diff --git a/.github/workflows/lexer.yml b/.github/workflows/lexer.yml
index 3e646dd3d48..c65765c189c 100644
--- a/.github/workflows/lexer.yml
+++ b/.github/workflows/lexer.yml
@@ -11,6 +11,7 @@ on:
       - code/src/include/Rinlinedfuns.h
       - code/tests/reg-encodings.R
       - code/tests/ir-unicode-lexer.R
+      - code/tests/ir-package-compat.R
       - .github/workflows/lexer.yml
 
 jobs:
@@ -46,3 +47,14 @@ jobs:
 
       - name: Test Unicode lexer priority
         run: build/bin/R --vanilla < code/tests/ir-unicode-lexer.R
+
+      - name: Install package compatibility oracle
+        run: |
+          mkdir -p build/compat-lib
+          IR_COMPAT_LIB="$PWD/build/compat-lib" \
+            build/bin/Rscript -e 'install.packages(c("rlang", "testthat", "tibble", "dplyr"), lib = Sys.getenv("IR_COMPAT_LIB"), repos = "https://cloud.r-project.org", type = "source", Ncpus = 2L)'
+
+      - name: Test unmodified-package compatibility
+        env:
+          IR_COMPAT_LIB: ${{ github.workspace }}/build/compat-lib
+        run: build/bin/R --vanilla < code/tests/ir-package-compat.R
diff --git a/README.md b/README.md
index 77c6c4cdccb..1664b7978ad 100644
--- a/README.md
+++ b/README.md
@@ -1,8 +1,8 @@
-### fn λ → ÷ ≟ ←
+### λ ƒ → ÷ ≟ ←
 
 → and ← can now be used for assignment, not just `<-` and `->`.
 
-`function(x) x**3` can now be written `fn(x) x**3`, `λ(x) x**3`, or `ƒ(x) x**3`.
+`function(x) x**3` can now be written `λ(x) x**3` or `ƒ(x) x**3`.
 
 ÷ means division.
 
@@ -35,7 +35,7 @@ because ≟ tests equality.
 | `3 → x` | assign |
 | `3 ↠ x` | assign in an enclosing frame |
 | `left ≟ right` | test equality |
-| `fn(x) expression`, `λ(x) expression`, or `ƒ(x) expression` | construct a function |
+| `λ(x) expression` or `ƒ(x) expression` | construct a function |
 | `left ÷ right` | divide |
 
 
@@ -61,7 +61,7 @@ stopifnot((answer = 4))
 
 Code that used a single `=` as assignment must use an arrow in this R. The comma and argument-label rules have otherwise been left alone.
 
-`fn` also used to be an ordinary name. It is reserved here, so old code using bare `fn` as a variable or argument must choose another name. This source uses `fun`; in particular, write `optim(par, fun = ...)` rather than `optim(par, fn = ...)`.
+`fn` is an ordinary identifier, as in standard R. Code using `fn` as a variable, formal argument, or named argument does not need to change; for example, `optim(par, fn = ...)` remains valid. `fn(...)` is an ordinary call to a function named `fn`, not function-constructor syntax.
 
 
 ------
diff --git a/code/src/main/gram.c b/code/src/main/gram.c
index d15fd020d38..6e871b82262 100644
--- a/code/src/main/gram.c
+++ b/code/src/main/gram.c
@@ -4493,7 +4493,6 @@ static keywords[] = {
     { "NA_character_", NUM_CONST  },
     { "NA_complex_", NUM_CONST  },
     { "function",   FUNCTION   },
-    { "fn",         FUNCTION   },
     { "λ",          FUNCTION   },
     { "ƒ",          FUNCTION   },
     { "while",	    WHILE      },
diff --git a/code/src/main/gram.y b/code/src/main/gram.y
index cf39e473eb8..e15141d81cb 100644
--- a/code/src/main/gram.y
+++ b/code/src/main/gram.y
@@ -2179,7 +2179,6 @@ static keywords[] = {
     { "NA_character_", NUM_CONST  },
     { "NA_complex_", NUM_CONST  },
     { "function",   FUNCTION   },
-    { "fn",         FUNCTION   },
     { "λ",          FUNCTION   },
     { "ƒ",          FUNCTION   },
     { "while",	    WHILE      },
diff --git a/code/tests/ir-package-compat.R b/code/tests/ir-package-compat.R
new file mode 100644
index 00000000000..8658c538e4d
--- /dev/null
+++ b/code/tests/ir-package-compat.R
@@ -0,0 +1,40 @@
+## IR compatibility smoke: bare fn must remain ordinary R syntax.
+
+parse1 <- function(text) parse(text = text, keep.source = FALSE)[[1L]]
+
+## Regression for the shape used by rlang: fn is a formal and is assigned to.
+stopifnot(identical(parse1("fn"), as.name("fn")))
+call_trace_context <- eval(parse1(
+    "function(call, fn) {
+        fn <- fn
+        fn
+    }"
+))
+stopifnot(
+    identical(names(formals(call_trace_context)), c("call", "fn")),
+    identical(call_trace_context(NULL, 7L), 7L)
+)
+
+## The IR constructor spellings remain distinctive syntax.
+lambda_identity <- eval(parse1("λ(fn) fn"))
+florin_identity <- eval(parse1("ƒ(fn) fn"))
+stopifnot(
+    identical(lambda_identity(11L), 11L),
+    identical(florin_identity(12L), 12L)
+)
+
+## Upstream packages are the oracle: do not patch or translate them.
+compat_lib <- Sys.getenv("IR_COMPAT_LIB", "build/compat-lib")
+.libPaths(c(compat_lib, .libPaths()))
+
+packages <- c("rlang", "testthat", "tibble", "dplyr")
+for (package in packages) {
+    suppressPackageStartupMessages(
+        library(package, character.only = TRUE)
+    )
+}
+
+testthat::test_that("IR spellings work inside upstream testthat", {
+    answer ← 8 ÷ 2
+    testthat::expect_true(answer ≟ 4)
+})