diff --git a/src/mcp/runtime/truncate.ts b/src/mcp/runtime/truncate.ts
index d192eee0..5568d8e6 100644
--- a/src/mcp/runtime/truncate.ts
+++ b/src/mcp/runtime/truncate.ts
@@ -3,11 +3,17 @@
  * characters. OpenAPI-generated MCP servers regularly emit 30+ KB
  * descriptions; without truncation a single tool can blow up the system
  * prompt and break provider-side caches.
+ *
+ * The cap is a hard limit on the returned string: the truncation marker is
+ * reserved inside {@link MAX_MCP_TOOL_DESCRIPTION_LENGTH}, not appended past it.
  */
 
 export const MAX_MCP_TOOL_DESCRIPTION_LENGTH = 2048;
 
+const TRUNCATION_MARKER = "… [truncated]";
+
 export function truncateMcpToolDescription(value: string): string {
   if (value.length <= MAX_MCP_TOOL_DESCRIPTION_LENGTH) return value;
-  return value.slice(0, MAX_MCP_TOOL_DESCRIPTION_LENGTH) + "… [truncated]";
+  const headLength = MAX_MCP_TOOL_DESCRIPTION_LENGTH - TRUNCATION_MARKER.length;
+  return value.slice(0, headLength) + TRUNCATION_MARKER;
 }
diff --git a/tests/mcp/truncate.spec.ts b/tests/mcp/truncate.spec.ts
new file mode 100644
index 00000000..ab205b72
--- /dev/null
+++ b/tests/mcp/truncate.spec.ts
@@ -0,0 +1,32 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import {
+  MAX_MCP_TOOL_DESCRIPTION_LENGTH,
+  truncateMcpToolDescription,
+} from "../../src/mcp/runtime/truncate.js";
+
+test("truncateMcpToolDescription leaves descriptions at the limit unchanged", () => {
+  const atLimit = "x".repeat(MAX_MCP_TOOL_DESCRIPTION_LENGTH);
+  assert.equal(truncateMcpToolDescription(atLimit), atLimit);
+});
+
+test("truncateMcpToolDescription keeps one-over-limit output within the cap", () => {
+  const overLimit = "x".repeat(MAX_MCP_TOOL_DESCRIPTION_LENGTH + 1);
+  const result = truncateMcpToolDescription(overLimit);
+
+  assert.ok(
+    result.length <= MAX_MCP_TOOL_DESCRIPTION_LENGTH,
+    `expected <= ${MAX_MCP_TOOL_DESCRIPTION_LENGTH}, got ${result.length}`,
+  );
+  assert.equal(result.length, MAX_MCP_TOOL_DESCRIPTION_LENGTH);
+  assert.ok(result.endsWith("… [truncated]"));
+});
+
+test("truncateMcpToolDescription clamps very large descriptions to the limit", () => {
+  const huge = "x".repeat(30_000);
+  const result = truncateMcpToolDescription(huge);
+
+  assert.equal(result.length, MAX_MCP_TOOL_DESCRIPTION_LENGTH);
+  assert.ok(result.endsWith("… [truncated]"));
+});