Add transport property to Context by jlowin · Pull Request #2850 · PrefectHQ/fastmcp

jlowin · 2026-01-12T19:45:34Z

Tools sometimes need to know which transport the server is running on—for example, to adjust response formatting for STDIO or handle timeout behavior differently for HTTP transports.

This adds ctx.transport which returns the current transport type:

from fastmcp import FastMCP, Context

mcp = FastMCP("example")

@mcp.tool
def my_tool(ctx: Context) -> str:
    if ctx.transport == "stdio":
        return "short response"
    return "detailed response with more context"

Returns Literal["stdio", "sse", "streamable-http"] when running, or None outside a server context.

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coderabbitai · 2026-01-12T19:54:12Z

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Reviewing files that changed from the base of the PR and between 22c76d0 and 680709b.

⛔ Files ignored due to path filters (1)

tests/server/test_context.py is excluded by none and included by none

📒 Files selected for processing (1)

src/fastmcp/server/http.py

Walkthrough

This PR introduces transport tracking capabilities to the FastMCP server. A new TransportType literal type is defined supporting "stdio", "sse", and "streamable-http". A ContextVar stores the current transport type, with set_transport() and reset_transport() functions managing its lifecycle. The transport type is exposed as a read-only property on both LogData and Context classes. Transport state is initialized at server startup in the stdio, SSE, and streamable-http implementations. Documentation describing the transport semantics and usage is added.

Possibly related PRs

SEP-1699: Add SSE polling support with EventStore #2564: Also modifies the Context class in src/fastmcp/server/context.py with new public APIs, extending context tracking capabilities

🚥 Pre-merge checks | ✅ 1 | ❌ 2

❌ Failed checks (1 warning, 1 inconclusive)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 76.92% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Description check	❓ Inconclusive	The description explains the motivation, provides a clear code example, and documents the return type. However, it lacks the Contributors Checklist items required by the template (issue closure, testing, documentation updates).	Complete the Contributors Checklist section by checking off items and confirming the issue number, test coverage, and documentation updates are in place.

✅ Passed checks (1 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Add transport property to Context' accurately summarizes the main change: adding a transport property to the Context class to expose the current server transport type.

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Actionable comments posted: 5

🧹 Nitpick comments (1)

src/fastmcp/client/transports.py (1)

211-223: Docstring update is helpful; consider explicitly stating what happens when timeout is not provided.

Given the behavior change (default timeouts now come from create_mcp_http_client), calling out the “no factory + no read_timeout_seconds” case would reduce surprises.

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📥 Commits

Reviewing files that changed from the base of the PR and between 18c71f9 and 89ee4f4.

⛔ Files ignored due to path filters (3)

loq.toml is excluded by none and included by none
tests/integration_tests/test_timeout_fix.py is excluded by none and included by none
tests/server/test_context.py is excluded by none and included by none

📒 Files selected for processing (5)

docs/servers/context.mdx
src/fastmcp/client/transports.py
src/fastmcp/server/context.py
src/fastmcp/server/http.py
src/fastmcp/server/server.py

🧰 Additional context used

📓 Path-based instructions (2)

docs/**/*.mdx

📄 CodeRabbit inference engine (docs/.cursor/rules/mintlify.mdc)

docs/**/*.mdx: Use clear, direct language appropriate for technical audiences
Write in second person ('you') for instructions and procedures in MDX documentation
Use active voice over passive voice in MDX technical documentation
Employ present tense for current states and future tense for outcomes in MDX documentation
Maintain consistent terminology throughout all MDX documentation
Keep sentences concise while providing necessary context in MDX documentation
Use parallel structure in lists, headings, and procedures in MDX documentation
Lead with the most important information using inverted pyramid structure in MDX documentation
Use progressive disclosure in MDX documentation: present basic concepts before advanced ones
Break complex procedures into numbered steps in MDX documentation
Include prerequisites and context before instructions in MDX documentation
Provide expected outcomes for each major step in MDX documentation
End sections with next steps or related information in MDX documentation
Use descriptive, keyword-rich headings for navigation and SEO in MDX documentation
Focus on user goals and outcomes rather than system features in MDX documentation
Anticipate common questions and address them proactively in MDX documentation
Include troubleshooting for likely failure points in MDX documentation
Provide multiple pathways (beginner vs advanced) but offer an opinionated path to avoid overwhelming users in MDX documentation
Always include complete, runnable code examples that users can copy and execute in MDX documentation
Show proper error handling and edge case management in MDX code examples
Use realistic data instead of placeholder values in MDX code examples
Include expected outputs and results for verification in MDX code examples
Test all code examples thoroughly before publishing in MDX documentation
Specify language and include filename when relevant in MDX code examples
Add explanatory comments for complex logic in MDX code examples
Document all API...

Files:

docs/servers/context.mdx

src/**/*.py

📄 CodeRabbit inference engine (AGENTS.md)

src/**/*.py: Python source code must use Python ≥3.10 with full type annotations
Never use bare except - be specific with exception types
Prioritize readable, understandable code - clarity over cleverness; avoid obfuscated or confusing patterns
Follow existing patterns and maintain consistency across the codebase

Files:

src/fastmcp/server/context.py
src/fastmcp/server/server.py
src/fastmcp/server/http.py
src/fastmcp/client/transports.py

🧬 Code graph analysis (2)

src/fastmcp/server/server.py (2)

src/fastmcp/server/context.py (3)

fastmcp (186-191)

reset_transport (93-95)

set_transport (86-90)

src/fastmcp/utilities/logging.py (1)

temporary_log_level (103-151)

src/fastmcp/server/http.py (1)

src/fastmcp/server/context.py (3)

fastmcp (186-191)

reset_transport (93-95)

set_transport (86-90)

🪛 LanguageTool

docs/servers/context.mdx

[style] ~307-~307: This phrase is redundant. Consider using “outside”.
Context: ...lifetime. It returns None when called outside of a server context (for example, in unit ...

(OUTSIDE_OF)

[style] ~307-~307: This phrase is redundant. Consider using “outside”.
Context: ...ple, in unit tests or when running code outside of an MCP request). ```python from fastmc...

(OUTSIDE_OF)

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (4)

GitHub Check: Run tests: Python 3.10 on windows-latest
GitHub Check: Run tests: Python 3.13 on ubuntu-latest
GitHub Check: Run tests: Python 3.10 on ubuntu-latest
GitHub Check: Run tests with lowest-direct dependencies

🔇 Additional comments (3)

src/fastmcp/server/context.py (1)

421-429: Context.transport property is straightforward and matches the intended API.

Returning _current_transport.get() with a defaulted ContextVar cleanly yields None outside a transport-scoped context.

src/fastmcp/server/server.py (1)

2117-2149: stdio transport scoping + finally-reset is solid; initialization options addition looks reasonable.

Setting set_transport("stdio") at entry and resetting in finally is the right pattern, and threading experimental_capabilities + NotificationOptions(tools_changed=True) through initialization is consistent with the PR goals.

src/fastmcp/client/transports.py (1)

28-29: No action needed on this import; mcp.shared._httpx_utils is a documented public utility in the MCP SDK. [resolve]

Despite the leading-underscore naming convention, the MCP SDK explicitly exposes McpHttpClientFactory and create_mcp_http_client as documented utilities for external use. These are stable, supported APIs with clear signatures and documented defaults (30s connect timeout, 5min read timeout for HTTP clients).

coderabbitai · 2026-01-12T19:54:15Z

+### Transport
+
+The `ctx.transport` property indicates which transport is being used to run the server. This is useful when your tool needs to behave differently depending on whether the server is running over STDIO, SSE, or Streamable HTTP. For example, you might want to return shorter responses over STDIO or adjust timeout behavior based on transport characteristics.
+
+The transport type is set once when the server starts and remains constant for the server's lifetime. It returns `None` when called outside of a server context (for example, in unit tests or when running code outside of an MCP request).
+
+```python
+from fastmcp import FastMCP, Context
+
+mcp = FastMCP("example")
+
+@mcp.tool
+def connection_info(ctx: Context) -> str:
+    if ctx.transport == "stdio":
+        return "Connected via STDIO"
+    elif ctx.transport == "sse":
+        return "Connected via SSE"
+    elif ctx.transport == "streamable-http":
+        return "Connected via Streamable HTTP"
+    else:
+        return "Transport unknown"
+```
+
+**Property signature:** `ctx.transport -> Literal["stdio", "sse", "streamable-http"] | None`
+


⚠️ Potential issue | 🟡 Minor

Tighten phrasing and add a tiny “verify” hint for readers.

Line 307: “outside of a server context” → “outside a server context” (per static analysis hint).

Consider adding a one-liner after the code block like “Verify: call the tool and confirm it returns ‘Connected via …’” to align with the docs guideline of giving an expected outcome.

Proposed edit

-The transport type is set once when the server starts and remains constant for the server's lifetime. It returns `None` when called outside of a server context (for example, in unit tests or when running code outside of an MCP request). +The transport type is set once when the server starts and remains constant for the server's lifetime. It returns `None` when called outside a server context (for example, in unit tests or when running code outside an MCP request).

🧰 Tools

🪛 LanguageTool

[style] ~307-~307: This phrase is redundant. Consider using “outside”.
Context: ...lifetime. It returns None when called outside of a server context (for example, in unit ...

(OUTSIDE_OF)

[style] ~307-~307: This phrase is redundant. Consider using “outside”.
Context: ...ple, in unit tests or when running code outside of an MCP request). ```python from fastmc...

(OUTSIDE_OF)

Expose the current transport type (stdio, sse, streamable-http) via ctx.transport, enabling transport-specific behavior in tools.

coderabbitai

Actionable comments posted: 0

🧹 Nitpick comments (1)

docs/servers/context.mdx (1)
314-324: Minor: Consider adding async to the example function for consistency.

The transport property access doesn't require await, so the sync function works correctly. However, most other Context examples in this file use async def since tools typically use async context methods. For consistency with other examples, you might consider:
 @mcp.tool
-def connection_info(ctx: Context) -> str:
+async def connection_info(ctx: Context) -> str:
This is purely a consistency suggestion—the current sync version is functionally correct.

📜 Review details

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Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 89ee4f4 and 22c76d0.

⛔ Files ignored due to path filters (2)

loq.toml is excluded by none and included by none
tests/server/test_context.py is excluded by none and included by none

📒 Files selected for processing (4)

docs/servers/context.mdx
src/fastmcp/server/context.py
src/fastmcp/server/http.py
src/fastmcp/server/server.py

🚧 Files skipped from review as they are similar to previous changes (1)

src/fastmcp/server/http.py

🧰 Additional context used

📓 Path-based instructions (2)

src/**/*.py

📄 CodeRabbit inference engine (AGENTS.md)

src/**/*.py: Python source code must use Python ≥3.10 with full type annotations
Never use bare except - be specific with exception types
Prioritize readable, understandable code - clarity over cleverness; avoid obfuscated or confusing patterns
Follow existing patterns and maintain consistency across the codebase

Files:

src/fastmcp/server/context.py
src/fastmcp/server/server.py

docs/**/*.mdx

📄 CodeRabbit inference engine (docs/.cursor/rules/mintlify.mdc)

docs/**/*.mdx: Use clear, direct language appropriate for technical audiences
Write in second person ('you') for instructions and procedures in MDX documentation
Use active voice over passive voice in MDX technical documentation
Employ present tense for current states and future tense for outcomes in MDX documentation
Maintain consistent terminology throughout all MDX documentation
Keep sentences concise while providing necessary context in MDX documentation
Use parallel structure in lists, headings, and procedures in MDX documentation
Lead with the most important information using inverted pyramid structure in MDX documentation
Use progressive disclosure in MDX documentation: present basic concepts before advanced ones
Break complex procedures into numbered steps in MDX documentation
Include prerequisites and context before instructions in MDX documentation
Provide expected outcomes for each major step in MDX documentation
End sections with next steps or related information in MDX documentation
Use descriptive, keyword-rich headings for navigation and SEO in MDX documentation
Focus on user goals and outcomes rather than system features in MDX documentation
Anticipate common questions and address them proactively in MDX documentation
Include troubleshooting for likely failure points in MDX documentation
Provide multiple pathways (beginner vs advanced) but offer an opinionated path to avoid overwhelming users in MDX documentation
Always include complete, runnable code examples that users can copy and execute in MDX documentation
Show proper error handling and edge case management in MDX code examples
Use realistic data instead of placeholder values in MDX code examples
Include expected outputs and results for verification in MDX code examples
Test all code examples thoroughly before publishing in MDX documentation
Specify language and include filename when relevant in MDX code examples
Add explanatory comments for complex logic in MDX code examples
Document all API...

Files:

docs/servers/context.mdx

🧬 Code graph analysis (1)

src/fastmcp/server/server.py (2)

src/fastmcp/server/context.py (3)

fastmcp (186-191)

reset_transport (93-95)

set_transport (86-90)

src/fastmcp/server/tasks/capabilities.py (1)

get_task_capabilities (12-34)

🪛 LanguageTool

docs/servers/context.mdx

[style] ~307-~307: This phrase is redundant. Consider using “outside”.
Context: ...lifetime. It returns None when called outside of a server context (for example, in unit ...

(OUTSIDE_OF)

[style] ~307-~307: This phrase is redundant. Consider using “outside”.
Context: ...ple, in unit tests or when running code outside of an MCP request). ```python from fastmc...

(OUTSIDE_OF)

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (4)

GitHub Check: Run tests: Python 3.10 on windows-latest
GitHub Check: Run tests: Python 3.13 on ubuntu-latest
GitHub Check: Run tests: Python 3.10 on ubuntu-latest
GitHub Check: Run tests with lowest-direct dependencies

🔇 Additional comments (4)

src/fastmcp/server/context.py (2)

80-95: LGTM! Clean transport state management API.

The TransportType literal, ContextVar, and the set_transport/reset_transport pair provide a well-designed, thread-safe mechanism for tracking transport state. The token-based reset pattern ensures proper cleanup and supports nested contexts correctly.

421-428: LGTM! Well-documented transport property.

The property correctly exposes the transport type with clear documentation of possible values and the None case for out-of-context usage.

docs/servers/context.mdx (1)

303-327: LGTM! Clear and well-structured documentation.

The Transport section follows the documentation patterns established in the file:

Clear explanation of purpose and use cases

Lifecycle behavior documented

Complete, runnable code example with all transport cases

Property signature at the end

The code example demonstrates practical usage by showing how to branch behavior based on transport type.

src/fastmcp/server/server.py (1)

2117-2148: LGTM! Correct transport lifecycle management for stdio.

The implementation properly:

Sets transport state before entering the server context

Uses try/finally to guarantee cleanup even on exceptions

Captures the token for proper reset

This pattern is consistently applied across all transport types: stdio, SSE, and streamable-http transports all follow the same lifecycle management approach.

ContextVars set in lifespan don't propagate to Starlette request handler tasks. Move transport setting to RequestContextMiddleware which runs per-request, and store transport_type on app.state.

jlowin added the feature Major new functionality. Reserved for 2-4 significant PRs per release. Not for issues. label Jan 12, 2026

jlowin added feature Major new functionality. Reserved for 2-4 significant PRs per release. Not for issues. and removed enhancement Improvement to existing functionality. For issues and smaller PR improvements. labels Jan 12, 2026

chatgpt-codex-connector Bot reviewed Jan 12, 2026

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Comment thread src/fastmcp/server/http.py Outdated

coderabbitai Bot reviewed Jan 12, 2026

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Add transport property to Context

22c76d0

Expose the current transport type (stdio, sse, streamable-http) via ctx.transport, enabling transport-specific behavior in tools.

jlowin force-pushed the add-transport-to-context branch from 89ee4f4 to 22c76d0 Compare January 12, 2026 20:04

mintlify Bot deployed to staging - docs January 12, 2026 20:05 View deployment

coderabbitai Bot reviewed Jan 12, 2026

View reviewed changes

Fix HTTP transport: set per-request instead of in lifespan

680709b

ContextVars set in lifespan don't propagate to Starlette request handler tasks. Move transport setting to RequestContextMiddleware which runs per-request, and store transport_type on app.state.

jlowin merged commit bf3df4d into main Jan 12, 2026
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jlowin added this to the 3.0 milestone Jan 13, 2026

jlowin mentioned this pull request Jan 16, 2026

Add missing v3 features to tracking docs #2888

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coderabbitai Bot mentioned this pull request Jan 16, 2026

Add session-scoped state persistence #2873

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jlowin added enhancement Improvement to existing functionality. For issues and smaller PR improvements. and removed feature Major new functionality. Reserved for 2-4 significant PRs per release. Not for issues. labels Jan 20, 2026

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Add transport property to Context#2850

Add transport property to Context#2850
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