diff --git a/workspace/a2a_cli.py b/workspace/a2a_cli.py
index 5ba7381c..0b2ce03c 100644
--- a/workspace/a2a_cli.py
+++ b/workspace/a2a_cli.py
@@ -28,7 +28,7 @@ WORKSPACE_ID = _WORKSPACE_ID_raw
 if os.path.exists("/.dockerenv") or os.environ.get("DOCKER_VERSION"):
     PLATFORM_URL = os.environ.get("PLATFORM_URL", "http://host.docker.internal:8080")
 else:
-    PLATFORM_URL = os.environ.get("PLATFORM_URL", "http://localhost:8080")
+    PLATFORM_URL = os.environ.get("PLATFORM_URL", "http://host.docker.internal:8080")
 
 
 async def discover(target_id: str) -> dict | None:
diff --git a/workspace/a2a_client.py b/workspace/a2a_client.py
index 8e499f40..07674e58 100644
--- a/workspace/a2a_client.py
+++ b/workspace/a2a_client.py
@@ -29,7 +29,7 @@ WORKSPACE_ID = _WORKSPACE_ID_raw
 if os.path.exists("/.dockerenv") or os.environ.get("DOCKER_VERSION"):
     PLATFORM_URL = os.environ.get("PLATFORM_URL", "http://host.docker.internal:8080")
 else:
-    PLATFORM_URL = os.environ.get("PLATFORM_URL", "http://localhost:8080")
+    PLATFORM_URL = os.environ.get("PLATFORM_URL", "http://host.docker.internal:8080")
 
 # Cache workspace ID → name mappings (populated by list_peers calls)
 _peer_names: dict[str, str] = {}
diff --git a/workspace/builtin_tools/temporal_workflow.py b/workspace/builtin_tools/temporal_workflow.py
index 8f8e6f41..12db0e60 100644
--- a/workspace/builtin_tools/temporal_workflow.py
+++ b/workspace/builtin_tools/temporal_workflow.py
@@ -54,6 +54,16 @@ import httpx
 
 logger = logging.getLogger(__name__)
 
+
+def _platform_url() -> str:
+    """Return the platform URL, defaulting to host.docker.internal when running
+    inside a Docker container (where localhost refers to the container, not the
+    host).  External callers can always override via the PLATFORM_URL env var.
+    """
+    if os.path.exists("/.dockerenv") or os.environ.get("DOCKER_VERSION"):
+        return os.environ.get("PLATFORM_URL", "http://host.docker.internal:8080")
+    return os.environ.get("PLATFORM_URL", "http://host.docker.internal:8080")
+
 # ─────────────────────────────────────────────────────────────────────────────
 # Constants
 # ─────────────────────────────────────────────────────────────────────────────
@@ -79,12 +89,12 @@ async def _fetch_latest_checkpoint(workspace_id: str) -> Optional[dict]:
         workspace_id: The workspace to query.
 
     Reads:
-        PLATFORM_URL  Platform base URL (default ``http://localhost:8080``).
+        PLATFORM_URL  Platform base URL (default ``http://host.docker.internal:8080``).
     """
     try:
         from platform_auth import auth_headers as _auth_headers  # type: ignore[import]
 
-        platform_url = os.environ.get("PLATFORM_URL", "http://localhost:8080")
+        platform_url = _platform_url()
         url = f"{platform_url}/workspaces/{workspace_id}/checkpoints/latest"
         async with httpx.AsyncClient(timeout=5.0) as client:
             resp = await client.get(url, headers=_auth_headers())
@@ -125,12 +135,12 @@ async def _save_checkpoint(
         payload:       Optional JSON-serialisable dict stored as JSONB.
 
     Reads:
-        PLATFORM_URL   Platform base URL (default ``http://localhost:8080``).
+        PLATFORM_URL   Platform base URL (default ``http://host.docker.internal:8080``).
     """
     try:
         from platform_auth import auth_headers as _auth_headers  # type: ignore[import]
 
-        platform_url = os.environ.get("PLATFORM_URL", "http://localhost:8080")
+        platform_url = _platform_url()
         url = f"{platform_url}/workspaces/{workspace_id}/checkpoints"
         body: dict = {
             "workflow_id": workflow_id,
diff --git a/workspace/consolidation.py b/workspace/consolidation.py
index 81e9ec88..edd9c72f 100644
--- a/workspace/consolidation.py
+++ b/workspace/consolidation.py
@@ -21,7 +21,7 @@ logger = logging.getLogger(__name__)
 if os.path.exists("/.dockerenv") or os.environ.get("DOCKER_VERSION"):
     PLATFORM_URL = os.environ.get("PLATFORM_URL", "http://host.docker.internal:8080")
 else:
-    PLATFORM_URL = os.environ.get("PLATFORM_URL", "http://localhost:8080")
+    PLATFORM_URL = os.environ.get("PLATFORM_URL", "http://host.docker.internal:8080")
 _WORKSPACE_ID_raw = os.environ.get("WORKSPACE_ID")
 if not _WORKSPACE_ID_raw:
     raise RuntimeError("WORKSPACE_ID environment variable is required but not set")
diff --git a/workspace/coordinator.py b/workspace/coordinator.py
index 12d317ef..70ac7aa4 100644
--- a/workspace/coordinator.py
+++ b/workspace/coordinator.py
@@ -25,7 +25,7 @@ logger = logging.getLogger(__name__)
 if os.path.exists("/.dockerenv") or os.environ.get("DOCKER_VERSION"):
     PLATFORM_URL = os.environ.get("PLATFORM_URL", "http://host.docker.internal:8080")
 else:
-    PLATFORM_URL = os.environ.get("PLATFORM_URL", "http://localhost:8080")
+    PLATFORM_URL = os.environ.get("PLATFORM_URL", "http://host.docker.internal:8080")
 _WORKSPACE_ID_raw = os.environ.get("WORKSPACE_ID")
 if not _WORKSPACE_ID_raw:
     raise RuntimeError("WORKSPACE_ID environment variable is required but not set")
diff --git a/workspace/main.py b/workspace/main.py
index 77c2d2d6..8b40bc4e 100644
--- a/workspace/main.py
+++ b/workspace/main.py
@@ -63,7 +63,7 @@ async def main():  # pragma: no cover
     if os.path.exists("/.dockerenv") or os.environ.get("DOCKER_VERSION"):
         platform_url = os.environ.get("PLATFORM_URL", "http://host.docker.internal:8080")
     else:
-        platform_url = os.environ.get("PLATFORM_URL", "http://localhost:8080")
+        platform_url = os.environ.get("PLATFORM_URL", "http://host.docker.internal:8080")
     awareness_config = get_awareness_config()
 
     # 0. Initialise OpenTelemetry (no-op if packages not installed)