diff --git a/tests/test_agent_prompt.py b/tests/test_agent_prompt.py
index 9cc12984..50bcd9ce 100644
--- a/tests/test_agent_prompt.py
+++ b/tests/test_agent_prompt.py
@@ -12,6 +12,7 @@
 
 from __future__ import annotations
 
+import re
 from pathlib import Path
 
 import pytest
@@ -37,8 +38,33 @@
 # exhaustive per-command detail belongs in `AGENT_CONTEXT` (loaded on demand),
 # and the real answer to sustained growth is splitting keboola-expert into
 # per-domain specialists, not another bump. Trim before you add.
+#
+# This is the SINGLE SOURCE OF TRUTH for the budget. Prose elsewhere that
+# hardcodes its own figure instead of quoting this one drifts silently:
+# CONTRIBUTING.md and kbagent-pr-reviewer.md both said "60 KB" long after
+# v0.48.0 moved the ceiling to 62 000 B, and both still said "62 000 B"
+# after v0.88.0 moved it again to 70 000 B, until each was caught by hand.
+# test_documented_budget_matches_enforced_budget below is the gate that
+# keeps them honest going forward.
 PROMPT_BYTE_BUDGET = 70_000
 
+# Docs that state the prompt budget in prose. Each must quote it as
+# "<PROMPT_BYTE_BUDGET> B" (space-grouped, e.g. "70 000 B") next to the
+# word "budget" -- see _BUDGET_MENTION_RE below.
+BUDGET_DOC_SITES = [
+    "CONTRIBUTING.md",
+    "plugins/kbagent/agents/kbagent-pr-reviewer.md",
+]
+
+# Matches a budget figure immediately followed by "budget" (optionally
+# "prompt budget"), in either the enforced byte form ("70 000 B" / "70000 B")
+# or the deprecated kilobyte form ("60 KB"). Grouping punctuation (spaces or
+# commas) inside the number is tolerated so "70 000", "70,000" and "70000"
+# all match -- only the digits are compared against PROMPT_BYTE_BUDGET.
+_BUDGET_MENTION_RE = re.compile(
+    r"(?P<number>\d[\d ,]*\d|\d)\s*(?P<unit>KB|B)\s+(?:prompt\s+)?budget"
+)
+
 
 @pytest.fixture(scope="module")
 def agent_body() -> str:
@@ -81,6 +107,42 @@ def test_agent_prompt_under_token_budget(self, agent_body: str) -> None:
             f"budget is {PROMPT_BYTE_BUDGET} bytes. Trim or split into specialists."
         )
 
+    def test_documented_budget_matches_enforced_budget(self) -> None:
+        """Every doc site's budget figure must match PROMPT_BYTE_BUDGET.
+
+        A doc site that hardcodes its own figure instead of quoting this
+        module's constant is a silent-drift surface: CONTRIBUTING.md and
+        kbagent-pr-reviewer.md both sat on a stale figure for a full release
+        span after the constant moved, until someone caught it by hand. The
+        expected string is derived from PROMPT_BYTE_BUDGET rather than a
+        second hardcoded literal, so a legitimate future budget change (the
+        constant and the docs moving together) never fails this test --
+        only a doc site left behind does.
+
+        _BUDGET_MENTION_RE also matches the deprecated "KB" form, so a site
+        that regresses to kilobytes is caught even though its digits happen
+        to equal PROMPT_BYTE_BUDGET's byte figure (which would never
+        legitimately occur, but the point is to catch the wrong *unit* too).
+        """
+        expected_bytes = str(PROMPT_BYTE_BUDGET)
+        for relative_path in BUDGET_DOC_SITES:
+            text = (REPO_ROOT / relative_path).read_text(encoding="utf-8")
+            mentions = _BUDGET_MENTION_RE.findall(text)
+            assert mentions, (
+                f"{relative_path} does not mention the prompt budget in the "
+                f"expected '<number> B budget' form. It must quote the "
+                f"enforced value (PROMPT_BYTE_BUDGET in {Path(__file__).name}), "
+                f"not its own figure."
+            )
+            for number, unit in mentions:
+                digits = re.sub(r"[ ,]", "", number)
+                assert unit == "B" and digits == expected_bytes, (
+                    f"{relative_path} states the prompt budget as "
+                    f"'{number} {unit}', but PROMPT_BYTE_BUDGET is "
+                    f"{expected_bytes} B ({Path(__file__).name} is the single "
+                    f"source of truth). Update the doc to match."
+                )
+
 
 # ---------------------------------------------------------------------
 # 2. Non-negotiable rules are all present (plan §6.1 + §14.8)