# MCP Hangar MCP Server

Policy enforcement plane for MCP: every tool call ends in a verdict. Self-hosted, MIT.

**Publisher claimed.** No tool list reported, and Pod has not connected to this server.

## Status

Pod has not dialled MCP Hangar yet, so everything on this page is what its publisher reported rather than what we observed. Registries describe servers; they do not connect to them. Until a check runs, treat the tool list below as a claim.

## Connect

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

## Known issues

**335 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

### T4 feat(core): gate mid-flight task consent (input_required / tasks/update)

## Parent

Follow-up to the closed WS-4 epic #302.

## Status

**Serving path shipped in `2.0.0rc1`; blocked on [python-sdk#3005](https://github.com/modelcontextprotocol/python-sdk/pull/3005) for a native SEP-2663 backend.**

This issue's previous body said it was *"blocked by the relay-only task decision captured in ADR-008 ... no supported `input_required` / `tasks/update` serving path exists to gate."* That is out of date in both halves:

- ADR-008's relay-only absolutism was superseded in pa

[Read the thread](https://github.com/mcp-hangar/mcp-hangar/issues/322) · 2026-06-30 · closed · 6 comments

### T8 feat(core): best-effort tools/list_changed per session (INKR 4)

## Status

Blocked pending an upstream MCP library capability. Reconfirmed against MCP 1.26.0 during the 2026-07-08 review.

## Why It Is Blocked

Hangar has no supported access to active `ServerSession` instances, so it cannot send a per-session `tools/list_changed` notification safely:

- `StreamableHTTPSessionManager` exposes no public session lookup or session enumeration API.
- `ServerSession` is created and destroyed inside the library run loop; the manager transport map does not retain it

[Read the thread](https://github.com/mcp-hangar/mcp-hangar/issues/234) · 2026-06-23 · closed · 6 comments

### [BUG] `Mcp-Param-*` validation skips are unobserved, and some skip the check while the call still proceeds

## Short summary

SEP-2243's header–body agreement is what stops a caller from routing on one value
while executing another. The SDK enforces it for `Mcp-Param-*`, but the
enforcement is fail-open by design: if the pre-dispatch `tools/list` cannot
produce a schema, validation is skipped and the call proceeds.

**Rewritten after 2.14.0.** The original text said "Hangar does not currently
route or authorize on `Mcp-Param-*`", so the worst case was a deployment that put
an intermediary in front of 

[Read the thread](https://github.com/mcp-hangar/mcp-hangar/issues/1053) · 2026-08-24 · closed · 5 comments

### Streamable HTTP sessions are per-replica, so a coordinated deployment is not one server to a client

A Streamable HTTP session lives in one replica's memory. Three replicas of one
coordinated gateway — one database, one management lease, one identity by
ADR-020 — are therefore three different servers to any MCP client, and the
Service in front of them hands each request to whichever one it feels like.

Measured on core `2.5.2`, `replicaCount: 3`, through the chart's own ClusterIP
Service. Each attempt is one client doing `initialize` → `tools/list` →
session teardown:

```
15 attempts through t

[Read the thread](https://github.com/mcp-hangar/mcp-hangar/issues/877) · 2026-08-11 · closed · 5 comments

### feat(core): migrate to mcp SDK v2 beta (2.0.0b2) on mcp2

## Verdict (audit, WS-1)

**The stateless alignment was hand-rolled on SDK v1 (1.28.x), not on v2 beta.** So migrating to the mcp SDK v2 beta is a *separate* workstream — and the largest one in this preparation. Confirmed in code:

- Pin is `mcp>=1.28.1`; `uv.lock` resolves **1.28.1**; venv runs 1.28.1.
- `src/mcp_hangar/protocol.py` is a leaf module that **manually** builds the reverse-DNS `_meta` keys (`io.modelcontextprotocol/protocolVersion` / `/clientInfo` / `/capabilities`) and merges dict

[Read the thread](https://github.com/mcp-hangar/mcp-hangar/issues/547) · 2026-07-20 · closed · 5 comments

### Most recent

### [TASK] make the per-tenant resource_link cap configurable

Part of #1139, depends on the per-tenant capacity task. **This is the `feat:` part of the parent** -- it adds a config key, so it is what makes the release a minor rather than a patch. Land it separately for that reason.

## Goal

`_MAX_LINKS` is a module constant referenced by nothing else (verified: no other module names it), so an operator whose tenants legitimately hand out more links than the cap has no lever.

## Change

* Make the per-tenant cap configurable, defaulting to today's value s

[Read the thread](https://github.com/mcp-hangar/mcp-hangar/issues/1146) · 2026-08-29 · closed · 0 comments

### [TASK] count resource_link evictions

Part of #1139, depends on the per-tenant capacity task.

## Goal

Nothing counts link evictions, so the defect in the parent issue is invisible to an operator: the victim's `resources/list` simply gets shorter. This is the shape #1128 argued against on the egress path -- an outcome with no record.

## Change

`mcp_hangar_resource_links_evicted_total`, incremented wherever an eviction happens (per-tenant cap and tenant-map cap alike).

**It is appended in `_register_all_metrics` or it does not ex

[Read the thread](https://github.com/mcp-hangar/mcp-hangar/issues/1145) · 2026-08-29 · closed · 0 comments

### [TASK] bound the handed-out resource_link map per tenant, not globally

Part of #1139. **Do this one first**; the metric and the config key in the same parent build on it.

## Goal

`_links` (`src/mcp_hangar/fastmcp_server/resource_link_read_through.py:81`) is one process-wide `OrderedDict` capped at `_MAX_LINKS = 4096` (`:79`) and evicted oldest-first across all tenants (`_remember`, `:152-158`). One tenant's traffic therefore expires another's links.

## Change

An outer map `tenant_id -> OrderedDict[uri, (server, block)]`:

* each inner map capped at `_MAX_LINKS_

[Read the thread](https://github.com/mcp-hangar/mcp-hangar/issues/1144) · 2026-08-29 · closed · 0 comments

### [TASK] remove_group_policy / remove_member_policy remove every kind

Part of #1138. Independent of the other task in this parent -- **no production caller reaches either function today**, which is why nothing has broken and why the first caller would inherit the bug.

## Goal

`remove_group_policy` (`tool_access_resolver.py:304-312`) pops `(group_id, DEFAULT_KIND)` only. `remove_member_policy` (`:314-324`) pops `(group_id, member_id, DEFAULT_KIND)` and the `DEFAULT_KIND` cache entry only. Both are kind-incomplete in exactly the way #1034 fixed for servers.

## Ch

[Read the thread](https://github.com/mcp-hangar/mcp-hangar/issues/1143) · 2026-08-29 · closed · 0 comments

### [TASK] unloading an mcp_server also retires its per-tenant policies

Part of #1138.

## Goal

`remove_mcp_server_policy` (`src/mcp_hangar/domain/services/tool_access_resolver.py:283-298`) clears `_mcp_server_policies` and never touches `_standalone_member_policies` (declared `:94`, written from `tool_access.member.<tenant>`, read on the resolve path `:425`). An id reloaded after a hot-unload inherits its predecessor's per-tenant rules.

Verified on 2.16.0, through the public API:

```python
r.set_standalone_member_policy("billing", "tenant:a", ToolAccessPolicy(de

[Read the thread](https://github.com/mcp-hangar/mcp-hangar/issues/1142) · 2026-08-29 · closed · 0 comments

### [TASK] the withdraw/restore endpoints accept a kind, and a resource URI can reach them

Part of #1137, depends on the events task in the same parent.

## Goal

`server/api/admin_tools.py` never accepts or passes a `kind`, so both endpoints are pinned to the registry's default. A prompt or resource cannot be withdrawn at runtime, the response says it was, and -- verified -- the write withdraws the **same-named tool** instead.

## Two traps, both verified on 2.16.0

**1. A resource URI cannot ride in the current route.** The path is `/{server:str}/{tool:str}/withdraw`, and Starlette'

[Read the thread](https://github.com/mcp-hangar/mcp-hangar/issues/1141) · 2026-08-29 · closed · 0 comments

### [TASK] ToolWithdrawn / ToolRestored carry the kind that was withdrawn

Part of #1137. **Do this one first** -- #1137's endpoint work depends on the field existing.

## Goal

`ToolWithdrawn` and `ToolRestored` (`src/mcp_hangar/domain/events/approvals.py:71`, `:90`) carry `tenant_id`, `mcp_server`, `tool`, `schema_version` -- and no kind. A consumer rebuilding a `LOCAL_VIEW` from the log therefore replays a prompt withdrawal as a tool one, and will keep doing so even after the endpoint is fixed.

## Change

1. Both events grow `kind: str = "tool"`, documented in the 

[Read the thread](https://github.com/mcp-hangar/mcp-hangar/issues/1140) · 2026-08-29 · closed · 0 comments

[See all 24 reports Pod holds for MCP Hangar](/mcp/mcp-hangar/issues) — of 335 qualified upstream.

## Firsthand observations

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