Python: Fix header_provider headers not reaching streamable HTTP transport requests

[robbiebusinessacc]

Motivation & Context

MCPStreamableHTTPTool.header_provider headers never reach the MCP server on real
streamable HTTP connections. call_tool() stores the provider's output in a
contextvars.ContextVar, but the transport sends the actual HTTP requests from tasks
the MCP SDK spawns at connect time (post_writer handing requests to tg.start_soon).
Those tasks copied their context before call_tool() ran, so the httpx request hook
always reads an empty dict and the per-call headers (e.g. Authorization) are silently
dropped. The existing unit tests read the ContextVar from the caller's task, which is
why they pass while real connections fail.

Description & Review Guide

• What are the major changes?
  • Keep the per-call headers in an instance-level snapshot for the duration of
    call_tool(), in addition to the ContextVar. The request hook falls back to the
    snapshot when the ContextVar is empty, so headers reach requests sent from
    transport tasks. The previous snapshot value is restored when the call completes.
  • Added a regression test that drives the real streamable_http_client transport
    against an in-process httpx.MockTransport server (initialize, tools/list,
    tools/call — no network) and asserts the Authorization header arrives on the
    tools/call request. It fails without the fix. Also added a test that the
    snapshot is set during a call and cleared afterwards.
• What is the impact of these changes? header_provider now works on real
  streamable HTTP connections. No public API changes; existing tests are unchanged
  and passing.
• What do you want reviewers to focus on? Concurrent call_tool() invocations on
  the same tool instance follow last-writer-wins for the snapshot while their requests
  are in flight. Sequential calls are always attributed correctly, and this is strictly
  better than current behavior where the headers are never applied — flagging it in
  case a stricter scoping is preferred.

Related Issue

Fixes #7161

Open PR #7080 touches the same header machinery but addresses a different gap
(headers during the initialize handshake, #7079). This PR fixes per-call headers
for tools/call; the two are complementary.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Pull request overview

Fixes a Python MCP Streamable HTTP integration bug where MCPStreamableHTTPTool.header_provider headers set during call_tool() were not applied to HTTP requests sent from transport tasks spawned during connect(), causing per-call headers (e.g., Authorization) to be dropped in real connections.

Changes:

• Add an instance-level “active call headers” snapshot and have the httpx request hook fall back to it when the ContextVar is empty.
• Update call_tool() to manage the snapshot lifecycle for the duration of a tool call.
• Add regression tests using a real streamable_http_client + httpx.MockTransport to ensure Authorization reaches tools/call, plus a test asserting the snapshot is restored after the call.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
python/packages/core/agent_framework/_mcp.py	Adds instance-level header snapshot fallback for streamable HTTP request hooks during call_tool().
python/packages/core/tests/core/test_mcp.py	Adds regression coverage validating headers reach tools/call over the real streamable HTTP transport.

[github-actions]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_mcp.py	1369	113	91%	241, 247, 356, 375, 596, 675–676, 808, 863, 978, 981, 991, 995, 1042–1043, 1048, 1055–1056, 1063, 1068–1069, 1076–1077, 1081, 1086–1087, 1096, 1103–1104, 1122, 1135, 1159–1160, 1179–1182, 1184–1185, 1189, 1215, 1249–1251, 1253, 1306–1308, 1367–1368, 1637, 1678–1679, 1692, 1695, 1704–1705, 1710–1711, 1717, 1771–1772, 1792–1793, 1802–1803, 1808–1809, 1815, 1908, 1911, 1938, 1961–1965, 1988–1990, 1995, 1999–2000, 2102, 2109, 2111, 2182, 2197–2198, 2205–2206, 2211–2212, 2217, 2221, 2236, 2298, 2481, 2483, 2505, 2507–2510, 2523–2524, 2568, 2630, 3055–3056, 3293–3294, 3312
TOTAL	44839	5103	88%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
9140	33 💤	0 ❌	0 🔥	2m 20s ⏱️

[robbiebusinessacc]

@microsoft-github-policy-service agree