fix(compaction): count user-message image blocks in cut-point estimator (#95128)

estimateTokens charged 4800 chars per image in the toolResult branch but
counted only text in the user branch, so image blocks in recent user turns
scored zero. findCutPoint never reached keepRecentTokens and left the cut at
the earliest point, so image-heavy sessions compacted to a no-op and looped on
context overflow. Fold the per-image accounting into one shared helper used by
both branches.

packages/agent-core/src/harness/compaction/compaction-image-tokens.test.ts

@@ -0,0 +1,106 @@
+import { describe, expect, it } from "vitest";
+import type { ImageContent } from "../../llm.js";
+import type { AgentMessage } from "../../types.js";
+import type { SessionTreeEntry } from "../types.js";
+import { estimateTokens, findCutPoint } from "./compaction.js";
+
+const IMAGE_PAYLOAD = "a".repeat(1_500_000);
+
+function imageBlock(): ImageContent {
+  return { type: "image", data: IMAGE_PAYLOAD, mimeType: "image/png" };
+}
+
+function userImage(timestamp: number): AgentMessage {
+  return { role: "user", content: [imageBlock()], timestamp };
+}
+
+function userText(text: string, timestamp: number): AgentMessage {
+  return { role: "user", content: [{ type: "text", text }], timestamp };
+}
+
+function toolResultImage(timestamp: number): AgentMessage {
+  return {
+    role: "toolResult",
+    toolCallId: "call-1",
+    toolName: "screenshot",
+    content: [imageBlock()],
+    isError: false,
+    timestamp,
+  };
+}
+
+function assistantText(text: string, timestamp: number): AgentMessage {
+  return {
+    role: "assistant",
+    content: [{ type: "text", text }],
+    api: "anthropic-messages",
+    provider: "anthropic",
+    model: "claude-fable-5",
+    usage: {
+      input: 0,
+      output: 0,
+      cacheRead: 0,
+      cacheWrite: 0,
+      totalTokens: 0,
+      cost: { input: 0, output: 0, cacheRead: 0, cacheWrite: 0, total: 0 },
+    },
+    stopReason: "stop",
+    timestamp,
+  };
+}
+
+function messageEntry(message: AgentMessage, index: number): SessionTreeEntry {
+  return {
+    type: "message",
+    id: `entry-${index}`,
+    parentId: index === 0 ? null : `entry-${index - 1}`,
+    timestamp: new Date(message.timestamp).toISOString(),
+    message,
+  };
+}
+
+function buildTranscript(recentUserTurns: AgentMessage[]): SessionTreeEntry[] {
+  const messages: AgentMessage[] = [userText("start of the conversation", 1)];
+  let timestamp = 2;
+  for (const turn of recentUserTurns) {
+    messages.push(assistantText("ok", timestamp++));
+    messages.push(turn);
+  }
+  return messages.map((message, index) => messageEntry(message, index));
+}
+
+describe("estimateTokens image accounting", () => {
+  it("charges a user-message image block the same as a tool-result image block", () => {
+    const userTokens = estimateTokens(userImage(1));
+    const toolTokens = estimateTokens(toolResultImage(1));
+
+    expect(userTokens).toBe(toolTokens);
+    expect(userTokens).toBeGreaterThanOrEqual(1200);
+  });
+});
+
+describe("findCutPoint with image-heavy recent turns", () => {
+  it("trims image-dominated user turns instead of keeping the whole transcript", () => {
+    const entries = buildTranscript([userImage(10), userImage(20), userImage(30)]);
+
+    const result = findCutPoint(entries, 0, entries.length, 1500);
+
+    expect(result.firstKeptEntryIndex).toBeGreaterThan(0);
+  });
+
+  it("matches the cut point of an equivalent text-cost control", () => {
+    const equivalentText = "x".repeat(4800);
+    const imageEntries = buildTranscript([userImage(10), userImage(20), userImage(30)]);
+    const textEntries = buildTranscript([
+      userText(equivalentText, 10),
+      userText(equivalentText, 20),
+      userText(equivalentText, 30),
+    ]);
+
+    const imageResult = findCutPoint(imageEntries, 0, imageEntries.length, 1500);
+    const textResult = findCutPoint(textEntries, 0, textEntries.length, 1500);
+
+    expect(textResult.firstKeptEntryIndex).toBeGreaterThan(0);
+    expect(imageResult.firstKeptEntryIndex).toBe(textResult.firstKeptEntryIndex);
+  });
+});

packages/agent-core/src/harness/compaction/compaction.ts

@@ -244,6 +244,20 @@ export function shouldCompact(
   return contextTokens > contextWindow - settings.reserveTokens;
 }
 
+const IMAGE_BLOCK_CHARS = 4800;
+
+function countContentBlockChars(content: Array<{ type: string; text?: string }>): number {
+  let chars = 0;
+  for (const block of content) {
+    if (block.type === "text" && block.text) {
+      chars += block.text.length;
+    } else if (block.type === "image") {
+      chars += IMAGE_BLOCK_CHARS;
+    }
+  }
+  return chars;
+}
+
 /** Estimate token count for one message using a conservative character heuristic. */
 export function estimateTokens(message: AgentMessage): number {
   let chars = 0;
@@ -257,11 +271,7 @@ export function estimateTokens(message: AgentMessage): number {
       if (typeof content === "string") {
         chars = content.length;
       } else if (Array.isArray(content)) {
-        for (const block of content) {
-          if (block.type === "text" && block.text) {
-            chars += block.text.length;
-          }
-        }
+        chars = countContentBlockChars(content);
       }
       return Math.ceil(chars / 4);
     }
@@ -283,14 +293,7 @@ export function estimateTokens(message: AgentMessage): number {
       if (typeof harnessMessage.content === "string") {
         chars = harnessMessage.content.length;
       } else {
-        for (const block of harnessMessage.content) {
-          if (block.type === "text" && block.text) {
-            chars += block.text.length;
-          }
-          if (block.type === "image") {
-            chars += 4800;
-          }
-        }
+        chars = countContentBlockChars(harnessMessage.content);
       }
       return Math.ceil(chars / 4);
     }