From db9e942398a210fb02d3ce9d1d8a2f8ae231cdd6 Mon Sep 17 00:00:00 2001
From: Kit Langton <kit.langton@gmail.com>
Date: Fri, 31 Jul 2026 13:24:05 -0400
Subject: [PATCH] fix(core): resize large image attachments (#39919)

---
 packages/core/src/session.ts              |  6 ++--
 packages/core/test/session-prompt.test.ts | 36 +++++++++++++++++++++--
 2 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/packages/core/src/session.ts b/packages/core/src/session.ts
index 9f5f8e4aa5a..e438e67bb26 100644
--- a/packages/core/src/session.ts
+++ b/packages/core/src/session.ts
@@ -938,7 +938,7 @@ const materializeAttachment = Effect.fn("Session.materializeAttachment")(functio
       : resolved.bytes
   const normalized = yield* normalizeImageAttachment(
     input,
-    Base64.make(Buffer.from(content).toString("base64")),
+    Buffer.from(content).toString("base64"),
     mime,
     image,
   )
@@ -954,11 +954,11 @@ const materializeAttachment = Effect.fn("Session.materializeAttachment")(functio
 
 const normalizeImageAttachment = Effect.fn("Session.normalizeImageAttachment")(function* (
   input: PromptInput.FileAttachment,
-  data: Base64,
+  data: string,
   mime: string,
   image: Effect.Effect<Image.Interface>,
 ) {
-  if (!mime.startsWith("image/")) return { data, mime }
+  if (!mime.startsWith("image/")) return { data: Base64.make(data), mime }
   const service = yield* image
   const label = input.name ?? (input.uri.startsWith("data:") ? "inline attachment" : input.uri)
   const content = { uri: label, content: data, encoding: "base64" as const, mime }
diff --git a/packages/core/test/session-prompt.test.ts b/packages/core/test/session-prompt.test.ts
index 9f30efe9c06..b292a11c434 100644
--- a/packages/core/test/session-prompt.test.ts
+++ b/packages/core/test/session-prompt.test.ts
@@ -26,8 +26,8 @@ import { SessionPendingTable, SessionMessageTable, SessionTable } from "@opencod
 import { SessionStore } from "@opencode-ai/core/session/store"
 import { LocationServiceMap } from "@opencode-ai/core/location-service-map"
 import type { LocationServices } from "@opencode-ai/core/location-services"
+import { Image } from "@opencode-ai/core/image"
 import { testEffect } from "./lib/effect"
-import { imagePassthrough } from "./lib/image"
 
 const executionCalls: Session.ID[] = []
 const interruptCalls: Session.ID[] = []
@@ -58,7 +58,10 @@ const locations = Layer.effect(
     () =>
       // Attachment admission only needs the location-scoped Image service.
       // oxlint-disable-next-line typescript-eslint/no-unsafe-type-assertion
-      imagePassthrough as unknown as Layer.Layer<LocationServices>,
+      Layer.mock(Image.Service, {
+        normalize: (_resource, content) =>
+          Effect.succeed(content.content.length > 5 * 1024 * 1024 ? { ...content, content: "AA==" } : content),
+      }) as unknown as Layer.Layer<LocationServices>,
   ),
 )
 const it = testEffect(
@@ -386,6 +389,35 @@ describe("Session.prompt", () => {
     }),
   )
 
+  it.effect("normalizes large image content before validating persisted Base64", () =>
+    Effect.gen(function* () {
+      yield* setup
+      const session = yield* Session.Service
+      const pixel = Buffer.from(
+        "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNk+A8AAQUBAScY42YAAAAASUVORK5CYII=",
+        "base64",
+      )
+      const bytes = Buffer.concat([pixel, Buffer.alloc(4_323_030 - pixel.length)])
+      const data = bytes.toString("base64")
+      expect(data).toHaveLength(5_764_040)
+
+      const message = yield* session.prompt({
+        sessionID,
+        text: "Inspect this image",
+        files: [{ uri: `data:image/png;base64,${data}` }],
+        resume: false,
+      })
+
+      expect(message.data.files).toEqual([
+        {
+          data: "AA==",
+          mime: "image/png",
+          source: { type: "inline" },
+        },
+      ])
+    }),
+  )
+
   it.effect("sniffs data URL content instead of trusting its declared MIME", () =>
     Effect.gen(function* () {
       yield* setup