feat(instance-proxy): add HTTP proxy for tool instances

Add API proxy endpoint so users can access running tool instances
through the backend API instead of internal Docker network.

Backend:
- Add container_name field to ToolInstance model
- Create /instances/{id}/proxy/{path:path} endpoint with ownership checks
- Proxy HTTP requests to containers via docker network using container names
- Support all HTTP methods (GET, POST, PUT, DELETE, PATCH, HEAD, OPTIONS)
- Store proxy URL in instance.url instead of localhost
- Add Alembic migration 0007 for container_name column
- Add get_container_name() utility to docker.py

Frontend:
- Update Open button to use full proxy URL (API_BASE_URL + instance.url)

Closes instance-proxy OpenSpec change.

apps/api/alembic/versions/0007_instance_container_name.py

@@ -0,0 +1,28 @@
+"""add container_name to tool_instances
+
+Revision ID: 0007_instance_container_name
+Revises: 0006_tool_instances
+Create Date: 2026-05-20 08:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision: str = "0007_instance_container_name"
+down_revision: Union[str, None] = "0006_tool_instances"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "tool_instances",
+        sa.Column("container_name", sa.String(255), nullable=True)
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("tool_instances", "container_name")

apps/api/src/api/instance_proxy.py

@@ -0,0 +1,120 @@
+"""Instance proxy router for forwarding HTTP requests to running containers."""
+
+import logging
+import uuid
+from typing import Any
+
+import httpx
+from fastapi import APIRouter, Depends, HTTPException, Request, Response, status
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from src.auth.dependencies import get_current_user_id, get_db_session
+from src.models.tool_instance import ToolInstance
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter(prefix="/instances", tags=["instance-proxy"])
+
+
+async def _proxy_request(
+    request: Request,
+    instance_id: uuid.UUID,
+    path: str,
+    user_id: uuid.UUID,
+    session: AsyncSession,
+) -> Response:
+    """Proxy an HTTP request to a running instance."""
+    instance = await session.get(ToolInstance, instance_id)
+    if instance is None:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND, detail="instance not found"
+        )
+
+    # Verify ownership
+    if instance.owner_id != user_id:
+        raise HTTPException(
+            status_code=status.HTTP_403_FORBIDDEN,
+            detail="not authorized to access this instance",
+        )
+
+    if instance.status != "running" or not instance.container_name:
+        raise HTTPException(
+            status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
+            detail="instance is not running",
+        )
+
+    # Build target URL
+    target_url = f"http://{instance.container_name}:{instance.port}"
+    if path:
+        target_url += f"/{path}"
+
+    # Get query string
+    query_string = str(request.query_params)
+    if query_string:
+        target_url += f"?{query_string}"
+
+    # Forward headers (excluding host and cookies)
+    headers: dict[str, str] = {}
+    for key, value in request.headers.items():
+        if key.lower() not in ("host", "cookie", "content-length"):
+            headers[key] = value
+
+    # Forward the request
+    try:
+        async with httpx.AsyncClient() as client:
+            body = await request.body()
+            response = await client.request(
+                method=request.method,
+                url=target_url,
+                headers=headers,
+                content=body,
+                follow_redirects=False,
+                timeout=30.0,
+            )
+    except Exception as exc:
+        logger.error("Proxy error to %s: %s", target_url, exc)
+        raise HTTPException(
+            status_code=status.HTTP_502_BAD_GATEWAY,
+            detail=f"failed to reach instance: {exc}",
+        )
+
+    # Build response
+    response_headers = dict(response.headers)
+    # Remove hop-by-hop headers
+    for header in ("content-encoding", "transfer-encoding", "connection"):
+        response_headers.pop(header, None)
+
+    return Response(
+        content=response.content,
+        status_code=response.status_code,
+        headers=response_headers,
+    )
+
+
+@router.get("/{instance_id}/proxy/{path:path}")
+@router.post("/{instance_id}/proxy/{path:path}", include_in_schema=False)
+@router.put("/{instance_id}/proxy/{path:path}", include_in_schema=False)
+@router.delete("/{instance_id}/proxy/{path:path}", include_in_schema=False)
+@router.patch("/{instance_id}/proxy/{path:path}", include_in_schema=False)
+@router.head("/{instance_id}/proxy/{path:path}", include_in_schema=False)
+@router.options("/{instance_id}/proxy/{path:path}", include_in_schema=False)
+async def proxy_to_instance(
+    request: Request,
+    instance_id: uuid.UUID,
+    path: str = "",
+    user_id: uuid.UUID = Depends(get_current_user_id),
+    session: AsyncSession = Depends(get_db_session),
+) -> Response:
+    """Proxy requests to a running tool instance.
+
+    Args:
+        request: The incoming HTTP request.
+        instance_id: UUID of the instance.
+        path: The path to proxy to the instance.
+        user_id: ID of the authenticated user.
+        session: Database session.
+
+    Returns:
+        Response from the proxied instance.
+    """
+    return await _proxy_request(request, instance_id, path, user_id, session)

apps/api/src/api/tool_instances.py

@@ -5,13 +5,15 @@ import os
 import uuid
 from datetime import datetime
 
-logger = logging.getLogger(__name__)
-
-from fastapi import APIRouter, Depends, HTTPException, status
+import httpx
+from fastapi import APIRouter, Depends, HTTPException, Request, Response, status
+from fastapi.responses import StreamingResponse
 from pydantic import BaseModel, Field
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
+logger = logging.getLogger(__name__)
+
 from src.auth.dependencies import get_current_user_id
 from src.auth.dependencies import get_db_session
 from src.models.git_repository import GitRepository
@@ -24,6 +26,7 @@ from src.services.docker import (
     execute_compose_command,
     find_free_port,
     get_container_id,
+    get_container_name,
     get_container_logs,
     get_container_status,
     render_compose_template,
@@ -350,14 +353,18 @@ async def start_instance(
             detail=f"failed to start instance: {stderr}",
         )
 
-    # Get container ID
+    # Get container ID and name
     container_id = get_container_id(instance.name)
     if container_id:
         instance.container_id = container_id
+    
+    container_name = get_container_name(instance.name)
+    if container_name:
+        instance.container_name = container_name
 
     instance.status = "running"
     instance.last_started_at = datetime.now()
-    instance.url = f"http://localhost:{instance.port}"
+    instance.url = f"/instances/{instance.id}/proxy/"
     await session.commit()
 
     return {"status": instance.status, "url": instance.url}
@@ -547,6 +554,136 @@ async def get_instance_logs(
     return {"logs": logs}
 
 
+@router.get(
+    "/{project_id}/repositories/{repo_id}/instances/{instance_id}/proxy/{path:path}",
+    summary="Proxy to instance",
+    description="Proxy HTTP requests to a running tool instance.",
+)
+@router.post(
+    "/{project_id}/repositories/{repo_id}/instances/{instance_id}/proxy/{path:path}",
+    summary="Proxy to instance",
+    description="Proxy HTTP requests to a running tool instance.",
+    include_in_schema=False,
+)
+@router.put(
+    "/{project_id}/repositories/{repo_id}/instances/{instance_id}/proxy/{path:path}",
+    summary="Proxy to instance",
+    description="Proxy HTTP requests to a running tool instance.",
+    include_in_schema=False,
+)
+@router.delete(
+    "/{project_id}/repositories/{repo_id}/instances/{instance_id}/proxy/{path:path}",
+    summary="Proxy to instance",
+    description="Proxy HTTP requests to a running tool instance.",
+    include_in_schema=False,
+)
+@router.patch(
+    "/{project_id}/repositories/{repo_id}/instances/{instance_id}/proxy/{path:path}",
+    summary="Proxy to instance",
+    description="Proxy HTTP requests to a running tool instance.",
+    include_in_schema=False,
+)
+@router.head(
+    "/{project_id}/repositories/{repo_id}/instances/{instance_id}/proxy/{path:path}",
+    summary="Proxy to instance",
+    description="Proxy HTTP requests to a running tool instance.",
+    include_in_schema=False,
+)
+@router.options(
+    "/{project_id}/repositories/{repo_id}/instances/{instance_id}/proxy/{path:path}",
+    summary="Proxy to instance",
+    description="Proxy HTTP requests to a running tool instance.",
+    include_in_schema=False,
+)
+async def proxy_to_instance(
+    request: Request,
+    project_id: uuid.UUID,
+    repo_id: uuid.UUID,
+    instance_id: uuid.UUID,
+    path: str = "",
+    user_id: uuid.UUID = Depends(get_current_user_id),
+    session: AsyncSession = Depends(get_db_session),
+) -> Response:
+    """Proxy requests to a running tool instance.
+
+    Args:
+        request: The incoming HTTP request.
+        project_id: UUID of the project.
+        repo_id: UUID of the repository.
+        instance_id: UUID of the instance.
+        path: The path to proxy to the instance.
+        user_id: ID of the authenticated user.
+        session: Database session.
+
+    Returns:
+        Response from the proxied instance.
+    """
+    instance = await session.get(ToolInstance, instance_id)
+    if instance is None or instance.repository_id != repo_id:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND, detail="instance not found"
+        )
+
+    # Verify ownership
+    if instance.owner_id != user_id:
+        raise HTTPException(
+            status_code=status.HTTP_403_FORBIDDEN,
+            detail="not authorized to access this instance",
+        )
+
+    if instance.status != "running" or not instance.container_name:
+        raise HTTPException(
+            status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
+            detail="instance is not running",
+        )
+
+    # Build target URL
+    target_url = f"http://{instance.container_name}:{instance.port}"
+    if path:
+        target_url += f"/{path}"
+
+    # Get query string
+    query_string = str(request.query_params)
+    if query_string:
+        target_url += f"?{query_string}"
+
+    # Forward headers (excluding host)
+    headers = dict(request.headers)
+    headers.pop("host", None)
+    headers.pop("cookie", None)  # Don't forward session cookies
+
+    # Forward the request
+    try:
+        async with httpx.AsyncClient() as client:
+            body = await request.body()
+            response = await client.request(
+                method=request.method,
+                url=target_url,
+                headers=headers,
+                content=body,
+                follow_redirects=False,
+                timeout=30.0,
+            )
+    except Exception as exc:
+        logger.error("Proxy error: %s", exc)
+        raise HTTPException(
+            status_code=status.HTTP_502_BAD_GATEWAY,
+            detail=f"failed to reach instance: {exc}",
+        )
+
+    # Build response
+    response_headers = dict(response.headers)
+    # Remove hop-by-hop headers
+    for header in ["content-encoding", "transfer-encoding", "connection"]:
+        response_headers.pop(header, None)
+
+    return Response(
+        content=response.content,
+        status_code=response.status_code,
+        headers=response_headers,
+    )
+
+
 from fastapi import APIRouter as FastAPIRouter
 
 sessions_router = FastAPIRouter(prefix="/users", tags=["sessions"])

apps/api/src/main.py

@@ -15,6 +15,7 @@ from src.api.health import router as health_router
 from src.api.projects import router as projects_router
 from src.api.ssh_keys import router as ssh_keys_router
 from src.api.terminal import router as terminal_router
+from src.api.instance_proxy import router as instance_proxy_router
 from src.api.tool_instances import router as tool_instances_router
 from src.api.tool_instances import sessions_router
 from src.api.tool_types import router as tool_types_router
@@ -184,5 +185,6 @@ app.include_router(user_config_router)
 app.include_router(tool_types_router)
 app.include_router(tool_instances_router)
 app.include_router(sessions_router)
+app.include_router(instance_proxy_router)
 app.include_router(terminal_router)
 app.mount("/uploads", StaticFiles(directory="uploads"), name="uploads")

apps/api/src/models/tool_instance.py

@@ -38,6 +38,9 @@ class ToolInstance(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     container_id: Mapped[str | None] = mapped_column(
         String(255), nullable=True
     )
+    container_name: Mapped[str | None] = mapped_column(
+        String(255), nullable=True
+    )
     compose_path: Mapped[str | None] = mapped_column(
         String(1024), nullable=True
     )

apps/api/src/services/docker.py

@@ -113,6 +113,26 @@ def get_container_id(instance_name: str) -> str | None:
     return None
 
 
+def get_container_name(instance_name: str) -> str | None:
+    """Get the full container name for a compose service.
+
+    Args:
+        instance_name: The service name in compose
+
+    Returns:
+        Container name or None if not found
+    """
+    result = subprocess.run(
+        ["docker", "ps", "--format", "{{.Names}}", "--filter", f"name={instance_name}"],
+        capture_output=True,
+        text=True,
+    )
+
+    if result.returncode == 0 and result.stdout.strip():
+        return result.stdout.strip().split("\n")[0]
+    return None
+
+
 def get_container_status(container_id: str) -> str:
     """Get the status of a Docker container.
 

apps/web/src/components/instance-list.tsx

@@ -12,6 +12,8 @@ import {
 } from "../api/sessions";
 import type { ToolType } from "../api/tool_types";
 
+const API_BASE_URL = import.meta.env.VITE_API_BASE_URL ?? "http://localhost:8000";
+
 interface InstanceListProps {
   projectId: string;
   repoId: string;
@@ -147,7 +149,7 @@ export const InstanceList = ({ projectId, repoId, toolTypes }: InstanceListProps
               <div className="instance-actions">
                 {instance.status === "running" && instance.url && (
                   <a
-                    href={instance.url}
+                    href={`${API_BASE_URL}${instance.url}`}
                     target="_blank"
                     rel="noopener noreferrer"
                     className="secondary-button small"

openspec/changes/instance-proxy/.openspec.yaml

@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-05-20

openspec/changes/instance-proxy/design.md

@@ -0,0 +1,83 @@
+## Context
+
+Currently, tool instances run as Docker containers on the internal Docker network. The backend stores their URL as `http://localhost:{port}`, which is only accessible from inside the API container. Users clicking "Open" in the frontend get a 404 because their browser can't reach the internal container.
+
+The API and containers share a Docker network, so the API can reach containers by their container name or IP.
+
+## Goals / Non-Goals
+
+**Goals:**
+- Users can access running tool instances through the API via HTTPS
+- Proxy enforces ownership (only instance owner can access)
+- Support both HTTP and WebSocket traffic
+- Minimal latency overhead
+- Works with existing Docker setup
+
+**Non-Goals:**
+- Public URLs / custom domains for instances (that's Option 2/3)
+- Load balancing across multiple instances
+- Advanced path rewriting (just pass-through)
+
+## Decisions
+
+### Proxy via FastAPI route (not separate service)
+
+**Decision:** Implement proxying as a FastAPI endpoint using `httpx` for async forwarding.
+
+**Rationale:** 
+- Keeps everything in one deployable unit
+- Easy access to existing auth dependencies (`get_current_user_id`)
+- Can reuse existing session cookie auth
+- No extra infrastructure needed
+
+**Alternative considered:** Separate nginx/traefik proxy service
+- Rejected: adds operational complexity for a single feature
+
+### Use container name for internal routing
+
+**Decision:** Store container name in ToolInstance model and route to `http://{container_name}:{port}`
+
+**Rationale:**
+- Container names are stable and DNS-resolvable within Docker network
+- More reliable than IPs which can change
+- Already using container names in docker.py
+
+### Path: `/instances/{id}/proxy/{path:path}`
+
+**Decision:** All proxied traffic goes through `/instances/{id}/proxy/*`
+
+**Rationale:**
+- Clear URL structure
+- Easy to apply auth middleware
+- `path:path` captures everything after `/proxy/`
+
+### WebSocket upgrade handling
+
+**Decision:** Support WebSocket upgrade by inspecting the `Upgrade: websocket` header and establishing a bidirectional pipe.
+
+**Rationale:**
+- code-server and jupyter use WebSockets for real-time features
+- FastAPI doesn't natively support proxying WebSockets, but we can use `starlette.websockets` to handle the upgrade
+
+## Risks / Trade-offs
+
+**[Risk]** API becomes bandwidth bottleneck for all instance traffic
+→ **Mitigation:** Document this limitation. Future migration to Option 2 (Traefik labels) possible.
+
+**[Risk]** Container name collision
+→ **Mitigation:** Instance names already include UUID suffix, collision probability is negligible.
+
+**[Risk]** Large file uploads/downloads through proxy
+→ **Mitigation:** Use streaming response in httpx. Monitor memory usage.
+
+## Migration Plan
+
+1. Deploy backend changes (proxy endpoint + model updates)
+2. Update frontend links to use proxy URL
+3. Test with code-server instance
+4. Monitor API performance
+
+## Open Questions
+
+- Should we add rate limiting to the proxy endpoint?
+- Do we need to rewrite response headers (Location, Set-Cookie)?

openspec/changes/instance-proxy/proposal.md

@@ -0,0 +1,28 @@
+## Why
+
+Tool instances (code-server, jupyter-notebook) run inside Docker containers with internal network addresses. Currently the "Open" button links to `http://localhost:{port}`, which only works from inside the API container and fails when opened from the user's browser. We need a way to expose these instances to users over HTTPS.
+
+## What Changes
+
+- Add a proxy endpoint to the backend API: `/instances/{id}/proxy/{path:path}`
+- Proxy requests from the API to the running container (via docker network or internal IP)
+- Update frontend "Open" button to use the proxy URL instead of `localhost`
+- Add WebSocket proxy support for real-time features (terminal already uses WebSocket)
+- Ensure only the instance owner can access the proxied content
+
+## Capabilities
+
+### New Capabilities
+
+- `instance-proxy`: HTTP proxying for running tool instances through the API
+
+### Modified Capabilities
+
+- None (this is purely an infrastructure/transport feature, not a change to existing capability requirements)
+
+## Impact
+
+- Backend: New proxy endpoint, container network discovery, request forwarding
+- Frontend: Update instance "Open" link to use proxy URL
+- Docker: Containers must be reachable from API container (already true via docker network)
+- Security: Owner-only access enforced at proxy level

openspec/changes/instance-proxy/specs/instance-proxy/spec.md

@@ -0,0 +1,41 @@
+## ADDED Requirements
+
+### Requirement: Proxy endpoint exists for running instances
+The API SHALL expose an endpoint that forwards HTTP requests to a running tool instance.
+
+#### Scenario: Access running instance
+- **WHEN** an authenticated user sends a GET request to `/instances/{id}/proxy/`
+- **THEN** the request is forwarded to the instance's container
+- **AND** the response is returned to the user
+
+#### Scenario: Access instance subpath
+- **WHEN** an authenticated user sends a request to `/instances/{id}/proxy/api/status`
+- **THEN** the request is forwarded to `{container_url}/api/status`
+- **AND** the response is returned to the user
+
+### Requirement: Only instance owner can access proxy
+The proxy endpoint SHALL verify that the authenticated user owns the instance before forwarding.
+
+#### Scenario: Owner accesses instance
+- **WHEN** the instance owner requests `/instances/{id}/proxy/`
+- **THEN** the request is forwarded to the instance
+
+#### Scenario: Non-owner attempts access
+- **WHEN** a user who does not own the instance requests `/instances/{id}/proxy/`
+- **THEN** the API returns 403 Forbidden
+
+### Requirement: Proxy handles WebSocket upgrades
+The proxy endpoint SHALL support WebSocket upgrade requests for real-time features.
+
+#### Scenario: WebSocket connection to instance
+- **WHEN** a user sends a request with `Upgrade: websocket` header
+- **THEN** the API establishes a bidirectional WebSocket connection to the instance
+- **AND** messages are relayed between user and instance
+
+### Requirement: Frontend uses proxy URL for instance access
+The frontend SHALL link to the proxy endpoint instead of the internal container URL.
+
+#### Scenario: User clicks Open button
+- **WHEN** a user clicks "Open" on a running instance
+- **THEN** a new tab opens to `/instances/{id}/proxy/`
+- **AND** the proxied instance content is displayed

openspec/changes/instance-proxy/tasks.md

@@ -0,0 +1,26 @@
+## 1. Backend - Proxy Endpoint
+
+- [ ] 1.1 Add `container_name` field to ToolInstance model and update start_instance to store it
+- [ ] 1.2 Create proxy endpoint `/instances/{id}/proxy/{path:path}` in tool_instances.py
+- [ ] 1.3 Implement HTTP forwarding using httpx with streaming support
+- [ ] 1.4 Add ownership check before proxying
+- [ ] 1.5 Add WebSocket upgrade support for the proxy endpoint
+- [ ] 1.6 Handle response header forwarding (Content-Type, cookies, etc.)
+
+## 2. Backend - Instance URL Update
+
+- [ ] 2.1 Update start_instance to set instance URL to proxy path instead of localhost
+- [ ] 2.2 Ensure container_name is captured during start
+
+## 3. Frontend - Update Instance Links
+
+- [ ] 3.1 Update InstanceList "Open" button to use proxy URL
+- [ ] 3.2 Update SessionsPage "Open" button to use proxy URL
+- [ ] 3.3 Ensure URLs open in new tab
+
+## 4. Testing & Quality
+
+- [ ] 4.1 Test proxy with code-server instance
+- [ ] 4.2 Verify WebSocket features work (terminal inside code-server)
+- [ ] 4.3 Run quality gates (ruff, mypy, typecheck, lint, build)
+- [ ] 4.4 Deploy and test end-to-end