fix(backend): validate installation tokens against the installation, not GET /user

[simmi-tdh]

Fixes #1661

GitHub App installation tokens (ghs_) fail repository discovery because getGitHubReposFromConfig attempts to verify credentials via GET /user. Since installation tokens authenticate as an integration rather than a user, GitHub returns a 403 Resource not accessible by integration error, halting the discovery process. Because the isAuthenticated: !!token guard cannot be bypassed, this check prevents installation tokens from functioning. I looked at the source code and it seems the token type is already recognized (detectGitHubTokenType returns app_installation), and getRepoAuth already constructs x-access-token Git credentials from it. Currently, only the preflight check rejects it.

Fix:
Validate against the endpoint appropriate for the token type—GET /installation/repositories for installation tokens, and GET /user for others. Unrecognized prefixes will attempt GET /user and fall back on a 403. Since invalid credentials still trigger an error, the validation remains robust. Only one line at the call site needs to be changed.

Added 8 new tests to github.test.ts (bringing the total to 29 in the file and 313 across the backend suite). Verified by unit test; the 403 was originally observed on a live v5.1.12 deployment. I haven't yet run a patched build end-to-end against a live installation token.

I have omitted repos.listForAuthenticatedUser and rest.search.repos, as they fail in the same manner when users: is configured. I kept them out to maintain focus, but I am happy to include them if you would prefer.

Summary by CodeRabbit

• Bug Fixes

  • Improved GitHub credential validation for user and installation tokens.
  • Repository configuration authentication now supports installation credentials and handles unsupported or unauthorized credentials more reliably.

• Tests

  • Added coverage for credential type detection, installation access validation, fallback behavior, and authentication failures.

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Note

Medium Risk
Changes authentication preflight for all GitHub connections; behavior is well-tested but misclassification could reject valid tokens or accept bad ones briefly before discovery fails elsewhere.

Overview
Fixes GitHub App installation token (ghs_) preflight auth so repository discovery no longer fails when a valid installation token gets 403 from GET /user.

Credential checks in getGitHubReposFromConfig now go through verifyCredential, which picks the endpoint by token type: user-context tokens (classic/OAuth/app-user/fine-grained PAT) still use GET /user; installation tokens use GET /installation/repositories with per_page: 1. Unrecognized prefixes try GET /user first and only fall back to the installation endpoint on 403; other errors still fail fast.

Adds supportsUserIntrospection / USER_INTROSPECTABLE_TOKEN_TYPES and unit tests for type classification, installation validation, unknown-token fallback, and invalid credentials.

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• packages/backend/src/github.test.ts

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• packages/backend/src/github.test.ts

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Walkthrough

GitHub credential validation now supports user tokens, installation tokens, and unknown token types. Repository configuration uses the token-aware verifier, with tests covering endpoint selection and error handling.

Changes

GitHub credential validation

Layer / File(s)	Summary
Token classification and verification packages/backend/src/github.ts, packages/backend/src/github.test.ts	The code identifies token types that support GET /user. verifyCredential validates user tokens through GET /user, installation tokens through GET /installation/repositories, and unknown tokens through a 403 fallback. Tests cover these paths and 401 rejection.
Repository configuration authentication packages/backend/src/github.ts	Repository configuration calls verifyCredential instead of directly calling GET /user.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant RepositoryConfig
  participant verifyCredential
  participant GitHubAPI
  RepositoryConfig->>verifyCredential: Verify configured token
  verifyCredential->>GitHubAPI: Validate through token-specific endpoint
  GitHubAPI-->>verifyCredential: Return success or HTTP error
  verifyCredential-->>RepositoryConfig: Resolve or reject validation

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Merge Risk: ⚪ Minimal · up to 35eb8

Installation tokens can now pass credential preflight through their compatible GitHub endpoint while invalid credentials continue to be rejected. No actionable merge-blocking risk was identified.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: validating GitHub installation tokens against the installation endpoint instead of GET /user.
Linked Issues check	✅ Passed	The changes satisfy the coding requirements in [#1661]. verifyCredential uses GET /user for user-context tokens and GET /installation/repositories for ghs_ tokens. Unknown prefixes try `GET /u…
Out of Scope Changes check	✅ Passed	The changes stay within [#1661]. Token classification, credential verification, repository discovery integration, and focused tests directly address the preflight bug. The users: repository paths re…
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 2…

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
In `@packages/backend/src/github.test.ts`:
- Around line 420-425: Extend the unknown-token tests around verifyCredential to
assert users.getAuthenticated is called once before the
installation-repositories request in the 403 case, and add coverage for an
unknown token receiving a non-403 response such as 401: verifyCredential must
reject and must not call octokit.request.

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• packages/backend/src/github.test.ts
• packages/backend/src/github.ts

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