fix: add default values to Literal type fields in content types

Content types like TextContent and ImageContent have single-value Literal type
fields (e.g., type: Literal["text"]) that now default automatically so users
don't have to pass them explicitly.

Before: TextContent(type="text", text="hello")
After: TextContent(text="hello")

Github-Issue: #1731

Author: felixweinberger

src/mcp/types.py

@@ -1016,7 +1016,7 @@ class GetPromptRequest(Request[GetPromptRequestParams, Literal["prompts/get"]]):
 class TextContent(BaseModel):
     """Text content for a message."""
 
-    type: Literal["text"]
+    type: Literal["text"] = "text"
     text: str
     """The text content of the message."""
     annotations: Annotations | None = None
@@ -1031,7 +1031,7 @@ class TextContent(BaseModel):
 class ImageContent(BaseModel):
     """Image content for a message."""
 
-    type: Literal["image"]
+    type: Literal["image"] = "image"
     data: str
     """The base64-encoded image data."""
     mimeType: str
@@ -1051,7 +1051,7 @@ class ImageContent(BaseModel):
 class AudioContent(BaseModel):
     """Audio content for a message."""
 
-    type: Literal["audio"]
+    type: Literal["audio"] = "audio"
     data: str
     """The base64-encoded audio data."""
     mimeType: str
@@ -1077,7 +1077,7 @@ class ToolUseContent(BaseModel):
     in the next user message.
     """
 
-    type: Literal["tool_use"]
+    type: Literal["tool_use"] = "tool_use"
     """Discriminator for tool use content."""
 
     name: str
@@ -1105,7 +1105,7 @@ class ToolResultContent(BaseModel):
     from the assistant. It contains the output of executing the requested tool.
     """
 
-    type: Literal["tool_result"]
+    type: Literal["tool_result"] = "tool_result"
     """Discriminator for tool result content."""
 
     toolUseId: str
@@ -1172,7 +1172,7 @@ class EmbeddedResource(BaseModel):
     of the LLM and/or the user.
     """
 
-    type: Literal["resource"]
+    type: Literal["resource"] = "resource"
     resource: TextResourceContents | BlobResourceContents
     annotations: Annotations | None = None
     meta: dict[str, Any] | None = Field(alias="_meta", default=None)
@@ -1190,7 +1190,7 @@ class ResourceLink(Resource):
     Note: resource links returned by tools are not guaranteed to appear in the results of `resources/list` requests.
     """
 
-    type: Literal["resource_link"]
+    type: Literal["resource_link"] = "resource_link"
 
 
 ContentBlock = TextContent | ImageContent | AudioContent | ResourceLink | EmbeddedResource
@@ -1584,7 +1584,7 @@ def content_as_list(self) -> list[SamplingMessageContentBlock]:
 class ResourceTemplateReference(BaseModel):
     """A reference to a resource or resource template definition."""
 
-    type: Literal["ref/resource"]
+    type: Literal["ref/resource"] = "ref/resource"
     uri: str
     """The URI or URI template of the resource."""
     model_config = ConfigDict(extra="allow")
@@ -1598,7 +1598,7 @@ class ResourceReference(ResourceTemplateReference):
 class PromptReference(BaseModel):
     """Identifies a prompt."""
 
-    type: Literal["ref/prompt"]
+    type: Literal["ref/prompt"] = "ref/prompt"
     name: str
     """The name of the prompt or prompt template"""
     model_config = ConfigDict(extra="allow")

tests/test_types.py

@@ -361,3 +361,62 @@ def test_list_tools_result_preserves_json_schema_2020_12_fields():
     assert tool.inputSchema["$schema"] == "https://json-schema.org/draft/2020-12/schema"
     assert "$defs" in tool.inputSchema
     assert tool.inputSchema["additionalProperties"] is False
+
+
+def test_content_types_have_default_literal_type_fields():
+    """Test that content types default their Literal type fields.
+
+    Content types like TextContent have single-value Literal type fields
+    (e.g., type: Literal["text"]) that should default automatically so users
+    don't have to pass them explicitly.
+
+    Before fix: TextContent(type="text", text="hello") - type required
+    After fix: TextContent(text="hello") - type defaults to "text"
+
+    See: https://github.com/modelcontextprotocol/python-sdk/issues/1731
+    """
+    from mcp.types import (
+        AudioContent,
+        EmbeddedResource,
+        ImageContent,
+        PromptReference,
+        ResourceLink,
+        ResourceTemplateReference,
+        TextResourceContents,
+    )
+
+    # TextContent should default type to "text"
+    text_content = TextContent(text="hello")
+    assert text_content.type == "text"
+
+    # ImageContent should default type to "image"
+    image_content = ImageContent(data="base64data", mimeType="image/png")
+    assert image_content.type == "image"
+
+    # AudioContent should default type to "audio"
+    audio_content = AudioContent(data="base64data", mimeType="audio/mp3")
+    assert audio_content.type == "audio"
+
+    # ToolUseContent should default type to "tool_use"
+    tool_use = ToolUseContent(name="my_tool", id="call_123", input={"arg": "value"})
+    assert tool_use.type == "tool_use"
+
+    # ToolResultContent should default type to "tool_result"
+    tool_result = ToolResultContent(toolUseId="call_123")
+    assert tool_result.type == "tool_result"
+
+    # EmbeddedResource should default type to "resource"
+    embedded = EmbeddedResource(resource=TextResourceContents(uri="file:///test.txt", text="content"))
+    assert embedded.type == "resource"
+
+    # ResourceLink should default type to "resource_link"
+    resource_link = ResourceLink(name="test", uri="file:///test.txt")
+    assert resource_link.type == "resource_link"
+
+    # ResourceTemplateReference should default type to "ref/resource"
+    resource_ref = ResourceTemplateReference(uri="file:///test.txt")
+    assert resource_ref.type == "ref/resource"
+
+    # PromptReference should default type to "ref/prompt"
+    prompt_ref = PromptReference(name="my_prompt")
+    assert prompt_ref.type == "ref/prompt"