diff --git a/src/agents/embedded-agent-runner/run/preemptive-compaction.test.ts b/src/agents/embedded-agent-runner/run/preemptive-compaction.test.ts
index 806226c6cabd..f2274486158b 100644
--- a/src/agents/embedded-agent-runner/run/preemptive-compaction.test.ts
+++ b/src/agents/embedded-agent-runner/run/preemptive-compaction.test.ts
@@ -455,4 +455,133 @@ describe("preemptive-compaction", () => {
     expect(result.route).toBe("truncate_tool_results_only");
     expect(result.shouldCompact).toBe(false);
   });
+
+  it("estimates CJK tool results at roughly one token per character", () => {
+    const cjkText = "中".repeat(85_000);
+    const toolResultTokens = estimateLlmBoundaryTokenPressure({
+      messages: [makeToolResultMessage(cjkText)],
+      systemPrompt: "sys",
+      prompt: "continue",
+    });
+    const assistantTokens = estimateLlmBoundaryTokenPressure({
+      messages: [makeAssistantHistory(cjkText)],
+      systemPrompt: "sys",
+      prompt: "continue",
+    });
+    const result = shouldPreemptivelyCompactBeforePrompt({
+      messages: [makeToolResultMessage(cjkText)],
+      systemPrompt: "sys",
+      prompt: "continue",
+      contextTokenBudget: 128_000,
+      reserveTokens: 20_000,
+    });
+
+    expect(toolResultTokens).toBeGreaterThanOrEqual(assistantTokens);
+    expect(toolResultTokens - assistantTokens).toBeLessThanOrEqual(5);
+    expect(result.estimatedPromptTokens).toBe(toolResultTokens);
+    expect(result.promptBudgetBeforeReserve).toBeGreaterThan(result.estimatedPromptTokens);
+    expect(result.route).toBe("fits");
+    expect(result.shouldCompact).toBe(false);
+    expect(result.overflowTokens).toBe(0);
+  });
+
+  it("avoids false overflow when CJK is less than half of a tool result", () => {
+    const mixedContent = "中".repeat(40_000) + "a".repeat(60_000);
+    const result = shouldPreemptivelyCompactBeforePrompt({
+      messages: [makeToolResultMessage(mixedContent)],
+      systemPrompt: "sys",
+      prompt: "continue",
+      contextTokenBudget: 100_000,
+      reserveTokens: 20_000,
+    });
+
+    expect(result.estimatedPromptTokens).toBeLessThan(result.promptBudgetBeforeReserve);
+    expect(result.route).toBe("fits");
+    expect(result.shouldCompact).toBe(false);
+  });
+
+  it("keeps mixed-script estimates monotonic across the former CJK cutoff", () => {
+    const estimate = (cjkChars: number) =>
+      estimateLlmBoundaryTokenPressure({
+        messages: [makeToolResultMessage("中".repeat(cjkChars) + "a".repeat(10_000 - cjkChars))],
+        systemPrompt: "sys",
+        prompt: "continue",
+      });
+
+    const belowCutoff = estimate(4_999);
+    const atCutoff = estimate(5_000);
+    const aboveCutoff = estimate(5_001);
+
+    expect(atCutoff).toBeGreaterThanOrEqual(belowCutoff);
+    expect(aboveCutoff).toBeGreaterThanOrEqual(atCutoff);
+    expect(aboveCutoff - belowCutoff).toBeLessThanOrEqual(2);
+  });
+
+  it("keeps the conservative ratio for non-CJK tool results", () => {
+    const latinText = "alpha beta gamma delta epsilon ".repeat(1000);
+    const toolResultTokens = estimateLlmBoundaryTokenPressure({
+      messages: [makeToolResultMessage(latinText)],
+      systemPrompt: "sys",
+      prompt: "continue",
+    });
+    const assistantTokens = estimateLlmBoundaryTokenPressure({
+      messages: [makeAssistantHistory(latinText)],
+      systemPrompt: "sys",
+      prompt: "continue",
+    });
+
+    expect(toolResultTokens).toBeGreaterThan(assistantTokens * 1.5);
+    expect(toolResultTokens).toBeLessThan(assistantTokens * 2.5);
+  });
+
+  it("applies the CJK-aware ratio to JSON tool-result payloads", () => {
+    const cjkPayload = {
+      summary: "中文内容".repeat(5_000),
+      note: "更多中文文本".repeat(2_000),
+    };
+    const messages = [makeJsonToolResultMessage(cjkPayload)];
+
+    const estimatedPromptTokens = estimateLlmBoundaryTokenPressure({
+      messages,
+      systemPrompt: "sys",
+      prompt: "continue",
+    });
+
+    expect(estimatedPromptTokens).toBeLessThan(90_000);
+
+    const result = shouldPreemptivelyCompactBeforePrompt({
+      messages,
+      systemPrompt: "sys",
+      prompt: "continue",
+      contextTokenBudget: 128_000,
+      reserveTokens: 20_000,
+    });
+
+    expect(result.route).toBe("fits");
+    expect(result.shouldCompact).toBe(false);
+    expect(result.overflowTokens).toBe(0);
+  });
+
+  it("does not throw when tool-result content cannot be serialized", () => {
+    const circular: Record<string, unknown> = { self: undefined };
+    circular.self = circular;
+    const message = {
+      role: "toolResult",
+      toolCallId: "call_circular",
+      toolName: "bad_tool",
+      content: circular,
+      isError: false,
+      timestamp: timestamp++,
+    } as unknown as AgentMessage;
+
+    const result = shouldPreemptivelyCompactBeforePrompt({
+      messages: [message],
+      systemPrompt: "sys",
+      prompt: "continue",
+      contextTokenBudget: 128_000,
+      reserveTokens: 20_000,
+    });
+
+    expect(Number.isFinite(result.estimatedPromptTokens)).toBe(true);
+  });
 });
diff --git a/src/agents/embedded-agent-runner/run/preemptive-compaction.ts b/src/agents/embedded-agent-runner/run/preemptive-compaction.ts
index 8a811d1de1bb..ce7250efa0b7 100644
--- a/src/agents/embedded-agent-runner/run/preemptive-compaction.ts
+++ b/src/agents/embedded-agent-runner/run/preemptive-compaction.ts
@@ -112,18 +112,50 @@ function estimateContentBlockTokenPressure(
   return CONTENT_BLOCK_OVERHEAD_TOKENS + estimateJsonPayloadTokenPressure(block, charsPerToken);
 }
 
+function estimateToolResultStringTokenPressure(text: string): number {
+  const conservativeToolResultEstimate = Math.ceil(text.length / TOOL_RESULT_CHARS_PER_TOKEN);
+  const cjkAwareEstimate = estimateStringTokenPressure(text);
+  return Math.max(conservativeToolResultEstimate, cjkAwareEstimate);
+}
+
+function estimateToolResultJsonTokenPressure(value: unknown): number {
+  try {
+    const serialized = JSON.stringify(value);
+    return typeof serialized === "string" ? estimateToolResultStringTokenPressure(serialized) : 1;
+  } catch {
+    return 256;
+  }
+}
+
+function estimateToolResultBlockTokenPressure(block: unknown): number {
+  if (typeof block === "string") {
+    return estimateToolResultStringTokenPressure(block);
+  }
+  if (!isRecord(block)) {
+    return estimateToolResultJsonTokenPressure(block);
+  }
+
+  if (block.type === "text" && typeof block.text === "string") {
+    return CONTENT_BLOCK_OVERHEAD_TOKENS + estimateToolResultStringTokenPressure(block.text);
+  }
+  if (block.type === "thinking" && typeof block.thinking === "string") {
+    return CONTENT_BLOCK_OVERHEAD_TOKENS + estimateToolResultStringTokenPressure(block.thinking);
+  }
+  if (block.type === "image") {
+    return IMAGE_BLOCK_TOKENS;
+  }
+  return CONTENT_BLOCK_OVERHEAD_TOKENS + estimateToolResultJsonTokenPressure(block);
+}
+
 function estimateToolResultContentTokenPressure(content: unknown): number {
   if (typeof content === "string") {
-    return estimateStringTokenPressure(content, TOOL_RESULT_CHARS_PER_TOKEN);
+    return estimateToolResultStringTokenPressure(content);
   }
   if (Array.isArray(content)) {
-    return content.reduce(
-      (sum, block) => sum + estimateContentBlockTokenPressure(block, TOOL_RESULT_CHARS_PER_TOKEN),
-      0,
-    );
+    return content.reduce((sum, block) => sum + estimateToolResultBlockTokenPressure(block), 0);
   }
   if (content !== undefined) {
-    return estimateJsonPayloadTokenPressure(content, TOOL_RESULT_CHARS_PER_TOKEN);
+    return estimateToolResultJsonTokenPressure(content);
   }
   return 0;
 }