feat(extension): bound native messaging frame codec

[seonghobae]

Partial implementation of #27, stacked on exact current #82 head c639cd78e3acad235be4cbbfdef67b84ce7ddbfa.

Buyer/security boundary

This Draft adds the next separately reviewed native-messaging boundary after exact extension-to-host authority: a bounded Chrome native-messaging frame codec. It uses the official Chrome framing contract without promoting transport bytes into process, registration, Agent, or secret authority.

The production boundary:

• prefixes outbound UTF-8 text with one native-endian unsigned 32-bit byte length;
• enforces Chrome's direction-specific maximums: 1 MiB native-host → Chrome and 64 MiB Chrome → native-host;
• validates the declared size before interpreting the body;
• requires the supplied input to contain exactly one complete frame, rejecting both truncation and trailing bytes;
• rejects non-UTF-8 payloads fail closed; and
• emits deterministic error text that does not reflect attacker-controlled payload bytes.

This layer deliberately does not spawn or supervise a native process, parse JSON syntax or schemas, authenticate operating-system host registration, trust Chrome's process arguments as OriginWeave authorization, grant Agent capability, carry secrets, or weaken #82's exact extension-to-host authority boundary.

Test-first evidence

The realistic regression was established before production support. Exact test-only head 3d9b8a2f0468848f7f3eeaeb13ac75c0798da0e8 reached the intended compiler boundary: CI checked out that exact head and failed cargo check --locked --workspace --all-targets with unresolved production imports for NativeMessagingFrameDirection, NativeMessagingFrameError, decode_native_messaging_frame, and encode_native_messaging_frame.

The same canonical branch then received the minimal production implementation, exact-limit encoding coverage, all deterministic error-display branches, the primary-source doctoring update, and the Unreleased changelog record. A source-only rustfmt defect surfaced on an intermediate exact head and was repaired by applying the canonical formatter output rather than changing semantics.

Exact-current verification

Current exact contributor head is 57dc0b4886cff1dc22a2ab1c251c5e5bff84158e; GitHub reports this Draft open and mergeable against exact prerequisite #82.

On this unchanged exact head:

• CI run 32461885323: success;
  • Rust contracts job 96710344984: repository contracts, canonical formatting, locked workspace/all-target check, full tests, strict Clippy, and rustdoc success;
  • Production coverage job 96710345129: exact owned-production function, line, region, and branch coverage enforcement success;
• Manifest V3 Compatibility run 32461885241: success;
• SAST Semgrep run 32461885233: queued at the latest bounded refetch; and
• Security Scan run 32461885346: queued at the latest bounded refetch.

Queued scanners are continuation evidence and are not represented as passing. Fresh formal-review inventory contains no submitted reviews and fresh inline-review inventory contains no threads. No predecessor, prerequisite, queued, skipped, cancelled, synthetic, status-only, or model-only result is promoted as current proof.

Standards / truth boundary

docs/doctoring/browser-agent-protocols.md now records Chrome's current native-messaging contract and keeps framing distinct from manifest/OS registration, process identity/supervision, caller-origin binding, JSON schema validation, extension-to-host authority, Agent capability, and secret disclosure. The current framing layer is only a reusable byte-level transport primitive.

Keep this PR Draft while #82 remains active. Protected-main AGENTS.md forbids this scheduled actor from merging, self-approving, altering workflows, adding secrets, weakening checks, tagging, or publishing.

[coderabbitai]

Important

Review skipped

Draft detected.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: f85c6048-a62e-49bc-aced-44c722a8b43b

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[seonghobae]

Closing as a proven duplicate of the earlier canonical framing owner #154 (feat/native-messaging-framing-boundary, exact head 4a71b7dd357974f791f8d7f4a0be5c4c0b9ea1b1). Fresh comparison against the shared exact #82 base shows #154 already owns the production boundary this branch attempted to add: native-endian 32-bit framing, 1 MiB host→browser / 64 MiB browser→host limits, oversized-length rejection before body slicing/allocation, exact total-frame-length equality, UTF-8 text validation, deterministic source-free errors, TDD evidence, changelog, and primary-source doctoring. Its current tests also cover Unicode UTF-8 text and every error-display variant; the #213 exact-limit/diagnostic regressions do not establish a distinct missing production contract. #169 already depends on #154, confirming #154 as the dependency-root owner. No #213 checks/reviews are transferred to #154, and #154's existing exact-head evidence remains independently authoritative.