@@ -182,16 +182,17 @@ export const ReadTool = Tool.define<
             const splitsPair = (index: number) =>
               index > 0 && /[\uD800-\uDBFF]/.test(text[index - 1]) && /[\uDC00-\uDFFF]/.test(text[index] ?? "")
             const windowStart = splitsPair(charOffset) ? charOffset - 1 : charOffset
-            let stop = Math.min(text.length, windowStart + maxChars)
+            const byteLimit = MAX_BYTES - 512
+            let stop = Math.min(text.length, windowStart + maxChars, windowStart + byteLimit)
             if (splitsPair(stop)) stop += stop === windowStart + 1 ? 1 : -1
             let chunk = text.slice(windowStart, stop)
             // Reserve room for the continuation message within the same byte cap.
-            if (Buffer.byteLength(chunk, "utf-8") > MAX_BYTES - 512) {
+            if (Buffer.byteLength(chunk, "utf-8") > byteLimit) {
               let bytes = 0
               let chars = 0
               for (const char of chunk) {
                 const size = Buffer.byteLength(char, "utf-8")
-                if (bytes + size > MAX_BYTES - 512) break
+                if (bytes + size > byteLimit) break
                 bytes += size
                 chars += char.length
               }
 
@@ -1,5 +1,8 @@
 import { PermissionV1 } from "@opencode-ai/core/v1/permission"
 import { afterEach, describe, expect } from "bun:test"
+// altimate_change start — observe character-window slice bounds
+import { spyOn } from "bun:test"
+// altimate_change end
 import { Cause, Effect, Exit, Layer, Stream } from "effect"
 import path from "path"
 import { Agent } from "../../src/agent/agent"
@@ -382,6 +385,42 @@ describe("tool.read truncation", () => {
     }),
   )
 
+  it.instance("bounds slice length for a 20 MB single-line character window", () =>
+    Effect.gen(function* () {
+      const test = yield* TestInstance
+      const filepath = path.join(test.directory, "long.txt")
+      const content = "0123456789".repeat(2 * 1024 * 1024)
+      const charOffset = 123
+      const byteLimit = 50 * 1024 - 512
+      yield* put(filepath, content)
+
+      const slice = String.prototype.slice
+      const windows: number[] = []
+      const spy = spyOn(String.prototype, "slice").mockImplementation(function (this: string, start, end) {
+        if (this.length === content.length && start === charOffset && end !== undefined) {
+          windows.push(end - start)
+        }
+        return slice.call(this, start, end)
+      })
+      const result = yield* run({
+        filePath: filepath,
+        limit: 1,
+        char_offset: charOffset,
+        max_chars: 10_000_000,
+      }).pipe(Effect.ensuring(Effect.sync(() => spy.mockRestore())))
+
+      const line =
+        content.slice(charOffset, charOffset + byteLimit) +
+        `... (line truncated to ${byteLimit} chars) (Use offset=1, limit=1, char_offset=${charOffset + byteLimit}, max_chars=${byteLimit} to continue this line.)`
+      expect(result.output).toBe(
+        `<path>${full(filepath)}</path>\n<type>file</type>\n<content>\n1: ${line}\n\n(End of file - total 1 lines)\n</content>`,
+      )
+      expect(result.metadata.truncated).toBe(true)
+      expect(windows.length).toBeGreaterThan(0)
+      for (const length of windows) expect(length).toBeLessThanOrEqual(byteLimit)
+    }),
+  )
+
   it.instance("bounds oversized multibyte character windows without losing the continuation", () =>
     Effect.gen(function* () {
       const test = yield* TestInstance