fix(core): budget retained image tokens

packages/core/src/session/compaction.ts

@@ -23,6 +23,7 @@ const DEFAULT_BUFFER = 20_000
 const DEFAULT_KEEP_TOKENS = 15_000
 const OUTPUT_TOKEN_MAX = 32_000
 const TOOL_OUTPUT_MAX_CHARS = 2_000
+const IMAGE_TOKEN_ESTIMATE = 2_000
 const SUMMARY_TEMPLATE = `Output exactly the Markdown structure shown inside <template> and keep the section order unchanged. Do not include the <template> tags in your response.
 <template>
 ## Objective
@@ -126,6 +127,22 @@ export const serializeToolContent = (content: SessionMessage.ToolStateCompleted[
     )
     .join("\n")
 
+export const estimateImageTokens = (message: SessionMessage.Info) => {
+  if (message.type === "user")
+    return (message.files?.filter((file) => file.mime.toLowerCase().startsWith("image/")).length ?? 0) * IMAGE_TOKEN_ESTIMATE
+  if (message.type !== "assistant") return 0
+  return (
+    message.content
+      .flatMap((part) =>
+        part.type === "tool" && (part.state.status === "completed" || part.state.status === "error")
+          ? (part.state.content ?? [])
+          : [],
+      )
+      .filter((content) => content.type === "file" && content.mime.toLowerCase().startsWith("image/")).length *
+    IMAGE_TOKEN_ESTIMATE
+  )
+}
+
 const serialize = (message: SessionMessage.Info) => {
   if (message.type === "user") {
     const files =
@@ -188,7 +205,7 @@ const select = (
   let total = 0
   let split = conversation.length
   for (let index = conversation.length - 1; index >= 0; index--) {
-    const next = total + Token.estimate(conversation[index].text)
+    const next = total + Token.estimate(conversation[index].text) + estimateImageTokens(conversation[index].message)
     if (split < conversation.length && next > tokens) break
     total = next
     split = index

packages/core/test/session-compaction.test.ts

@@ -114,6 +114,24 @@ test("compaction describes tool media without embedding base64", () => {
   expect(serialized).not.toContain(base64)
 })
 
+test("compaction estimates image context without counting base64", () => {
+  const image = FileAttachment.make({
+    data: Base64.make("a".repeat(10_000)),
+    mime: "image/png",
+    source: { type: "inline" },
+    name: "image.png",
+  })
+  const message = SessionMessage.User.make({
+    id: SessionMessage.ID.create(),
+    type: "user",
+    text: "Compare these images.",
+    files: [image, image],
+    time: { created: DateTime.makeUnsafe(0) },
+  })
+
+  expect(SessionCompaction.estimateImageTokens(message)).toBe(4_000)
+})
+
 test("compaction prompt requires the checkpoint headings in order", () => {
   const prompt = SessionCompaction.buildPrompt({ context: ["Conversation history"] })
   expect(prompt.match(/^#{2,3} .+$/gm)).toEqual([

packages/www/content/docs/(Configure)/compaction.mdx

@@ -111,7 +111,9 @@ and relevant files.
 The newest serialized context up to `keep.tokens` is retained separately. This
 is not a byte-for-byte transcript: tool output is limited to 2000 characters,
 and non-media attachments become textual descriptors. Media in the retained
-context is attached to the checkpoint in the same order as its descriptors. On
+context is attached to the checkpoint in the same order as its descriptors.
+Tail selection budgets 2000 additional tokens per image as a provider-neutral
+planning estimate; it does not count base64 request bytes as text tokens. On
 later compactions, V2 updates the previous summary and carries forward its
 retained recent context before selecting a new tail.