diff --git a/src/agents/prometheus/prometheus-byte-exactness.test.ts b/src/agents/prometheus/prometheus-byte-exactness.test.ts
new file mode 100644
index 000000000..882c33e63
--- /dev/null
+++ b/src/agents/prometheus/prometheus-byte-exactness.test.ts
@@ -0,0 +1,81 @@
+/// <reference types="bun-types" />
+
+import { describe, expect, test } from "bun:test"
+import { createHash } from "node:crypto"
+import { getPrometheusPrompt } from "./system-prompt"
+
+type PrometheusPromptBaseline = {
+  readonly name: string
+  readonly model: string | undefined
+  readonly disabledTools: readonly string[]
+  readonly sha256: string
+  readonly shouldContainQuestionTool: boolean
+}
+
+const PROMETHEUS_PROMPT_BASELINES: readonly PrometheusPromptBaseline[] = [
+  {
+    name: "default-enabled",
+    model: undefined,
+    disabledTools: [],
+    sha256: "7cd6dcc764c4b6c7cca61cf3878a1a2b2fb91836b38cbd0ed348d3e778cea4d9",
+    shouldContainQuestionTool: true,
+  },
+  {
+    name: "default-question-disabled",
+    model: undefined,
+    disabledTools: ["question"],
+    sha256: "db181638b60c222e5238daa8c090b8b908235d4a1cef7ff760839d4200195f59",
+    shouldContainQuestionTool: false,
+  },
+  {
+    name: "gpt-enabled",
+    model: "gpt-5.5",
+    disabledTools: [],
+    sha256: "95e42fb8112a6aac3d702fa40ec4a8f89923acd239e1041646ed5a0fd2a9feb9",
+    shouldContainQuestionTool: true,
+  },
+  {
+    name: "gpt-question-disabled",
+    model: "gpt-5.5",
+    disabledTools: ["question"],
+    sha256: "8792637920d271caec5675e63ed6685b1e5e6e292824fd6cf3f9a699e83a42fe",
+    shouldContainQuestionTool: false,
+  },
+  {
+    name: "gemini-enabled",
+    model: "gemini-3.1-pro",
+    disabledTools: [],
+    sha256: "df846993f69aef852bfe14569231453c673d369d69229f6e67f5af0915c05a1d",
+    shouldContainQuestionTool: true,
+  },
+  {
+    name: "gemini-question-disabled",
+    model: "gemini-3.1-pro",
+    disabledTools: ["question"],
+    sha256: "f9f9b7498c681a7d98a388a2e2aaf875227a145b4f824277ef54ed3a8d80106b",
+    shouldContainQuestionTool: false,
+  },
+]
+
+describe("Prometheus prompt byte exactness", () => {
+  test("#given captured Prometheus prompt baselines #then every variant keeps the same bytes", () => {
+    for (const baseline of PROMETHEUS_PROMPT_BASELINES) {
+      const prompt = getPrometheusPrompt(baseline.model, baseline.disabledTools)
+
+      expect(prompt.length, baseline.name).toBeGreaterThan(0)
+      expect(hashPrompt(prompt), baseline.name).toBe(baseline.sha256)
+    }
+  })
+
+  test("#given Question tool availability changes #then Question examples follow disabledTools", () => {
+    for (const baseline of PROMETHEUS_PROMPT_BASELINES) {
+      const prompt = getPrometheusPrompt(baseline.model, baseline.disabledTools)
+
+      expect(prompt.includes("Question({"), baseline.name).toBe(baseline.shouldContainQuestionTool)
+    }
+  })
+})
+
+function hashPrompt(prompt: string): string {
+  return createHash("sha256").update(prompt).digest("hex")
+}