diff --git a/agent/bedrock_adapter.py b/agent/bedrock_adapter.py
index 50f05e3aa29f..c8cff3f76e13 100644
--- a/agent/bedrock_adapter.py
+++ b/agent/bedrock_adapter.py
@@ -1393,9 +1393,22 @@ BEDROCK_CONTEXT_LENGTHS: Dict[str, int] = {
 # Default for unknown Bedrock models
 BEDROCK_DEFAULT_CONTEXT_LENGTH = 128_000
 
+# Probe tiers (in tokens).  We send a request padded just past each tier and
+# read the real window from Bedrock's length-validation error.  Two reasons
+# this is tiered rather than one giant request:
+#   1. A wildly oversized payload (e.g. 5M tokens) makes Bedrock return an
+#      opaque InternalServerException after retries instead of a clean
+#      ValidationException — so we must stay within a sane overage.
+#   2. Stepping up lets us discover larger windows (2M+) without over-padding
+#      smaller ones.
+# Each tier value is the *padding target*; the error reports the true maximum,
+# which is what we actually return.
+_BEDROCK_PROBE_TIERS = (1_300_000, 2_200_000)
+_WORDS_PER_TOKEN = 0.9  # conservative: ensures the padded prompt clears the tier
 
-def get_bedrock_context_length(model_id: str) -> int:
-    """Look up the context window size for a Bedrock model.
+
+def _static_bedrock_context_length(model_id: str) -> int:
+    """Longest-substring-match lookup against the static fallback table.
 
     Uses substring matching so versioned IDs like
     ``anthropic.claude-sonnet-4-6-20250514-v1:0`` resolve correctly.
@@ -1408,3 +1421,103 @@ def get_bedrock_context_length(model_id: str) -> int:
             best_key = key
             best_val = val
     return best_val
+
+
+def probe_bedrock_context_length(model_id: str, region: str) -> Optional[int]:
+    """Discover a Bedrock model's real context window by provoking a length error.
+
+    Bedrock does not expose the context window via any metadata API
+    (``get-foundation-model`` omits it, ``Converse`` metrics omit it,
+    ``CountTokens`` is unsupported on several models).  The only authoritative
+    source is the ``ValidationException`` raised when a prompt exceeds the
+    window:
+
+        "The model returned the following errors: prompt is too long:
+         1300032 tokens > 1000000 maximum"
+
+    Length validation happens *before* inference, so an oversized request is
+    rejected immediately and cheaply — no tokens are generated and no input is
+    actually processed.  We pad a request just past each tier in
+    ``_BEDROCK_PROBE_TIERS`` and parse the reported ``maximum``.  Tiers exist
+    because (a) a *wildly* oversized payload makes Bedrock fail with an opaque
+    InternalServerException instead of a clean length error, and (b) stepping
+    up discovers larger windows without over-padding smaller ones.
+
+    Returns the detected window, or ``None`` if the probe could not run
+    (missing credentials, network error, or no parseable limit) so the caller
+    can fall back to the static table.
+    """
+    try:
+        from agent.model_metadata import parse_context_limit_from_error
+    except ImportError:  # pragma: no cover — same package
+        return None
+
+    try:
+        client = _get_bedrock_runtime_client(region)
+    except Exception as exc:  # boto3 missing / credential resolution failure
+        logger.debug("Bedrock context probe skipped for %s: %s", model_id, exc)
+        return None
+
+    last_error = ""
+    for tier_tokens in _BEDROCK_PROBE_TIERS:
+        pad_words = int(tier_tokens / _WORDS_PER_TOKEN)
+        oversized = "data " * pad_words
+        try:
+            client.converse(
+                modelId=model_id,
+                messages=[{"role": "user", "content": [{"text": oversized}]}],
+                inferenceConfig={"maxTokens": 8},
+            )
+            # Accepted a prompt this large → the window is at least this tier.
+            # Returning the tier as a lower bound is safe and avoids inventing
+            # a number we can't confirm.
+            logger.debug(
+                "Bedrock context probe for %s accepted ~%s-token prompt; "
+                "window is at least that", model_id, f"{tier_tokens:,}",
+            )
+            return tier_tokens
+        except Exception as exc:
+            msg = str(exc)
+            last_error = msg
+            limit = parse_context_limit_from_error(msg)
+            if limit and limit >= 1024:
+                logger.info(
+                    "Probed Bedrock context window for %s: %s tokens",
+                    model_id, f"{limit:,}",
+                )
+                return limit
+            # No parseable limit at this tier (opaque server error, auth,
+            # throttle).  Try the next, smaller-overage strategy is N/A here —
+            # tiers ascend — so just continue; if all fail we return None.
+            continue
+
+    logger.debug(
+        "Bedrock context probe for %s returned no parseable limit: %s",
+        model_id, last_error[:200],
+    )
+    return None
+
+
+def get_bedrock_context_length(model_id: str, region: str = "", probe: bool = True) -> int:
+    """Resolve the context window for a Bedrock model.
+
+    Resolution order:
+      1. Live probe against Bedrock (authoritative; cached by the caller).
+      2. Static fallback table (longest-substring match).
+      3. Conservative default.
+
+    The static table is intentionally a *fallback*, not the primary source:
+    AWS ships new model versions (opus-4-7, opus-4-8, ...) faster than the
+    table can track, and a stale entry silently caps the window (e.g. a
+    1M-token Opus pinned to 200K via an ``opus-4`` substring match).  The
+    probe asks Bedrock directly so every model — current or future — gets its
+    real window with no table maintenance.
+
+    ``probe=False`` (or an empty ``region``) skips the network call and uses
+    the static table only — used by pure-offline/display code paths.
+    """
+    if probe and region:
+        probed = probe_bedrock_context_length(model_id, region)
+        if probed:
+            return probed
+    return _static_bedrock_context_length(model_id)
diff --git a/agent/model_metadata.py b/agent/model_metadata.py
index 7e193aab09a6..bd3505c251d3 100644
--- a/agent/model_metadata.py
+++ b/agent/model_metadata.py
@@ -2287,10 +2287,42 @@ def get_model_context_length(
     # back to the 128K default before reaching the original step 4b branch.
     if is_bedrock_context:
         try:
-            from agent.bedrock_adapter import get_bedrock_context_length
-            return get_bedrock_context_length(model)
+            from agent.bedrock_adapter import (
+                get_bedrock_context_length,
+                resolve_bedrock_region,
+            )
         except ImportError:
             pass  # boto3 not installed — fall through to generic resolution
+        else:
+            # Bedrock does not expose the context window via any metadata API,
+            # so get_bedrock_context_length() probes the live endpoint (one
+            # fast, pre-inference length rejection) to read the real window.
+            # Cache the probe result per model so we pay that cost once, not
+            # every turn — keyed by base_url when present, else a synthetic
+            # bedrock:// key so display/offline paths share the entry.
+            cache_key_url = base_url or "bedrock://"
+            cached = get_cached_context_length(model, cache_key_url)
+            if cached is not None:
+                return cached
+            # Resolve region from the base_url host first, then the standard
+            # AWS region chain.  An empty region disables probing (table only).
+            region = ""
+            if base_url:
+                _m = re.search(r"bedrock-runtime\.([a-z0-9-]+)\.", base_url)
+                if _m:
+                    region = _m.group(1)
+            if not region:
+                try:
+                    region = resolve_bedrock_region()
+                except Exception:
+                    region = ""
+            ctx = get_bedrock_context_length(model, region=region, probe=bool(region))
+            if ctx and region:
+                # Only persist probe-derived values (region present); a pure
+                # table fallback shouldn't poison the cache against a later
+                # successful probe.
+                save_context_length(model, cache_key_url, ctx)
+            return ctx
 
     if provider == "novita" or (base_url and base_url_host_matches(base_url, "api.novita.ai")):
         ctx = _resolve_endpoint_context_length(model, base_url or "https://api.novita.ai/openai/v1", api_key=api_key)
diff --git a/tests/agent/test_bedrock_adapter.py b/tests/agent/test_bedrock_adapter.py
index 2f8a6dc652fa..56b99838d0be 100644
--- a/tests/agent/test_bedrock_adapter.py
+++ b/tests/agent/test_bedrock_adapter.py
@@ -1238,6 +1238,71 @@ class TestBedrockContextLength:
         # "anthropic.claude-3-5-sonnet" should match before "anthropic.claude-3"
         assert get_bedrock_context_length("anthropic.claude-3-5-sonnet-20240620-v1:0") == 200_000
 
+    def test_no_region_skips_probe_uses_table(self):
+        # Default call (no region) must NOT hit the network — returns the
+        # static table value.  Guards backward compatibility for callers that
+        # still invoke get_bedrock_context_length(model_id) with one arg.
+        from agent.bedrock_adapter import get_bedrock_context_length
+        with patch("agent.bedrock_adapter.probe_bedrock_context_length") as mock_probe:
+            assert get_bedrock_context_length("anthropic.claude-opus-4-6") == 200_000
+            mock_probe.assert_not_called()
+
+
+class TestBedrockContextProbe:
+    """Test the live context-window probe that reads the real window from
+    Bedrock's 'prompt is too long' validation error."""
+
+    def _client_raising(self, message):
+        client = MagicMock()
+        client.converse.side_effect = Exception(message)
+        return client
+
+    def test_probe_parses_real_window_from_error(self):
+        from agent.bedrock_adapter import probe_bedrock_context_length
+        err = (
+            "An error occurred (ValidationException) when calling the Converse "
+            "operation: The model returned the following errors: prompt is too "
+            "long: 5000032 tokens > 1000000 maximum"
+        )
+        with patch("agent.bedrock_adapter._get_bedrock_runtime_client",
+                   return_value=self._client_raising(err)):
+            assert probe_bedrock_context_length(
+                "eu.anthropic.claude-opus-4-8", "eu-central-1") == 1_000_000
+
+    def test_probe_returns_none_on_unparseable_error(self):
+        from agent.bedrock_adapter import probe_bedrock_context_length
+        err = "An error occurred (AccessDeniedException): not authorized"
+        with patch("agent.bedrock_adapter._get_bedrock_runtime_client",
+                   return_value=self._client_raising(err)):
+            assert probe_bedrock_context_length(
+                "eu.anthropic.claude-opus-4-8", "eu-central-1") is None
+
+    def test_probe_returns_none_when_client_unavailable(self):
+        from agent.bedrock_adapter import probe_bedrock_context_length
+        with patch("agent.bedrock_adapter._get_bedrock_runtime_client",
+                   side_effect=RuntimeError("boto3 missing")):
+            assert probe_bedrock_context_length("any.model", "eu-central-1") is None
+
+    def test_probe_result_beats_static_table(self):
+        # A successful probe (1M) must override the stale table value (200K
+        # via the 'anthropic.claude-opus-4' substring match).
+        from agent.bedrock_adapter import get_bedrock_context_length
+        err = "prompt is too long: 5000032 tokens > 1000000 maximum"
+        with patch("agent.bedrock_adapter._get_bedrock_runtime_client",
+                   return_value=self._client_raising(err)):
+            assert get_bedrock_context_length(
+                "eu.anthropic.claude-opus-4-8",
+                region="eu-central-1") == 1_000_000
+
+    def test_probe_failure_falls_back_to_table(self):
+        from agent.bedrock_adapter import get_bedrock_context_length
+        err = "AccessDeniedException: nope"
+        with patch("agent.bedrock_adapter._get_bedrock_runtime_client",
+                   return_value=self._client_raising(err)):
+            # opus-4-6 is in the table at 200K; probe fails → table wins.
+            assert get_bedrock_context_length(
+                "anthropic.claude-opus-4-6", region="eu-central-1") == 200_000
+
 
 # ---------------------------------------------------------------------------
 # Tool-calling capability detection