[u9g]

Problem

Some MCP apps render as an empty box in T3. These apps first load their embed URL in a nested iframe. Inside T3's opaque-origin frame that embed can't boot, for example because its module scripts need CORS and the app's server only grants that to its own origin. The app then falls back to a self-contained HTML preview, which it already holds from the tool result, in a blob: iframe. T3's generated policy only lists the app's declared frameDomains in frame-src, so the browser blocks the fallback as well and nothing renders.

Change

mcpAppContentSecurityPolicy now always allows blob: in frame-src, next to any declared frame origins. A blob: frame can only show a document the app built itself, and it inherits the app's opaque origin, sandbox and CSP. Network access stays limited to the declared connectDomains. img-src and media-src already allow blob: for the same reason.

Snapshots written before this change keep their old policy, because the CSP is injected when a snapshot is saved.

Scope and approval

This is a small, focused fix with no prior issue. Apps that build their UI in a local blob: frame currently render blank, and the fix is a one-directive change to the policy T3 already generates. No new capability or setting is added.

Verification

• vp test run src/mcpApp.test.ts in packages/shared passes (15 tests). The two new or updated assertions cover frame-src blob: with and without declared frame origins.
• Manual check in Chrome: an app document wrapped with injectMcpAppCsp from main and from this branch, hosted in an <iframe sandbox="allow-scripts allow-forms"> like McpAppFrame. The app creates a blob: frame. On main the frame is blocked; with this change it renders.

Before (main)	After

• I also checked a real third-party app snapshot captured by T3, replayed with its stored tool result. On main it shows the blocked-frame page. With blob: added, the app renders fully. Those screenshots contain internal data, so they aren't attached.
• Not checked: the mobile client's MCP app host.

Agent: Claude Opus 5.5 (claude-opus-5-5) via Claude Code in T3 Code.

[macroscopeapp]

Approvability

Verdict: Needs Human Approval

Macroscope's review found this PR requires human approval — This fixes blob-frame fallbacks by allowing blob: frames for MCP apps, even when no frame domains are declared. A human must confirm that default policy expansion; existing sandbox and network restrictions remain unchanged.

^{You can add or adjust custom eligibility rules. Learn more.}

[coderabbitai]

No actionable comments were generated in the recent review.

Walkthrough

The MCP app content security policy now permits blob: frames and includes declared frame domains. Tests cover cases with and without declared frame domains.

Changes: MCP app frame policy

Layer / File(s)	Summary
Update frame-src policy and tests packages/shared/src/mcpApp.ts, packages/shared/src/mcpApp.test.ts	The frame-src directive now includes blob: and appends declared frame domains. Tests check the policy with and without a declared domain.

Estimated code review effort: 2 (Simple) | ~8 minutes

Change: Bug fix

Possibly related PRs

• pingdotgg/t3code#28739: This PR stores CSP in MCP app snapshots, which can use the updated policy to permit blob frames.

Suggested reviewers: juliusmarminge

Merge Risk: Low · up to db691

MCP app inline previews in blob: frames now render. The policy is slightly looser for apps that declare no frame origins, so the owner should confirm that exception is intended. No concrete failure is evident.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly describes the fix for blank MCP apps that use blob frames.
Description check		The description covers the problem, change, scope, verification, screenshots, and unchecked mobile client. It also identifies the agent and harness.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Finishing Touches

Generate unit tests (beta)

• Create a new PR

• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

Recent review info

Run configuration

• Source mode: Auto
• Repository: pingdotgg/t3code
• Branch: main
• File: .coderabbit.config.ts
• Commit: 6577c6b
• Layers: Repository TS config
• Review profile: CHILL
• Plan: Advanced
• Run ID: 4c7b14f1-08a5-490a-96ea-5212e9976240

Commits

Reviewing files that changed from the base of the PR and between 92a33e6 and db69191.

Files selected for processing: 1

• packages/shared/src/mcpApp.ts

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 8 remain after this review.

[maria-rcks]

Note

Written by gpt-6.1-sol on behalf of Maria

partial verification only: the original blob-frame fallback remains unverified.

Verified in a real dev environment

• ran github/main (6f06f2675d) in a separate worktree, then the exact pr head db6919140b672b1dc7307d1b4ce9aa68c2adf6e3. used one dev server at a time, isolated task data, repository playwright, and system chrome on linux.
• created the project and threads through the web app. real codex gpt-6.1-sol turns called the public draw.io mcp service and rendered the returned diagram on both revisions. no mocked provider or injected snapshots.
• the actual saved mcp app document changes from frame-src 'none' on main to frame-src blob: on the head. the declared connect-src origins and allow-scripts allow-forms sandbox remain unchanged. a diagnostic fetch from the app frame to t3's own api was blocked by csp before any network request.
• on the head, zoom, entering full screen, returning inline, and reopening the saved thread worked. blacksmith ran vp test run src/mcpApp.test.ts in packages/shared: 15 tests passed.

runtime policy transcript

these captures show neighboring app rendering, not a before/after reproduction of the blob fallback.

Not verified

• the affected third-party app is unnamed and unavailable here. the public apps inspected do not create nested blob frames, so i could not reproduce the original blank frame on main or prove its fallback on the head through a real user flow. the policy check and neighboring app checks do not establish that end-to-end result.
• native mobile, electron, macos/windows, claude, and remote/tunnel connections were not exercised.

no source changes or pushes. this is not a complete verification sign-off.