fix: replace or with is not None checks for config/override merging (#3833)

* fix: replace `or` with `is not None` checks for config/override merging

Falsy-but-valid values like port=0 (OS-assigned), host="" (all interfaces),
and description="" (explicitly cleared) were silently dropped by `x or default`
patterns across CLI, transport, and component registration.

Fixes #3832

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: replace `or` with `is not None` for description in FunctionResourceTemplate

🤖 Generated with Claude Code

Co-authored-by: Jeremiah Lowin <jlowin@users.noreply.github.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
Co-authored-by: Jeremiah Lowin <jlowin@users.noreply.github.com>

src/fastmcp/cli/cli.py

@@ -521,13 +521,13 @@ async def run(
 
         # Warn about options that are ignored in module mode
         ignored_options: list[str] = []
-        if transport:
+        if transport is not None:
             ignored_options.append("--transport")
-        if host:
+        if host is not None:
             ignored_options.append("--host")
-        if port:
+        if port is not None:
             ignored_options.append("--port")
-        if path:
+        if path is not None:
             ignored_options.append("--path")
         if ignored_options:
             logger.warning(
@@ -601,11 +601,15 @@ async def run(
         sys.exit(1)
 
     # Get effective values (CLI overrides take precedence)
-    final_transport = transport or config.deployment.transport
-    final_host = host or config.deployment.host
-    final_port = port or config.deployment.port
-    final_path = path or config.deployment.path
-    final_log_level = log_level or config.deployment.log_level
+    final_transport = (
+        transport if transport is not None else config.deployment.transport
+    )
+    final_host = host if host is not None else config.deployment.host
+    final_port = port if port is not None else config.deployment.port
+    final_path = path if path is not None else config.deployment.path
+    final_log_level = (
+        log_level if log_level is not None else config.deployment.log_level
+    )
     final_server_args = server_args or config.deployment.args
     # Use CLI override if provided, otherwise use settings
     # no_banner CLI flag overrides the show_server_banner setting
@@ -642,11 +646,11 @@ async def run(
             if final_transport:
                 reload_cmd.extend(["--transport", final_transport])
             if final_transport != "stdio":
-                if final_host:
+                if final_host is not None:
                     reload_cmd.extend(["--host", final_host])
-                if final_port:
+                if final_port is not None:
                     reload_cmd.extend(["--port", str(final_port)])
-                if final_path:
+                if final_path is not None:
                     reload_cmd.extend(["--path", final_path])
             if final_log_level:
                 reload_cmd.extend(["--log-level", final_log_level])
@@ -691,11 +695,11 @@ async def run(
             inner_cmd.extend(["--transport", final_transport])
         # Only add HTTP-specific options for non-stdio transports
         if final_transport != "stdio":
-            if final_host:
+            if final_host is not None:
                 inner_cmd.extend(["--host", final_host])
-            if final_port:
+            if final_port is not None:
                 inner_cmd.extend(["--port", str(final_port)])
-            if final_path:
+            if final_path is not None:
                 inner_cmd.extend(["--path", final_path])
         if final_log_level:
             inner_cmd.extend(["--log-level", final_log_level])

src/fastmcp/cli/run.py

@@ -181,11 +181,15 @@ async def run_command(
             config = load_mcp_server_config(config_path)
 
             # Merge deployment config with CLI arguments (CLI takes precedence)
-            transport = transport or config.deployment.transport
-            host = host or config.deployment.host
-            port = port or config.deployment.port
-            path = path or config.deployment.path
-            log_level = log_level or config.deployment.log_level
+            transport = (
+                transport if transport is not None else config.deployment.transport
+            )
+            host = host if host is not None else config.deployment.host
+            port = port if port is not None else config.deployment.port
+            path = path if path is not None else config.deployment.path
+            log_level = (
+                log_level if log_level is not None else config.deployment.log_level
+            )
             server_args = (
                 server_args if server_args is not None else config.deployment.args
             )
@@ -234,15 +238,17 @@ async def run_command(
         return
 
     kwargs = {}
-    if transport:
+    if transport is not None:
         kwargs["transport"] = transport
-    if host:
-        kwargs["host"] = host
-    if port:
-        kwargs["port"] = port
-    if path:
-        kwargs["path"] = path
-    if log_level:
+    # Only pass HTTP-specific options for non-stdio transports
+    if transport != "stdio":
+        if host is not None:
+            kwargs["host"] = host
+        if port is not None:
+            kwargs["port"] = port
+        if path is not None:
+            kwargs["path"] = path
+    if log_level is not None:
         kwargs["log_level"] = log_level
     if stateless:
         kwargs["stateless"] = True
@@ -310,9 +316,9 @@ async def run_v1_server_async(
         port: Port to bind to
         transport: Transport protocol to use
     """
-    if host:
+    if host is not None:
         server.settings.host = host
-    if port:
+    if port is not None:
         server.settings.port = port
 
     match transport:

src/fastmcp/prompts/function_prompt.py

@@ -152,7 +152,11 @@ class FunctionPrompt(Prompt):
             if param.kind == inspect.Parameter.VAR_KEYWORD:
                 raise ValueError("Functions with **kwargs are not supported as prompts")
 
-        description = metadata.description or inspect.getdoc(fn)
+        description = (
+            metadata.description
+            if metadata.description is not None
+            else inspect.getdoc(fn)
+        )
 
         # Normalize task to TaskConfig and validate
         task_value = metadata.task

src/fastmcp/resources/function_resource.py

@@ -196,7 +196,9 @@ class FunctionResource(Resource):
             name=func_name,
             version=str(metadata.version) if metadata.version is not None else None,
             title=metadata.title,
-            description=metadata.description or inspect.getdoc(fn),
+            description=metadata.description
+            if metadata.description is not None
+            else inspect.getdoc(fn),
             icons=metadata.icons,
             mime_type=resolved_mime or "text/plain",
             tags=metadata.tags or set(),

src/fastmcp/resources/template.py

@@ -553,7 +553,7 @@ class FunctionResourceTemplate(ResourceTemplate):
                     f"URI parameters {all_uri_params} must be a subset of the function arguments: {func_params}"
                 )
 
-        description = description or inspect.getdoc(fn)
+        description = description if description is not None else inspect.getdoc(fn)
 
         # Normalize task to TaskConfig and validate
         if task is None:

src/fastmcp/server/mixins/transport.py

@@ -263,9 +263,11 @@ class TransportMixin:
         if stateless_http and transport == "sse":
             raise ValueError("SSE transport does not support stateless mode")
 
-        host = host or fastmcp.settings.host
-        port = port or fastmcp.settings.port
-        default_log_level_to_use = (log_level or fastmcp.settings.log_level).lower()
+        host = host if host is not None else fastmcp.settings.host
+        port = port if port is not None else fastmcp.settings.port
+        default_log_level_to_use = (
+            log_level if log_level is not None else fastmcp.settings.log_level
+        ).lower()
 
         app = self.http_app(
             path=path,
@@ -335,7 +337,9 @@ class TransportMixin:
         if transport in ("streamable-http", "http"):
             return create_streamable_http_app(
                 server=self,
-                streamable_http_path=path or fastmcp.settings.streamable_http_path,
+                streamable_http_path=path
+                if path is not None
+                else fastmcp.settings.streamable_http_path,
                 event_store=event_store,
                 retry_interval=retry_interval,
                 auth=self.auth,
@@ -356,7 +360,7 @@ class TransportMixin:
             return create_sse_app(
                 server=self,
                 message_path=fastmcp.settings.message_path,
-                sse_path=path or fastmcp.settings.sse_path,
+                sse_path=path if path is not None else fastmcp.settings.sse_path,
                 auth=self.auth,
                 debug=fastmcp.settings.debug,
                 middleware=middleware,

src/fastmcp/server/sampling/sampling_tool.py

@@ -116,7 +116,7 @@ class SamplingTool(FastMCPBaseModel):
 
         return cls(
             name=name or parsed.name,
-            description=description or parsed.description,
+            description=description if description is not None else parsed.description,
             parameters=parsed.input_schema,
             fn=parsed.fn,
             sequential=sequential,

src/fastmcp/tools/function_tool.py

@@ -222,7 +222,9 @@ class FunctionTool(Tool):
             name=metadata.name or parsed_fn.name,
             version=str(metadata.version) if metadata.version is not None else None,
             title=metadata.title,
-            description=metadata.description or parsed_fn.description,
+            description=metadata.description
+            if metadata.description is not None
+            else parsed_fn.description,
             icons=metadata.icons,
             parameters=parsed_fn.input_schema,
             output_schema=final_output_schema,