\//i.test(normalizedPath)) {
+    normalizedPath = `//${normalizedPath.slice(8)}`;
+  } else if (/^\/\/\?\/[A-Za-z]:\//.test(normalizedPath)) {
+    normalizedPath = normalizedPath.slice(4);
+  }
+
+  if (/^[A-Za-z]:\//.test(normalizedPath)) {
+    const drive = normalizedPath.slice(0, 2);
+    return `file:///${drive}${encodeFilePath(normalizedPath.slice(2))}`;
+  }
+  if (isWindowsNetworkPath && normalizedPath.startsWith("//")) {
+    return `file:${encodeFilePath(normalizedPath)}`;
+  }
+  return null;
+}
+
 function markdownImageSource(value: string): string {
-  if (/^[A-Za-z]:[\\/]/.test(value)) {
-    return `file:///${value.replace(/\\/g, "/")}`;
+  const windowsUri = windowsFileUri(value);
+  if (windowsUri) {
+    return windowsUri;
+  }
+  if (value.startsWith("/")) {
+    return `file://${encodeFilePath(value)}`;
   }
   return value;
 }