diff --git a/src/parse.jl b/src/parse.jl
index 3468ea9..7f8a264 100644
--- a/src/parse.jl
+++ b/src/parse.jl
@@ -188,15 +188,24 @@ function jsonreadstyle(::Type{T}, ::Type{O}, null, style::StructStyle, unknown_f
     ignore_unknown_fields =
         unknown_fields === :ignore ? true :
         unknown_fields === :error ? false :
-        throw(ArgumentError("`unknown_fields` must be `:ignore` or `:error`, got `$(repr(unknown_fields))`"))
+        # plain symbol interpolation (not `repr`): `show(::Symbol)` routes through the
+        # `@nospecialize`d Expr-show machinery, which drags hundreds of unresolvable
+        # dynamic calls into `--trim` builds from this error path alone
+        throw(ArgumentError("`unknown_fields` must be `:ignore` or `:error`, got `:$(unknown_fields)`"))
     if T === Any && !ignore_unknown_fields
         throw(ArgumentError("`unknown_fields` is only supported when parsing into a target type or existing object"))
     end
     return JSONReadStyle{O}(null, style, ignore_unknown_fields)
 end
 
+# explicit key formatting (not `repr` or plain interpolation): keys arrive as
+# `PtrString`, which has no `print`/`show` of its own, so both routes fall back to the
+# generic struct `show` — an unresolvable dynamic site under `--trim`. Keys can also be
+# `Int` (array-into-struct with extra elements) or other scalars, hence the fallback.
+unknownfieldkey(key::PtrString) = convert(String, key)
+unknownfieldkey(key) = string(key)
 @noinline unknownfielderror(::Type{T}, key) where {T} =
-    ArgumentError("encountered unknown JSON member $(repr(key)) while parsing `$T`")
+    ArgumentError("encountered unknown JSON member \"$(unknownfieldkey(key))\" while parsing `$T`")
 
 function StructUtils.unknownfield(st::JSONReadStyle, ::Type{T}, key, value) where {T}
     st.ignore_unknown_fields || throw(unknownfielderror(T, key))
@@ -372,7 +381,9 @@ function StructUtils.make(st::StructStyle, ::Type{Any}, x::LazyValues)
     elseif type == JSONTypes.TRUE || type == JSONTypes.FALSE
         return StructUtils.lift(st, Bool, x)
     else
-        throw(ArgumentError("cannot parse $x"))
+        # don't interpolate the lazy value: its `show` pulls the array-display machinery
+        # into `--trim` builds from this (unreachable in practice) fallback branch
+        throw(ArgumentError("cannot parse json"))
     end
 end
 
diff --git a/src/write.jl b/src/write.jl
index 5262175..374f938 100644
--- a/src/write.jl
+++ b/src/write.jl
@@ -34,6 +34,31 @@ const StringLike = Union{Enum, AbstractChar, VersionNumber, Cstring, Cwstring, U
 StructUtils.lower(::JSONStyle, ::Missing) = nothing
 StructUtils.lower(::JSONStyle, x::Symbol) = String(x)
 StructUtils.lower(::JSONStyle, x::StringLike) = string(x)
+# trim-friendly ISO renderings: string(::TimeType) routes through Dates'
+# DateFormat machinery (dynamic lpad/repeat) under juliac --trim
+StructUtils.lower(::JSONStyle, x::Dates.Date) = _iso_date_string(x)
+StructUtils.lower(::JSONStyle, x::Dates.DateTime) = _iso_datetime_string(x)
+StructUtils.lower(::JSONStyle, x::Dates.Time) = _iso_time_string(x)
+
+_two_dig(x::Int)::String = x < 10 ? string('0', x) : string(x)
+_three_dig(x::Int)::String = x < 10 ? string("00", x) : x < 100 ? string('0', x) : string(x)
+_four_dig(x::Int)::String = x < 10 ? string("000", x) : x < 100 ? string("00", x) : x < 1000 ? string('0', x) : string(x)
+
+_iso_date_string(d::Dates.Date)::String =
+    string(_four_dig(Dates.year(d)), '-', _two_dig(Dates.month(d)), '-', _two_dig(Dates.day(d)))
+
+function _iso_datetime_string(dt::Dates.DateTime)::String
+    ms = Dates.millisecond(dt)
+    base = string(_iso_date_string(Dates.Date(dt)), 'T',
+                  _two_dig(Dates.hour(dt)), ':', _two_dig(Dates.minute(dt)), ':', _two_dig(Dates.second(dt)))
+    return ms == 0 ? base : string(base, '.', _three_dig(ms))
+end
+
+function _iso_time_string(t::Dates.Time)::String
+    ms = Dates.millisecond(t)
+    base = string(_two_dig(Dates.hour(t)), ':', _two_dig(Dates.minute(t)), ':', _two_dig(Dates.second(t)))
+    return ms == 0 ? base : string(base, '.', _three_dig(ms))
+end
 StructUtils.lower(::JSONStyle, x::Regex) = x.pattern
 StructUtils.lower(::JSONStyle, x::Complex) = (re=real(x), im=imag(x))
 StructUtils.lower(::JSONStyle, x::AbstractArray{<:Any,0}) = x[1]
diff --git a/test/json_trim_workload.jl b/test/json_trim_workload.jl
new file mode 100644
index 0000000..05467cb
--- /dev/null
+++ b/test/json_trim_workload.jl
@@ -0,0 +1,137 @@
+# Trim-compile workload: covers typed and untyped parse, JSON writing, and —
+# specifically for the trim-verifier fixes — the parse error paths (unknown
+# fields with PtrString keys, the bad `unknown_fields` option message) and the
+# hand-formatted Date/DateTime/Time lowers. Compiled by
+# test/trim_compile_tests.jl with `juliac --trim=safe` (error budget zero),
+# then executed so the assertions also prove runtime behavior.
+using JSON
+using Dates
+
+struct TrimUser
+    id::Int
+    name::String
+end
+
+@defaults struct TrimConfig
+    enabled::Bool = false
+    label::String = "none"
+    count::Int = 0
+end
+
+function _assert_typed_parse()::Nothing
+    u = JSON.parse("""{"id":1,"name":"ab"}""", TrimUser)
+    u.id == 1 || error("typed id")
+    u.name == "ab" || error("typed name")
+
+    c = JSON.parse("""{"enabled":true,"count":7}""", TrimConfig)
+    c.enabled || error("kwdef bool")
+    c.label == "none" || error("kwdef default")
+    c.count == 7 || error("kwdef count")
+
+    v = JSON.parse("""[1,2,3]""", Vector{Int})
+    v == [1, 2, 3] || error("typed vector")
+
+    d = JSON.parse("""{"a":1,"b":2}""", Dict{String,Int})
+    d["a"] == 1 || error("typed dict")
+    return nothing
+end
+
+# the error paths the trim fixes rewrote: these must not only compile under
+# --trim but produce the exact messages at runtime
+function _assert_error_paths()::Nothing
+    threw = false
+    try
+        JSON.parse("""{"id":1,"name":"a","extra":2}""", TrimUser; unknown_fields = :error)
+    catch e
+        e isa ArgumentError || error("unknown-field error type")
+        m = e.msg
+        m isa String || error("unknown-field msg type")
+        # PtrString key formatting (unknownfieldkey)
+        occursin("unknown JSON member \"extra\"", m) || error("unknown-field msg")
+        occursin("TrimUser", m) || error("unknown-field target type")
+        threw = true
+    end
+    threw || error("unknown-field did not throw")
+
+    threw = false
+    try
+        JSON.parse("""{"id":1,"name":"a"}""", TrimUser; unknown_fields = :bogus)
+    catch e
+        e isa ArgumentError || error("bad-option error type")
+        m = e.msg
+        m isa String || error("bad-option msg type")
+        # plain symbol interpolation (not `repr`/Expr-show)
+        occursin(":bogus", m) || error("bad-option msg")
+        threw = true
+    end
+    threw || error("bad-option did not throw")
+    return nothing
+end
+
+function _assert_untyped_parse()::Nothing
+    # untyped parse returns Any by design (any JSON value type) — narrow the
+    # top-level to the concrete Object before indexing, as trim-compiled
+    # consumers must
+    raw = JSON.parse("""{"a":[1,2],"b":{"c":null},"s":"x","f":1.5,"t":true}""")
+    raw isa JSON.Object{String,Any} || error("untyped root type")
+    o = raw
+    a = o["a"]
+    a isa Vector{Any} || error("untyped array type")
+    length(a) == 2 || error("untyped array len")
+    a1 = a[1]
+    (a1 isa Int && a1 == 1) || error("untyped array elem")
+    rawb = o["b"]
+    rawb isa JSON.Object{String,Any} || error("untyped nested type")
+    c = rawb["c"]
+    c === nothing || error("untyped null")
+    s = o["s"]
+    (s isa String && s == "x") || error("untyped string")
+    f = o["f"]
+    (f isa Float64 && f == 1.5) || error("untyped float")
+    t = o["t"]
+    (t === true) || error("untyped bool")
+    return nothing
+end
+
+function _assert_json_write()::Nothing
+    JSON.json((; a = [1, 2], b = "x")) == "{\"a\":[1,2],\"b\":\"x\"}" || error("json nt")
+    JSON.json([1, 2, 3]) == "[1,2,3]" || error("json vector")
+    JSON.json(Dict("k" => 1)) == "{\"k\":1}" || error("json dict")
+    JSON.json("q\"uote") == "\"q\\\"uote\"" || error("json escape")
+    JSON.json(nothing) == "null" || error("json null")
+    u = TrimUser(3, "n")
+    JSON.json(u) == "{\"id\":3,\"name\":\"n\"}" || error("json struct")
+    round = JSON.parse(JSON.json(u), TrimUser)
+    round.id == 3 || error("roundtrip")
+    return nothing
+end
+
+# hand-formatted ISO lowers: exact renderings, including millisecond padding
+# and the ms-free forms
+function _assert_date_lowers()::Nothing
+    JSON.json(Date(2026, 7, 3)) == "\"2026-07-03\"" || error("date lower")
+    JSON.json(DateTime(2026, 7, 3, 4, 5, 6)) == "\"2026-07-03T04:05:06\"" || error("datetime lower")
+    JSON.json(DateTime(2026, 7, 3, 4, 5, 6, 70)) == "\"2026-07-03T04:05:06.070\"" || error("datetime ms pad")
+    JSON.json(Time(1, 2, 3)) == "\"01:02:03\"" || error("time lower")
+    JSON.json(Time(1, 2, 3, 45)) == "\"01:02:03.045\"" || error("time ms pad")
+    JSON.json((; d = Date(2026, 1, 2), t = Time(23, 59, 59))) ==
+        "{\"d\":\"2026-01-02\",\"t\":\"23:59:59\"}" || error("dates in object")
+    return nothing
+end
+
+function run_json_trim_workload()::Nothing
+    _assert_typed_parse()
+    _assert_error_paths()
+    _assert_untyped_parse()
+    _assert_json_write()
+    _assert_date_lowers()
+    return nothing
+end
+
+function @main(args::Vector{String})::Cint
+    _ = args
+    run_json_trim_workload()
+    return 0
+end
+
+Base.Experimental.entrypoint(main, (Vector{String},))
diff --git a/test/runtests.jl b/test/runtests.jl
index ca1498a..3891ea4 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -105,3 +105,5 @@ end
     ]
     @test JSON.json(x, 2) isa String
 end
+
+include(joinpath(dirname(pathof(JSON)), "../test/trim_compile_tests.jl"))
diff --git a/test/trim_compile_tests.jl b/test/trim_compile_tests.jl
new file mode 100644
index 0000000..a567c01
--- /dev/null
+++ b/test/trim_compile_tests.jl
@@ -0,0 +1,184 @@
+using Test
+
+const _TRIM_SAFE_ERROR_BUDGET = 0
+const _TRIM_SUPPORTED = VERSION >= v"1.12.0-rc1"
+const _TRIM_PRE_RELEASE = !isempty(VERSION.prerelease)
+const _TRIM_COMPILE_TIMEOUT_S = Sys.iswindows() ? 600.0 : 120.0
+const _TRIM_EXECUTABLE_TIMEOUT_S = Sys.iswindows() ? 120.0 : 30.0
+const _TRIM_USE_BUNDLE = Sys.iswindows()
+const _JULIAC_ENTRYPOINT_EXPR = "using JuliaC; if isdefined(JuliaC, :main); JuliaC.main(ARGS); else JuliaC._main_cli(ARGS); end"
+
+# Pkg.test() sets JULIA_LOAD_PATH restrictively, which prevents subprocesses
+# from finding stdlib packages like Pkg.  Remove it so subprocesses get the
+# default load path.
+function _clean_cmd(cmd::Cmd)
+    env = Dict{String,String}(k => v for (k, v) in ENV if k != "JULIA_LOAD_PATH")
+    return setenv(cmd, env)
+end
+
+function _setup_trim_env()
+    # JuliaC requires Julia 1.12+ and can't be in [extras] without breaking
+    # Pkg.test() on older Julia versions.  Create a temp project that dev's
+    # JSON from the local checkout and adds JuliaC.
+    pkg_path = normpath(joinpath(@__DIR__, ".."))
+    env_path = mktempdir()
+    julia = joinpath(Sys.BINDIR, Base.julia_exename())
+    setup_script = joinpath(env_path, "setup.jl")
+    write(setup_script, """
+    import Pkg
+    Pkg.develop(path=$(repr(pkg_path)))
+    Pkg.add("JuliaC")
+    # TODO: drop once Parsers.jl#207 (trim fixes for the float overflow-widening
+    # ladder) is released — JSON's number parsing pulls Parsers' float path into
+    # the trim verify set, and the registered release still fails verification.
+    Pkg.add(url="https://github.com/JuliaData/Parsers.jl", rev="trim-verifier-fixes")
+    """)
+    println("[trim] setting up temp environment with JuliaC...")
+    flush(stdout)
+    exit_code, output, timed_out = _run_command_with_timeout(
+        _clean_cmd(`$julia --startup-file=no --history-file=no --project=$env_path $setup_script`);
+        timeout_s = 120.0, log_label = "setup"
+    )
+    rm(setup_script; force = true)
+    if exit_code != 0 || timed_out
+        println("[trim] setup FAILED (exit=$exit_code, timed_out=$timed_out)")
+        println(output)
+        error("failed to set up trim test environment")
+    end
+    println("[trim] temp environment ready")
+    return env_path
+end
+
+function _run_trim_compile(project_path::String, script_path::String, output_name::String; timeout_s::Float64 = _TRIM_COMPILE_TIMEOUT_S, bundle_dir::Union{Nothing, String} = nothing)
+    julia_exe = joinpath(Sys.BINDIR, Base.julia_exename())
+    cmd = if bundle_dir === nothing
+        _clean_cmd(`$julia_exe --startup-file=no --history-file=no --code-coverage=none --project=$project_path -e $(_JULIAC_ENTRYPOINT_EXPR) -- --output-exe $output_name --project=$project_path --experimental --trim=safe $script_path`)
+    else
+        _clean_cmd(`$julia_exe --startup-file=no --history-file=no --code-coverage=none --project=$project_path -e $(_JULIAC_ENTRYPOINT_EXPR) -- --output-exe $output_name --bundle $bundle_dir --project=$project_path --experimental --trim=safe $script_path`)
+    end
+    return _run_command_with_timeout(cmd; timeout_s = timeout_s, log_label = "compile")
+end
+
+function _run_command_with_timeout(cmd::Cmd; timeout_s::Float64, log_label::String)
+    output_path = tempname()
+    out = open(output_path, "w")
+    exit_code = -1
+    timed_out = false
+    try
+        proc = run(pipeline(ignorestatus(cmd), stdout = out, stderr = out); wait = false)
+        timed_out = _wait_process_with_timeout!(proc; timeout_s = timeout_s, log_label = log_label)
+        exit_code = something(proc.exitcode, -1)
+    finally
+        close(out)
+    end
+    output = try
+        read(output_path, String)
+    catch
+        ""
+    finally
+        rm(output_path; force = true)
+    end
+    return exit_code, output, timed_out
+end
+
+function _wait_process_with_timeout!(proc::Base.Process; timeout_s::Float64, log_label::String)
+    started_at = time()
+    next_log_at = started_at + 10.0
+    timed_out = false
+    while Base.process_running(proc)
+        now = time()
+        if now - started_at >= timeout_s
+            timed_out = true
+            try; kill(proc); catch; end
+            break
+        end
+        if now >= next_log_at
+            elapsed = round(now - started_at; digits = 1)
+            println("[trim] $(log_label) WAIT $(elapsed)s")
+            flush(stdout)
+            next_log_at = now + 10.0
+        end
+        sleep(0.1)
+    end
+    try; wait(proc); catch; end
+    return timed_out
+end
+
+function _parse_trim_verify_totals(output::String)
+    m = match(r"Trim verify finished with\s+(\d+)\s+errors,\s+(\d+)\s+warnings\.", output)
+    m === nothing && return nothing
+    return parse(Int, m.captures[1]), parse(Int, m.captures[2])
+end
+
+function _run_trim_case(project_path::String, script_file::String, output_name::String)
+    script_path = joinpath(@__DIR__, script_file)
+    @test isfile(script_path)
+    println("[trim] compile START $(script_file)")
+    start_t = time()
+    mktempdir() do tmpdir
+        cd(tmpdir) do
+            bundle_dir = _TRIM_USE_BUNDLE ? joinpath(tmpdir, "bundle") : nothing
+            exit_code, output, timed_out = _run_trim_compile(project_path, script_path, output_name; bundle_dir = bundle_dir)
+            if timed_out
+                println("[trim] compile TIMED OUT for $(script_file)")
+                println(output)
+                @test false
+                return
+            end
+            totals = _parse_trim_verify_totals(output)
+            trim_errors, trim_warnings = if totals === nothing
+                exit_code == 0 ? (0, 0) : error("failed to parse trim verifier summary:\n$output")
+            else
+                totals
+            end
+            if trim_errors > 0 || trim_warnings > 0
+                println("---- trim compile output ($(script_file)) ----")
+                println(output)
+                println("---- end output ----")
+            end
+            @test trim_errors <= _TRIM_SAFE_ERROR_BUDGET
+            @test trim_warnings == 0
+            output_path = Sys.iswindows() ? "$(output_name).exe" : output_name
+            if trim_errors == 0
+                run_path = bundle_dir === nothing ? output_path : joinpath(bundle_dir, "bin", output_path)
+                @test exit_code == 0
+                @test isfile(run_path)
+                run_cmd = `$(abspath(run_path))`
+                run_exit, run_output, run_timed_out = _run_command_with_timeout(run_cmd; timeout_s = _TRIM_EXECUTABLE_TIMEOUT_S, log_label = "run")
+                if run_timed_out
+                    println("[trim] executable TIMED OUT for $(script_file)")
+                    println(run_output)
+                end
+                if run_exit != 0
+                    println("---- trim executable output ($(script_file)) ----")
+                    println(run_output)
+                    println("---- end output ----")
+                end
+                @test !run_timed_out
+                @test run_exit == 0
+            else
+                @test exit_code != 0
+            end
+        end
+    end
+    println("[trim] compile DONE $(script_file) ($(round(time() - start_t; digits = 2))s)")
+    return nothing
+end
+
+@testset "Trim compile" begin
+    if !_TRIM_SUPPORTED
+        println("[trim] skip Julia < 1.12: JuliaC trim compilation is unavailable")
+        @test true
+    elseif _TRIM_PRE_RELEASE
+        println("[trim] skip prerelease Julia: trim verifier behavior is not stable yet")
+        @test true
+    else
+        project_path = _setup_trim_env()
+        trim_workloads = [
+            ("json_trim_workload.jl", "json_trim_workload"),
+        ]
+        for (script_file, output_name) in trim_workloads
+            _run_trim_case(project_path, script_file, output_name)
+        end
+    end
+end