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SEP-2243 HTTP Standardization#2243

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mikekistler wants to merge 27 commits intomodelcontextprotocol:mainfrom
mikekistler:mdk/http-standardization-sep
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SEP-2243 HTTP Standardization#2243
mikekistler wants to merge 27 commits intomodelcontextprotocol:mainfrom
mikekistler:mdk/http-standardization-sep

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@mikekistler mikekistler commented Feb 13, 2026

Motivation and Context

This PR introduces a SEP for incorporating HTTP features into the HTTP Transport that support processing by "middle boxes" on the internet, for tasks like routing, tracing, and priorization.

How Has This Been Tested?

I have a reference implementation built on the C# SDK that illustrates the basic features in this SEP.

https://github.com/mikekistler/csharp-sdk/tree/mdk/http-standardization

Breaking Changes

All changes are gated by protocol version and thus not breaking.

Types of changes

  • Bug fix (non-breaking change which fixes an issue)
  • New feature (non-breaking change which adds functionality)
  • Breaking change (fix or feature that would cause existing functionality to change)
  • Documentation update

Checklist

  • I have read the MCP Documentation
  • My code follows the repository's style guidelines
  • New and existing tests pass locally
  • I have added appropriate error handling
  • I have added or updated documentation as needed

Additional context

This PR was previously reviewed in the Transports Working Group and updated to address their feedback.
It has been approved to proceed to review by the core maintainers.

@mikekistler mikekistler changed the title SEP-xxxx HTTP Standardization SEP-2243 HTTP Standardization Feb 13, 2026
LucaButBoring
LucaButBoring reviewed Feb 13, 2026
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SamMorrowDrums
SamMorrowDrums reviewed Feb 16, 2026
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This is cool, I’d consider splitting out the extension from this proposal and making it separate because I think there’s little to no controversy in the method based routing part of the proposal. Virtually all my questions/comments pertain to the extended part.

I say that as somebody with existing production use-cases for extension as I mention in a comment, where I'm currently double parsing json.

pja-ant
pja-ant reviewed Feb 17, 2026
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pja-ant
pja-ant reviewed Feb 17, 2026
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mikekistler
mikekistler commented Feb 17, 2026
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000-000-000-000-000
000-000-000-000-000 reviewed Feb 18, 2026
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@localden localden added proposal SEP proposal without a sponsor. SEP labels Feb 21, 2026
@CaitieM20 CaitieM20 mentioned this pull request Feb 23, 2026
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@chemicL chemicL mentioned this pull request Feb 24, 2026
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@CaitieM20 CaitieM20 added the in-review SEP proposal ready for review. label Feb 25, 2026
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CaitieM20 commented Feb 25, 2026

This was voted on by core maintainers on 2/19 as Accept with Changes.

SamMorrowDrums
SamMorrowDrums approved these changes Feb 27, 2026
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@mikekistler thanks for thoughtful changes, looking good to me!

kurtisvg
kurtisvg previously approved these changes Mar 2, 2026
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@kurtisvg kurtisvg mentioned this pull request Mar 3, 2026
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@kurtisvg kurtisvg self-assigned this Mar 4, 2026
@sep-automation-bot sep-automation-bot bot added draft SEP proposal with a sponsor. and removed proposal SEP proposal without a sponsor. labels Mar 4, 2026
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sep-automation-bot bot commented Mar 4, 2026

State Transition: proposal → draft

This SEP has been transitioned from proposal to draft.

@kurtisvg has been assigned as the sponsor for this SEP.

This is an automated message from the SEP lifecycle bot.

@kurtisvg kurtisvg removed the draft SEP proposal with a sponsor. label Mar 4, 2026
@kurtisvg kurtisvg mentioned this pull request Mar 5, 2026
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SamMorrowDrums commented Mar 8, 2026

@kurtisvg I tagged you to review #2355
and @mikekistler I think you'd find it interesting too, because in a sense this is something where it might go the other way:

  • we can add such a recommendation to http spec already
  • we can eventually look at communicating the same data in a different place (such as initialization) for other transports

This PR sets a precedent for JSON RPC duplication in headers, and in my mind it's not crazy to propose the other direction so we can lean into the http transport for stuff it is already good at we are not using.

Unlike auth, where there are complex reasons to keep it in the transport, i18n feels a simple case.

Curious if you agree or have thoughts?

@pja-ant pja-ant mentioned this pull request Mar 11, 2026
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@localden localden mentioned this pull request Mar 11, 2026
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kurtisvg commented Mar 13, 2026

Here are the final vote after discussion in 3/4 Core Maintainer's meeting:

  • SEP-2243: HTTP Standardization
    • Outcome: Accept
    • Votes: 🟢 7 | 🟡 0 | 🔴 0

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kurtisvg commented Mar 13, 2026

/lgtm

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github-actions bot commented Mar 24, 2026

New commits were pushed — removed the approved label. Re-approve with /lgtm.

@mikekistler
Convert proposal from Google Doc to markdown doc in seps folder
b5837fb
@mikekistler
Fix markdownlint and cspell issues
a86b856
@mikekistler
Add literal examples
2a1b11b
@mikekistler
Add rationale for location of custom header metadata
0ca096c
@mikekistler
Define value encoding rules
30afbb4
@mikekistler
Add section on Edge Cases & Conformance Tests
4a3bff5
@mikekistler
Clarify requirement on clients to support custom headers
baff541
@mikekistler
Define JSON RPC error code for header mismatch errors
caf47f4
@mikekistler
Add clear error handling for missing custom headers
22e3b54
@mikekistler
Added tradeoffs on passing metadata in headers vs path
ac3c345
@mikekistler
Added section on options for header number/size limits
e797a60
@mikekistler
Address PR review comments
563a659
@mikekistler
Apply suggestion from @mikekistler
a8b84a1
@mikekistler
Collapse Mcp-Tool-Name, Mcp-Resource, Mcp-Prompt-Name into Mcp-Name
35bf6fa
@mikekistler
Specify Base64 encoding is of UTF-8 representation
3f3b37c
@mikekistler
Add rationale for encoding approach with alternatives evaluation
aabbb54
@mikekistler
Drop caching as advantage of path-based routing
d49bc73
@mikekistler
Clarify client behavior for setting standard and custom headers
ac3ebcb
@mikekistler
Allow intermediaries to reject invalid requests without JSON-RPC erro…
042cfc6 
…r response
@mikekistler
Add security guidance for trusting header values
3b08ec8
@mikekistler
Add rationale for scoping x-mcp-header to tools only
9a6109c
@mikekistler
Remove limit on header value size
7a35a75
@mikekistler
Rename sep file to match PR number
2670ec0
@mikekistler mikekistler force-pushed the mdk/http-standardization-sep branch from 11401e1 to d4e1393 Compare March 24, 2026 13:16
@mikekistler mikekistler force-pushed the mdk/http-standardization-sep branch from d4e1393 to 6b42ef7 Compare March 24, 2026 13:20
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@SamMorrowDrums SamMorrowDrums SamMorrowDrums approved these changes

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