feat: Add CORS configuration support for server (#3201)

Adds flexible CORS (Cross-Origin Resource Sharing) configuration support
to the FastAPI
  server with both local development and explicit configuration modes:

- **Local development mode**: `cors: true` enables localhost-only access
with regex
  pattern `https?://localhost:\d+`
- **Explicit configuration mode**: Specific origins configuration with
credential support
   and validation
   
- Prevents insecure combinations (wildcards with credentials)
  
- FastAPI CORSMiddleware integration via `model_dump()`

Addresses the need for configurable CORS policies to support web
frontends and
  cross-origin API access while maintaining security.

  Closes #2119

  ## Test Plan

  1.  Ran Unit Tests.

2. Manual tests: FastAPI middleware integration with actual HTTP
requests
    - Local development mode localhost access validation
    - Explicit configuration mode origins validation
    - Preflight OPTIONS request handling

Some screenshots of manual tests.
<img width="1920" height="927" alt="image"
src="https://github.com/user-attachments/assets/79322338-40c7-45c9-a9ea-e3e8d8e2f849"
/>

<img width="1911" height="1037" alt="image"
src="https://github.com/user-attachments/assets/1683524e-b0c9-48c9-a0a5-782e949cde01"
/>

cc: @leseb @rhuss @franciscojavierarceo

llama_stack/core/datatypes.py

@@ -318,6 +318,41 @@ class QuotaConfig(BaseModel):
     period: QuotaPeriod = Field(default=QuotaPeriod.DAY, description="Quota period to set")
 
 
+class CORSConfig(BaseModel):
+    allow_origins: list[str] = Field(default_factory=list)
+    allow_origin_regex: str | None = Field(default=None)
+    allow_methods: list[str] = Field(default=["OPTIONS"])
+    allow_headers: list[str] = Field(default_factory=list)
+    allow_credentials: bool = Field(default=False)
+    expose_headers: list[str] = Field(default_factory=list)
+    max_age: int = Field(default=600, ge=0)
+
+    @model_validator(mode="after")
+    def validate_credentials_config(self) -> Self:
+        if self.allow_credentials and (self.allow_origins == ["*"] or "*" in self.allow_origins):
+            raise ValueError("Cannot use wildcard origins with credentials enabled")
+        return self
+
+
+def process_cors_config(cors_config: bool | CORSConfig | None) -> CORSConfig | None:
+    if cors_config is False or cors_config is None:
+        return None
+
+    if cors_config is True:
+        # dev mode: allow localhost on any port
+        return CORSConfig(
+            allow_origins=[],
+            allow_origin_regex=r"https?://localhost:\d+",
+            allow_methods=["GET", "POST", "PUT", "DELETE", "OPTIONS"],
+            allow_headers=["Content-Type", "Authorization", "X-Requested-With"],
+        )
+
+    if isinstance(cors_config, CORSConfig):
+        return cors_config
+
+    raise ValueError(f"Expected bool or CORSConfig, got {type(cors_config).__name__}")
+
+
 class ServerConfig(BaseModel):
     port: int = Field(
         default=8321,
@@ -349,6 +384,12 @@ class ServerConfig(BaseModel):
         default=None,
         description="Per client quota request configuration",
     )
+    cors: bool | CORSConfig | None = Field(
+        default=None,
+        description="CORS configuration for cross-origin requests. Can be:\n"
+        "- true: Enable localhost CORS for development\n"
+        "- {allow_origins: [...], allow_methods: [...], ...}: Full configuration",
+    )
 
 
 class StackRunConfig(BaseModel):

llama_stack/core/server/server.py

@@ -28,6 +28,7 @@ from aiohttp import hdrs
 from fastapi import Body, FastAPI, HTTPException, Request, Response
 from fastapi import Path as FastapiPath
 from fastapi.exceptions import RequestValidationError
+from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import JSONResponse, StreamingResponse
 from openai import BadRequestError
 from pydantic import BaseModel, ValidationError
@@ -40,6 +41,7 @@ from llama_stack.core.datatypes import (
     AuthenticationRequiredError,
     LoggingConfig,
     StackRunConfig,
+    process_cors_config,
 )
 from llama_stack.core.distribution import builtin_automatically_routed_apis
 from llama_stack.core.external import ExternalApiSpec, load_external_apis
@@ -483,6 +485,12 @@ def main(args: argparse.Namespace | None = None):
             window_seconds=window_seconds,
         )
 
+    if config.server.cors:
+        logger.info("Enabling CORS")
+        cors_config = process_cors_config(config.server.cors)
+        if cors_config:
+            app.add_middleware(CORSMiddleware, **cors_config.model_dump())
+
     if Api.telemetry in impls:
         setup_logger(impls[Api.telemetry])
     else: