diff --git a/kani-compiler/src/kani_compiler.rs b/kani-compiler/src/kani_compiler.rs
index 3f2b3d0c7722..2261e696fb15 100644
--- a/kani-compiler/src/kani_compiler.rs
+++ b/kani-compiler/src/kani_compiler.rs
@@ -32,9 +32,58 @@ use rustc_public::rustc_internal;
 use rustc_session::config::ErrorOutputType;
 use tracing::debug;
 
+/// rustc flags kani-compiler requires for correct verification semantics, set
+/// unconditionally on every kani-mode invocation. kani's MIR analysis and
+/// goto-c codegen assume abort-on-panic, checked overflow, v0 mangling,
+/// encoded MIR, storage markers, and `cfg(kani)`. `cargo kani` already passes
+/// these (`kani_rustc_flags()` in kani-driver); for that path appending is
+/// idempotent — scalar `-C`/`-Z` flags are last-flag-wins and `--cfg`/
+/// `--check-cfg` are additive. For any other caller — a build system that
+/// drives kani-compiler directly, or a contributor running
+/// `RUSTC=kani-compiler cargo build` to debug — omitting one of these flags
+/// previously produced an incorrect or failed verification: missing
+/// `--cfg=kani` is a vacuous 0-harness pass, missing `-Coverflow-checks=on`
+/// or `-Zmir-enable-passes` proves a different program (silent), and the
+/// rest are hard errors. Now the compiler enforces the correct values.
+///
+/// Appended (not prepended): rustc is last-flag-wins for scalar `-C`/`-Z`
+/// options, so appending after the caller's args makes these non-overridable.
+///
+/// Single-token `--flag=value` encoding: rustc's getopts-based CLI parses
+/// `--cfg=kani` and `--cfg kani` identically, and this is the same encoding
+/// kani-driver has always passed (`base_rustc_flags()`). The integration test
+/// at `tests/script-based-pre/compiler_defaults/` pins that both cfg flags
+/// are parsed and applied in this exact form.
+///
+/// Deliberately NOT included:
+/// - `-Clinker=echo` — would clobber a real linker a build system provides
+///   for rlib output (last-flag-wins).
+/// - `-Zcrate-attr=feature(register_tool)` / `-Zcrate-attr=register_tool(kanitool)`
+///   — `-Zcrate-attr` ERRORS on a duplicate registration. `cargo kani` already
+///   passes these via `base_rustc_flags()`; appending again would break every
+///   `cargo kani` invocation (`error: tool 'kanitool' was already registered`).
+///   Omitting them is a loud compile error (`unrecognized tool kanitool`), not
+///   silent unsoundness — caller responsibility is acceptable here.
+/// - Conditional flags (`-Cinstrument-coverage`, `-Zno-codegen`,
+///   `-Cdebug-assertions=off`, `-Zrandomize-layout`) — encode session intent;
+///   the caller chooses them.
+/// - Diagnostic-format flags (`-Ztrim-diagnostic-paths`, `-Zhuman_readable_cgu_names`,
+///   `-Zunstable-options`) — caller preferences, not invariants.
+const KANI_REQUIRED_RUSTC_ARGS: &[&str] = &[
+    "-Cpanic=abort",
+    "-Coverflow-checks=on",
+    "-Csymbol-mangling-version=v0",
+    "-Zalways-encode-mir",
+    "-Zpanic_abort_tests=yes",
+    "-Zmir-enable-passes=-RemoveStorageMarkers",
+    "--cfg=kani",
+    "--check-cfg=cfg(kani)",
+];
+
 /// Run the Kani flavour of the compiler.
 /// This may require multiple runs of the rustc driver ([`rustc_driver::run_compiler`]).
-pub fn run(args: Vec<String>) {
+pub fn run(mut args: Vec<String>) {
+    args.extend(KANI_REQUIRED_RUSTC_ARGS.iter().map(|s| s.to_string()));
     let mut kani_compiler = KaniCompiler::new();
     kani_compiler.run(args);
 }
@@ -134,3 +183,43 @@ impl Callbacks for KaniCompiler {
         Compilation::Continue
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::KANI_REQUIRED_RUSTC_ARGS;
+
+    /// The required-flags list must never include a linker override — a build
+    /// system that needs a real linker for rlib output would have it silently
+    /// clobbered (last-flag-wins).
+    #[test]
+    fn required_args_do_not_clobber_linker() {
+        assert!(
+            !KANI_REQUIRED_RUSTC_ARGS.iter().any(|f| f.starts_with("-Clinker")),
+            "KANI_REQUIRED_RUSTC_ARGS must not set -Clinker (build systems own that)"
+        );
+    }
+
+    /// The soundness invariants kani's MIR analysis assumes.
+    #[test]
+    fn required_args_cover_soundness_invariants() {
+        for must_have in
+            ["-Cpanic=abort", "-Coverflow-checks=on", "-Zalways-encode-mir", "--cfg=kani"]
+        {
+            assert!(
+                KANI_REQUIRED_RUSTC_ARGS.contains(&must_have),
+                "missing soundness-critical flag: {must_have}"
+            );
+        }
+    }
+
+    /// `-Zcrate-attr` errors on duplicate registration. `cargo kani` already
+    /// passes `-Z crate-attr=...`; including it here would break every
+    /// `cargo kani` invocation with `error: tool 'kanitool' was already registered`.
+    #[test]
+    fn required_args_do_not_duplicate_crate_attr() {
+        assert!(
+            !KANI_REQUIRED_RUSTC_ARGS.iter().any(|f| f.contains("crate-attr")),
+            "KANI_REQUIRED_RUSTC_ARGS must not include -Zcrate-attr (cargo kani passes it; rustc errors on duplicate)"
+        );
+    }
+}
diff --git a/tests/script-based-pre/compiler_defaults/compiler_defaults.expected b/tests/script-based-pre/compiler_defaults/compiler_defaults.expected
new file mode 100644
index 000000000000..d8a4843e4433
--- /dev/null
+++ b/tests/script-based-pre/compiler_defaults/compiler_defaults.expected
@@ -0,0 +1,8 @@
+[TEST] minimal: kani-compiler exited 0
+[TEST] minimal proof_harnesses: 2
+[TEST] minimal mangled names are v0
+[TEST] unexpected_cfgs fired for undeclared cfg
+[TEST] no unexpected_cfgs warning for cfg(kani)
+[TEST] conflict: kani-compiler exited 0
+[TEST] conflict proof_harnesses: 2
+[TEST] conflict mangled names are v0
diff --git a/tests/script-based-pre/compiler_defaults/compiler_defaults.sh b/tests/script-based-pre/compiler_defaults/compiler_defaults.sh
new file mode 100755
index 000000000000..ff408d9b0693
--- /dev/null
+++ b/tests/script-based-pre/compiler_defaults/compiler_defaults.sh
@@ -0,0 +1,148 @@
+#!/usr/bin/env bash
+# Copyright Kani Contributors
+# SPDX-License-Identifier: Apache-2.0 OR MIT
+
+# Test that kani-compiler sets its required rustc flags unconditionally.
+#
+# Case 1 (minimal): invoke kani-compiler directly with the MINIMAL flag set
+# and assert the #[cfg(kani)]-gated harnesses appear in the metadata. Without
+# the defaults, --cfg=kani would be unset, the harness module would be
+# invisible, and the metadata would have 0 proof_harnesses: a vacuous
+# "verification" with nothing verified.
+#
+# The single-token `--flag=value` encoding of the cfg defaults must be parsed
+# AND applied, not just accepted:
+# - --cfg=kani: harnesses appear at all — the whole module is gated on it.
+# - --check-cfg=cfg(kani): fixture.rs also carries a deliberately undeclared
+#   cfg. rustc only checks cfg names when at least one --check-cfg argument
+#   is present, and none is passed below — so that cfg drawing an
+#   unexpected_cfgs warning proves the default was parsed as a check-cfg
+#   spec, and #[cfg(kani)] drawing none proves `kani` is the name it
+#   registered.
+#
+# Case 2 (conflict): the caller explicitly passes the opposite of three
+# required flags and the required values must still win — see the run_case
+# call below for how each winner is observed.
+
+set -eu
+
+OUTDIR="tmp_compiler_defaults"
+rm -rf "${OUTDIR}"
+mkdir "${OUTDIR}"
+
+# Locate kani-compiler and the kani sysroot in the dev build: `cargo build-dev`
+# populates target/kani/ (KANI_SYSROOT in .cargo/config.toml) with bin/ and
+# lib/, mirroring the release install layout. Same pattern as
+# std_codegen/codegen_std.sh. KANI_HOME is what `kani` itself passes as
+# --sysroot: LibConfig::new() uses the parent of the lib/ folder.
+KANI_DIR=$(git rev-parse --show-toplevel)
+KANI_HOME="${KANI_DIR}/target/kani"
+KANI_COMPILER="${KANI_HOME}/bin/kani-compiler"
+KANI_LIB="${KANI_HOME}/lib"
+
+[[ -x "${KANI_COMPILER}" ]] || {
+    echo "ERROR: kani-compiler not found at ${KANI_COMPILER}"
+    echo "Run 'cargo build-dev' first."
+    exit 1
+}
+
+# Args every case shares. Deliberately omits every flag
+# KANI_REQUIRED_RUSTC_ARGS now defaults: -Cpanic=abort, -Coverflow-checks=on,
+# -Csymbol-mangling-version, -Zalways-encode-mir, -Zpanic_abort_tests,
+# -Zmir-enable-passes, --cfg=kani, --check-cfg=cfg(kani). What remains is
+# what every caller still has to pass: install paths, the routing marker,
+# -Zunstable-options (gates `--extern noprelude:`), -Zcrate-attr (errors on
+# duplicate so it stays caller-supplied), and the reachability intent.
+COMMON_ARGS=(
+    --kani-compiler
+    --crate-type lib
+    --sysroot "${KANI_HOME}"
+    -L "${KANI_LIB}"
+    -Z unstable-options
+    --extern kani
+    --extern noprelude:std="${KANI_LIB}/libstd.rlib"
+    -Z crate-attr="feature(register_tool)"
+    -Z crate-attr="register_tool(kanitool)"
+    -Cllvm-args=--reachability=harnesses
+)
+
+# The case's metadata must list all of fixture.rs's harnesses (the count
+# observes which cfgs resolved — see fixture.rs) with v0-mangled
+# (`_R`-prefixed; legacy is `_ZN`) symbol names.
+check_metadata() {
+    local label="$1"
+    local metas=("${OUTDIR}/${label}"/*.kani-metadata.json)
+    [[ ${#metas[@]} -eq 1 && -f "${metas[0]}" ]] || {
+        echo "ERROR: ${label}: expected exactly one metadata file, got: ${metas[*]}"
+        exit 1
+    }
+    local count
+    count=$(jq '.proof_harnesses | length' "${metas[0]}")
+    echo "[TEST] ${label} proof_harnesses: ${count}"
+    if ! jq -e '(.proof_harnesses | length) > 0 and all(.proof_harnesses[]; .mangled_name | startswith("_R"))' \
+        "${metas[0]}" > /dev/null; then
+        echo "ERROR: ${label}: missing or non-v0 mangled harness names:"
+        jq -r '.proof_harnesses[].mangled_name' "${metas[0]}"
+        exit 1
+    fi
+    echo "[TEST] ${label} mangled names are v0"
+}
+
+# Compile fixture.rs as one test case: `label` names the case (and the
+# crate), remaining args are the caller flags under test. kani-compiler
+# appends KANI_REQUIRED_RUSTC_ARGS after everything passed here.
+run_case() {
+    local label="$1"
+    shift
+    mkdir "${OUTDIR}/${label}"
+    "${KANI_COMPILER}" "${COMMON_ARGS[@]}" \
+        --crate-name "${label}" \
+        --out-dir "${OUTDIR}/${label}" \
+        "$@" \
+        fixture.rs 2> "${OUTDIR}/${label}.stderr" || {
+        cat "${OUTDIR}/${label}.stderr"
+        exit 1
+    }
+    echo "[TEST] ${label}: kani-compiler exited 0"
+    check_metadata "${label}"
+}
+
+# Case 1: minimal invocation — every required flag missing; the defaults
+# must supply them all.
+run_case minimal
+
+# Negative control: fixture.rs's undeclared cfg must draw unexpected_cfgs.
+# Should rustc ever start checking cfg names without any --check-cfg being
+# passed, this leg turns vacuous — but the absence leg below still catches a
+# dropped or mis-encoded default, because #[cfg(kani)] would then warn.
+WARN='unexpected `cfg` condition name'
+if ! grep -q "${WARN}: \`not_a_kani_cfg\`" "${OUTDIR}/minimal.stderr"; then
+    echo 'ERROR: undeclared cfg drew no unexpected_cfgs warning — cfg checking is not active'
+    cat "${OUTDIR}/minimal.stderr"
+    exit 1
+fi
+echo "[TEST] unexpected_cfgs fired for undeclared cfg"
+
+# ...and `kani` must be the name --check-cfg=cfg(kani) registered: with cfg
+# checking proven active, #[cfg(kani)] must not warn.
+if grep -q "${WARN}: \`kani\`" "${OUTDIR}/minimal.stderr"; then
+    echo 'ERROR: unexpected_cfgs fired for cfg(kani) — --check-cfg=cfg(kani) did not register `kani`'
+    cat "${OUTDIR}/minimal.stderr"
+    exit 1
+fi
+echo "[TEST] no unexpected_cfgs warning for cfg(kani)"
+
+# Case 2: the caller explicitly passes conflicting values and the required
+# values must still win (today: defaults are appended after caller args and
+# rustc is last-flag-wins for scalar -C/-Z flags; these assertions pin the
+# outcome, not the mechanism). Each winner is observable without CBMC:
+# - -Cpanic=unwind or -Coverflow-checks=off winning is a hard error: kani-
+#   compiler's own session gates refuse to run ("Kani can only handle abort
+#   panic strategy", "Kani requires overflow checks"), so exiting 0 proves
+#   the required values resolved. The #[cfg(panic = "abort")]-gated harness
+#   doubles as a count backstop should those gates ever move.
+# - -Csymbol-mangling-version=legacy winning flips mangled names off v0's
+#   `_R` prefix, failing check_metadata.
+run_case conflict -Cpanic=unwind -Coverflow-checks=off -Csymbol-mangling-version=legacy
+
+rm -rf "${OUTDIR}"
diff --git a/tests/script-based-pre/compiler_defaults/config.yml b/tests/script-based-pre/compiler_defaults/config.yml
new file mode 100644
index 000000000000..4ab0c253ea3b
--- /dev/null
+++ b/tests/script-based-pre/compiler_defaults/config.yml
@@ -0,0 +1,5 @@
+# Copyright Kani Contributors
+# SPDX-License-Identifier: Apache-2.0 OR MIT
+script: compiler_defaults.sh
+expected: compiler_defaults.expected
+exit_code: 0
diff --git a/tests/script-based-pre/compiler_defaults/fixture.rs b/tests/script-based-pre/compiler_defaults/fixture.rs
new file mode 100644
index 000000000000..c7bc1b5368ee
--- /dev/null
+++ b/tests/script-based-pre/compiler_defaults/fixture.rs
@@ -0,0 +1,35 @@
+// Copyright Kani Contributors
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+//! `#[cfg(kani)]`-gated harnesses. If kani-compiler sets `--cfg=kani` as a
+//! default, the harnesses compile; if not, the module is invisible and 0
+//! harnesses appear in the metadata — a vacuous "verification" with nothing
+//! verified.
+//!
+//! `control` is a negative control for the `--check-cfg=cfg(kani)` default:
+//! its cfg is deliberately undeclared, so the unexpected_cfgs warning it
+//! draws proves cfg checking is active, while `#[cfg(kani)]` drawing no
+//! warning proves `kani` is the name the default registered.
+
+#[cfg(kani)]
+mod verify {
+    #[kani::proof]
+    fn check_with_defaults() {
+        let x: u8 = kani::any();
+        assert_eq!(x.wrapping_mul(1), x);
+    }
+
+    /// rustc derives `cfg(panic = "abort")` from the RESOLVED panic
+    /// strategy, so this harness is in the metadata count only if
+    /// `-Cpanic=abort` won the session — a backstop for the conflict case
+    /// alongside kani-compiler's own abort-strategy gate.
+    #[cfg(panic = "abort")]
+    #[kani::proof]
+    fn check_panic_abort_wins() {
+        let x: u8 = kani::any();
+        assert_eq!(x.wrapping_mul(1), x);
+    }
+}
+
+#[cfg(not_a_kani_cfg)]
+mod control {}