diff --git a/app/api/streak/route.test.ts b/app/api/streak/route.test.ts
index bc3786984..728b52321 100644
--- a/app/api/streak/route.test.ts
+++ b/app/api/streak/route.test.ts
@@ -197,6 +197,26 @@ describe('GET /api/streak', () => {
       expect(response.headers.get('Content-Type')).toContain('image/svg+xml');
     });
 
+    it('returns JSON 400 when the user parameter is missing and format=json', async () => {
+      const response = await GET(makeRequest({ format: 'json' }));
+      expect(response.status).toBe(400);
+      const body = await response.json();
+      expect(body).toHaveProperty('error');
+      expect(body.error).toContain('Missing user parameter');
+      expect(response.headers.get('Content-Type')).toContain('application/json');
+    });
+
+    it('returns 404 JSON for a nonexistent user when format=json', async () => {
+      vi.mocked(fetchGitHubContributions).mockRejectedValue(
+        new Error("Could not resolve to a User with the login of 'nonexistentuser'")
+      );
+      const response = await GET(makeRequest({ user: 'nonexistentuser', format: 'json' }));
+      expect(response.status).toBe(404);
+      const body = await response.json();
+      expect(body).toHaveProperty('error');
+      expect(response.headers.get('Content-Type')).toContain('application/json');
+    });
+
     it('returns 400 when org parameter contains spaces and invalid characters', async () => {
       const response = await GET(
         makeRequest({ user: 'octocat', org: 'invalid_org_name_with_spaces' })
diff --git a/app/api/streak/route.ts b/app/api/streak/route.ts
index 44e5e205c..cb45c205c 100644
--- a/app/api/streak/route.ts
+++ b/app/api/streak/route.ts
@@ -101,6 +101,21 @@ export async function GET(request: Request) {
         Object.values(fieldErrors.fieldErrors).flat()[0] ??
         fieldErrors.formErrors[0] ??
         'Invalid parameters';
+
+      if (searchParams.get('format') === 'json') {
+        return NextResponse.json(
+          { error: firstError },
+          {
+            status: 400,
+            headers: {
+              'Content-Type': 'application/json',
+              'Cache-Control': 'no-store',
+              'X-Request-ID': requestId,
+            },
+          }
+        );
+      }
+
       const errTheme = resolveErrorTheme(searchParams);
       const errorSvg = buildInlineErrorSVG(firstError, {
         bg: errTheme.bg,
@@ -1035,7 +1050,7 @@ function buildErrorResponse(
   }
 
   if (isNotFound) {
-    const match = message.match(/"([^"]+)"|login of '([^']+)'/);
+    const match = rawMessage.match(/"([^"]+)"|login of '([^']+)'/);
     const fallbackTarget = parseResult.success
       ? parseResult.data.org || parseResult.data.user
       : 'unknown';
diff --git a/lib/validations.streakParamsSchema.test.ts b/lib/validations.streakParamsSchema.test.ts
index 75c8fb777..85d645fda 100644
--- a/lib/validations.streakParamsSchema.test.ts
+++ b/lib/validations.streakParamsSchema.test.ts
@@ -65,15 +65,24 @@ describe('streakParamsSchema', () => {
     expect(result.success).toBe(true);
   });
 
+  it('accepts input with org but missing user', () => {
+    const result = streakParamsSchema.safeParse({ org: 'github' });
+    expect(result.success).toBe(true);
+    if (result.success) {
+      expect(result.data.org).toBe('github');
+      expect(result.data.user).toBe(''); // default empty string
+    }
+  });
+
   // ── Invalid / negative cases ──────────────────────────────────────────────
 
-  it('fails when user is missing', () => {
+  it('fails when both user and org are missing', () => {
     const result = streakParamsSchema.safeParse({});
     expect(result.success).toBe(false);
   });
 
-  it('fails when user is an empty string', () => {
-    const result = streakParamsSchema.safeParse({ user: '' });
+  it('fails when user and org are empty strings', () => {
+    const result = streakParamsSchema.safeParse({ user: '', org: '' });
     expect(result.success).toBe(false);
   });
 
diff --git a/lib/validations.ts b/lib/validations.ts
index b1834c1d1..32734bcbb 100644
--- a/lib/validations.ts
+++ b/lib/validations.ts
@@ -233,8 +233,9 @@ const baseStreakParamsSchema = z.object({
   // Required — missing user surfaces as "Missing" to match existing tests
   user: z
     .string({ error: 'Missing user parameter' })
-    .min(1, { message: 'Missing user parameter' })
+    .default('')
     .superRefine((val, ctx) => {
+      if (val === '') return;
       const users = val.split(',').map((u) => u.trim());
       if (users.length === 0) {
         ctx.addIssue({
@@ -693,6 +694,11 @@ const baseStreakParamsSchema = z.object({
 const TWO_YEARS_MS = 2 * 365.25 * 24 * 60 * 60 * 1000;
 
 export const streakParamsSchema = baseStreakParamsSchema
+  .refine(
+    (data) =>
+      (data.user && data.user.trim().length > 0) || (data.org && data.org.trim().length > 0),
+    { message: 'Missing user parameter', path: ['user'] }
+  )
   .refine((data) => !data.from || !data.to || Date.parse(data.from) <= Date.parse(data.to), {
     message: '"to" date must be after or equal to "from" date',
     path: ['to'],
@@ -1240,6 +1246,11 @@ export const animatedStreakParamsSchema = baseStreakParamsSchema
       .optional()
       .transform((val) => val || 'rise'),
   })
+  .refine(
+    (data) =>
+      (data.user && data.user.trim().length > 0) || (data.org && data.org.trim().length > 0),
+    { message: 'Missing user parameter', path: ['user'] }
+  )
   .refine((data) => !data.from || !data.to || Date.parse(data.from) <= Date.parse(data.to), {
     message: '"to" date must be after or equal to "from" date',
     path: ['to'],