Remove deprecated WSTransport

[jlowin]

WSTransport was deprecated and directed users to StreamableHttpTransport. It was never widely used and had no tests. This removes it from the client module and documents the removal in the v3 upgrade guide.

[coderabbitai]

Walkthrough

This change removes WebSocket transport (WSTransport) from the FastMCP client library. The WSTransport class is deleted from the transports module, and its export is removed from the public API. Documentation is updated to reflect this removal and recommends using StreamableHttpTransport as an alternative. The public API is reorganized to include FastMCPTransport, PythonStdioTransport, SSETransport, and related transports while removing WSTransport exports.

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description is incomplete. It lacks the required checklist items and proper section structure from the template, missing issue reference and self-review confirmation.	Complete the Contributors and Review Checklists with issue number reference, testing confirmation, and self-review acknowledgment as required by the repository template.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Remove deprecated WSTransport' clearly and concisely describes the main change of removing the WSTransport class from the codebase.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[coderabbitai]

Actionable comments posted: 0

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

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

16-30: SSETransport lacks test coverage and user-facing documentation.

SSETransport is exported in the public API but has:

1. No dedicated test coverage (zero test files found)
2. No user-facing documentation in docs/
3. A well-implemented but completely untested transport

This pattern mirrors the removed WSTransport. Before exposing SSETransport as a public API, add test coverage and document its usage.

🧹 Nitpick comments (2)

docs/development/v3-notes/v3-features.mdx (1)

307-310: Consider adding migration guidance or context.

The documentation correctly states that WSTransport has been removed and directs users to StreamableHttpTransport. However, it could benefit from:

1. A brief explanation of why WSTransport was removed (mentioned in PR description: "never widely used, had no tests")
2. A code example showing the migration path, similar to other breaking change sections in this file

Based on coding guidelines for MDX documentation.

📝 Suggested enhancement

 ### WSTransport Removed
 
-The deprecated `WSTransport` client transport has been removed. Use `StreamableHttpTransport` instead.
+The deprecated `WSTransport` client transport has been removed from the client module. WSTransport was never widely used and had no tests. Use `StreamableHttpTransport` for WebSocket-based communication:
+
+```python
+# Before
+from fastmcp.client import WSTransport
+client = Client(WSTransport("ws://server/mcp"))
+
+# After
+from fastmcp.client import StreamableHttpTransport
+client = Client(StreamableHttpTransport("http://server/mcp"))
+```

docs/development/upgrade-guide.mdx (1)

13-16: Consider adding migration example for consistency.

While the documentation clearly states the replacement, all other breaking changes in this guide include before/after code examples (see Mount Namespace Parameter at lines 30-40, Component Enable/Disable at lines 43-57, etc.). Adding a migration example would maintain consistency and help users understand the practical impact.

Based on coding guidelines: "Include complete, runnable code examples that users can copy and execute in MDX documentation."

📝 Suggested enhancement for consistency

 ### WSTransport Removed
 
 The deprecated `WSTransport` client transport has been removed. Use `StreamableHttpTransport` instead.
+
+<CodeGroup>
+```python Before
+from fastmcp.client import WSTransport
+
+transport = WSTransport("ws://localhost:8080/mcp")
+client = Client(transport)
+```
+
+```python After
+from fastmcp.client import StreamableHttpTransport
+
+transport = StreamableHttpTransport("http://localhost:8080/mcp")
+client = Client(transport)
+```
+</CodeGroup>

📜 Review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 87f2edd and 9717b81.

📒 Files selected for processing (4)

• docs/development/upgrade-guide.mdx
• docs/development/v3-notes/v3-features.mdx
• src/fastmcp/client/__init__.py
• src/fastmcp/client/transports.py

💤 Files with no reviewable changes (1)

• src/fastmcp/client/transports.py

🧰 Additional context used

📓 Path-based instructions (4)

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
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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/development/upgrade-guide.mdx
• docs/development/v3-notes/v3-features.mdx

docs/**/*.{md,mdx,json}

📄 CodeRabbit inference engine (AGENTS.md)

Documentation uses Mintlify framework. Files must be in docs.json to be included. Never modify docs/python-sdk/** (auto-generated)

Files:

• docs/development/upgrade-guide.mdx
• docs/development/v3-notes/v3-features.mdx

docs/**/*.{md,mdx}

📄 CodeRabbit inference engine (AGENTS.md)

docs/**/*.{md,mdx}: Code examples in documentation must explain before showing code and make blocks fully runnable (include imports)
Documentation structure: Headers form navigation guide with logical H2/H3 hierarchy. Content should be user-focused with sections motivating features (why) before mechanics (how). Use prose over code comments for important information
Never use 'This isn't...' or 'not just...' constructions in writing - state what something IS directly. Avoid defensive writing patterns

Files:

• docs/development/upgrade-guide.mdx
• docs/development/v3-notes/v3-features.mdx

src/fastmcp/**/*.py

📄 CodeRabbit inference engine (AGENTS.md)

src/fastmcp/**/*.py: Python ≥ 3.10 with full type annotations required
Prioritize readable, understandable code - clarity over cleverness. Avoid obfuscated or confusing patterns even if shorter
Follow existing patterns and maintain consistency in code implementation
Be intentional about re-exports - don't blindly re-export everything to parent namespaces. Core types defining a module's purpose should be exported. Specialized features can live in submodules. Only re-export to fastmcp.* for most fundamental types
Never use bare except - be specific with exception types

Files:

• src/fastmcp/client/__init__.py

🧠 Learnings (4)

📓 Common learnings

Learnt from: CR
Repo: jlowin/fastmcp PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-12-25T15:53:07.656Z
Learning: Applies to tests/**/*.py : Pass FastMCP servers directly to clients for testing without network complexity; only use HTTP transport when explicitly testing network features

📚 Learning: 2025-12-25T15:53:07.656Z

Learnt from: CR
Repo: jlowin/fastmcp PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-12-25T15:53:07.656Z
Learning: Applies to src/fastmcp/**/*.py : Be intentional about re-exports - don't blindly re-export everything to parent namespaces. Core types defining a module's purpose should be exported. Specialized features can live in submodules. Only re-export to fastmcp.* for most fundamental types

Applied to files:

• src/fastmcp/client/__init__.py

📚 Learning: 2025-12-25T15:53:07.656Z

Learnt from: CR
Repo: jlowin/fastmcp PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-12-25T15:53:07.656Z
Learning: Applies to tests/**/*.py : Pass FastMCP servers directly to clients for testing without network complexity; only use HTTP transport when explicitly testing network features

Applied to files:

• src/fastmcp/client/__init__.py

📚 Learning: 2025-12-25T15:53:07.656Z

Learnt from: CR
Repo: jlowin/fastmcp PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-12-25T15:53:07.656Z
Learning: Applies to src/fastmcp/**/*.py : Python ≥ 3.10 with full type annotations required

Applied to files:

• src/fastmcp/client/__init__.py

🧬 Code graph analysis (1)

src/fastmcp/client/__init__.py (3)

src/fastmcp/client/auth/oauth.py (1)

• OAuth (138-329)

src/fastmcp/client/auth/bearer.py (1)

• BearerAuth (11-17)

src/fastmcp/client/transports.py (7)

• ClientTransport (77-119)
• FastMCPTransport (820-889)
• NodeStdioTransport (561-611)
• PythonStdioTransport (479-529)
• StdioTransport (296-420)
• UvStdioTransport (614-690)
• UvxStdioTransport (693-755)

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• GitHub Check: Run tests: Python 3.10 on ubuntu-latest
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• GitHub Check: Run tests: Python 3.13 on ubuntu-latest

🔇 Additional comments (1)

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

1-14: LGTM! Clean removal of WSTransport and intentional expansion of public API.

The changes correctly:

• Remove WSTransport from imports and exports
• Add FastMCPTransport and several Stdio transport variants to the public API
• Maintain alphabetical ordering for readability
• Keep imports and __all__ in sync

The expanded transport exports (FastMCPTransport, PythonStdioTransport, SSETransport, StdioTransport, UvStdioTransport, UvxStdioTransport) align with the provider-based architecture mentioned in v3 features, making these core transports more discoverable for users.

Based on learnings about intentional re-exports.