bake: fix "Runtime file not found" panic in production builds of custom frameworks

[robobun]

Fixes #32142

bun build --app with any custom Bake.Framework panicked after "Rendering routes":

panic: Runtime file not found. This is an unexpected bug in Bun. Please file a bug report on GitHub.

Repro (from the issue): a bun.app.ts with fileSystemRouterTypes pointing at custom serverEntryPoint/clientEntryPoint files. The client chunk was written, then bun crashed before running prerender, so dist/index.html was never produced.

Cause

generateChunksInParallel only populated OutputFile.bake_extra when framework.is_built_in_react was true. For custom frameworks every chunk's bake_extra.bake_is_runtime stayed false, so production.rs never found a runtime file index and hit the panic.

Two problems hid behind that gate:

1. bake_is_runtime (and the route kind) must be set for any framework, not just built-in React.
2. BakeRouteKind::FullyStaticRoute feeds the noClient flag in renderRoutesForProdStatic, which drops the client entry URL from RouteMetadata.modules. The "use client" analysis it relies on only produces boundaries when server components are enabled; un-gating it blindly would have classified every route of a non-server-components framework as fully static and silently dropped its declared clientEntryPoint from prerendered pages.

There was also a second latent bug: when no bundled module needs the runtime's wrapper helpers (easy to hit with a small custom-framework app), the runtime is live in no chunk at all, so even with bake_extra populated there is legitimately no runtime file. That case now skips the disk write instead of panicking.

Fix

• src/bundler/linker_context/generateChunksInParallel.rs: populate bake_extra for every framework; a server entry point is FullyStaticRoute only when server_components is configured and the visitor finds no client boundary, otherwise Route (built-in React always configures server components, so its classification is unchanged).
• src/runtime/bake/production.rs: a missing runtime chunk is not an error; only write it to disk when it exists.
• Removed the now-unread is_built_in_react mirrors from the bundler-side and parser-side framework views (the runtime-side field stays; it still gates the React install hint).

Verification

New test in test/bake/dev/production.test.ts builds the issue's custom-framework app and asserts the build exits 0, dist/index.html contains the prerendered content, and the client entry script URL is passed to prerender via meta.modules and exists on disk. With the src/ changes reverted it fails with the panic above (re-checked after the rebase below).

Existing production tests (including "don't include client code if fully static route", which covers the React fully-static path) and test/bake/framework-router.test.ts pass.

Rebase notes

Rebased onto main after BakeExtra.is_route/fully_static were replaced by the BakeRouteKind enum. Conflicts resolved:

• generateChunksInParallel.rs: the server_components gate is now expressed as the FullyStaticRoute vs Route choice shown above.
• bundle_v2.rs: kept main's pub(crate) fields and its removal of client_css_in_js; dropped is_built_in_react as before.

Everything else merged cleanly.

---

no test proof · iteration 4 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/bake/dev/production.test.ts

[robobun]

^{Updated 11:49 PM PT - Aug 28th, 2026}

❌ @robobun, your commit d976a21 has 1 failures in Build #108173 (All Failures):

• test/js/web/url/url.test.ts - code 1: TypeError: Invalid URL on 🍎 any x64

---

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bunx bun-pr 32143

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bun-32143 --bun

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• test/bake/dev/production.test.ts

Walkthrough

This PR removes is_built_in_react from framework types and projections. It updates chunk metadata and route classification, skips missing runtime files instead of panicking, and adds a custom framework production build test.

Changes

Custom framework build corrections

Layer / File(s)	Summary
Framework contract updates src/bundler/bundle_v2.rs, src/js_parser/parser.rs, src/bundler/ParseTask.rs, src/runtime/bake/...	Removes is_built_in_react from framework types, constructor arguments, and runtime-to-bundler projections.
Chunk metadata and route classification src/bundler/linker_context/generateChunksInParallel.rs	Removes the React-only BakeExtra gate. Fully static route classification now requires server components and no transitive "use client" boundary.
Runtime file output handling src/runtime/bake/production.rs	Skips runtime output when no runtime file index exists and updates framework view documentation.
Custom framework build validation test/bake/dev/production.test.ts	Tests custom framework builds, rendered HTML, client script markup, and the referenced bundle on disk.

Suggested reviewers: jarred-sumner, dylan-conway

Merge Risk: ⚪ Minimal · up to 36535

The PR fixes production builds for custom frameworks, while the regression test has a minor validation gap because it checks the process exit code before validating generated files; this weakens failure diagnostics but is not merge-blocking after normal checks.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes address issue #32142 by populating runtime metadata for custom frameworks, avoiding invalid runtime-file assumptions, preserving client entry metadata, and adding a regression test for suc…
Out of Scope Changes check	✅ Passed	The changes remain within scope. Framework-field cleanup, route classification, runtime-file handling, and the regression test directly support the reported production-build panic and its fix.
Title check	✅ Passed	The title clearly and concisely identifies the primary change: fixing the "Runtime file not found" panic in production builds that use custom frameworks.
Description check	✅ Passed	The description explains the issue, cause, fix, scope, and verification. It provides detailed test evidence, although it uses "Cause," "Fix," and "Verification" headings instead of the template headin…

Full details: Linked Issues check

Explanation

The changes address issue #32142 by populating runtime metadata for custom frameworks, avoiding invalid runtime-file assumptions, preserving client entry metadata, and adding a regression test for successful prerendered output.

Full details: Description check

Explanation

The description explains the issue, cause, fix, scope, and verification. It provides detailed test evidence, although it uses "Cause," "Fix," and "Verification" headings instead of the template headings.

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[claude]

I didn't find any bugs, but this changes bundler chunk-generation semantics (un-gating bake_extra, redefining fully_static for non-server-components frameworks) and relaxes a panic into a silent skip — worth a human look at those two judgment calls.

Extended reasoning...

Overview

The PR fixes a panic in bun build --app for custom (non-React) Bake frameworks by (1) populating OutputFile.bake_extra for all frameworks instead of only when is_built_in_react is true, (2) gating the fully_static route classification on server_components.is_some() so custom frameworks without server components don't have their client entry silently dropped, (3) converting the "Runtime file not found" panic in production.rs to a no-op when no chunk contains the runtime, and (4) removing the now-unread is_built_in_react mirror field from the bundler/parser-side Framework projections (4 files). A regression test reproduces the issue and asserts the client entry script reaches prerender.

Security risks

None apparent. This is build-time bundler/SSG logic with no auth, crypto, network, or untrusted-input parsing changes.

Level of scrutiny

Medium. Bake is explicitly experimental, and the field removals are mechanical. However, two changes encode judgment calls that a maintainer should sanity-check: the new fully_static = server_components.is_some() && … semantics (deciding that a framework without server components can never have a fully-static route — i.e. its clientEntryPoint is always emitted), and replacing an assertion-panic with a silent skip (relaxing an invariant that previously guarded against the runtime chunk going missing). Both are well-argued in the PR description, but they're design decisions rather than mechanical fixes.

Other factors

The PR is well-written with a clear cause analysis, the existing "don't include client code if fully static route" test covers the built-in-React path that must remain unchanged, and the new test exercises the custom-framework path end-to-end. No CODEOWNERS apply to the touched files. CI build was still in progress at review time.

[claude]

I didn't find any bugs, but this changes chunk-classification semantics (fully_static now keys off server_components instead of is_built_in_react) and replaces a panic with a silent skip in the production build path, so it's worth a human look from someone familiar with bake.

Extended reasoning...

Overview

This PR fixes a panic in bun build --app for custom (non-React) Bake frameworks. It touches 8 files across the bundler (generateChunksInParallel.rs, bundle_v2.rs, ParseTask.rs), parser (parser.rs), and runtime bake layers (mod.rs, bake_body.rs, production.rs), plus a new regression test. The core changes are: (1) drop the is_built_in_react gate so BakeExtra is populated for every framework, (2) re-gate fully_static on framework.server_components.is_some() instead, (3) replace the "Runtime file not found" panic with a conditional skip, and (4) remove the now-dead is_built_in_react field from the bundler/parser Framework projections.

Security risks

None identified. This is build-time chunk metadata and output-file bookkeeping; no auth, crypto, network, or untrusted-input handling is involved.

Level of scrutiny

Medium-high. While each individual edit is small, the change to fully_static is a semantic shift in how routes are classified for prerendering — it now depends on whether server components are configured rather than whether the framework is built-in React. The PR argues built-in React always configures server components so its behavior is unchanged, and that looks correct from Framework::react(), but this invariant deserves confirmation from someone who owns the bake subsystem. Similarly, converting a panic to a silent skip is well-justified here (small custom apps may genuinely have no runtime chunk) but removes a guardrail that could otherwise catch future regressions in bake_is_runtime tagging.

Other factors

The new test exercises the exact failing scenario from the linked issue and asserts both the prerendered HTML and the client-entry script emission, and the existing "don't include client code if fully static route" test covers the React path. The cross-crate field removal is mechanical and the compiler enforces it. CI failures appear to be unrelated flakes (http backpressure, terminal, fetch-abort tests on macOS). Overall the fix looks correct and well-reasoned, but the behavioral surface (route static-ness classification + panic removal in production builds) is large enough that I'd rather a human sign off.

[robobun]

CI status: 285 of 286 jobs pass. The single failure on darwin-14-aarch64 is test/js/bun/io/fetch/fetch-abort-slow-connect.test.ts ("TimeoutError" vs "Error"), which is unrelated to this diff.

That test expects connects to TEST-NET-1 (192.0.2.1) to hang in EINPROGRESS so AbortSignal.timeout(50) wins; on these mac agents the connect fails instantly (the test fails after ~2ms), so a connection error surfaces before the timeout. The same failure appears on build 61983 for an unrelated branch. All bake and bundler tests pass on every platform.

[robobun]

Still reproduces on main at b5afcac (and on the 1.4.0 release): a framework object that only sets fileSystemRouterTypes with a serverEntryPoint makes bun build --app panic with Runtime file not found right after "Rendering routes", exit code 134. The cause is unchanged: generateChunksInParallel.rs still only fills bake_extra when is_built_in_react is set, and production.rs still panics when no output file has bake_is_runtime.

This branch currently conflicts with main in src/bundler/bundle_v2.rs and src/bundler/linker_context/generateChunksInParallel.rs; the other files merge cleanly, so it only needs a rebase.

[robobun]

Rebased onto main. Two conflicts, both from the switch to the BakeRouteKind enum, resolved as described in the rebase notes in the PR description; the regression test still fails without the src/ changes and passes with them on the new base. The follow-up commit only shortens two comments.

[claude]

I reviewed this PR and didn't find any bugs. Because it changes bake route classification semantics (FullyStaticRoute vs Route for frameworks without server components) and replaces a panic guard with a silent skip, a human look would still be worthwhile to confirm those framework-behavior decisions.

Checked: the server_components.is_some() gate preserves built-in React's classification (React always sets server_components); the sibling writeOutputFilesToDisk.rs path already uses BakeExtra::default() and is unreachable from bake (output_dir is forced empty), so no missed twin site; runtime-side is_built_in_react is retained for the install hint in production.rs and DevServer.rs.

Extended reasoning...

Overview

The PR fixes a panic in bun build --app for custom (non-React) bake frameworks. It touches 7 source files across the bundler (generateChunksInParallel.rs, bundle_v2.rs, ParseTask.rs), parser (parser.rs), and bake runtime (production.rs, bake_body.rs, mod.rs), plus adds a test in test/bake/dev/production.test.ts. The core changes: (1) drop the is_built_in_react gate so bake_extra is populated for every framework, (2) gate FullyStaticRoute classification on server_components.is_some(), (3) replace the "Runtime file not found" panic with a conditional skip, (4) remove the now-dead is_built_in_react field from the bundler-side and parser-side Framework projections.

Security risks

None identified. This is bundler/build-time metadata plumbing for an experimental feature (BUN_FEATURE_FLAG_EXPERIMENTAL_BAKE). No user input parsing, no auth, no network.

Level of scrutiny

Medium-high. Bake is experimental so blast radius is limited, but the change makes two behavior decisions a maintainer should confirm:

• Route classification: for frameworks without server components, every server entry point is now classified as Route (never FullyStaticRoute), meaning the client entry URL is always passed to prerender. The reasoning (no "use client" boundaries exist without server components, so StaticRouteVisitor would vacuously call everything static) is sound and conservative, but it's a semantic choice about what "fully static" means for custom frameworks.
• Panic → skip: the removed panic was a safety net. The PR argues the runtime chunk is legitimately absent when nothing uses its helpers, which is true, but if a future bug causes bake_is_runtime to go unset on a chunk that does contain the runtime, the write will now be silently skipped instead of crashing loudly.

Other factors

• Verified the built-in React path is unchanged: Framework::react() always sets server_components: Some(...), so the new server_components.is_some() && !has_transitive_use_client(...) predicate reduces to the old !has_transitive_use_client(...) there. The existing "don't include client code if fully static route" test covers that.
• Checked the sibling disk-write path (writeOutputFilesToDisk.rs): it already uses BakeExtra::default() unconditionally and is unreachable from bake production (both init_transpiler variants force output_dir empty), so there's no missed parallel site.
• The runtime-side Framework.is_built_in_react field is kept — still read at production.rs:509 and DevServer.rs:780 for the React install hint.
• CI on the earlier revision was green except an unrelated darwin fetch-abort flake; a new build (#99510) is running post-rebase.
• The comment-cop bot's remaining flag on production.rs:785 is on a two-line comment that states a fact, not a workaround justification; the author already responded.
• The new test asserts positive behavior (exit 0, HTML content, client script written) rather than absence-of-panic, which is the right shape.

[alii]

Why do we remove is_built_in_react?

[robobun]

Only the two bundler-facing mirrors are removed, not the flag itself.

bake::Framework.is_built_in_react (the runtime-side struct) stays. production.rs and DevServer.rs still read it for the React install hint when framework resolution fails.

What the PR deletes is the copy of that flag in two lower-tier views:

• bake_types::Framework.is_built_in_react in bundle_v2.rs. Its only reader was the if !is_built_in_react { break BakeExtra::default() } gate in generateChunksInParallel.rs, which is the bug. Once the gate is gone, nothing in the bundler reads the field.
• js_parser::options::Framework.is_built_in_react. The parser never read it. ParseTask.rs copied it across, and that was the end of the chain.

Both structs are documented as the subset of Framework that the bundler and parser consult, so I dropped the unread field rather than keep plumbing it through Framework::new and both as_bundler_view projections.

It is a separable part of the diff. If you would rather keep the bundler-side mirror for a future use, I can restore it, and the PR is then only the generateChunksInParallel.rs and production.rs changes plus the test.

[alii]

please can you make it two commits instead of two prs? and ill look at the diff of each commit

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Note

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[robobun]

Done, the branch is now two commits on the same base:

1. 265e9b6aee: the fix and the test. generateChunksInParallel.rs (drop the gate, classify the route), production.rs (no panic when the runtime chunk is absent), test/bake/dev/production.test.ts.
2. acaf2cb4be: removal of the unread is_built_in_react mirrors. bundle_v2.rs, ParseTask.rs, parser.rs, bake_body.rs, mod.rs, and the stale comment in production.rs.

The first commit type-checks on its own. The combined tree is byte-identical to the previous head (6e5ae19), so the earlier test runs still apply.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[coderabbitai]

Note

GitHub couldn't provide a complete incremental comparison for this pull request, so CodeRabbit is performing a full review instead. This review may take a little longer.

[robobun]

Rebased onto current main, no conflicts. The two commits are unchanged in content, only the SHAs moved:

1. 01762b4621: the fix and the test.
2. 365351161c: removal of the unread is_built_in_react mirrors.

The previous CI run failed on three things that the old base (446 commits behind) did not have yet: the renewed localhost cert fixture (#40488), the DevServer teardown fix that test/bake/deinitialization.test.ts exercises (#39630), and the binary size reductions from the dead code removals. None of them touch this diff. test/bake/dev/production.test.ts and test/bake/deinitialization.test.ts pass locally on the new base.

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Inline comments:
In `@test/bake/dev/production.test.ts`:
- Line 697: Move the exitCode assertion for the production build command to
after the dist/index.html and referenced client bundle filesystem checks,
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📥 Commits

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📒 Files selected for processing (8)

• src/bundler/ParseTask.rs
• src/bundler/bundle_v2.rs
• src/bundler/linker_context/generateChunksInParallel.rs
• src/js_parser/parser.rs
• src/runtime/bake/bake_body.rs
• src/runtime/bake/mod.rs
• src/runtime/bake/production.rs
• test/bake/dev/production.test.ts

💤 Files with no reviewable changes (3)

• src/bundler/ParseTask.rs
• src/js_parser/parser.rs
• src/bundler/bundle_v2.rs

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[robobun]

Pushed a small test-only follow-up, d976a21: the regression test now asserts the build exit code after the output checks, per the review note. The two commits above are unchanged.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

CI on the rebased head (d976a21, build 108173): 176 jobs pass so far, one lane red. The red lane is test/js/web/url/url.test.ts on darwin x64 (the Unicode 16 IDNA table test throws Invalid URL), which also fails on the main builds 108134 and 108145, so it is not from this diff. The cert, deinitialization, and binary size failures from the previous run are gone after the rebase.