fix: auto-detect Bun runtime when not explicitly set in trigger.config.ts

[abhay0132]

Fixes #3105

Problem

When a user runs their project with Bun (e.g. via a pre-build command like
bun run build) but does not explicitly set runtime: "bun" in their
trigger.config.ts, the deployment UI incorrectly shows "Node.js" as the runtime.

This happened because the default runtime was hardcoded to "node" regardless
of what runtime was actually executing the process.

Fix

Auto-detect the runtime by checking process.versions.bun at config resolution
time. If Bun is detected and the user has not explicitly set a runtime in their
config, we now correctly default to "bun" instead of "node".

The explicit user config always takes priority — this only affects the fallback.

Changes

• packages/cli-v3/src/config.ts: replaced hardcoded "node" default with
  runtime auto-detection using process.versions.bun

[changeset-bot]

⚠️ No Changeset found

Latest commit: ae4e2b5

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[github-actions]

Hi @abhay0132, thanks for your interest in contributing!

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This PR will be closed automatically. See https://github.com/triggerdotdev/trigger.dev/blob/main/CONTRIBUTING.md for more details.

[coderabbitai]

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Reviewing files that changed from the base of the PR and between 35298ac and ae4e2b5.

📒 Files selected for processing (1)

• packages/cli-v3/src/config.ts

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Walkthrough

The resolveConfig function in packages/cli-v3/src/config.ts has been modified to change how the runtime is determined. A new detectedRuntime value is introduced that checks for Bun's presence via typeof process.versions.bun === "string", defaulting to "node" otherwise. The defaultRuntime is now calculated by prioritizing user-provided config.runtime, then falling back to detectedRuntime, rather than deriving it solely from the run_engine_v2 compatibility feature. The resolved defaultRuntime continues to be written to the merged config.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

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[devin-ai-integration]

Devin Review found 1 potential issue.

View 1 additional finding in Devin Review.

[devin-ai-integration]

🚩 Behavioral change: deploy from Bun CLI will now default to Bun runtime in production images

Previously the default runtime was always "node" (since run_engine_v2 was unconditionally forced true at packages/cli-v3/src/config.ts:191). Now, if a user runs trigger deploy from a Bun process without explicitly setting runtime in their trigger.config.ts, the deployed Docker image will use the Bun runtime. This is intentional per the commit message ("detect Bun runtime automatically when not explicitly configured"), but it's a meaningful behavioral change for users who happen to use Bun as their local shell/script runner but don't intend for their deployed tasks to run under Bun (which is marked as experimental per packages/cli-v3/src/config.ts:351-356). The bun experimental warning at line 351-356 only fires when config.runtime === "bun" is explicitly set in the config file, so auto-detected Bun will skip the warning.

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