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MCP servers, over streamable HTTP

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A server is a row with a URL; its tools are discovered by a button and
cached, then offered beside the built-in ones. Written by hand rather than
taken from the reference SDK, because that SDK's transport does its own
connecting -- and the one thing that must not be bypassed is check_url on
every hop. Owning the transport is the point; the framing beside it is the
small part.

Sessions are per call: initialize, initialized, the call, a best-effort
DELETE. Caching one wants an owner, a TTL, eviction, a lock and a shutdown
hook, and the server may expire it under all of that anyway -- ToolContext
is a session-free snapshot precisely so nothing in a tool holds live state.

A server's names and descriptions reach the model as instructions and are
bounded before they do; what it returns is escaped preformatted text, never
markdown. Tools are namespaced per server, so two servers exposing "search"
do not collide and neither shadows a built-in.

Also: a round's calls now run together under a semaphore, results indexed
so each tool turn stays paired with its call, and generation.status names
what is running -- a remote tool is latency-bound, and a silent pause is
what a hang looks like.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Jaroslav Beneš
2026-08-01 16:44:29 +02:00
co-authored by Claude Opus 5
parent d4cefb066a
commit ecadb66414
15 changed files with 2166 additions and 12 deletions
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src/lembas/web/templates/admin/mcp_detail.html
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{% extends "admin/_layout.html" %}
{% from "_macros.html" import icon %}
{% set section = "mcp" %}
{% block title %}{{ "New server" if is_new else server.name }} - LLeMbas{% endblock %}
{% block heading %}{{ "New MCP server" if is_new else server.name }}{% endblock %}
{% block admin_content %}
<nav class="crumbs">
<a class="crumbs__back" href="/admin/mcp">
{{ icon("chevron-right", "icon--sm crumbs__icon") }} All servers
</a>
</nav>
{% if error %}
<div class="alert alert--error">{{ icon("warning", "icon--sm") }} <span>{{ error }}</span></div>
{% endif %}
{% if server.last_error %}
<div class="alert alert--error">
{{ icon("warning", "icon--sm") }}
<span>Last contacted unsuccessfully: {{ server.last_error }}</span>
</div>
{% endif %}
<form method="post" action="{{ '/admin/mcp' if is_new else '/admin/mcp/' ~ server.id }}"
class="form-grid">
<section class="card">
<h2 class="card__title">The server</h2>
<div class="field">
<label class="field__label" for="name">Name</label>
<input class="input" id="name" name="name" value="{{ server.name }}" required
maxlength="120" placeholder="GitHub">
</div>
<div class="field">
<label class="field__label" for="slug">Identifier</label>
<input class="input input--mono" id="slug" name="slug" value="{{ server.slug }}" required
maxlength="24" pattern="[a-z0-9][a-z0-9_\-]*" placeholder="github">
<p class="field__hint">
Prefixed onto every tool name this server offers, so that two servers
both exposing <code>search</code> do not collide.
</p>
</div>
<div class="field">
<label class="field__label" for="url">Endpoint URL</label>
<input class="input input--mono" id="url" name="url" value="{{ server.url }}" required
placeholder="https://mcp.example.com/mcp">
<p class="field__hint">
The streamable-HTTP endpoint itself, the one that accepts a POST. A
server that answers with a redirect to somewhere else will be refused.
</p>
</div>
<div class="field">
<label class="field__label" for="headers">Extra headers</label>
<textarea class="textarea input--mono" id="headers" name="headers" rows="3"
spellcheck="false">{{ headers_text }}</textarea>
<p class="field__hint">One <code>Name: value</code> per line.</p>
</div>
<div class="field">
<label class="field__label" for="timeout">Timeout (seconds)</label>
<input class="input" id="timeout" name="timeout" value="{{ server.timeout }}"
inputmode="numeric">
</div>
<div class="field">
<label class="field__label" for="max_chars">Most characters to keep per call</label>
<input class="input" id="max_chars" name="max_chars" value="{{ server.max_chars }}"
inputmode="numeric">
</div>
</section>
<section class="card">
<h2 class="card__title">Credential</h2>
<div class="field">
<label class="field__label" for="secret_placement">How it is sent</label>
<select class="select" id="secret_placement" name="secret_placement">
{% for value, label in secret_placements %}
<option value="{{ value }}" {{ 'selected' if value == server.secret_placement }}>
{{ label }}
</option>
{% endfor %}
</select>
</div>
<div class="field">
<label class="field__label" for="secret_name">Header or parameter name</label>
<input class="input input--mono" id="secret_name" name="secret_name"
value="{{ server.secret_name }}" maxlength="120">
</div>
<div class="field">
<label class="field__label" for="secret">Secret</label>
<input class="input input--mono" id="secret" name="secret" type="password"
autocomplete="off" placeholder="No secret set"
value="{{ unchanged if server.secret_encrypted else '' }}">
<p class="field__hint">
{% if server.secret_encrypted %}
Currently <code>{{ masked }}</code>. Leave the dots alone to keep it,
or clear the field to remove it.
{% else %}
Encrypted at rest and never shown again.
{% endif %}
</p>
</div>
</section>
{% if tools %}
<section class="card">
<h2 class="card__title">Tools it offers</h2>
<p class="field__hint">
Discovered at the last refresh. Untick one to withhold it — a tool this
server adds later is offered by default.
</p>
<input type="hidden" name="tool_choices" value="1">
<div class="checkbox-row checkbox-row--stacked">
{% for tool in tools %}
<label class="checkbox">
<input type="hidden" name="tool_names" value="{{ tool.name }}">
<input type="checkbox" name="tool_names_on" value="{{ tool.name }}"
{{ 'checked' if tool.on }}>
<span>
<code>{{ tool.offer_name or tool.name }}</code>
{% if tool.offer_name and tool.offer_name != tool.name %}
<span class="faint text-xs">({{ tool.name }} on the server)</span>
{% endif %}
{% if tool.description %}
<br><span class="faint text-xs">{{ tool.description }}</span>
{% endif %}
</span>
</label>
{% endfor %}
</div>
</section>
{% elif not is_new %}
<section class="card">
<h2 class="card__title">Tools it offers</h2>
<p class="field__hint">
Nothing discovered yet. Save, then press <strong>Test &amp; refresh</strong>
on the <a href="/admin/mcp">list</a>.
</p>
</section>
{% endif %}
<section class="card">
<h2 class="card__title">Guidance</h2>
<div class="field">
<textarea class="textarea" name="guidance" rows="4"
placeholder="- Use the GitHub tools for anything about our repositories."
>{{ server.guidance }}</textarea>
<p class="field__hint">
Added to the system message whenever any tool from this server is
offered. One piece of guidance for the server, not one per tool —
the tools carry their own descriptions.
{% if prompt_overridden %}
<br><strong>Someone has overridden this wording under
<a href="/admin/prompts">Prompts</a></strong> — that is what the model
sees, not this.
{% endif %}
</p>
</div>
</section>
<section class="card">
<h2 class="card__title">Availability</h2>
<div class="field">
<div class="checkbox-row">
<label class="checkbox">
<input type="checkbox" name="enabled" value="true" {{ 'checked' if server.enabled }}>
<span>Enabled — offered in chats</span>
</label>
<label class="checkbox">
<input type="checkbox" name="allow_private" value="true"
{{ 'checked' if server.allow_private }}>
<span>May reach private and loopback addresses</span>
</label>
</div>
<p class="field__hint">
Tick the second only for a server on your own network. It is what stops
this being aimed at LLeMbas itself, a router, or a metadata endpoint.
</p>
</div>
<div class="field">
<label class="checkbox">
<input type="checkbox" name="public" value="true" {{ 'checked' if server.public }}>
<span>Available to everyone</span>
</label>
</div>
<div class="field">
<span class="field__label">Groups with access</span>
{% if groups %}
<div class="checkbox-row">
{% for group in groups %}
<label class="checkbox">
<input type="checkbox" name="group_ids" value="{{ group.id }}"
{{ 'checked' if group.id in selected_groups }}>
<span>{{ group.name }}</span>
</label>
{% endfor %}
</div>
<p class="field__hint">Ignored while the server is available to everyone.</p>
{% else %}
<p class="field__hint">
No groups yet — <a href="/admin/groups">create one</a> to restrict access.
</p>
{% endif %}
</div>
<div class="field">
<label class="field__label" for="position">Position</label>
<input class="input" id="position" name="position" value="{{ server.position }}"
inputmode="numeric">
</div>
</section>
<div class="btn-row">
<button class="btn btn--primary" type="submit">
{{ "Add server" if is_new else "Save changes" }}
</button>
<a class="btn btn--ghost" href="/admin/mcp">Back to all servers</a>
{% if not is_new %}
<button class="btn btn--danger" type="submit" formnovalidate
formaction="/admin/mcp/{{ server.id }}/delete"
data-confirm-button="Delete the server “{{ server.name }}”? Chats that used its tools keep their transcripts.">
Delete
</button>
{% endif %}
</div>
</form>
{% endblock %}