diff --git a/Common/source/langscan.c b/Common/source/langscan.c
index 7bdbd03e2..2e0670738 100644
--- a/Common/source/langscan.c
+++ b/Common/source/langscan.c
@@ -1380,34 +1380,39 @@ boolean langstripstructuremarkers (Handle hin, Handle *hout) {
 
 			/*scan forward from the indent boundary to find the start of any
 			trailing comment (« or //) outside a string literal. UserTalk
-			strings use ASCII " (chdoublequote, 0x22) OR the Mac smart-quote
-			pair chopencurlyquote (0xD2) / chclosecurlyquote (0xD3); the
-			scanner accepts both forms (langscan.c parsepopstringconst), so
-			we track both here. Without that, a string like "a // b" with
-			"" delimiters works, but "a // b" with «...» smart quotes would
-			get its // misread as a comment start and the rest of the line
-			truncated. flqcurly distinguishes the closing delimiter to use.*/
+			has three literal delimiters, and all three must be tracked here
+			or a comment marker inside one gets misread as a real comment:
+			ASCII " (chdoublequote, 0x22); the Mac smart-quote pair
+			chopencurlyquote (0xD2) / chclosecurlyquote (0xD3); and
+			chsinglequote (0x27) for character and string4 constants
+			(parsepopstringconst / the single-quote branch below both accept
+			these). Without that, a string like "a // b" with "" delimiters
+			works, but «a // b» or 'Ç' would get its comment byte misread as
+			a comment start, truncating the rest of the line. Losing the
+			trailing structural-marker strip that way leaves a `{` in the
+			stored node text that the outline export re-emits from the level
+			transition, so each reinstall adds another brace (#866).
+			chclose is the delimiter that ends the literal we are inside.*/
 			long i = linestart + indent;
-			boolean flqcurly = false;
+			byte chclose = 0;
 			while (i < lineend) {
 				byte ch = buf [i];
 				if (flinstring) {
 					/*\ escapes the next byte (e.g. \" inside ASCII strings).
-					Same convention applies inside curly-quote strings; the
-					scanner doesn't distinguish.*/
+					Same convention applies inside curly-quote and
+					single-quote strings; the scanner doesn't distinguish.*/
 					if (ch == '\\' && i + 1 < lineend) {
 						i += 2;
 						continue;
 						}
-					if ((!flqcurly && ch == '"')
-					    || (flqcurly && ch == (byte) chclosecurlyquote))
+					if (ch == chclose)
 						flinstring = false;
 					++i;
 					continue;
 					}
-				if (ch == '"' || ch == chopencurlyquote) {
+				if (ch == '"' || ch == chopencurlyquote || ch == (byte) chsinglequote) {
 					flinstring = true;
-					flqcurly = (ch == chopencurlyquote);
+					chclose = (ch == chopencurlyquote) ? (byte) chclosecurlyquote : ch;
 					++i;
 					continue;
 					}
@@ -1453,13 +1458,17 @@ boolean langstripstructuremarkers (Handle hin, Handle *hout) {
 					surplus closing braces. If not, the brace we are about
 					to strip is needed to close an inline one-line block;
 					stop. Count {/} in the kept content, respecting
-					string-literal state for both " and « forms (escape
-					with backslash applies to both).*/
+					string-literal state for the ", « and ' forms (escape
+					with backslash applies to all three). Single quotes
+					must be tracked here for the same reason as in the
+					comment-start scan above: a brace inside a character
+					or string4 constant is not structural (#866).
+					k_close is the delimiter ending the current literal.*/
 					long try_from = stripfrom - 1;
 					long open_ct = 0;
 					long close_ct = 0;
 					boolean in_str = false;
-					boolean qcurly = false;
+					byte k_close = 0;
 					boolean esc = false;
 					long k;
 					for (k = linestart + indent; k < try_from; ++k) {
@@ -1467,14 +1476,13 @@ boolean langstripstructuremarkers (Handle hin, Handle *hout) {
 						if (esc) { esc = false; continue; }
 						if (in_str) {
 							if (kc == '\\') esc = true;
-							else if ((!qcurly && kc == '"')
-							         || (qcurly && kc == (byte) chclosecurlyquote))
+							else if (kc == k_close)
 								in_str = false;
 							continue;
 							}
-						if (kc == '"' || kc == chopencurlyquote) {
+						if (kc == '"' || kc == chopencurlyquote || kc == (byte) chsinglequote) {
 							in_str = true;
-							qcurly = (kc == chopencurlyquote);
+							k_close = (kc == chopencurlyquote) ? (byte) chclosecurlyquote : kc;
 							continue;
 							}
 						if (kc == '{') ++open_ct;
diff --git a/tests/integration/test_cases/script_install_roundtrip.yaml b/tests/integration/test_cases/script_install_roundtrip.yaml
index ca0640ae2..31d5626cd 100644
--- a/tests/integration/test_cases/script_install_roundtrip.yaml
+++ b/tests/integration/test_cases/script_install_roundtrip.yaml
@@ -111,8 +111,72 @@ tests:
     expected_success: true
     expected_result: "true"
 
+  - name: "round-trip: comment marker inside a single-quoted literal is not a comment (#866)"
+    description: "A single-quoted character/string4 constant holding the comment-marker byte (0xC7) must round-trip unchanged. The strip pass tracked \" and the curly-quote pair but not ', so the 0xC7 inside 'C' was mis-read as the start of a trailing comment. That set commentstart mid-literal, skipped the trailing structural-marker strip, and left a { in the stored node text that the export pass re-emitted from the level transition -- adding one extra { per install, cumulatively, until the script no longer compiled."
+    script: |
+      local (src = "on f () {\r\tlocal (s = \"x\");\r\tif s contains '\xc7' {\r\t\ts = \"y\"};\r\treturn (s)}");
+      new (tabletype, @system.temp.rt);
+      script.newScriptObject (src, @system.temp.rt.f);
+      local (back = string (system.temp.rt.f));
+      delete (@system.temp.rt);
+      return (back == src)
+    expected_success: true
+    expected_result: "true"
+
+  - name: "round-trip: single-quoted comment marker stays idempotent across reinstall (#866)"
+    description: "Reinstalling a script's own unmodified source must be idempotent. With the single-quote gap, each install appended another { to the same line, so the second read-back differed from the first and the script stopped compiling."
+    script: |
+      local (src = "on f () {\r\tlocal (s = \"x\");\r\tif s contains '\xc7' {\r\t\ts = \"y\"};\r\treturn (s)}");
+      new (tabletype, @system.temp.rt);
+      script.newScriptObject (src, @system.temp.rt.f);
+      local (first = string (system.temp.rt.f));
+      script.newScriptObject (first, @system.temp.rt.f);
+      local (second = string (system.temp.rt.f));
+      delete (@system.temp.rt);
+      return (first == second)
+    expected_success: true
+    expected_result: "true"
+
+  - name: "round-trip: single-quoted comment marker still compiles and runs (#866)"
+    description: "End-to-end behavioral check: after install the script must actually execute, not merely read back. suites.commercial.parseAete in Virgin.root is the real-world instance of this shape."
+    script: |
+      local (src = "on f (s) {\r\tif s contains '\xc7' {\r\t\treturn (1)};\r\treturn (0)}");
+      new (tabletype, @system.temp.rt);
+      script.newScriptObject (src, @system.temp.rt.f);
+      local (hit = system.temp.rt.f ("a\xc7b"));
+      local (miss = system.temp.rt.f ("ab"));
+      delete (@system.temp.rt);
+      return (hit == 1 and miss == 0)
+    expected_success: true
+    expected_result: "true"
+
+  - name: "round-trip: brace inside a single-quoted constant is not counted as structure (#866)"
+    description: "Exercises the brace-balance counter rather than the comment-start scan. That counter decides whether a trailing } is load-bearing (the #621 inline-block check); it tracked the same two delimiters and so counted a { or } inside a character constant as structural. Here the constant holds a brace and the line ends with an inline one-line block, so a miscount changes the strip decision and unbalances the stored node text. Both scanners now track ' as a delimiter."
+    script: |
+      local (src = "on f (c) {\r\tif c == '{' {\r\t\ttry {local (x = 1)}};\r\treturn (1)}");
+      new (tabletype, @system.temp.rt);
+      script.newScriptObject (src, @system.temp.rt.f);
+      local (back = string (system.temp.rt.f));
+      local (result = system.temp.rt.f ("{"));
+      delete (@system.temp.rt);
+      return (back == src and result == 1)
+    expected_success: true
+    expected_result: "true"
+
+  - name: "round-trip: comment marker inside a curly-quote string literal (0xD2/0xD3)"
+    description: "Coverage gap predating #866: the curly-quote string form was tracked by both scanners but never exercised by a test, and the delimiter refactor touches that path. A comment marker inside a Mac smart-quote string must not be read as a trailing comment. Uses the \\xd2 and \\xd3 escapes so the bytes survive the runner writing the script file as UTF-8."
+    script: |
+      local (src = "on f () {\r\tlocal (s = \xd2a \xc7 b\xd3);\r\treturn (s)}");
+      new (tabletype, @system.temp.rt);
+      script.newScriptObject (src, @system.temp.rt.f);
+      local (back = string (system.temp.rt.f));
+      delete (@system.temp.rt);
+      return (back == src)
+    expected_success: true
+    expected_result: "true"
+
   - name: "round-trip: inline one-line nested block at outermost depth (#621)"
-    description: "Same shape as the previous test but the inline block sits at the outermost body level (no enclosing if/while). Confirms the brace-balance check generalizes across nesting depths and isn't depth-1-specific."
+    description: "Same shape as the depth-1 inline-block case above but the inline block sits at the outermost body level (no enclosing if/while). Confirms the brace-balance check generalizes across nesting depths and isn't depth-1-specific."
     script: |
       local (src = "on f () {\r\ttry {local (x = 7)};\r\treturn (1)}");
       new (tabletype, @system.temp.rt);