Check the Lua bridge and return project tempo, play state, cursor_beats, tracks (selected=true on selected ones), time_selection {start_beats, end_beats} when there is one, selected_items [{track_index, item_index}], any_solo, master_mute and dirty (unsaved changes). Call first; clear a lingering solo before rendering. detail=true adds arrangement facts (time signature at the start, tempo_map of tempo/meter changes, length in beats, markers, MIDI vs audio per track) without guessing chords.

Playback control. action: play, stop, pause, record, toggle_repeat, goto_start, goto (edit cursor to position_beats, or to a marker/region by number or name; moves playback too when playing). Returns play_state, position_sec and position_beats.

Track operations. action: list | add | delete | update | bus. update uses name, volume_db, pan (-1..1), mute, solo. bus creates a submix named name and routes source_indices through it (their direct send to the master/parent is turned off).

MIDI on a track item. Times are absolute project beats. action: create_item | add | get | replace | update | delete | cc | get_cc. replace rewrites the whole take in one undo step; add only appends. cc inserts CC (cc+value) or program change (program) via ccs[]; get_cc reads CC, program and pitch_bend back in that shape. get omits index (use array offset), channel if 0, muted/selected if false.

Track FX. action: list | add | params | set | delete | bypass | move | preset. params takes filter (case-insensitive part of the parameter name) to list only matching ones. track_index -1 is the master. set takes value 0..1 (REAPER normalized) or formatted: the value as the plugin displays it (number in its unit, e.g. -18 for dB, 4 for 4:1, 120 for ms; or a label like "3.2k"), resolved by searching the display text. set returns the formatted value actually set.

Track sends. action: list | add | delete | update. track_index is the source (-1 master). dest_index is the destination track. add/update take volume_db, pan, mute, mode (post_fader | pre_fader | pre_fx) and channel routing: src_channel / dst_channel = first channel of the stereo pair, 0-based (dst_channel 2 = channels 3/4, a compressor's sidechain input; the destination's channel count is raised to fit).

Media items on a track. action: list | update | import | split | delete | duplicate. Times are beats except fade_in_sec / fade_out_sec (seconds, REAPER native). update: gain_db, fades and fade_*_shape, loop, mute, position_beats (move), length_beats (trim the end), left_edge_beats (trim or extend the start; the content stays in place), dest_track_index (move to another track; returns the new indices), active_take. list shows take name, source file and take count. split cuts at at_beats (inside the item) and returns the pieces' item_indices. delete takes item_index or item_indices. duplicate copies the item (unpooled) to position_beats (default: right after it), optionally on dest_track_index. import uses a path already on the REAPER host; as_new_track=true makes new tracks. Item indices follow position, so re-list after edits that move items.

Track envelopes. action: list | get | set. name is REAPER envelope name (Volume, Pan, Mute, ...). set replaces all points, or with start_beats / end_beats only the points in that range (ends included; every new point must lie inside it); get with a range returns only those points. value is the envelope's value; Volume is linear gain (1.0 = 0 dB, 0.5 = -6 dB), also on fader-scaled envelopes. unit=db reads/writes volume-type envelopes (Volume, Trim Volume, send Volume) in dB. set re-activates an envelope left inactive (ACT 0).

Project-level edits. action: tempo | time_signature | time_selection | marker | save. save with path creates missing folders and writes a copy (no dialog); an untitled project adopts that path. save without path needs an already-named project.

Render. No undo. Long timeout. bounds: project | time_selection | custom | items. Default: the master mix. stems=true renders the selected tracks as stems instead (pass track_indices to select them). path is the output folder; the file name comes from pattern. Sets all media online (action 40101) first unless media_online=false. Refuses when a target file already exists (REAPER's overwrite prompt would hang the bridge) and when tracks are soloed (pass allow_solo=true, or solo_indices / mute_indices to solo/mute tracks for this render only; they are restored afterwards). Returns files [{path, bytes, length_sec, peak_db, lufs_i}] and warnings (near-silent, missing file, master muted); measure=false skips the loudness scan. format wav (bit_depth 16 | 24 | 32 = float) or mp3 (320 kbps) sets the file type for this render only; without it the project's render format is used.

Run a REAPER Main_OnCommand by numeric command id.

Call any ReaScript function by name. Pointers come back as {"__handle":"hN"} and can be passed back in. Example: func=CountTracks args=[0].

Run a Lua snippet inside REAPER (reaper in scope). Use return to send a value. Best for multi-step work that would otherwise be many round-trips. Errors carry the failing line. timeout (seconds, default 10, max 600): a snippet still running Lua after it is stopped with an error (what it did until then stays done); a call blocked inside REAPER (ExecProcess, a render, a dialog) can't be stopped. REAPER is frozen while it runs, so start long host commands in the background (io.popen('cmd > log 2>&1 &')) and poll instead.

Many operations in one IPC hop and one undo point. Each call is {func, args} or {func: 'run_lua', code, timeout} or {func, arguments} for a grouped tool name (track/midi/fx/...). Each slot is the inner result, or {error}. There is no rollback: by default one failure does not abort the rest; stop_on_error=true skips the slots after the first failure ({skipped: true}). Handles from earlier calls work later.