Preserve content blocks and thinking origin in C store append

Round-trip persistent message content through the C session store bridge
instead of dropping it, including tool-use, tool-result, system, and
thinking blocks. Reconstruct borrowed C content blocks with arena-backed
helpers, carry message identity fields like API style/model/reasoning, and
add tests covering append marshalling and thinking signature origin
round-tripping.

1 files changed, 58 insertions, 1 deletions

diff --git a/libpanto-c/include/panto.h b/libpanto-c/include/panto.h
index d0c272f..c43b39f 100644
--- a/libpanto-c/include/panto.h
+++ b/libpanto-c/include/panto.h
@@ -58,7 +58,36 @@ typedef struct { size_t message_index; } PantoToolDispatchComplete;
 
 typedef struct { PantoEventTag tag; union { PantoMessageRole message_start; PantoBlockStart block_start; PantoToolDetails tool_details; PantoContentDelta content_delta; PantoBlockComplete block_complete; PantoMessageComplete message_complete; PantoProviderRetry provider_retry; PantoToolDispatchStart tool_dispatch_start; PantoToolDispatchComplete tool_dispatch_complete; } data; } PantoEvent;
 
-typedef struct { PantoMessageRole role; bool has_usage; PantoUsage usage; bool has_metadata; PantoSlice metadata; } PantoPersistentMessage;
+/* One content block of a message being appended. A flattened, borrow-only
+ * view: every slice points into the in-memory message and is valid only for
+ * the duration of the append_messages call. `text` carries the Text body,
+ * Thinking text, ToolUse input JSON, concatenated ToolResult text, or
+ * System/CompactionSummary text depending on `tag`. `tool_id` is the ToolUse
+ * id or ToolResult tool_use_id; `tool_name` is the ToolUse name; `is_error`
+ * is the ToolResult error flag; `system_mode` is the System block mode;
+ * `thinking_signature` is the Thinking signature (empty when absent), and the
+ * `thinking_signature_*` fields record the provider/style/model that emitted
+ * it. To replay a thinking block faithfully (Anthropic drops thinking blocks
+ * whose signature lacks a matching origin) a store must persist the signature
+ * AND all three origin fields, then feed them back via the message builder. */
+typedef struct {
+  PantoContentBlockTag tag;
+  PantoSlice text;
+  PantoSlice tool_id;
+  PantoSlice tool_name;
+  bool is_error;
+  PantoSystemMode system_mode;
+  PantoSlice thinking_signature;
+  PantoAPIStyle thinking_signature_api_style;
+  PantoSlice thinking_signature_base_url;
+  PantoSlice thinking_signature_model;
+} PantoOwnedContentBlock;
+
+/* `api_style`/`base_url`/`model`/`reasoning` are the per-message wire identity
+ * (the provider that produced this message), which can drift within a single
+ * conversation (compaction on a different model, a mid-chat model switch). A
+ * store should persist these per message rather than from static config. */
+typedef struct { PantoMessageRole role; bool has_usage; PantoUsage usage; bool has_metadata; PantoSlice metadata; PantoAPIStyle api_style; PantoSlice base_url; PantoSlice model; PantoReasoningEffort reasoning; const PantoOwnedContentBlock *content; size_t content_len; } PantoPersistentMessage;
 
 typedef struct {
   PantoStatus (*create)(void *ctx, PantoSession **out);
@@ -93,6 +122,34 @@ PANTO_EXPORT PantoStatus panto_conversation_replace_system_message(PantoConversa
 PANTO_EXPORT PantoStatus panto_conversation_add_user_text(PantoConversation *conversation, const char *text);
 PANTO_EXPORT PantoStatus panto_conversation_add_assistant_text(PantoConversation *conversation, const char *text, bool has_usage, PantoUsage usage);
 PANTO_EXPORT PantoStatus panto_conversation_add_compaction_summary(PantoConversation *conversation, const char *text);
+
+/* Rich, block-level message builder. Accumulate content blocks, then commit
+ * the whole message to a conversation in one call — the path for rebuilding a
+ * persisted conversation that contains tool calls or thinking. The builder
+ * uses the same allocator as the conversation it is committed to.
+ *
+ * Usage: create -> add_* (one per block, in order) -> conversation_add_message.
+ * `panto_conversation_add_message` ALWAYS consumes the builder (it is freed on
+ * both success and failure); do not destroy it afterwards. Call
+ * `panto_message_builder_destroy` only to discard a builder you never commit.
+ * Each `text`/`signature`/etc. argument is copied. */
+typedef struct PantoMessageBuilder PantoMessageBuilder;
+PANTO_EXPORT PantoMessageBuilder *panto_message_builder_create(PantoMessageRole role);
+PANTO_EXPORT void panto_message_builder_destroy(PantoMessageBuilder *builder);
+PANTO_EXPORT PantoStatus panto_message_builder_add_text(PantoMessageBuilder *builder, const char *text);
+/* `signature` may be NULL/empty when the provider emitted no signature, in
+ * which case the `signature_*` origin args are ignored. When a signature is
+ * supplied, pass the origin (style/base_url/model) it was produced under so
+ * the block can be replayed; an Anthropic request drops thinking blocks whose
+ * signature origin does not match the request's base_url/model. */
+PANTO_EXPORT PantoStatus panto_message_builder_add_thinking(PantoMessageBuilder *builder, const char *text, const char *signature, PantoAPIStyle signature_api_style, const char *signature_base_url, const char *signature_model);
+PANTO_EXPORT PantoStatus panto_message_builder_add_tool_use(PantoMessageBuilder *builder, const char *id, const char *name, const char *input);
+PANTO_EXPORT PantoStatus panto_message_builder_add_tool_result(PantoMessageBuilder *builder, const char *tool_use_id, const char *text, bool is_error);
+PANTO_EXPORT PantoStatus panto_message_builder_add_system(PantoMessageBuilder *builder, const char *text, PantoSystemMode mode);
+/* Commit the builder's blocks as one message of the builder's role. Consumes
+ * (frees) the builder. `usage` applies to assistant turns; pass has_usage=false
+ * otherwise. */
+PANTO_EXPORT PantoStatus panto_conversation_add_message(PantoConversation *conversation, PantoMessageBuilder *builder, bool has_usage, PantoUsage usage);
 PANTO_EXPORT PantoStatus panto_session_create_null(PantoSession **out);
 PANTO_EXPORT PantoStatus panto_session_create(PantoSessionStore *store, PantoSessionInfo info, PantoSession **out);
 PANTO_EXPORT void panto_session_destroy(PantoSession *session);