Falsy-but-valid values silently dropped by `or` operator in config merging

[strawgate]

Several places use x or default to merge user-provided values with defaults. This silently drops falsy-but-valid values like port=0 (OS-assigned port), host="" (all interfaces), and description="" (explicitly cleared).

run_http() — port=0 and host="" ignored

# src/fastmcp/server/mixins/transport.py:266-267
host = host or fastmcp.settings.host    # host="" becomes "127.0.0.1"
port = port or fastmcp.settings.port    # port=0 becomes 8000

port=0 is the standard way to request an OS-assigned port. host="" binds to all interfaces. Both are silently replaced.

CLI run command — same pattern

# src/fastmcp/cli/cli.py:604-609
final_host = host or config.deployment.host
final_port = port or config.deployment.port
final_path = path or config.deployment.path

fastmcp run server.py --port 0 silently uses the config port instead.

Tool/resource/prompt description — empty string falls through to docstring

# src/fastmcp/tools/function_tool.py:225
description=metadata.description or parsed_fn.description

# src/fastmcp/resources/function_resource.py:199
description=metadata.description or inspect.getdoc(fn)

# src/fastmcp/prompts/function_prompt.py:155
description = metadata.description or inspect.getdoc(fn)

@mcp.tool(description="")
def my_tool() -> str:
    """This docstring should NOT be used as description"""
    return "hello"

# description is "This docstring should NOT be used as description"

Fix

Replace x or default with x if x is not None else default in all affected locations.

🤖 Generated with Claude Code

[marvin-context-protocol]

Claude finished @strawgate's task in 4m 26s —— View job

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Recommendation

Bug confirmed across all reported locations plus two additional ones in cli/run.py and resources/template.py. The fix is straightforward: replace x or default with x if x is not None else default at each affected site.

The most impactful bugs are port=0 being silently replaced (prevents OS-assigned ports) and description="" falling through to docstrings (prevents clearing descriptions).

Findings

All reported locations verified. I also found 2 additional affected files not mentioned in the issue.

I ran a test script that confirms all 3 description bugs — passing description="" to @mcp.tool, @mcp.resource, and @mcp.prompt causes the docstring to be used instead:

Tool description: 'This docstring should NOT be used as description'  # should be ''
Resource description: 'This docstring should NOT be used'             # should be ''
Prompt description: 'This docstring should NOT be used'               # should be ''

Complete list of affected locations (8 total):

File	Line	Pattern	Impact
src/fastmcp/server/mixins/transport.py	266	host = host or fastmcp.settings.host	host="" silently becomes "127.0.0.1"
src/fastmcp/server/mixins/transport.py	267	port = port or fastmcp.settings.port	port=0 silently becomes 8000
src/fastmcp/cli/cli.py	605-608	final_host = host or config... (host, port, path, log_level)	Same as above via CLI
src/fastmcp/cli/run.py	185-188	host = host or config... (host, port, path, log_level)	Same issue, not in original report
src/fastmcp/tools/function_tool.py	225	description=metadata.description or parsed_fn.description	description="" falls through to docstring
src/fastmcp/resources/function_resource.py	199	description=metadata.description or inspect.getdoc(fn)	Same
src/fastmcp/prompts/function_prompt.py	155	description = metadata.description or inspect.getdoc(fn)	Same
src/fastmcp/resources/template.py	556	description = description or inspect.getdoc(fn)	Same, not in original report

Not affected: tool_transform.py already uses a NotSet sentinel correctly for description (line 577-578). The name or patterns (e.g., metadata.name or parsed_fn.name) are acceptable since name="" is not a useful value.

Verification

I wrote and ran a test that directly creates tools/resources/prompts with description="":

# Tool: description="" falls through to docstring
tool = FunctionTool.from_function(my_tool, description="")
assert tool.description == ""  # FAILS: got docstring instead

# Resource: same
metadata = ResourceMeta(uri="resource://test", description="")
resource = FunctionResource.from_function(my_resource, metadata=metadata)
assert resource.description == ""  # FAILS: got docstring instead

# Prompt: same
metadata = PromptMeta(description="")
prompt = FunctionPrompt.from_function(my_prompt, metadata=metadata)
assert prompt.description == ""  # FAILS: got docstring instead

All 3 assertions failed, confirming the bug. The port=0 and host="" bugs are confirmed by code inspection — the function signatures accept None as default, and the or operator treats 0 and "" as falsy.

Detailed Action Plan

Replace x or default with x if x is not None else default at all 8 locations. Each change is mechanical.

1. src/fastmcp/server/mixins/transport.py (lines 266-267)

# Before
host = host or fastmcp.settings.host
port = port or fastmcp.settings.port

# After
host = host if host is not None else fastmcp.settings.host
port = port if port is not None else fastmcp.settings.port

2. src/fastmcp/cli/cli.py (lines 604-608)

# Before
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

# After
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

3. src/fastmcp/cli/run.py (lines 184-188)

# Before
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

# After
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

4. src/fastmcp/tools/function_tool.py (line 225)

# Before
description=metadata.description or parsed_fn.description,

# After
description=metadata.description if metadata.description is not None else parsed_fn.description,

5. src/fastmcp/resources/function_resource.py (line 199)

# Before
description=metadata.description or inspect.getdoc(fn),

# After
description=metadata.description if metadata.description is not None else inspect.getdoc(fn),

6. src/fastmcp/prompts/function_prompt.py (line 155)

# Before
description = metadata.description or inspect.getdoc(fn)

# After
description = metadata.description if metadata.description is not None else inspect.getdoc(fn)

7. src/fastmcp/resources/template.py (line 556)

# Before
description = description or inspect.getdoc(fn)

# After
description = description if description is not None else inspect.getdoc(fn)

Tests to add: Write tests covering each category:

• port=0 is preserved in run_http_async (mock uvicorn to capture args)
• host="" is preserved
• description="" doesn't fall through to docstring for tools, resources, prompts, and resource templates
• CLI --port 0 is respected

Related Files

File	Relevance	Lines
src/fastmcp/server/mixins/transport.py	host/port merging for HTTP transport	266-267
src/fastmcp/cli/cli.py	CLI run command config merging	604-608
src/fastmcp/cli/run.py	Config file merging (not in original report)	184-188
src/fastmcp/tools/function_tool.py	Tool description fallback	225
src/fastmcp/resources/function_resource.py	Resource description fallback	199
src/fastmcp/prompts/function_prompt.py	Prompt description fallback	155
src/fastmcp/resources/template.py	Resource template description fallback (not in original report)	556
src/fastmcp/tools/tool_transform.py	Already uses NotSet sentinel correctly — reference implementation	575-578

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