diff --git a/.changeset/oauth-profile-error-codes.md b/.changeset/oauth-profile-error-codes.md
new file mode 100644
index 0000000..b501269
--- /dev/null
+++ b/.changeset/oauth-profile-error-codes.md
@@ -0,0 +1,15 @@
+---
+'seamless-auth-api': minor
+---
+
+Surface actionable OAuth callback failure codes.
+
+The OAuth callback previously collapsed every profile failure into a generic
+`400 { error: 'OAuth login failed' }`, so a user whose provider returned no email
+(the most common case, for example a GitHub account with no public email) had no
+way to know what to fix. The callback now returns a stable machine-readable `code`
+alongside the existing `error` string for the curated, user-actionable cases:
+`oauth_missing_email`, `oauth_email_not_verified`, and `oauth_missing_subject`.
+Unexpected internal failures still return the generic message with no detail, and
+the audit event records the specific reason instead of the blanket
+`callback_failed` for the known cases.
diff --git a/src/controllers/oauth.ts b/src/controllers/oauth.ts
index cdec4de..e943ab6 100644
--- a/src/controllers/oauth.ts
+++ b/src/controllers/oauth.ts
@@ -18,6 +18,7 @@ import {
   fetchOAuthProfile,
   getEnabledOAuthProviders,
   getOAuthProvider,
+  OAuthProfileError,
   resolveOAuthRedirectUri,
   resolveOAuthUser,
   serializeOAuthProvider,
@@ -157,6 +158,20 @@ export async function finishOAuthLogin(req: Request, res: Response) {
     });
   } catch (error) {
     logger.error(`OAuth callback failed for provider ${provider.id}: ${error}`);
+
+    if (error instanceof OAuthProfileError) {
+      await AuthEventService.log({
+        type: 'oauth_login_failed',
+        req,
+        metadata: { providerId: provider.id, reason: error.code },
+      });
+
+      return res.status(400).json({
+        error: error.message,
+        code: error.code,
+      });
+    }
+
     await AuthEventService.log({
       type: 'oauth_login_failed',
       req,
diff --git a/src/routes/oauth.routes.ts b/src/routes/oauth.routes.ts
index a10bb8f..9d35f58 100644
--- a/src/routes/oauth.routes.ts
+++ b/src/routes/oauth.routes.ts
@@ -14,6 +14,7 @@ import {
   StartOAuthLoginRequestSchema,
 } from '../schemas/oauth.requests.js';
 import {
+  OAuthLoginErrorResponseSchema,
   OAuthLoginSuccessResponseSchema,
   OAuthProvidersResponseSchema,
   StartOAuthLoginResponseSchema,
@@ -65,7 +66,7 @@ oauthRouter.post(
       body: FinishOAuthLoginRequestSchema,
       response: {
         200: OAuthLoginSuccessResponseSchema,
-        400: InternalErrorSchema,
+        400: OAuthLoginErrorResponseSchema,
         403: InternalErrorSchema,
         404: InternalErrorSchema,
       },
diff --git a/src/schemas/oauth.responses.ts b/src/schemas/oauth.responses.ts
index 022e82c..e4085a3 100644
--- a/src/schemas/oauth.responses.ts
+++ b/src/schemas/oauth.responses.ts
@@ -27,3 +27,11 @@ export const StartOAuthLoginResponseSchema = z.object({
 export const OAuthLoginSuccessResponseSchema = RefreshSuccessResponseSchema.omit({
   sessionId: true,
 });
+
+export const OAuthLoginErrorResponseSchema = z.object({
+  message: z.string().optional(),
+  error: z.string(),
+  code: z
+    .enum(['oauth_missing_email', 'oauth_email_not_verified', 'oauth_missing_subject'])
+    .optional(),
+});
diff --git a/src/services/oauthService.ts b/src/services/oauthService.ts
index 14cb6ea..c4dace6 100644
--- a/src/services/oauthService.ts
+++ b/src/services/oauthService.ts
@@ -36,6 +36,23 @@ export type OAuthProfile = {
   raw: Record<string, unknown>;
 };
 
+export type OAuthProfileErrorCode =
+  | 'oauth_missing_subject'
+  | 'oauth_missing_email'
+  | 'oauth_email_not_verified';
+
+// Curated, user-actionable profile failures. The controller forwards the code
+// to the caller; every other failure stays a bare Error and returns generic.
+export class OAuthProfileError extends Error {
+  readonly code: OAuthProfileErrorCode;
+
+  constructor(code: OAuthProfileErrorCode, message: string) {
+    super(message);
+    this.name = 'OAuthProfileError';
+    this.code = code;
+  }
+}
+
 function stateSecret() {
   const explicit = process.env.OAUTH_STATE_SECRET?.trim();
   if (explicit) return explicit;
@@ -379,11 +396,17 @@ export async function fetchOAuthProfile(provider: OAuthProviderConfig, accessTok
   const name = getJsonPathValue(raw, provider.nameJsonPath);
 
   if (typeof subject !== 'string' && typeof subject !== 'number') {
-    throw new Error('OAuth profile did not include a provider subject');
+    throw new OAuthProfileError(
+      'oauth_missing_subject',
+      'OAuth profile did not include a provider subject',
+    );
   }
 
   if (!email) {
-    throw new Error('OAuth profile did not include an email address');
+    throw new OAuthProfileError(
+      'oauth_missing_email',
+      'OAuth profile did not include an email address',
+    );
   }
 
   const emailVerified =
@@ -394,7 +417,7 @@ export async function fetchOAuthProfile(provider: OAuthProviderConfig, accessTok
         : undefined;
 
   if (emailVerified === false || (provider.requireEmailVerified && emailVerified !== true)) {
-    throw new Error('OAuth profile email is not verified');
+    throw new OAuthProfileError('oauth_email_not_verified', 'OAuth profile email is not verified');
   }
 
   return {
diff --git a/tests/integration/oauth/oauth.spec.ts b/tests/integration/oauth/oauth.spec.ts
index 782a0bf..ed5a909 100644
--- a/tests/integration/oauth/oauth.spec.ts
+++ b/tests/integration/oauth/oauth.spec.ts
@@ -302,4 +302,80 @@ describe('OAuth routes', () => {
       }),
     );
   });
+
+  const callbackWithProfile = async (profile: Record<string, unknown>) => {
+    const start = await request(app).post('/oauth/google/start').send({
+      redirectUri: 'http://localhost:5174/oauth/callback',
+    });
+
+    const fetchMock = vi.fn();
+    vi.stubGlobal('fetch', fetchMock);
+    fetchMock
+      .mockResolvedValueOnce({
+        ok: true,
+        json: async () => ({ access_token: 'provider-token' }),
+      })
+      .mockResolvedValueOnce({
+        ok: true,
+        json: async () => profile,
+      });
+
+    return request(app).post('/oauth/google/callback').send({
+      code: 'oauth-code',
+      state: start.body.state,
+    });
+  };
+
+  it('surfaces oauth_missing_email when the profile has no email', async () => {
+    const res = await callbackWithProfile({ sub: 'provider-user', email_verified: true });
+
+    expect(res.status).toBe(400);
+    expect(res.body).toEqual({
+      error: 'OAuth profile did not include an email address',
+      code: 'oauth_missing_email',
+    });
+    expect(Session.create).not.toHaveBeenCalled();
+    expect(AuthEventService.log).toHaveBeenCalledWith(
+      expect.objectContaining({
+        type: 'oauth_login_failed',
+        metadata: { providerId: 'google', reason: 'oauth_missing_email' },
+      }),
+    );
+  });
+
+  it('surfaces oauth_email_not_verified when the provider reports an unverified email', async () => {
+    const res = await callbackWithProfile({
+      sub: 'provider-user',
+      email: 'person@example.com',
+      email_verified: false,
+    });
+
+    expect(res.status).toBe(400);
+    expect(res.body).toEqual({
+      error: 'OAuth profile email is not verified',
+      code: 'oauth_email_not_verified',
+    });
+    expect(AuthEventService.log).toHaveBeenCalledWith(
+      expect.objectContaining({
+        type: 'oauth_login_failed',
+        metadata: { providerId: 'google', reason: 'oauth_email_not_verified' },
+      }),
+    );
+  });
+
+  it('surfaces oauth_missing_subject when the profile has no provider subject', async () => {
+    const res = await callbackWithProfile({ email: 'person@example.com', email_verified: true });
+
+    expect(res.status).toBe(400);
+    expect(res.body).toEqual({
+      error: 'OAuth profile did not include a provider subject',
+      code: 'oauth_missing_subject',
+    });
+    expect(AuthEventService.log).toHaveBeenCalledWith(
+      expect.objectContaining({
+        type: 'oauth_login_failed',
+        metadata: { providerId: 'google', reason: 'oauth_missing_subject' },
+      }),
+    );
+  });
 });