diff --git a/packages/cli/src/config/config.ts b/packages/cli/src/config/config.ts
index a4b55cac8..ebed6ebbd 100755
--- a/packages/cli/src/config/config.ts
+++ b/packages/cli/src/config/config.ts
@@ -231,6 +231,19 @@ export function resolveJsonSchemaArg(
     );
   }
 
+  // The schema becomes the parameter schema of the synthetic
+  // structured_output tool, and tool-call arguments are object-shaped.
+  // A schema like `{"type":"string"}` would compile fine but be
+  // unsatisfiable as a tool-call argument — fail at parse time so the
+  // user sees the contract violation immediately instead of at runtime.
+  const schemaType = (parsed as { type?: unknown }).type;
+  if (schemaType !== undefined && schemaType !== 'object') {
+    throw new FatalConfigError(
+      `--json-schema top-level type must be "object" (got "${String(schemaType)}"); ` +
+        'wrap your value under an object property if you need a non-object payload.',
+    );
+  }
+
   return parsed as Record<string, unknown>;
 }
 
@@ -540,7 +553,7 @@ export async function parseArguments(): Promise<CliArgs> {
         .option('json-schema', {
           type: 'string',
           description:
-            'JSON Schema that the model\'s final output must conform to ' +
+            "JSON Schema that the model's final output must conform to " +
             '(headless mode only). Accepts a JSON literal or "@path/to/schema.json". ' +
             'Registers a synthetic `structured_output` tool; the session ends on ' +
             'the first valid call.',
diff --git a/packages/cli/src/config/jsonSchemaArg.test.ts b/packages/cli/src/config/jsonSchemaArg.test.ts
index 87b700d55..f507cb34e 100644
--- a/packages/cli/src/config/jsonSchemaArg.test.ts
+++ b/packages/cli/src/config/jsonSchemaArg.test.ts
@@ -42,9 +42,7 @@ describe('resolveJsonSchemaArg', () => {
   });
 
   it('throws on invalid JSON', () => {
-    expect(() => resolveJsonSchemaArg('{not json}')).toThrow(
-      /not valid JSON/,
-    );
+    expect(() => resolveJsonSchemaArg('{not json}')).toThrow(/not valid JSON/);
   });
 
   it('throws when the parsed value is not an object', () => {
@@ -78,4 +76,15 @@ describe('resolveJsonSchemaArg', () => {
     );
     expect(schema).toBeDefined();
   });
+
+  it('rejects schemas whose top-level type is not "object"', () => {
+    // The schema becomes structured_output's parameter schema; tool args
+    // are object-shaped, so non-object roots can never be satisfied.
+    expect(() => resolveJsonSchemaArg('{"type":"string"}')).toThrow(
+      /top-level type must be "object"/,
+    );
+    expect(() => resolveJsonSchemaArg('{"type":"array"}')).toThrow(
+      /top-level type must be "object"/,
+    );
+  });
 });
diff --git a/packages/core/src/utils/schemaValidator.test.ts b/packages/core/src/utils/schemaValidator.test.ts
index 98b05b1ca..9d32ff38a 100644
--- a/packages/core/src/utils/schemaValidator.test.ts
+++ b/packages/core/src/utils/schemaValidator.test.ts
@@ -478,5 +478,28 @@ describe('SchemaValidator', () => {
       expect(SchemaValidator.compileStrict(undefined)).toMatch(/JSON object/);
       expect(SchemaValidator.compileStrict('a string')).toMatch(/JSON object/);
     });
+
+    it('rejects arrays even though typeof === "object"', () => {
+      // Arrays satisfy `typeof === 'object'` but are not valid JSON Schema
+      // root values; the prior guard accepted them and let the misleading
+      // error surface from Ajv much later.
+      expect(SchemaValidator.compileStrict([])).toMatch(/JSON object/);
+      expect(SchemaValidator.compileStrict([{ type: 'string' }])).toMatch(
+        /JSON object/,
+      );
+    });
+
+    it('flags unknown keywords (typos) under strict mode', () => {
+      // The shared SchemaValidator.validate is intentionally lenient
+      // (`strictSchema: false`) so MCP-style custom keywords don't break
+      // runtime validation. compileStrict is the explicit user-supplied
+      // surface and should NOT swallow typos like `propertees`.
+      const err = SchemaValidator.compileStrict({
+        type: 'object',
+        propertees: { foo: { type: 'string' } },
+      });
+      expect(err).not.toBeNull();
+      expect(err).toMatch(/propert/i);
+    });
   });
 });
diff --git a/packages/core/src/utils/schemaValidator.ts b/packages/core/src/utils/schemaValidator.ts
index 55395d66b..f451e238c 100644
--- a/packages/core/src/utils/schemaValidator.ts
+++ b/packages/core/src/utils/schemaValidator.ts
@@ -72,12 +72,24 @@ export class SchemaValidator {
    * to surface invalid schemas instead of letting them no-op at runtime.
    */
   static compileStrict(schema: unknown): string | null {
-    if (!schema || typeof schema !== 'object') {
+    if (!schema || typeof schema !== 'object' || Array.isArray(schema)) {
       return 'schema must be a JSON object';
     }
-    const validator = getValidator(schema as AnySchema);
+    // Use a dedicated strict-mode Ajv so typos like `propertees` raise
+    // instead of being silently ignored. The shared ajvDefault/ajv2020
+    // instances run with `strictSchema: false` so unknown MCP keywords
+    // don't break runtime validation — that leniency is wrong for
+    // explicit user-supplied schemas where `compileStrict` is exactly
+    // the surface meant to surface mistakes.
+    const isDraft2020 =
+      '$schema' in schema &&
+      (schema as { $schema?: string }).$schema === DRAFT_2020_12_SCHEMA;
+    const strictAjv: Ajv = isDraft2020
+      ? new Ajv2020Class({ strict: true })
+      : new AjvClass({ strict: true });
+    addFormatsFunc(strictAjv);
     try {
-      validator.compile(schema as AnySchema);
+      strictAjv.compile(schema as AnySchema);
       return null;
     } catch (error) {
       return error instanceof Error ? error.message : String(error);