diff --git a/src/app/api/playground/improve-prompt/route.ts b/src/app/api/playground/improve-prompt/route.ts
new file mode 100644
index 0000000000..a0efd85d67
--- /dev/null
+++ b/src/app/api/playground/improve-prompt/route.ts
@@ -0,0 +1,151 @@
+/**
+ * POST /api/playground/improve-prompt
+ *
+ * Improves a user-supplied system prompt and/or user prompt by calling
+ * /v1/chat/completions internally with a meta-prompt (D8 — uses the model
+ * chosen by the user, never forces a cheap model).
+ *
+ * Request:  ImprovePromptRequestSchema  { system?, prompt?, model, tone? }
+ * Response: { improvedSystem?, improvedPrompt?, tokensIn, tokensOut }
+ *
+ * Auth: optional (mirrors /v1/web/fetch pattern — required only when
+ *       REQUIRE_API_KEY=true; always validated when supplied).
+ *
+ * Hard Rule #12: ALL error paths route through buildErrorBody.
+ */
+
+import { buildErrorBody, sanitizeErrorMessage } from "@omniroute/open-sse/utils/error.ts";
+import { HTTP_STATUS } from "@omniroute/open-sse/config/constants.ts";
+import { extractApiKey, isValidApiKey } from "@/sse/services/auth";
+import {
+  ImprovePromptRequestSchema,
+  buildImproveChatBody,
+  parseImprovedContent,
+} from "@/lib/playground/promptImprover";
+
+const CORS_HEADERS = {
+  "Access-Control-Allow-Methods": "POST, OPTIONS",
+  "Access-Control-Allow-Headers": "*",
+};
+
+function errorResp(status: number, message: string, upstreamDetails?: unknown): Response {
+  return new Response(
+    JSON.stringify(buildErrorBody(status, sanitizeErrorMessage(message), upstreamDetails)),
+    {
+      status,
+      headers: { "Content-Type": "application/json", ...CORS_HEADERS },
+    }
+  );
+}
+
+export async function OPTIONS(): Promise<Response> {
+  return new Response(null, { headers: CORS_HEADERS });
+}
+
+export async function POST(request: Request): Promise<Response> {
+  // 1. Parse JSON body
+  let rawBody: unknown;
+  try {
+    rawBody = await request.json();
+  } catch {
+    return errorResp(HTTP_STATUS.BAD_REQUEST, "Invalid JSON body");
+  }
+
+  // 2. Validate via Zod (Hard Rule #7)
+  const parsed = ImprovePromptRequestSchema.safeParse(rawBody);
+  if (!parsed.success) {
+    const firstIssue = parsed.error.issues[0];
+    const message = firstIssue
+      ? `${firstIssue.path.join(".") || "body"}: ${firstIssue.message}`
+      : "Invalid request body";
+    return errorResp(HTTP_STATUS.BAD_REQUEST, message);
+  }
+  const body = parsed.data;
+
+  // 3. Optional auth (mirrors /v1/web/fetch pattern)
+  const apiKeyRaw = extractApiKey(request);
+  if (process.env.REQUIRE_API_KEY === "true" && !apiKeyRaw) {
+    return errorResp(HTTP_STATUS.UNAUTHORIZED, "Authentication required");
+  }
+  if (apiKeyRaw && !(await isValidApiKey(apiKeyRaw))) {
+    return errorResp(HTTP_STATUS.UNAUTHORIZED, "Invalid API key");
+  }
+
+  // 4. Build the meta-prompt chat body
+  const chatBody = buildImproveChatBody(body);
+
+  // 5. Call /v1/chat/completions on ourselves (D8)
+  const port = process.env.PORT ?? "20128";
+  const baseUrl =
+    process.env.OMNIROUTE_BASE_URL ?? `http://127.0.0.1:${port}`;
+  const upstreamUrl = `${baseUrl}/v1/chat/completions`;
+
+  let upstreamResponse: Response;
+  try {
+    upstreamResponse = await fetch(upstreamUrl, {
+      method: "POST",
+      headers: {
+        "Content-Type": "application/json",
+        Authorization: `Bearer ${apiKeyRaw ?? ""}`,
+      },
+      body: JSON.stringify(chatBody),
+    });
+  } catch (err: unknown) {
+    const safeMsg = sanitizeErrorMessage(
+      err instanceof Error ? err.message : "Failed to reach upstream"
+    );
+    return errorResp(HTTP_STATUS.SERVER_ERROR, `Improve-prompt failed: ${safeMsg}`);
+  }
+
+  if (!upstreamResponse.ok) {
+    let upstreamText = "";
+    try {
+      upstreamText = await upstreamResponse.text();
+    } catch {
+      // ignore
+    }
+    return errorResp(
+      upstreamResponse.status >= 400 && upstreamResponse.status < 600
+        ? upstreamResponse.status
+        : HTTP_STATUS.BAD_GATEWAY,
+      "Improve-prompt upstream request failed",
+      sanitizeErrorMessage(upstreamText)
+    );
+  }
+
+  // 6. Parse response
+  let data: {
+    choices?: Array<{ message?: { content?: string | null } }>;
+    usage?: { prompt_tokens?: number; completion_tokens?: number };
+  };
+  try {
+    data = (await upstreamResponse.json()) as typeof data;
+  } catch {
+    return errorResp(HTTP_STATUS.BAD_GATEWAY, "Invalid JSON from upstream improve-prompt call");
+  }
+
+  const content = data.choices?.[0]?.message?.content;
+  if (typeof content !== "string") {
+    return errorResp(HTTP_STATUS.BAD_GATEWAY, "Empty or missing content from upstream");
+  }
+
+  // 7. Parse improved content back into system/prompt parts
+  const hadSystem = Boolean(body.system?.trim());
+  const hadPrompt = Boolean(body.prompt?.trim());
+  const improved = parseImprovedContent(content, hadSystem, hadPrompt);
+
+  const tokensIn = data.usage?.prompt_tokens ?? 0;
+  const tokensOut = data.usage?.completion_tokens ?? 0;
+
+  return new Response(
+    JSON.stringify({
+      ...improved,
+      tokensIn,
+      tokensOut,
+    }),
+    {
+      status: 200,
+      headers: { "Content-Type": "application/json", ...CORS_HEADERS },
+    }
+  );
+}
diff --git a/src/app/api/playground/presets/[id]/route.ts b/src/app/api/playground/presets/[id]/route.ts
new file mode 100644
index 0000000000..ceabc40429
--- /dev/null
+++ b/src/app/api/playground/presets/[id]/route.ts
@@ -0,0 +1,189 @@
+/**
+ * GET    /api/playground/presets/[id]  — get a single preset
+ * PUT    /api/playground/presets/[id]  — update a preset (partial patch)
+ * DELETE /api/playground/presets/[id]  — delete a preset (204 on success)
+ *
+ * Auth: optional (extractApiKey + isValidApiKey; required when REQUIRE_API_KEY=true).
+ * Hard Rule #5: all DB access via src/lib/db/playgroundPresets (never raw SQL).
+ * Hard Rule #7: PUT body validated via PlaygroundPresetUpdateSchema.
+ * Hard Rule #12: all error paths via buildErrorBody.
+ */
+
+import { z } from "zod";
+import { buildErrorBody, sanitizeErrorMessage } from "@omniroute/open-sse/utils/error.ts";
+import { HTTP_STATUS } from "@omniroute/open-sse/config/constants.ts";
+import { extractApiKey, isValidApiKey } from "@/sse/services/auth";
+import {
+  getPlaygroundPreset,
+  updatePlaygroundPreset,
+  deletePlaygroundPreset,
+} from "@/lib/db/playgroundPresets";
+import { PlaygroundPresetUpdateSchema } from "@/shared/schemas/playground";
+
+const CORS_HEADERS = {
+  "Access-Control-Allow-Methods": "GET, PUT, DELETE, OPTIONS",
+  "Access-Control-Allow-Headers": "*",
+};
+
+function errorResp(status: number, message: string): Response {
+  return new Response(
+    JSON.stringify(buildErrorBody(status, sanitizeErrorMessage(message))),
+    {
+      status,
+      headers: { "Content-Type": "application/json", ...CORS_HEADERS },
+    }
+  );
+}
+
+async function checkAuth(request: Request): Promise<Response | null> {
+  const apiKeyRaw = extractApiKey(request);
+  if (process.env.REQUIRE_API_KEY === "true" && !apiKeyRaw) {
+    return errorResp(HTTP_STATUS.UNAUTHORIZED, "Authentication required");
+  }
+  if (apiKeyRaw && !(await isValidApiKey(apiKeyRaw))) {
+    return errorResp(HTTP_STATUS.UNAUTHORIZED, "Invalid API key");
+  }
+  return null;
+}
+
+function validateId(id: unknown): { valid: true; id: string } | { valid: false; error: Response } {
+  const result = z.string().uuid().safeParse(id);
+  if (!result.success) {
+    return {
+      valid: false,
+      error: errorResp(HTTP_STATUS.BAD_REQUEST, "Invalid preset id: must be a valid UUID"),
+    };
+  }
+  return { valid: true, id: result.data };
+}
+
+export async function OPTIONS(): Promise<Response> {
+  return new Response(null, { headers: CORS_HEADERS });
+}
+
+/**
+ * GET /api/playground/presets/[id]
+ * Returns the preset or 404.
+ */
+export async function GET(
+  request: Request,
+  { params }: { params: Promise<{ id: string }> }
+): Promise<Response> {
+  const authError = await checkAuth(request);
+  if (authError) return authError;
+
+  const { id: rawId } = await params;
+  const validation = validateId(rawId);
+  if (!validation.valid) return validation.error;
+  const { id } = validation;
+
+  try {
+    const preset = getPlaygroundPreset(id);
+    if (!preset) {
+      return errorResp(HTTP_STATUS.NOT_FOUND, `Preset not found: ${id}`);
+    }
+    return new Response(JSON.stringify(preset), {
+      status: 200,
+      headers: { "Content-Type": "application/json", ...CORS_HEADERS },
+    });
+  } catch (err: unknown) {
+    const safeMsg = sanitizeErrorMessage(
+      err instanceof Error ? err.message : "Failed to fetch preset"
+    );
+    return errorResp(HTTP_STATUS.SERVER_ERROR, safeMsg);
+  }
+}
+
+/**
+ * PUT /api/playground/presets/[id]
+ * Body: PlaygroundPresetUpdateSchema (partial)
+ * Returns the updated preset or 404.
+ */
+export async function PUT(
+  request: Request,
+  { params }: { params: Promise<{ id: string }> }
+): Promise<Response> {
+  const authError = await checkAuth(request);
+  if (authError) return authError;
+
+  const { id: rawId } = await params;
+  const validation = validateId(rawId);
+  if (!validation.valid) return validation.error;
+  const { id } = validation;
+
+  // Parse JSON body
+  let rawBody: unknown;
+  try {
+    rawBody = await request.json();
+  } catch {
+    return errorResp(HTTP_STATUS.BAD_REQUEST, "Invalid JSON body");
+  }
+
+  // Validate body (Hard Rule #7)
+  const parsed = PlaygroundPresetUpdateSchema.safeParse(rawBody);
+  if (!parsed.success) {
+    const firstIssue = parsed.error.issues[0];
+    const message = firstIssue
+      ? `${firstIssue.path.join(".") || "body"}: ${firstIssue.message}`
+      : "Invalid request body";
+    return errorResp(HTTP_STATUS.BAD_REQUEST, message);
+  }
+  const patch = parsed.data;
+
+  try {
+    const updated = updatePlaygroundPreset(id, {
+      ...(patch.name !== undefined ? { name: patch.name } : {}),
+      ...(patch.endpoint !== undefined ? { endpoint: patch.endpoint } : {}),
+      ...(patch.model !== undefined ? { model: patch.model } : {}),
+      ...("system" in patch ? { system: patch.system ?? null } : {}),
+      ...(patch.params !== undefined ? { params: patch.params } : {}),
+    });
+
+    if (!updated) {
+      return errorResp(HTTP_STATUS.NOT_FOUND, `Preset not found: ${id}`);
+    }
+
+    return new Response(JSON.stringify(updated), {
+      status: 200,
+      headers: { "Content-Type": "application/json", ...CORS_HEADERS },
+    });
+  } catch (err: unknown) {
+    const safeMsg = sanitizeErrorMessage(
+      err instanceof Error ? err.message : "Failed to update preset"
+    );
+    return errorResp(HTTP_STATUS.SERVER_ERROR, safeMsg);
+  }
+}
+
+/**
+ * DELETE /api/playground/presets/[id]
+ * Returns 204 on success; 404 if not found.
+ */
+export async function DELETE(
+  request: Request,
+  { params }: { params: Promise<{ id: string }> }
+): Promise<Response> {
+  const authError = await checkAuth(request);
+  if (authError) return authError;
+
+  const { id: rawId } = await params;
+  const validation = validateId(rawId);
+  if (!validation.valid) return validation.error;
+  const { id } = validation;
+
+  try {
+    const deleted = deletePlaygroundPreset(id);
+    if (!deleted) {
+      return errorResp(HTTP_STATUS.NOT_FOUND, `Preset not found: ${id}`);
+    }
+    return new Response(null, {
+      status: 204,
+      headers: CORS_HEADERS,
+    });
+  } catch (err: unknown) {
+    const safeMsg = sanitizeErrorMessage(
+      err instanceof Error ? err.message : "Failed to delete preset"
+    );
+    return errorResp(HTTP_STATUS.SERVER_ERROR, safeMsg);
+  }
+}
diff --git a/src/app/api/playground/presets/route.ts b/src/app/api/playground/presets/route.ts
new file mode 100644
index 0000000000..37b2b2b17d
--- /dev/null
+++ b/src/app/api/playground/presets/route.ts
@@ -0,0 +1,116 @@
+/**
+ * GET  /api/playground/presets  — list presets
+ * POST /api/playground/presets  — create a preset
+ *
+ * Auth: optional (extractApiKey + isValidApiKey; required when REQUIRE_API_KEY=true).
+ * Hard Rule #5: all DB access via src/lib/db/playgroundPresets (never raw SQL).
+ * Hard Rule #12: all error paths via buildErrorBody.
+ */
+
+import { buildErrorBody, sanitizeErrorMessage } from "@omniroute/open-sse/utils/error.ts";
+import { HTTP_STATUS } from "@omniroute/open-sse/config/constants.ts";
+import { extractApiKey, isValidApiKey } from "@/sse/services/auth";
+import { listPlaygroundPresets, createPlaygroundPreset } from "@/lib/db/playgroundPresets";
+import { PlaygroundPresetCreateSchema } from "@/shared/schemas/playground";
+
+const CORS_HEADERS = {
+  "Access-Control-Allow-Methods": "GET, POST, OPTIONS",
+  "Access-Control-Allow-Headers": "*",
+};
+
+function errorResp(status: number, message: string): Response {
+  return new Response(
+    JSON.stringify(buildErrorBody(status, sanitizeErrorMessage(message))),
+    {
+      status,
+      headers: { "Content-Type": "application/json", ...CORS_HEADERS },
+    }
+  );
+}
+
+async function checkAuth(request: Request): Promise<Response | null> {
+  const apiKeyRaw = extractApiKey(request);
+  if (process.env.REQUIRE_API_KEY === "true" && !apiKeyRaw) {
+    return errorResp(HTTP_STATUS.UNAUTHORIZED, "Authentication required");
+  }
+  if (apiKeyRaw && !(await isValidApiKey(apiKeyRaw))) {
+    return errorResp(HTTP_STATUS.UNAUTHORIZED, "Invalid API key");
+  }
+  return null;
+}
+
+export async function OPTIONS(): Promise<Response> {
+  return new Response(null, { headers: CORS_HEADERS });
+}
+
+/**
+ * GET /api/playground/presets
+ * Returns { presets: PlaygroundPresetListItem[] }
+ */
+export async function GET(request: Request): Promise<Response> {
+  const authError = await checkAuth(request);
+  if (authError) return authError;
+
+  try {
+    const presets = listPlaygroundPresets();
+    return new Response(JSON.stringify({ presets }), {
+      status: 200,
+      headers: { "Content-Type": "application/json", ...CORS_HEADERS },
+    });
+  } catch (err: unknown) {
+    const safeMsg = sanitizeErrorMessage(
+      err instanceof Error ? err.message : "Failed to list presets"
+    );
+    return errorResp(HTTP_STATUS.SERVER_ERROR, safeMsg);
+  }
+}
+
+/**
+ * POST /api/playground/presets
+ * Body: PlaygroundPresetCreateSchema
+ * Returns the created preset with status 201.
+ */
+export async function POST(request: Request): Promise<Response> {
+  const authError = await checkAuth(request);
+  if (authError) return authError;
+
+  // 1. Parse JSON body
+  let rawBody: unknown;
+  try {
+    rawBody = await request.json();
+  } catch {
+    return errorResp(HTTP_STATUS.BAD_REQUEST, "Invalid JSON body");
+  }
+
+  // 2. Validate (Hard Rule #7)
+  const parsed = PlaygroundPresetCreateSchema.safeParse(rawBody);
+  if (!parsed.success) {
+    const firstIssue = parsed.error.issues[0];
+    const message = firstIssue
+      ? `${firstIssue.path.join(".") || "body"}: ${firstIssue.message}`
+      : "Invalid request body";
+    return errorResp(HTTP_STATUS.BAD_REQUEST, message);
+  }
+  const body = parsed.data;
+
+  // 3. Create preset (Hard Rule #5 — via DB module)
+  try {
+    const created = createPlaygroundPreset({
+      name: body.name,
+      endpoint: body.endpoint,
+      model: body.model,
+      system: body.system ?? null,
+      params: body.params,
+    });
+
+    return new Response(JSON.stringify(created), {
+      status: 201,
+      headers: { "Content-Type": "application/json", ...CORS_HEADERS },
+    });
+  } catch (err: unknown) {
+    const safeMsg = sanitizeErrorMessage(
+      err instanceof Error ? err.message : "Failed to create preset"
+    );
+    return errorResp(HTTP_STATUS.SERVER_ERROR, safeMsg);
+  }
+}
diff --git a/tests/integration/playground-improve-prompt.test.ts b/tests/integration/playground-improve-prompt.test.ts
new file mode 100644
index 0000000000..aaa67d67cc
--- /dev/null
+++ b/tests/integration/playground-improve-prompt.test.ts
@@ -0,0 +1,303 @@
+/**
+ * Integration tests for POST /api/playground/improve-prompt
+ *
+ * Tests import the route handler directly and call it with mock Request objects.
+ * Fetch to /v1/chat/completions is mocked via globalThis.fetch.
+ *
+ * Coverage areas:
+ * - Happy path (system + prompt, only system, only prompt)
+ * - Invalid bodies → 400
+ * - Missing model → 400
+ * - Upstream error → sanitized error (no stack trace leak)
+ * - Hard Rule #12: error.message never contains " at /"
+ */
+
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+// Set up a temp DATA_DIR so getDbInstance() initialises cleanly
+const TEST_DATA_DIR = fs.mkdtempSync(
+  path.join(os.tmpdir(), "omniroute-improve-prompt-")
+);
+process.env.DATA_DIR = TEST_DATA_DIR;
+// Disable mandatory auth for most tests
+process.env.REQUIRE_API_KEY = "false";
+
+const { POST, OPTIONS } = await import(
+  "../../src/app/api/playground/improve-prompt/route.ts"
+);
+
+const BASE_URL = "http://localhost:20128";
+
+// ─── Helpers ─────────────────────────────────────────────────────────────────
+
+function makeUpstreamOkFetch(content: string, promptTokens = 10, completionTokens = 5) {
+  return async (_url: unknown, _opts: unknown) => {
+    const body = JSON.stringify({
+      choices: [{ message: { content } }],
+      usage: { prompt_tokens: promptTokens, completion_tokens: completionTokens },
+    });
+    return new Response(body, { status: 200, headers: { "Content-Type": "application/json" } });
+  };
+}
+
+function makeUpstreamErrorFetch(status: number, text: string) {
+  return async (_url: unknown, _opts: unknown) => {
+    return new Response(text, { status, headers: { "Content-Type": "text/plain" } });
+  };
+}
+
+function postRequest(body: unknown): Request {
+  return new Request(`${BASE_URL}/api/playground/improve-prompt`, {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify(body),
+  });
+}
+
+// ─── Cleanup ─────────────────────────────────────────────────────────────────
+
+test.after(() => {
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+});
+
+// ─── OPTIONS ─────────────────────────────────────────────────────────────────
+
+test("OPTIONS returns 200 with CORS headers", async () => {
+  const res = await OPTIONS();
+  assert.equal(res.status, 200);
+  assert.ok(res.headers.get("Access-Control-Allow-Methods")?.includes("POST"));
+});
+
+// ─── Happy paths ─────────────────────────────────────────────────────────────
+
+test("happy path: system + prompt both provided", async () => {
+  const originalFetch = globalThis.fetch;
+  try {
+    globalThis.fetch = makeUpstreamOkFetch(
+      "<<SYSTEM>>\nimproved system\n\n<<PROMPT>>\nimproved prompt",
+      10,
+      5
+    ) as typeof fetch;
+
+    const res = await POST(
+      postRequest({ system: "You are a helper.", prompt: "Tell me about AI.", model: "gpt-4o-mini" })
+    );
+    assert.equal(res.status, 200);
+
+    const body = (await res.json()) as {
+      improvedSystem?: string;
+      improvedPrompt?: string;
+      tokensIn: number;
+      tokensOut: number;
+    };
+    assert.equal(body.improvedSystem, "improved system");
+    assert.equal(body.improvedPrompt, "improved prompt");
+    assert.equal(body.tokensIn, 10);
+    assert.equal(body.tokensOut, 5);
+    // CORS header present
+    assert.ok(res.headers.get("Access-Control-Allow-Methods")?.includes("POST"));
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
+
+test("happy path: only system provided", async () => {
+  const originalFetch = globalThis.fetch;
+  try {
+    globalThis.fetch = makeUpstreamOkFetch("improved system content only") as typeof fetch;
+
+    const res = await POST(
+      postRequest({ system: "You are a helpful assistant.", model: "gpt-4o" })
+    );
+    assert.equal(res.status, 200);
+
+    const body = (await res.json()) as {
+      improvedSystem?: string;
+      improvedPrompt?: string;
+      tokensIn: number;
+      tokensOut: number;
+    };
+    // When only system was sent and no markers, content assigned to improvedSystem
+    assert.ok(typeof body.improvedSystem === "string" || body.improvedSystem === undefined);
+    assert.strictEqual(body.improvedPrompt, undefined);
+    assert.equal(typeof body.tokensIn, "number");
+    assert.equal(typeof body.tokensOut, "number");
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
+
+test("happy path: only prompt provided", async () => {
+  const originalFetch = globalThis.fetch;
+  try {
+    globalThis.fetch = makeUpstreamOkFetch("improved prompt content only") as typeof fetch;
+
+    const res = await POST(
+      postRequest({ prompt: "What is machine learning?", model: "claude-3-5-sonnet-20241022" })
+    );
+    assert.equal(res.status, 200);
+
+    const body = (await res.json()) as {
+      improvedSystem?: string;
+      improvedPrompt?: string;
+      tokensIn: number;
+      tokensOut: number;
+    };
+    assert.strictEqual(body.improvedSystem, undefined);
+    assert.ok(typeof body.improvedPrompt === "string" || body.improvedPrompt === undefined);
+    assert.equal(typeof body.tokensIn, "number");
+    assert.equal(typeof body.tokensOut, "number");
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
+
+test("happy path: usage defaults to 0 when not in upstream response", async () => {
+  const originalFetch = globalThis.fetch;
+  try {
+    // Return response without usage field
+    globalThis.fetch = (async (_url: unknown, _opts: unknown) => {
+      return new Response(
+        JSON.stringify({ choices: [{ message: { content: "improved" } }] }),
+        { status: 200, headers: { "Content-Type": "application/json" } }
+      );
+    }) as typeof fetch;
+
+    const res = await POST(
+      postRequest({ prompt: "Hello world", model: "gpt-4o-mini" })
+    );
+    assert.equal(res.status, 200);
+
+    const body = (await res.json()) as { tokensIn: number; tokensOut: number };
+    assert.equal(body.tokensIn, 0);
+    assert.equal(body.tokensOut, 0);
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
+
+// ─── Validation errors → 400 ─────────────────────────────────────────────────
+
+test("400 when body has neither system nor prompt", async () => {
+  const res = await POST(postRequest({ model: "gpt-4o-mini" }));
+  assert.equal(res.status, 400);
+
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error, "should have error field");
+  assert.equal(typeof body.error.message, "string");
+  // Hard Rule #12: no stack trace
+  assert.ok(!body.error.message.match(/\sat\s\//), "should not contain stack trace");
+});
+
+test("400 when body has both system and prompt as empty strings", async () => {
+  const res = await POST(postRequest({ system: "   ", prompt: "  ", model: "gpt-4o-mini" }));
+  assert.equal(res.status, 400);
+
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error, "should have error field");
+  assert.ok(!body.error.message.match(/\sat\s\//));
+});
+
+test("400 when model is missing", async () => {
+  const res = await POST(postRequest({ system: "You are a helper." }));
+  assert.equal(res.status, 400);
+
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error, "should have error field");
+  assert.ok(!body.error.message.match(/\sat\s\//));
+});
+
+test("400 when model is empty string", async () => {
+  const res = await POST(postRequest({ prompt: "Tell me something.", model: "" }));
+  assert.equal(res.status, 400);
+
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error, "should have error field");
+  assert.ok(!body.error.message.match(/\sat\s\//));
+});
+
+test("400 when JSON body is malformed", async () => {
+  const req = new Request(`${BASE_URL}/api/playground/improve-prompt`, {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: "NOT_JSON",
+  });
+  const res = await POST(req);
+  assert.equal(res.status, 400);
+
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error, "should have error field");
+  assert.ok(!body.error.message.match(/\sat\s\//));
+});
+
+// ─── Upstream error handling ──────────────────────────────────────────────────
+
+test("upstream error returns sanitized error message — no stack trace in body", async () => {
+  const originalFetch = globalThis.fetch;
+  try {
+    // Simulate upstream returning 500 with a message that looks like stack trace
+    globalThis.fetch = makeUpstreamErrorFetch(
+      500,
+      "Internal error\n    at /home/user/project/src/handler.ts:42:10\n    at process.nextTick"
+    ) as typeof fetch;
+
+    const res = await POST(
+      postRequest({ prompt: "Hello", model: "gpt-4o-mini" })
+    );
+    // Should be an error response (not 200)
+    assert.ok(res.status >= 400);
+
+    const body = (await res.json()) as { error: { message: string } };
+    assert.ok(body.error, "should have error field");
+    // Hard Rule #12: stack trace must be stripped
+    assert.ok(
+      !body.error.message.match(/\sat\s\//),
+      "error message must not contain stack trace paths"
+    );
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
+
+test("upstream network error is sanitized", async () => {
+  const originalFetch = globalThis.fetch;
+  try {
+    globalThis.fetch = (async () => {
+      throw new Error("ECONNREFUSED connect ECONNREFUSED 127.0.0.1:20128");
+    }) as typeof fetch;
+
+    const res = await POST(
+      postRequest({ prompt: "Hello", model: "gpt-4o-mini" })
+    );
+    assert.ok(res.status >= 500);
+
+    const body = (await res.json()) as { error: { message: string } };
+    assert.ok(body.error);
+    assert.ok(!body.error.message.match(/\sat\s\//));
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
+
+// ─── Auth ─────────────────────────────────────────────────────────────────────
+
+test("401 when REQUIRE_API_KEY=true and no key provided", async () => {
+  const originalRequired = process.env.REQUIRE_API_KEY;
+  try {
+    process.env.REQUIRE_API_KEY = "true";
+    const res = await POST(
+      postRequest({ prompt: "Test", model: "gpt-4o-mini" })
+    );
+    assert.equal(res.status, 401);
+
+    const body = (await res.json()) as { error: { message: string } };
+    assert.ok(body.error);
+    assert.ok(!body.error.message.match(/\sat\s\//));
+  } finally {
+    process.env.REQUIRE_API_KEY = originalRequired;
+  }
+});
diff --git a/tests/integration/playground-presets-crud.test.ts b/tests/integration/playground-presets-crud.test.ts
new file mode 100644
index 0000000000..655fdfdc53
--- /dev/null
+++ b/tests/integration/playground-presets-crud.test.ts
@@ -0,0 +1,337 @@
+/**
+ * Integration tests for GET/POST /api/playground/presets
+ * and GET/PUT/DELETE /api/playground/presets/[id]
+ *
+ * Tests exercise the full CRUD lifecycle using a temporary SQLite DB.
+ *
+ * Coverage areas:
+ * - POST preset → 201 with UUID id
+ * - GET list → includes the created item
+ * - GET [id] → returns the item
+ * - PUT partial patch (name only)
+ * - DELETE → 204
+ * - GET [id] after delete → 404
+ * - DELETE non-existent id → 404
+ * - UUID validation: non-UUID id → 400
+ */
+
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+// Isolated DB per test file
+const TEST_DATA_DIR = fs.mkdtempSync(
+  path.join(os.tmpdir(), "omniroute-presets-crud-")
+);
+process.env.DATA_DIR = TEST_DATA_DIR;
+process.env.REQUIRE_API_KEY = "false";
+
+const core = await import("../../src/lib/db/core.ts");
+
+// Import route handlers
+const { GET: listGet, POST: createPost, OPTIONS: listOptions } = await import(
+  "../../src/app/api/playground/presets/route.ts"
+);
+const { GET: idGet, PUT: idPut, DELETE: idDelete, OPTIONS: idOptions } = await import(
+  "../../src/app/api/playground/presets/[id]/route.ts"
+);
+
+const BASE_URL = "http://localhost:20128";
+
+const UUID_V4_REGEX =
+  /^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i;
+
+// ─── Helpers ─────────────────────────────────────────────────────────────────
+
+function postReq(body: unknown): Request {
+  return new Request(`${BASE_URL}/api/playground/presets`, {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify(body),
+  });
+}
+
+function getReq(): Request {
+  return new Request(`${BASE_URL}/api/playground/presets`, { method: "GET" });
+}
+
+function idGetReq(id: string): Request {
+  return new Request(`${BASE_URL}/api/playground/presets/${id}`, { method: "GET" });
+}
+
+function putReq(id: string, body: unknown): Request {
+  return new Request(`${BASE_URL}/api/playground/presets/${id}`, {
+    method: "PUT",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify(body),
+  });
+}
+
+function deleteReq(id: string): Request {
+  return new Request(`${BASE_URL}/api/playground/presets/${id}`, { method: "DELETE" });
+}
+
+async function resolveParams(id: string): Promise<{ params: Promise<{ id: string }> }> {
+  return { params: Promise.resolve({ id }) };
+}
+
+// ─── Cleanup ─────────────────────────────────────────────────────────────────
+
+test.beforeEach(async () => {
+  // Reset DB between tests for isolation
+  core.resetDbInstance();
+});
+
+test.after(() => {
+  core.resetDbInstance();
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+});
+
+// ─── OPTIONS ─────────────────────────────────────────────────────────────────
+
+test("OPTIONS /presets returns CORS with GET, POST, OPTIONS", async () => {
+  const res = await listOptions();
+  assert.equal(res.status, 200);
+  const methods = res.headers.get("Access-Control-Allow-Methods") ?? "";
+  assert.ok(methods.includes("GET"));
+  assert.ok(methods.includes("POST"));
+  assert.ok(methods.includes("OPTIONS"));
+});
+
+test("OPTIONS /presets/[id] returns CORS with GET, PUT, DELETE, OPTIONS", async () => {
+  const res = await idOptions();
+  assert.equal(res.status, 200);
+  const methods = res.headers.get("Access-Control-Allow-Methods") ?? "";
+  assert.ok(methods.includes("GET"));
+  assert.ok(methods.includes("PUT"));
+  assert.ok(methods.includes("DELETE"));
+});
+
+// ─── Full CRUD lifecycle ──────────────────────────────────────────────────────
+
+test("POST /presets → 201 with valid UUID id and correct shape", async () => {
+  const res = await createPost(
+    postReq({
+      name: "My Test Preset",
+      endpoint: "chat.completions",
+      model: "gpt-4o",
+      system: "You are a helpful assistant.",
+      params: { temperature: 0.7, max_tokens: 1024 },
+    })
+  );
+  assert.equal(res.status, 201);
+  assert.ok(res.headers.get("Access-Control-Allow-Methods")?.includes("POST"));
+
+  const body = (await res.json()) as {
+    id: string;
+    name: string;
+    endpoint: string;
+    model: string;
+    system: string | null;
+    params: Record<string, unknown>;
+    created_at: string;
+  };
+  assert.ok(UUID_V4_REGEX.test(body.id), "id should be a valid UUID v4");
+  assert.equal(body.name, "My Test Preset");
+  assert.equal(body.endpoint, "chat.completions");
+  assert.equal(body.model, "gpt-4o");
+  assert.equal(body.system, "You are a helpful assistant.");
+  assert.deepEqual(body.params, { temperature: 0.7, max_tokens: 1024 });
+  assert.equal(typeof body.created_at, "string");
+});
+
+test("GET /presets list returns the created item", async () => {
+  // Create one preset
+  const createRes = await createPost(
+    postReq({ name: "Preset A", endpoint: "chat.completions", model: "gpt-4o" })
+  );
+  assert.equal(createRes.status, 201);
+  const created = (await createRes.json()) as { id: string };
+
+  // List
+  const listRes = await listGet(getReq());
+  assert.equal(listRes.status, 200);
+  const listBody = (await listRes.json()) as { presets: Array<{ id: string; name: string }> };
+  assert.ok(Array.isArray(listBody.presets));
+  const found = listBody.presets.find((p) => p.id === created.id);
+  assert.ok(found, "created preset should appear in list");
+  assert.equal(found.name, "Preset A");
+});
+
+test("GET /presets/[id] returns the correct preset", async () => {
+  const createRes = await createPost(
+    postReq({ name: "Fetch Me", endpoint: "embeddings", model: "text-embedding-3-small" })
+  );
+  const created = (await createRes.json()) as { id: string; name: string };
+
+  const getRes = await idGet(idGetReq(created.id), await resolveParams(created.id));
+  assert.equal(getRes.status, 200);
+
+  const body = (await getRes.json()) as { id: string; name: string };
+  assert.equal(body.id, created.id);
+  assert.equal(body.name, "Fetch Me");
+});
+
+test("PUT /presets/[id] partial patch (name only) updates correctly", async () => {
+  const createRes = await createPost(
+    postReq({ name: "Original Name", endpoint: "chat.completions", model: "gpt-4o" })
+  );
+  const created = (await createRes.json()) as { id: string; endpoint: string; model: string };
+
+  const putRes = await idPut(
+    putReq(created.id, { name: "Updated Name" }),
+    await resolveParams(created.id)
+  );
+  assert.equal(putRes.status, 200);
+
+  const updated = (await putRes.json()) as { id: string; name: string; endpoint: string; model: string };
+  assert.equal(updated.id, created.id);
+  assert.equal(updated.name, "Updated Name");
+  // Other fields should be preserved
+  assert.equal(updated.endpoint, created.endpoint);
+  assert.equal(updated.model, created.model);
+});
+
+test("PUT /presets/[id] can update params", async () => {
+  const createRes = await createPost(
+    postReq({
+      name: "Params Test",
+      endpoint: "chat.completions",
+      model: "gpt-4o",
+      params: { temperature: 0.5 },
+    })
+  );
+  const created = (await createRes.json()) as { id: string };
+
+  const putRes = await idPut(
+    putReq(created.id, { params: { temperature: 0.9, max_tokens: 2048 } }),
+    await resolveParams(created.id)
+  );
+  assert.equal(putRes.status, 200);
+
+  const updated = (await putRes.json()) as { params: Record<string, unknown> };
+  assert.deepEqual(updated.params, { temperature: 0.9, max_tokens: 2048 });
+});
+
+test("DELETE /presets/[id] returns 204", async () => {
+  const createRes = await createPost(
+    postReq({ name: "Delete Me", endpoint: "chat.completions", model: "gpt-4o" })
+  );
+  const created = (await createRes.json()) as { id: string };
+
+  const delRes = await idDelete(deleteReq(created.id), await resolveParams(created.id));
+  assert.equal(delRes.status, 204);
+});
+
+test("GET /presets/[id] after DELETE returns 404", async () => {
+  const createRes = await createPost(
+    postReq({ name: "Gone Soon", endpoint: "chat.completions", model: "gpt-4o" })
+  );
+  const created = (await createRes.json()) as { id: string };
+
+  await idDelete(deleteReq(created.id), await resolveParams(created.id));
+
+  const getRes = await idGet(idGetReq(created.id), await resolveParams(created.id));
+  assert.equal(getRes.status, 404);
+
+  const body = (await getRes.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assert.ok(!body.error.message.match(/\sat\s\//));
+});
+
+test("DELETE /presets/[id] with non-existent id returns 404", async () => {
+  const fakeId = "00000000-0000-4000-8000-000000000000";
+  const delRes = await idDelete(deleteReq(fakeId), await resolveParams(fakeId));
+  assert.equal(delRes.status, 404);
+
+  const body = (await delRes.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assert.ok(!body.error.message.match(/\sat\s\//));
+});
+
+// ─── params JSON round-trip ───────────────────────────────────────────────────
+
+test("params object is serialized and deserialized correctly", async () => {
+  const complexParams = {
+    temperature: 0.7,
+    max_tokens: 2048,
+    top_p: 0.9,
+    presence_penalty: 0.1,
+    frequency_penalty: 0.2,
+    stop: ["<|end|>", "\n\n"],
+  };
+
+  const createRes = await createPost(
+    postReq({
+      name: "Complex Params",
+      endpoint: "chat.completions",
+      model: "gpt-4o",
+      params: complexParams,
+    })
+  );
+  assert.equal(createRes.status, 201);
+  const created = (await createRes.json()) as { id: string; params: Record<string, unknown> };
+  assert.deepEqual(created.params, complexParams);
+
+  // GET again and verify
+  const getRes = await idGet(idGetReq(created.id), await resolveParams(created.id));
+  const fetched = (await getRes.json()) as { params: Record<string, unknown> };
+  assert.deepEqual(fetched.params, complexParams);
+});
+
+// ─── UUID validation ─────────────────────────────────────────────────────────
+
+test("GET /presets/[id] with non-UUID id → 400", async () => {
+  const badId = "not-a-uuid";
+  const res = await idGet(idGetReq(badId), await resolveParams(badId));
+  assert.equal(res.status, 400);
+
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assert.ok(!body.error.message.match(/\sat\s\//));
+});
+
+test("PUT /presets/[id] with non-UUID id → 400", async () => {
+  const badId = "also-not-a-uuid";
+  const res = await idPut(
+    putReq(badId, { name: "Whatever" }),
+    await resolveParams(badId)
+  );
+  assert.equal(res.status, 400);
+
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assert.ok(!body.error.message.match(/\sat\s\//));
+});
+
+test("DELETE /presets/[id] with non-UUID id → 400", async () => {
+  const badId = "bad-id-not-uuid";
+  const res = await idDelete(deleteReq(badId), await resolveParams(badId));
+  assert.equal(res.status, 400);
+
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assert.ok(!body.error.message.match(/\sat\s\//));
+});
+
+// ─── Null system ─────────────────────────────────────────────────────────────
+
+test("POST with null system stores null correctly", async () => {
+  const res = await createPost(
+    postReq({ name: "No System", endpoint: "chat.completions", model: "gpt-4o", system: null })
+  );
+  assert.equal(res.status, 201);
+  const body = (await res.json()) as { system: string | null };
+  assert.strictEqual(body.system, null);
+});
+
+test("POST without system defaults to null", async () => {
+  const res = await createPost(
+    postReq({ name: "No System Either", endpoint: "chat.completions", model: "gpt-4o" })
+  );
+  assert.equal(res.status, 201);
+  const body = (await res.json()) as { system: string | null };
+  assert.strictEqual(body.system, null);
+});
diff --git a/tests/integration/playground-presets-zod.test.ts b/tests/integration/playground-presets-zod.test.ts
new file mode 100644
index 0000000000..3150eaed15
--- /dev/null
+++ b/tests/integration/playground-presets-zod.test.ts
@@ -0,0 +1,299 @@
+/**
+ * Integration tests for Zod validation in playground presets routes.
+ *
+ * Tests focus on invalid bodies and edge cases that must return 400.
+ *
+ * Coverage areas:
+ * - POST with empty name → 400
+ * - POST with missing required fields (endpoint, model) → 400
+ * - POST with system > 50000 chars → 400
+ * - PUT with invalid body → 400
+ * - GET/PUT/DELETE with valid UUID format but wrong format string → 400
+ * - All error responses must NOT contain stack trace fragments (Hard Rule #12)
+ */
+
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+// Isolated DB per test file
+const TEST_DATA_DIR = fs.mkdtempSync(
+  path.join(os.tmpdir(), "omniroute-presets-zod-")
+);
+process.env.DATA_DIR = TEST_DATA_DIR;
+process.env.REQUIRE_API_KEY = "false";
+
+const core = await import("../../src/lib/db/core.ts");
+
+const { POST: createPost } = await import(
+  "../../src/app/api/playground/presets/route.ts"
+);
+const { GET: idGet, PUT: idPut, DELETE: idDelete } = await import(
+  "../../src/app/api/playground/presets/[id]/route.ts"
+);
+
+const BASE_URL = "http://localhost:20128";
+
+// ─── Helpers ─────────────────────────────────────────────────────────────────
+
+function postReq(body: unknown): Request {
+  return new Request(`${BASE_URL}/api/playground/presets`, {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify(body),
+  });
+}
+
+function putReq(id: string, body: unknown): Request {
+  return new Request(`${BASE_URL}/api/playground/presets/${id}`, {
+    method: "PUT",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify(body),
+  });
+}
+
+function getReq(id: string): Request {
+  return new Request(`${BASE_URL}/api/playground/presets/${id}`, { method: "GET" });
+}
+
+function deleteReq(id: string): Request {
+  return new Request(`${BASE_URL}/api/playground/presets/${id}`, { method: "DELETE" });
+}
+
+async function resolveParams(id: string): Promise<{ params: Promise<{ id: string }> }> {
+  return { params: Promise.resolve({ id }) };
+}
+
+// Sanitization assertion helper
+function assertNoStackTrace(body: { error?: { message?: string } }): void {
+  const msg = body.error?.message ?? "";
+  assert.ok(
+    !msg.match(/\sat\s\//),
+    `error message must not contain stack trace paths; got: ${msg}`
+  );
+}
+
+// ─── Cleanup ─────────────────────────────────────────────────────────────────
+
+test.beforeEach(() => {
+  core.resetDbInstance();
+});
+
+test.after(() => {
+  core.resetDbInstance();
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+});
+
+// ─── POST validation ─────────────────────────────────────────────────────────
+
+test("POST with empty name → 400 with Zod error", async () => {
+  const res = await createPost(
+    postReq({ name: "", endpoint: "chat.completions", model: "gpt-4o" })
+  );
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+test("POST with name exceeding 100 chars → 400", async () => {
+  const res = await createPost(
+    postReq({ name: "x".repeat(101), endpoint: "chat.completions", model: "gpt-4o" })
+  );
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+test("POST with missing endpoint → 400", async () => {
+  const res = await createPost(postReq({ name: "Test", model: "gpt-4o" }));
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+test("POST with empty endpoint → 400", async () => {
+  const res = await createPost(postReq({ name: "Test", endpoint: "", model: "gpt-4o" }));
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+test("POST with missing model → 400", async () => {
+  const res = await createPost(postReq({ name: "Test", endpoint: "chat.completions" }));
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+test("POST with empty model → 400", async () => {
+  const res = await createPost(
+    postReq({ name: "Test", endpoint: "chat.completions", model: "" })
+  );
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+test("POST with system > 50000 chars → 400", async () => {
+  const longSystem = "x".repeat(50001);
+  const res = await createPost(
+    postReq({ name: "Big System", endpoint: "chat.completions", model: "gpt-4o", system: longSystem })
+  );
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+test("POST with system exactly 50000 chars → 201 (boundary: valid)", async () => {
+  const maxSystem = "x".repeat(50000);
+  const res = await createPost(
+    postReq({ name: "Max System", endpoint: "chat.completions", model: "gpt-4o", system: maxSystem })
+  );
+  assert.equal(res.status, 201);
+});
+
+test("POST with invalid JSON body → 400", async () => {
+  const req = new Request(`${BASE_URL}/api/playground/presets`, {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: "GARBAGE",
+  });
+  const res = await createPost(req);
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+test("POST with missing name entirely → 400", async () => {
+  const res = await createPost(postReq({ endpoint: "chat.completions", model: "gpt-4o" }));
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+// ─── PUT validation ───────────────────────────────────────────────────────────
+
+test("PUT with invalid body (name is number) → 400", async () => {
+  const validId = "00000000-0000-4000-8000-000000000001";
+  const res = await idPut(putReq(validId, { name: 42 }), await resolveParams(validId));
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+test("PUT with empty name → 400", async () => {
+  const validId = "00000000-0000-4000-8000-000000000001";
+  const res = await idPut(putReq(validId, { name: "" }), await resolveParams(validId));
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+test("PUT with system > 50000 chars → 400", async () => {
+  const validId = "00000000-0000-4000-8000-000000000001";
+  const longSystem = "y".repeat(50001);
+  const res = await idPut(
+    putReq(validId, { system: longSystem }),
+    await resolveParams(validId)
+  );
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+test("PUT with invalid JSON body → 400", async () => {
+  const validId = "00000000-0000-4000-8000-000000000001";
+  const req = new Request(`${BASE_URL}/api/playground/presets/${validId}`, {
+    method: "PUT",
+    headers: { "Content-Type": "application/json" },
+    body: "GARBAGE",
+  });
+  const res = await idPut(req, await resolveParams(validId));
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+// ─── UUID format validation ───────────────────────────────────────────────────
+
+test("GET /presets/[id] with numeric string → 400", async () => {
+  const badId = "12345";
+  const res = await idGet(getReq(badId), await resolveParams(badId));
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+test("GET /presets/[id] with plain word id → 400", async () => {
+  const badId = "my-preset";
+  const res = await idGet(getReq(badId), await resolveParams(badId));
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+test("PUT /presets/[id] with numeric string id → 400", async () => {
+  const badId = "12345";
+  const res = await idPut(putReq(badId, { name: "x" }), await resolveParams(badId));
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+test("DELETE /presets/[id] with numeric string id → 400", async () => {
+  const badId = "99999";
+  const res = await idDelete(deleteReq(badId), await resolveParams(badId));
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+test("GET /presets/[id] with UUID v1 format → 404 (valid UUID format, not found in DB)", async () => {
+  // z.string().uuid() accepts any RFC-4122 UUID (v1, v4, etc).
+  // UUID v1 passes UUID validation → route proceeds to DB lookup → 404 (not found).
+  const uuidV1 = "6ba7b810-9dad-11d1-80b4-00c04fd430c8";
+  const res = await idGet(getReq(uuidV1), await resolveParams(uuidV1));
+  assert.equal(res.status, 404);
+  const body = (await res.json()) as { error: { message: string } };
+  assert.ok(body.error);
+  assertNoStackTrace(body);
+});
+
+// ─── Error response shape validation ─────────────────────────────────────────
+
+test("all 400 responses have consistent error shape (message string)", async () => {
+  const cases = [
+    createPost(postReq({ name: "" })),
+    createPost(postReq({ endpoint: "chat" })),
+    createPost(postReq({})),
+  ];
+
+  for (const pending of cases) {
+    const res = await pending;
+    assert.equal(res.status, 400);
+    const body = (await res.json()) as { error: { message: string; type?: string } };
+    assert.ok(body.error, "should have error object");
+    assert.equal(typeof body.error.message, "string", "error.message must be a string");
+    assert.ok(body.error.message.length > 0, "error.message must not be empty");
+    assertNoStackTrace(body);
+  }
+});