test(atlas): add prompt byte preservation baselines

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/agents/atlas/prompt-byte-preservation.test.ts

@@ -0,0 +1,153 @@
+import { createHash } from "node:crypto"
+import { describe, expect, test } from "bun:test"
+import { createAtlasAgent, type AtlasPromptSource, type OrchestratorContext } from "./agent"
+
+type VariantPromptCase = {
+  readonly variant: AtlasPromptSource
+  readonly model: string
+  readonly expectedHash: string
+  readonly expectedLength: number
+}
+
+const BASE_CONTEXT = {
+  availableAgents: [
+    {
+      name: "oracle",
+      description: "Read-only architecture reviewer",
+      metadata: {
+        category: "advisor",
+        cost: "EXPENSIVE",
+        triggers: [{ domain: "Architecture", trigger: "Need design review" }],
+        promptAlias: "Oracle",
+      },
+    },
+    {
+      name: "explore",
+      description: "Fast codebase searcher",
+      metadata: {
+        category: "exploration",
+        cost: "CHEAP",
+        triggers: [{ domain: "Code search", trigger: "Need repository context" }],
+        promptAlias: "Explore",
+      },
+    },
+  ],
+  availableSkills: [
+    {
+      name: "programming",
+      description: "Strict TypeScript implementation discipline",
+      location: "user",
+    },
+    {
+      name: "git-master",
+      description: "Atomic git operations",
+      location: "plugin",
+    },
+    {
+      name: "frontend-ui-ux",
+      description: "Premium UI guidance",
+      location: "project",
+    },
+  ],
+  userCategories: {
+    custom: { description: "Custom deterministic category", temperature: 0.7 },
+    quick: { description: "User quick override", temperature: 0.2 },
+  },
+} satisfies OrchestratorContext
+
+const VARIANT_PROMPT_CASES = [
+  {
+    variant: "default",
+    model: "anthropic/claude-sonnet-4-6",
+    expectedHash: "b29612f266994284487c37342c8e253f158b5d08daf95266e71651cbfcf1b9f9",
+    expectedLength: 25847,
+  },
+  {
+    variant: "gpt",
+    model: "openai/gpt-5.5",
+    expectedHash: "187a6d5f63dd166c88b568e9c2e142205eb4d8537386e1c81a38707e4ac59efb",
+    expectedLength: 24707,
+  },
+  {
+    variant: "gemini",
+    model: "google/gemini-3.1-pro",
+    expectedHash: "194f4508da8c5a885a44a8d253cb6f6504190cf60d634cc42801a794bc4c8d33",
+    expectedLength: 27579,
+  },
+  {
+    variant: "kimi",
+    model: "moonshotai/kimi-k2.6",
+    expectedHash: "2d1d3e3fb665493e624f5d810a693e2df637346b3dab7800b9a689b6ed7932bf",
+    expectedLength: 26107,
+  },
+  {
+    variant: "opus-4-7",
+    model: "anthropic/claude-opus-4-7",
+    expectedHash: "353bd5d9ceaeb2b4eb53cb851d65d206a777643c6542505ab32e0bd1993c3de2",
+    expectedLength: 26729,
+  },
+] satisfies readonly VariantPromptCase[]
+
+const RUNTIME_PLACEHOLDERS = [
+  "{CATEGORY_SECTION}",
+  "{AGENT_SECTION}",
+  "{DECISION_MATRIX}",
+  "{SKILLS_SECTION}",
+  "{{CATEGORY_SKILLS_DELEGATION_GUIDE}}",
+] as const
+
+describe("Atlas prompt byte preservation", () => {
+  for (const promptCase of VARIANT_PROMPT_CASES) {
+    test(`#given ${promptCase.variant} model #when Atlas prompt renders #then hash matches the baseline`, () => {
+      const prompt = getAtlasPromptText({ ...BASE_CONTEXT, model: promptCase.model })
+
+      expect(createHash("sha256").update(prompt).digest("hex")).toBe(promptCase.expectedHash)
+      expect(prompt.length).toBe(promptCase.expectedLength)
+    })
+  }
+})
+
+describe("Atlas prompt runtime section injection", () => {
+  test("#given unique live context markers #when prompt renders #then placeholders are resolved", () => {
+    const prompt = getAtlasPromptText({
+      model: "anthropic/claude-sonnet-4-6",
+      availableAgents: [
+        {
+          name: "unique-agent-section-marker",
+          description: "UNIQUE_AGENT_SECTION_VALUE",
+          metadata: {
+            category: "advisor",
+            cost: "EXPENSIVE",
+            triggers: [{ domain: "Runtime", trigger: "Unique agent marker" }],
+          },
+        },
+      ],
+      availableSkills: [
+        {
+          name: "unique-guide-skill-marker",
+          description: "Unique guide skill marker",
+          location: "user",
+        },
+      ],
+      userCategories: {
+        "unique-category-section-marker": {
+          description: "UNIQUE_CATEGORY_SECTION_VALUE",
+          temperature: 0.4,
+        },
+      },
+    })
+
+    expect(prompt).toContain("UNIQUE_CATEGORY_SECTION_VALUE")
+    expect(prompt).toContain("UNIQUE_AGENT_SECTION_VALUE")
+    expect(prompt).toContain("unique-guide-skill-marker")
+    for (const placeholder of RUNTIME_PLACEHOLDERS) {
+      expect(prompt).not.toContain(placeholder)
+    }
+  })
+})
+
+function getAtlasPromptText(ctx: OrchestratorContext): string {
+  const prompt = createAtlasAgent(ctx).prompt
+  if (typeof prompt === "string") return prompt
+  throw new TypeError("Atlas prompt must be a string")
+}