diff --git a/.codespellrc b/.codespellrc
index 6a20d6e8..43f1415e 100644
--- a/.codespellrc
+++ b/.codespellrc
@@ -64,7 +64,9 @@
 
 # ACI - Azure Container Instances abbreviation
 
-ignore-words-list = numer,wit,aks,edn,ser,ois,gir,rouge,categor,aline,ative,afterall,deques,dateA,dateB,TE,FillIn,alle,vai,LOD,InOut,pixelX,aNULL,Wee,Sherif,queston,extenions,Vertexes,nin,FO,CAF,Parth,ans,gud,Vally,vally,ACI
+# soruce - intentional misspelling for tests
+
+ignore-words-list = numer,wit,aks,edn,ser,ois,gir,rouge,categor,aline,ative,afterall,deques,dateA,dateB,TE,FillIn,alle,vai,LOD,InOut,pixelX,aNULL,Wee,Sherif,queston,extenions,Vertexes,nin,FO,CAF,Parth,ans,gud,Vally,vally,ACI,soruce
 
 # Skip certain files and directories
 
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 075101c2..fc3d43ad 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -234,13 +234,16 @@ The repository's canonical validation rules live in `eng/external-plugin-validat
 
 For entries committed to `plugins/external.json`, the current marketplace validation requires:
 
-- `name`, `description`, and `version`
-- `author.name`
+- `name`, `description`, and `version` (a valid [semantic version](https://semver.org), e.g. `1.2.3`)
+- `author.name` (and, when present, a valid `author.url` and `author.email`)
 - `repository` as an HTTPS GitHub URL
 - `keywords` as lowercase hyphenated tags
+- `license`, when provided, is recommended to be an [SPDX identifier or expression](https://spdx.org/licenses) (e.g. `MIT`, `Apache-2.0`, `MIT OR Apache-2.0`), but a non-SPDX or proprietary license is allowed and only produces a warning
 - `source.source: "github"` plus `source.repo` in `owner/repo` format
 - optional `source.path` values of `/` for repository root, or a repository-relative folder where the plugin structure starts (do not point to `plugin.json` directly)
 
+Validation also emits non-fatal warnings to catch mistakes early: unknown/misspelled top-level, `author`, or `source` fields; `license` values that are not recognized SPDX identifiers (non-SPDX/proprietary licenses are permitted, not rejected); and marketplace entries whose `source` omits an immutable `ref`/`sha` locator. The same `license` validation is shared with local plugin `plugin.json` manifests so both are checked consistently.
+
 The public-submission policy builds on those rules and also requires `license` plus at least one immutable source locator: `source.ref`, `source.sha`, or both.
 
 ##### Review workflow
diff --git a/eng/external-plugin-validation.mjs b/eng/external-plugin-validation.mjs
index 0a5ce2c1..07f14c25 100644
--- a/eng/external-plugin-validation.mjs
+++ b/eng/external-plugin-validation.mjs
@@ -1,6 +1,8 @@
 import fs from "fs";
 import path from "path";
 import { ROOT_FOLDER } from "./constants.mjs";
+import { validateLicenseField } from "./lib/license.mjs";
+import { inlineCode } from "./lib/markdown.mjs";
 
 export const EXTERNAL_PLUGINS_FILE = path.join(ROOT_FOLDER, "plugins", "external.json");
 
@@ -12,6 +14,7 @@ export const EXTERNAL_PLUGIN_POLICIES = Object.freeze({
     requireKeywords: true,
     requireLicense: false,
     requireImmutableLocator: false,
+    warnMissingImmutableLocator: true,
   }),
   publicSubmission: Object.freeze({
     allowedSourceTypes: ["github"],
@@ -20,9 +23,34 @@ export const EXTERNAL_PLUGIN_POLICIES = Object.freeze({
     requireKeywords: true,
     requireLicense: true,
     requireImmutableLocator: true,
+    warnMissingImmutableLocator: false,
   }),
 });
 
+// Allowed keys for typo detection. Kept intentionally permissive: unknown keys
+// produce warnings (not errors) so the schema stays forward-compatible.
+const ALLOWED_PLUGIN_KEYS = Object.freeze([
+  "name",
+  "description",
+  "version",
+  "author",
+  "repository",
+  "homepage",
+  "license",
+  "keywords",
+  "tags",
+  "source",
+]);
+
+const ALLOWED_AUTHOR_KEYS = Object.freeze(["name", "url", "email"]);
+
+const ALLOWED_SOURCE_KEYS = Object.freeze(["source", "repo", "path", "ref", "sha"]);
+
+// Semantic Versioning 2.0.0 (https://semver.org). Anchored: major.minor.patch
+// with optional -prerelease and +build metadata.
+const SEMVER_PATTERN =
+  /^(0|[1-9]\d*)\.(0|[1-9]\d*)\.(0|[1-9]\d*)(?:-((?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*)(?:\.(?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*))*))?(?:\+([0-9a-zA-Z-]+(?:\.[0-9a-zA-Z-]+)*))?$/;
+
 // NOTE: Keep in sync with PLUGIN_JSON_CANDIDATES in external-plugin-quality-gates.mjs
 const EXTERNAL_PLUGIN_ROOT_MANIFEST_PATHS = Object.freeze([
   "plugin.json",
@@ -97,6 +125,12 @@ function validateVersion(version, prefix, errors) {
   if (version.length > 100) {
     errors.push(`${prefix}: "version" must be 100 characters or fewer`);
   }
+
+  if (!SEMVER_PATTERN.test(version)) {
+    errors.push(
+      `${prefix}: "version" must be a valid semantic version (e.g. "1.2.3" or "1.2.3-beta.1"); see https://semver.org`
+    );
+  }
 }
 
 function validateKeywords(keywords, prefix, errors, warnings, required) {
@@ -164,7 +198,7 @@ function validateHttpsUrl(value, fieldName, prefix, errors, options = {}) {
   }
 }
 
-function validateAuthor(author, prefix, errors, required) {
+function validateAuthor(author, prefix, errors, warnings, required) {
   if (author === undefined) {
     if (required) {
       errors.push(`${prefix}: "author" is required`);
@@ -184,18 +218,43 @@ function validateAuthor(author, prefix, errors, required) {
   if (author.url !== undefined) {
     validateHttpsUrl(author.url, "author.url", prefix, errors);
   }
+
+  if (author.email !== undefined) {
+    validateEmail(author.email, "author.email", prefix, errors);
+  }
+
+  validateKnownFields(author, ALLOWED_AUTHOR_KEYS, "author", prefix, warnings);
 }
 
-function validateLicense(license, prefix, errors, required) {
-  if (license === undefined) {
-    if (required) {
-      errors.push(`${prefix}: "license" is required`);
-    }
+function validateEmail(value, fieldName, prefix, errors) {
+  if (!isNonEmptyString(value)) {
+    errors.push(`${prefix}: "${fieldName}" must be a non-empty string`);
     return;
   }
 
-  if (!isNonEmptyString(license)) {
-    errors.push(`${prefix}: "license" must be a non-empty string`);
+  // Pragmatic email check: single "@", non-empty local part, and a dotted domain.
+  if (!/^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(value)) {
+    errors.push(`${prefix}: "${fieldName}" must be a valid email address`);
+  }
+}
+
+function validateLicense(license, prefix, errors, warnings, required) {
+  const result = validateLicenseField(license, { prefix, required });
+  errors.push(...result.errors);
+  warnings.push(...result.warnings);
+}
+
+function validateKnownFields(value, allowedKeys, scope, prefix, warnings) {
+  if (!value || typeof value !== "object" || Array.isArray(value)) {
+    return;
+  }
+
+  const allowed = new Set(allowedKeys);
+  const label = scope ? `${scope}.` : "";
+  for (const key of Object.keys(value)) {
+    if (!allowed.has(key)) {
+      warnings.push(`${prefix}: unknown ${scope || "top-level"} field ${inlineCode(`${label}${key}`)} (possible typo)`);
+    }
   }
 }
 
@@ -289,7 +348,7 @@ function validateCommitSha(sha, prefix, errors) {
   }
 }
 
-function validateGitHubSource(source, prefix, errors, requireImmutableLocator) {
+function validateGitHubSource(source, prefix, errors, warnings, policy) {
   if (!source || typeof source !== "object" || Array.isArray(source)) {
     errors.push(`${prefix}: "source" must be an object`);
     return;
@@ -317,8 +376,15 @@ function validateGitHubSource(source, prefix, errors, requireImmutableLocator) {
     validateCommitSha(source.sha, prefix, errors);
   }
 
-  if (requireImmutableLocator && source.ref === undefined && source.sha === undefined) {
-    errors.push(`${prefix}: one of "source.ref" or "source.sha" is required for public external plugin submissions`);
+  const missingLocator = source.ref === undefined && source.sha === undefined;
+  if (missingLocator) {
+    if (policy.requireImmutableLocator) {
+      errors.push(`${prefix}: one of "source.ref" or "source.sha" is required for public external plugin submissions`);
+    } else if (policy.warnMissingImmutableLocator) {
+      warnings.push(
+        `${prefix}: "source" has no "source.ref" or "source.sha"; an immutable tag ref or commit SHA is recommended for reproducible installs`
+      );
+    }
   }
 }
 
@@ -338,11 +404,12 @@ export function validateExternalPlugin(plugin, index, options = {}) {
   validatePluginName(plugin.name, prefix, errors);
   validateDescription(plugin.description, prefix, errors);
   validateVersion(plugin.version, prefix, errors);
-  validateAuthor(plugin.author, prefix, errors, policy.requireAuthor);
+  validateAuthor(plugin.author, prefix, errors, warnings, policy.requireAuthor);
   validateRepository(plugin.repository, prefix, errors, policy.requireRepository);
   validateHomepage(plugin.homepage, prefix, errors);
-  validateLicense(plugin.license, prefix, errors, policy.requireLicense);
+  validateLicense(plugin.license, prefix, errors, warnings, policy.requireLicense);
   validateKeywords(plugin.keywords ?? plugin.tags, prefix, errors, warnings, policy.requireKeywords);
+  validateKnownFields(plugin, ALLOWED_PLUGIN_KEYS, "", prefix, warnings);
 
   if (plugin.tags !== undefined && plugin.keywords === undefined) {
     warnings.push(`${prefix}: prefer "keywords" over legacy "tags"`);
@@ -350,12 +417,15 @@ export function validateExternalPlugin(plugin, index, options = {}) {
 
   if (!plugin.source) {
     errors.push(`${prefix}: "source" is required`);
-  } else if (typeof plugin.source === "string") {
+  } else if (typeof plugin.source !== "object" || Array.isArray(plugin.source)) {
     errors.push(`${prefix}: "source" must be an object (local file paths are not allowed for external plugins)`);
-  } else if (!policy.allowedSourceTypes.includes(plugin.source.source)) {
-    errors.push(`${prefix}: "source.source" must be one of: ${policy.allowedSourceTypes.join(", ")}`);
-  } else if (plugin.source.source === "github") {
-    validateGitHubSource(plugin.source, prefix, errors, policy.requireImmutableLocator);
+  } else {
+    validateKnownFields(plugin.source, ALLOWED_SOURCE_KEYS, "source", prefix, warnings);
+    if (!policy.allowedSourceTypes.includes(plugin.source.source)) {
+      errors.push(`${prefix}: "source.source" must be one of: ${policy.allowedSourceTypes.join(", ")}`);
+    } else if (plugin.source.source === "github") {
+      validateGitHubSource(plugin.source, prefix, errors, warnings, policy);
+    }
   }
 
   return { errors, warnings };
diff --git a/eng/external-plugin-validation.test.mjs b/eng/external-plugin-validation.test.mjs
new file mode 100644
index 00000000..8144c984
--- /dev/null
+++ b/eng/external-plugin-validation.test.mjs
@@ -0,0 +1,245 @@
+import assert from "node:assert/strict";
+import fs from "fs";
+import { test } from "node:test";
+import {
+  EXTERNAL_PLUGINS_FILE,
+  validateExternalPlugin,
+  validateExternalPlugins,
+} from "./external-plugin-validation.mjs";
+import { validateLicenseField } from "./lib/license.mjs";
+
+function basePlugin(overrides = {}) {
+  return {
+    name: "example-plugin",
+    description: "An example external plugin used for validation tests.",
+    version: "1.2.3",
+    author: { name: "Example Author", url: "https://github.com/example" },
+    repository: "https://github.com/example/example-plugin",
+    license: "MIT",
+    keywords: ["example", "testing"],
+    source: {
+      source: "github",
+      repo: "example/example-plugin",
+      ref: "v1.2.3",
+    },
+    ...overrides,
+  };
+}
+
+function hasError(result, needle) {
+  return result.errors.some((message) => message.includes(needle));
+}
+
+function hasWarning(result, needle) {
+  return result.warnings.some((message) => message.includes(needle));
+}
+
+function longestBacktickRun(value) {
+  return Math.max(0, ...Array.from(value.matchAll(/`+/g), (match) => match[0].length));
+}
+
+test("valid plugin has no errors under both policies", () => {
+  const marketplace = validateExternalPlugin(basePlugin(), 0, { policy: "marketplace" });
+  assert.deepEqual(marketplace.errors, []);
+
+  const publicSubmission = validateExternalPlugin(
+    basePlugin({ source: { source: "github", repo: "example/example-plugin", ref: "v1.2.3" } }),
+    0,
+    { policy: "publicSubmission" }
+  );
+  assert.deepEqual(publicSubmission.errors, []);
+});
+
+test("version must be semver", () => {
+  assert.deepEqual(validateExternalPlugin(basePlugin({ version: "1.2.3" }), 0).errors, []);
+  // Prerelease metadata is allowed (matches committed 1.0.1161-preview1).
+  assert.deepEqual(validateExternalPlugin(basePlugin({ version: "1.0.1161-preview1" }), 0).errors, []);
+  assert.deepEqual(validateExternalPlugin(basePlugin({ version: "1.2.3-beta.1+build.5" }), 0).errors, []);
+
+  assert.ok(hasError(validateExternalPlugin(basePlugin({ version: "1.0" }), 0), "semantic version"));
+  assert.ok(hasError(validateExternalPlugin(basePlugin({ version: "v1.2.3" }), 0), "semantic version"));
+  assert.ok(hasError(validateExternalPlugin(basePlugin({ version: "latest" }), 0), "semantic version"));
+});
+
+test("license accepts SPDX ids and expressions", () => {
+  assert.deepEqual(validateExternalPlugin(basePlugin({ license: "Apache-2.0" }), 0).errors, []);
+  assert.deepEqual(validateExternalPlugin(basePlugin({ license: "MIT OR Apache-2.0" }), 0).errors, []);
+  assert.deepEqual(validateExternalPlugin(basePlugin({ license: "(MIT OR Apache-2.0)" }), 0).errors, []);
+  assert.deepEqual(validateExternalPlugin(basePlugin({ license: "LicenseRef-Custom" }), 0).errors, []);
+
+  const recognized = validateExternalPlugin(basePlugin({ license: "MIT" }), 0);
+  assert.equal(recognized.warnings.filter((m) => m.includes("license")).length, 0);
+});
+
+test("non-SPDX license is a warning, never an error", () => {
+  for (const license of ["SSAL-1.0", "Proprietary", "UNLICENSED", "SEE LICENSE IN LICENSE.md", "MIT OR"]) {
+    const result = validateExternalPlugin(basePlugin({ license }), 0);
+    assert.deepEqual(result.errors, [], `expected no errors for license "${license}"`);
+    assert.ok(
+      hasWarning(result, "not a recognized SPDX identifier"),
+      `expected a warning for license "${license}"`
+    );
+  }
+});
+
+test("empty/non-string license is an error; required license is enforced", () => {
+  assert.ok(hasError(validateExternalPlugin(basePlugin({ license: "" }), 0), "non-empty string"));
+  assert.ok(hasError(validateExternalPlugin(basePlugin({ license: 42 }), 0), "non-empty string"));
+
+  // publicSubmission requires a license.
+  const noLicense = basePlugin();
+  delete noLicense.license;
+  assert.ok(
+    hasError(validateExternalPlugin(noLicense, 0, { policy: "publicSubmission" }), '"license" is required')
+  );
+});
+
+test("validateLicenseField is reusable for local plugin.json", () => {
+  assert.deepEqual(validateLicenseField("MIT"), { errors: [], warnings: [] });
+
+  const proprietary = validateLicenseField("Proprietary");
+  assert.deepEqual(proprietary.errors, []);
+  assert.equal(proprietary.warnings.length, 1);
+
+  // Optional by default: absent license produces nothing.
+  assert.deepEqual(validateLicenseField(undefined), { errors: [], warnings: [] });
+  assert.ok(validateLicenseField(undefined, { required: true }).errors.length === 1);
+});
+
+test("license grammar validates refs, parentheses, WITH exceptions, and warning sanitization", () => {
+  for (const license of [
+    "GPL-2.0-only WITH Classpath-exception-2.0",
+    "LicenseRef-Custom",
+    "DocumentRef-spdx-tool:LicenseRef-MyLicense",
+    "(MIT OR Apache-2.0)",
+    "MIT AND Apache-2.0",
+  ]) {
+    const result = validateLicenseField(license);
+    assert.deepEqual(result.errors, [], `expected no errors for recognized license "${license}"`);
+    assert.equal(result.warnings.length, 0, `expected no warnings for recognized license "${license}"`);
+  }
+
+  for (const license of [
+    "LicenseRef-",
+    "LicenseRef-@@",
+    "DocumentRef-foo",
+    "(MIT",
+    "MIT)",
+    "MIT OR (Apache-2.0))",
+    "(MIT OR Apache-2.0",
+    "MIT WITH Apache-2.0",
+    "GPL-2.0-only WITH MIT",
+  ]) {
+    const result = validateLicenseField(license);
+    assert.deepEqual(result.errors, [], `expected malformed license "${license}" to warn, not error`);
+    assert.equal(result.warnings.length, 1, `expected one warning for malformed license "${license}"`);
+    assert.ok(hasWarning(result, "not a recognized SPDX identifier"));
+  }
+
+  const injected = validateLicenseField("MIT\n\n## Injected\n| a | b |");
+  assert.deepEqual(injected.errors, []);
+  assert.equal(injected.warnings.length, 1);
+  assert.ok(!injected.warnings[0].includes("\n"));
+  assert.ok(hasWarning(injected, "not a recognized SPDX identifier"));
+
+  const backslashed = validateLicenseField("MIT \\ Custom \\`code\\`");
+  assert.deepEqual(backslashed.errors, []);
+  assert.equal(backslashed.warnings.length, 1);
+  assert.ok(backslashed.warnings[0].includes("`` MIT \\ Custom \\`code\\` ``"));
+});
+
+test("license warning wraps backticks and markdown link in an unbreakable code span", () => {
+  const license = "x` [pwn](https://evil.example)";
+  const result = validateLicenseField(license);
+
+  assert.deepEqual(result.errors, []);
+  assert.equal(result.warnings.length, 1);
+
+  const warning = result.warnings[0];
+  assert.ok(warning.includes("is not a recognized SPDX identifier"));
+  const [, wrappedValue] = warning.match(/"license" value (.+?) is not a recognized SPDX identifier/u);
+  const fence = wrappedValue.match(/^`+/u)[0];
+  const content = wrappedValue.slice(fence.length, -fence.length);
+
+  assert.ok(fence.length > longestBacktickRun(content));
+  assert.ok(content.includes("[pwn](https://evil.example)"));
+});
+
+test("author.email is validated only when present", () => {
+  assert.deepEqual(
+    validateExternalPlugin(basePlugin({ author: { name: "Example", email: "dev@example.com" } }), 0).errors,
+    []
+  );
+  assert.ok(
+    hasError(
+      validateExternalPlugin(basePlugin({ author: { name: "Example", email: "not-an-email" } }), 0),
+      "author.email"
+    )
+  );
+  // Absent email must not raise an error.
+  assert.deepEqual(validateExternalPlugin(basePlugin({ author: { name: "Example" } }), 0).errors, []);
+});
+
+test("unknown fields produce warnings, not errors", () => {
+  const topLevel = validateExternalPlugin(basePlugin({ licence: "MIT" }), 0);
+  assert.deepEqual(topLevel.errors, []);
+  assert.ok(hasWarning(topLevel, "unknown top-level field `licence`"));
+
+  const authorTypo = validateExternalPlugin(
+    basePlugin({ author: { name: "Example", emial: "dev@example.com" } }),
+    0
+  );
+  assert.ok(hasWarning(authorTypo, "unknown author field `author.emial`"));
+
+  const sourceTypo = validateExternalPlugin(
+    basePlugin({ source: { source: "github", repo: "example/example-plugin", ref: "v1.2.3", branch: "main" } }),
+    0
+  );
+  assert.ok(hasWarning(sourceTypo, "unknown source field `source.branch`"));
+
+  // A typo in the source discriminator itself is still surfaced as an unknown
+  // source field (plus the discriminator error), because the unknown-field check
+  // now runs for every source object rather than only recognized github sources.
+  const sourceDiscriminatorTypo = validateExternalPlugin(
+    basePlugin({ source: { soruce: "github", repo: "example/example-plugin", ref: "v1.2.3" } }),
+    0
+  );
+  assert.ok(hasWarning(sourceDiscriminatorTypo, "unknown source field `source.soruce`"));
+  assert.ok(hasError(sourceDiscriminatorTypo, '"source.source" must be one of'));
+
+  // Supported fields never warn.
+  const clean = validateExternalPlugin(basePlugin({ homepage: "https://github.com/example/example-plugin" }), 0);
+  assert.equal(
+    clean.warnings.filter((message) => message.includes("unknown")).length,
+    0
+  );
+});
+
+test("unknown field warning collapses malicious key markdown into one inline code span", () => {
+  const maliciousKey = "evil\n- injected [x](https://evil.example)";
+  const result = validateExternalPlugin(basePlugin({ [maliciousKey]: true }), 0);
+
+  assert.deepEqual(result.errors, []);
+  const warning = result.warnings.find((message) => message.includes("unknown top-level field"));
+  assert.ok(warning);
+  assert.ok(!warning.includes("\n"));
+  assert.ok(warning.includes("unknown top-level field `evil - injected [x](https://evil.example)`"));
+  assert.ok(warning.includes("(possible typo)"));
+});
+
+test("immutable locator: marketplace warns, publicSubmission errors", () => {
+  const noLocator = basePlugin({ source: { source: "github", repo: "example/example-plugin" } });
+
+  const marketplace = validateExternalPlugin(noLocator, 0, { policy: "marketplace" });
+  assert.deepEqual(marketplace.errors, []);
+  assert.ok(hasWarning(marketplace, "immutable tag ref or commit SHA is recommended"));
+
+  const publicSubmission = validateExternalPlugin(noLocator, 0, { policy: "publicSubmission" });
+  assert.ok(hasError(publicSubmission, 'one of "source.ref" or "source.sha" is required'));
+});
+
+test("committed external.json passes marketplace policy with zero errors", () => {
+  const plugins = JSON.parse(fs.readFileSync(EXTERNAL_PLUGINS_FILE, "utf8"));
+  const { errors } = validateExternalPlugins(plugins, { policy: "marketplace" });
+  assert.deepEqual(errors, [], `Committed external.json produced errors:\n${errors.join("\n")}`);
+});
diff --git a/eng/lib/license.mjs b/eng/lib/license.mjs
new file mode 100644
index 00000000..4fb9e312
--- /dev/null
+++ b/eng/lib/license.mjs
@@ -0,0 +1,261 @@
+// Shared license validation used by both the external plugin catalog validator
+// (eng/external-plugin-validation.mjs) and the local plugin.json validator
+// (eng/validate-plugins.mjs). Keeping it here avoids one validator importing from
+// the other and gives license rules a single, dedicated home.
+//
+// The agent-plugins-spec schema (https://github.com/agentplugins/agent-plugins-spec)
+// does not enforce SPDX, and plugins may legitimately use proprietary or non-OSS
+// licenses. A non-SPDX license is therefore a warning, never an error.
+
+import { inlineCode } from "./markdown.mjs";
+
+// A single SPDX license identifier token (e.g. "MIT", "Apache-2.0", "LicenseRef-Foo").
+const SPDX_ID_PATTERN = /^[A-Za-z0-9][A-Za-z0-9.-]*\+?$/;
+const SPDX_IDSTRING_PATTERN = /^[A-Za-z0-9.-]+$/;
+
+// Curated set of common SPDX license identifiers. Not exhaustive: unrecognized
+// but well-formed identifiers produce a warning rather than an error.
+const KNOWN_SPDX_IDS = new Set([
+  "0BSD",
+  "AGPL-3.0",
+  "AGPL-3.0-only",
+  "AGPL-3.0-or-later",
+  "Apache-2.0",
+  "BSD-2-Clause",
+  "BSD-3-Clause",
+  "BSL-1.0",
+  "CC-BY-4.0",
+  "CC-BY-SA-4.0",
+  "CC0-1.0",
+  "EPL-2.0",
+  "GPL-2.0",
+  "GPL-2.0-only",
+  "GPL-2.0-or-later",
+  "GPL-3.0",
+  "GPL-3.0-only",
+  "GPL-3.0-or-later",
+  "ISC",
+  "LGPL-2.1",
+  "LGPL-2.1-only",
+  "LGPL-2.1-or-later",
+  "LGPL-3.0",
+  "LGPL-3.0-only",
+  "LGPL-3.0-or-later",
+  "MIT",
+  "MPL-2.0",
+  "Unlicense",
+  "WTFPL",
+  "Zlib",
+]);
+
+const KNOWN_SPDX_EXCEPTIONS = new Set([
+  "Classpath-exception-2.0",
+  "GCC-exception-3.1",
+  "LLVM-exception",
+  "Autoconf-exception-3.0",
+  "Bison-exception-2.2",
+  "Font-exception-2.0",
+  "GPL-3.0-linking-exception",
+  "Linux-syscall-note",
+  "OpenSSL-exception",
+  "Qt-GPL-exception-1.0",
+]);
+
+function isNonEmptyString(value) {
+  return typeof value === "string" && value.trim().length > 0;
+}
+
+function isSpdxIdString(value) {
+  return SPDX_IDSTRING_PATTERN.test(value);
+}
+
+function isRecognizedLicenseRef(token) {
+  if (token.startsWith("LicenseRef-")) {
+    return isSpdxIdString(token.slice("LicenseRef-".length));
+  }
+
+  if (!token.startsWith("DocumentRef-")) {
+    return false;
+  }
+
+  const separatorIndex = token.indexOf(":LicenseRef-");
+  if (separatorIndex === -1) {
+    return false;
+  }
+
+  const documentRef = token.slice("DocumentRef-".length, separatorIndex);
+  const licenseRef = token.slice(separatorIndex + ":LicenseRef-".length);
+  return isSpdxIdString(documentRef) && isSpdxIdString(licenseRef);
+}
+
+function isRecognizedSpdxIdToken(token) {
+  if (isRecognizedLicenseRef(token)) {
+    return true;
+  }
+
+  if (!SPDX_ID_PATTERN.test(token)) {
+    return false;
+  }
+
+  const normalized = token.endsWith("+") ? token.slice(0, -1) : token;
+  return KNOWN_SPDX_IDS.has(normalized);
+}
+
+function isRecognizedSpdxExceptionToken(token) {
+  return isSpdxIdString(token) && KNOWN_SPDX_EXCEPTIONS.has(token);
+}
+
+function tokenizeSpdxExpression(license) {
+  return license
+    .replace(/([()])/g, " $1 ")
+    .trim()
+    .split(/\s+/)
+    .filter((token) => token.length > 0);
+}
+
+// Returns true only for a well-formed SPDX license expression composed of
+// recognized identifiers joined by SPDX operators (optionally parenthesized).
+// Anything else (proprietary strings, free text, unrecognized ids) returns false so
+// the caller can warn without rejecting it — the plugin spec does not enforce SPDX.
+export function isRecognizedSpdxExpression(license) {
+  if (typeof license !== "string") {
+    return false;
+  }
+
+  const tokens = tokenizeSpdxExpression(license);
+
+  if (tokens.length === 0) {
+    return false;
+  }
+
+  let position = 0;
+
+  function peek() {
+    return tokens[position];
+  }
+
+  function consume() {
+    return tokens[position++];
+  }
+
+  function parsePrimary() {
+    const token = peek();
+
+    if (token === "(") {
+      consume();
+      if (!parseOrExpression()) {
+        return false;
+      }
+      if (peek() !== ")") {
+        return false;
+      }
+      consume();
+      return true;
+    }
+
+    if (!token || token === ")" || ["AND", "OR", "WITH"].includes(token.toUpperCase())) {
+      return false;
+    }
+
+    consume();
+    return isRecognizedSpdxIdToken(token);
+  }
+
+  function parseSimpleExpression() {
+    const token = peek();
+    if (!token || token === ")" || ["AND", "OR", "WITH"].includes(token.toUpperCase())) {
+      return false;
+    }
+
+    consume();
+    return isRecognizedSpdxIdToken(token);
+  }
+
+  function parseWithExpression() {
+    if (peek() === "(") {
+      return parsePrimary() && peek()?.toUpperCase() !== "WITH";
+    }
+
+    if (!parseSimpleExpression()) {
+      return false;
+    }
+
+    if (peek()?.toUpperCase() !== "WITH") {
+      return true;
+    }
+
+    consume();
+    const exceptionToken = peek();
+    if (!exceptionToken || exceptionToken === ")" || ["AND", "OR", "WITH"].includes(exceptionToken.toUpperCase())) {
+      return false;
+    }
+    consume();
+    return isRecognizedSpdxExceptionToken(exceptionToken);
+  }
+
+  function parseAndExpression() {
+    if (!parseWithExpression()) {
+      return false;
+    }
+
+    while (peek()?.toUpperCase() === "AND") {
+      consume();
+      if (!parseWithExpression()) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+  function parseOrExpression() {
+    if (!parseAndExpression()) {
+      return false;
+    }
+
+    while (peek()?.toUpperCase() === "OR") {
+      consume();
+      if (!parseAndExpression()) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+
+  return parseOrExpression() && position === tokens.length;
+}
+
+// Canonical license validation. A non-SPDX license is a warning (never an error)
+// so authors may use proprietary or non-OSS licenses. Returns collected
+// errors/warnings so each caller can integrate them into its own reporting.
+//
+// Options:
+//   required — when true, a missing license is an error (default false).
+//   prefix   — optional message prefix (e.g. "external.json[3]") for context.
+export function validateLicenseField(license, options = {}) {
+  const { required = false } = options;
+  const prefix = options.prefix ? `${options.prefix}: ` : "";
+  const errors = [];
+  const warnings = [];
+
+  if (license === undefined) {
+    if (required) {
+      errors.push(`${prefix}"license" is required`);
+    }
+    return { errors, warnings };
+  }
+
+  if (!isNonEmptyString(license)) {
+    errors.push(`${prefix}"license" must be a non-empty string`);
+    return { errors, warnings };
+  }
+
+  if (!isRecognizedSpdxExpression(license)) {
+    warnings.push(
+      `${prefix}"license" value ${inlineCode(license)} is not a recognized SPDX identifier; prefer a standard SPDX id (https://spdx.org/licenses), though non-SPDX or proprietary licenses are allowed`
+    );
+  }
+
+  return { errors, warnings };
+}
diff --git a/eng/lib/markdown.mjs b/eng/lib/markdown.mjs
new file mode 100644
index 00000000..86189049
--- /dev/null
+++ b/eng/lib/markdown.mjs
@@ -0,0 +1,11 @@
+export function inlineCode(value) {
+  const collapsed = String(value).replace(/\s+/g, " ").trim();
+  const content = collapsed.length > 80 ? `${collapsed.slice(0, 77)}...` : collapsed;
+  const longestBacktickRun = Math.max(0, ...Array.from(content.matchAll(/`+/g), (match) => match[0].length));
+  const fence = "`".repeat(longestBacktickRun + 1);
+  const pad = content.length === 0 || content.startsWith("`") || content.endsWith("`") ? " " : "";
+
+  // These values are rendered verbatim into Markdown bot comments; using a fence
+  // longer than any backtick run in the content prevents closing the code span.
+  return `${fence}${pad}${content}${pad}${fence}`;
+}
diff --git a/eng/lib/markdown.test.mjs b/eng/lib/markdown.test.mjs
new file mode 100644
index 00000000..4b1edb5f
--- /dev/null
+++ b/eng/lib/markdown.test.mjs
@@ -0,0 +1,25 @@
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { inlineCode } from "./markdown.mjs";
+
+test("inlineCode wraps plain values in a single-backtick code span", () => {
+  assert.equal(inlineCode("plain value"), "`plain value`");
+});
+
+test("inlineCode uses a fence longer than the longest backtick run", () => {
+  assert.equal(inlineCode("a `` b"), "```a `` b```");
+});
+
+test("inlineCode pads values starting or ending with a backtick", () => {
+  assert.equal(inlineCode("`leading"), "`` `leading ``");
+  assert.equal(inlineCode("trailing`"), "`` trailing` ``");
+});
+
+test("inlineCode collapses whitespace and pads empty values", () => {
+  assert.equal(inlineCode("a\n\t b"), "`a b`");
+  assert.equal(inlineCode(" \n\t "), "`  `");
+});
+
+test("inlineCode truncates values to 80 characters", () => {
+  assert.equal(inlineCode("x".repeat(81)), `\`${"x".repeat(77)}...\``);
+});
diff --git a/eng/validate-plugins.mjs b/eng/validate-plugins.mjs
index 7161df06..e8cbf1a6 100755
--- a/eng/validate-plugins.mjs
+++ b/eng/validate-plugins.mjs
@@ -4,6 +4,7 @@ import fs from "fs";
 import path from "path";
 import { ROOT_FOLDER } from "./constants.mjs";
 import { readExternalPlugins } from "./external-plugin-validation.mjs";
+import { validateLicenseField } from "./lib/license.mjs";
 
 const PLUGINS_DIR = path.join(ROOT_FOLDER, "plugins");
 const EXTENSIONS_DIR = path.join(ROOT_FOLDER, "extensions");
@@ -239,6 +240,12 @@ function validatePlugin(folderName) {
   const keywordsError = validateKeywords(plugin.keywords ?? plugin.tags);
   if (keywordsError) errors.push(keywordsError);
 
+  // Rule 5b: license (shared with external plugins). Non-SPDX is a warning, not an error.
+  const warnings = [];
+  const licenseResult = validateLicenseField(plugin.license, { required: false });
+  errors.push(...licenseResult.errors);
+  warnings.push(...licenseResult.warnings);
+
   // Rule 6: agents, commands, skills paths
   const specErrors = validateSpecPaths(plugin);
   errors.push(...specErrors);
@@ -246,7 +253,7 @@ function validatePlugin(folderName) {
   const extensionRefErrors = validateCuratedPluginExtensionRefs(plugin);
   errors.push(...extensionRefErrors);
 
-  return { errors, plugin: parsedPlugin };
+  return { errors, warnings, plugin: parsedPlugin };
 }
 
 function validateExtensionScreenshotPath(extensionDir, pathValue, fieldName, errors) {
@@ -342,7 +349,7 @@ function validatePlugins() {
   for (const dir of pluginDirs) {
     console.log(`Validating ${dir}...`);
 
-    const { errors, plugin } = validatePlugin(dir);
+    const { errors, warnings, plugin } = validatePlugin(dir);
 
     if (errors.length > 0) {
       console.error(`❌ ${dir}:`);
@@ -352,6 +359,10 @@ function validatePlugins() {
       console.log(`✅ ${dir} is valid`);
     }
 
+    if (warnings?.length > 0) {
+      warnings.forEach((w) => console.warn(`⚠️  ${dir}: ${w}`));
+    }
+
     if (plugin?.name) {
       if (seenNames.has(plugin.name)) {
         console.error(`❌ Duplicate plugin name "${plugin.name}"`);