# MCP Stdio MCP Server

Stdio-to-HTTP gateway — connects MCP clients to remote HTTP MCP servers

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## Status

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## Connect

Published as `mcp-stdio` on pypi. Runs locally.

## Known issues

**72 problems reported by people outside the maintainer team.** Issues filed by the project's own owners, members and collaborators are excluded — those are release checklists and internal refactors, not things that will go wrong for you. Showing 12.

### Most discussed

### Design: dual-mode Streamable HTTP for the MCP 2026-07-28 draft revision (stateless transport, SEP-2575)

## Summary

The next MCP spec revision — draft dated **2026-07-28** — rewrites the Streamable HTTP transport into a stateless, per-request-metadata model ([draft changelog](https://modelcontextprotocol.io/specification/draft/changelog)). Most of the changes land squarely on gateway code paths. This issue captures the design for supporting both the current revision (2025-11-25, sessioned) and the draft revision (stateless) side by side.

> **Revised — see [the design revision comment](https://git

[Read the thread](https://github.com/shigechika/mcp-stdio/issues/270) · 2026-07-01 · closed · 26 comments

### Firestore token store: revoked-family tokens can be resurrected by a stale concurrent writer (residual to #406)

## Summary

While implementing the #406 fix (union-merge inside a Firestore transaction for `--token-store-firestore`'s blind-overwrite race), I audited every place that removes an entry from one of `_OAuthProvider`'s six in-memory stores (`src/mcp_stdio/server.py`). Two categories exist:

1. **Expiry-driven deletes** (`_gc_locked`, the lazy expiry check in `_live_entry`, the expired-token branch of `_token_refresh`) — safe to resurrect via merge, because expiry is re-checked independently on ev

[Read the thread](https://github.com/shigechika/mcp-stdio/issues/428) · 2026-08-23 · closed · 3 comments

### Phase 3.5-C′: modern pool polish (reaper, --modern-only, advisories, docs truth-up)

Phase 3.5-C′ per the #270 Phase 3 Re-scope (pool polish + carried advisories).

## Scope

- Idle-TTL/eviction reaper for modern pool children (today: cap-based eviction only, no reaper — the cap is a soft policy that can be exceeded when every child is pending/busy, by design).
- `--modern-only` posture flag (re-scope decision D2): opt-in 405 on GET/DELETE for deployments with no legacy clients (405 is SHOULD-strength and scoped to modern-only servers; a dual-era default must keep GET/SSE + DELE

[Read the thread](https://github.com/shigechika/mcp-stdio/issues/376) · 2026-07-31 · closed · 3 comments

### Phase 3.5-E: reverse MRTR bridge (child-initiated sampling/elicitation/roots on the modern face)

Phase 3.5-E per the #270 Phase 3 Re-scope (deferred from Phase 3 by decision D4).

## Scope

The reverse MRTR bridge: a pooled legacy child's server-initiated `sampling`/`elicitation`/`roots` requests, translated to `InputRequiredResult` + client retry on the modern face — the mirror of the relay's Phase 2 PR C.

- O14 obligations: server-to-client requests ONLY via MRTR; `InputRequiredResult` only on `tools/call`/`resources/read`/`prompts/get`; `requestState` is attacker-controlled input — inte

[Read the thread](https://github.com/shigechika/mcp-stdio/issues/375) · 2026-07-31 · closed · 3 comments

### Phase 3.5-D: serve-side subscriptions/listen (modern notification channel)

Phase 3.5-D per the #270 Phase 3 Re-scope (deferred from Phase 3 by decision D4 — not required by AC1/AC2/AC4).

## Scope

Serve-side `subscriptions/listen`: the modern face's notification channel, mirroring what the relay's client side consumes (Phase 2 PR A).

- O17 obligations (spec-verified in the re-scope): `notifications/subscriptions/acknowledged` MUST be the first message; `io.modelcontextprotocol/subscriptionId` stamping on every stream message; filter honoring (echo the honored subset)

[Read the thread](https://github.com/shigechika/mcp-stdio/issues/374) · 2026-07-31 · closed · 3 comments

### Most recent

### Firestore token store: cap-eviction and client-recycling resurrection (residual to #428)

## Summary

Follow-up to #406/#428/#429: while adding per-token tombstones for `_revoke_family_locked` (#428, closing the security-relevant residual — a family-wide revocation is now protected the same way an individual rotation/code-redemption already was), two lower-severity bare-delete paths were deliberately left out to keep that fix focused:

1. **Cap-based eviction** (`_evict_to_capacity_locked`, called from `_issue()` on `access`/`refresh`/`consumed_codes`/`consumed_refresh`/`consumed_acc

[Read the thread](https://github.com/shigechika/mcp-stdio/issues/433) · 2026-08-23 · open · 1 comment

### OAuth HTTP traffic (oauth.py, cli.py) is not covered by --max-message-size

## Background

PR #417 (`--max-message-size`, closing #416) bounds unbounded response reads in `relay.py`'s MCP JSON-RPC traffic and `serve.py`'s incoming request body. `/code-review medium` on that PR found the same CWE-770 class is still open on a different code path: the OAuth HTTP client traffic never goes through `_read_bounded`/`_iter_text_bounded` or gets the `Accept-Encoding: identity` default.

Concretely, unbounded (buffered `client.get()`/`client.post()`) reads remain in:

- `src/mcp_

[Read the thread](https://github.com/shigechika/mcp-stdio/issues/419) · 2026-08-22 · closed · 0 comments

### Support bounded (zip-bomb-resistant) decompression for --max-message-size

## Background

PR #417 (`--max-message-size`, closing #416) bounds how much of an upstream response `relay` buffers before parsing, and how large a request body `serve` accepts. During review, [ai-review flagged](https://github.com/shigechika/mcp-stdio/pull/417) that the cap can be bypassed: `httpx.Client()` advertises `Accept-Encoding: gzip, deflate` by default, and `resp.iter_bytes()`/`resp.iter_text()` transparently decompress a `Content-Encoding`-compressed response before the byte count is 

[Read the thread](https://github.com/shigechika/mcp-stdio/issues/418) · 2026-08-22 · closed · 0 comments

### Unbounded response/request body reads can OOM the gateway (relay + serve)

## Problem

Neither direction of the gateway bounds the size of a message it reads into memory before parsing:

- **`relay` (client side)**: every response-consuming call site — `resp.read()` (used by `_post_probe`, `_post_and_stream`'s JSON path, `_post_parsed`, and the initial probe reader) and `resp.iter_text()` (used by every SSE-consuming path, including `run_sse`'s long-lived reader and the modern era's `subscriptions/listen` stream) — buffers the remote MCP server's response with no upper

[Read the thread](https://github.com/shigechika/mcp-stdio/issues/416) · 2026-08-22 · closed · 0 comments

### test_poison_cancel_lines_log_once fails on CPython 3.14.7 (passed on 3.14.6)

## Symptom

`tests/test_relay.py::TestRunModernStdinHandoff::test_poison_cancel_lines_log_once`
fails on the `test (ubuntu-latest, 3.14)` job:

```
FAILED tests/test_relay.py::TestRunModernStdinHandoff::test_poison_cancel_lines_log_once
  - AssertionError: assert 0 == 1
1 failed, 1941 passed
```

The assertion is `err.count("cancel detection failed") == 1`, and it
gets 0 — the failure path never fires at all, so nothing is logged.

## Root cause: a CPython patch bump, not a code change

The runn

[Read the thread](https://github.com/shigechika/mcp-stdio/issues/411) · 2026-08-13 · closed · 0 comments

### --token-store-firestore: blind overwrite races during Cloud Run instance overlap

## Context

\`--token-store-firestore\`'s own \`--help\` text documents "no lock against two processes sharing one document -- safe ONLY when exactly one serve process ever writes to a given document at a time." A caller can approximate that with Cloud Run's \`min/max-instances=1\`, but that setting only bounds *concurrent* instances of a single revision -- it does not prevent the outgoing and incoming instance from running simultaneously during Cloud Run's normal instance lifecycle (a redeploy'

[Read the thread](https://github.com/shigechika/mcp-stdio/issues/406) · 2026-08-09 · closed · 1 comment

### ci(docs): PR runs can evict a queued Pages deployment (shared concurrency group)

Found by AI review on shigechika/zapi-mcp#54, where `docs.yml` was copied verbatim from here. The template is the right place to fix it.

## Problem

`docs.yml` declares the concurrency group at workflow level:

```yaml
on:
  push:
    branches: [main]
    paths: [...]
  pull_request:
    paths: [...]

concurrency:
  group: pages
  cancel-in-progress: false
```

Both event types share the group `pages`. GitHub keeps **only one pending run per group** — a newly queued run replaces the previously 

[Read the thread](https://github.com/shigechika/mcp-stdio/issues/397) · 2026-08-06 · closed · 0 comments

[See all 19 reports Pod holds for MCP Stdio](/mcp/mcp-stdio/issues) — of 72 qualified upstream.

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