diff --git a/packages/core/src/agents/agentLoader.test.ts b/packages/core/src/agents/agentLoader.test.ts
index 917628f7e7..661f08d76d 100644
--- a/packages/core/src/agents/agentLoader.test.ts
+++ b/packages/core/src/agents/agentLoader.test.ts
@@ -242,6 +242,99 @@ Body`);
         /Name must be a valid slug/,
       );
     });
+
+    describe('error formatting and kind inference', () => {
+      it('should only show local agent errors when kind is inferred as local (via kind field)', async () => {
+        const filePath = await writeAgentMarkdown(`---
+kind: local
+name: invalid-local
+# missing description
+---
+Body`);
+        const error = await parseAgentMarkdown(filePath).catch((e) => e);
+        expect(error).toBeInstanceOf(AgentLoadError);
+        expect(error.message).toContain('Validation failed');
+        expect(error.message).toContain('description: Required');
+        expect(error.message).not.toContain('Remote Agent');
+      });
+
+      it('should only show local agent errors when kind is inferred as local (via local-specific keys)', async () => {
+        const filePath = await writeAgentMarkdown(`---
+name: invalid-local
+# missing description
+tools:
+  - run_shell_command
+---
+Body`);
+        const error = await parseAgentMarkdown(filePath).catch((e) => e);
+        expect(error).toBeInstanceOf(AgentLoadError);
+        expect(error.message).toContain('Validation failed');
+        expect(error.message).toContain('description: Required');
+        expect(error.message).not.toContain('Remote Agent');
+      });
+
+      it('should only show remote agent errors when kind is inferred as remote (via kind field)', async () => {
+        const filePath = await writeAgentMarkdown(`---
+kind: remote
+name: invalid-remote
+# missing agent_card_url
+---
+Body`);
+        const error = await parseAgentMarkdown(filePath).catch((e) => e);
+        expect(error).toBeInstanceOf(AgentLoadError);
+        expect(error.message).toContain('Validation failed');
+        expect(error.message).toContain('agent_card_url: Required');
+        expect(error.message).not.toContain('Local Agent');
+      });
+
+      it('should only show remote agent errors when kind is inferred as remote (via remote-specific keys)', async () => {
+        const filePath = await writeAgentMarkdown(`---
+name: invalid-remote
+auth:
+  type: apiKey
+  key: my_key
+# missing agent_card_url
+---
+Body`);
+        const error = await parseAgentMarkdown(filePath).catch((e) => e);
+        expect(error).toBeInstanceOf(AgentLoadError);
+        expect(error.message).toContain('Validation failed');
+        expect(error.message).toContain('agent_card_url: Required');
+        expect(error.message).not.toContain('Local Agent');
+      });
+
+      it('should show errors for both types when kind cannot be inferred', async () => {
+        const filePath = await writeAgentMarkdown(`---
+name: invalid-unknown
+# missing description and missing agent_card_url, no specific keys
+---
+Body`);
+        const error = await parseAgentMarkdown(filePath).catch((e) => e);
+        expect(error).toBeInstanceOf(AgentLoadError);
+        expect(error.message).toContain('Validation failed');
+        expect(error.message).toContain('(Local Agent)');
+        expect(error.message).toContain('(Remote Agent)');
+        expect(error.message).toContain('description: Required');
+        expect(error.message).toContain('agent_card_url: Required');
+      });
+
+      it('should format errors without a stray colon when the path is empty (e.g. strict object with unknown keys)', async () => {
+        const filePath = await writeAgentMarkdown(`---
+kind: local
+name: my-agent
+description: test
+unknown_field: true
+---
+Body`);
+        const error = await parseAgentMarkdown(filePath).catch((e) => e);
+        expect(error).toBeInstanceOf(AgentLoadError);
+        expect(error.message).toContain(
+          "Unrecognized key(s) in object: 'unknown_field'",
+        );
+        expect(error.message).not.toContain(': Unrecognized key(s)');
+        expect(error.message).not.toContain('Required');
+      });
+    });
   });
 
   describe('markdownToAgentDefinition', () => {
@@ -744,5 +837,24 @@ auth:
         },
       });
     });
+
+    it('should throw an error for an unknown auth type in markdownToAgentDefinition', () => {
+      const markdown = {
+        kind: 'remote' as const,
+        name: 'unknown-auth-agent',
+        agent_card_url: 'https://example.com/card',
+        auth: {
+          type: 'apiKey' as const,
+          key: 'some-key',
+        },
+      };
+
+      // Mutate the object at runtime to bypass TypeScript compile-time checks cleanly
+      Object.assign(markdown.auth, { type: 'some-unknown-type' });
+
+      expect(() => markdownToAgentDefinition(markdown)).toThrow(
+        /Unknown auth type: some-unknown-type/,
+      );
+    });
   });
 });
diff --git a/packages/core/src/agents/agentLoader.ts b/packages/core/src/agents/agentLoader.ts
index 1b9eb1ea4e..eac0985f2d 100644
--- a/packages/core/src/agents/agentLoader.ts
+++ b/packages/core/src/agents/agentLoader.ts
@@ -21,79 +21,6 @@ import { isValidToolName } from '../tools/tool-names.js';
 import { FRONTMATTER_REGEX } from '../skills/skillLoader.js';
 import { getErrorMessage } from '../utils/errors.js';
 
-/**
- * DTO for Markdown parsing - represents the structure from frontmatter.
- */
-interface FrontmatterBaseAgentDefinition {
-  name: string;
-  display_name?: string;
-}
-
-interface FrontmatterMCPServerConfig {
-  command?: string;
-  args?: string[];
-  env?: Record<string, string>;
-  cwd?: string;
-  url?: string;
-  http_url?: string;
-  headers?: Record<string, string>;
-  tcp?: string;
-  type?: 'sse' | 'http';
-  timeout?: number;
-  trust?: boolean;
-  description?: string;
-  include_tools?: string[];
-  exclude_tools?: string[];
-}
-
-interface FrontmatterLocalAgentDefinition
-  extends FrontmatterBaseAgentDefinition {
-  kind: 'local';
-  description: string;
-  tools?: string[];
-  mcp_servers?: Record<string, FrontmatterMCPServerConfig>;
-  system_prompt: string;
-  model?: string;
-  temperature?: number;
-  max_turns?: number;
-  timeout_mins?: number;
-}
-
-/**
- * Authentication configuration for remote agents in frontmatter format.
- */
-interface FrontmatterAuthConfig {
-  type: 'apiKey' | 'http' | 'google-credentials' | 'oauth';
-  // API Key
-  key?: string;
-  name?: string;
-  // HTTP
-  scheme?: string;
-  token?: string;
-  username?: string;
-  password?: string;
-  value?: string;
-  // Google Credentials
-  scopes?: string[];
-  // OAuth2
-  client_id?: string;
-  client_secret?: string;
-  authorization_url?: string;
-  token_url?: string;
-}
-
-interface FrontmatterRemoteAgentDefinition
-  extends FrontmatterBaseAgentDefinition {
-  kind: 'remote';
-  description?: string;
-  agent_card_url: string;
-  auth?: FrontmatterAuthConfig;
-}
-
-type FrontmatterAgentDefinition =
-  | FrontmatterLocalAgentDefinition
-  | FrontmatterRemoteAgentDefinition;
-
 /**
  * Error thrown when an agent definition is invalid or cannot be loaded.
  */
@@ -159,15 +86,13 @@ const localAgentSchema = z
   })
   .strict();
 
-/**
- * Base fields shared by all auth configs.
- */
+type FrontmatterLocalAgentDefinition = z.infer<typeof localAgentSchema> & {
+  system_prompt: string;
+};
+
+// Base fields shared by all auth configs.
 const baseAuthFields = {};
 
-/**
- * API Key auth schema.
- * Supports sending key in header, query parameter, or cookie.
- */
 const apiKeyAuthSchema = z.object({
   ...baseAuthFields,
   type: z.literal('apiKey'),
@@ -175,11 +100,6 @@ const apiKeyAuthSchema = z.object({
   name: z.string().optional(),
 });
 
-/**
- * HTTP auth schema (Bearer or Basic).
- * Note: Validation for scheme-specific fields is applied in authConfigSchema
- * since discriminatedUnion doesn't support refined schemas directly.
- */
 const httpAuthSchema = z.object({
   ...baseAuthFields,
   type: z.literal('http'),
@@ -190,19 +110,12 @@ const httpAuthSchema = z.object({
   value: z.string().min(1).optional(),
 });
 
-/**
- * Google Credentials auth schema.
- */
 const googleCredentialsAuthSchema = z.object({
   ...baseAuthFields,
   type: z.literal('google-credentials'),
   scopes: z.array(z.string()).optional(),
 });
 
-/**
- * OAuth2 auth schema.
- * authorization_url and token_url can be discovered from the agent card if omitted.
- */
 const oauth2AuthSchema = z.object({
   ...baseAuthFields,
   type: z.literal('oauth'),
@@ -222,18 +135,16 @@ const authConfigSchema = z
   ])
   .superRefine((data, ctx) => {
     if (data.type === 'http') {
-      if (data.value) {
-        // Raw mode - only scheme and value are needed
-        return;
-      }
-      if (data.scheme === 'Bearer' && !data.token) {
-        ctx.addIssue({
-          code: z.ZodIssueCode.custom,
-          message: 'Bearer scheme requires "token"',
-          path: ['token'],
-        });
-      }
-      if (data.scheme === 'Basic') {
+      if (data.value) return;
+      if (data.scheme === 'Bearer') {
+        if (!data.token) {
+          ctx.addIssue({
+            code: z.ZodIssueCode.custom,
+            message: 'Bearer scheme requires "token"',
+            path: ['token'],
+          });
+        }
+      } else if (data.scheme === 'Basic') {
         if (!data.username) {
           ctx.addIssue({
             code: z.ZodIssueCode.custom,
@@ -248,10 +159,18 @@ const authConfigSchema = z
             path: ['password'],
           });
         }
+      } else {
+        ctx.addIssue({
+          code: z.ZodIssueCode.custom,
+          message: `HTTP scheme "${data.scheme}" requires "value"`,
+          path: ['value'],
+        });
       }
     }
   });
 
+type FrontmatterAuthConfig = z.infer<typeof authConfigSchema>;
+
 const remoteAgentSchema = z
   .object({
     kind: z.literal('remote').optional().default('remote'),
@@ -263,8 +182,12 @@ const remoteAgentSchema = z
   })
   .strict();
 
-// Use a Zod union to automatically discriminate between local and remote
-// agent types.
+type FrontmatterRemoteAgentDefinition = z.infer<typeof remoteAgentSchema>;
+
+type FrontmatterAgentDefinition =
+  | FrontmatterLocalAgentDefinition
+  | FrontmatterRemoteAgentDefinition;
+
 const agentUnionOptions = [
   { schema: localAgentSchema, label: 'Local Agent' },
   { schema: remoteAgentSchema, label: 'Remote Agent' },
@@ -277,23 +200,62 @@ const markdownFrontmatterSchema = z.union([
   agentUnionOptions[1].schema,
 ]);
 
-function formatZodError(error: z.ZodError, context: string): string {
+function guessIntendedKind(rawInput: unknown): 'local' | 'remote' | undefined {
+  if (typeof rawInput !== 'object' || rawInput === null) return undefined;
+  const input = rawInput as Partial<FrontmatterLocalAgentDefinition> &
+    Partial<FrontmatterRemoteAgentDefinition>;
+
+  if (input.kind === 'local') return 'local';
+  if (input.kind === 'remote') return 'remote';
+
+  const hasLocalKeys =
+    'tools' in input ||
+    'mcp_servers' in input ||
+    'model' in input ||
+    'temperature' in input ||
+    'max_turns' in input ||
+    'timeout_mins' in input;
+  const hasRemoteKeys = 'agent_card_url' in input || 'auth' in input;
+
+  if (hasLocalKeys && !hasRemoteKeys) return 'local';
+  if (hasRemoteKeys && !hasLocalKeys) return 'remote';
+
+  return undefined;
+}
+
+function formatZodError(
+  error: z.ZodError,
+  context: string,
+  rawInput?: unknown,
+): string {
+  const intendedKind = rawInput ? guessIntendedKind(rawInput) : undefined;
+
   const issues = error.issues
     .map((i) => {
-      // Handle union errors specifically to give better context
       if (i.code === z.ZodIssueCode.invalid_union) {
         return i.unionErrors
           .map((unionError, index) => {
             const label =
               agentUnionOptions[index]?.label ?? `Agent type #${index + 1}`;
+
+            if (intendedKind === 'local' && label === 'Remote Agent')
+              return null;
+            if (intendedKind === 'remote' && label === 'Local Agent')
+              return null;
+
             const unionIssues = unionError.issues
-              .map((u) => `${u.path.join('.')}: ${u.message}`)
+              .map((u) => {
+                const pathStr = u.path.join('.');
+                return pathStr ? `${pathStr}: ${u.message}` : u.message;
+              })
               .join(', ');
             return `(${label}) ${unionIssues}`;
           })
+          .filter(Boolean)
           .join('\n');
       }
-      return `${i.path.join('.')}: ${i.message}`;
+      const pathStr = i.path.join('.');
+      return pathStr ? `${pathStr}: ${i.message}` : i.message;
     })
     .join('\n');
   return `${context}:\n${issues}`;
@@ -343,8 +305,7 @@ export async function parseAgentMarkdown(
   } catch (error) {
     throw new AgentLoadError(
       filePath,
-      // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-      `YAML frontmatter parsing failed: ${(error as Error).message}`,
+      `YAML frontmatter parsing failed: ${getErrorMessage(error)}`,
     );
   }
 
@@ -368,7 +329,7 @@ export async function parseAgentMarkdown(
   if (!result.success) {
     throw new AgentLoadError(
       filePath,
-      `Validation failed: ${formatZodError(result.error, 'Agent Definition')}`,
+      `Validation failed: ${formatZodError(result.error, 'Agent Definition', rawFrontmatter)}`,
     );
   }
 
@@ -383,17 +344,14 @@ export async function parseAgentMarkdown(
     ];
   }
 
-  // Local agent validation
-  // Validate tools
-
   // Construct the local agent definition
-  const agentDef: FrontmatterLocalAgentDefinition = {
-    ...frontmatter,
-    kind: 'local',
-    system_prompt: body.trim(),
-  };
-
-  return [agentDef];
+  return [
+    {
+      ...frontmatter,
+      kind: 'local',
+      system_prompt: body.trim(),
+    },
+  ];
 }
 
 /**
@@ -403,15 +361,9 @@ export async function parseAgentMarkdown(
 function convertFrontmatterAuthToConfig(
   frontmatter: FrontmatterAuthConfig,
 ): A2AAuthConfig {
-  const base = {};
-
   switch (frontmatter.type) {
     case 'apiKey':
-      if (!frontmatter.key) {
-        throw new Error('Internal error: API key missing after validation.');
-      }
       return {
-        ...base,
         type: 'apiKey',
         key: frontmatter.key,
         name: frontmatter.name,
@@ -419,20 +371,13 @@ function convertFrontmatterAuthToConfig(
 
     case 'google-credentials':
       return {
-        ...base,
         type: 'google-credentials',
         scopes: frontmatter.scopes,
       };
 
-    case 'http': {
-      if (!frontmatter.scheme) {
-        throw new Error(
-          'Internal error: HTTP scheme missing after validation.',
-        );
-      }
+    case 'http':
       if (frontmatter.value) {
         return {
-          ...base,
           type: 'http',
           scheme: frontmatter.scheme,
           value: frontmatter.value,
@@ -440,40 +385,29 @@ function convertFrontmatterAuthToConfig(
       }
       switch (frontmatter.scheme) {
         case 'Bearer':
-          if (!frontmatter.token) {
-            throw new Error(
-              'Internal error: Bearer token missing after validation.',
-            );
-          }
+          // Token is required by schema validation
           return {
-            ...base,
             type: 'http',
             scheme: 'Bearer',
-            token: frontmatter.token,
+
+            token: frontmatter.token!,
           };
         case 'Basic':
-          if (!frontmatter.username || !frontmatter.password) {
-            throw new Error(
-              'Internal error: Basic auth credentials missing after validation.',
-            );
-          }
+          // Username/password are required by schema validation
           return {
-            ...base,
             type: 'http',
             scheme: 'Basic',
-            username: frontmatter.username,
-            password: frontmatter.password,
+
+            username: frontmatter.username!,
+
+            password: frontmatter.password!,
           };
-        default: {
-          // Other IANA schemes without a value should not reach here after validation
+        default:
           throw new Error(`Unknown HTTP scheme: ${frontmatter.scheme}`);
-        }
       }
-    }
 
     case 'oauth':
       return {
-        ...base,
         type: 'oauth2',
         client_id: frontmatter.client_id,
         client_secret: frontmatter.client_secret,
@@ -483,8 +417,12 @@ function convertFrontmatterAuthToConfig(
       };
 
     default: {
-      const exhaustive: never = frontmatter.type;
-      throw new Error(`Unknown auth type: ${exhaustive}`);
+      const exhaustive: never = frontmatter;
+      const raw: unknown = exhaustive;
+      if (typeof raw === 'object' && raw !== null && 'type' in raw) {
+        throw new Error(`Unknown auth type: ${String(raw['type'])}`);
+      }
+      throw new Error('Unknown auth type');
     }
   }
 }
@@ -533,7 +471,7 @@ export function markdownToAgentDefinition(
   const modelName = markdown.model || 'inherit';
 
   const mcpServers: Record<string, MCPServerConfig> = {};
-  if (markdown.kind === 'local' && markdown.mcp_servers) {
+  if (markdown.mcp_servers) {
     for (const [name, config] of Object.entries(markdown.mcp_servers)) {
       mcpServers[name] = new MCPServerConfig(
         config.command,
@@ -606,15 +544,13 @@ export async function loadAgentsFromDirectory(
     dirEntries = await fs.readdir(dir, { withFileTypes: true });
   } catch (error) {
     // If directory doesn't exist, just return empty
-    // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-    if ((error as NodeJS.ErrnoException).code === 'ENOENT') {
+    if (error instanceof Error && 'code' in error && error.code === 'ENOENT') {
       return result;
     }
     result.errors.push(
       new AgentLoadError(
         dir,
-        // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-        `Could not list directory: ${(error as Error).message}`,
+        `Could not list directory: ${getErrorMessage(error)}`,
       ),
     );
     return result;
@@ -644,8 +580,7 @@ export async function loadAgentsFromDirectory(
         result.errors.push(
           new AgentLoadError(
             filePath,
-            // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-            `Unexpected error: ${(error as Error).message}`,
+            `Unexpected error: ${getErrorMessage(error)}`,
           ),
         );
       }