diff --git a/packages/core/src/subagents/model-selection.test.ts b/packages/core/src/subagents/model-selection.test.ts
index aaa2624f8..ce7c5ca96 100644
--- a/packages/core/src/subagents/model-selection.test.ts
+++ b/packages/core/src/subagents/model-selection.test.ts
@@ -36,9 +36,17 @@ describe('parseSubagentModelSelection', () => {
     });
   });
 
-  it('rejects invalid authType prefixes', () => {
-    expect(() => parseSubagentModelSelection('invalid:glm-5')).toThrow(
-      /Invalid authType prefix/,
-    );
+  it('treats unknown prefix as bare model ID (colon in model ID)', () => {
+    expect(parseSubagentModelSelection('invalid:glm-5')).toEqual({
+      modelId: 'invalid:glm-5',
+      inherits: false,
+    });
+  });
+
+  it('treats model IDs with colons as bare model IDs', () => {
+    expect(parseSubagentModelSelection('gpt-4o:online')).toEqual({
+      modelId: 'gpt-4o:online',
+      inherits: false,
+    });
   });
 });
diff --git a/packages/core/src/subagents/model-selection.ts b/packages/core/src/subagents/model-selection.ts
index cea1ef97d..f9c909f78 100644
--- a/packages/core/src/subagents/model-selection.ts
+++ b/packages/core/src/subagents/model-selection.ts
@@ -38,10 +38,10 @@ export function parseSubagentModelSelection(
   const maybeAuthType = trimmed.slice(0, colonIndex).trim();
   const modelId = trimmed.slice(colonIndex + 1).trim();
 
+  // If the prefix isn't a known AuthType, treat the whole string as a bare
+  // model ID. Model IDs can legitimately contain colons (e.g. gpt-4o:online).
   if (!AUTH_TYPES.has(maybeAuthType as AuthType)) {
-    throw new Error(
-      `Invalid authType prefix "${maybeAuthType}". Expected one of: ${Object.values(AuthType).join(', ')}`,
-    );
+    return { modelId: trimmed, inherits: false };
   }
 
   if (!modelId) {
diff --git a/packages/core/src/subagents/validation.test.ts b/packages/core/src/subagents/validation.test.ts
index 5b525db29..99f2de30f 100644
--- a/packages/core/src/subagents/validation.test.ts
+++ b/packages/core/src/subagents/validation.test.ts
@@ -247,13 +247,17 @@ describe('SubagentValidator', () => {
       expect(result.errors).toContain('Model must be a non-empty string');
     });
 
-    it('should reject invalid authType prefixes', () => {
+    it('should accept model IDs containing colons with unknown prefix', () => {
       const result = validator.validateModel('invalid:glm-5');
-      expect(result.isValid).toBe(false);
-      expect(result.errors[0]).toContain('Invalid authType prefix');
+      expect(result.isValid).toBe(true);
     });
 
-    it('should reject missing model IDs after authType prefixes', () => {
+    it('should accept model IDs with colons (e.g. gpt-4o:online)', () => {
+      const result = validator.validateModel('gpt-4o:online');
+      expect(result.isValid).toBe(true);
+    });
+
+    it('should reject missing model IDs after valid authType prefixes', () => {
       const result = validator.validateModel('openai:');
       expect(result.isValid).toBe(false);
       expect(result.errors).toContain(