diff --git a/packages/cli/src/commands/serve.ts b/packages/cli/src/commands/serve.ts
index 67bc891..acd7e82 100644
--- a/packages/cli/src/commands/serve.ts
+++ b/packages/cli/src/commands/serve.ts
@@ -14,11 +14,12 @@
  * Docs: /docs/cli § "penguin server / penguin web".
  */
 import { spawn } from "node:child_process";
+import { DEFAULT_SERVER_PORT } from "@prismshadow/penguin-core";
 import type { Command } from "commander";
 import type { Messages } from "../i18n.js";
 
-/** Default service port (deliberately avoids common defaults like 3000/8080). */
-export const DEFAULT_PORT = 7364;
+/** Default service port — core's DEFAULT_SERVER_PORT (7364), the single source of truth. */
+export const DEFAULT_PORT = DEFAULT_SERVER_PORT;
 /** Default service listen host. */
 export const DEFAULT_HOST = "127.0.0.1";
 
diff --git a/packages/cli/test/serve.test.ts b/packages/cli/test/serve.test.ts
index 3cd9702..deffc4a 100644
--- a/packages/cli/test/serve.test.ts
+++ b/packages/cli/test/serve.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, it } from "vitest";
 import { Command } from "commander";
+import { DEFAULT_SERVER_PORT } from "@prismshadow/penguin-core";
 import {
   DEFAULT_HOST,
   DEFAULT_PORT,
@@ -11,6 +12,9 @@ import {
 import { getMessages } from "../src/i18n.js";
 
 describe("resolvePort (option > env var > default 7364)", () => {
+  it("derives DEFAULT_PORT from core's DEFAULT_SERVER_PORT", () => {
+    expect(DEFAULT_PORT).toBe(DEFAULT_SERVER_PORT);
+  });
   it("uses the default 7364 when neither is given", () => {
     expect(DEFAULT_PORT).toBe(7364);
     expect(resolvePort(undefined, undefined)).toBe(7364);
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index fef74c0..430d953 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -19,6 +19,9 @@
 export * from "./omnimessage/index.js";
 export * from "./interfaces.js";
 
+// Only the default server port leaves internal: the CLI / server default-port source of truth.
+export { DEFAULT_SERVER_PORT } from "./internal/ports.js";
+
 // Submodules
 export * from "./state/index.js";
 export * from "./llm/index.js";
diff --git a/packages/core/src/internal/ports.ts b/packages/core/src/internal/ports.ts
new file mode 100644
index 0000000..9f29840
--- /dev/null
+++ b/packages/core/src/internal/ports.ts
@@ -0,0 +1,10 @@
+/**
+ * Default PenguinHarness server port (internal shared constant; the barrel re-exports
+ * only DEFAULT_SERVER_PORT, as the CLI `penguin server` / `penguin web` and server
+ * default-port source of truth — previously each hardcoded the number). It is a
+ * fallback only: the `--port` flag and the PORT environment variable override it at
+ * runtime.
+ */
+
+/** Default main server / Web UI port; deliberately avoids common defaults like 3000/8080. */
+export const DEFAULT_SERVER_PORT = 7364;
diff --git a/packages/core/src/state/default-config.ts b/packages/core/src/state/default-config.ts
index c2242e5..1068265 100644
--- a/packages/core/src/state/default-config.ts
+++ b/packages/core/src/state/default-config.ts
@@ -92,13 +92,15 @@ Communicate with the user precisely and concisely, yet with warmth. Do not repea
 # Constraints
 - Make the smallest change that satisfies the request; do not modify unrelated files.
 - Destructive operations are forbidden.
-- Never kill a process you did not start yourself (e.g. to free a busy port) unless the user explicitly asks you to.
+- Never kill a process you did not start yourself unless the user explicitly asks you to, including PenguinHarness's own services. Never take a PenguinHarness service port for your own servers; when a port you want is busy, pick another free port instead of killing the listener.
 - If a tool call fails, read the error, adjust, and retry; never repeat the same failing input.
+- On an API authentication/authorization or API-key error (401/403, invalid or missing key), retry at most once.
 
 # Stop rules
 - Stop and give the final answer once the success criteria are met.
 - If the request is ambiguous, stop and ask the user for clarification instead of guessing their intent.
 - If you hit an error you cannot resolve, stop and report the blocker to the user.
+- If an API auth/key error persists after that one retry, stop calling tools and ask the user to update the key in the agent's vault or the model settings outside the chat — the secret value must never be pasted into the conversation. Updated secrets only take effect in the next conversation, so further retries cannot succeed.
 
 # Tool use
 - Prefer solving problems with your tools: inspect the real files and environment and run real commands instead of answering from memory or guessing.
diff --git a/packages/core/test/state.test.ts b/packages/core/test/state.test.ts
index bf5f6cf..32365cb 100644
--- a/packages/core/test/state.test.ts
+++ b/packages/core/test/state.test.ts
@@ -315,6 +315,22 @@ describe("assembleSystemPrompt", () => {
     expect(prompt).not.toContain(DATE_PLACEHOLDER);
   });
 
+  it("default prompt carries the port and API-key guardrails", async () => {
+    const state = await loadOrInitAgentState();
+    const prompt = assembleSystemPrompt(state);
+    // Ports: never kill listeners or take PenguinHarness service ports; numbers are deliberately not listed.
+    expect(prompt).toContain("pick another free port");
+    expect(prompt).toContain("PenguinHarness service port");
+    expect(prompt).not.toContain("7364");
+    // Auth/key failures: retry at most once (Constraints), then stop and ask the user to update
+    // the key outside the chat (Stop rules) — no CLI commands, no secret values in the conversation.
+    expect(prompt).toContain("retry at most once");
+    expect(prompt).toContain("stop calling tools");
+    expect(prompt).toContain("never be pasted into the conversation");
+    expect(prompt).toContain("next conversation");
+    expect(prompt).not.toContain("penguin config vault set");
+  });
+
   it("replaces AGENTS.md and specific Session environment fields at template locations", () => {
     const state = {
       root: tmpRoot,
diff --git a/packages/docs/vite.config.ts b/packages/docs/vite.config.ts
index 3203475..fae4d6d 100644
--- a/packages/docs/vite.config.ts
+++ b/packages/docs/vite.config.ts
@@ -16,5 +16,7 @@ import { defineConfig } from "vite";
 export default defineConfig({
   base: process.env.BASE_PATH ?? "/",
   plugins: [react(), tailwindcss()],
+  // Fixed PenguinHarness dev port (stands alone — only the main server default is
+  // shared, as DEFAULT_SERVER_PORT in core; vite configs cannot import core TS).
   server: { port: 7367 },
 });
diff --git a/packages/landing/vite.config.ts b/packages/landing/vite.config.ts
index e41c441..3d05923 100644
--- a/packages/landing/vite.config.ts
+++ b/packages/landing/vite.config.ts
@@ -13,5 +13,7 @@ import { defineConfig } from "vite";
 export default defineConfig({
   base: process.env.BASE_PATH ?? "/",
   plugins: [react(), tailwindcss()],
+  // Fixed PenguinHarness dev port (stands alone — only the main server default is
+  // shared, as DEFAULT_SERVER_PORT in core; vite configs cannot import core TS).
   server: { port: 7366 },
 });
diff --git a/packages/server/src/config.ts b/packages/server/src/config.ts
index 5644530..b7c4486 100644
--- a/packages/server/src/config.ts
+++ b/packages/server/src/config.ts
@@ -12,7 +12,7 @@
 import fs from "node:fs";
 import path from "node:path";
 import { fileURLToPath } from "node:url";
-import { resolveRoot } from "@prismshadow/penguin-core";
+import { DEFAULT_SERVER_PORT, resolveRoot } from "@prismshadow/penguin-core";
 
 export interface ServerConfig {
   /** Local data root directory (shared with the SDK/CLI). */
@@ -52,7 +52,7 @@ export function resolveServerConfig(env: NodeJS.ProcessEnv = process.env): Serve
   // An empty PORT string is treated as unset (the common `.env` case of an empty
   // `PORT=`): Number("") === 0 would pass the range check and bind to a random
   // port; this matches the CLI's resolvePort convention.
-  const port = Number(env.PORT || 7364);
+  const port = Number(env.PORT || DEFAULT_SERVER_PORT);
   if (!Number.isInteger(port) || port < 0 || port > 65535) {
     throw new Error(`Invalid port configuration PORT=${env.PORT}`);
   }
diff --git a/packages/web/vite.config.ts b/packages/web/vite.config.ts
index bcd307c..64d595c 100644
--- a/packages/web/vite.config.ts
+++ b/packages/web/vite.config.ts
@@ -14,6 +14,8 @@ import { defineConfig } from "vite";
 export default defineConfig({
   plugins: [react(), tailwindcss()],
   server: {
+    // Fixed PenguinHarness dev port (stands alone — only the main server default is
+    // shared, as DEFAULT_SERVER_PORT in core; vite configs cannot import core TS).
     port: 7365,
     proxy: {
       "/api": {