diff --git a/packages/@n8n/instance-ai/evaluations/README.md b/packages/@n8n/instance-ai/evaluations/README.md
index 2c741e853d7..6144b599e36 100644
--- a/packages/@n8n/instance-ai/evaluations/README.md
+++ b/packages/@n8n/instance-ai/evaluations/README.md
@@ -147,7 +147,7 @@ dotenvx run -f ../../../.env.local -- pnpm eval:instance-ai --iterations 3
 | `--tier` | — | Filter to test cases whose `datasets` array contains this value (e.g. `--tier pr` for the PR-time set). Combines with `--filter`/`--exclude`. |
 | `--source` | `disk` | Where test cases come from. `disk` (default) reads `data/workflows/`; `langtracer` pulls a suite from LangTracer's REST API — see [Sourcing from LangTracer](#sourcing-test-cases-from-langtracer) |
 | `--suite` | — | LangTracer suite slug (or numeric id) to pull when `--source langtracer` (required in that mode) |
-| `--build-via-mcp` | `false` | Build each workflow by driving the lane's MCP server with `claude -p`, then verify it on that same lane — see [Building via MCP (`--build-via-mcp`)](#building-via-mcp---build-via-mcp). Works across multiple `--base-url` lanes; mutually exclusive with `--prebuilt-workflows` |
+| `--build-via-mcp` | `false` | Build each workflow by driving the lane's MCP server with `claude -p`, then verify it on that same lane — see [Building via MCP (`--build-via-mcp`)](#building-via-mcp---build-via-mcp). Works across multiple `--base-url` lanes; requires `LANGSMITH_API_KEY`; mutually exclusive with `--prebuilt-workflows` |
 | `--mcp-server` | `n8n-local` | MCP server name for the staged `claude` config + tool allowlist (`--build-via-mcp`) |
 | `ANTHROPIC_MODEL` (env) | `claude-opus-4-8` | Anthropic model for the `claude` MCP build (`--build-via-mcp`); distinct from the verifier model. Not a flag: it rides `claude`'s native env var, and the CLI pins the default when unset so builds never float with claude-code's bundled default |
 | `--build-cwd` | — | Working directory for the `claude` build subprocess (`--build-via-mcp`); loads that project's Claude config/skills |
@@ -405,11 +405,14 @@ How it differs from the manifest flow:
   `claude` prints, so a build that times out or never emits the trailer can leave
   its workflow behind on the lane even though cleanup is on.
 
-**Prerequisites**: the `claude` CLI installed and authenticated (the build
-subprocess reads `ANTHROPIC_API_KEY`; set `ANTHROPIC_MODEL` to pick the build
-model, defaulting to `claude-opus-4-8` when unset); each lane reachable and
-seeded with the E2E owner. The MCP module is on by default, so no server-side
-config is needed beyond a running instance.
+**Prerequisites**: `LANGSMITH_API_KEY` set — MCP builds only run on the
+LangSmith path, whose lane allocator caps builds at 4 per lane globally (the
+keyless direct loop parallelizes iterations, which would multiply concurrent
+`claude` sessions by the iteration count). Plus the `claude` CLI installed and
+authenticated (the build subprocess reads `ANTHROPIC_API_KEY`; set
+`ANTHROPIC_MODEL` to pick the build model, defaulting to `claude-opus-4-8` when
+unset); each lane reachable and seeded with the E2E owner. The MCP module is on
+by default, so no server-side config is needed beyond a running instance.
 
 Local run against a pool of container lanes (reuses `scripts/run-eval-lanes.sh`,
 which starts + seeds the lanes and forwards everything after `--`):
diff --git a/packages/@n8n/instance-ai/evaluations/__tests__/args.test.ts b/packages/@n8n/instance-ai/evaluations/__tests__/args.test.ts
index 98af34b7c45..b0ff5444589 100644
--- a/packages/@n8n/instance-ai/evaluations/__tests__/args.test.ts
+++ b/packages/@n8n/instance-ai/evaluations/__tests__/args.test.ts
@@ -178,10 +178,21 @@ describe('parseCliArgs --source langtracer dataset isolation', () => {
 });
 
 describe('parseCliArgs --build-via-mcp', () => {
+	beforeEach(() => {
+		// --build-via-mcp is LangSmith-only; give every test in this suite a key
+		// so the requirement doesn't drown out what each test actually asserts.
+		vi.stubEnv('LANGSMITH_API_KEY', 'test-key');
+	});
+
 	afterEach(() => {
 		vi.unstubAllEnvs();
 	});
 
+	it('requires LANGSMITH_API_KEY (the direct loop does not support MCP builds)', () => {
+		vi.stubEnv('LANGSMITH_API_KEY', '');
+		expect(() => parseCliArgs(['--build-via-mcp'])).toThrow(/--build-via-mcp requires LangSmith/);
+	});
+
 	it('defaults to disabled with sensible build knobs', () => {
 		vi.stubEnv('ANTHROPIC_MODEL', '');
 		const args = parseCliArgs([]);
diff --git a/packages/@n8n/instance-ai/evaluations/cli/args.ts b/packages/@n8n/instance-ai/evaluations/cli/args.ts
index 90ccd5a3161..6771c8c7697 100644
--- a/packages/@n8n/instance-ai/evaluations/cli/args.ts
+++ b/packages/@n8n/instance-ai/evaluations/cli/args.ts
@@ -180,6 +180,16 @@ export function parseCliArgs(argv: string[]): CliArgs {
 			'--delete-prebuilt-workflows applies to --prebuilt-workflows. --build-via-mcp already cleans up the workflows it builds unless --keep-workflows is set.',
 		);
 	}
+	// MCP builds are LangSmith-only: the keyless direct loop parallelizes
+	// iterations without the lane allocator, so its 4-per-lane build cap applies
+	// PER ITERATION — concurrent `claude` sessions would scale with
+	// lanes × iterations × 4 and flood the shared Anthropic budget. Fail fast
+	// instead of teaching the direct loop (tech debt slated for removal) MCP builds.
+	if (validated.buildViaMcp && !process.env.LANGSMITH_API_KEY) {
+		throw new Error(
+			'--build-via-mcp requires LangSmith experiment tracking — set LANGSMITH_API_KEY. The no-LangSmith direct loop does not support MCP builds.',
+		);
+	}
 	// Build knobs without --build-via-mcp would parse fine and then be silently
 	// ignored — the run would look like it honored them. Fail loudly instead.
 	if (!validated.buildViaMcp && raw.buildOnlyFlags.length > 0) {
diff --git a/packages/@n8n/instance-ai/evaluations/cli/index.ts b/packages/@n8n/instance-ai/evaluations/cli/index.ts
index 7ca497fb017..fea5951b6a2 100644
--- a/packages/@n8n/instance-ai/evaluations/cli/index.ts
+++ b/packages/@n8n/instance-ai/evaluations/cli/index.ts
@@ -1250,16 +1250,8 @@ async function runDirectLoop(config: RunConfig): Promise<{
 	evaluation: MultiRunEvaluation;
 	slugByTestCase: Map<WorkflowTestCase, string>;
 }> {
-	const {
-		args,
-		lanes,
-		logger,
-		testCasesWithFiles,
-		prebuiltManifest,
-		cleanupBuiltWorkflows,
-		mcpBuildLogDir,
-		mcpBuildSpend,
-	} = config;
+	const { args, lanes, logger, testCasesWithFiles, prebuiltManifest, cleanupBuiltWorkflows } =
+		config;
 
 	const slugByTestCase = new Map<WorkflowTestCase, string>(
 		testCasesWithFiles.map(({ testCase, fileSlug }) => [testCase, fileSlug]),
@@ -1283,7 +1275,11 @@ async function runDirectLoop(config: RunConfig): Promise<{
 	const indexed = testCasesWithFiles.map((tc, origIdx) => ({ tc, origIdx }));
 	const buckets = partitionRoundRobin(indexed, lanes.length);
 
-	// Iterations are independent — run them in parallel.
+	// Iterations are independent — run them in parallel. NOTE: this makes the
+	// 4-per-lane build cap apply per iteration (lanes × iterations × 4 builds at
+	// peak), which is why --build-via-mcp never reaches this loop: parseCliArgs
+	// requires LangSmith for that mode, whose allocator caps builds per lane
+	// globally.
 	const allRunResults: WorkflowTestCaseResult[][] = await Promise.all(
 		Array.from({ length: args.iterations }, async (_unused, iter) => {
 			if (args.iterations > 1) {
@@ -1302,23 +1298,6 @@ async function runDirectLoop(config: RunConfig): Promise<{
 								tc.fileSlug,
 								iter,
 							);
-							// --build-via-mcp: build this case on THIS lane via `claude`,
-							// then verify it here. The static lane partitioning already
-							// keeps build + verify on the same lane (required — the
-							// workflow only exists on the lane that built it).
-							const buildOverride = args.buildViaMcp
-								? async () =>
-										await buildWorkflowViaMcpOnLane({
-											lane,
-											conversation: tc.testCase.conversation,
-											slug: tc.fileSlug,
-											iteration: iter,
-											args,
-											logDir: mcpBuildLogDir ?? process.cwd(),
-											logger,
-											buildSpend: mcpBuildSpend,
-										})
-								: undefined;
 							const result = await runWorkflowTestCase({
 								client: lane.client,
 								baseUrl: lane.baseUrl,
@@ -1331,11 +1310,8 @@ async function runDirectLoop(config: RunConfig): Promise<{
 								keepWorkflows: args.keepWorkflows,
 								laneTag,
 								prebuiltWorkflowId,
-								buildOverride,
 								pinAiRoots: args.pinAiRoots,
 							});
-							// Prebuilt only: MCP builds are registered for cleanup inside
-							// buildWorkflowViaMcpOnLane as soon as `claude` reports the id.
 							if (
 								prebuiltWorkflowId !== undefined &&
 								cleanupBuiltWorkflows &&
diff --git a/packages/@n8n/instance-ai/evaluations/harness/runner.ts b/packages/@n8n/instance-ai/evaluations/harness/runner.ts
index eb94a25f1a1..3465bd6719a 100644
--- a/packages/@n8n/instance-ai/evaluations/harness/runner.ts
+++ b/packages/@n8n/instance-ai/evaluations/harness/runner.ts
@@ -176,12 +176,6 @@ interface WorkflowTestCaseConfig {
 	/** When set, skip the orchestrator build and verify this existing workflow
 	 *  instead. The harness leaves it in place — caller owns its lifecycle. */
 	prebuiltWorkflowId?: string;
-	/** When set, obtain the BuildResult from this callback instead of building via
-	 *  the orchestrator or fetching `prebuiltWorkflowId`. Used by `--build-via-mcp`
-	 *  to build the workflow on this lane via `claude`, then verify it here. Treated
-	 *  like prebuilt (no transcript) for downstream judging. Never combined with
-	 *  `prebuiltWorkflowId`. */
-	buildOverride?: () => Promise<BuildResult>;
 	/** AI root nodes (Agent, Chain) to keep pinned — opt-out from the default-on
 	 *  wire-server interception path. Omit (or pass empty) to intercept every
 	 *  interceptable AI root the workflow contains. Server-side gated by the
@@ -207,31 +201,27 @@ export async function runWorkflowTestCase(
 		n8nBaseUrl: config.baseUrl,
 	};
 
-	// A build override (--build-via-mcp) and prebuilt lookup both yield a
-	// workflow with no build transcript, so they share the prebuilt judging path.
-	const isPrebuilt = config.prebuiltWorkflowId !== undefined || config.buildOverride !== undefined;
-	const build = config.buildOverride
-		? await config.buildOverride()
-		: config.prebuiltWorkflowId
-			? await fetchPrebuiltBuild(client, config.prebuiltWorkflowId, logger)
-			: await buildWorkflow({
-					client,
-					conversation: testCase.conversation,
-					messageBudget: testCase.messageBudget,
-					credentials: testCase.credentials,
-					seedFile: testCase.seedFile,
-					priorConversation: testCase.priorConversation,
-					seedThread: testCase.seedThread,
-					createdCredentialIds: config.createdCredentialIds,
-					timeoutMs,
-					preRunWorkflowIds: config.preRunWorkflowIds,
-					claimedWorkflowIds: config.claimedWorkflowIds,
-					logger,
-					laneTag: config.laneTag,
-				});
+	const isPrebuilt = config.prebuiltWorkflowId !== undefined;
+	const build = config.prebuiltWorkflowId
+		? await fetchPrebuiltBuild(client, config.prebuiltWorkflowId, logger)
+		: await buildWorkflow({
+				client,
+				conversation: testCase.conversation,
+				messageBudget: testCase.messageBudget,
+				credentials: testCase.credentials,
+				seedFile: testCase.seedFile,
+				priorConversation: testCase.priorConversation,
+				seedThread: testCase.seedThread,
+				createdCredentialIds: config.createdCredentialIds,
+				timeoutMs,
+				preRunWorkflowIds: config.preRunWorkflowIds,
+				claimedWorkflowIds: config.claimedWorkflowIds,
+				logger,
+				laneTag: config.laneTag,
+			});
 
 	if (isPrebuilt && build.success && !build.workflowChecks) {
-		// No transcript in prebuilt/MCP mode, but the authored conversation still
+		// No transcript in prebuilt mode, but the authored conversation still
 		// carries the user's request — feed it so prompt-aware checks (e.g.
 		// fulfills_user_request) grade against real intent instead of "".
 		build.workflowChecks = await runWorkflowChecks({
diff --git a/packages/cli/src/modules/mcp/evaluations/README.md b/packages/cli/src/modules/mcp/evaluations/README.md
index ef53463322d..8813025b9ae 100644
--- a/packages/cli/src/modules/mcp/evaluations/README.md
+++ b/packages/cli/src/modules/mcp/evaluations/README.md
@@ -11,8 +11,8 @@ troubleshooting, use the [Instance AI workflow evaluation README](../../../../..
 
 - **Fused — recommended.** `eval:instance-ai --build-via-mcp` builds each
   workflow through the MCP server and verifies it in the **same run**, across one
-  or more lanes. One command, one LangSmith experiment — and it's what CI
-  (`ci-mcp-evals.yml`) runs. See
+  or more lanes. One command, one LangSmith experiment (`LANGSMITH_API_KEY`
+  required) — and it's what CI (`ci-mcp-evals.yml`) runs. See
   [Building via MCP](../../../../../@n8n/instance-ai/evaluations/README.md#building-via-mcp---build-via-mcp).
 - **Two-phase — decoupled (documented below).** `eval:build-mcp-manifest` writes
   a manifest of workflow IDs, then `eval:instance-ai --prebuilt-workflows` scores