diff --git a/CHANGELOG.md b/CHANGELOG.md
index dd66442..e8f6c09 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -30,6 +30,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- **`reconcile_client_history` now runs foundation's transcript-repair pass before persisting** the client's view. Catches broken chat-completions clients (orphaned `tool_use` without paired `tool_result`, ordering violations, incomplete assistant turns) that would otherwise cause Anthropic to reject the next LLM call with HTTP 400. Mirrors `_runtime.py:_repair_loaded_transcript_if_needed` from the CLI face. Healthy transcripts pass through unchanged with negligible overhead (<10ms diagnostic).
+
 - **`X-Session-Id` header is now recognized as a fallback** for the existing `X-Client-Session-Id` correlation mechanism (PR #71). opencode and other Vercel AI SDK-based clients send `X-Session-Id` by default; amplifier-agent now picks it up automatically, so session-resume + client-authoritative reconciliation works for opencode with zero config. `X-Client-Session-Id` remains authoritative when both headers are present.
 
 - **Workspace name is no longer suffixed with the client session id.** Previously, `X-Client-Session-Id: abc` would route requests into `workspaces/<base>-abc/`. Now the workspace stays at `<base>` and per-client distinction is purely at the session_id level (`workspaces/<base>/sessions/http-abc/`). This keeps workspace-level hook state (context-intelligence, etc.) shared across all sessions of the same server process, where it belongs.
diff --git a/src/amplifier_agent_http/_reconciler.py b/src/amplifier_agent_http/_reconciler.py
index e394987..a757a46 100644
--- a/src/amplifier_agent_http/_reconciler.py
+++ b/src/amplifier_agent_http/_reconciler.py
@@ -14,6 +14,8 @@
 import logging
 from typing import TYPE_CHECKING, Any
 
+from amplifier_foundation.session import diagnose_transcript, repair_transcript
+
 if TYPE_CHECKING:
     from amplifier_agent_lib.session_store import SessionStore
 
@@ -26,16 +28,25 @@ def reconcile_client_history(
     session_id: str,
     store: SessionStore,
 ) -> list[dict[str, Any]]:
-    """Persist the client's view as authoritative and return it for replay.
+    """Repair the client-sent transcript (if broken), then persist as authoritative.
+
+    Step 1 — Layer-1 repair: run foundation's ``diagnose_transcript`` /
+    ``repair_transcript`` against the incoming messages. Catches orphaned
+    ``tool_use`` blocks (no paired ``tool_result``), ordering violations, and
+    incomplete assistant turns that would otherwise cause Anthropic to reject
+    the next LLM call with HTTP 400. Healthy transcripts pass through
+    unchanged.
+
+    Step 2 — Persist the (now repaired) client view to the session store so
+    the kernel's resume path loads from a clean state.
 
-    Chat-completions is client-authoritative: opencode (and any conforming
-    OpenAI-compatible client) sends the full conversation every turn.  Whatever
-    we have stored locally is at best a copy.  On any divergence the client's
-    view wins.
+    Mirrors the CLI face's ``_runtime.py:_repair_loaded_transcript_if_needed``
+    pattern, but for the HTTP wire's client-authoritative model: we trust the
+    client's view of the conversation, while still defending Anthropic's
+    API contract before replay.
 
-    We persist over the stored copy (idempotent on healthy resumes — same
-    content) so the next turn's load is consistent, then return for replay.
-    No divergence detection, no special events, no ceremony.
+    The repair runs every turn but is essentially free on healthy
+    transcripts (Layer 1, pure, <10ms — annotate + diagnose).
 
     Parameters
     ----------
@@ -51,10 +62,36 @@ def reconcile_client_history(
     Returns
     -------
     list[dict]
-        The client's messages, unchanged.  Returned for caller's convenience
-        so the caller doesn't have to re-reference ``client_messages``
+        The client's messages, repaired if broken and stripped of any
+        ``line_num`` annotations.  Returned for caller's convenience so
+        the caller doesn't have to re-reference ``client_messages``
         downstream.
     """
+    if client_messages:
+        # Foundation's diagnose_transcript prefers line_num annotations for
+        # the incomplete-turns fallback path. SessionStore doesn't annotate
+        # them, so add them to shallow copies before diagnosing. repair_transcript's
+        # output strips line_num itself; we strip again here defensively in case
+        # the healthy path is hit (no repair invocation).
+        annotated = [{**m, "line_num": i + 1} for i, m in enumerate(client_messages)]
+        diagnosis = diagnose_transcript(annotated)
+
+        if diagnosis["status"] != "healthy":
+            repaired = repair_transcript(annotated, diagnosis)
+            client_messages = [{k: v for k, v in m.items() if k != "line_num"} for m in repaired]
+            logger.warning(
+                "Client-sent transcript was broken — repaired before reconcile. "
+                "failure_modes=%s orphaned_tool_ids=%s misplaced_tool_ids=%s "
+                "incomplete_turns=%d entries_before=%d entries_after=%d session=%s",
+                diagnosis["failure_modes"],
+                diagnosis["orphaned_tool_ids"],
+                diagnosis["misplaced_tool_ids"],
+                len(diagnosis["incomplete_turns"]),
+                len(annotated),
+                len(client_messages),
+                session_id,
+            )
+
     store.save(
         session_id,
         client_messages,
diff --git a/tests/http/test_reconciler.py b/tests/http/test_reconciler.py
index 9330a26..ac24c24 100644
--- a/tests/http/test_reconciler.py
+++ b/tests/http/test_reconciler.py
@@ -333,3 +333,144 @@ async def _fake_run(**kwargs: Any) -> str:
     assert any(msg.get("content") == "edited message" for msg in transcript if isinstance(msg.get("content"), str)), (
         f"Expected 'edited message' in transcript, got: {transcript}"
     )
+
+
+# ---------------------------------------------------------------------------
+# Unit tests: repair step in reconcile_client_history
+# ---------------------------------------------------------------------------
+
+
+def test_reconciler_repairs_orphaned_tool_use_before_persist(tmp_path: Path) -> None:
+    """Client sends a transcript with an orphaned tool_call (no matching tool result).
+
+    The foundation's diagnose_transcript / repair_transcript operates on the
+    OpenAI wire format: ``tool_calls`` on the assistant message and ``role:
+    "tool"`` response messages with ``tool_call_id``.
+
+    reconcile_client_history runs the foundation repair pass, synthesises a
+    synthetic ``role: "tool"`` result, persists the cleaned version to the
+    store, and returns the repaired list — not the original broken one.
+    """
+    store = SessionStore(tmp_path)
+    sid = "http-repair-orphan"
+
+    broken: list[dict[str, Any]] = [
+        {"role": "user", "content": "do the thing"},
+        {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [
+                {
+                    "id": "call-123",
+                    "type": "function",
+                    "function": {"name": "bash", "arguments": '{"command": "ls"}'},
+                }
+            ],
+        },
+        # missing role:"tool" tool_call_id:"call-123" result — that's the break
+        {"role": "user", "content": "follow up"},
+    ]
+
+    result = reconcile_client_history(
+        client_messages=broken,
+        session_id=sid,
+        store=store,
+    )
+
+    # The returned transcript must no longer have an orphaned tool_call.
+    # After repair the stored and returned transcripts should be consistent.
+    loaded = store.load(sid)
+    assert loaded is not None
+    transcript, metadata = loaded
+    assert metadata == {"last_turn": "client_reconciled"}
+
+    # Returned value must be the repaired (stored) version.
+    assert result == transcript
+
+    # Every tool_call id in the repaired transcript must have a paired
+    # role:"tool" result message.
+    tool_call_ids: set[str] = set()
+    tool_result_ids: set[str] = set()
+    for msg in result:
+        if msg.get("role") == "assistant":
+            for tc in msg.get("tool_calls") or []:
+                tool_call_ids.add(tc["id"])
+        elif msg.get("role") == "tool":
+            tool_result_ids.add(msg.get("tool_call_id", ""))
+    unmatched = tool_call_ids - tool_result_ids
+    assert not unmatched, f"Orphaned tool_call ids remain after repair: {unmatched}"
+
+
+def test_reconciler_healthy_transcript_passes_through_unchanged(tmp_path: Path) -> None:
+    """Client sends a well-formed transcript.
+
+    diagnose_transcript reports healthy — no repair is invoked — and
+    store.save sees a transcript identity-equal to the original input.
+    """
+    store = SessionStore(tmp_path)
+    sid = "http-healthy-passthrough"
+
+    healthy: list[dict[str, Any]] = [
+        {"role": "user", "content": "hello"},
+        {"role": "assistant", "content": "hi there"},
+        {"role": "user", "content": "goodbye"},
+    ]
+
+    result = reconcile_client_history(
+        client_messages=healthy,
+        session_id=sid,
+        store=store,
+    )
+
+    # Healthy transcript: returned value is the same object (identity check).
+    assert result is healthy
+
+    # Persisted view matches input.
+    loaded = store.load(sid)
+    assert loaded is not None
+    transcript, _ = loaded
+    assert transcript == healthy
+
+
+def test_reconciler_logs_warning_with_failure_modes_on_repair(tmp_path: Path, caplog: Any) -> None:
+    """When a repair happens, a warning is logged containing 'failure_modes='
+    and the session_id — operator visibility for production debugging.
+
+    Uses the OpenAI wire format (``tool_calls`` / ``role: "tool"``) which is
+    what foundation's diagnose_transcript understands.
+    """
+    import logging
+
+    store = SessionStore(tmp_path)
+    sid = "http-warn-on-repair"
+
+    broken: list[dict[str, Any]] = [
+        {"role": "user", "content": "do the thing"},
+        {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [
+                {
+                    "id": "call-456",
+                    "type": "function",
+                    "function": {"name": "bash", "arguments": '{"command": "pwd"}'},
+                }
+            ],
+        },
+        # missing role:"tool" tool_call_id:"call-456" result
+        {"role": "user", "content": "next step"},
+    ]
+
+    with caplog.at_level(logging.WARNING, logger="amplifier_agent_http._reconciler"):
+        reconcile_client_history(
+            client_messages=broken,
+            session_id=sid,
+            store=store,
+        )
+
+    # At least one WARNING record must contain the expected fields.
+    warning_texts = [r.getMessage() for r in caplog.records if r.levelno == logging.WARNING]
+    assert warning_texts, "Expected at least one WARNING log entry from the reconciler"
+    combined = " ".join(warning_texts)
+    assert "failure_modes=" in combined, f"'failure_modes=' not found in: {combined}"
+    assert sid in combined, f"session_id '{sid}' not found in warning: {combined}"