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Claude Code Internal Analysis

Audio capture native module

This page is the artifact-level reverse-engineering writeup of audio-capture.node, the Bun-embedded addon that exposes recording and playback functions. It is the companion page to Audio capture and voice mode: that page documents the source-confirmed recording consumer and JS state machine; this page inventories the exact Linux-x64 binary boundary. No readable Claude Code call site for the playback wrappers was found, so playback is an exported capability rather than a confirmed active product path.

The .node binary is regenerated locally by scripts/extract-claude-code-final-artifacts.mjs and is held outside git via *.node in .gitignore. This page works off the linux-x64 build of @anthropic-ai/claude-code@2.1.215.

ELF metadata

ItemValue
Pathclaude-code-pkg/audio-capture.node
TypeELF 64-bit LSB shared object, x86-64, dynamically linked, stripped
Size492,184 bytes (481 KiB)
SHA-256185f990044394fbd4811284cfe9812d261453571c4dfbfa27dadd299c53036eb
Bun graph path/$bunfs/root/audio-capture.node
Defined dynamic symbolnapi_register_module_v1 (single export)

ABI surface

LayerEvidence
Dynamic library deps (readelf -d)libasound.so.2, libpthread.so.0, libc.so.6, libdl.so.2, ld-linux-x86-64.so.2 — ALSA is linked dynamically; no PulseAudio, PipeWire, JACK, Wayland, CoreAudio, or WASAPI dependencies are present.
Minimum glibcGLIBC_2.17 (same floor as image-processor.node).
ALSA symbol versions usedALSA_0.9 (base) and ALSA_0.9.0rc4 / ALSA_0.9.0rc8 (for *_set_period_time_near, *_set_buffer_time_near, *_get_*_min/max, *_status_get_htstamp, *_status_get_trigger_htstamp).
Rust toolchainSame rustc commit 01f6ddf7588f42ae2d7eb0a2f21d44e8e96674cf as the image addon.
N-API imports36 undefined napi_*. Notable absences: no napi_create_promise / napi_resolve_deferred / napi_reject_deferred. Threadsafe-function imports are present. This constrains the ABI shape, but does not by itself recover when callbacks are queued or how worker threads are managed.

Source provenance — statically linked Rust crates

Recovered from panic-location source paths in .rodata:

crateVersionRole
napi2.16.17napi-rs binding layer (function / class / threadsafe-fn / external buffer).
cpal0.15.3Cross-Platform Audio Library — the only audio API the addon talks to from Rust.
alsa0.9.1Rust ALSA FFI bindings, the Linux backend that cpal delegates to.
once_cell1.21.3Lazy/global storage support; the concrete values stored are not recovered here.

The recovered cpal-0.15.3 source paths include src/host/alsa/mod.rs, src/lib.rs, and src/traits.rs; the recovered alsa-0.9.1 paths include src/pcm.rs and src/poll.rs. No CoreAudio, WASAPI, JACK, or Oboe paths or corresponding system libraries were observed in this Linux artifact. That classifies this build, not the implementation of packages for other operating systems.

addr2line / gimli / hashbrown / rustc-demangle are present too but only service Rust panic backtraces; they are not part of the audio pipeline.

Implication: the artifact contains a focused napi-rs/cpal/ALSA dependency surface. Dependency presence does not establish the addon’s exact stream lifecycle or the composition of uninspected macOS and Windows packages.

ALSA surface

The undefined-symbol list has 49 distinct snd_pcm_* entries spanning PCM setup, I/O, status, recovery, and polling:

open / close / prepare / start / pause / recover / status / avail / bytes_to_frames
readi / writei                              ← capture and playback I/O
hw_params{,_any,_malloc,_free}
hw_params_set_{access,format,channels,rate,buffer_size,buffer_time_near,
              period_size_near,period_time_near}
hw_params_get_{rate_{min,max},channels_{min,max},buffer_size_{min,max}}
hw_params_test_{format,channels,rate}        ← parameter probing
hw_params_can_pause                          ← capability check
sw_params{,_current,_malloc,_free}
sw_params_set_{avail_min,start_threshold,tstamp_mode,tstamp_type}
poll_descriptors{,_count,_revents}           ← event-driven wakeup
status / status_sizeof / status_get_{delay,htstamp,trigger_htstamp}

Two bounded takeaways:

  • Both direction-specific imports are present: snd_pcm_readi and snd_pcm_writei. This is consistent with the recording and playback exports.
  • Parameter-test, min/max, recovery, and poll imports show that linked code can use those ALSA operations. An import table does not prove which functions execute for a given device, in what order, or before which JS callback.

JavaScript surface

Module exports (registered by napi_register_module_v1)

Recovered exactly from Failed to register function \…“ error strings:

Rust nameJS namePurpose
start_recordingstartRecording(...)Recording entry; readable CLI wrappers pass chunk and completion callbacks.
stop_recordingstopRecording()Recording stop entry.
is_recordingisRecording() → boolRecording-state query; returned false while inert in the exact-artifact probe.
start_playbackstartPlayback(...)Playback entry; exported, but no downstream readable CLI consumer was found.
write_playback_datawritePlaybackData(buffer)Playback-data entry; exported, but no downstream readable CLI consumer was found.
stop_playbackstopPlayback()Playback stop entry; exported, but no downstream readable CLI consumer was found.
is_playingisPlaying() → boolPlayback-state query; returned false while inert in the exact-artifact probe.
microphone_authorization_statusmicrophoneAuthorizationStatus() → numberNumeric authorization query. The inspected Linux-x64 artifact returned 3; the semantic enum mapping and other-platform behavior are not established by this binary.

No class is exposed — the eight functions hold the entire surface. The napi_define_class import slot exists but stays unused on this build (likely a side-effect of napi-rs’s macro template; the strings Failed to register class \`` / Failed to register export `are emitted but no class name follows them in.rodata`).

Call path from cli.renamed.js

The bundle wrapper at cli.renamed.js#L95 loads the addon via require("/$bunfs/root/audio-capture.node") (see the shim at claude-code-pkg/audio-capture.js). The voice runtime exposes isNativeAudioAvailable() around line 562780; the audio-capture-napi loaded in Xms log at line 562844 follows the first successful lazy load.

The runtime path (covered fully in Audio capture and voice mode) selects this addon before arecord / SoX rec when the addon loads and /proc/asound/cards reports a sound card. The readable call sites consume the recording wrappers. Searches for the playback wrapper names found their definitions/exports but no downstream CLI playback call site.

Concurrency evidence and limit

The binary imports N-API threadsafe-function calls and ALSA poll functions, while no Tokio crate path was observed. This is consistent with native work communicating asynchronously with JS callbacks, but the stripped binary has not been disassembled. The available evidence does not prove:

  • one dedicated thread per stream;
  • which component owns or schedules that thread;
  • whether every ALSA wait uses poll descriptors;
  • whether snd_pcm_recover runs before a callback;
  • whether fatal N-API imports are reachable for a device-disconnect case;
  • exact stop, drain, release, or callback ordering.

Defensive / error surfaces

Recovered Rust-side error categories:

CategorySource string (excerpt)
ALSA wrappingAudio capture error:, Audio playback error:, host supplied incorrect sample type, chunk size must be non-zero, capacity overflow
cpal stream invariants\capture` is earlier than representation supported by `StreamInstant`, `playback` occurs beyond representation supported by `StreamInstant`, stream duration has exceeded `StreamInstant` representation`
Poll-loop sanity checkexpected input stream, but polling descriptors indicated output
Internal invariantassertion failed: unsafe { alsa::snd_pcm_status_sizeof() } as usize <= STATUS_SIZE (cpal asserts that the ABI-fixed snd_pcm_status_t fits its inline buffer)
Borrow-check at runtime..RefCell already borrowed (cpal stream-state contention)

These strings and imports identify possible error categories, not a recovered propagation graph. The readable voice code surfaces capture/start or stream failures as voiceError; backend fallback is selected before recording, and no automatic post-start native-to-recorder handoff is established.

Microphone authorization observation

The JS wrapper forwards the native return value without translating it. A safe runtime probe of this exact @anthropic-ai/claude-code@2.1.215 Linux-x64 artifact returned 3 from microphoneAuthorizationStatus().

That observation corrects the earlier claim that Linux always returns 0, but it does not prove what 3 means internally. A familiar four-value authorization enum would map 3 to “authorized,” yet no readable native implementation or platform contract was recovered, so that mapping remains a hypothesis. The JS wrapper itself returns 0 only when the addon is unavailable.

Recovery script

Terminal window
# regenerate the .node files
node scripts/extract-claude-code-final-artifacts.mjs --refresh-package
# inspect
file claude-code-pkg/audio-capture.node
readelf -d claude-code-pkg/audio-capture.node | grep NEEDED
nm -D --undefined-only claude-code-pkg/audio-capture.node | grep -c snd_pcm_   # → 49
nm -D --undefined-only claude-code-pkg/audio-capture.node | grep -c napi_      # → 36
strings -n 6 claude-code-pkg/audio-capture.node | grep -E "^[a-z][a-z0-9_-]+-[0-9]+\.[0-9]+\.[0-9]+/" | sed 's|/.*||' | sort -u
# expected: alsa-0.9.1, cpal-0.15.3, napi-2.16.17, once_cell-1.21.3

Caveats

  • This page works only from the Linux-x64 artifact. It makes no claim that another platform has the same dependencies, export behavior, or authorization result.
  • The .text section is not disassembled. Panic-location paths, imports, exports, and runtime observations are strong artifact evidence, but they do not recover native function bodies or lifecycle order.
  • Crate-version strings identify code linked into the artifact; they do not prove every linked path is reached during Claude Code voice capture.

Created and maintained by Yingting Huang.