diff --git a/packages/agent-core/src/harness/compaction/compaction-image-tokens.test.ts b/packages/agent-core/src/harness/compaction/compaction-image-tokens.test.ts
new file mode 100644
index 000000000000..a0153bf13f85
--- /dev/null
+++ b/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);
+  });
+});
diff --git a/packages/agent-core/src/harness/compaction/compaction.ts b/packages/agent-core/src/harness/compaction/compaction.ts
index bbde2b19edea..26e86a3f842e 100644
--- a/packages/agent-core/src/harness/compaction/compaction.ts
+++ b/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);
     }