diff --git a/wasm/c/lumbda-c.loader.js b/wasm/c/lumbda-c.loader.js
index 181d4fe..1e2e8e2 100644
--- a/wasm/c/lumbda-c.loader.js
+++ b/wasm/c/lumbda-c.loader.js
@@ -19,6 +19,14 @@ async function _bootstrap() {
     // through immediately so the worker can postMessage a chunk to
     // the UI as each line emerges (vs the user staring at a blank
     // panel for 18 seconds during the heavy bend call).
+    //
+    // Emscripten's print convention is "one call per line WITHOUT
+    // the trailing newline" — the caller is expected to add a \n
+    // when reassembling. Our outBuf join adds it back, but the
+    // streaming path needs it explicitly or successive chunks
+    // concatenate horizontally in the output panel (saw with
+    // (let loop ((i 0)) ... (display i) (newline) ... (loop ...))
+    // — every tick ended up on the same line).
     let outBuf = [];
     let errBuf = [];
     let currentOnChunk = null;
@@ -26,11 +34,11 @@ async function _bootstrap() {
         locateFile: (p) => wasmDir + p,
         print: (line) => {
             outBuf.push(line);
-            if (currentOnChunk) currentOnChunk(line);
+            if (currentOnChunk) currentOnChunk(line + "\n");
         },
         printErr: (line) => {
             errBuf.push(line);
-            if (currentOnChunk) currentOnChunk(line);
+            if (currentOnChunk) currentOnChunk(line + "\n");
         },
     });
 
diff --git a/wasm/dist-repl/c/lumbda-c.loader.js b/wasm/dist-repl/c/lumbda-c.loader.js
index 181d4fe..1e2e8e2 100644
--- a/wasm/dist-repl/c/lumbda-c.loader.js
+++ b/wasm/dist-repl/c/lumbda-c.loader.js
@@ -19,6 +19,14 @@ async function _bootstrap() {
     // through immediately so the worker can postMessage a chunk to
     // the UI as each line emerges (vs the user staring at a blank
     // panel for 18 seconds during the heavy bend call).
+    //
+    // Emscripten's print convention is "one call per line WITHOUT
+    // the trailing newline" — the caller is expected to add a \n
+    // when reassembling. Our outBuf join adds it back, but the
+    // streaming path needs it explicitly or successive chunks
+    // concatenate horizontally in the output panel (saw with
+    // (let loop ((i 0)) ... (display i) (newline) ... (loop ...))
+    // — every tick ended up on the same line).
     let outBuf = [];
     let errBuf = [];
     let currentOnChunk = null;
@@ -26,11 +34,11 @@ async function _bootstrap() {
         locateFile: (p) => wasmDir + p,
         print: (line) => {
             outBuf.push(line);
-            if (currentOnChunk) currentOnChunk(line);
+            if (currentOnChunk) currentOnChunk(line + "\n");
         },
         printErr: (line) => {
             errBuf.push(line);
-            if (currentOnChunk) currentOnChunk(line);
+            if (currentOnChunk) currentOnChunk(line + "\n");
         },
     });
 
diff --git a/www/playground/c/lumbda-c.loader.js b/www/playground/c/lumbda-c.loader.js
index 181d4fe..1e2e8e2 100644
--- a/www/playground/c/lumbda-c.loader.js
+++ b/www/playground/c/lumbda-c.loader.js
@@ -19,6 +19,14 @@ async function _bootstrap() {
     // through immediately so the worker can postMessage a chunk to
     // the UI as each line emerges (vs the user staring at a blank
     // panel for 18 seconds during the heavy bend call).
+    //
+    // Emscripten's print convention is "one call per line WITHOUT
+    // the trailing newline" — the caller is expected to add a \n
+    // when reassembling. Our outBuf join adds it back, but the
+    // streaming path needs it explicitly or successive chunks
+    // concatenate horizontally in the output panel (saw with
+    // (let loop ((i 0)) ... (display i) (newline) ... (loop ...))
+    // — every tick ended up on the same line).
     let outBuf = [];
     let errBuf = [];
     let currentOnChunk = null;
@@ -26,11 +34,11 @@ async function _bootstrap() {
         locateFile: (p) => wasmDir + p,
         print: (line) => {
             outBuf.push(line);
-            if (currentOnChunk) currentOnChunk(line);
+            if (currentOnChunk) currentOnChunk(line + "\n");
         },
         printErr: (line) => {
             errBuf.push(line);
-            if (currentOnChunk) currentOnChunk(line);
+            if (currentOnChunk) currentOnChunk(line + "\n");
         },
     });
 
diff --git a/www/repl/c/lumbda-c.loader.js b/www/repl/c/lumbda-c.loader.js
index 64e78c2..1e2e8e2 100644
--- a/www/repl/c/lumbda-c.loader.js
+++ b/www/repl/c/lumbda-c.loader.js
@@ -13,12 +13,33 @@ async function _bootstrap() {
     const wasmDir = new URL("./", import.meta.url).href;
     const { default: createLumbdaC } = await import(/* @vite-ignore */ factoryURL);
 
+    // Streaming output: every printf in the wasm fires Module.print
+    // synchronously. We buffer normally for the final return, but if
+    // the current eval registered an onChunk callback we also call
+    // through immediately so the worker can postMessage a chunk to
+    // the UI as each line emerges (vs the user staring at a blank
+    // panel for 18 seconds during the heavy bend call).
+    //
+    // Emscripten's print convention is "one call per line WITHOUT
+    // the trailing newline" — the caller is expected to add a \n
+    // when reassembling. Our outBuf join adds it back, but the
+    // streaming path needs it explicitly or successive chunks
+    // concatenate horizontally in the output panel (saw with
+    // (let loop ((i 0)) ... (display i) (newline) ... (loop ...))
+    // — every tick ended up on the same line).
     let outBuf = [];
     let errBuf = [];
+    let currentOnChunk = null;
     const module = await createLumbdaC({
         locateFile: (p) => wasmDir + p,
-        print: (line) => outBuf.push(line),
-        printErr: (line) => errBuf.push(line),
+        print: (line) => {
+            outBuf.push(line);
+            if (currentOnChunk) currentOnChunk(line + "\n");
+        },
+        printErr: (line) => {
+            errBuf.push(line);
+            if (currentOnChunk) currentOnChunk(line + "\n");
+        },
     });
 
     const _init = module.cwrap("lumbda_wasm_init", null, []);
@@ -56,10 +77,12 @@ async function _bootstrap() {
             const path = `/tmp/${name}.portal`;
             module.FS.writeFile(path, blob);
         },
-        async evalLisp(src) {
+        async evalLisp(src, onChunk) {
             outBuf = [];
             errBuf = [];
+            currentOnChunk = onChunk || null;
             const errPtr = _eval(src);
+            currentOnChunk = null;
             let errMsg = "";
             if (errPtr) {
                 errMsg = module.UTF8ToString(errPtr);