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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.

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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 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 That is out of date in both halves:

Read the thread · 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:

Read the thread · 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 · 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.

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