diff --git a/.chronus/changes/copilot-add-spector-case-serialization-2026-5-1-18-57-48.md b/.chronus/changes/copilot-add-spector-case-serialization-2026-5-1-18-57-48.md
new file mode 100644
index 00000000000..6a25bdb8a4a
--- /dev/null
+++ b/.chronus/changes/copilot-add-spector-case-serialization-2026-5-1-18-57-48.md
@@ -0,0 +1,7 @@
+---
+changeKind: internal
+packages:
+  - "@typespec/http-specs"
+---
+
+Add encode/duration lossy Spector scenarios verifying a duration whose value carries more precision than the target integer encoding (fractional seconds and sub-millisecond milliseconds) is serialized as an integer
\ No newline at end of file
diff --git a/packages/http-specs/spec-summary.md b/packages/http-specs/spec-summary.md
index c623a91cc95..44cb91ad026 100644
--- a/packages/http-specs/spec-summary.md
+++ b/packages/http-specs/spec-summary.md
@@ -910,6 +910,60 @@ Expected header `duration: P40D`
 Test iso8601 encode for a duration array header.
 Expected header `duration: [P40D,P50D]`
 
+### Encode_Duration_Lossy_headerInt32Milliseconds
+
+- Endpoint: `get /encode/duration/lossy/header/int32-milliseconds`
+
+Test int32 milliseconds encode for a duration header whose value has a sub-millisecond fractional component.
+The duration is 36250.25 milliseconds, e.g. TimeSpan.FromMilliseconds(36250.25) in C#.
+The client must serialize the value as an integer (not a floating point number such as `36250.25`), discarding the sub-millisecond precision.
+Because emitters may floor, round, or ceil when discarding that precision, the expected header is `duration: 36250` or `duration: 36251`.
+
+### Encode_Duration_Lossy_headerInt32Seconds
+
+- Endpoint: `get /encode/duration/lossy/header/int32-seconds`
+
+Test int32 seconds encode for a duration header whose value has a fractional (sub-second) component.
+The duration is 36.25 seconds, e.g. TimeSpan.FromSeconds(36.25) in C#.
+The client must serialize the value as an integer (not a floating point number such as `36.25`), discarding the sub-second precision.
+Because emitters may floor, round, or ceil when discarding that precision, the expected header is `duration: 36` or `duration: 37`.
+
+### Encode_Duration_Lossy_propertyInt32Milliseconds
+
+- Endpoint: `post /encode/duration/lossy/property/int32-milliseconds`
+
+Test int32 milliseconds encode for a duration property whose value has a sub-millisecond fractional component.
+The duration is 36250.25 milliseconds, e.g. TimeSpan.FromMilliseconds(36250.25) in C#.
+The client must serialize the value as an integer (not a floating point number such as `36250.25`), discarding the sub-millisecond precision.
+Because emitters may floor, round, or ceil when discarding that precision, the expected request body `value` is `36250` or `36251`.
+
+### Encode_Duration_Lossy_propertyInt32Seconds
+
+- Endpoint: `post /encode/duration/lossy/property/int32-seconds`
+
+Test int32 seconds encode for a duration property whose value has a fractional (sub-second) component.
+The duration is 36.25 seconds, e.g. TimeSpan.FromSeconds(36.25) in C#.
+The client must serialize the value as an integer (not a floating point number such as `36.25`), discarding the sub-second precision.
+Because emitters may floor, round, or ceil when discarding that precision, the expected request body `value` is `36` or `37`.
+
+### Encode_Duration_Lossy_queryInt32Milliseconds
+
+- Endpoint: `get /encode/duration/lossy/query/int32-milliseconds`
+
+Test int32 milliseconds encode for a duration query parameter whose value has a sub-millisecond fractional component.
+The duration is 36250.25 milliseconds, e.g. TimeSpan.FromMilliseconds(36250.25) in C#.
+The client must serialize the value as an integer (not a floating point number such as `36250.25`), discarding the sub-millisecond precision.
+Because emitters may floor, round, or ceil when discarding that precision, the expected query parameter is `input=36250` or `input=36251`.
+
+### Encode_Duration_Lossy_queryInt32Seconds
+
+- Endpoint: `get /encode/duration/lossy/query/int32-seconds`
+
+Test int32 seconds encode for a duration query parameter whose value has a fractional (sub-second) component.
+The duration is 36.25 seconds, e.g. TimeSpan.FromSeconds(36.25) in C#.
+The client must serialize the value as an integer (not a floating point number such as `36.25`), discarding the sub-second precision.
+Because emitters may floor, round, or ceil when discarding that precision, the expected query parameter is `input=36` or `input=37`.
+
 ### Encode_Duration_Property_default
 
 - Endpoint: `post /encode/duration/property/default`
diff --git a/packages/http-specs/specs/encode/duration/main.tsp b/packages/http-specs/specs/encode/duration/main.tsp
index cb0cf16aee0..faeffbd1c85 100644
--- a/packages/http-specs/specs/encode/duration/main.tsp
+++ b/packages/http-specs/specs/encode/duration/main.tsp
@@ -729,3 +729,105 @@ namespace Header {
     duration: Int32MillisecondsDuration[],
   ): NoContentResponse;
 }
+
+/**
+ * Lossy encode scenarios.
+ *
+ * These scenarios cover the case where the source `duration` carries more precision than the
+ * target encoding can represent (e.g. a sub-second value encoded as integer seconds). The client
+ * must still serialize the value using the target number type (an integer), discarding the extra
+ * precision, rather than emitting a floating point number. This is distinct from arbitrary type
+ * mismatches, which are already covered by the round-trip scenarios above.
+ */
+@route("/lossy")
+namespace Lossy {
+  model Int32SecondsDurationProperty {
+    @encode(DurationKnownEncoding.seconds, int32)
+    value: duration;
+  }
+
+  model Int32MillisecondsDurationProperty {
+    @encode(DurationKnownEncoding.milliseconds, int32)
+    value: duration;
+  }
+
+  @route("/query/int32-seconds")
+  @scenario
+  @scenarioDoc("""
+    Test int32 seconds encode for a duration query parameter whose value has a fractional (sub-second) component.
+    The duration is 36.25 seconds, e.g. TimeSpan.FromSeconds(36.25) in C#.
+    The client must serialize the value as an integer (not a floating point number such as `36.25`), discarding the sub-second precision.
+    Because emitters may floor, round, or ceil when discarding that precision, the expected query parameter is `input=36` or `input=37`.
+    """)
+  op queryInt32Seconds(
+    @query
+    @encode(DurationKnownEncoding.seconds, int32)
+    input: duration,
+  ): NoContentResponse;
+
+  @route("/query/int32-milliseconds")
+  @scenario
+  @scenarioDoc("""
+    Test int32 milliseconds encode for a duration query parameter whose value has a sub-millisecond fractional component.
+    The duration is 36250.25 milliseconds, e.g. TimeSpan.FromMilliseconds(36250.25) in C#.
+    The client must serialize the value as an integer (not a floating point number such as `36250.25`), discarding the sub-millisecond precision.
+    Because emitters may floor, round, or ceil when discarding that precision, the expected query parameter is `input=36250` or `input=36251`.
+    """)
+  op queryInt32Milliseconds(
+    @query
+    @encode(DurationKnownEncoding.milliseconds, int32)
+    input: duration,
+  ): NoContentResponse;
+
+  @route("/property/int32-seconds")
+  @scenario
+  @scenarioDoc("""
+    Test int32 seconds encode for a duration property whose value has a fractional (sub-second) component.
+    The duration is 36.25 seconds, e.g. TimeSpan.FromSeconds(36.25) in C#.
+    The client must serialize the value as an integer (not a floating point number such as `36.25`), discarding the sub-second precision.
+    Because emitters may floor, round, or ceil when discarding that precision, the expected request body `value` is `36` or `37`.
+    """)
+  @post
+  op propertyInt32Seconds(@body body: Int32SecondsDurationProperty): Int32SecondsDurationProperty;
+
+  @route("/property/int32-milliseconds")
+  @scenario
+  @scenarioDoc("""
+    Test int32 milliseconds encode for a duration property whose value has a sub-millisecond fractional component.
+    The duration is 36250.25 milliseconds, e.g. TimeSpan.FromMilliseconds(36250.25) in C#.
+    The client must serialize the value as an integer (not a floating point number such as `36250.25`), discarding the sub-millisecond precision.
+    Because emitters may floor, round, or ceil when discarding that precision, the expected request body `value` is `36250` or `36251`.
+    """)
+  @post
+  op propertyInt32Milliseconds(
+    @body body: Int32MillisecondsDurationProperty,
+  ): Int32MillisecondsDurationProperty;
+
+  @route("/header/int32-seconds")
+  @scenario
+  @scenarioDoc("""
+    Test int32 seconds encode for a duration header whose value has a fractional (sub-second) component.
+    The duration is 36.25 seconds, e.g. TimeSpan.FromSeconds(36.25) in C#.
+    The client must serialize the value as an integer (not a floating point number such as `36.25`), discarding the sub-second precision.
+    Because emitters may floor, round, or ceil when discarding that precision, the expected header is `duration: 36` or `duration: 37`.
+    """)
+  op headerInt32Seconds(
+    @header
+    @encode(DurationKnownEncoding.seconds, int32)
+    duration: duration,
+  ): NoContentResponse;
+
+  @route("/header/int32-milliseconds")
+  @scenario
+  @scenarioDoc("""
+    Test int32 milliseconds encode for a duration header whose value has a sub-millisecond fractional component.
+    The duration is 36250.25 milliseconds, e.g. TimeSpan.FromMilliseconds(36250.25) in C#.
+    The client must serialize the value as an integer (not a floating point number such as `36250.25`), discarding the sub-millisecond precision.
+    Because emitters may floor, round, or ceil when discarding that precision, the expected header is `duration: 36250` or `duration: 36251`.
+    """)
+  op headerInt32Milliseconds(
+    @header
+    @encode(DurationKnownEncoding.milliseconds, int32)
+    duration: duration,
+  ): NoContentResponse;
+}
diff --git a/packages/http-specs/specs/encode/duration/mockapi.ts b/packages/http-specs/specs/encode/duration/mockapi.ts
index c74a77d300b..868f5c54947 100644
--- a/packages/http-specs/specs/encode/duration/mockapi.ts
+++ b/packages/http-specs/specs/encode/duration/mockapi.ts
@@ -23,6 +23,45 @@ function createBodyServerTests(uri: string, data: any, value: any) {
     kind: "MockApiDefinition",
   });
 }
+
+// Validates that a duration whose value carries more precision than the target encoding (a lossy
+// encode) is serialized as an integer. The allowed values cover floor, round and ceil so the test
+// does not take a position on an emitter's rounding mode while still rejecting floating point output.
+function createLossyBodyServerTests(uri: string, allowed: number[]) {
+  return passOnSuccess({
+    uri,
+    method: "post",
+    request: {
+      body: json({ value: allowed[0] }),
+    },
+    response: {
+      status: 200,
+      body: json({ value: allowed[0] }),
+    },
+    handler: (req: MockRequest) => {
+      const value = req.body?.value;
+      if (typeof value !== "number" || !Number.isInteger(value)) {
+        throw new ValidationError(
+          `Expected body property "value" to be serialized as an integer but got ${value}`,
+          "an integer",
+          value,
+        );
+      }
+      if (!allowed.includes(value)) {
+        throw new ValidationError(
+          `Expected body property "value" to be one of ${allowed.join(", ")} but got ${value}`,
+          allowed.join(" | "),
+          value,
+        );
+      }
+      return {
+        status: 200,
+        body: json({ value }),
+      };
+    },
+    kind: "MockApiDefinition",
+  });
+}
 Scenarios.Encode_Duration_Property_default = createBodyServerTests(
   "/encode/duration/property/default",
   {
@@ -175,6 +214,45 @@ function createQueryFloatServerTests(uri: string, paramData: any, value: number)
     kind: "MockApiDefinition",
   });
 }
+
+// Validates that a duration whose value carries more precision than the target encoding (a lossy
+// encode) is serialized as an integer. The allowed values cover floor, round and ceil so the test
+// does not take a position on an emitter's rounding mode while still rejecting floating point output.
+function createLossyQueryServerTests(uri: string, allowed: number[]) {
+  return passOnSuccess({
+    uri,
+    method: "get",
+    request: {
+      query: {
+        input: allowed[0],
+      },
+    },
+    response: {
+      status: 204,
+    },
+    handler: (req: MockRequest) => {
+      const actual = req.query["input"] as string;
+      if (!/^[-+]?\d+$/.test(actual)) {
+        throw new ValidationError(
+          `Expected query param input to be serialized as an integer but got ${actual}`,
+          "an integer",
+          actual,
+        );
+      }
+      if (!allowed.map(String).includes(actual)) {
+        throw new ValidationError(
+          `Expected query param input to be one of ${allowed.join(", ")} but got ${actual}`,
+          allowed.join(" | "),
+          actual,
+        );
+      }
+      return {
+        status: 204,
+      };
+    },
+    kind: "MockApiDefinition",
+  });
+}
 Scenarios.Encode_Duration_Query_default = createQueryServerTests(
   "/encode/duration/query/default",
   {
@@ -321,6 +399,45 @@ function createHeaderFloatServerTests(uri: string, value: number) {
   });
 }
 
+// Validates that a duration whose value carries more precision than the target encoding (a lossy
+// encode) is serialized as an integer. The allowed values cover floor, round and ceil so the test
+// does not take a position on an emitter's rounding mode while still rejecting floating point output.
+function createLossyHeaderServerTests(uri: string, allowed: number[]) {
+  return passOnSuccess({
+    uri,
+    method: "get",
+    request: {
+      headers: {
+        duration: String(allowed[0]),
+      },
+    },
+    response: {
+      status: 204,
+    },
+    handler: (req: MockRequest) => {
+      const actual = req.headers["duration"];
+      if (!/^[-+]?\d+$/.test(actual)) {
+        throw new ValidationError(
+          `Expected header duration to be serialized as an integer but got ${actual}`,
+          "an integer",
+          actual,
+        );
+      }
+      if (!allowed.map(String).includes(actual)) {
+        throw new ValidationError(
+          `Expected header duration to be one of ${allowed.join(", ")} but got ${actual}`,
+          allowed.join(" | "),
+          actual,
+        );
+      }
+      return {
+        status: 204,
+      };
+    },
+    kind: "MockApiDefinition",
+  });
+}
+
 Scenarios.Encode_Duration_Header_default = createHeaderServerTests(
   "/encode/duration/header/default",
   {
@@ -408,3 +525,30 @@ Scenarios.Encode_Duration_Header_floatMillisecondsLargerUnit = createHeaderFloat
   "/encode/duration/header/float-milliseconds-larger-unit",
   210000,
 );
+
+// Lossy encode scenarios: the source duration carries more precision than the target integer
+// encoding, so floor/round/ceil are all acceptable results (e.g. 36.25s -> 36 or 37).
+Scenarios.Encode_Duration_Lossy_queryInt32Seconds = createLossyQueryServerTests(
+  "/encode/duration/lossy/query/int32-seconds",
+  [36, 37],
+);
+Scenarios.Encode_Duration_Lossy_queryInt32Milliseconds = createLossyQueryServerTests(
+  "/encode/duration/lossy/query/int32-milliseconds",
+  [36250, 36251],
+);
+Scenarios.Encode_Duration_Lossy_propertyInt32Seconds = createLossyBodyServerTests(
+  "/encode/duration/lossy/property/int32-seconds",
+  [36, 37],
+);
+Scenarios.Encode_Duration_Lossy_propertyInt32Milliseconds = createLossyBodyServerTests(
+  "/encode/duration/lossy/property/int32-milliseconds",
+  [36250, 36251],
+);
+Scenarios.Encode_Duration_Lossy_headerInt32Seconds = createLossyHeaderServerTests(
+  "/encode/duration/lossy/header/int32-seconds",
+  [36, 37],
+);
+Scenarios.Encode_Duration_Lossy_headerInt32Milliseconds = createLossyHeaderServerTests(
+  "/encode/duration/lossy/header/int32-milliseconds",
+  [36250, 36251],
+);