Fix HTTP transport timeout defaulting to 5 seconds

[jlowin]

When StreamableHttpTransport was updated to use the new streamable_http_client API, the code path for creating an httpx client when no factory is provided used httpx.AsyncClient() directly. Without an explicit timeout, httpx defaults to 5 seconds for all operations.

This caused tools taking longer than 5 seconds to fail with a 409 Conflict error as the client's SSE stream would timeout and trigger reconnection logic that conflicts with the existing GET stream.

The fix uses create_mcp_http_client() from the MCP SDK which sets appropriate defaults (30s connect, 5min read timeout) and enables follow_redirects.

Fixes #2842, fixes #2845

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep test runtime under pytest 5s timeout

This test forces a 6-second tool run (duration=6), but pytest-timeout is globally set to 5 seconds in pyproject.toml ([tool.pytest.ini_options].timeout = 5). As written, the test will be terminated by pytest-timeout before the tool completes, so CI will fail even when the HTTP fix works. Use a shorter duration or add a per-test timeout marker to keep it within the configured limit.

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[coderabbitai]

Walkthrough

This PR updates src/fastmcp/client/transports.py to import create_mcp_http_client, compute a timeout from read_timeout_seconds in StreamableHttpTransport.connect_session, and pass that timeout (optionally) into the HTTP client creation path. If httpx_client_factory is provided it is invoked with explicit headers/auth/follow_redirects and the optional timeout; otherwise create_mcp_http_client is used as the default client creation path. Comments/documentation were updated to reflect MCP-appropriate timeouts.

Possibly related PRs

• PR 2620 (jlowin/fastmcp): Modifies StreamableHttpTransport client construction and timeout handling in src/fastmcp/client/transports.py, changing how factories/default clients are created and how timeout/header parameters are passed.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically summarizes the main change: fixing HTTP transport timeout defaulting to 5 seconds.
Description check	✅ Passed	The description provides a clear explanation of the problem, root cause, and solution, but does not follow the template checklist structure with checkbox items.
Linked Issues check	✅ Passed	The PR directly addresses the coding objectives from both linked issues #2842 and #2845: fixing timeout handling for long-running tools by using MCP-appropriate defaults instead of httpx's 5-second default.
Out of Scope Changes check	✅ Passed	All changes in src/fastmcp/client/transports.py are directly scoped to fixing the HTTP transport timeout issue described in the linked issues; no out-of-scope modifications detected.

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• src/fastmcp/client/transports.py

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Actionable comments posted: 0

🧹 Nitpick comments (1)

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

287-293: Consider preserving MCP's granular timeout defaults when user provides read_timeout_seconds.

Using httpx.Timeout(read_timeout_seconds.total_seconds()) sets all timeout components (connect, read, write, pool) to the same value. This means a user setting read_timeout_seconds=timedelta(minutes=5) would also get a 5-minute connect timeout, losing the MCP SDK's sensible 30-second connect default.

If preserving granular control is desired:

💡 Suggested refinement

-            timeout = httpx.Timeout(read_timeout_seconds.total_seconds())
+            timeout = httpx.Timeout(
+                connect=30.0,  # Preserve MCP default connect timeout
+                read=read_timeout_seconds.total_seconds(),
+                write=read_timeout_seconds.total_seconds(),
+                pool=read_timeout_seconds.total_seconds(),
+            )

Alternatively, this may be intentional to give users full control. If so, consider documenting that read_timeout_seconds overrides all timeout components.

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• src/fastmcp/client/transports.py

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src/**/*.py

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src/**/*.py: Python source code must use Python ≥3.10 with full type annotations
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Files:

• src/fastmcp/client/transports.py

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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 (3)

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

298-305: Factory path correctly forwards configuration with conditional timeout.

The conditional dict unpacking **({"timeout": timeout} if timeout else {}) cleanly avoids passing timeout=None to factories that may not handle it. The explicit follow_redirects=True ensures consistent behavior between factory and default paths.

---

28-28: Private API usage is the official pattern in the MCP SDK—no changes needed.

The import from mcp.shared._httpx_utils is the recommended approach for HTTP client creation in the MCP Python SDK (also used in official MCP client implementations like SSE and streamable-http). The code correctly uses create_mcp_http_client() to get MCP-standard timeout defaults (30s connect, 5min read) and follows redirects, as documented in the comment at lines 287-289. This is consistent with the existing McpHttpClientFactory import and represents the intended way to handle HTTP clients in the SDK. While the module path uses the private naming convention, this is the stable interface provided by MCP for this purpose.

---

306-311: Correct approach using MCP SDK's client factory with appropriate defaults.

This properly replaces the raw httpx.AsyncClient() with create_mcp_http_client(), which provides MCP-appropriate defaults (30s connect, 5min read timeout, and follow_redirects=True). The parameters passed (headers, timeout, auth) are correctly handled—when timeout is None, the factory's defaults apply; when provided, they override the defaults. This addresses the PR objective of fixing premature timeouts from the original 5-second default.