diff --git a/README.md b/README.md
index edd62dd..1a9ba9b 100644
--- a/README.md
+++ b/README.md
@@ -268,6 +268,8 @@ Live cost $ and context % are tracked per run and surfaced in the Tiers view. Ov
 
 Invalid values and unknown keys produce a startup warning naming the file, the field, and the bad value — never a silent no-op. Absent files are normal.
 
+Agent frontmatter `thinkingLevel` is validated with the same strictness (#90): an invalid value (typo'd level, wrong type) fails that agent's load with an actionable warning naming the file, the bad value, and the valid levels (`off | minimal | low | medium | high | xhigh | max`) — never a silent no-op. A YAML-empty value (`thinkingLevel:` with nothing after it) counts as absent.
+
 ### Operational runtime
 
 `src/runtime/` — the async/scheduling spine:
diff --git a/src/registry/frontmatter.ts b/src/registry/frontmatter.ts
index 8d6c681..da73d84 100644
--- a/src/registry/frontmatter.ts
+++ b/src/registry/frontmatter.ts
@@ -5,6 +5,14 @@ import { basename, extname } from "node:path";
 export type AgentSource = "builtin" | "project" | "global";
 export type ThinkingLevel = "off" | "minimal" | "low" | "medium" | "high" | "xhigh" | "max";
 
+export const THINKING_LEVELS: readonly ThinkingLevel[] = ["off", "minimal", "low", "medium", "high", "xhigh", "max"];
+
+/** Type guard for the closed ThinkingLevel enum. #90: canonical home is HERE, next to
+ *  the type; fleet-settings imports + re-exports it for its settings-field guard. */
+export function isThinkingLevel(v: unknown): v is ThinkingLevel {
+  return typeof v === "string" && (THINKING_LEVELS as readonly string[]).includes(v);
+}
+
 export interface AgentDef {
   name: string;
   description: string;
@@ -70,12 +78,27 @@ export function parseAgentFile(content: string, filePath: string, source: AgentS
   const backend = rawBackend as "pi" | "claude";
   const sessionKey = typeof raw.sessionKey === "string" && raw.sessionKey.trim() ? raw.sessionKey.trim() : name;
 
+  // #90: thinkingLevel is a closed enum — same value class as backend, same contract:
+  // trim-then-validate, an invalid value throws FrontmatterError (discovery catches →
+  // warning + skip), never a silent no-op. `null` (YAML empty value) counts as absent.
+  const rawThinking: unknown = raw.thinkingLevel;
+  let thinkingLevel: ThinkingLevel | undefined;
+  if (rawThinking !== undefined && rawThinking !== null) {
+    const candidate = typeof rawThinking === "string" ? rawThinking.trim() : rawThinking;
+    if (!isThinkingLevel(candidate)) {
+      throw new FrontmatterError(
+        `${filePath}: invalid thinkingLevel ${JSON.stringify(rawThinking)} (must be one of ${THINKING_LEVELS.join("|")})`,
+      );
+    }
+    thinkingLevel = candidate;
+  }
+
   return {
     name,
     description,
     model: typeof raw.model === "string" ? raw.model : undefined,
     tier: typeof raw.tier === "string" ? raw.tier : undefined,
-    thinkingLevel: typeof raw.thinkingLevel === "string" ? (raw.thinkingLevel as ThinkingLevel) : undefined,
+    thinkingLevel,
     tools: strList(raw.tools),
     skills: strList(raw.skills),
     rolePrompt: body,
diff --git a/src/settings/fleet-settings.ts b/src/settings/fleet-settings.ts
index 10a1267..4bbe77b 100644
--- a/src/settings/fleet-settings.ts
+++ b/src/settings/fleet-settings.ts
@@ -14,13 +14,11 @@
 // - The schema is intentionally small and additive; unknown keys warn so typos
 //   surface instead of no-oping.
 import { readFileSync } from "node:fs";
-import type { ThinkingLevel } from "../registry/frontmatter.ts";
+import { THINKING_LEVELS, isThinkingLevel, type ThinkingLevel } from "../registry/frontmatter.ts";
 
-const THINKING_LEVELS: readonly ThinkingLevel[] = ["off", "minimal", "low", "medium", "high", "xhigh", "max"];
-
-export function isThinkingLevel(v: unknown): v is ThinkingLevel {
-  return typeof v === "string" && (THINKING_LEVELS as readonly string[]).includes(v);
-}
+// #90: isThinkingLevel + THINKING_LEVELS moved to registry/frontmatter.ts (next to the
+// canonical type). Re-exported here so existing importers keep working.
+export { isThinkingLevel, THINKING_LEVELS };
 
 /** Fleet-wide defaults. Intentionally additive — new fields land here. */
 export interface FleetSettings {
diff --git a/test/discovery.test.mts b/test/discovery.test.mts
index 2bec76c..6fed244 100644
--- a/test/discovery.test.mts
+++ b/test/discovery.test.mts
@@ -58,4 +58,17 @@ test("missing dirs are tolerated (no throw)", () => {
   const r = discoverAgents({ projectDir: "/nonexistent", globalDir: "/nonexistent", builtinDir: null });
   strictEqual(r.agents.size, 0);
   strictEqual(r.errors.length, 0);
-});
\ No newline at end of file
+});
+test("#90: invalid thinkingLevel file skipped + actionable warning, siblings load", () => {
+  const proj = mkdtempSync(join(tmpdir(), "proj5-"));
+  writeFileSync(join(proj, "bad.md"), "---\nname: bad\ndescription: d\nthinkingLevel: ultra\n---\nbody\n");
+  writeFileSync(join(proj, "good.md"), agentFile("good"));
+  const r = discoverAgents({ projectDir: proj, globalDir: null, builtinDir: null });
+  ok(
+    r.warnings.some((w) => w.includes("bad.md") && w.includes("invalid thinkingLevel")),
+    "actionable warning surfaced",
+  );
+  ok(!r.agents.has("bad"), "invalid agent skipped");
+  ok(r.agents.has("good"), "sibling loaded");
+  rmSync(proj, { recursive: true, force: true });
+});
diff --git a/test/frontmatter.test.mts b/test/frontmatter.test.mts
index 211c834..11775dc 100644
--- a/test/frontmatter.test.mts
+++ b/test/frontmatter.test.mts
@@ -1,6 +1,6 @@
 // test/frontmatter.test.mts
 import { test } from "node:test";
-import { strictEqual, deepStrictEqual, throws } from "node:assert";
+import { strictEqual, deepStrictEqual, throws, ok } from "node:assert";
 import { parseAgentFile } from "../src/registry/frontmatter.ts";
 
 const BASE = `---
@@ -97,4 +97,59 @@ test("userMemory: true parses true", () => {
 test("userMemory: false parses false", () => {
   const a = parseAgentFile("---\nname: a\ndescription: d\nuserMemory: false\n---\nrole", "/tmp/agent.md", "builtin");
   strictEqual(a.userMemory, false);
-});
\ No newline at end of file
+});
+// --- #90: thinkingLevel is a closed enum — invalid values throw (like backend),
+// never silently no-op. Same value class, same contract. ---
+
+test("#90: invalid thinkingLevel string throws FrontmatterError with actionable message", () => {
+  const bad = "---\nname: g\ndescription: d\nthinkingLevel: ultra\n---\nbody\n";
+  throws(
+    () => parseAgentFile(bad, "/x/g.md", "project"),
+    (e: unknown) => {
+      ok(e instanceof Error && e.name === "FrontmatterError", `FrontmatterError, got ${String(e)}`);
+      ok(e instanceof Error && e.message.includes("invalid thinkingLevel"), `names the field: ${e.message}`);
+      ok(e instanceof Error && e.message.includes("ultra"), `names the bad value: ${e.message}`);
+      ok(e instanceof Error && e.message.includes("off|minimal|low|medium|high|xhigh|max"), `lists valid levels: ${e.message}`);
+      return true;
+    },
+  );
+});
+
+test("#90: non-string thinkingLevel throws FrontmatterError", () => {
+  const bad = "---\nname: g\ndescription: d\nthinkingLevel: 5\n---\nbody\n";
+  throws(() => parseAgentFile(bad, "/x/g.md", "project"), { name: "FrontmatterError" });
+});
+
+test("#90: thinkingLevel absent → undefined (back-compat)", () => {
+  const a = parseAgentFile("---\nname: a\ndescription: d\n---\nrole", "/tmp/agent.md", "builtin");
+  strictEqual(a.thinkingLevel, undefined);
+});
+
+test("#90: thinkingLevel null (YAML empty value) → treated as absent", () => {
+  const a = parseAgentFile("---\nname: a\ndescription: d\nthinkingLevel:\n---\nrole", "/tmp/agent.md", "builtin");
+  strictEqual(a.thinkingLevel, undefined);
+});
+
+test("#90: all seven thinking levels parse", () => {
+  for (const level of ["off", "minimal", "low", "medium", "high", "xhigh", "max"] as const) {
+    const md = `---\nname: a\ndescription: d\nthinkingLevel: ${level}\n---\nrole\n`;
+    strictEqual(parseAgentFile(md, "/tmp/agent.md", "builtin").thinkingLevel, level, `level ${level}`);
+  }
+});
+
+test("#90: quoted padded value trims (symmetric with backend, review NIT 1)", () => {
+  const a = parseAgentFile("---\nname: a\ndescription: d\nthinkingLevel: \" low\"\n---\nrole", "/tmp/agent.md", "builtin");
+  strictEqual(a.thinkingLevel, "low");
+});
+
+test("#90: array value throws with the value rendered (review NIT 2)", () => {
+  const bad = "---\nname: g\ndescription: d\nthinkingLevel: [low]\n---\nbody\n";
+  throws(
+    () => parseAgentFile(bad, "/x/g.md", "project"),
+    (e: unknown) => {
+      ok(e instanceof Error && e.name === "FrontmatterError", `FrontmatterError, got ${String(e)}`);
+      ok(e instanceof Error && e.message.includes('["low"]'), `renders the array: ${e.message}`);
+      return true;
+    },
+  );
+});