fix(github): emit PROMPT_TOO_LARGE error on context overflow (#14166)

packages/opencode/test/cli/github-action.test.ts

@@ -1,5 +1,5 @@
 import { test, expect, describe } from "bun:test"
-import { extractResponseText } from "../../src/cli/cmd/github"
+import { extractResponseText, formatPromptTooLargeError } from "../../src/cli/cmd/github"
 import type { MessageV2 } from "../../src/session/message-v2"
 
 // Helper to create minimal valid parts
@@ -159,3 +159,39 @@ describe("extractResponseText", () => {
     expect(extractResponseText(parts)).toBe("Here's what I found")
   })
 })
+
+describe("formatPromptTooLargeError", () => {
+  test("formats error without files", () => {
+    const result = formatPromptTooLargeError([])
+    expect(result).toBe("PROMPT_TOO_LARGE: The prompt exceeds the model's context limit.")
+  })
+
+  test("formats error with files (base64 content)", () => {
+    // Base64 is ~33% larger than original, so we multiply by 0.75 to get original size
+    // 400 KB base64 = 300 KB original, 200 KB base64 = 150 KB original
+    const files = [
+      { filename: "screenshot.png", content: "a".repeat(400 * 1024) },
+      { filename: "diagram.png", content: "b".repeat(200 * 1024) },
+    ]
+    const result = formatPromptTooLargeError(files)
+
+    expect(result).toStartWith("PROMPT_TOO_LARGE: The prompt exceeds the model's context limit.")
+    expect(result).toInclude("Files in prompt:")
+    expect(result).toInclude("screenshot.png (300 KB)")
+    expect(result).toInclude("diagram.png (150 KB)")
+  })
+
+  test("lists all files when multiple present", () => {
+    // Base64 sizes: 4KB -> 3KB, 8KB -> 6KB, 12KB -> 9KB
+    const files = [
+      { filename: "img1.png", content: "x".repeat(4 * 1024) },
+      { filename: "img2.jpg", content: "y".repeat(8 * 1024) },
+      { filename: "img3.gif", content: "z".repeat(12 * 1024) },
+    ]
+    const result = formatPromptTooLargeError(files)
+
+    expect(result).toInclude("img1.png (3 KB)")
+    expect(result).toInclude("img2.jpg (6 KB)")
+    expect(result).toInclude("img3.gif (9 KB)")
+  })
+})