diff --git a/.changeset/validate-generated-workflow-yaml.md b/.changeset/validate-generated-workflow-yaml.md
new file mode 100644
index 000000000..29a47fc79
--- /dev/null
+++ b/.changeset/validate-generated-workflow-yaml.md
@@ -0,0 +1,5 @@
+---
+"radius": patch
+---
+
+Validate generated verify, deploy, and delete workflows as YAML before they can be committed, surfacing malformed upstream templates or quote-bearing environment and application-file values as local generation errors instead of GitHub Actions HTTP 422 failures.
diff --git a/.github/workflows/live-tests.yml b/.github/workflows/live-tests.yml
new file mode 100644
index 000000000..6298822ed
--- /dev/null
+++ b/.github/workflows/live-tests.yml
@@ -0,0 +1,67 @@
+# yaml-language-server: $schema=https://www.schemastore.org/github-workflow.json
+---
+name: Live upstream tests
+
+# Runs the opt-in `*.live.test.ts` suites that fetch the real workflow templates
+# from radius-project/radius at runtime and assert YAML validity plus the OIDC
+# environment-binding contract. A failure means an upstream template change
+# broke what the extension generates (e.g. the invalid-YAML deploy regression
+# in #407).
+#
+# This is a SEPARATE workflow from the hermetic Build suite (build.yml), which
+# stays fully offline and deterministic. These live suites are intentionally
+# excluded from that suite and from the publish/release pipeline because they are
+# non-hermetic: they depend on an external repo's moving `main` ref and on
+# network access to raw.githubusercontent.com.
+#
+# They run on every pull request and every push to `main` so an upstream
+# template regression is flagged immediately as a failed check, plus nightly as a
+# catch-all in case no PR lands between an upstream change and the next run.
+#
+# Tradeoff to be aware of: because they fetch from radius-project/radius@main, a
+# regression pushed upstream can turn an otherwise-good ai-extensions PR red until
+# upstream is fixed. That is intentional here — surfacing the break early is the
+# point — but a failure in this job should be triaged against the current
+# upstream templates, not assumed to be caused by the PR's own changes.
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+  schedule:
+    # 07:00 UTC daily (~midnight US Pacific), as a catch-all between pushes.
+    - cron: "0 7 * * *"
+  workflow_dispatch:
+
+permissions: {}
+
+jobs:
+  live:
+    name: Live upstream template tests
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    env:
+      # Enables the `describe.skipIf(!LIVE)` blocks in the workflow live tests
+      # (workflow-yaml.live.test.ts and oidc-environment-contract.live.test.ts).
+      RUN_LIVE_WORKFLOW_TESTS: "1"
+    steps:
+      - name: Checkout
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+
+      - name: Install pnpm
+        uses: pnpm/action-setup@0977fd99725f1db4007ccb2928dbb4e90d06cc86 # v6.0.10
+
+      - name: Setup Node.js
+        uses: actions/setup-node@820762786026740c76f36085b0efc47a31fe5020 # v7.0.0
+        with:
+          node-version-file: .node-version
+          cache: pnpm
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Run live upstream workflow tests
+        run: pnpm test -- src/workflows/workflow-yaml.live.test.ts src/workflows/oidc-environment-contract.live.test.ts
diff --git a/packages/adapter-canvas/build.mjs b/packages/adapter-canvas/build.mjs
index fee2b1b48..78b6f44ef 100644
--- a/packages/adapter-canvas/build.mjs
+++ b/packages/adapter-canvas/build.mjs
@@ -31,6 +31,10 @@ import { homedir } from "node:os";
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
 const repoRoot = resolve(__dirname, "../..");
+const yamlBrowserEntry = resolve(
+  dirname(fileURLToPath(import.meta.resolve("yaml"))),
+  "../browser/index.js"
+);
 
 // .node-version is the one place the supported Node major is declared; deriving
 // the compile target from it keeps them from drifting apart.
@@ -351,6 +355,9 @@ const options = {
   format: "esm",
   platform: "node",
   target,
+  // yaml's Node entry is CommonJS and leaves a dynamic require("process") in
+  // the ESM bundle. Its browser entry is equivalent pure ESM parser code.
+  alias: { yaml: yamlBrowserEntry },
   // The runtime is known to be Node, so emit UTF-8 rather than \uXXXX escapes.
   charset: "utf8",
   minify: true,
diff --git a/packages/adapter-canvas/package.json b/packages/adapter-canvas/package.json
index 16c65baa8..0f56227c9 100644
--- a/packages/adapter-canvas/package.json
+++ b/packages/adapter-canvas/package.json
@@ -26,7 +26,8 @@
   },
   "dependencies": {
     "@radius-project/adapter-shared": "workspace:*",
-    "@radius-project/core": "workspace:*"
+    "@radius-project/core": "workspace:*",
+    "yaml": "catalog:"
   },
   "devDependencies": {
     "@github/copilot-sdk": "catalog:",
diff --git a/packages/adapter-canvas/src/infra.test.ts b/packages/adapter-canvas/src/infra.test.ts
index 137df6264..0ef0046c5 100644
--- a/packages/adapter-canvas/src/infra.test.ts
+++ b/packages/adapter-canvas/src/infra.test.ts
@@ -118,6 +118,42 @@ const STALE_AZURE_VERIFY =
 const STALE_AWS_VERIFY =
   "name: verify\njobs:\n  v:\n    run: echo STALE ${{ vars.AWS_ROLE_ARN }}\n";
 
+describe("generated workflow YAML validation", () => {
+  beforeEach(() => {
+    h.upstream = { ...BASE_UPSTREAM };
+  });
+
+  it.each([
+    {
+      name: "deploy environment",
+      generate: () => generateDeployWorkflow("it's-prod", ".radius/app.bicep"),
+      context: 'deploy workflow "run-rad-commands.yml"'
+    },
+    {
+      name: "deploy app file",
+      generate: () => generateDeployWorkflow("prod", ".radius/app's.bicep"),
+      context: 'deploy workflow "run-rad-commands-azure.yml"'
+    },
+    {
+      name: "delete environment",
+      generate: () => generateDeleteWorkflow("it's-prod"),
+      context: 'delete workflow "delete-application.yml"'
+    },
+    {
+      name: "verify environment",
+      generate: () => generateVerifyWorkflow("it's-prod", "azure"),
+      context: 'verify workflow "verify-azure.yml"'
+    }
+  ])(
+    "rejects an invalid $name scalar before commit",
+    async ({ generate, context }) => {
+      await expect(generate()).rejects.toThrow(
+        `Generated ${context} is invalid YAML`
+      );
+    }
+  );
+});
+
 describe("GHCR verification probe", () => {
   it("checks push permission with a non-mutating upload session", () => {
     const workflow = configureVerifyGhcrProbe(
diff --git a/packages/adapter-canvas/src/infra.ts b/packages/adapter-canvas/src/infra.ts
index f9191cda8..b525d7152 100644
--- a/packages/adapter-canvas/src/infra.ts
+++ b/packages/adapter-canvas/src/infra.ts
@@ -22,6 +22,7 @@ import {
   DELETE_AWS_FILE
 } from "@radius-project/core";
 import type { DeployWorkflowOptions } from "@radius-project/core";
+import { parse as parseYaml } from "yaml";
 import {
   fetchFileFromRepoResult,
   fetchFileFromRepo,
@@ -84,6 +85,17 @@ function errorMessage(error: unknown): string {
   return error instanceof Error ? error.message : String(error);
 }
 
+function assertValidWorkflowYaml(workflow: string, context: string): void {
+  try {
+    parseYaml(workflow);
+  } catch (error) {
+    throw new Error(
+      `Generated ${context} is invalid YAML: ${errorMessage(error)}`,
+      { cause: error }
+    );
+  }
+}
+
 export { DEPLOY_DISPATCHER_FILE, DEPLOY_AZURE_FILE, DEPLOY_AWS_FILE };
 export { DELETE_APP_DISPATCHER_FILE, DELETE_AZURE_FILE, DELETE_AWS_FILE };
 
@@ -139,9 +151,11 @@ export async function generateVerifyWorkflow(
   if (!fileName)
     throw new Error(`No verify template for provider "${provider}".`);
   const upstream = await fetchRadiusTemplate(fileName);
-  return configureVerifyGhcrProbe(
+  const workflow = configureVerifyGhcrProbe(
     coreGenerateVerifyWorkflow(env, platform, upstream)
   );
+  assertValidWorkflowYaml(workflow, `verify workflow "${fileName}"`);
+  return workflow;
 }
 
 /**
@@ -238,6 +252,9 @@ export async function generateDeployWorkflow(
       generated[DEPLOY_DISPATCHER_FILE]
     );
   }
+  for (const [file, workflow] of Object.entries(generated)) {
+    assertValidWorkflowYaml(workflow, `deploy workflow "${file}"`);
+  }
   return generated;
 }
 
@@ -271,6 +288,9 @@ export async function generateDeleteWorkflow(
       generated[DELETE_APP_DISPATCHER_FILE]
     );
   }
+  for (const [file, workflow] of Object.entries(generated)) {
+    assertValidWorkflowYaml(workflow, `delete workflow "${file}"`);
+  }
   return generated;
 }
 
diff --git a/packages/core/package.json b/packages/core/package.json
index 0d09b6d2e..afc35a6da 100644
--- a/packages/core/package.json
+++ b/packages/core/package.json
@@ -22,6 +22,7 @@
     "lint": "eslint \"src/**/*.ts\" vitest.config.ts"
   },
   "devDependencies": {
-    "vitest": "catalog:"
+    "vitest": "catalog:",
+    "yaml": "catalog:"
   }
 }
diff --git a/packages/core/src/workflows/deploy.test.ts b/packages/core/src/workflows/deploy.test.ts
index 2a4c4c6e6..d2896fa88 100644
--- a/packages/core/src/workflows/deploy.test.ts
+++ b/packages/core/src/workflows/deploy.test.ts
@@ -1,4 +1,5 @@
 import { describe, expect, it } from "vitest";
+import { parse as parseYaml } from "yaml";
 import {
   DEFAULT_TARGET_CLUSTER_ARCH_FALLBACK_PLATFORMS,
   DEFAULT_TARGET_CLUSTER_ARCH_MODE,
@@ -182,3 +183,120 @@ describe("generateDeployWorkflow", () => {
     );
   });
 });
+
+// Fixtures that FAITHFULLY MIRROR the quoting of the real radius-project/radius
+// deploy templates (`.github/extension/run-rad-commands*.yml`), unlike the
+// inline fixtures above which use double quotes throughout. Reproducing the real
+// mix is what lets these tests catch the issue #407 class of bug:
+//   - plain values are injected into SINGLE-quoted scalars (`APP_FILE`, `ENV`)
+//     and into unquoted GHA expressions (`APP_IMAGE`, dispatcher `environment`);
+//   - the arch placeholders are injected into DOUBLE-quoted scalars, because the
+//     value core injects is itself a GHA expression whose default is
+//     single-quoted (`${{ vars.X || 'detect' }}`). A single-quoted scalar there
+//     nests the quotes and produces invalid YAML (the #407 regression).
+const REALISTIC_TEMPLATES = {
+  [DEPLOY_DISPATCHER_FILE]: `name: deploy
+on:
+  workflow_dispatch:
+    inputs:
+      environment:
+        default: '{{ENV}}'
+jobs:
+  detect:
+    runs-on: ubuntu-latest
+    environment: \${{ inputs.environment || '{{ENV}}' }}
+    env:
+      TARGET_CLUSTER_ARCH_MODE: "{{TARGET_CLUSTER_ARCH_MODE}}"
+      TARGET_CLUSTER_ARCH_FALLBACK_PLATFORMS: "{{TARGET_CLUSTER_ARCH_FALLBACK_PLATFORMS}}"
+    steps:
+      - run: echo \${{ github.sha }}
+`,
+  [DEPLOY_AZURE_FILE]: `name: deploy-azure
+env:
+  APP_FILE: '{{APP_FILE}}'
+  APP_IMAGE: \${{ inputs.image || github.sha || 'latest' }}
+  TARGET_CLUSTER_ARCH_MODE: "{{TARGET_CLUSTER_ARCH_MODE}}"
+  TARGET_CLUSTER_ARCH_FALLBACK_PLATFORMS: "{{TARGET_CLUSTER_ARCH_FALLBACK_PLATFORMS}}"
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: radius-project/radius/.github/extension/actions/run-rad-commands@{{RADIUS_REF}}
+`,
+  [DEPLOY_AWS_FILE]: `name: deploy-aws
+env:
+  APP_FILE: '{{APP_FILE}}'
+  APP_IMAGE: \${{ inputs.image || github.sha || 'latest' }}
+  TARGET_CLUSTER_ARCH_MODE: "{{TARGET_CLUSTER_ARCH_MODE}}"
+  TARGET_CLUSTER_ARCH_FALLBACK_PLATFORMS: "{{TARGET_CLUSTER_ARCH_FALLBACK_PLATFORMS}}"
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: radius-project/radius/.github/extension/actions/run-rad-commands@{{RADIUS_REF}}
+`
+};
+
+describe("generateDeployWorkflow YAML validity", () => {
+  // Positive guard: with the real templates' quoting and the default arch vars
+  // (single-quoted GHA expressions), every rendered file must parse as valid
+  // YAML. Prior tests only did `.toContain(...)` substring checks and never
+  // parsed the output, so they stayed green against invalid YAML.
+  it("renders valid YAML for every file when arch scalars are double-quoted", () => {
+    const files = generateDeployWorkflow(
+      "prod",
+      ".radius/app.bicep",
+      REALISTIC_TEMPLATES
+    );
+
+    for (const [name, body] of Object.entries(files)) {
+      expect(
+        () => parseYaml(body),
+        `${name} should parse as valid YAML`
+      ).not.toThrow();
+    }
+  });
+
+  // The injected GHA expression (with its single-quoted default) must survive
+  // substitution intact inside the double-quoted scalar.
+  it("preserves the injected GHA arch expressions as YAML string values", () => {
+    const files = generateDeployWorkflow(
+      "prod",
+      ".radius/app.bicep",
+      REALISTIC_TEMPLATES
+    );
+    const azure = parseYaml(files[DEPLOY_AZURE_FILE]) as {
+      env: Record<string, string>;
+    };
+
+    expect(azure.env.TARGET_CLUSTER_ARCH_MODE).toBe(
+      `\${{ vars.${RADIUS_BUILD_ARCH_MODE_VAR} || '${DEFAULT_TARGET_CLUSTER_ARCH_MODE}' }}`
+    );
+    expect(azure.env.TARGET_CLUSTER_ARCH_FALLBACK_PLATFORMS).toBe(
+      `\${{ vars.${RADIUS_BUILD_PLATFORMS_VAR} || '${DEFAULT_TARGET_CLUSTER_ARCH_FALLBACK_PLATFORMS}' }}`
+    );
+    // The plain single-quoted scalars stay intact too.
+    expect(azure.env.APP_FILE).toBe(".radius/app.bicep");
+  });
+
+  // Negative regression guard: this is exactly the #407 bug. When the arch
+  // scalar is SINGLE-quoted (as the radius templates were before #12721), the
+  // single-quoted default inside the injected GHA expression nests and YAML
+  // terminates the scalar early, so the rendered file is invalid YAML. This test
+  // locks in WHY the scalar must be double-quoted: if someone reintroduces the
+  // single-quoted scalar (or an injected value that nests quotes), it fails.
+  it("produces invalid YAML when an arch scalar is single-quoted (issue #407)", () => {
+    const broken = {
+      ...REALISTIC_TEMPLATES,
+      [DEPLOY_AZURE_FILE]: REALISTIC_TEMPLATES[DEPLOY_AZURE_FILE].replace(
+        'TARGET_CLUSTER_ARCH_MODE: "{{TARGET_CLUSTER_ARCH_MODE}}"',
+        "TARGET_CLUSTER_ARCH_MODE: '{{TARGET_CLUSTER_ARCH_MODE}}'"
+      )
+    };
+    // Substitution still succeeds (no unresolved placeholder), so generation
+    // does not throw — the breakage only surfaces when the YAML is parsed.
+    const files = generateDeployWorkflow("prod", ".radius/app.bicep", broken);
+
+    expect(() => parseYaml(files[DEPLOY_AZURE_FILE])).toThrow();
+  });
+});
diff --git a/packages/core/src/workflows/oidc-environment-contract.live.test.ts b/packages/core/src/workflows/oidc-environment-contract.live.test.ts
index 19c59bcaf..8b89196ac 100644
--- a/packages/core/src/workflows/oidc-environment-contract.live.test.ts
+++ b/packages/core/src/workflows/oidc-environment-contract.live.test.ts
@@ -15,10 +15,10 @@
 // no template copy, so it would only assert against a fixture we wrote here.
 //
 // This hits the network and depends on an external repo's moving ref, so it is
-// NOT part of the default hermetic suite: it runs only when
-// RUN_LIVE_WORKFLOW_TESTS is set (e.g. locally or in a scheduled job), never in
-// normal CI. Runtime day-to-day protection is the verify-credentials flow, which
-// fails loudly if the FIC does not match; this test catches the drift earlier.
+// NOT part of the default hermetic suite. A separate non-required CI workflow
+// sets RUN_LIVE_WORKFLOW_TESTS on pull requests, pushes to main, and nightly.
+// Runtime day-to-day protection is the verify-credentials flow, which fails
+// loudly if the FIC does not match; this test catches the drift earlier.
 import { describe, it, expect } from "vitest";
 import {
   RADIUS_WORKFLOW_REPO,
diff --git a/packages/core/src/workflows/workflow-yaml.live.test.ts b/packages/core/src/workflows/workflow-yaml.live.test.ts
new file mode 100644
index 000000000..128aa0736
--- /dev/null
+++ b/packages/core/src/workflows/workflow-yaml.live.test.ts
@@ -0,0 +1,125 @@
+// Opt-in live regression test. Fetches the CURRENT workflow templates from
+// radius-project/radius `.github/extension/` and asserts that, once the
+// extension fills them, every generated file parses as valid YAML.
+//
+// Why this matters: the extension renders templates fetched from
+// radius-project/radius at RUNTIME (RADIUS_REF, currently `main`) — it bundles
+// no copy. `generateDeployWorkflow` injects GitHub Actions expressions whose
+// string defaults are single-quoted (`${{ vars.RADIUS_BUILD_ARCH_MODE ||
+// 'detect' }}`). If an upstream template wraps such a placeholder in a
+// single-quoted YAML scalar, the injected quotes nest and the rendered file
+// becomes invalid YAML — every deploy dispatch then fails upstream with
+// `HTTP 422: failed to parse workflow`. That is exactly issue #407
+// (radius-project/ai-extensions#407), caused by radius-project/radius#12640 and
+// fixed by radius-project/radius#12721. The hermetic tests in deploy.test.ts use
+// inline fixtures, so only a LIVE render of the real templates catches an
+// upstream quoting (or any other YAML) regression.
+//
+// Coverage is not limited to the deploy workflows: every generator the
+// extension ships (deploy, delete, verify) is rendered from its real upstream
+// templates and asserted to produce valid YAML, so a malformed scalar,
+// indentation, or quoting change in ANY of those upstream templates is caught.
+//
+// This hits the network and depends on an external repo's moving ref, so it is
+// NOT part of the default hermetic suite. A separate non-required CI workflow
+// sets RUN_LIVE_WORKFLOW_TESTS on pull requests, pushes to main, and nightly.
+import { describe, it, expect } from "vitest";
+import { parse as parseYaml } from "yaml";
+import { azure } from "../platforms/azure.js";
+import { aws } from "../platforms/aws.js";
+import {
+  RADIUS_WORKFLOW_REPO,
+  RADIUS_WORKFLOW_DIR,
+  RADIUS_REF,
+  DEPLOY_DISPATCHER_FILE,
+  DEPLOY_AZURE_FILE,
+  DEPLOY_AWS_FILE,
+  generateDeployWorkflow
+} from "./deploy.js";
+import {
+  DELETE_RADIUS_REF,
+  DELETE_APP_DISPATCHER_FILE,
+  DELETE_AZURE_FILE,
+  DELETE_AWS_FILE,
+  generateDeleteWorkflow
+} from "./delete.js";
+import {
+  VERIFY_AZURE_FILE,
+  VERIFY_AWS_FILE,
+  generateVerifyWorkflow
+} from "./verify.js";
+
+const LIVE = !!process.env.RUN_LIVE_WORKFLOW_TESTS;
+
+async function fetchWorkflow(file: string, ref: string): Promise<string> {
+  const url = `https://raw.githubusercontent.com/${RADIUS_WORKFLOW_REPO}/${ref}/${RADIUS_WORKFLOW_DIR}/${file}`;
+  const res = await fetch(url);
+  if (!res.ok) {
+    throw new Error(`failed to fetch ${url}: ${res.status} ${res.statusText}`);
+  }
+  return res.text();
+}
+
+async function fetchTemplates(
+  files: string[],
+  ref: string
+): Promise<Record<string, string>> {
+  const bodies = await Promise.all(files.map((f) => fetchWorkflow(f, ref)));
+  const templates: Record<string, string> = {};
+  files.forEach((f, i) => {
+    templates[f] = bodies[i];
+  });
+  return templates;
+}
+
+function assertAllValidYaml(
+  generated: Record<string, string>,
+  ref: string
+): void {
+  for (const [name, body] of Object.entries(generated)) {
+    expect(
+      () => parseYaml(body),
+      `${name} rendered from upstream ${ref} should parse as valid YAML`
+    ).not.toThrow();
+  }
+}
+
+describe.skipIf(!LIVE)(
+  "live workflow YAML validity (opt-in: set RUN_LIVE_WORKFLOW_TESTS)",
+  () => {
+    // The deploy workflows are the ones that inject single-quoted GHA arch
+    // expressions, so this is the case that would have caught radius#12640.
+    it("renders valid YAML from the current upstream deploy templates", async () => {
+      const templates = await fetchTemplates(
+        [DEPLOY_DISPATCHER_FILE, DEPLOY_AZURE_FILE, DEPLOY_AWS_FILE],
+        RADIUS_REF
+      );
+      const generated = generateDeployWorkflow(
+        "prod",
+        ".radius/app.bicep",
+        templates
+      );
+      assertAllValidYaml(generated, RADIUS_REF);
+    }, 30_000);
+
+    it("renders valid YAML from the current upstream delete templates", async () => {
+      const templates = await fetchTemplates(
+        [DELETE_APP_DISPATCHER_FILE, DELETE_AZURE_FILE, DELETE_AWS_FILE],
+        DELETE_RADIUS_REF
+      );
+      const generated = generateDeleteWorkflow("prod", templates);
+      assertAllValidYaml(generated, DELETE_RADIUS_REF);
+    }, 30_000);
+
+    it("renders valid YAML from the current upstream verify templates", async () => {
+      for (const [platform, file] of [
+        [azure, VERIFY_AZURE_FILE],
+        [aws, VERIFY_AWS_FILE]
+      ] as const) {
+        const template = await fetchWorkflow(file, RADIUS_REF);
+        const rendered = generateVerifyWorkflow("prod", platform, template);
+        assertAllValidYaml({ [file]: rendered }, RADIUS_REF);
+      }
+    }, 30_000);
+  }
+);
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 062420f29..499b94701 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -24,6 +24,9 @@ catalogs:
     vitest:
       specifier: ^4.1.10
       version: 4.1.10
+    yaml:
+      specifier: ^2.9.0
+      version: 2.9.0
 
 overrides:
   read-yaml-file@<2: ^2.1.0
@@ -92,6 +95,9 @@ importers:
       '@radius-project/core':
         specifier: workspace:*
         version: link:../core
+      yaml:
+        specifier: 'catalog:'
+        version: 2.9.0
     devDependencies:
       '@github/copilot-sdk':
         specifier: 'catalog:'
@@ -154,6 +160,9 @@ importers:
       vitest:
         specifier: 'catalog:'
         version: 4.1.10(@types/node@24.13.3)(@vitest/browser-playwright@4.1.10)(@vitest/coverage-v8@4.1.10)(vite@8.2.0(@types/node@24.13.3)(esbuild@0.28.2)(yaml@2.9.0))
+      yaml:
+        specifier: 'catalog:'
+        version: 2.9.0
 
   plugins/radius:
     dependencies:
diff --git a/pnpm-workspace.yaml b/pnpm-workspace.yaml
index e57cf1032..cc2c7e1fb 100644
--- a/pnpm-workspace.yaml
+++ b/pnpm-workspace.yaml
@@ -27,5 +27,9 @@ catalog:
   "@vitest/coverage-v8": ^4.1.10
   typescript: ^7.0.2
   vitest: ^4.1.10
+  # YAML parser used only by @radius-project/core tests to assert generated
+  # workflow files parse as valid YAML (dev-only; core ships no runtime deps).
+  # Caret range compatible with the 2.9.0 already resolved in the lockfile.
+  yaml: ^2.9.0
 
 minimumReleaseAge: 11520   # 8 days