diff --git a/llmcore.py b/llmcore.py
index 88879416e..d9b545b2f 100644
--- a/llmcore.py
+++ b/llmcore.py
@@ -99,13 +99,57 @@ def safeprint(*argv):
 
 STATS = {}
 
+# Default chars-per-token heuristic for the legacy `context_win * 3` rule.
+# Kept as a named module constant so a future model-aware estimator only has
+# to swap the value here (or override per-session via
+# `sess.token_chars_per_token` / `sess.token_estimator`). `trim_messages_history`
+# still uses this as a char-equivalent multiplier — the trim gate is char-based
+# to keep behaviour byte-identical with the previous implementation.
+DEFAULT_CHARS_PER_TOKEN = 3
+
+def estimate_context_tokens(history, sess=None):
+    """Estimate `history`'s "tokens" for debug / diagnostics, *not* the trim gate.
+
+    This is a pure function of `(history, sess)` that returns an int token
+    estimate. It is **not** the trigger that fires `trim_messages_history`
+    (the trigger is still `cost(history) > context_win * 3` chars) — the
+    estimator exists so we can:
+
+      1. Stop burying the heuristic inside `trim_messages_history`. Issue
+         #750: `3 chars/token` is wrong for CJK/code/JSON; today there is no
+         single place to swap it.
+      2. Surface the estimate alongside the real `input_tokens` recorded by
+         `_record_usage()` so future debugging can compare them.
+      3. Give sessions a clean override entry (`sess.token_estimator` callable
+         OR `sess.token_chars_per_token` float) without leaking provider-
+         specific knobs into the trim path.
+
+    Default behaviour (v1, byte-identical with the prior code):
+      `int(sum(len(json.dumps(m, ensure_ascii=False)) for m in history) / 3)`
+    i.e. the same `cost() / 3` the prior code used implicitly for its log.
+    `sess` may be None — used by unit tests and by callers that have no
+    session object handy yet.
+    """
+    estimator = getattr(sess, 'token_estimator', None) if sess is not None else None
+    if estimator is not None:
+        try: return max(0, int(estimator(history) or 0))
+        except Exception: pass                                       # never let an override crash the trim path
+    cpt = getattr(sess, 'token_chars_per_token', DEFAULT_CHARS_PER_TOKEN) if sess is not None else DEFAULT_CHARS_PER_TOKEN
+    try: cpt = float(cpt)
+    except (TypeError, ValueError): cpt = DEFAULT_CHARS_PER_TOKEN
+    if cpt <= 0: cpt = DEFAULT_CHARS_PER_TOKEN                       # guard against a misconfigured zero / negative divisor
+    base = sum(len(json.dumps(m, ensure_ascii=False)) for m in history)
+    return int(base / cpt)
+
 def trim_messages_history(history, sess):
-    cap = sess.context_win * 3
+    cap = sess.context_win * DEFAULT_CHARS_PER_TOKEN
     target = int(cap * getattr(sess, 'trim_keep_rate', 0.6))
     kp = sess.trim_keep_prefix
     def cost(ms): return sum(len(json.dumps(m, ensure_ascii=False)) for m in ms)
     compress_history_tags(history, interval=getattr(sess, 'cut_msg_interval', 7))
-    STATS.update(ctx=(c := cost(history)), msgs=len(history)); print(f'[Debug] Current context: {c} chars, {len(history)} messages.')
+    c = cost(history)
+    STATS.update(ctx=c, tokens_est=estimate_context_tokens(history, sess), msgs=len(history))
+    print(f'[Debug] Current context: {c} chars (~{STATS["tokens_est"]} tokens est), {len(history)} messages.')
     if c <= cap: return
     compress_history_tags(history, keep_recent=4, force=True)
     if cost(history) <= target: return
@@ -122,7 +166,9 @@ def cost(ms): return sum(len(json.dumps(m, ensure_ascii=False)) for m in ms)
         gap = [{"role": "assistant", "content": _d()}] if m.get('role') == 'user' else [{"role": "user", "content": _d()}, {"role": "assistant", "content": _d()}]
         history[:] = pre + gap + post
     else: history[:] = pre + post
-    STATS.update(ctx=(c := cost(history)), msgs=len(history)); print(f'[Debug] Trimmed context, current: {c} chars, {len(history)} messages.')
+    c = cost(history)
+    STATS.update(ctx=c, tokens_est=estimate_context_tokens(history, sess), msgs=len(history))
+    print(f'[Debug] Trimmed context, current: {c} chars (~{STATS["tokens_est"]} tokens est), {len(history)} messages.')
 
 def auto_make_url(base, path):
     b, p = base.rstrip('/'), path.strip('/')
diff --git a/tests/test_llmcore_estimate_context_tokens.py b/tests/test_llmcore_estimate_context_tokens.py
new file mode 100644
index 000000000..80f840619
--- /dev/null
+++ b/tests/test_llmcore_estimate_context_tokens.py
@@ -0,0 +1,255 @@
+"""Regression tests for `llmcore.estimate_context_tokens` (#750).
+
+Background: `trim_messages_history` historically used the heuristic
+``3 chars == 1 token`` inlined into the cap calculation
+(``cap = sess.context_win * 3``). Issue #750 called for that heuristic to
+be moved into a single named place, so a future model-aware estimator can
+swap it without touching the trim path. This file pins the v1 behaviour
+and exercises both extension points:
+``sess.token_estimator`` (callable override) and
+``sess.token_chars_per_token`` (float multiplier override).
+
+The tests load `llmcore.py` in isolation (heavy deps stubbed) — same trick
+used by `tests/test_llmcore_record_usage_messages.py`.
+"""
+
+import importlib.util
+import json
+import sys
+import types
+import unittest
+from pathlib import Path
+
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+LLMCORE_PATH = REPO_ROOT / "llmcore.py"
+
+
+def _load_llmcore_module():
+    """Load `llmcore` without spinning up the agent runtime.
+
+    `llmcore` is a top-level script that imports `agentmain`, `requests`,
+    and `websockets` at module load time. We stub just enough to import the
+    module and access the helpers we want to test, then return it.
+    """
+    stubs = {
+        "requests": types.ModuleType("requests"),
+        "websockets": types.ModuleType("websockets"),
+        "agentmain": types.ModuleType("agentmain"),
+    }
+    stubs["requests"].post = lambda *a, **kw: None
+    stubs["requests"].Session = type("Session", (), {})
+    for name, mod in stubs.items():
+        sys.modules.setdefault(name, mod)
+    spec = importlib.util.spec_from_file_location("llmcore_under_test", LLMCORE_PATH)
+    mod = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(mod)
+    return mod
+
+
+llmcore = _load_llmcore_module()
+
+
+def _history(*pairs):
+    """Build a history list of `{role, content}` dicts from (role, text) pairs."""
+    return [{"role": role, "content": text} for role, text in pairs]
+
+
+def _sess(**attrs):
+    """Build a throw-away session stub. Only the attributes our estimator
+    reads are set; everything else is irrelevant."""
+    return types.SimpleNamespace(**attrs)
+
+
+class EstimateContextTokensDefaultHeuristicTests(unittest.TestCase):
+    """Pin the v1 behaviour: `int(char_count / 3)` rounded down."""
+
+    def _json_chars(self, *pairs):
+        # The estimator sums `len(json.dumps(m, ensure_ascii=False)) for m in history`
+        # i.e. it json-serialises each message *individually* and sums. Match that
+        # here so we don't bake in the wrong assumption about list-vs-per-msg.
+        return sum(len(json.dumps({"role": r, "content": c}, ensure_ascii=False)) for r, c in pairs)
+
+    def test_empty_history_returns_zero(self):
+        self.assertEqual(llmcore.estimate_context_tokens([]), 0)
+        self.assertEqual(llmcore.estimate_context_tokens([], None), 0)
+
+    def test_short_english_history(self):
+        # {"role":"user","content":"hello"} -> 36 chars -> 36/3 = 12 tokens.
+        hist = _history(("user", "hello"))
+        self.assertEqual(llmcore.estimate_context_tokens(hist), self._json_chars(("user", "hello")) // 3)
+        self.assertEqual(llmcore.estimate_context_tokens(hist), 12)
+
+    def test_cjk_history_is_one_token_per_three_chars(self):
+        # CJK glyphs are still 1 char each under ensure_ascii=False — that
+        # is the v1 bug #750 calls out. Pin the behaviour so a future
+        # tokenizer-aware fix doesn't silently regress English history.
+        hist = _history(("user", "你好世界你好"))
+        self.assertEqual(llmcore.estimate_context_tokens(hist), self._json_chars(("user", "你好世界你好")) // 3)
+        self.assertEqual(llmcore.estimate_context_tokens(hist), 12)
+
+    def test_uses_ensure_ascii_false_so_cjk_counts_in_chars(self):
+        # `json.dumps(..., ensure_ascii=False)` keeps the original chars
+        # rather than escaping them to \uXXXX (which would inflate CJK).
+        # With escaped form "你好" would become "\u4f60\u597d" (12 chars);
+        # with ensure_ascii=False the content stays at 2 chars. We pin the
+        # unescaped form (the value used by the original `cost()` too).
+        hist = _history(("user", "你好"))
+        self.assertEqual(llmcore.estimate_context_tokens(hist), self._json_chars(("user", "你好")) // 3)
+        # Sanity: escaped form would yield a different number.
+        escaped_chars = len(json.dumps({"role": "user", "content": "你好"}, ensure_ascii=True))
+        self.assertNotEqual(self._json_chars(("user", "你好")), escaped_chars)
+
+    def test_history_with_multiple_messages(self):
+        # The estimator sums per-message json.dumps chars, then divides.
+        hist = _history(("user", "abc"), ("assistant", "defg"))
+        self.assertEqual(
+            llmcore.estimate_context_tokens(hist),
+            self._json_chars(("user", "abc"), ("assistant", "defg")) // 3,
+        )
+
+    def test_non_json_serialisable_messages_are_handled_by_json_dumps(self):
+        # `cost()` (and the estimator) use `json.dumps` directly. A
+        # non-serialisable value will raise inside the estimator — that's
+        # acceptable because the trim path catches and treats it as
+        # over-cap (the original code also relied on this). Pin it.
+        bad = [{"role": "user", "content": {"nested": object()}}]
+        with self.assertRaises(TypeError):
+            llmcore.estimate_context_tokens(bad)
+
+
+class EstimateContextTokensCharsPerTokenOverrideTests(unittest.TestCase):
+    """`sess.token_chars_per_token` is the float-multiplier override."""
+
+    def test_string_is_coerced_to_float(self):
+        sess = _sess(token_chars_per_token="1.5")
+        # 36 chars / 1.5 = 24 tokens
+        self.assertEqual(llmcore.estimate_context_tokens(_history(("user", "hello")), sess), 24)
+
+    def test_invalid_value_falls_back_to_default(self):
+        # Garbage values must not crash the trim path; the fallback is the
+        # module default of 3.
+        sess = _sess(token_chars_per_token="not-a-number")
+        # 38 chars / 3 = 12 tokens
+        self.assertEqual(llmcore.estimate_context_tokens(_history(("user", "hello")), sess), 12)
+
+    def test_zero_divisor_is_replaced_by_default(self):
+        # A misconfigured zero / negative must not produce a ZeroDivisionError
+        # or a wildly negative estimate — it falls back to the default.
+        sess = _sess(token_chars_per_token=0)
+        self.assertEqual(llmcore.estimate_context_tokens(_history(("user", "hello")), sess), 12)
+        sess = _sess(token_chars_per_token=-2)
+        self.assertEqual(llmcore.estimate_context_tokens(_history(("user", "hello")), sess), 12)
+
+    def test_none_value_falls_back_to_default(self):
+        sess = _sess(token_chars_per_token=None)
+        self.assertEqual(llmcore.estimate_context_tokens(_history(("user", "hello")), sess), 12)
+
+
+class EstimateContextTokensCallableOverrideTests(unittest.TestCase):
+    """`sess.token_estimator` is the full-callable override."""
+
+    def test_callable_takes_precedence_over_chars_per_token(self):
+        sess = _sess(token_estimator=lambda h: 42, token_chars_per_token=1.5)
+        # The callable wins even when chars_per_token would also be set.
+        self.assertEqual(llmcore.estimate_context_tokens(_history(("user", "hi")), sess), 42)
+
+    def test_callable_returning_zero_or_none_is_clamped(self):
+        # `int(estimator(history) or 0)` then `max(0, ...)` — defensive
+        # against a buggy estimator that returns None or 0 or a negative.
+        sess = _sess(token_estimator=lambda h: 0)
+        self.assertEqual(llmcore.estimate_context_tokens(_history(("user", "hi")), sess), 0)
+        sess = _sess(token_estimator=lambda h: None)
+        self.assertEqual(llmcore.estimate_context_tokens(_history(("user", "hi")), sess), 0)
+        sess = _sess(token_estimator=lambda h: -7)
+        self.assertEqual(llmcore.estimate_context_tokens(_history(("user", "hi")), sess), 0)
+
+    def test_callable_returning_non_int_is_truncated(self):
+        # Floats must be coerced to int — the estimator contract says int.
+        sess = _sess(token_estimator=lambda h: 1.9)
+        self.assertEqual(llmcore.estimate_context_tokens(_history(("user", "hi")), sess), 1)
+
+    def test_buggy_callable_is_swallowed_and_falls_back(self):
+        # The estimator must never crash the trim path — a buggy override
+        # should fall back to the chars-per-token path.
+        sess = _sess(token_estimator=lambda h: 1 / 0, token_chars_per_token="3")
+        # 38 chars / 3 = 12 tokens (fallback to chars_per_token path)
+        self.assertEqual(llmcore.estimate_context_tokens(_history(("user", "hello")), sess), 12)
+
+    def test_callable_receives_full_history(self):
+        captured = []
+        sess = _sess(token_estimator=lambda h: captured.append(list(h)) or 99)
+        hist = _history(("user", "hi"), ("assistant", "yo"))
+        llmcore.estimate_context_tokens(hist, sess)
+        # The estimator sees the whole list, not a slice.
+        self.assertEqual(len(captured[0]), 2)
+        self.assertEqual(captured[0][0]["role"], "user")
+
+
+class TrimMessagesHistoryBehaviorPreservedTests(unittest.TestCase):
+    """The trim *gate* must keep its char-based behaviour byte-identical
+    with the previous implementation — only the debug log gains the new
+    `tokens_est` field."""
+
+    def _make_sess(self, context_win=100, trim_keep_prefix=0, trim_keep_rate=0.6):
+        return _sess(
+            context_win=context_win,
+            trim_keep_prefix=trim_keep_prefix,
+            trim_keep_rate=trim_keep_rate,
+        )
+
+    def test_small_history_does_not_trim_and_logs_both_stats(self):
+        sess = self._make_sess(context_win=1000)
+        hist = _history(("user", "hi"))
+        llmcore.trim_messages_history(hist, sess)
+        # gate says: under cap -> return without trimming
+        self.assertEqual(len(hist), 1)
+        # stats now expose both `ctx` (chars, for cost_tracker compat) and
+        # `tokens_est` (the new field).
+        self.assertIn("ctx", llmcore.STATS)
+        self.assertIn("tokens_est", llmcore.STATS)
+        # tokens_est is the same as ctx / 3 under the default heuristic.
+        self.assertEqual(llmcore.STATS["tokens_est"], llmcore.STATS["ctx"] // 3)
+
+    def test_large_history_triggers_hard_cut(self):
+        # context_win * 3 chars is the trim trigger. Build a history
+        # whose post-compress cost > target (the trim target), then
+        # confirm:
+        #   * length decreases
+        #   * tokens_est tracks ctx / 3 under the default heuristic
+        # compress_history_tags() shrinks <thinking>/<tool_use> blocks in
+        # older messages but does not drop messages, so plain content is
+        # left alone and the trim's hard-cut loop (which loops while
+        # `len(post) > 9`) is the only thing that can drop messages.
+        sess = self._make_sess(context_win=2, trim_keep_prefix=0, trim_keep_rate=0.3)
+        # 25 user messages of 30-char content. context_win*3 = 6 cap,
+        # target = 6 * 0.3 = 1. hard-cut loop: pops until either
+        # len(post) <= 9 OR cost <= target. With 30-char msgs at >1
+        # char target the loop terminates by len(post) > 9, ending at 9.
+        hist = _history(*[("user", "x" * 30)] * 25)
+        before = len(hist)
+        llmcore.trim_messages_history(hist, sess)
+        self.assertLess(len(hist), before)
+        # The trim path always leaves the last <=9 messages (that's the
+        # only shape the existing hard-cut loop can produce). Pin it.
+        self.assertLessEqual(len(hist), 9)
+        # tokens_est still tracks ctx / 3 under the default heuristic.
+        self.assertEqual(llmcore.STATS["tokens_est"], llmcore.STATS["ctx"] // 3)
+
+    def test_session_with_custom_chars_per_token_is_honoured_in_log_only(self):
+        # The estimator must reflect the per-session override in STATS,
+        # but the trim gate must still use the default 3-char multiplier
+        # (so behaviour stays byte-identical for v1).
+        sess = self._make_sess(context_win=1000)
+        sess.token_chars_per_token = 1.5
+        hist = _history(("user", "hi"))
+        llmcore.trim_messages_history(hist, sess)
+        # ctx stays char-based.
+        self.assertGreater(llmcore.STATS["ctx"], 0)
+        # tokens_est uses the per-session override (1.5x larger than /3).
+        # exact formula: ctx / 1.5 (rounded down by int()).
+        self.assertEqual(llmcore.STATS["tokens_est"], llmcore.STATS["ctx"] // 1.5)
+
+
+if __name__ == "__main__":
+    unittest.main()
\ No newline at end of file