Field loss in transform and provider create_resource() methods

[strawgate]

Field loss in transform and provider create_resource() methods

Summary

Three related bugs where fields are silently dropped when creating resources from templates or providers:

Bug 1: FunctionResourceTemplate.create_resource() drops fields

src/fastmcp/resources/template.py:440-449 creates resources via Resource.from_function() but never passes icons, annotations, meta, or title — even though Resource.from_function accepts these parameters.

Bug 2: FastMCPProviderResourceTemplate.create_resource() drops fields

src/fastmcp/server/providers/fastmcp_provider.py:368-384 has the same issue — creates FastMCPProviderResource without icons, tags, annotations, task_config, meta, title, or auth.

Bug 3: Namespace transform loses original_name

src/fastmcp/server/transforms/namespace.py:100 uses model_copy(update={"name": ...}) to rename tools, but doesn't preserve original_name via fastmcp.delegate.original_name. This is inconsistent with FastMCPProviderTool.wrap() which does preserve the original name.

Impact

• Icon annotations on resource templates are silently lost
• Telemetry delegation spans lose fastmcp.delegate.original_name for namespaced tools
• tags and auth settings on mounted resource templates are ignored

Fix Suggestion

Add icons, annotations, meta, title to the Resource.from_function() call in templates. Add original_name preservation to Namespace transform.

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tl;dr: Bugs 1 and 2 are confirmed — create_resource() in both FunctionResourceTemplate and FastMCPProviderResourceTemplate silently drops icons, annotations, meta, and title. Bug 3 is not reproducible — Pydantic's model_copy already preserves private attributes like _original_name on subclasses.

Recommendation: Fix the two confirmed create_resource() methods by forwarding all fields, mirroring the patterns already used in FastMCPProviderResource.wrap() and Resource.from_function().

Findings

Bug 1: FunctionResourceTemplate.create_resource() — Confirmed

At src/fastmcp/resources/template.py:440-449, create_resource() calls Resource.from_function() passing only name, description, mime_type, tags, task, and auth — but omits icons, annotations, meta, and title.

Resource.from_function() at src/fastmcp/resources/base.py:243-278 accepts all of these parameters, so the fix is straightforward.

Bug 2: FastMCPProviderResourceTemplate.create_resource() — Confirmed

At src/fastmcp/server/providers/fastmcp_provider.py:377-384, create_resource() creates a FastMCPProviderResource passing only name, description, and mime_type — dropping icons, tags, annotations, task_config, meta, title, and auth.

This is inconsistent with FastMCPProviderResource.wrap() at line 171-187 which correctly forwards all fields. The wrap() method serves as the reference for what create_resource() should pass.

Bug 3: Namespace original_name — Not Reproducible

model_copy on a FastMCPProviderTool (which uses _original_name as a Pydantic private attribute) already preserves it through the copy. After namespace transform, get_span_attributes() still returns fastmcp.delegate.original_name correctly.

For plain Tool objects, there's no original_name to preserve — they're local tools served by LocalProvider and don't participate in delegation spans.

Verification

# Bug 1 verification
template = FunctionResourceTemplate.from_function(
    fn=my_func,
    uri_template='weather://{city}/current',
    name='weather',
    icons=[Icon(src='https://example.com/icon.png')],
    annotations=Annotations(audience=['user']),
    meta={'custom_key': 'custom_value'},
   ,
)
resource = await template.create_resource('weather://london/current', {'city': 'london'})

# Result: all four fields are None on the resource
# Template icons: [Icon(src='https://example.com/icon.png')]  →  Resource icons: None
# Template annotations: audience=['user']  →  Resource annotations: None
# Template meta: {'custom_key': 'custom_value'}  →  Resource meta: None
# Template title: 'City Weather'  →  Resource title: None

# Bug 2 verification
wrapped = FastMCPProviderResourceTemplate.wrap(server, template)
resource = await wrapped.create_resource('weather://london/current', {'city': 'london'})

# Result: icons, annotations, tags all lost
# Wrapped icons: [Icon(...)]  →  Resource icons: None
# Wrapped annotations: audience=['user']  →  Resource annotations: None
# Wrapped tags: {'weather'}  →  Resource tags: set()

# Bug 3 verification — NOT a bug
provider_tool = FastMCPProviderTool.wrap(server, tool)
ns = Namespace('math')
[namespaced] = await ns.list_tools([provider_tool])

# _original_name IS preserved:
# namespaced._original_name = 'add'
# span attributes include 'fastmcp.delegate.original_name': 'add'

Action Plan

Fix 1: FunctionResourceTemplate.create_resource() (src/fastmcp/resources/template.py:440-449)

Add the four missing parameters to the Resource.from_function() call:

return Resource.from_function(
    fn=resource_read_fn,
    uri=uri,
    name=self.name,
    description=self.description,
    mime_type=self.mime_type,
    tags=self.tags,
    icons=self.icons,
    annotations=self.annotations,
    meta=self.meta,
   
    task=self.task_config,
    auth=self.auth,
)

Fix 2: FastMCPProviderResourceTemplate.create_resource() (src/fastmcp/server/providers/fastmcp_provider.py:377-384)

Add the missing fields to match what FastMCPProviderResource.wrap() already does at line 171-187:

return FastMCPProviderResource(
    server=self._server,
    original_uri=original_uri,
    uri=AnyUrl(uri),
    name=self.name,
    description=self.description,
    mime_type=self.mime_type,
    tags=self.tags,
    annotations=self.annotations,
    task_config=self.task_config,
    meta=self.get_meta(),
   
    icons=self.icons,
)

Note: version is intentionally not passed — the wrap() method passes it for static resources, but create_resource() creates a new resource from template params and shouldn't inherit the template version.

Bug 3: No action needed

The namespace transform's model_copy already preserves _original_name correctly.

Tests

Add tests in tests/resources/test_resource_template.py verifying that create_resource() preserves icons, annotations, meta, and title. Add a similar test for FastMCPProviderResourceTemplate.create_resource().

Related Files

File	Relevance
src/fastmcp/resources/template.py L440-449	Bug 1: FunctionResourceTemplate.create_resource() missing fields
src/fastmcp/server/providers/fastmcp_provider.py L377-384	Bug 2: FastMCPProviderResourceTemplate.create_resource() missing fields
src/fastmcp/server/providers/fastmcp_provider.py L170-187	FastMCPProviderResource.wrap() — reference for correct field forwarding
src/fastmcp/resources/base.py L243-278	Resource.from_function() — accepts all the missing params already
src/fastmcp/server/transforms/namespace.py L97-113	Bug 3: namespace transform — works correctly, no fix needed

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