diff --git a/README.md b/README.md
index b01444d..d57e559 100644
--- a/README.md
+++ b/README.md
@@ -23,7 +23,7 @@ Implemented:
 - Audit events for shift creation, updates, cancellation, leave approval and leave cancellation
 - Before/after state snapshots stored as JSONB, with actor and timestamp
 - Audit writes participate in the same transaction as the business mutation
-- Validation and API error handling
+- Validation and API error handling, including PostgreSQL exclusion violations mapped to stable HTTP 409 responses
 - Unit and integration tests
 - GitHub Actions CI
 
@@ -83,7 +83,6 @@ Migration `V4__audit_events.sql` adds the audit event table. Audit writes use th
 - leave and time-off
 - department constraints
 - schedule queries
-- map PostgreSQL exclusion violations to HTTP 409 consistently for create and update paths
 - extend integration coverage to concurrent updates and leave/shift interactions
 
 ### Milestone 3 — Workforce Operations
diff --git a/src/main/kotlin/dev/whrite/opsgrid/api/ApiExceptionHandler.kt b/src/main/kotlin/dev/whrite/opsgrid/api/ApiExceptionHandler.kt
index 07e5eff..0af52ae 100644
--- a/src/main/kotlin/dev/whrite/opsgrid/api/ApiExceptionHandler.kt
+++ b/src/main/kotlin/dev/whrite/opsgrid/api/ApiExceptionHandler.kt
@@ -6,10 +6,14 @@ import dev.whrite.opsgrid.shift.EmployeeInactiveException
 import dev.whrite.opsgrid.shift.EmployeeNotFoundException
 import dev.whrite.opsgrid.shift.ShiftConflictException
 import dev.whrite.opsgrid.shift.ShiftNotFoundException
+import org.hibernate.exception.ConstraintViolationException
+import org.springframework.dao.DataIntegrityViolationException
 import org.springframework.http.HttpStatus
+import org.springframework.http.ResponseEntity
 import org.springframework.web.bind.annotation.ExceptionHandler
 import org.springframework.web.bind.annotation.ResponseStatus
 import org.springframework.web.bind.annotation.RestControllerAdvice
+import java.sql.SQLException
 
 data class ApiError(val code: String, val message: String)
 
@@ -45,8 +49,39 @@ class ApiExceptionHandler {
     fun leaveConflict(ex: LeaveConflictException) =
         ApiError("LEAVE_CONFLICT", ex.message ?: "Leave conflict")
 
+    @ExceptionHandler(DataIntegrityViolationException::class)
+    fun dataIntegrityViolation(ex: DataIntegrityViolationException): ResponseEntity<ApiError> {
+        val causes = generateSequence<Throwable>(ex) { it.cause }.toList()
+        val sqlState = causes.filterIsInstance<SQLException>().firstOrNull()?.sqlState
+        val constraintName = causes
+            .filterIsInstance<ConstraintViolationException>()
+            .firstOrNull()
+            ?.constraintName
+
+        val error = when {
+            sqlState == "23P01" && constraintName == SHIFT_EXCLUSION_CONSTRAINT ->
+                ApiError("SHIFT_CONFLICT", "The shift overlaps an existing scheduled shift")
+
+            sqlState == "23P01" && constraintName == LEAVE_EXCLUSION_CONSTRAINT ->
+                ApiError("LEAVE_CONFLICT", "The leave window overlaps existing approved leave")
+
+            sqlState == "23P01" ->
+                ApiError("SCHEDULE_CONFLICT", "The requested time window conflicts with an existing schedule")
+
+            else ->
+                ApiError("DATA_INTEGRITY_VIOLATION", "The request violates a data integrity constraint")
+        }
+
+        return ResponseEntity.status(HttpStatus.CONFLICT).body(error)
+    }
+
     @ExceptionHandler(IllegalArgumentException::class)
     @ResponseStatus(HttpStatus.BAD_REQUEST)
     fun badRequest(ex: IllegalArgumentException) =
         ApiError("INVALID_REQUEST", ex.message ?: "Invalid request")
+
+    private companion object {
+        const val SHIFT_EXCLUSION_CONSTRAINT = "shifts_no_overlapping_employee_windows"
+        const val LEAVE_EXCLUSION_CONSTRAINT = "leave_no_overlapping_employee_windows"
+    }
 }
diff --git a/src/test/kotlin/dev/whrite/opsgrid/api/ApiExceptionHandlerTest.kt b/src/test/kotlin/dev/whrite/opsgrid/api/ApiExceptionHandlerTest.kt
new file mode 100644
index 0000000..3b7a3ea
--- /dev/null
+++ b/src/test/kotlin/dev/whrite/opsgrid/api/ApiExceptionHandlerTest.kt
@@ -0,0 +1,57 @@
+package dev.whrite.opsgrid.api
+
+import org.hibernate.exception.ConstraintViolationException
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+import org.springframework.dao.DataIntegrityViolationException
+import org.springframework.http.HttpStatus
+import java.sql.SQLException
+
+class ApiExceptionHandlerTest {
+    private val handler = ApiExceptionHandler()
+
+    @Test
+    fun mapsShiftExclusionViolationToShiftConflict() {
+        val response = handler.dataIntegrityViolation(
+            dataIntegrityFailure("23P01", "shifts_no_overlapping_employee_windows")
+        )
+
+        assertEquals(HttpStatus.CONFLICT, response.statusCode)
+        assertEquals("SHIFT_CONFLICT", response.body?.code)
+        assertEquals("The shift overlaps an existing scheduled shift", response.body?.message)
+    }
+
+    @Test
+    fun mapsLeaveExclusionViolationToLeaveConflict() {
+        val response = handler.dataIntegrityViolation(
+            dataIntegrityFailure("23P01", "leave_no_overlapping_employee_windows")
+        )
+
+        assertEquals(HttpStatus.CONFLICT, response.statusCode)
+        assertEquals("LEAVE_CONFLICT", response.body?.code)
+    }
+
+    @Test
+    fun doesNotExposeDatabaseDetailsForOtherIntegrityViolations() {
+        val response = handler.dataIntegrityViolation(
+            dataIntegrityFailure("23505", "employees_employee_number_key")
+        )
+
+        assertEquals(HttpStatus.CONFLICT, response.statusCode)
+        assertEquals("DATA_INTEGRITY_VIOLATION", response.body?.code)
+        assertEquals(
+            "The request violates a data integrity constraint",
+            response.body?.message
+        )
+    }
+
+    private fun dataIntegrityFailure(sqlState: String, constraint: String) =
+        DataIntegrityViolationException(
+            "database write failed",
+            ConstraintViolationException(
+                "constraint violation",
+                SQLException("database detail must not reach the client", sqlState),
+                constraint
+            )
+        )
+}