diff --git a/packages/sdk/src/cli/build.ts b/packages/sdk/src/cli/build.ts
index e0defc4f..7bff80a7 100644
--- a/packages/sdk/src/cli/build.ts
+++ b/packages/sdk/src/cli/build.ts
@@ -6,28 +6,44 @@ import { canonicalize } from '../canonical.js';
 import { compileSpec, toKernelSpec } from '../compile.js';
 import { preflight } from '../preflight.js';
 import { buildTypescript } from '../bundle-typescript.js';
-import { readProjectConfig } from './check.js';
+import { checkBuildableFlow, readProjectConfig, type CheckReport } from './check.js';
 import type { CliIo } from '../cli.js';
 import type { FlowSpec } from '../spec.js';
 
-export interface BuildArgs { command: 'build'; value: string; out?: string; verify: boolean }
+export interface BuildArgs { command: 'build'; value: string; out?: string; verify: boolean; json: boolean }
 
 export function parseBuildArgs(args: readonly string[]): BuildArgs | undefined {
   if (args[0] === '--verify') {
     return args.length === 2 && !args[1]!.startsWith('-')
-      ? { command: 'build', value: args[1]!, verify: true } : undefined;
+      ? { command: 'build', value: args[1]!, verify: true, json: false } : undefined;
   }
   let out: string | undefined;
   let value: string | undefined;
+  let json = false;
   for (let i = 0; i < args.length; i++) {
     const arg = args[i]!;
     if (arg === '--out') {
       if (out !== undefined || args[i + 1] === undefined || args[i + 1]!.startsWith('-')) return undefined;
       out = args[++i];
+    } else if (arg === '--json') {
+      if (json) return undefined;
+      json = true;
     } else if (arg.startsWith('-') || value !== undefined) return undefined;
     else value = arg;
   }
-  return value === undefined ? undefined : { command: 'build', value, out, verify: false };
+  return value === undefined ? undefined : { command: 'build', value, out, verify: false, json };
+}
+
+/**
+ * Emit a check report in the same shape `flows check` uses so build-time
+ * refusals are consumable by the same tooling.
+ */
+function emitBuildCheckReport(report: CheckReport, json: boolean, io: CliIo): void {
+  for (const diagnostic of report.diagnostics) {
+    if (diagnostic.severity !== 'refusal') continue;
+    io.stderr(`REFUSED [${diagnostic.kind}] ${diagnostic.message}`);
+  }
+  if (json) io.stdout(JSON.stringify(report));
 }
 
 export async function runBuild(args: BuildArgs, io: CliIo): Promise<0 | 2> {
@@ -36,6 +52,16 @@ export async function runBuild(args: BuildArgs, io: CliIo): Promise<0 | 2> {
       io.stdout(`VERIFIED sha256:${await verifyBundle(args.value)}`);
       return 0;
     }
+    // Gate the build on the same preflight pipeline `flows check` uses.
+    // Refusals never leave partial artifacts — no file capture, no canonical
+    // spec write, no digest computation, no bundle directory creation. A
+    // previous `dist/flows/<name>@sha256:<hex>/` directory from an earlier
+    // successful build is not touched (buildFlow is never called).
+    const gate = await checkBuildableFlow(args.value);
+    if (!gate.report.ok) {
+      emitBuildCheckReport(gate.report, args.json, io);
+      return 2;
+    }
     io.stdout(await buildFlow(args.value, args.out ?? 'dist/flows', io.stderr));
     return 0;
   } catch (error) {
diff --git a/packages/sdk/src/cli/check.ts b/packages/sdk/src/cli/check.ts
index 0228710e..b96e435c 100644
--- a/packages/sdk/src/cli/check.ts
+++ b/packages/sdk/src/cli/check.ts
@@ -143,6 +143,72 @@ export function checkAuthoredFlow(authoring: FlowSpec, path: string, projectConf
   }
 }
 
+/**
+ * Build-time gate that runs the same preflight pipeline as `checkFlow`
+ * but with deferred probes — a build machine is not the deployment target,
+ * so `cli`/`command`/`executor` existence is checked at run-time, not here.
+ *
+ * "Build-provable" refusals (unknown model, `use:` unresolved, invalid
+ * verification, missing bundle assets, budget syntax, etc.) still surface,
+ * because they are properties of the flow spec, not of the build host.
+ */
+export async function checkBuildableFlow(path: string): Promise<CheckExecution> {
+  const absolutePath = resolve(path);
+  try {
+    const authored = /\.(?:[cm]?[jt]s)$/.test(path);
+    if (authored) {
+      // TS flows are gated by `buildFlow` itself, which runs
+      // `buildTypescript` to compile the authored spec and then invokes
+      // preflight with deferred probes at the same refusal threshold as
+      // this gate. Running `checkTypeScriptFlow` here would need
+      // `@relayflows/surface` to be resolvable from the flow's directory,
+      // which is not a build-time invariant. Return an ok report so the
+      // gate delegates to `buildFlow`'s inline preflight.
+      return {
+        report: { ok: true, path, gates: [], resolutions: [], diagnostics: [] },
+      };
+    }
+    const source = readFlowSource(absolutePath);
+    const authoring: FlowSpec = readFlow(source, absolutePath);
+    const config = readProjectConfig(dirname(absolutePath));
+    const deferred: PreflightProbes = {
+      cli: () => { throw new Error('deferred to deployment'); },
+      executor: () => { throw new Error('deferred to deployment'); },
+      command: () => { throw new Error('deferred to deployment'); },
+    };
+    const result = preflight(authoring, {
+      ...(config.cli !== undefined ? { projectCli: config.cli } : {}),
+      ...(config.path !== undefined ? { projectConfigPath: config.path } : {}),
+      projectSearchStart: dirname(absolutePath),
+      models: config.models,
+      ...(config.path !== undefined ? { modelRegistryPath: config.path } : {}),
+      probes: deferred,
+    });
+    // Refusals rooted in build-machine environment probes (`probe_failed`)
+    // are excluded from the build gate — the build host is not the
+    // deployment target, and those checks are re-run at `flows run`.
+    const diagnostics = result.diagnostics.filter(
+      (d) => !(d.severity === 'refusal' && d.kind === 'probe_failed'),
+    );
+    const ok = !diagnostics.some((d) => d.severity === 'refusal');
+    return {
+      report: {
+        ok,
+        path,
+        ...(config.path !== undefined ? { projectConfigPath: config.path } : {}),
+        gates: result.gates,
+        resolutions: result.resolutions,
+        diagnostics,
+      },
+    };
+  } catch (error) {
+    const failure = error instanceof CheckFailure
+      ? error
+      : new CheckFailure('invalid_spec', `Flow "${path}" could not be checked as a Relayflow spec.`);
+    return { report: inputFailureReport(failure, path) };
+  }
+}
+
 export function inputFailureReport(
   failure: { kind: CheckFailureKind; message: string },
   path?: string,
diff --git a/packages/sdk/tests/build-gate.test.ts b/packages/sdk/tests/build-gate.test.ts
new file mode 100644
index 00000000..2fa346eb
--- /dev/null
+++ b/packages/sdk/tests/build-gate.test.ts
@@ -0,0 +1,100 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import { mkdtemp, readdir, readFile, rm, writeFile } from 'node:fs/promises';
+import { existsSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { basename, dirname, join, resolve } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { spawnSync } from 'node:child_process';
+
+const sdk = resolve(dirname(fileURLToPath(import.meta.url)), '..');
+const repo = resolve(sdk, '../..');
+const cli = join(sdk, 'dist/cli.js');
+const key = Buffer.alloc(32, 7).toString('base64');
+const temporary: string[] = [];
+
+async function temp(): Promise<string> {
+  const path = await mkdtemp(join(tmpdir(), 'flows-build-gate-test-'));
+  temporary.push(path);
+  return path;
+}
+
+function invoke(args: string[], cwd = repo, env: NodeJS.ProcessEnv = {}) {
+  return spawnSync(process.execPath, [cli, 'build', ...args], {
+    cwd, encoding: 'utf8', timeout: 120_000,
+    env: { PATH: process.env['PATH'], FLOWS_BUILD_KEY: key, ...env },
+  });
+}
+
+afterEach(async () => {
+  await Promise.all(temporary.splice(0).map(path => rm(path, { force: true, recursive: true })));
+});
+
+describe('flows build gates on flows check green (#318)', () => {
+  it('refuses a flow with an unresolvable named-agent CLI and leaves no artifacts', async () => {
+    const cwd = await temp();
+    await writeFile(join(cwd, 'bad.yaml'), JSON.stringify({
+      version: '0.1.0', name: 'unbuildable',
+      steps: [{ id: 'ask', type: 'agent', instruction: 'hello' }],
+    }));
+    const out = join(cwd, 'dist/flows');
+    const result = invoke(['--out', out, 'bad.yaml'], cwd);
+    expect(result.status).toBe(2);
+    expect(result.stderr).toContain('cli_unresolved');
+    // No `dist/flows/<name>@sha256:<hex>/` — the refusal path never leaves
+    // partial artifacts.
+    expect(existsSync(out)).toBe(false);
+  });
+
+  it('--json emits one CheckReport object on stdout on refusal, exits 2, no artifacts', async () => {
+    const cwd = await temp();
+    await writeFile(join(cwd, 'bad.yaml'), JSON.stringify({
+      version: '0.1.0', name: 'unbuildable-json',
+      steps: [{ id: 'ask', type: 'agent', instruction: 'hello' }],
+    }));
+    const out = join(cwd, 'dist/flows');
+    const result = invoke(['--json', '--out', out, 'bad.yaml'], cwd);
+    expect(result.status).toBe(2);
+    // Exactly one JSON object on stdout — same shape `flows check --json` emits.
+    const trimmed = result.stdout.trim();
+    expect(() => JSON.parse(trimmed)).not.toThrow();
+    const report = JSON.parse(trimmed) as {
+      ok: boolean;
+      diagnostics: Array<{ severity: string; kind: string }>;
+      gates: unknown[];
+      resolutions: unknown[];
+    };
+    expect(report.ok).toBe(false);
+    expect(Array.isArray(report.diagnostics)).toBe(true);
+    expect(report.diagnostics.some(d => d.severity === 'refusal' && d.kind === 'cli_unresolved')).toBe(true);
+    expect(Array.isArray(report.gates)).toBe(true);
+    expect(Array.isArray(report.resolutions)).toBe(true);
+    expect(existsSync(out)).toBe(false);
+  });
+
+  it('builds the bundle on success (regression: gate must not block valid flows)', async () => {
+    const cwd = await temp();
+    // Deterministic-only YAML flow — no agent, no CLI required. Uses the
+    // same shape as the fixture at testdata/hello-deterministic.flow.yaml
+    // but keeps this test self-contained.
+    await writeFile(join(cwd, 'script.sh'), '#!/bin/sh\necho hello\n');
+    await writeFile(join(cwd, 'good.yaml'), JSON.stringify({
+      version: '0.1.0', name: 'buildable',
+      steps: [{ id: 'run', type: 'deterministic', command: './script.sh' }],
+    }));
+    const out = join(cwd, 'out');
+    const result = invoke(['--out', out, 'good.yaml'], cwd);
+    expect(result.status).toBe(0);
+    const bundle = result.stdout.trim();
+    expect(bundle).toContain('buildable@sha256:');
+    const files = await readdir(bundle);
+    // Full bundle emitted: canonical spec, preflight, manifest, identity.
+    expect(files).toContain('spec.canonical.json');
+    expect(files).toContain('preflight.json');
+    expect(files).toContain('manifest.json');
+    expect(files).toContain('identity.json');
+    // Bundle name shape matches the digest-addressing convention.
+    expect(basename(bundle).startsWith('buildable@sha256:')).toBe(true);
+    // Sanity: assets captured from build-time file references.
+    expect(await readFile(join(bundle, 'assets/script.sh'), 'utf8')).toContain('echo hello');
+  });
+});
diff --git a/packages/sdk/tsconfig.tests.json b/packages/sdk/tsconfig.tests.json
index 5ef64519..7e8adf11 100644
--- a/packages/sdk/tsconfig.tests.json
+++ b/packages/sdk/tsconfig.tests.json
@@ -31,7 +31,8 @@
     "tests/close-pr-flow.test.ts",
     "tests/direct-input.test.ts",
     "tests/fixtures/needs-human.flow.ts",
-    "tests/named-gates.test.ts"
+    "tests/named-gates.test.ts",
+    "tests/build-gate.test.ts"
   ],
   "exclude": ["node_modules", "dist"]
 }