chore: Create the context cache based on the token count of previous request

before this change, we estimate the token count of the contents to cache and use it to compare with the threshold user set. but that's not precise , so we use the actual prompt token count of previous llm request.

We won't create cache for the very initial request

PiperOrigin-RevId: 814484840

src/google/adk/flows/llm_flows/context_cache_processor.py

@@ -62,9 +62,11 @@ class ContextCacheRequestProcessor(BaseLlmRequestProcessor):
     # Set cache config to request
     llm_request.cache_config = invocation_context.context_cache_config
 
-    # Find latest cache metadata from session events
-    latest_cache_metadata = self._find_latest_cache_metadata(
-        invocation_context, agent.name, invocation_context.invocation_id
+    # Find latest cache metadata and previous token count from session events
+    latest_cache_metadata, previous_token_count = (
+        self._find_cache_info_from_events(
+            invocation_context, agent.name, invocation_context.invocation_id
+        )
     )
 
     if latest_cache_metadata:
@@ -77,51 +79,78 @@ class ContextCacheRequestProcessor(BaseLlmRequestProcessor):
           latest_cache_metadata.cached_contents_count,
       )
 
+    if previous_token_count is not None:
+      llm_request.cacheable_contents_token_count = previous_token_count
+      logger.debug(
+          'Found previous prompt token count for agent %s: %d',
+          agent.name,
+          previous_token_count,
+      )
+
     logger.debug('Context caching enabled for agent %s', agent.name)
 
     # This processor yields no events
     return
     yield  # AsyncGenerator requires a yield in function body
 
-  def _find_latest_cache_metadata(
+  def _find_cache_info_from_events(
       self,
       invocation_context: 'InvocationContext',
       agent_name: str,
       current_invocation_id: str,
-  ) -> Optional[CacheMetadata]:
-    """Find the latest cache metadata from session events.
+  ) -> tuple[Optional[CacheMetadata], Optional[int]]:
+    """Find cache metadata and previous token count from session events.
 
     Args:
         invocation_context: Context containing session with events
-        agent_name: Name of agent to find cache metadata for
+        agent_name: Name of agent to find cache info for
         current_invocation_id: Current invocation ID to compare for increment
 
     Returns:
-        Latest cache metadata for the agent (with updated invocations_used
-        if needed), or None if not found
+        Tuple of (cache_metadata, previous_prompt_token_count)
+        cache_metadata: Latest cache metadata with updated invocations_used if needed
+        previous_prompt_token_count: Most recent prompt token count from LLM response
     """
     if not invocation_context.session or not invocation_context.session.events:
-      return None
+      return None, None
+
+    cache_metadata = None
+    previous_token_count = None
 
     # Search events from most recent to oldest using index traversal
     events = invocation_context.session.events
     for i in range(len(events) - 1, -1, -1):
       event = events[i]
-      if event.cache_metadata is not None and event.author == agent_name:
-
-        cache_metadata = event.cache_metadata
+      if event.author != agent_name:
+        continue
 
+      # Look for cache metadata (only in actual LLM response events)
+      if cache_metadata is None and event.cache_metadata is not None:
         # Check if this is a different invocation - increment invocations_used
         if event.invocation_id and event.invocation_id != current_invocation_id:
           # Different invocation - increment invocations_used
-          return cache_metadata.model_copy(
-              update={'invocations_used': cache_metadata.invocations_used + 1}
+          cache_metadata = event.cache_metadata.model_copy(
+              update={
+                  'invocations_used': event.cache_metadata.invocations_used + 1
+              }
           )
         else:
           # Same invocation or no invocation_id - return as-is
-          return cache_metadata
+          cache_metadata = event.cache_metadata
 
-    return None
+      # Look for previous prompt token count (from actual LLM response events)
+      if (
+          previous_token_count is None
+          and event.usage_metadata
+          and event.usage_metadata.prompt_token_count is not None
+      ):
+        previous_token_count = event.usage_metadata.prompt_token_count
+
+      # Stop early if we found both pieces of information
+      if cache_metadata is not None and previous_token_count is not None:
+        break
+
+    return cache_metadata, previous_token_count
 
 
 # Create processor instance for use in flows

src/google/adk/models/gemini_context_cache_manager.py

@@ -257,12 +257,21 @@ class GeminiContextCacheManager:
     Returns:
         Cache metadata if successful, None otherwise
     """
-    # Estimate token count for minimum cache size check
-    estimated_tokens = self._estimate_request_tokens(llm_request)
-    if estimated_tokens < llm_request.cache_config.min_tokens:
+    # Check if we have token count from previous response for cache size validation
+    if llm_request.cacheable_contents_token_count is None:
       logger.info(
-          "Request too small for caching (%d < %d tokens)",
-          estimated_tokens,
+          "No previous token count available, skipping cache creation for"
+          " initial request"
+      )
+      return None
+
+    if (
+        llm_request.cacheable_contents_token_count
+        < llm_request.cache_config.min_tokens
+    ):
+      logger.info(
+          "Previous request too small for caching (%d < %d tokens)",
+          llm_request.cacheable_contents_token_count,
           llm_request.cache_config.min_tokens,
       )
       return None

src/google/adk/models/llm_request.py

@@ -88,6 +88,9 @@ class LlmRequest(BaseModel):
   cache_metadata: Optional[CacheMetadata] = None
   """Cache metadata from previous requests, used for cache management."""
 
+  cacheable_contents_token_count: Optional[int] = None
+  """Token count from previous request's prompt, used for cache size validation."""
+
   def append_instructions(
       self, instructions: Union[list[str], types.Content]
   ) -> list[types.Content]: