diff --git a/packages/pi-plugin/src/tools/index.test.ts b/packages/pi-plugin/src/tools/index.test.ts
index 46ea1a32..02527780 100644
--- a/packages/pi-plugin/src/tools/index.test.ts
+++ b/packages/pi-plugin/src/tools/index.test.ts
@@ -1,6 +1,10 @@
 import { describe, expect, it } from "bun:test";
-import { readFileSync } from "node:fs";
-import { join } from "node:path";
+import {
+	A1_HASH_BASELINE_HEADING,
+	A1_TOOL_SECTION_HEADING,
+	a1GoldenSectionOffset,
+	readA1GoldenDocument,
+} from "@magic-context/core/shared/prompt-surface-a1-golden";
 import {
 	createPromptSurfaceRuntime,
 	LIGHT_TOOL_DESCRIPTIONS,
@@ -240,16 +244,10 @@ function readA1GoldenTools(): Record<
 	string,
 	{ description: string; parameters: Record<string, unknown> }
 > {
-	const document = readFileSync(
-		join(
-			import.meta.dir,
-			"../../../plugin/src/shared/prompt-surface-a1-golden.md",
-		),
-		"utf8",
-	);
+	const document = readA1GoldenDocument();
 	const toolSection = document.slice(
-		document.indexOf("## 2. Tool surface"),
-		document.indexOf("## 3. System-prompt hash baseline"),
+		a1GoldenSectionOffset(document, A1_TOOL_SECTION_HEADING),
+		a1GoldenSectionOffset(document, A1_HASH_BASELINE_HEADING),
 	);
 	const headings = [...toolSection.matchAll(/^### (ctx_[a-z_]+) —.*$/gm)];
 	return Object.fromEntries(
diff --git a/packages/plugin/scripts/export-agent-surface.ts b/packages/plugin/scripts/export-agent-surface.ts
index 547d0732..e0e8db6b 100644
--- a/packages/plugin/scripts/export-agent-surface.ts
+++ b/packages/plugin/scripts/export-agent-surface.ts
@@ -8,6 +8,7 @@
  * Usage: bun packages/plugin/scripts/export-agent-surface.ts [outPath]
  * Default out: .alfonso/agent-surface-export.md (repo root)
  */
+import { createHash } from "node:crypto";
 import { mkdirSync, writeFileSync } from "node:fs";
 import { dirname, resolve } from "node:path";
 import Tokenizer from "ai-tokenizer";
@@ -112,6 +113,32 @@ for (const [name, definition] of Object.entries(definitions)) {
     out.push("");
 }
 
+// ── Section 3: system-prompt hash baseline ───────────────────────────────────
+// Derived, not transcribed: the hash handler persists the MD5 of
+// `output.system.join("\n")`, and in the no-host-prefix baseline each guidance
+// section IS the whole system array — so the hash is just the MD5 of the
+// guidance bytes emitted in section 1.
+out.push("## 3. System-prompt hash baseline");
+out.push("");
+out.push(
+    'The hash handler persists the MD5 of `output.system.join("\\\\n")`. The values below use each captured guidance section as the complete system array, which is the deterministic no-host-prefix baseline for later compatibility tests. The guidance bytes above are the source bytes; this table records the hash bytes that must remain unchanged when the default prompt surface is selected.',
+);
+out.push("");
+out.push("| Variant | Guidance bytes | MD5 system-prompt hash |");
+out.push("|---|---:|---|");
+for (const [label, text] of variants) {
+    // Table labels are the short form ("PRIMARY full"), not the full heading.
+    const shortLabel = label.split(" (")[0];
+    const bytes = Buffer.byteLength(text, "utf8");
+    const md5 = createHash("md5").update(text).digest("hex");
+    out.push(`| ${shortLabel} | ${bytes} | \`${md5}\` |`);
+}
+out.push("");
+out.push(
+    'The OpenCode and Pi runtime compatibility tests consume this snapshot for omitted `prompt_surface` and explicit `{ default: "full" }`: both assert guidance, registered tool descriptions, tool IDs, and hashes; OpenCode also asserts these parameter schemas directly, while Pi asserts its TypeBox-owned schemas stay byte-identical across both config forms.',
+);
+out.push("");
+
 const outPath = resolve(process.argv[2] ?? "../../.alfonso/agent-surface-export.md");
 mkdirSync(dirname(outPath), { recursive: true });
 writeFileSync(outPath, out.join("\n"));
diff --git a/packages/plugin/src/plugin/tool-registry.test.ts b/packages/plugin/src/plugin/tool-registry.test.ts
index 4a3fa092..d845e0d6 100644
--- a/packages/plugin/src/plugin/tool-registry.test.ts
+++ b/packages/plugin/src/plugin/tool-registry.test.ts
@@ -1,13 +1,19 @@
 /// <reference types="bun-types" />
 
 import { afterEach, describe, expect, it } from "bun:test";
-import { mkdtempSync, readFileSync, rmSync } from "node:fs";
+import { mkdtempSync, rmSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { type ToolDefinition, tool } from "@opencode-ai/plugin";
 import type { MagicContextPluginConfig } from "../config";
 import { closeDatabase, openDatabase } from "../features/magic-context/storage";
 import { resetCtxReduceRegisteredGloballyForTest } from "../hooks/magic-context/ctx-reduce-availability";
+import {
+    A1_HASH_BASELINE_HEADING,
+    A1_TOOL_SECTION_HEADING,
+    a1GoldenSectionOffset,
+    readA1GoldenDocument,
+} from "../shared/prompt-surface-a1-golden";
 import type { PromptSurfaceRuntime } from "../shared/prompt-surface-runtime";
 import {
     createPromptSurfaceRuntime,
@@ -204,13 +210,10 @@ describe("createToolRegistry — compaction-off mode (#266 S4)", () => {
 type GoldenTool = { description: string; parameters: Record<string, unknown> };
 
 function readA1GoldenTools(): Record<string, GoldenTool> {
-    const document = readFileSync(
-        join(import.meta.dir, "../shared/prompt-surface-a1-golden.md"),
-        "utf8",
-    );
+    const document = readA1GoldenDocument();
     const toolSection = document.slice(
-        document.indexOf("## 2. Tool surface"),
-        document.indexOf("## 3. System-prompt hash baseline"),
+        a1GoldenSectionOffset(document, A1_TOOL_SECTION_HEADING),
+        a1GoldenSectionOffset(document, A1_HASH_BASELINE_HEADING),
     );
     const headings = [...toolSection.matchAll(/^### (ctx_[a-z_]+) —.*$/gm)];
     return Object.fromEntries(
diff --git a/packages/plugin/src/shared/prompt-surface-a1-golden.ts b/packages/plugin/src/shared/prompt-surface-a1-golden.ts
new file mode 100644
index 00000000..1c743e43
--- /dev/null
+++ b/packages/plugin/src/shared/prompt-surface-a1-golden.ts
@@ -0,0 +1,41 @@
+import { readFileSync } from "node:fs";
+import { join } from "node:path";
+
+/**
+ * Shared accessors for the A1 prompt-surface golden.
+ *
+ * The golden (`prompt-surface-a1-golden.md`, generated by
+ * `scripts/export-agent-surface.ts`) is read by tests in BOTH packages, and its
+ * section headings are the contract between the generator and those readers.
+ * Keeping the path, the heading strings, and the offset guard here means a
+ * change to the golden's section format is a one-file edit rather than a hunt
+ * across packages.
+ *
+ * This module resolves the golden relative to ITSELF, so consumers never carry
+ * their own `../..` path arithmetic — that arithmetic differs per package and is
+ * exactly the kind of thing that rots silently when files move.
+ */
+
+export const A1_TOOL_SECTION_HEADING = "## 2. Tool surface";
+export const A1_HASH_BASELINE_HEADING = "## 3. System-prompt hash baseline";
+
+export function readA1GoldenDocument(): string {
+    return readFileSync(join(import.meta.dir, "prompt-surface-a1-golden.md"), "utf8");
+}
+
+/**
+ * Locate a golden section heading, refusing to continue when it is absent.
+ *
+ * A bare `indexOf` returns -1 for a missing heading, and `slice(start, -1)`
+ * silently yields an empty (or truncated) section rather than failing: the
+ * golden then parses as ZERO tools and the consuming test passes by comparing
+ * empty to empty. Failing loudly here keeps a malformed golden from reading as a
+ * clean run.
+ */
+export function a1GoldenSectionOffset(document: string, heading: string): number {
+    const offset = document.indexOf(heading);
+    if (offset === -1) {
+        throw new Error(`A1 golden is missing the "${heading}" section heading`);
+    }
+    return offset;
+}