opencode web: many API routes return SPA HTML instead of JSON, causing ClientError: UnsupportedContentType

[Winsoftpedia]

Description

When running opencode web and accessing the interface via a browser (e.g. http://localhost:18000), several API endpoints return text/html (the SPA shell) instead of application/json. This causes the frontend to throw ClientError: UnsupportedContentType in the C$ function (content-type check in the fetch wrapper), logged as "Failed to finish bootstrap instance".

Impact

• Console error on every page load: Failed to finish bootstrap instance ClientError: UnsupportedContentType
• Toast notification: "Failed to reload {project}"
• The UI still works despite the error (the bootstrap runner catches it gracefully)
• Desktop app and CLI are not affected — only opencode web

Affected endpoints

The following endpoints return text/html (SPA fallback) instead of the expected application/json or text/event-stream:

Endpoint	Expected	Actual
GET /api/server	JSON	HTML
GET /api/path?location=...	JSON	HTML
GET /api/vcs?location=...	JSON	HTML
GET /api/model/default?location=...	JSON	HTML
GET /api/plugin	JSON	HTML
GET /api/permission/request/list?location=...	JSON	HTML
GET /api/question/request/list?location=...	JSON	HTML
POST /api/session/{id}/command	JSON	HTML

Meanwhile, these endpoints work correctly:

• GET /api/health ✅
• GET /api/session ✅
• GET /api/agent ✅
• GET /api/location ✅
• GET /api/session/active ✅
• GET /global/config ✅
• GET /global/health ✅
• All /experimental/... endpoints ✅

Notes

• The experimental API endpoints (/experimental/...) work correctly on the same server
• Running with --pure (no plugins) does not fix it
• Clean reinstall of v1.18.5 does not fix it
• Version: 1.18.5 (latest)
• Root cause: the C$ function in the frontend bundle checks Content-Type and throws for non-JSON/non-SSE responses

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [Bug] Desktop v1.18.5: UnsupportedContentType error on project reload after update #38789: Same UnsupportedContentType / "Failed to reload project" error in Desktop v1.18.5 — same root cause and symptoms
• Web UI: New Session creates in wrong project, /project/init returns HTML, Settings missing Projects tab, Auto-accept toggle broken #31272: Web UI API endpoints returning HTML instead of JSON (overlapping scope)

Please review those issues before this one is investigated further. If your issue is distinct, feel free to clarify what differs.

[opencode-agent]

Fixed in v1.18.6. The release restores protocol-aware compatibility across the affected API flows, so API requests resolve through their JSON/SSE handlers instead of falling through to the SPA HTML response. The final legacy bootstrap gap was addressed by #38816.