diff --git a/SPARC/api.py b/SPARC/api.py
index 1b29d38..ea0674a 100644
--- a/SPARC/api.py
+++ b/SPARC/api.py
@@ -5,8 +5,9 @@ Provides REST API endpoints for analyzing company patent portfolios.
 
 from __future__ import annotations
 
+from collections import deque
 from contextlib import asynccontextmanager
-from datetime import datetime
+from datetime import datetime, timedelta, timezone
 from typing import TYPE_CHECKING, Annotated, List
 
 if TYPE_CHECKING:
@@ -29,9 +30,11 @@ from SPARC.auth import (
     close_db_client,
     create_tokens,
     decode_token,
+    generate_api_key,
     get_current_admin,
     get_current_user,
     get_db_client,
+    hash_api_key,
     init_db_client,
 )
 from SPARC.types import BatchAnalysisResult, CompanyAnalysisResult
@@ -139,6 +142,31 @@ class HealthResponse(BaseModel):
     timestamp: datetime
 
 
+# Historical diff models
+class AnalysisDiffResponse(BaseModel):
+    """Response model for diffing two analysis runs of the same company."""
+
+    company_name: str
+    from_id: int
+    to_id: int
+    from_timestamp: datetime
+    to_timestamp: datetime
+    patent_count_delta: int
+    added_patents: list[str]
+    removed_patents: list[str]
+    changed_fields: dict[str, dict]
+    summary: str
+
+
+class CompanyAnalysisHistoryItem(BaseModel):
+    """A summary item from a company's analysis history."""
+
+    id: int
+    analysis_type: str | None = None
+    model: str | None = None
+    timestamp: datetime
+
+
 # Auth request/response models
 class RegisterRequest(BaseModel):
     """User registration request."""
@@ -248,6 +276,9 @@ app.state.limiter = limiter
 # In-memory rate limit statistics
 _rate_limit_stats: dict[str, dict] = {}
 
+# Time-series log of rejected requests (capped to last 24 h worth of entries).
+_rejected_log: deque[dict] = deque(maxlen=100_000)
+
 
 def _track_rate_limit_request(endpoint: str, ip: str, rejected: bool = False) -> None:
     """Record a request against a rate-limited endpoint."""
@@ -262,6 +293,11 @@ def _track_rate_limit_request(endpoint: str, ip: str, rejected: bool = False) ->
     _rate_limit_stats[key]["total_requests"] += 1
     if rejected:
         _rate_limit_stats[key]["rejected_requests"] += 1
+        _rejected_log.append({
+            "endpoint": endpoint,
+            "ip": ip,
+            "timestamp": datetime.now(timezone.utc).isoformat(),
+        })
     ip_stats = _rate_limit_stats[key].setdefault("by_ip", {})
     if ip not in ip_stats:
         ip_stats[ip] = {"total": 0, "rejected": 0}
@@ -378,6 +414,92 @@ async def get_me(current_user: UserResponse = Depends(get_current_user)):
     return current_user
 
 
+# ============== API Key Endpoints ==============
+
+
+class CreateApiKeyRequest(BaseModel):
+    """Request to create a new API key."""
+
+    label: str | None = Field(default=None, max_length=100, description="Optional label for the key")
+
+
+class ApiKeyResponse(BaseModel):
+    """Response after creating an API key (includes plaintext key)."""
+
+    id: int
+    key: str  # plaintext key, shown only at creation time
+    label: str | None = None
+    created_at: datetime
+
+
+class ApiKeyInfo(BaseModel):
+    """API key metadata (no secret)."""
+
+    id: int
+    label: str | None = None
+    created_at: datetime
+
+
+@app.post("/auth/apikeys", response_model=ApiKeyResponse, tags=["Auth"])
+async def create_api_key_endpoint(
+    body: CreateApiKeyRequest | None = None,
+    current_user: UserResponse = Depends(get_current_user),
+):
+    """Generate a new API key for the authenticated user.
+
+    The plaintext key is returned **only once** in the response.
+    Store it securely; it cannot be retrieved again.
+    """
+    plaintext_key = generate_api_key()
+    key_hash = hash_api_key(plaintext_key)
+
+    db = get_db_client()
+    label = body.label if body else None
+    row = db.create_api_key(
+        user_id=current_user.id,
+        key_hash=key_hash,
+        label=label,
+    )
+
+    return ApiKeyResponse(
+        id=row["id"],
+        key=plaintext_key,
+        label=row["label"],
+        created_at=row["created_at"],
+    )
+
+
+@app.get("/auth/apikeys", response_model=list[ApiKeyInfo], tags=["Auth"])
+async def list_api_keys_endpoint(
+    current_user: UserResponse = Depends(get_current_user),
+):
+    """List active API key IDs and labels for the authenticated user.
+
+    Does **not** return the secret keys.
+    """
+    db = get_db_client()
+    keys = db.list_api_keys(current_user.id)
+    return [ApiKeyInfo(**k) for k in keys]
+
+
+@app.delete("/auth/apikeys/{key_id}", tags=["Auth"])
+async def revoke_api_key_endpoint(
+    key_id: int,
+    current_user: UserResponse = Depends(get_current_user),
+):
+    """Revoke (delete) an API key by its ID.
+
+    The key must belong to the authenticated user.
+    """
+    db = get_db_client()
+    deleted = db.delete_api_key(key_id, current_user.id)
+
+    if not deleted:
+        raise HTTPException(status_code=404, detail="API key not found")
+
+    return {"message": "API key revoked"}
+
+
 # ============== Admin Endpoints ==============
 
 
@@ -507,10 +629,12 @@ async def get_rate_limit_stats(
     """Get rate limit status and usage statistics (admin only).
 
     Returns current rate limit configuration and request statistics
-    for all rate-limited endpoints.
+    for all rate-limited endpoints, including per-IP breakdown and
+    a time-series of throttled (rejected) requests in the last 24 hours.
 
     Returns:
-        List of rate limit stats per endpoint with total/rejected counts
+        Rate limit stats per endpoint, per-IP breakdown, and throttled
+        request history bucketed by hour.
     """
     rate_limits_config = {
         "/auth/register": {"limit": "5/minute"},
@@ -520,14 +644,45 @@ async def get_rate_limit_stats(
     results = []
     for endpoint, conf in rate_limits_config.items():
         stats = _rate_limit_stats.get(endpoint, {})
+        by_ip_raw = stats.get("by_ip", {})
+        by_ip = [
+            {"ip": ip, "total": counts["total"], "rejected": counts["rejected"]}
+            for ip, counts in by_ip_raw.items()
+        ]
         results.append({
             "endpoint": endpoint,
             "limit": conf["limit"],
             "total_requests": stats.get("total_requests", 0),
             "rejected_requests": stats.get("rejected_requests", 0),
+            "by_ip": by_ip,
         })
 
-    return {"rate_limits": results}
+    # Build hourly buckets of throttled requests for the last 24 hours
+    now = datetime.now(timezone.utc)
+    cutoff = now - timedelta(hours=24)
+    hourly_buckets: dict[str, int] = {}
+    throttled_24h = 0
+    for entry in _rejected_log:
+        ts_str = entry["timestamp"]
+        try:
+            ts = datetime.fromisoformat(ts_str)
+        except (ValueError, TypeError):
+            continue
+        if ts >= cutoff:
+            throttled_24h += 1
+            bucket = ts.strftime("%Y-%m-%dT%H:00:00Z")
+            hourly_buckets[bucket] = hourly_buckets.get(bucket, 0) + 1
+
+    throttled_over_time = [
+        {"timestamp": k, "count": v}
+        for k, v in sorted(hourly_buckets.items())
+    ]
+
+    return {
+        "rate_limits": results,
+        "throttled_24h": throttled_24h,
+        "throttled_over_time": throttled_over_time,
+    }
 
 
 @app.get("/admin/alerts", tags=["Admin"])
@@ -927,6 +1082,147 @@ async def analyze_company(
     return _convert_result(result)
 
 
+def _extract_patent_ids(response_text: str) -> set[str]:
+    """Extract patent IDs from an analysis response text.
+
+    Looks for patterns like US-12345678-B2, US12345678B2, etc.
+    """
+    import re
+    pattern = r"US[-\s]?\d{7,8}[-\s]?[A-Z]\d?"
+    return set(re.findall(pattern, response_text or ""))
+
+
+def _compute_analysis_diff(from_rec: dict, to_rec: dict) -> AnalysisDiffResponse:
+    """Compute a structured diff between two analysis records."""
+    from_patents = _extract_patent_ids(from_rec.get("response", "") or "")
+    to_patents = _extract_patent_ids(to_rec.get("response", "") or "")
+
+    added = sorted(to_patents - from_patents)
+    removed = sorted(from_patents - to_patents)
+
+    patent_count_delta = len(to_patents) - len(from_patents)
+
+    changed_fields: dict[str, dict] = {}
+    if from_rec.get("model") != to_rec.get("model"):
+        changed_fields["model"] = {
+            "from": from_rec.get("model"),
+            "to": to_rec.get("model"),
+        }
+    if from_rec.get("analysis_type") != to_rec.get("analysis_type"):
+        changed_fields["analysis_type"] = {
+            "from": from_rec.get("analysis_type"),
+            "to": to_rec.get("analysis_type"),
+        }
+
+    # Build a human-readable summary
+    parts: list[str] = []
+    if added:
+        parts.append(f"{len(added)} new patent(s) appeared")
+    if removed:
+        parts.append(f"{len(removed)} patent(s) no longer referenced")
+    if patent_count_delta > 0:
+        parts.append(f"patent mention count increased by {patent_count_delta}")
+    elif patent_count_delta < 0:
+        parts.append(f"patent mention count decreased by {abs(patent_count_delta)}")
+    if changed_fields:
+        parts.append(f"field(s) changed: {', '.join(changed_fields.keys())}")
+    summary = "; ".join(parts) if parts else "No significant differences detected."
+
+    return AnalysisDiffResponse(
+        company_name=to_rec["company_name"],
+        from_id=from_rec["id"],
+        to_id=to_rec["id"],
+        from_timestamp=from_rec["timestamp"],
+        to_timestamp=to_rec["timestamp"],
+        patent_count_delta=patent_count_delta,
+        added_patents=added,
+        removed_patents=removed,
+        changed_fields=changed_fields,
+        summary=summary,
+    )
+
+
+@app.get(
+    "/analyze/{company_name}/history",
+    response_model=list[CompanyAnalysisHistoryItem],
+    tags=["Analysis"],
+)
+async def list_company_analysis_history(
+    company_name: Annotated[str, Path(min_length=2, max_length=100, pattern=r"^[a-zA-Z0-9][a-zA-Z0-9 \-&.]*$")],
+    limit: int = Query(default=20, ge=1, le=100),
+    _: UserResponse = Depends(get_current_user),
+):
+    """List previous analysis runs for a company.
+
+    Returns a list of analysis records ordered by timestamp descending,
+    useful for selecting which runs to compare via the diff endpoint.
+
+    Args:
+        company_name: Company name to look up
+        limit: Maximum number of results
+
+    Returns:
+        List of analysis history items
+    """
+    db = _get_job_db()
+    rows = db.list_company_analyses(company_name, limit=limit)
+    return [
+        CompanyAnalysisHistoryItem(
+            id=r["id"],
+            analysis_type=r.get("analysis_type"),
+            model=r.get("model"),
+            timestamp=r["timestamp"],
+        )
+        for r in rows
+    ]
+
+
+@app.get(
+    "/analyze/{company_name}/diff",
+    response_model=AnalysisDiffResponse,
+    tags=["Analysis"],
+)
+async def diff_company_analyses(
+    company_name: Annotated[str, Path(min_length=2, max_length=100, pattern=r"^[a-zA-Z0-9][a-zA-Z0-9 \-&.]*$")],
+    from_id: int = Query(..., alias="from", description="Analysis ID of the older run"),
+    to_id: int = Query(..., alias="to", description="Analysis ID of the newer run"),
+    _: UserResponse = Depends(get_current_user),
+):
+    """Compare two analysis runs for the same company.
+
+    Returns a structured diff showing added/removed patents, score delta,
+    and a summary narrative.
+
+    Args:
+        company_name: Company name (must match both analysis records)
+        from_id: ID of the older analysis run
+        to_id: ID of the newer analysis run
+
+    Returns:
+        AnalysisDiffResponse with added/removed/changed fields
+
+    Raises:
+        404: If either analysis ID does not exist or belongs to a different company
+    """
+    db = _get_job_db()
+
+    from_rec = db.get_analysis_by_id(from_id)
+    if not from_rec or (from_rec["company_name"] or "").lower() != company_name.lower():
+        raise HTTPException(
+            status_code=404,
+            detail=f"Analysis ID {from_id} not found for company '{company_name}'",
+        )
+
+    to_rec = db.get_analysis_by_id(to_id)
+    if not to_rec or (to_rec["company_name"] or "").lower() != company_name.lower():
+        raise HTTPException(
+            status_code=404,
+            detail=f"Analysis ID {to_id} not found for company '{company_name}'",
+        )
+
+    return _compute_analysis_diff(from_rec, to_rec)
+
+
 @app.get(
     "/analyze/patent/{patent_id}",
     tags=["Analysis"],
diff --git a/SPARC/auth.py b/SPARC/auth.py
index 890d286..932ae53 100644
--- a/SPARC/auth.py
+++ b/SPARC/auth.py
@@ -1,11 +1,13 @@
-"""JWT authentication utilities for SPARC API."""
+"""JWT and API key authentication utilities for SPARC API."""
 
 import os
+import secrets
 from datetime import datetime, timedelta, timezone
 from typing import Optional
 
+import bcrypt
 import jwt
-from fastapi import Depends, HTTPException, status
+from fastapi import Depends, HTTPException, Request, status
 from fastapi.security import HTTPAuthorizationCredentials, HTTPBearer
 from pydantic import BaseModel
 
@@ -32,7 +34,7 @@ def check_jwt_secret() -> None:
             "Set a secure JWT_SECRET environment variable before running in non-development environments."
         )
 
-security = HTTPBearer()
+security = HTTPBearer(auto_error=False)
 
 
 class TokenPayload(BaseModel):
@@ -178,20 +180,107 @@ def get_db_client() -> DatabaseClient:
     return _db_client
 
 
-async def get_current_user(
-    credentials: HTTPAuthorizationCredentials = Depends(security),
-) -> UserResponse:
-    """Get the current authenticated user from JWT token.
+def generate_api_key() -> str:
+    """Generate a random 32-byte hex API key.
+
+    Returns:
+        64-character hex string
+    """
+    return secrets.token_hex(32)
+
+
+def hash_api_key(key: str) -> str:
+    """Hash an API key using bcrypt.
 
     Args:
-        credentials: Bearer token from request
+        key: Plaintext API key
+
+    Returns:
+        bcrypt hash string
+    """
+    return bcrypt.hashpw(key.encode(), bcrypt.gensalt()).decode()
+
+
+def verify_api_key(key: str, key_hash: str) -> bool:
+    """Verify a plaintext API key against its bcrypt hash.
+
+    Args:
+        key: Plaintext API key
+        key_hash: Stored bcrypt hash
+
+    Returns:
+        True if key matches
+    """
+    return bcrypt.checkpw(key.encode(), key_hash.encode())
+
+
+def _authenticate_via_api_key(api_key: str) -> Optional[UserResponse]:
+    """Look up a user by raw API key.
+
+    Iterates over all stored key hashes (small table) and returns the
+    corresponding user when a match is found.
+
+    Args:
+        api_key: Plaintext API key from X-API-Key header
+
+    Returns:
+        UserResponse if valid key, None otherwise
+    """
+    db = get_db_client()
+    key_rows = db.get_all_api_key_hashes()
+
+    for row in key_rows:
+        if verify_api_key(api_key, row["key_hash"]):
+            user = db.get_user_by_id(row["user_id"])
+            if user:
+                return UserResponse(
+                    id=user["id"],
+                    email=user["email"],
+                    role=user["role"],
+                    created_at=user["created_at"],
+                )
+    return None
+
+
+async def get_current_user(
+    request: Request,
+    credentials: Optional[HTTPAuthorizationCredentials] = Depends(security),
+) -> UserResponse:
+    """Get the current authenticated user from JWT token or API key.
+
+    Supports two authentication methods:
+    1. Bearer JWT token via Authorization header
+    2. API key via X-API-Key header
+
+    Args:
+        request: The incoming request (used for X-API-Key header)
+        credentials: Optional Bearer token from request
 
     Returns:
         UserResponse with user details
 
     Raises:
-        HTTPException: If token is invalid or expired
+        HTTPException: If no valid credentials are provided
     """
+    # Try X-API-Key header first
+    api_key = request.headers.get("X-API-Key")
+    if api_key:
+        user = _authenticate_via_api_key(api_key)
+        if user:
+            return user
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="Invalid API key",
+        )
+
+    # Fall back to JWT Bearer token
+    if not credentials:
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="Missing authentication credentials",
+            headers={"WWW-Authenticate": "Bearer"},
+        )
+
     token = credentials.credentials
     payload = decode_token(token)
 
diff --git a/SPARC/database.py b/SPARC/database.py
index 0759a66..45e87e6 100644
--- a/SPARC/database.py
+++ b/SPARC/database.py
@@ -221,6 +221,27 @@ class DatabaseClient:
                 ON alerts(company_name)
             """)
 
+            # Create API keys table for programmatic access
+            cursor.execute("""
+                CREATE TABLE IF NOT EXISTS api_keys (
+                    id SERIAL PRIMARY KEY,
+                    user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+                    key_hash VARCHAR(255) NOT NULL,
+                    label VARCHAR(100),
+                    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+                )
+            """)
+
+            cursor.execute("""
+                CREATE INDEX IF NOT EXISTS idx_api_keys_user_id
+                ON api_keys(user_id)
+            """)
+
+            cursor.execute("""
+                CREATE INDEX IF NOT EXISTS idx_api_keys_key_hash
+                ON api_keys(key_hash)
+            """)
+
             self.conn.commit()
 
     @staticmethod
@@ -977,3 +998,138 @@ class DatabaseClient:
                     (limit,),
                 )
                 return [dict(row) for row in cursor.fetchall()]
+
+    # Historical Analysis Diff Methods
+
+    def get_analysis_by_id(self, analysis_id: int) -> Optional[Dict]:
+        """Get a single analysis record by its ID.
+
+        Args:
+            analysis_id: The primary key of the llm_messages row.
+
+        Returns:
+            Dict with analysis fields, or None if not found.
+        """
+        with self.get_conn() as conn:
+            with conn.cursor(cursor_factory=RealDictCursor) as cursor:
+                cursor.execute(
+                    """
+                    SELECT id, company_name, analysis_type, model, response, timestamp
+                    FROM llm_messages
+                    WHERE id = %s AND is_cached = FALSE
+                    """,
+                    (analysis_id,),
+                )
+                row = cursor.fetchone()
+                return dict(row) if row else None
+
+    def list_company_analyses(
+        self, company_name: str, limit: int = 20
+    ) -> List[Dict]:
+        """List past analysis runs for a given company.
+
+        Returns records ordered by timestamp descending so callers can
+        identify which previous runs are available for diffing.
+
+        Args:
+            company_name: Company name (case-insensitive match).
+            limit: Maximum number of records.
+
+        Returns:
+            List of analysis dicts.
+        """
+        with self.get_conn() as conn:
+            with conn.cursor(cursor_factory=RealDictCursor) as cursor:
+                cursor.execute(
+                    """
+                    SELECT id, company_name, analysis_type, model, response, timestamp
+                    FROM llm_messages
+                    WHERE LOWER(company_name) = LOWER(%s) AND is_cached = FALSE
+                    ORDER BY timestamp DESC
+                    LIMIT %s
+                    """,
+                    (company_name, limit),
+                )
+                return [dict(row) for row in cursor.fetchall()]
+
+    # API Key Methods
+
+    def create_api_key(
+        self,
+        user_id: int,
+        key_hash: str,
+        label: Optional[str] = None,
+    ) -> Dict:
+        """Store a new API key hash for a user.
+
+        Args:
+            user_id: The owning user's ID
+            key_hash: bcrypt hash of the plaintext key
+            label: Optional human-readable label
+
+        Returns:
+            Dict with id, user_id, label, created_at
+        """
+        with self.get_conn() as conn:
+            with conn.cursor(cursor_factory=RealDictCursor) as cursor:
+                cursor.execute(
+                    """
+                    INSERT INTO api_keys (user_id, key_hash, label)
+                    VALUES (%s, %s, %s)
+                    RETURNING id, user_id, label, created_at
+                    """,
+                    (user_id, key_hash, label),
+                )
+                row = cursor.fetchone()
+            conn.commit()
+            return dict(row)
+
+    def list_api_keys(self, user_id: int) -> List[Dict]:
+        """List active API key metadata for a user (no secrets).
+
+        Args:
+            user_id: The user's ID
+
+        Returns:
+            List of dicts with id, label, created_at
+        """
+        with self.get_conn() as conn:
+            with conn.cursor(cursor_factory=RealDictCursor) as cursor:
+                cursor.execute(
+                    "SELECT id, label, created_at FROM api_keys WHERE user_id = %s ORDER BY created_at DESC",
+                    (user_id,),
+                )
+                return [dict(row) for row in cursor.fetchall()]
+
+    def delete_api_key(self, key_id: int, user_id: int) -> bool:
+        """Revoke an API key by ID (must belong to user).
+
+        Args:
+            key_id: The API key row ID
+            user_id: The owning user's ID
+
+        Returns:
+            True if a key was deleted
+        """
+        with self.get_conn() as conn:
+            with conn.cursor() as cursor:
+                cursor.execute(
+                    "DELETE FROM api_keys WHERE id = %s AND user_id = %s",
+                    (key_id, user_id),
+                )
+                deleted = cursor.rowcount > 0
+            conn.commit()
+            return deleted
+
+    def get_all_api_key_hashes(self) -> List[Dict]:
+        """Return all API key hashes with their associated user IDs.
+
+        Used by the auth layer to validate an incoming API key.
+
+        Returns:
+            List of dicts with key_hash, user_id
+        """
+        with self.get_conn() as conn:
+            with conn.cursor(cursor_factory=RealDictCursor) as cursor:
+                cursor.execute("SELECT key_hash, user_id FROM api_keys")
+                return [dict(row) for row in cursor.fetchall()]
diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index d7ec5ba..d2fcc54 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -11,7 +11,9 @@ import { Batch } from './pages/Batch';
 import { AnalyticsPage } from './pages/Analytics';
 import { About } from './pages/About';
 import { AdminUsers } from './pages/AdminUsers';
+import { AdminRateLimits } from './pages/AdminRateLimits';
 import { Compare } from './pages/Compare';
+import { HistoryDiff } from './pages/HistoryDiff';
 
 const queryClient = new QueryClient({
   defaultOptions: {
@@ -45,6 +47,7 @@ function App() {
               <Route path="/batch" element={<Batch />} />
               <Route path="/analytics" element={<AnalyticsPage />} />
               <Route path="/compare" element={<Compare />} />
+              <Route path="/history-diff" element={<HistoryDiff />} />
               <Route path="/about" element={<About />} />
 
               {/* Admin routes */}
@@ -56,6 +59,14 @@ function App() {
                   </ProtectedRoute>
                 }
               />
+              <Route
+                path="/admin/rate-limits"
+                element={
+                  <ProtectedRoute requireAdmin>
+                    <AdminRateLimits />
+                  </ProtectedRoute>
+                }
+              />
             </Route>
 
             {/* Default redirect */}
diff --git a/frontend/src/api/client.ts b/frontend/src/api/client.ts
index 09a4ae6..b29a736 100644
--- a/frontend/src/api/client.ts
+++ b/frontend/src/api/client.ts
@@ -148,8 +148,43 @@ export const analysisApi = {
     const response = await api.get<JobStatus[]>(`/jobs?${params}`);
     return response.data;
   },
+
+  getCompanyHistory: async (companyName: string, limit = 20): Promise<AnalysisHistoryItem[]> => {
+    const response = await api.get<AnalysisHistoryItem[]>(
+      `/analyze/${encodeURIComponent(companyName)}/history?limit=${limit}`
+    );
+    return response.data;
+  },
+
+  diffAnalyses: async (companyName: string, fromId: number, toId: number): Promise<AnalysisDiff> => {
+    const response = await api.get<AnalysisDiff>(
+      `/analyze/${encodeURIComponent(companyName)}/diff?from=${fromId}&to=${toId}`
+    );
+    return response.data;
+  },
 };
 
+// Analysis diff types
+export interface AnalysisHistoryItem {
+  id: number;
+  analysis_type: string | null;
+  model: string | null;
+  timestamp: string;
+}
+
+export interface AnalysisDiff {
+  company_name: string;
+  from_id: number;
+  to_id: number;
+  from_timestamp: string;
+  to_timestamp: string;
+  patent_count_delta: number;
+  added_patents: string[];
+  removed_patents: string[];
+  changed_fields: Record<string, { from: string | null; to: string | null }>;
+  summary: string;
+}
+
 // Export API
 export const exportApi = {
   exportCsv: async (companyName: string): Promise<void> => {
@@ -201,6 +236,32 @@ export const analyticsApi = {
   },
 };
 
+// Rate limit types
+export interface RateLimitIpEntry {
+  ip: string;
+  total: number;
+  rejected: number;
+}
+
+export interface RateLimitEndpointStats {
+  endpoint: string;
+  limit: string;
+  total_requests: number;
+  rejected_requests: number;
+  by_ip: RateLimitIpEntry[];
+}
+
+export interface ThrottledBucket {
+  timestamp: string;
+  count: number;
+}
+
+export interface RateLimitStatsResponse {
+  rate_limits: RateLimitEndpointStats[];
+  throttled_24h: number;
+  throttled_over_time: ThrottledBucket[];
+}
+
 // Admin API
 export const adminApi = {
   listUsers: async (limit = 100, offset = 0): Promise<User[]> => {
@@ -216,6 +277,11 @@ export const adminApi = {
   deleteUser: async (userId: number): Promise<void> => {
     await api.delete(`/admin/users/${userId}`);
   },
+
+  getRateLimits: async (): Promise<RateLimitStatsResponse> => {
+    const response = await api.get<RateLimitStatsResponse>('/admin/rate-limits');
+    return response.data;
+  },
 };
 
 export default api;
diff --git a/frontend/src/components/Layout.tsx b/frontend/src/components/Layout.tsx
index d0df715..d4b11b7 100644
--- a/frontend/src/components/Layout.tsx
+++ b/frontend/src/components/Layout.tsx
@@ -1,7 +1,7 @@
 import { Outlet, NavLink, useNavigate } from 'react-router-dom';
 import { useAuth } from '../context/AuthContext';
 import { useTheme } from '../context/ThemeContext';
-import { Search, Layers, BarChart3, Info, Users, LogOut, GitCompareArrows, Sun, Moon } from 'lucide-react';
+import { Search, Layers, BarChart3, Info, Users, LogOut, GitCompareArrows, Sun, Moon, History, ShieldAlert } from 'lucide-react';
 
 export function Layout() {
   const { user, isAdmin, logout } = useAuth();
@@ -18,11 +18,13 @@ export function Layout() {
     { to: '/batch', icon: Layers, label: 'Batch' },
     { to: '/analytics', icon: BarChart3, label: 'Analytics' },
     { to: '/compare', icon: GitCompareArrows, label: 'Compare' },
+    { to: '/history-diff', icon: History, label: 'Diff' },
     { to: '/about', icon: Info, label: 'About' },
   ];
 
   if (isAdmin) {
     navItems.push({ to: '/admin/users', icon: Users, label: 'Users' });
+    navItems.push({ to: '/admin/rate-limits', icon: ShieldAlert, label: 'Rate Limits' });
   }
 
   return (
diff --git a/frontend/src/pages/AdminRateLimits.tsx b/frontend/src/pages/AdminRateLimits.tsx
new file mode 100644
index 0000000..97b41c4
--- /dev/null
+++ b/frontend/src/pages/AdminRateLimits.tsx
@@ -0,0 +1,240 @@
+import { useState } from 'react';
+import { useQuery } from '@tanstack/react-query';
+import { adminApi } from '../api/client';
+import type { RateLimitStatsResponse } from '../api/client';
+import { ShieldAlert, Activity, AlertCircle, RefreshCw, Clock } from 'lucide-react';
+
+const REFRESH_OPTIONS = [
+  { label: '15s', value: 15_000 },
+  { label: '30s', value: 30_000 },
+  { label: '1m', value: 60_000 },
+  { label: 'Off', value: 0 },
+];
+
+export function AdminRateLimits() {
+  const [refreshInterval, setRefreshInterval] = useState(30_000);
+
+  const { data, isLoading, isError, dataUpdatedAt } = useQuery<RateLimitStatsResponse>({
+    queryKey: ['admin-rate-limits'],
+    queryFn: () => adminApi.getRateLimits(),
+    refetchInterval: refreshInterval || false,
+  });
+
+  if (isLoading) {
+    return (
+      <div className="flex items-center justify-center min-h-[400px]">
+        <div className="animate-spin rounded-full h-12 w-12 border-t-2 border-b-2 border-primary"></div>
+      </div>
+    );
+  }
+
+  if (isError) {
+    return (
+      <div className="flex items-center gap-2 bg-error/10 border border-error/20 text-error rounded-xl px-4 py-3">
+        <AlertCircle size={18} />
+        <span>Failed to load rate limit statistics.</span>
+      </div>
+    );
+  }
+
+  const maxThrottledCount = data?.throttled_over_time?.length
+    ? Math.max(...data.throttled_over_time.map((b) => b.count))
+    : 0;
+
+  return (
+    <div className="space-y-6">
+      {/* Header */}
+      <div className="flex items-center justify-between flex-wrap gap-4">
+        <div>
+          <h2 className="text-xl font-semibold text-text-primary border-b-2 border-primary/30 pb-2 mb-2">
+            Rate Limiting Dashboard
+          </h2>
+          <p className="text-text-secondary">Monitor API rate limits and throttled requests.</p>
+        </div>
+        <div className="flex items-center gap-3">
+          {/* Last updated */}
+          {dataUpdatedAt > 0 && (
+            <span className="text-xs text-text-secondary flex items-center gap-1">
+              <Clock size={12} />
+              Updated {new Date(dataUpdatedAt).toLocaleTimeString()}
+            </span>
+          )}
+          {/* Refresh interval selector */}
+          <div className="flex items-center gap-1 bg-bg-card/60 border border-primary/15 rounded-xl p-1">
+            <RefreshCw size={14} className="text-text-secondary ml-2" />
+            {REFRESH_OPTIONS.map((opt) => (
+              <button
+                key={opt.value}
+                onClick={() => setRefreshInterval(opt.value)}
+                className={`px-3 py-1 rounded-lg text-xs font-medium transition-all ${
+                  refreshInterval === opt.value
+                    ? 'bg-primary text-white'
+                    : 'text-text-secondary hover:text-text-primary hover:bg-bg-card-hover'
+                }`}
+              >
+                {opt.label}
+              </button>
+            ))}
+          </div>
+        </div>
+      </div>
+
+      {/* Summary cards */}
+      <div className="grid grid-cols-1 md:grid-cols-3 gap-4">
+        <div className="bg-bg-card/60 border border-primary/15 rounded-2xl p-5">
+          <div className="flex items-center gap-2 mb-2">
+            <Activity size={18} className="text-primary" />
+            <span className="text-sm font-semibold text-text-secondary uppercase tracking-wider">
+              Total Requests
+            </span>
+          </div>
+          <div className="text-3xl font-bold text-text-primary">
+            {data?.rate_limits.reduce((sum, rl) => sum + rl.total_requests, 0) ?? 0}
+          </div>
+        </div>
+        <div className="bg-bg-card/60 border border-error/15 rounded-2xl p-5">
+          <div className="flex items-center gap-2 mb-2">
+            <ShieldAlert size={18} className="text-error" />
+            <span className="text-sm font-semibold text-text-secondary uppercase tracking-wider">
+              Throttled (24h)
+            </span>
+          </div>
+          <div className="text-3xl font-bold text-error">
+            {data?.throttled_24h ?? 0}
+          </div>
+        </div>
+        <div className="bg-bg-card/60 border border-secondary/15 rounded-2xl p-5">
+          <div className="flex items-center gap-2 mb-2">
+            <ShieldAlert size={18} className="text-secondary" />
+            <span className="text-sm font-semibold text-text-secondary uppercase tracking-wider">
+              Rate-Limited Endpoints
+            </span>
+          </div>
+          <div className="text-3xl font-bold text-text-primary">
+            {data?.rate_limits.length ?? 0}
+          </div>
+        </div>
+      </div>
+
+      {/* Throttled over time chart (simple bar chart) */}
+      {data?.throttled_over_time && data.throttled_over_time.length > 0 && (
+        <div className="bg-bg-card/60 border border-primary/15 rounded-2xl p-5">
+          <h3 className="text-sm font-semibold text-text-secondary uppercase tracking-wider mb-4">
+            Throttled Requests Over Time (Last 24h)
+          </h3>
+          <div className="flex items-end gap-1 h-32">
+            {data.throttled_over_time.map((bucket) => {
+              const height = maxThrottledCount > 0 ? (bucket.count / maxThrottledCount) * 100 : 0;
+              const hour = new Date(bucket.timestamp).getHours();
+              return (
+                <div key={bucket.timestamp} className="flex-1 flex flex-col items-center gap-1">
+                  <span className="text-xs text-text-secondary">{bucket.count}</span>
+                  <div
+                    className="w-full bg-error/70 rounded-t-sm min-h-[2px] transition-all"
+                    style={{ height: `${Math.max(height, 2)}%` }}
+                    title={`${bucket.timestamp}: ${bucket.count} throttled`}
+                  />
+                  <span className="text-[10px] text-text-secondary">{hour}:00</span>
+                </div>
+              );
+            })}
+          </div>
+        </div>
+      )}
+
+      {/* Per-endpoint table */}
+      <div className="bg-bg-card/60 border border-primary/15 rounded-2xl overflow-hidden">
+        <div className="overflow-x-auto">
+          <table className="w-full">
+            <thead>
+              <tr className="border-b border-primary/10">
+                <th className="text-left px-6 py-4 text-sm font-semibold text-text-secondary uppercase tracking-wider">
+                  Endpoint
+                </th>
+                <th className="text-left px-6 py-4 text-sm font-semibold text-text-secondary uppercase tracking-wider">
+                  Limit
+                </th>
+                <th className="text-right px-6 py-4 text-sm font-semibold text-text-secondary uppercase tracking-wider">
+                  Total Requests
+                </th>
+                <th className="text-right px-6 py-4 text-sm font-semibold text-text-secondary uppercase tracking-wider">
+                  Rejected
+                </th>
+              </tr>
+            </thead>
+            <tbody className="divide-y divide-primary/10">
+              {data?.rate_limits.map((rl) => (
+                <tr key={rl.endpoint} className="hover:bg-bg-card-hover/50 transition-colors">
+                  <td className="px-6 py-4 font-mono text-sm text-text-primary">{rl.endpoint}</td>
+                  <td className="px-6 py-4">
+                    <span className="inline-flex px-2 py-0.5 rounded-full text-xs font-medium bg-primary/10 text-primary border border-primary/20">
+                      {rl.limit}
+                    </span>
+                  </td>
+                  <td className="px-6 py-4 text-right text-text-primary font-semibold">
+                    {rl.total_requests}
+                  </td>
+                  <td className="px-6 py-4 text-right">
+                    <span className={rl.rejected_requests > 0 ? 'text-error font-semibold' : 'text-text-secondary'}>
+                      {rl.rejected_requests}
+                    </span>
+                  </td>
+                </tr>
+              ))}
+            </tbody>
+          </table>
+        </div>
+      </div>
+
+      {/* Per-IP breakdown */}
+      {data?.rate_limits.some((rl) => rl.by_ip.length > 0) && (
+        <div className="bg-bg-card/60 border border-primary/15 rounded-2xl overflow-hidden">
+          <div className="px-6 py-4 border-b border-primary/10">
+            <h3 className="text-sm font-semibold text-text-secondary uppercase tracking-wider">
+              Per-IP Breakdown
+            </h3>
+          </div>
+          <div className="overflow-x-auto">
+            <table className="w-full">
+              <thead>
+                <tr className="border-b border-primary/10">
+                  <th className="text-left px-6 py-3 text-sm font-semibold text-text-secondary uppercase tracking-wider">
+                    Endpoint
+                  </th>
+                  <th className="text-left px-6 py-3 text-sm font-semibold text-text-secondary uppercase tracking-wider">
+                    IP Address
+                  </th>
+                  <th className="text-right px-6 py-3 text-sm font-semibold text-text-secondary uppercase tracking-wider">
+                    Total
+                  </th>
+                  <th className="text-right px-6 py-3 text-sm font-semibold text-text-secondary uppercase tracking-wider">
+                    Rejected
+                  </th>
+                </tr>
+              </thead>
+              <tbody className="divide-y divide-primary/10">
+                {data.rate_limits.flatMap((rl) =>
+                  rl.by_ip.map((ipEntry) => (
+                    <tr
+                      key={`${rl.endpoint}-${ipEntry.ip}`}
+                      className="hover:bg-bg-card-hover/50 transition-colors"
+                    >
+                      <td className="px-6 py-3 font-mono text-sm text-text-primary">{rl.endpoint}</td>
+                      <td className="px-6 py-3 font-mono text-sm text-text-secondary">{ipEntry.ip}</td>
+                      <td className="px-6 py-3 text-right text-text-primary">{ipEntry.total}</td>
+                      <td className="px-6 py-3 text-right">
+                        <span className={ipEntry.rejected > 0 ? 'text-error font-semibold' : 'text-text-secondary'}>
+                          {ipEntry.rejected}
+                        </span>
+                      </td>
+                    </tr>
+                  ))
+                )}
+              </tbody>
+            </table>
+          </div>
+        </div>
+      )}
+    </div>
+  );
+}
diff --git a/frontend/src/pages/Analysis.tsx b/frontend/src/pages/Analysis.tsx
index 2f4fc35..bd6c2ea 100644
--- a/frontend/src/pages/Analysis.tsx
+++ b/frontend/src/pages/Analysis.tsx
@@ -1,10 +1,12 @@
 import { useState } from 'react';
+import { useNavigate } from 'react-router-dom';
 import { useMutation, useQuery } from '@tanstack/react-query';
 import { analysisApi, exportApi } from '../api/client';
-import { Search, CheckCircle, AlertCircle, Clock, FileText, Download, ChevronDown } from 'lucide-react';
+import { Search, CheckCircle, AlertCircle, Clock, FileText, Download, ChevronDown, History } from 'lucide-react';
 import type { CompanyAnalysis } from '../types';
 
 export function Analysis() {
+  const navigate = useNavigate();
   const [companyName, setCompanyName] = useState('');
   const [selectedModel, setSelectedModel] = useState('');
   const [result, setResult] = useState<CompanyAnalysis | null>(null);
@@ -157,6 +159,13 @@ export function Analysis() {
                     <FileText size={14} />
                     Export PDF
                   </button>
+                  <button
+                    onClick={() => navigate(`/history-diff?company=${encodeURIComponent(result.company_name)}`)}
+                    className="flex items-center gap-2 text-sm bg-secondary/20 hover:bg-secondary/30 text-secondary font-medium px-3 py-1.5 rounded-lg transition-colors"
+                  >
+                    <History size={14} />
+                    Compare with previous
+                  </button>
                 </div>
               </div>
               <div className="prose dark:prose-invert max-w-none">
diff --git a/frontend/src/pages/HistoryDiff.tsx b/frontend/src/pages/HistoryDiff.tsx
new file mode 100644
index 0000000..9019d1d
--- /dev/null
+++ b/frontend/src/pages/HistoryDiff.tsx
@@ -0,0 +1,249 @@
+import { useState } from 'react';
+import { useSearchParams } from 'react-router-dom';
+import { useQuery } from '@tanstack/react-query';
+import { analysisApi } from '../api/client';
+import type { AnalysisHistoryItem, AnalysisDiff } from '../api/client';
+import { History, ArrowRight, Plus, Minus, AlertCircle, Search } from 'lucide-react';
+
+export function HistoryDiff() {
+  const [searchParams, setSearchParams] = useSearchParams();
+  const [companyInput, setCompanyInput] = useState(searchParams.get('company') || '');
+
+  const company = searchParams.get('company') || '';
+  const fromId = searchParams.get('from');
+  const toId = searchParams.get('to');
+
+  // Fetch history when a company is selected
+  const historyQuery = useQuery({
+    queryKey: ['history', company],
+    queryFn: () => analysisApi.getCompanyHistory(company),
+    enabled: !!company,
+  });
+
+  // Fetch diff when both IDs are selected
+  const diffQuery = useQuery<AnalysisDiff>({
+    queryKey: ['diff', company, fromId, toId],
+    queryFn: () => analysisApi.diffAnalyses(company, Number(fromId), Number(toId)),
+    enabled: !!company && !!fromId && !!toId,
+  });
+
+  const handleSearch = (e: React.FormEvent) => {
+    e.preventDefault();
+    const name = companyInput.trim();
+    if (name) {
+      setSearchParams({ company: name });
+    }
+  };
+
+  const handleSelectRuns = (from: number, to: number) => {
+    setSearchParams({ company, from: String(from), to: String(to) });
+  };
+
+  const history: AnalysisHistoryItem[] = historyQuery.data || [];
+
+  return (
+    <div className="space-y-6">
+      {/* Header */}
+      <div>
+        <h2 className="text-xl font-semibold text-text-primary border-b-2 border-primary/30 pb-2 mb-2">
+          Historical Analysis Diff
+        </h2>
+        <p className="text-text-secondary">
+          Compare analysis runs for the same company to see what changed between them.
+        </p>
+      </div>
+
+      {/* Company Search */}
+      <form onSubmit={handleSearch} className="flex gap-4">
+        <div className="flex-1 relative">
+          <Search className="absolute left-4 top-1/2 -translate-y-1/2 text-text-secondary" size={18} />
+          <input
+            type="text"
+            value={companyInput}
+            onChange={(e) => setCompanyInput(e.target.value)}
+            placeholder="Enter company name (e.g., nvidia)"
+            className="w-full bg-bg-card/80 border border-primary/30 rounded-xl pl-12 pr-4 py-3 text-text-primary placeholder-text-secondary/50 focus:outline-none focus:border-primary focus:ring-2 focus:ring-primary/20 transition-all"
+          />
+        </div>
+        <button
+          type="submit"
+          disabled={!companyInput.trim()}
+          className="bg-gradient-to-r from-primary to-primary-dark text-white font-semibold py-3 px-6 rounded-xl hover:shadow-lg hover:shadow-primary/30 transition-all disabled:opacity-50 disabled:cursor-not-allowed flex items-center gap-2"
+        >
+          <History size={18} />
+          Load History
+        </button>
+      </form>
+
+      {/* History list */}
+      {company && historyQuery.isLoading && (
+        <div className="text-text-secondary animate-pulse">Loading analysis history...</div>
+      )}
+
+      {company && historyQuery.isError && (
+        <div className="flex items-center gap-2 bg-error/10 border border-error/20 text-error rounded-xl px-4 py-3">
+          <AlertCircle size={18} />
+          <span>Failed to load history. Check the company name and try again.</span>
+        </div>
+      )}
+
+      {company && history.length === 0 && !historyQuery.isLoading && (
+        <div className="text-text-secondary">No analysis history found for "{company}".</div>
+      )}
+
+      {history.length >= 2 && (
+        <div className="bg-bg-card/60 backdrop-blur-lg border border-primary/15 rounded-2xl p-6">
+          <h3 className="text-lg font-semibold text-text-primary border-b-2 border-primary/30 pb-2 mb-4">
+            Select Two Runs to Compare
+          </h3>
+          <div className="space-y-2">
+            {history.map((item, idx) => {
+              const next = history[idx + 1];
+              if (!next) return null;
+              const isSelected =
+                fromId === String(next.id) && toId === String(item.id);
+              return (
+                <button
+                  key={item.id}
+                  onClick={() => handleSelectRuns(next.id, item.id)}
+                  className={`w-full text-left flex items-center gap-3 px-4 py-3 rounded-xl border transition-all ${
+                    isSelected
+                      ? 'border-primary bg-primary/10'
+                      : 'border-primary/15 hover:border-primary/40 hover:bg-primary/5'
+                  }`}
+                >
+                  <span className="text-sm text-text-secondary font-mono">
+                    #{next.id}
+                  </span>
+                  <span className="text-xs text-text-secondary">
+                    {new Date(next.timestamp).toLocaleString()}
+                  </span>
+                  <ArrowRight size={14} className="text-primary" />
+                  <span className="text-sm text-text-secondary font-mono">
+                    #{item.id}
+                  </span>
+                  <span className="text-xs text-text-secondary">
+                    {new Date(item.timestamp).toLocaleString()}
+                  </span>
+                  {item.model && (
+                    <span className="ml-auto text-xs bg-primary/20 text-primary px-2 py-0.5 rounded">
+                      {item.model}
+                    </span>
+                  )}
+                </button>
+              );
+            })}
+          </div>
+        </div>
+      )}
+
+      {/* Diff Results */}
+      {diffQuery.isLoading && (
+        <div className="text-text-secondary animate-pulse">Computing diff...</div>
+      )}
+
+      {diffQuery.isError && (
+        <div className="flex items-center gap-2 bg-error/10 border border-error/20 text-error rounded-xl px-4 py-3">
+          <AlertCircle size={18} />
+          <span>Failed to compute diff. One or both analysis IDs may not exist.</span>
+        </div>
+      )}
+
+      {diffQuery.data && <DiffView diff={diffQuery.data} />}
+    </div>
+  );
+}
+
+function DiffView({ diff }: { diff: AnalysisDiff }) {
+  return (
+    <div className="bg-bg-card/60 backdrop-blur-lg border border-primary/15 rounded-2xl p-6 space-y-6">
+      <h3 className="text-lg font-semibold text-text-primary border-b-2 border-primary/30 pb-2">
+        Diff: #{diff.from_id} &rarr; #{diff.to_id}
+      </h3>
+
+      {/* Summary */}
+      <div className="bg-primary/5 border border-primary/20 rounded-xl p-4">
+        <div className="text-sm font-medium text-text-primary">{diff.summary}</div>
+        <div className="flex items-center gap-4 mt-2 text-xs text-text-secondary">
+          <span>{new Date(diff.from_timestamp).toLocaleString()}</span>
+          <ArrowRight size={12} />
+          <span>{new Date(diff.to_timestamp).toLocaleString()}</span>
+        </div>
+      </div>
+
+      {/* Patent count delta */}
+      <div className="flex items-center gap-3">
+        <span className="text-sm text-text-secondary">Patent mention delta:</span>
+        <span
+          className={`text-lg font-bold ${
+            diff.patent_count_delta > 0
+              ? 'text-success'
+              : diff.patent_count_delta < 0
+              ? 'text-error'
+              : 'text-text-secondary'
+          }`}
+        >
+          {diff.patent_count_delta > 0 ? '+' : ''}
+          {diff.patent_count_delta}
+        </span>
+      </div>
+
+      {/* Added patents */}
+      {diff.added_patents.length > 0 && (
+        <div>
+          <h4 className="text-sm font-semibold text-success flex items-center gap-1 mb-2">
+            <Plus size={14} />
+            New Patents ({diff.added_patents.length})
+          </h4>
+          <div className="flex flex-wrap gap-2">
+            {diff.added_patents.map((p) => (
+              <span
+                key={p}
+                className="text-xs bg-success/10 border border-success/20 text-success px-2 py-1 rounded font-mono"
+              >
+                {p}
+              </span>
+            ))}
+          </div>
+        </div>
+      )}
+
+      {/* Removed patents */}
+      {diff.removed_patents.length > 0 && (
+        <div>
+          <h4 className="text-sm font-semibold text-error flex items-center gap-1 mb-2">
+            <Minus size={14} />
+            Removed Patents ({diff.removed_patents.length})
+          </h4>
+          <div className="flex flex-wrap gap-2">
+            {diff.removed_patents.map((p) => (
+              <span
+                key={p}
+                className="text-xs bg-error/10 border border-error/20 text-error px-2 py-1 rounded font-mono"
+              >
+                {p}
+              </span>
+            ))}
+          </div>
+        </div>
+      )}
+
+      {/* Changed fields */}
+      {Object.keys(diff.changed_fields).length > 0 && (
+        <div>
+          <h4 className="text-sm font-semibold text-text-primary mb-2">Changed Fields</h4>
+          <div className="space-y-1">
+            {Object.entries(diff.changed_fields).map(([field, vals]) => (
+              <div key={field} className="flex items-center gap-2 text-sm">
+                <span className="text-text-secondary font-mono">{field}:</span>
+                <span className="text-error line-through">{vals.from || 'null'}</span>
+                <ArrowRight size={12} className="text-text-secondary" />
+                <span className="text-success">{vals.to || 'null'}</span>
+              </div>
+            ))}
+          </div>
+        </div>
+      )}
+    </div>
+  );
+}
diff --git a/tests/test_analysis_diff.py b/tests/test_analysis_diff.py
new file mode 100644
index 0000000..c867b55
--- /dev/null
+++ b/tests/test_analysis_diff.py
@@ -0,0 +1,244 @@
+"""Tests for historical analysis diff endpoint."""
+
+from datetime import datetime
+from unittest.mock import MagicMock, patch
+
+import pytest
+from fastapi.testclient import TestClient
+
+from SPARC.api import AnalysisDiffResponse, _compute_analysis_diff, _extract_patent_ids, app
+from SPARC.auth import UserResponse, get_current_user
+
+
+# ---------- helpers ----------
+
+def _mock_user():
+    """Return a fake authenticated user for dependency override."""
+    return UserResponse(
+        id=1,
+        email="test@example.com",
+        role="user",
+        created_at=datetime(2025, 1, 1),
+    )
+
+
+@pytest.fixture
+def auth_client():
+    """TestClient with auth dependency overridden."""
+    app.dependency_overrides[get_current_user] = _mock_user
+    client = TestClient(app, raise_server_exceptions=False)
+    yield client
+    app.dependency_overrides.clear()
+
+
+# ---------- unit tests for helpers ----------
+
+class TestExtractPatentIds:
+    """Test _extract_patent_ids utility."""
+
+    def test_extracts_standard_ids(self):
+        text = "Patent US-12345678-B2 covers the device. Also see US-9876543-A1."
+        ids = _extract_patent_ids(text)
+        assert "US-12345678-B2" in ids
+        assert "US-9876543-A1" in ids
+
+    def test_empty_text(self):
+        assert _extract_patent_ids("") == set()
+        assert _extract_patent_ids(None) == set()  # type: ignore[arg-type]
+
+
+class TestComputeAnalysisDiff:
+    """Test _compute_analysis_diff logic."""
+
+    def test_identical_analyses(self):
+        rec = {
+            "id": 1,
+            "company_name": "nvidia",
+            "analysis_type": "portfolio",
+            "model": "openai/gpt-4o",
+            "response": "Patent US-12345678-B2 is notable.",
+            "timestamp": datetime(2025, 5, 1),
+        }
+        diff = _compute_analysis_diff(rec, dict(rec, id=2, timestamp=datetime(2025, 5, 2)))
+        assert diff.patent_count_delta == 0
+        assert diff.added_patents == []
+        assert diff.removed_patents == []
+
+    def test_added_and_removed_patents(self):
+        from_rec = {
+            "id": 1,
+            "company_name": "nvidia",
+            "analysis_type": "portfolio",
+            "model": "openai/gpt-4o",
+            "response": "Patent US-12345678-B2 and US-11111111-A1.",
+            "timestamp": datetime(2025, 5, 1),
+        }
+        to_rec = {
+            "id": 2,
+            "company_name": "nvidia",
+            "analysis_type": "portfolio",
+            "model": "openai/gpt-4o",
+            "response": "Patent US-12345678-B2 and US-99999999-B1.",
+            "timestamp": datetime(2025, 5, 2),
+        }
+        diff = _compute_analysis_diff(from_rec, to_rec)
+        assert "US-99999999-B1" in diff.added_patents
+        assert "US-11111111-A1" in diff.removed_patents
+        assert diff.patent_count_delta == 0  # one added, one removed
+
+    def test_model_change_detected(self):
+        from_rec = {
+            "id": 1,
+            "company_name": "nvidia",
+            "analysis_type": "portfolio",
+            "model": "openai/gpt-4o",
+            "response": "",
+            "timestamp": datetime(2025, 5, 1),
+        }
+        to_rec = {
+            "id": 2,
+            "company_name": "nvidia",
+            "analysis_type": "portfolio",
+            "model": "anthropic/claude-3.5-sonnet",
+            "response": "",
+            "timestamp": datetime(2025, 5, 2),
+        }
+        diff = _compute_analysis_diff(from_rec, to_rec)
+        assert "model" in diff.changed_fields
+        assert diff.changed_fields["model"]["from"] == "openai/gpt-4o"
+        assert diff.changed_fields["model"]["to"] == "anthropic/claude-3.5-sonnet"
+
+
+# ---------- API endpoint tests ----------
+
+class TestDiffEndpoint:
+    """Test GET /analyze/{company_name}/diff."""
+
+    @patch("SPARC.api._get_job_db")
+    def test_happy_path(self, mock_get_db, auth_client):
+        """Diff returns structured response when both IDs exist."""
+        db = MagicMock()
+        mock_get_db.return_value = db
+
+        from_rec = {
+            "id": 10,
+            "company_name": "nvidia",
+            "analysis_type": "portfolio",
+            "model": "openai/gpt-4o",
+            "response": "Patent US-12345678-B2 found.",
+            "timestamp": datetime(2025, 5, 1),
+        }
+        to_rec = {
+            "id": 20,
+            "company_name": "nvidia",
+            "analysis_type": "portfolio",
+            "model": "openai/gpt-4o",
+            "response": "Patent US-12345678-B2 and US-99999999-A1 found.",
+            "timestamp": datetime(2025, 5, 10),
+        }
+        db.get_analysis_by_id.side_effect = lambda aid: from_rec if aid == 10 else to_rec
+
+        response = auth_client.get("/analyze/nvidia/diff?from=10&to=20")
+        assert response.status_code == 200
+        data = response.json()
+        assert data["company_name"] == "nvidia"
+        assert data["from_id"] == 10
+        assert data["to_id"] == 20
+        assert "US-99999999-A1" in data["added_patents"]
+        assert data["patent_count_delta"] == 1
+
+    @patch("SPARC.api._get_job_db")
+    def test_from_id_not_found(self, mock_get_db, auth_client):
+        """Returns 404 when 'from' analysis ID doesn't exist."""
+        db = MagicMock()
+        mock_get_db.return_value = db
+        db.get_analysis_by_id.return_value = None
+
+        response = auth_client.get("/analyze/nvidia/diff?from=999&to=1000")
+        assert response.status_code == 404
+        assert "999" in response.json()["detail"]
+
+    @patch("SPARC.api._get_job_db")
+    def test_to_id_not_found(self, mock_get_db, auth_client):
+        """Returns 404 when 'to' analysis ID doesn't exist."""
+        db = MagicMock()
+        mock_get_db.return_value = db
+
+        from_rec = {
+            "id": 10,
+            "company_name": "nvidia",
+            "analysis_type": "portfolio",
+            "model": "openai/gpt-4o",
+            "response": "",
+            "timestamp": datetime(2025, 5, 1),
+        }
+        db.get_analysis_by_id.side_effect = lambda aid: from_rec if aid == 10 else None
+
+        response = auth_client.get("/analyze/nvidia/diff?from=10&to=999")
+        assert response.status_code == 404
+        assert "999" in response.json()["detail"]
+
+    @patch("SPARC.api._get_job_db")
+    def test_company_mismatch(self, mock_get_db, auth_client):
+        """Returns 404 when analysis belongs to a different company."""
+        db = MagicMock()
+        mock_get_db.return_value = db
+
+        rec = {
+            "id": 10,
+            "company_name": "intel",
+            "analysis_type": "portfolio",
+            "model": "openai/gpt-4o",
+            "response": "",
+            "timestamp": datetime(2025, 5, 1),
+        }
+        db.get_analysis_by_id.return_value = rec
+
+        response = auth_client.get("/analyze/nvidia/diff?from=10&to=20")
+        assert response.status_code == 404
+
+
+class TestHistoryEndpoint:
+    """Test GET /analyze/{company_name}/history."""
+
+    @patch("SPARC.api._get_job_db")
+    def test_returns_history_list(self, mock_get_db, auth_client):
+        """History endpoint returns list of past analysis runs."""
+        db = MagicMock()
+        mock_get_db.return_value = db
+        db.list_company_analyses.return_value = [
+            {
+                "id": 20,
+                "company_name": "nvidia",
+                "analysis_type": "portfolio",
+                "model": "openai/gpt-4o",
+                "response": "...",
+                "timestamp": datetime(2025, 5, 10),
+            },
+            {
+                "id": 10,
+                "company_name": "nvidia",
+                "analysis_type": "portfolio",
+                "model": "openai/gpt-4o",
+                "response": "...",
+                "timestamp": datetime(2025, 5, 1),
+            },
+        ]
+
+        response = auth_client.get("/analyze/nvidia/history")
+        assert response.status_code == 200
+        data = response.json()
+        assert len(data) == 2
+        assert data[0]["id"] == 20
+        assert data[1]["id"] == 10
+
+    @patch("SPARC.api._get_job_db")
+    def test_empty_history(self, mock_get_db, auth_client):
+        """History endpoint returns empty list when no analyses exist."""
+        db = MagicMock()
+        mock_get_db.return_value = db
+        db.list_company_analyses.return_value = []
+
+        response = auth_client.get("/analyze/nvidia/history")
+        assert response.status_code == 200
+        assert response.json() == []
diff --git a/tests/test_api_keys.py b/tests/test_api_keys.py
new file mode 100644
index 0000000..3942f63
--- /dev/null
+++ b/tests/test_api_keys.py
@@ -0,0 +1,319 @@
+"""Tests for user-level API key generation, listing, revocation, and authentication.
+
+Covers all acceptance criteria from issue #1673:
+1. Users can create API keys (POST /auth/apikeys)
+2. Users can list their active key IDs (GET /auth/apikeys)
+3. Users can revoke keys (DELETE /auth/apikeys/{key_id})
+4. API requests authenticated with a valid API key work on protected endpoints
+5. Revoked keys are immediately rejected
+6. Plaintext key is shown only at creation time
+
+All tests use mocked DB fixtures and require no live database.
+"""
+
+from datetime import datetime, timezone
+from unittest.mock import MagicMock, patch
+
+import pytest
+from fastapi.testclient import TestClient
+
+from SPARC.api import app
+from SPARC.auth import (
+    create_access_token,
+    generate_api_key,
+    hash_api_key,
+    verify_api_key,
+)
+
+
+@pytest.fixture
+def client():
+    """Create test client."""
+    return TestClient(app)
+
+
+def _make_user():
+    return {
+        "id": 1,
+        "email": "user@test.com",
+        "role": "user",
+        "created_at": datetime(2025, 1, 1, tzinfo=timezone.utc),
+    }
+
+
+def _auth_header(user_dict):
+    """Create an Authorization header with a valid access token."""
+    token = create_access_token(user_dict["id"], user_dict["email"], user_dict["role"])
+    return {"Authorization": f"Bearer {token}"}
+
+
+@pytest.fixture(autouse=True)
+def mock_db(monkeypatch):
+    """Mock the database client used by auth and api endpoints."""
+    db = MagicMock()
+    db.get_user_count.return_value = 0
+    db.get_user_by_id.return_value = None
+    db.get_user_by_email.return_value = None
+    db.authenticate_user.return_value = None
+    db.create_user.return_value = None
+    db.get_all_users.return_value = []
+    db.update_user_role.return_value = None
+    db.delete_user.return_value = False
+    db.create_api_key.return_value = None
+    db.list_api_keys.return_value = []
+    db.delete_api_key.return_value = False
+    db.get_all_api_key_hashes.return_value = []
+
+    with patch("SPARC.api.get_db_client", return_value=db), \
+         patch("SPARC.auth.get_db_client", return_value=db):
+        yield db
+
+
+class TestCreateApiKey:
+    """POST /auth/apikeys"""
+
+    def test_create_key_returns_plaintext_and_id(self, client, mock_db):
+        """Creating a key returns the plaintext key and metadata."""
+        user = _make_user()
+        mock_db.get_user_by_id.return_value = user
+        mock_db.create_api_key.return_value = {
+            "id": 42,
+            "user_id": user["id"],
+            "label": "my-ci-key",
+            "created_at": datetime(2025, 6, 1, tzinfo=timezone.utc),
+        }
+
+        response = client.post(
+            "/auth/apikeys",
+            json={"label": "my-ci-key"},
+            headers=_auth_header(user),
+        )
+
+        assert response.status_code == 200
+        data = response.json()
+        assert data["id"] == 42
+        assert len(data["key"]) == 64  # 32 bytes hex = 64 chars
+        assert data["label"] == "my-ci-key"
+        assert "created_at" in data
+
+        # Verify the hash passed to DB is valid for the returned key
+        call_args = mock_db.create_api_key.call_args
+        stored_hash = call_args.kwargs.get("key_hash") or call_args[1].get("key_hash") or call_args[0][1]
+        assert verify_api_key(data["key"], stored_hash)
+
+    def test_create_key_without_label(self, client, mock_db):
+        """Creating a key without a label should work."""
+        user = _make_user()
+        mock_db.get_user_by_id.return_value = user
+        mock_db.create_api_key.return_value = {
+            "id": 1,
+            "user_id": user["id"],
+            "label": None,
+            "created_at": datetime(2025, 6, 1, tzinfo=timezone.utc),
+        }
+
+        response = client.post(
+            "/auth/apikeys",
+            headers=_auth_header(user),
+        )
+
+        assert response.status_code == 200
+        assert response.json()["label"] is None
+
+    def test_create_key_requires_auth(self, client):
+        """Creating a key without auth should fail."""
+        response = client.post("/auth/apikeys")
+        assert response.status_code == 401
+
+
+class TestListApiKeys:
+    """GET /auth/apikeys"""
+
+    def test_list_keys_returns_metadata_only(self, client, mock_db):
+        """Listing keys should return IDs and labels, not secrets."""
+        user = _make_user()
+        mock_db.get_user_by_id.return_value = user
+        mock_db.list_api_keys.return_value = [
+            {"id": 1, "label": "key-1", "created_at": datetime(2025, 1, 1, tzinfo=timezone.utc)},
+            {"id": 2, "label": None, "created_at": datetime(2025, 2, 1, tzinfo=timezone.utc)},
+        ]
+
+        response = client.get("/auth/apikeys", headers=_auth_header(user))
+
+        assert response.status_code == 200
+        data = response.json()
+        assert len(data) == 2
+        assert data[0]["id"] == 1
+        assert data[0]["label"] == "key-1"
+        # Ensure no secret key is exposed
+        for item in data:
+            assert "key" not in item
+            assert "key_hash" not in item
+
+    def test_list_keys_empty(self, client, mock_db):
+        """User with no keys gets an empty list."""
+        user = _make_user()
+        mock_db.get_user_by_id.return_value = user
+        mock_db.list_api_keys.return_value = []
+
+        response = client.get("/auth/apikeys", headers=_auth_header(user))
+
+        assert response.status_code == 200
+        assert response.json() == []
+
+
+class TestRevokeApiKey:
+    """DELETE /auth/apikeys/{key_id}"""
+
+    def test_revoke_existing_key(self, client, mock_db):
+        """Revoking an owned key should succeed."""
+        user = _make_user()
+        mock_db.get_user_by_id.return_value = user
+        mock_db.delete_api_key.return_value = True
+
+        response = client.delete("/auth/apikeys/42", headers=_auth_header(user))
+
+        assert response.status_code == 200
+        assert "revoked" in response.json()["message"].lower()
+        mock_db.delete_api_key.assert_called_once_with(42, user["id"])
+
+    def test_revoke_nonexistent_key_returns_404(self, client, mock_db):
+        """Revoking a key that doesn't exist (or isn't owned) returns 404."""
+        user = _make_user()
+        mock_db.get_user_by_id.return_value = user
+        mock_db.delete_api_key.return_value = False
+
+        response = client.delete("/auth/apikeys/999", headers=_auth_header(user))
+
+        assert response.status_code == 404
+
+
+class TestApiKeyAuthentication:
+    """Using X-API-Key header on protected endpoints."""
+
+    def test_valid_api_key_accesses_protected_endpoint(self, client, mock_db):
+        """A valid API key should authenticate and access /auth/me."""
+        user = _make_user()
+        plaintext = generate_api_key()
+        hashed = hash_api_key(plaintext)
+
+        mock_db.get_all_api_key_hashes.return_value = [
+            {"key_hash": hashed, "user_id": user["id"]},
+        ]
+        mock_db.get_user_by_id.return_value = user
+
+        response = client.get("/auth/me", headers={"X-API-Key": plaintext})
+
+        assert response.status_code == 200
+        data = response.json()
+        assert data["email"] == user["email"]
+        assert data["id"] == user["id"]
+
+    def test_invalid_api_key_returns_401(self, client, mock_db):
+        """An invalid API key should return 401."""
+        mock_db.get_all_api_key_hashes.return_value = []
+
+        response = client.get("/auth/me", headers={"X-API-Key": "bad-key"})
+
+        assert response.status_code == 401
+        assert "invalid api key" in response.json()["detail"].lower()
+
+    def test_revoked_key_returns_401(self, client, mock_db):
+        """After revocation, using the key should return 401."""
+        # Simulate revoked key: no matching hashes in DB
+        mock_db.get_all_api_key_hashes.return_value = []
+
+        response = client.get("/auth/me", headers={"X-API-Key": "a" * 64})
+
+        assert response.status_code == 401
+
+    def test_api_key_for_deleted_user_returns_401(self, client, mock_db):
+        """An API key whose user no longer exists should return 401."""
+        plaintext = generate_api_key()
+        hashed = hash_api_key(plaintext)
+
+        mock_db.get_all_api_key_hashes.return_value = [
+            {"key_hash": hashed, "user_id": 999},
+        ]
+        mock_db.get_user_by_id.return_value = None  # user deleted
+
+        response = client.get("/auth/me", headers={"X-API-Key": plaintext})
+
+        assert response.status_code == 401
+
+    def test_no_auth_at_all_returns_401(self, client, mock_db):
+        """No auth header at all should return 401."""
+        response = client.get("/auth/me")
+        assert response.status_code == 401
+
+
+class TestApiKeyFullFlow:
+    """End-to-end flow: create key, use it, revoke it, try again."""
+
+    def test_create_use_revoke_flow(self, client, mock_db):
+        """Simulate full lifecycle of an API key."""
+        user = _make_user()
+        mock_db.get_user_by_id.return_value = user
+
+        # Step 1: Create key
+        mock_db.create_api_key.return_value = {
+            "id": 10,
+            "user_id": user["id"],
+            "label": "test",
+            "created_at": datetime(2025, 6, 1, tzinfo=timezone.utc),
+        }
+
+        create_resp = client.post(
+            "/auth/apikeys",
+            json={"label": "test"},
+            headers=_auth_header(user),
+        )
+        assert create_resp.status_code == 200
+        plaintext = create_resp.json()["key"]
+
+        # Capture the hash that was stored
+        call_args = mock_db.create_api_key.call_args
+        stored_hash = call_args.kwargs.get("key_hash") or call_args[0][1]
+
+        # Step 2: Use key on protected endpoint
+        mock_db.get_all_api_key_hashes.return_value = [
+            {"key_hash": stored_hash, "user_id": user["id"]},
+        ]
+
+        use_resp = client.get("/auth/me", headers={"X-API-Key": plaintext})
+        assert use_resp.status_code == 200
+        assert use_resp.json()["email"] == user["email"]
+
+        # Step 3: Revoke key
+        mock_db.delete_api_key.return_value = True
+        revoke_resp = client.delete("/auth/apikeys/10", headers=_auth_header(user))
+        assert revoke_resp.status_code == 200
+
+        # Step 4: Try using revoked key
+        mock_db.get_all_api_key_hashes.return_value = []  # key removed from DB
+        rejected_resp = client.get("/auth/me", headers={"X-API-Key": plaintext})
+        assert rejected_resp.status_code == 401
+
+
+class TestApiKeyHelpers:
+    """Unit tests for key generation and hashing helpers."""
+
+    def test_generate_api_key_length(self):
+        """Generated key should be 64 hex characters (32 bytes)."""
+        key = generate_api_key()
+        assert len(key) == 64
+        # Should be valid hex
+        int(key, 16)
+
+    def test_generate_api_key_uniqueness(self):
+        """Two generated keys should be different."""
+        k1 = generate_api_key()
+        k2 = generate_api_key()
+        assert k1 != k2
+
+    def test_hash_and_verify(self):
+        """hash_api_key and verify_api_key should round-trip correctly."""
+        key = generate_api_key()
+        hashed = hash_api_key(key)
+        assert verify_api_key(key, hashed)
+        assert not verify_api_key("wrong-key", hashed)
diff --git a/tests/test_rate_limit_admin.py b/tests/test_rate_limit_admin.py
index bc63a5a..f10e9da 100644
--- a/tests/test_rate_limit_admin.py
+++ b/tests/test_rate_limit_admin.py
@@ -20,8 +20,10 @@ def client():
 def reset_stats():
     """Reset rate limit stats between tests."""
     api._rate_limit_stats.clear()
+    api._rejected_log.clear()
     yield
     api._rate_limit_stats.clear()
+    api._rejected_log.clear()
 
 
 def _mock_admin():
@@ -50,8 +52,7 @@ class TestRateLimitAdminEndpoint:
             app.dependency_overrides.clear()
 
     def test_non_admin_rejected(self, client):
-        """Non-admin users should get 403."""
-        # Without overriding the dependency, it should fail auth
+        """Non-admin users should get 401/403."""
         response = client.get("/admin/rate-limits")
         assert response.status_code in (401, 403)
 
@@ -77,6 +78,9 @@ class TestRateLimitAdminEndpoint:
             for rl in data["rate_limits"]:
                 assert rl["total_requests"] == 0
                 assert rl["rejected_requests"] == 0
+                assert rl["by_ip"] == []
+            assert data["throttled_24h"] == 0
+            assert data["throttled_over_time"] == []
         finally:
             app.dependency_overrides.clear()
 
@@ -107,3 +111,68 @@ class TestRateLimitAdminEndpoint:
                 assert isinstance(rl["limit"], str)
         finally:
             app.dependency_overrides.clear()
+
+    def test_per_ip_breakdown(self, client):
+        """Stats should include per-IP breakdown with total and rejected counts."""
+        api._track_rate_limit_request("/auth/login", "10.0.0.1")
+        api._track_rate_limit_request("/auth/login", "10.0.0.1", rejected=True)
+        api._track_rate_limit_request("/auth/login", "10.0.0.2")
+
+        app.dependency_overrides[api.get_current_admin] = _mock_admin
+        try:
+            response = client.get("/admin/rate-limits")
+            data = response.json()
+            login_stats = next(rl for rl in data["rate_limits"] if rl["endpoint"] == "/auth/login")
+            by_ip = login_stats["by_ip"]
+            assert len(by_ip) == 2
+            ip1 = next(entry for entry in by_ip if entry["ip"] == "10.0.0.1")
+            assert ip1["total"] == 2
+            assert ip1["rejected"] == 1
+            ip2 = next(entry for entry in by_ip if entry["ip"] == "10.0.0.2")
+            assert ip2["total"] == 1
+            assert ip2["rejected"] == 0
+        finally:
+            app.dependency_overrides.clear()
+
+    def test_throttled_24h_count(self, client):
+        """Should report total throttled requests in the last 24 hours."""
+        api._track_rate_limit_request("/auth/login", "10.0.0.1", rejected=True)
+        api._track_rate_limit_request("/auth/register", "10.0.0.2", rejected=True)
+
+        app.dependency_overrides[api.get_current_admin] = _mock_admin
+        try:
+            response = client.get("/admin/rate-limits")
+            data = response.json()
+            assert data["throttled_24h"] == 2
+        finally:
+            app.dependency_overrides.clear()
+
+    def test_throttled_over_time_structure(self, client):
+        """Throttled-over-time should be a list of {timestamp, count} buckets."""
+        api._track_rate_limit_request("/auth/login", "10.0.0.1", rejected=True)
+
+        app.dependency_overrides[api.get_current_admin] = _mock_admin
+        try:
+            response = client.get("/admin/rate-limits")
+            data = response.json()
+            assert len(data["throttled_over_time"]) >= 1
+            entry = data["throttled_over_time"][0]
+            assert "timestamp" in entry
+            assert "count" in entry
+            assert entry["count"] >= 1
+        finally:
+            app.dependency_overrides.clear()
+
+    def test_response_shape_matches_contract(self, client):
+        """The full response should match the expected shape for the frontend."""
+        app.dependency_overrides[api.get_current_admin] = _mock_admin
+        try:
+            response = client.get("/admin/rate-limits")
+            data = response.json()
+            # Top-level keys
+            assert set(data.keys()) == {"rate_limits", "throttled_24h", "throttled_over_time"}
+            # Each rate_limit entry
+            for rl in data["rate_limits"]:
+                assert set(rl.keys()) == {"endpoint", "limit", "total_requests", "rejected_requests", "by_ip"}
+        finally:
+            app.dependency_overrides.clear()