fix: Use inline data for local image URIs

Closes #231

Author: benbrandt

src/CodexAcpClient.ts

@@ -703,7 +703,7 @@ function buildPromptItems(prompt: acp.ContentBlock[]): UserInput[] {
             case "text":
                 return {type: "text", text: block.text, text_elements: []};
             case "image": {
-                const url = block.uri ?? `data:${block.mimeType};base64,${block.data}`;
+                const url = shouldUseImageUri(block.uri) ? block.uri : imageDataUrl(block);
                 return {type: "image", url};
             }
             case "resource_link":
@@ -729,6 +729,22 @@ function buildPromptItems(prompt: acp.ContentBlock[]): UserInput[] {
     }).filter((block): block is UserInput => block !== null);
 }
 
+function imageDataUrl(block: acp.ContentBlock & { type: "image" }): string {
+    return `data:${block.mimeType};base64,${block.data}`;
+}
+
+function shouldUseImageUri(uri: string | null | undefined): uri is string {
+    if (!uri) {
+        return false;
+    }
+    try {
+        const protocol = new URL(uri).protocol;
+        return protocol === "http:" || protocol === "https:";
+    } catch {
+        return false;
+    }
+}
+
 function isImageMimeType(mimeType: string | null | undefined): mimeType is string {
     return mimeType?.startsWith("image/") ?? false;
 }

src/__tests__/CodexACPAgent/CodexAcpClient.test.ts

@@ -1412,6 +1412,24 @@ describe('ACP server test', { timeout: 40_000 }, () => {
         }));
     });
 
+    it ('should use inline image data when image uri is a local file', async () => {
+        const { mockFixture, turnStartSpy } = setupPromptFixture({
+            supportedInputModalities: ["text", "image"],
+        });
+
+        const prompt: acp.ContentBlock[] = [
+            { type: "image", mimeType: "image/png", data: "abc123", uri: "file:///Users/test/Desktop/Screenshot%201.png" },
+        ];
+
+        await mockFixture.getCodexAcpAgent().prompt({ sessionId: "id", prompt });
+
+        expect(turnStartSpy).toHaveBeenCalledWith(expect.objectContaining({
+            input: [
+                { type: "image", url: "data:image/png;base64,abc123" },
+            ]
+        }));
+    });
+
     it ('should show rate limits from multiple sources in status', async () => {
         const rateLimits: RateLimitsMap = new Map();
         rateLimits.set("limit-1", {