feat: support `ui/update-model-context` for MCP Apps by aharvard · Pull Request #8044 · block/goose

aharvard · 2026-03-20T19:57:12Z

Summary

Implements ui/update-model-context from the MCP Apps specification, allowing MCP App views to push context updates to the host agent without triggering immediate action.

Closes #6472

Demo

goose-update-model-context.mov

How it works

When an MCP App sends ui/update-model-context:

McpAppRenderer (desktop) catches the request and POSTs to /agent/update_model_context
Agent stores the context in an in-memory HashMap, keyed by {extension}__{resourceUri} (last-write-wins)
On the next agent turn, stored contexts are formatted as XML blocks and injected alongside MOIM as a synthetic user message

MCP App (iframe)
  → postMessage("ui/update-model-context", { content, structuredContent })
    → McpAppRenderer → POST /agent/update_model_context
      → Agent.mcp_app_contexts HashMap (in-memory, ephemeral)
        → Next LLM turn: collect → format as XML → inject with MOIM

The model sees context like:

<mcp-app-context source="mcp-app-bench__inspect-model-context">
The user is interacting with a counter demo. Current counter value: 5
{"counter": 5}
</mcp-app-context>

Design decisions

Last-write-wins: Each app resource gets a single slot. Subsequent updates overwrite the previous one — no accumulation.
Ephemeral: Context lives only in memory. Not persisted to session storage. Clears on new session.
Non-triggering: Unlike ui/message, updating context does not kick off an agent turn. It passively waits for the next natural turn.
MOIM integration: App context is combined with existing MOIM (working dir, tool usage, etc.) into a single synthetic user message.

Changes

File	What
`crates/goose/src/agents/agent.rs`	`McpAppContext` struct, `mcp_app_contexts` HashMap, `update_mcp_app_context()`, `collect_mcp_app_contexts()`, 4 unit tests
`crates/goose/src/agents/moim.rs`	`inject_moim()` accepts `mcp_app_context` param, combines with MOIM text, 2 new tests
`crates/goose/src/agents/mod.rs`	Re-exports `McpAppContext`
`crates/goose-server/src/routes/agent.rs`	`POST /agent/update_model_context` endpoint
`crates/goose-server/src/openapi.rs`	Registers endpoint + schema
`ui/desktop/src/components/McpApps/McpAppRenderer.tsx`	Handles `ui/update-model-context` from MCP Apps
`ui/desktop/openapi.json`, `api/*`	Auto-generated

Testing

7 unit tests covering: empty state, store + collect, overwrite, structured content, MOIM + app context injection, app context only injection
Manual testing with mcp-app-bench Model Context Inspector (counter demo + manual send)

Implement the ui/update-model-context method from the MCP Apps specification, allowing MCP App views to push context updates to the host agent without triggering immediate action. Context updates are stored in-memory (last-write-wins per app key) and injected alongside MOIM as a synthetic user message on the next agent turn, giving the model awareness of app state. - Add McpAppContext store on Agent with update/collect methods - Add /agent/update_model_context server endpoint - Integrate with MOIM injection in moim.rs - Handle ui/update-model-context in McpAppRenderer - Add 7 unit tests covering store, overwrite, collect, and injection Signed-off-by: Andrew Harvard <aharvard@squareup.com> Co-authored-by: Goose <opensource@block.xyz> Ai-assisted: true

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crates/goose/src/agents/agent.rs

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ui/desktop/src/components/McpApps/McpAppRenderer.tsx

crates/goose/src/agents/agent.rs

Escape <, >, &, and " in app-provided text and keys before wrapping them in <mcp-app-context> XML blocks. Prevents malformed markup from benign payloads (e.g. code snippets) and blocks malicious apps from closing the tag to spoof extra context sections. Adds a test covering special character escaping. Signed-off-by: Andrew Harvard <aharvard@squareup.com> Co-authored-by: Goose <opensource@block.xyz> Ai-assisted: true

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crates/goose/src/agents/agent.rs

Content blocks without a text field (e.g. image, resource) were silently dropped during context collection. Now falls back to JSON serialization so the model sees all block types. Signed-off-by: Andrew Harvard <aharvard@squareup.com> Co-authored-by: Goose <opensource@block.xyz> Ai-assisted: true

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ui/desktop/src/components/McpApps/McpAppRenderer.tsx

crates/goose/src/agents/agent.rs

aharvard · 2026-03-20T20:42:39Z

Regarding the per-view key comment — this is a known design decision. The current key scheme ({extension}__{resourceUri}) intentionally uses last-write-wins per resource. This is the correct default for the common case (one instance per resource).

For the multi-instance question (same resource rendered multiple times in a session), we've opened an issue upstream to get spec guidance: modelcontextprotocol/ext-apps#558

Instance-scoped keys can be added later without breaking the existing API if multi-instance support is needed.

When serializing non-text content blocks (e.g. images), remove the `data` field before JSON-stringifying. This prevents base64 payloads from bloating the synthetic MOIM message with thousands of wasted tokens. The block metadata (type, mimeType, etc.) is preserved so the model knows a non-text block exists without seeing the raw data. Signed-off-by: Andrew Harvard <aharvard@squareup.com> Co-authored-by: Goose <opensource@block.xyz> Ai-assisted: true

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crates/goose/src/agents/agent.rs

structuredContent is view-only hydration data and should never be injected into the model context. Remove it from collect_mcp_app_contexts entirely. Also strip resource.blob from embedded resource content blocks alongside the existing data field stripping, preventing binary payloads from leaking into the synthetic message. Signed-off-by: Andrew Harvard <aharvard@squareup.com> Co-authored-by: Goose <opensource@block.xyz> Ai-assisted: true

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chatgpt-codex-connector · 2026-03-20T21:21:14Z

crates/goose/src/agents/agent.rs

+    s.replace('&', "&amp;")
+        .replace('<', "&lt;")
+        .replace('>', "&gt;")
+        .replace('"', "&quot;")
+}


Stop escaping quotes in app-context body text

collect_mcp_app_contexts() serializes non-text blocks to JSON and then runs them through xml_escape(). Because this helper also rewrites " to ", the fallback you inject into Message::user().with_text(...) is no longer valid JSON—apps that send image/resource blocks will reach the model as {"type":...} instead of something it can reliably inspect or copy. Escaping </& is enough for element text here; escaping quotes breaks the metadata this feature is trying to preserve.

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chatgpt-codex-connector · 2026-03-20T21:21:14Z

crates/goose-server/src/routes/agent.rs

+            goose::agents::McpAppContext {
+                content: payload.content,
+                structured_content: payload.structured_content,


Drop structuredContent instead of retaining it unused

This request stores structuredContent on McpAppContext, but the new collection path explicitly never injects that field into the prompt. I checked the production references after this change: outside tests, structured_content is only written here and on the struct, never read. For MCP apps that follow our own guidance and put full hydration payloads in structuredContent, each update now pins that potentially large JSON blob in mcp_app_contexts until another update or session teardown with no effect on model behavior.

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DOsinga

left some comments, but maybe we should talk about this. is moim the right mechanism? if the agent gets reset due to some mechanism, you lose the value the way it is setup right now. should we consider a user invisible message instead? and also what's the role of the structured content here?

DOsinga · 2026-03-21T13:01:33Z

crates/goose/src/agents/moim.rs

    }

-    if let Some(moim) = extension_manager
+    let moim = extension_manager


the way it works right now is that the extension manager does all the moim collection, moving part of that here but not everything makes this fragile. so I would either have the extension manager return a list of moim items and do the construction here (preferred) or send the mcp_app_context to the extension manager

DOsinga · 2026-03-21T13:02:28Z

crates/goose/src/agents/moim.rs

        );
    }
+
+    #[tokio::test]


I would drop these tests. all they do is confirm that if you add a string it makes it to the other end

DOsinga · 2026-03-21T13:09:41Z

crates/goose/src/agents/agent.rs

+                }
+            }
+            // structuredContent is intentionally not injected — it is
+            // view-only hydration data and should never reach the model.


this gives me pause - shouldn't the structured content go back to the widget then again?

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aharvard changed the title ~~feat: support ui/update-model-context for MCP Apps~~ feat: support ui/update-model-context for MCP Apps Mar 20, 2026

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aharvard requested review from DOsinga, alexhancock, jamadeo, jh-block and michaelneale March 20, 2026 21:01

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