[Bug] resolveConfigValue uses || instead of ?? — empty env vars return literal var name as credential

[RelaxJonh]

Affected area

Coding agent — credential resolution (resolve-config-value.ts)

What happened?

resolveConfigValue() and resolveConfigValueUncached() in packages/coding-agent/src/core/resolve-config-value.ts use || (logical OR) instead of ?? (nullish coalescing) when falling back from an environment variable to the literal config string:

// Line 22 (resolveConfigValue)
const envValue = process.env[config];
return envValue || config;

// Line 96 (resolveConfigValueUncached)
const envValue = process.env[config];
return envValue || config;

|| treats empty string "" as falsy, so when a user sets MY_API_KEY="" (explicitly empty), the function returns the literal environment variable name "MY_API_KEY" instead of "" or undefined.

Steps to reproduce

1. Set an environment variable to empty: export TEST_API_KEY=""
2. Configure auth.json with "key": "TEST_API_KEY"
3. Call resolveConfigValue("TEST_API_KEY")
4. Expected: undefined or "" (empty — should trigger missing-credential error)
5. Actual: "TEST_API_KEY" (the literal string, used as the API key)

Impact

• When a credential env var is set to empty string (common after unset in some shells, or .env files with KEY=), the literal env var name is sent as the API key to the provider.
• This produces confusing 401 errors from the provider ("invalid API key: TEST_API_KEY") instead of a clear "credential not found" message.
• resolveConfigValueOrThrow (line 99) calls resolveConfigValueUncached and checks !== undefined, so it never throws — the empty env var silently becomes the literal name.

Suggested fix

Replace || with ?? in both functions:

// resolveConfigValue (line 22)
return envValue ?? config;

// resolveConfigValueUncached (line 96)
return envValue ?? config;

This preserves the existing behavior for undefined (env var not set → use literal) while correctly handling empty strings (env var set to empty → return empty, triggering downstream missing-credential logic).

Environment

• Prime Agent v0.8.x (main branch, commit 2c34b82f8)
• Node.js, all platforms

[RelaxJonh]

I've investigated this and prepared a fix on my fork: https://github.com/RelaxJonh/prime-agent/tree/fix/resolve-config-value-nullish-coalescing

The fix is a 2-line change in packages/coding-agent/src/core/resolve-config-value.ts:

  const envValue = process.env[config];
- return envValue || config;
+ return envValue ?? config;

Applied to both resolveConfigValue (line 22) and resolveConfigValueUncached (line 96).

Happy to open a PR if the maintainers would like to review it.