diff --git a/Bdd/Targets/Common/BddTargetAppName.c b/Bdd/Targets/Common/BddTargetAppName.c
index 5505e9d1..06ec56c9 100644
--- a/Bdd/Targets/Common/BddTargetAppName.c
+++ b/Bdd/Targets/Common/BddTargetAppName.c
@@ -1,5 +1,5 @@
 #include "BddTargetAppName.h"
-#include "SolidSyslogFormatter.h"
+#include "SolidSyslogHeaderField.h"
 
 #include <ctype.h>
 #include <stdbool.h>
@@ -57,7 +57,8 @@ void BddTargetAppName_Set(const char* argv0)
     }
 }
 
-void BddTargetAppName_Get(struct SolidSyslogFormatter* formatter)
+void BddTargetAppName_Get(struct SolidSyslogHeaderField* field, void* context)
 {
-    SolidSyslogFormatter_BoundedString(formatter, appName, appNameLength);
+    (void) context;
+    SolidSyslogHeaderField_PrintUsAscii(field, appName, appNameLength);
 }
diff --git a/Bdd/Targets/Common/BddTargetAppName.h b/Bdd/Targets/Common/BddTargetAppName.h
index 1da4b2af..75d312da 100644
--- a/Bdd/Targets/Common/BddTargetAppName.h
+++ b/Bdd/Targets/Common/BddTargetAppName.h
@@ -5,10 +5,10 @@
 
 EXTERN_C_BEGIN
 
-    struct SolidSyslogFormatter;
+    struct SolidSyslogHeaderField;
 
     void BddTargetAppName_Set(const char* argv0);
-    void BddTargetAppName_Get(struct SolidSyslogFormatter * formatter);
+    void BddTargetAppName_Get(struct SolidSyslogHeaderField * field, void* context);
 
 EXTERN_C_END
 
diff --git a/Bdd/Targets/Common/BddTargetFreeRtosPipeline.c b/Bdd/Targets/Common/BddTargetFreeRtosPipeline.c
index a45485ec..14003347 100644
--- a/Bdd/Targets/Common/BddTargetFreeRtosPipeline.c
+++ b/Bdd/Targets/Common/BddTargetFreeRtosPipeline.c
@@ -27,6 +27,7 @@
 #include "SolidSyslogError.h"
 #include "SolidSyslogFileBlockDevice.h"
 #include "SolidSyslogFormatter.h"
+#include "SolidSyslogHeaderField.h"
 #include "SolidSyslogFreeRtosMutex.h"
 #include "SolidSyslogFreeRtosSysUpTime.h"
 #include "SolidSyslogMbedTlsAesGcmPolicy.h"
@@ -175,7 +176,7 @@ static void OnStoreFull(void* context);
 static size_t GetCapacityThreshold(void* context);
 static void OnThresholdCrossed(void* context);
 static void TeardownAll(void);
-static void GetAppName(struct SolidSyslogFormatter* formatter);
+static void GetAppName(struct SolidSyslogHeaderField* field, void* context);
 static void GetTimeQuality(struct SolidSyslogTimeQuality* timeQuality);
 static void ErrorHandlerEx(void* context, const struct SolidSyslogErrorEvent* event);
 
@@ -220,9 +221,10 @@ void BddTargetFreeRtosPipeline_SetConfig(const struct BddTargetFreeRtosPipelineC
 
 /* ---- SolidSyslog config callbacks ------------------------------------------ */
 
-static void GetAppName(struct SolidSyslogFormatter* formatter)
+static void GetAppName(struct SolidSyslogHeaderField* field, void* context)
 {
-    SolidSyslogFormatter_BoundedString(formatter, appName, strlen(appName));
+    (void) context;
+    SolidSyslogHeaderField_PrintUsAscii(field, appName, strlen(appName));
 }
 
 /* No RTC and no time-sync on these reference targets — RFC 5424 §6.2.3.1
diff --git a/Bdd/Targets/Common/BddTargetFreeRtosPipeline.h b/Bdd/Targets/Common/BddTargetFreeRtosPipeline.h
index ef3f08f3..c3865983 100644
--- a/Bdd/Targets/Common/BddTargetFreeRtosPipeline.h
+++ b/Bdd/Targets/Common/BddTargetFreeRtosPipeline.h
@@ -2,8 +2,8 @@
 #define BDD_TARGET_FREE_RTOS_PIPELINE_H
 
 #include "SolidSyslogEndpoint.h"
+#include "SolidSyslogHeaderFieldFunction.h"
 #include "SolidSyslogSender.h"
-#include "SolidSyslogStringFunction.h"
 
 #include <stdbool.h>
 #include <stdint.h>
@@ -37,7 +37,7 @@ struct BddTargetFreeRtosPipelineConfig
     struct SolidSyslogSender* (*BuildSender)(void);
     /* Emit the RFC 5424 HOSTNAME by reading the platform IP stack. Wired into
      * SolidSyslogConfig.GetHostname. */
-    SolidSyslogStringFunction GetHostname;
+    SolidSyslogHeaderFieldFunction GetHostname;
     /* Tear down the sender + platform adapters. Runs on the interactive task
      * after the shared pipeline teardown (SolidSyslog / SD / store / buffer). */
     void (*TeardownNetwork)(void);
diff --git a/Bdd/Targets/FreeRtos/main.c b/Bdd/Targets/FreeRtos/main.c
index fbdd8d61..1eede1c4 100644
--- a/Bdd/Targets/FreeRtos/main.c
+++ b/Bdd/Targets/FreeRtos/main.c
@@ -23,7 +23,7 @@
 #include "BddTargetTlsConfig.h"
 #include "BddTargetTlsSender.h"
 
-#include "SolidSyslogFormatter.h"
+#include "SolidSyslogHeaderField.h"
 #include "SolidSyslogPlusTcpAddress.h"
 #include "SolidSyslogPlusTcpDatagram.h"
 #include "SolidSyslogPlusTcpResolver.h"
@@ -96,7 +96,7 @@ static BaseType_t interactiveTaskCreated = pdFALSE;
 extern NetworkInterface_t* pxMPS2_FillInterfaceDescriptor(BaseType_t xEMACIndex, NetworkInterface_t* pxInterface);
 
 static void SetEthernetIrqPriority(void);
-static void GetHostname(struct SolidSyslogFormatter* formatter);
+static void GetHostname(struct SolidSyslogHeaderField* field, void* context);
 static struct SolidSyslogSender* BuildSender(void);
 static void TeardownNetwork(void);
 
@@ -229,16 +229,17 @@ static void SetEthernetIrqPriority(void)
     *ipr = ETHERNET_IRQ_PRIORITY;
 }
 
-static void GetHostname(struct SolidSyslogFormatter* formatter)
+static void GetHostname(struct SolidSyslogHeaderField* field, void* context)
 {
     /* RFC 5424 §6.2.4 rung 2 (static IP address). Read back from the IP stack so
      * a future DHCP / hostname slice satisfies the same rung without re-touching
      * this callback. */
     uint32_t ipAddress = 0U;
     char ipBuffer[16];
+    (void) context;
     FreeRTOS_GetEndPointConfiguration(&ipAddress, NULL, NULL, NULL, &networkEndPoint);
     FreeRTOS_inet_ntoa(ipAddress, ipBuffer);
-    SolidSyslogFormatter_BoundedString(formatter, ipBuffer, strlen(ipBuffer));
+    SolidSyslogHeaderField_PrintUsAscii(field, ipBuffer, strlen(ipBuffer));
 }
 
 /* Build the PlusTcp SwitchingSender: UDP datagram, octet-framed TCP, and a
diff --git a/Bdd/Targets/FreeRtosLwip/main.c b/Bdd/Targets/FreeRtosLwip/main.c
index 1ecca1a2..12692eb1 100644
--- a/Bdd/Targets/FreeRtosLwip/main.c
+++ b/Bdd/Targets/FreeRtosLwip/main.c
@@ -24,7 +24,7 @@
 #include "BddTargetSwitchConfig.h"
 #include "BddTargetTlsSender.h"
 
-#include "SolidSyslogFormatter.h"
+#include "SolidSyslogHeaderField.h"
 #include "SolidSyslogLwipRawAddress.h"
 #include "SolidSyslogLwipRawDatagram.h"
 #include "SolidSyslogLwipRawDnsResolver.h"
@@ -70,7 +70,7 @@ static void LwipTcpipMarshal(SolidSyslogLwipRawCallback callback, void* context)
 static void NetworkBringUp(void* context);
 static void WarmUpGatewayArp(void);
 static void GatewayResolvedQuery(void* context);
-static void GetHostname(struct SolidSyslogFormatter* formatter);
+static void GetHostname(struct SolidSyslogHeaderField* field, void* context);
 static struct SolidSyslogSender* BuildSender(void);
 static void TeardownNetwork(void);
 
@@ -233,12 +233,13 @@ void vApplicationStackOverflowHook(TaskHandle_t task, char* taskName)
     }
 }
 
-static void GetHostname(struct SolidSyslogFormatter* formatter)
+static void GetHostname(struct SolidSyslogHeaderField* field, void* context)
 {
     /* RFC 5424 §6.2.4 rung 2 (static IP) — read back from the netif so a future
      * DHCP slice satisfies the same rung without touching this callback. */
     const char* address = ip4addr_ntoa(netif_ip4_addr(&networkInterface));
-    SolidSyslogFormatter_BoundedString(formatter, address, strlen(address));
+    (void) context;
+    SolidSyslogHeaderField_PrintUsAscii(field, address, strlen(address));
 }
 
 /* Bring up the netif on the tcpip thread, warm the gateway ARP, then build the
diff --git a/Core/Interface/SolidSyslogConfig.h b/Core/Interface/SolidSyslogConfig.h
index a9cd3ffa..e579cdca 100644
--- a/Core/Interface/SolidSyslogConfig.h
+++ b/Core/Interface/SolidSyslogConfig.h
@@ -3,7 +3,7 @@
 
 #include <stddef.h>
 
-#include "SolidSyslogStringFunction.h"
+#include "SolidSyslogHeaderFieldFunction.h"
 #include "SolidSyslogTimestamp.h"
 #include "ExternC.h"
 
@@ -11,7 +11,7 @@ EXTERN_C_BEGIN
 
     struct SolidSyslog;
     struct SolidSyslogBuffer;
-    struct SolidSyslogFormatter;
+    struct SolidSyslogHeaderField;
     struct SolidSyslogSender;
     struct SolidSyslogStore;
     struct SolidSyslogStructuredData;
@@ -21,9 +21,12 @@ EXTERN_C_BEGIN
         struct SolidSyslogBuffer* Buffer;
         struct SolidSyslogSender* Sender;
         SolidSyslogClockFunction Clock;
-        SolidSyslogStringFunction GetHostname;
-        SolidSyslogStringFunction GetAppName;
-        SolidSyslogStringFunction GetProcessId;
+        SolidSyslogHeaderFieldFunction GetHostname;
+        void* GetHostnameContext;
+        SolidSyslogHeaderFieldFunction GetAppName;
+        void* GetAppNameContext;
+        SolidSyslogHeaderFieldFunction GetProcessId;
+        void* GetProcessIdContext;
         struct SolidSyslogStore* Store;
         struct SolidSyslogStructuredData** Sd;
         size_t SdCount;
diff --git a/Core/Interface/SolidSyslogHeaderField.h b/Core/Interface/SolidSyslogHeaderField.h
new file mode 100644
index 00000000..a2942a15
--- /dev/null
+++ b/Core/Interface/SolidSyslogHeaderField.h
@@ -0,0 +1,34 @@
+#ifndef SOLIDSYSLOGHEADERFIELD_H
+#define SOLIDSYSLOGHEADERFIELD_H
+
+#include "ExternC.h"
+
+#include <stddef.h>
+#include <stdint.h>
+
+EXTERN_C_BEGIN
+
+    /* The value sink for an RFC 5424 header field (HOSTNAME / APP-NAME /
+     * PROCID). A field producer is handed only a SolidSyslogHeaderField* — it
+     * can append PRINTUSASCII content (printable US-ASCII; any other byte,
+     * space included, is substituted) bounded to the field width, but cannot
+     * reach the raw formatter. Stack-transient, no pool (D.002). */
+    struct SolidSyslogHeaderField;
+
+    /* Appends up to maxLength bytes of source (stopping at a NUL terminator),
+     * substituting any byte outside printable US-ASCII ('!'..'~') — space
+     * included — with the library's substitute character. Output is further
+     * bounded by the field width the writer was created with. */
+    void SolidSyslogHeaderField_PrintUsAscii(
+        struct SolidSyslogHeaderField * field,
+        const char* source,
+        size_t maxLength
+    );
+
+    /* Appends the decimal digits of value (always printable US-ASCII), bounded
+     * by the field width. */
+    void SolidSyslogHeaderField_Uint32(struct SolidSyslogHeaderField * field, uint32_t value);
+
+EXTERN_C_END
+
+#endif /* SOLIDSYSLOGHEADERFIELD_H */
diff --git a/Core/Interface/SolidSyslogHeaderFieldFunction.h b/Core/Interface/SolidSyslogHeaderFieldFunction.h
new file mode 100644
index 00000000..26c87d71
--- /dev/null
+++ b/Core/Interface/SolidSyslogHeaderFieldFunction.h
@@ -0,0 +1,19 @@
+#ifndef SOLIDSYSLOGHEADERFIELDFUNCTION_H
+#define SOLIDSYSLOGHEADERFIELDFUNCTION_H
+
+#include "ExternC.h"
+
+EXTERN_C_BEGIN
+
+    struct SolidSyslogHeaderField;
+
+    /* Appends an RFC 5424 header-field value (HOSTNAME / APP-NAME / PROCID) into
+     * the field sink it is handed. The library owns the PRINTUSASCII charset
+     * (the sink applies it), so a callback cannot break the header framing.
+     * context is passed through unchanged from the config the callback was
+     * registered in. */
+    typedef void (*SolidSyslogHeaderFieldFunction)(struct SolidSyslogHeaderField* field, void* context);
+
+EXTERN_C_END
+
+#endif /* SOLIDSYSLOGHEADERFIELDFUNCTION_H */
diff --git a/Core/Interface/SolidSyslogStringFunction.h b/Core/Interface/SolidSyslogStringFunction.h
deleted file mode 100644
index 597a1644..00000000
--- a/Core/Interface/SolidSyslogStringFunction.h
+++ /dev/null
@@ -1,14 +0,0 @@
-#ifndef SOLIDSYSLOGSTRINGFUNCTION_H
-#define SOLIDSYSLOGSTRINGFUNCTION_H
-
-#include "ExternC.h"
-
-EXTERN_C_BEGIN
-
-    struct SolidSyslogFormatter;
-
-    typedef void (*SolidSyslogStringFunction)(struct SolidSyslogFormatter* formatter);
-
-EXTERN_C_END
-
-#endif /* SOLIDSYSLOGSTRINGFUNCTION_H */
diff --git a/Core/Source/CMakeLists.txt b/Core/Source/CMakeLists.txt
index 235f9460..2a84eb12 100644
--- a/Core/Source/CMakeLists.txt
+++ b/Core/Source/CMakeLists.txt
@@ -34,6 +34,7 @@ set(SOURCES
     SolidSyslogStructuredData.c
     SolidSyslogSdValue.c
     SolidSyslogSdElement.c
+    SolidSyslogHeaderField.c
     SolidSyslogTimeQualitySd.c
     SolidSyslogTimeQualitySdStatic.c
     SolidSyslogOriginSd.c
diff --git a/Core/Source/SolidSyslog.c b/Core/Source/SolidSyslog.c
index 6e70e8b5..b239eed6 100644
--- a/Core/Source/SolidSyslog.c
+++ b/Core/Source/SolidSyslog.c
@@ -15,27 +15,39 @@
 #include "SolidSyslogNullSender.h"
 #include "SolidSyslogNullStore.h"
 #include "SolidSyslogPrivate.h"
+#include "SolidSyslogHeaderFieldFunction.h"
 #include "SolidSyslogSender.h"
 #include "SolidSyslogStore.h"
-#include "SolidSyslogStringFunction.h"
 #include "SolidSyslogTimestamp.h"
 #include "SolidSyslogTunables.h"
 
 const struct SolidSyslogErrorSource SolidSyslogErrorSource = {"SolidSyslog"};
 
 struct SolidSyslogBuffer;
-struct SolidSyslogFormatter;
+struct SolidSyslogHeaderField;
 struct SolidSyslogSender;
 struct SolidSyslogStore;
 struct SolidSyslogStructuredData;
 
 static inline void SolidSyslog_DrainBufferIntoStore(struct SolidSyslog* self);
 static inline void SolidSyslog_SendOneFromStore(struct SolidSyslog* self);
-static void SolidSyslog_InstallAppName(struct SolidSyslog* self, SolidSyslogStringFunction configured);
+static void SolidSyslog_InstallAppName(
+    struct SolidSyslog* self,
+    SolidSyslogHeaderFieldFunction configured,
+    void* context
+);
 static void SolidSyslog_InstallBuffer(struct SolidSyslog* self, struct SolidSyslogBuffer* configured);
 static void SolidSyslog_InstallClock(struct SolidSyslog* self, SolidSyslogClockFunction configured);
-static void SolidSyslog_InstallHostname(struct SolidSyslog* self, SolidSyslogStringFunction configured);
-static void SolidSyslog_InstallProcessId(struct SolidSyslog* self, SolidSyslogStringFunction configured);
+static void SolidSyslog_InstallHostname(
+    struct SolidSyslog* self,
+    SolidSyslogHeaderFieldFunction configured,
+    void* context
+);
+static void SolidSyslog_InstallProcessId(
+    struct SolidSyslog* self,
+    SolidSyslogHeaderFieldFunction configured,
+    void* context
+);
 static void SolidSyslog_InstallSender(struct SolidSyslog* self, struct SolidSyslogSender* configured);
 static void SolidSyslog_InstallStore(struct SolidSyslog* self, struct SolidSyslogStore* configured);
 static void SolidSyslog_InstallStructuredData(
@@ -54,9 +66,9 @@ void SolidSyslog_Initialise(struct SolidSyslog* self, const struct SolidSyslogCo
     SolidSyslog_InstallSender(self, config->Sender);
     SolidSyslog_InstallStore(self, config->Store);
     SolidSyslog_InstallClock(self, config->Clock);
-    SolidSyslog_InstallHostname(self, config->GetHostname);
-    SolidSyslog_InstallAppName(self, config->GetAppName);
-    SolidSyslog_InstallProcessId(self, config->GetProcessId);
+    SolidSyslog_InstallHostname(self, config->GetHostname, config->GetHostnameContext);
+    SolidSyslog_InstallAppName(self, config->GetAppName, config->GetAppNameContext);
+    SolidSyslog_InstallProcessId(self, config->GetProcessId, config->GetProcessIdContext);
     SolidSyslog_InstallStructuredData(self, config->Sd, config->SdCount);
 }
 
@@ -74,9 +86,12 @@ static void SolidSyslog_ResetToDefaults(struct SolidSyslog* self)
     self->Sender = SolidSyslogNullSender_Get();
     self->Store = SolidSyslogNullStore_Get();
     self->Format.Clock = SolidSyslog_NullClock;
-    self->Format.GetHostname = SolidSyslog_NullStringFunction;
-    self->Format.GetAppName = SolidSyslog_NullStringFunction;
-    self->Format.GetProcessId = SolidSyslog_NullStringFunction;
+    self->Format.GetHostname = SolidSyslog_NullHeaderField;
+    self->Format.GetHostnameContext = NULL;
+    self->Format.GetAppName = SolidSyslog_NullHeaderField;
+    self->Format.GetAppNameContext = NULL;
+    self->Format.GetProcessId = SolidSyslog_NullHeaderField;
+    self->Format.GetProcessIdContext = NULL;
     self->Format.Sd = NULL;
     self->Format.SdCount = 0;
 }
@@ -137,27 +152,42 @@ static void SolidSyslog_InstallClock(struct SolidSyslog* self, SolidSyslogClockF
     }
 }
 
-static void SolidSyslog_InstallHostname(struct SolidSyslog* self, SolidSyslogStringFunction configured)
+static void SolidSyslog_InstallHostname(
+    struct SolidSyslog* self,
+    SolidSyslogHeaderFieldFunction configured,
+    void* context
+)
 {
     if (configured != NULL)
     {
         self->Format.GetHostname = configured;
+        self->Format.GetHostnameContext = context;
     }
 }
 
-static void SolidSyslog_InstallAppName(struct SolidSyslog* self, SolidSyslogStringFunction configured)
+static void SolidSyslog_InstallAppName(
+    struct SolidSyslog* self,
+    SolidSyslogHeaderFieldFunction configured,
+    void* context
+)
 {
     if (configured != NULL)
     {
         self->Format.GetAppName = configured;
+        self->Format.GetAppNameContext = context;
     }
 }
 
-static void SolidSyslog_InstallProcessId(struct SolidSyslog* self, SolidSyslogStringFunction configured)
+static void SolidSyslog_InstallProcessId(
+    struct SolidSyslog* self,
+    SolidSyslogHeaderFieldFunction configured,
+    void* context
+)
 {
     if (configured != NULL)
     {
         self->Format.GetProcessId = configured;
+        self->Format.GetProcessIdContext = context;
     }
 }
 
@@ -288,7 +318,8 @@ void SolidSyslog_NullClock(struct SolidSyslogTimestamp* ts)
     (void) ts;
 }
 
-void SolidSyslog_NullStringFunction(struct SolidSyslogFormatter* formatter)
+void SolidSyslog_NullHeaderField(struct SolidSyslogHeaderField* field, void* context)
 {
-    (void) formatter;
+    (void) field;
+    (void) context;
 }
diff --git a/Core/Source/SolidSyslogHeaderField.c b/Core/Source/SolidSyslogHeaderField.c
new file mode 100644
index 00000000..3ab2aaba
--- /dev/null
+++ b/Core/Source/SolidSyslogHeaderField.c
@@ -0,0 +1,48 @@
+#include "SolidSyslogHeaderFieldPrivate.h"
+
+#include <stddef.h>
+
+#include "SolidSyslogFormatter.h"
+
+static inline size_t HeaderField_Consumed(const struct SolidSyslogHeaderField* field, size_t before);
+
+void SolidSyslogHeaderField_FromFormatter(
+    struct SolidSyslogHeaderField* field,
+    struct SolidSyslogFormatter* formatter,
+    size_t maxLength
+)
+{
+    field->Formatter = formatter;
+    field->Remaining = maxLength;
+}
+
+void SolidSyslogHeaderField_PrintUsAscii(struct SolidSyslogHeaderField* field, const char* source, size_t maxLength)
+{
+    size_t limit = (maxLength < field->Remaining) ? maxLength : field->Remaining;
+    size_t before = SolidSyslogFormatter_Length(field->Formatter);
+
+    SolidSyslogFormatter_PrintUsAsciiString(field->Formatter, source, limit);
+    field->Remaining -= HeaderField_Consumed(field, before);
+}
+
+void SolidSyslogHeaderField_Uint32(struct SolidSyslogHeaderField* field, uint32_t value)
+{
+    /* Unlike _PrintUsAscii, _Uint32 cannot pre-clamp its output to the field
+     * budget (the formatter writes the whole number), so guard the budget
+     * directly: skip once the field is full, and clamp to zero rather than
+     * letting the size_t subtraction underflow when this number alone overruns
+     * the remainder. A header-field callback may mix any number of appends. */
+    if (field->Remaining > 0U)
+    {
+        size_t before = SolidSyslogFormatter_Length(field->Formatter);
+
+        SolidSyslogFormatter_Uint32(field->Formatter, value);
+        size_t consumed = HeaderField_Consumed(field, before);
+        field->Remaining = (consumed >= field->Remaining) ? 0U : (field->Remaining - consumed);
+    }
+}
+
+static inline size_t HeaderField_Consumed(const struct SolidSyslogHeaderField* field, size_t before)
+{
+    return SolidSyslogFormatter_Length(field->Formatter) - before;
+}
diff --git a/Core/Source/SolidSyslogHeaderFieldPrivate.h b/Core/Source/SolidSyslogHeaderFieldPrivate.h
new file mode 100644
index 00000000..df5ef3a3
--- /dev/null
+++ b/Core/Source/SolidSyslogHeaderFieldPrivate.h
@@ -0,0 +1,35 @@
+#ifndef SOLIDSYSLOGHEADERFIELDPRIVATE_H
+#define SOLIDSYSLOGHEADERFIELDPRIVATE_H
+
+#include "ExternC.h"
+
+#include <stddef.h>
+
+#include "SolidSyslogHeaderField.h"
+
+EXTERN_C_BEGIN
+
+    struct SolidSyslogFormatter;
+
+    /* Definition lives here (not the public header) so a field producer handed
+     * a SolidSyslogHeaderField* cannot reach the wrapped formatter. Remaining
+     * is the field-width budget still available across this field's appends. */
+    struct SolidSyslogHeaderField
+    {
+        struct SolidSyslogFormatter* Formatter;
+        size_t Remaining;
+    };
+
+    /* Internal constructor — wraps a message-buffer formatter and caps this
+     * field at maxLength bytes. MessageFormatter builds one per header field
+     * and passes it to the configured callback. Stack-transient: the caller
+     * owns the storage. */
+    void SolidSyslogHeaderField_FromFormatter(
+        struct SolidSyslogHeaderField * field,
+        struct SolidSyslogFormatter * formatter,
+        size_t maxLength
+    );
+
+EXTERN_C_END
+
+#endif /* SOLIDSYSLOGHEADERFIELDPRIVATE_H */
diff --git a/Core/Source/SolidSyslogMessageFormatter.c b/Core/Source/SolidSyslogMessageFormatter.c
index a6bd67d3..d71bff8e 100644
--- a/Core/Source/SolidSyslogMessageFormatter.c
+++ b/Core/Source/SolidSyslogMessageFormatter.c
@@ -6,9 +6,10 @@
 
 #include "SolidSyslog.h"
 #include "SolidSyslogFormatter.h"
+#include "SolidSyslogHeaderFieldFunction.h"
+#include "SolidSyslogHeaderFieldPrivate.h"
 #include "SolidSyslogPrival.h"
 #include "SolidSyslogSdElementPrivate.h"
-#include "SolidSyslogStringFunction.h"
 #include "SolidSyslogStructuredData.h"
 #include "SolidSyslogTimestamp.h"
 #include "SolidSyslogTimestampFormatter.h"
@@ -33,7 +34,8 @@ static inline bool MessageFormatter_SeverityIsValid(uint8_t severity);
 static inline void MessageFormatter_FormatTimestamp(struct SolidSyslogFormatter* f, SolidSyslogClockFunction clock);
 static inline void MessageFormatter_FormatStringField(
     struct SolidSyslogFormatter* f,
-    SolidSyslogStringFunction fn,
+    SolidSyslogHeaderFieldFunction fn,
+    void* context,
     size_t maxSize
 );
 static inline void MessageFormatter_FormatMsgId(struct SolidSyslogFormatter* f, const char* messageId);
@@ -57,11 +59,26 @@ void SolidSyslogMessageFormatter_Format(
     SolidSyslogFormatter_AsciiCharacter(f, ' ');
     MessageFormatter_FormatTimestamp(f, context->Clock);
     SolidSyslogFormatter_AsciiCharacter(f, ' ');
-    MessageFormatter_FormatStringField(f, context->GetHostname, SOLIDSYSLOG_MAX_HOSTNAME_SIZE);
+    MessageFormatter_FormatStringField(
+        f,
+        context->GetHostname,
+        context->GetHostnameContext,
+        SOLIDSYSLOG_MAX_HOSTNAME_SIZE
+    );
     SolidSyslogFormatter_AsciiCharacter(f, ' ');
-    MessageFormatter_FormatStringField(f, context->GetAppName, SOLIDSYSLOG_MAX_APP_NAME_SIZE);
+    MessageFormatter_FormatStringField(
+        f,
+        context->GetAppName,
+        context->GetAppNameContext,
+        SOLIDSYSLOG_MAX_APP_NAME_SIZE
+    );
     SolidSyslogFormatter_AsciiCharacter(f, ' ');
-    MessageFormatter_FormatStringField(f, context->GetProcessId, SOLIDSYSLOG_MAX_PROCESS_ID_SIZE);
+    MessageFormatter_FormatStringField(
+        f,
+        context->GetProcessId,
+        context->GetProcessIdContext,
+        SOLIDSYSLOG_MAX_PROCESS_ID_SIZE
+    );
     SolidSyslogFormatter_AsciiCharacter(f, ' ');
     MessageFormatter_FormatMsgId(f, message->MessageId);
     SolidSyslogFormatter_AsciiCharacter(f, ' ');
@@ -121,22 +138,21 @@ static inline void MessageFormatter_FormatTimestamp(struct SolidSyslogFormatter*
 
 static inline void MessageFormatter_FormatStringField(
     struct SolidSyslogFormatter* f,
-    SolidSyslogStringFunction fn,
+    SolidSyslogHeaderFieldFunction fn,
+    void* context,
     size_t maxSize
 )
 {
-    SolidSyslogFormatterStorage fieldStorage[SOLIDSYSLOG_FORMATTER_STORAGE_SIZE(SOLIDSYSLOG_MAX_HOSTNAME_SIZE)];
-    struct SolidSyslogFormatter* field = SolidSyslogFormatter_Create(fieldStorage, maxSize);
-
-    fn(field);
+    size_t lengthBefore = SolidSyslogFormatter_Length(f);
+    struct SolidSyslogHeaderField field;
 
-    size_t fieldLength = SolidSyslogFormatter_Length(field);
+    /* maxSize is the field's storage size (carries a NUL slot); the usable
+     * field width is one less — matching the RFC HOSTNAME / APP-NAME / PROCID
+     * caps the scratch-field formatter enforced before this writer existed. */
+    SolidSyslogHeaderField_FromFormatter(&field, f, maxSize - 1U);
+    fn(&field, context);
 
-    if (fieldLength > 0U)
-    {
-        SolidSyslogFormatter_PrintUsAsciiString(f, SolidSyslogFormatter_AsFormattedBuffer(field), fieldLength);
-    }
-    else
+    if (SolidSyslogFormatter_Length(f) == lengthBefore)
     {
         SolidSyslogFormatter_NilValue(f);
     }
diff --git a/Core/Source/SolidSyslogMessageFormatter.h b/Core/Source/SolidSyslogMessageFormatter.h
index 5da2f004..a1ff6e09 100644
--- a/Core/Source/SolidSyslogMessageFormatter.h
+++ b/Core/Source/SolidSyslogMessageFormatter.h
@@ -5,7 +5,7 @@
 
 #include <stddef.h>
 
-#include "SolidSyslogStringFunction.h"
+#include "SolidSyslogHeaderFieldFunction.h"
 #include "SolidSyslogTimestamp.h"
 
 EXTERN_C_BEGIN
@@ -20,9 +20,12 @@ EXTERN_C_BEGIN
     struct SolidSyslogMessageFormatterContext
     {
         SolidSyslogClockFunction Clock;
-        SolidSyslogStringFunction GetHostname;
-        SolidSyslogStringFunction GetAppName;
-        SolidSyslogStringFunction GetProcessId;
+        SolidSyslogHeaderFieldFunction GetHostname;
+        void* GetHostnameContext;
+        SolidSyslogHeaderFieldFunction GetAppName;
+        void* GetAppNameContext;
+        SolidSyslogHeaderFieldFunction GetProcessId;
+        void* GetProcessIdContext;
         struct SolidSyslogStructuredData** Sd;
         size_t SdCount;
     };
diff --git a/Core/Source/SolidSyslogPrivate.h b/Core/Source/SolidSyslogPrivate.h
index 66f56653..ce95d061 100644
--- a/Core/Source/SolidSyslogPrivate.h
+++ b/Core/Source/SolidSyslogPrivate.h
@@ -9,7 +9,7 @@
 
 struct SolidSyslogBuffer;
 struct SolidSyslogConfig;
-struct SolidSyslogFormatter;
+struct SolidSyslogHeaderField;
 struct SolidSyslogSender;
 struct SolidSyslogStore;
 
@@ -29,7 +29,7 @@ void SolidSyslog_Cleanup(struct SolidSyslog* self);
  * NullInstance). No public Null equivalent exists for the function-pointer
  * typedefs, so they stay TU-internal across this class. */
 void SolidSyslog_NullClock(struct SolidSyslogTimestamp* ts);
-void SolidSyslog_NullStringFunction(struct SolidSyslogFormatter* formatter);
+void SolidSyslog_NullHeaderField(struct SolidSyslogHeaderField* field, void* context);
 
 static inline void SolidSyslog_Report(enum SolidSyslogSeverity severity, uint16_t category, enum SolidSyslogErrors code)
 {
diff --git a/Core/Source/SolidSyslogStatic.c b/Core/Source/SolidSyslogStatic.c
index 89b7abb7..e16a777d 100644
--- a/Core/Source/SolidSyslogStatic.c
+++ b/Core/Source/SolidSyslogStatic.c
@@ -72,9 +72,12 @@ static void SolidSyslog_EnsureNullInstancePopulated(void)
         SolidSyslog_NullInstance.Sender = SolidSyslogNullSender_Get();
         SolidSyslog_NullInstance.Store = SolidSyslogNullStore_Get();
         SolidSyslog_NullInstance.Format.Clock = SolidSyslog_NullClock;
-        SolidSyslog_NullInstance.Format.GetHostname = SolidSyslog_NullStringFunction;
-        SolidSyslog_NullInstance.Format.GetAppName = SolidSyslog_NullStringFunction;
-        SolidSyslog_NullInstance.Format.GetProcessId = SolidSyslog_NullStringFunction;
+        SolidSyslog_NullInstance.Format.GetHostname = SolidSyslog_NullHeaderField;
+        SolidSyslog_NullInstance.Format.GetHostnameContext = NULL;
+        SolidSyslog_NullInstance.Format.GetAppName = SolidSyslog_NullHeaderField;
+        SolidSyslog_NullInstance.Format.GetAppNameContext = NULL;
+        SolidSyslog_NullInstance.Format.GetProcessId = SolidSyslog_NullHeaderField;
+        SolidSyslog_NullInstance.Format.GetProcessIdContext = NULL;
         SolidSyslog_NullInstance.Format.Sd = NULL;
         SolidSyslog_NullInstance.Format.SdCount = 0;
         populated = true;
diff --git a/DEVLOG.md b/DEVLOG.md
index 275ca80c..b4680980 100644
--- a/DEVLOG.md
+++ b/DEVLOG.md
@@ -14551,3 +14551,56 @@ MISRA rule — different category, doesn't set precedent.
 
 ### Open questions
 - None outstanding.
+
+## 2026-06-06 — S14.07: move header-field callbacks off the formatter
+
+### Decisions
+- **New `SolidSyslogHeaderField` writer** (TDD'd first, 10 tests): opaque,
+  stack-transient US-ASCII sink for the RFC 5424 HOSTNAME / APP-NAME / PROCID
+  fields. Wraps the message-buffer formatter; exposes `_PrintUsAscii(source,
+  maxLength)` (printable-US-ASCII filter, space substituted, NUL-stopped, bounded
+  by maxLength AND the field width) and `_Uint32` (PROCID). A different charset
+  rule from SD escaping → its own type, not `SolidSyslogSdValue`. Name chosen by
+  David over `SolidSyslogToken` / `SolidSyslogField`.
+- **Callbacks flipped** `SolidSyslogStringFunction` → `SolidSyslogHeaderFieldFunction`
+  (`void(SolidSyslogHeaderField*, void* context)`) in its own header, with three
+  paired context fields on `SolidSyslogConfig` + the MessageFormatter context.
+  MessageFormatter builds one HeaderField per field (cap = field-storage size − 1,
+  preserving the old scratch-formatter's null-slot width) and passes it; the
+  scratch-field + re-filter is gone. Byte-identical (existing tests stand).
+- **`SolidSyslogStringFunction.h` retired** — it was the last public consumer of
+  `SolidSyslogFormatter`, clearing the way for S14.08 to make the formatter
+  fully private.
+- **Surprises:** (1) `PosixMessageQueueBuffer` coincidentally reused the PROCID
+  helper to stamp its queue name with the pid — decoupled to a direct `getpid()`
+  + `Formatter_Uint32` rather than route a queue name through the header-field
+  helper (which would have forced a Platform source to include a Core/Source
+  private header). (2) ~16 positional `SolidSyslogConfig{}` test literals
+  silently misaligned once three context fields were interleaved (`store` landing
+  in a `void*` context slot) — fixed by inserting the context nullptrs.
+- Migrated the Posix/Windows hostname & processId helpers + all BDD header-field
+  callbacks (BddTargetAppName, FreeRtos/FreeRtosLwip GetHostname, pipeline
+  GetAppName). Updated the two stale `docs/rfc-compliance.md` `language`
+  references (already wrong since S14.04) to `SolidSyslogSdValueFunction`.
+- Checks: debug green (1469 tests), BddTargetTests 66/66; clang-format applied;
+  cppcheck-MISRA exit 0 (anchors moved: SolidSyslog.c 11.8 ×8, MessageFormatter.c
+  11.8 ×5 + 5.7 20→21; renumber clean).
+
+### Deferred
+- Moving `SolidSyslogFormatter.h` out of the public interface is S14.08, along
+  with the wholesale CLAUDE.md public-header-table revision (the new HeaderField /
+  HeaderFieldFunction rows land there).
+
+### Open questions
+- None outstanding.
+
+### S14.07 follow-up (CodeRabbit review)
+- `SolidSyslogHeaderField_Uint32` could underflow `Remaining` (size_t) if a number
+  overran the field budget. Initially looked unreachable (PROCID = a single ≤10-digit
+  uint32 ≤ 128 cap), but the header-field callbacks are arbitrary user code that may make
+  any number of mixed `_PrintUsAscii` / `_Uint32` appends on one field — so the budget must
+  hold across calls and `_Uint32` must respect it. Added the guard (skip when the field is
+  full; clamp to zero instead of underflowing) + two tests for the mixed-append cap. RFC
+  5424 §6 ABNF is the source of the per-field caps (HOSTNAME 255 / APP-NAME 48 / PROCID 128
+  / MSGID 32), and streaming straight to the message buffer is exactly why the writer must
+  carry the cap explicitly now (the old scratch formatter enforced it via its capacity).
diff --git a/Platform/Posix/Interface/SolidSyslogPosixHostname.h b/Platform/Posix/Interface/SolidSyslogPosixHostname.h
index 44028cb1..3e0f4c68 100644
--- a/Platform/Posix/Interface/SolidSyslogPosixHostname.h
+++ b/Platform/Posix/Interface/SolidSyslogPosixHostname.h
@@ -3,11 +3,11 @@
 
 #include "ExternC.h"
 
-struct SolidSyslogFormatter;
+struct SolidSyslogHeaderField;
 
 EXTERN_C_BEGIN
 
-    void SolidSyslogPosixHostname_Get(struct SolidSyslogFormatter * formatter);
+    void SolidSyslogPosixHostname_Get(struct SolidSyslogHeaderField * field, void* context);
 
 EXTERN_C_END
 
diff --git a/Platform/Posix/Interface/SolidSyslogPosixProcessId.h b/Platform/Posix/Interface/SolidSyslogPosixProcessId.h
index f557982a..deb53145 100644
--- a/Platform/Posix/Interface/SolidSyslogPosixProcessId.h
+++ b/Platform/Posix/Interface/SolidSyslogPosixProcessId.h
@@ -3,11 +3,11 @@
 
 #include "ExternC.h"
 
-struct SolidSyslogFormatter;
+struct SolidSyslogHeaderField;
 
 EXTERN_C_BEGIN
 
-    void SolidSyslogPosixProcessId_Get(struct SolidSyslogFormatter * formatter);
+    void SolidSyslogPosixProcessId_Get(struct SolidSyslogHeaderField * field, void* context);
 
 EXTERN_C_END
 
diff --git a/Platform/Posix/Source/SolidSyslogPosixHostname.c b/Platform/Posix/Source/SolidSyslogPosixHostname.c
index 3a7357d1..93a49138 100644
--- a/Platform/Posix/Source/SolidSyslogPosixHostname.c
+++ b/Platform/Posix/Source/SolidSyslogPosixHostname.c
@@ -2,22 +2,24 @@
 
 #include <unistd.h>
 
-#include "SolidSyslogFormatter.h"
+#include "SolidSyslogHeaderField.h"
 
-struct SolidSyslogFormatter;
+struct SolidSyslogHeaderField;
 
 enum
 {
     MAX_HOSTNAME_SIZE = 256U
 };
 
-void SolidSyslogPosixHostname_Get(struct SolidSyslogFormatter* formatter)
+void SolidSyslogPosixHostname_Get(struct SolidSyslogHeaderField* field, void* context)
 {
     char hostname[MAX_HOSTNAME_SIZE];
 
+    (void) context;
+
     if (gethostname(hostname, sizeof(hostname)) == 0)
     {
         hostname[sizeof(hostname) - 1U] = '\0';
-        SolidSyslogFormatter_PrintUsAsciiString(formatter, hostname, sizeof(hostname));
+        SolidSyslogHeaderField_PrintUsAscii(field, hostname, sizeof(hostname));
     }
 }
diff --git a/Platform/Posix/Source/SolidSyslogPosixMessageQueueBuffer.c b/Platform/Posix/Source/SolidSyslogPosixMessageQueueBuffer.c
index f7bc5dda..0d365bbe 100644
--- a/Platform/Posix/Source/SolidSyslogPosixMessageQueueBuffer.c
+++ b/Platform/Posix/Source/SolidSyslogPosixMessageQueueBuffer.c
@@ -7,6 +7,7 @@
 #include <stddef.h>
 #include <stdint.h>
 #include <sys/types.h>
+#include <unistd.h>
 
 #include "SolidSyslogBufferCategories.h"
 #include "SolidSyslogBufferDefinition.h"
@@ -16,7 +17,6 @@
 #include "SolidSyslogNullBuffer.h"
 #include "SolidSyslogPosixMessageQueueBufferErrors.h"
 #include "SolidSyslogPosixMessageQueueBufferPrivate.h"
-#include "SolidSyslogPosixProcessId.h"
 #include "SolidSyslogPrival.h"
 
 const struct SolidSyslogErrorSource PosixMessageQueueBufferErrorSource = {"PosixMessageQueueBuffer"};
@@ -54,7 +54,7 @@ bool PosixMessageQueueBuffer_Initialise(
     struct SolidSyslogFormatter* name =
         SolidSyslogFormatter_Create(self->NameStorage, POSIX_MESSAGE_QUEUE_BUFFER_MAX_NAME_SIZE);
     SolidSyslogFormatter_BoundedString(name, queueNamePrefix, sizeof(queueNamePrefix) - 1U);
-    SolidSyslogPosixProcessId_Get(name);
+    SolidSyslogFormatter_Uint32(name, (uint32_t) getpid());
     SolidSyslogFormatter_AsciiCharacter(name, '_');
     SolidSyslogFormatter_Uint32(name, (uint32_t) slotIndex);
 
diff --git a/Platform/Posix/Source/SolidSyslogPosixProcessId.c b/Platform/Posix/Source/SolidSyslogPosixProcessId.c
index 6d9a1803..0e437309 100644
--- a/Platform/Posix/Source/SolidSyslogPosixProcessId.c
+++ b/Platform/Posix/Source/SolidSyslogPosixProcessId.c
@@ -3,11 +3,12 @@
 #include <unistd.h>
 #include <stdint.h>
 
-#include "SolidSyslogFormatter.h"
+#include "SolidSyslogHeaderField.h"
 
-struct SolidSyslogFormatter;
+struct SolidSyslogHeaderField;
 
-void SolidSyslogPosixProcessId_Get(struct SolidSyslogFormatter* formatter)
+void SolidSyslogPosixProcessId_Get(struct SolidSyslogHeaderField* field, void* context)
 {
-    SolidSyslogFormatter_Uint32(formatter, (uint32_t) getpid());
+    (void) context;
+    SolidSyslogHeaderField_Uint32(field, (uint32_t) getpid());
 }
diff --git a/Platform/Windows/Interface/SolidSyslogWindowsHostname.h b/Platform/Windows/Interface/SolidSyslogWindowsHostname.h
index 8174ed8f..13ad44cd 100644
--- a/Platform/Windows/Interface/SolidSyslogWindowsHostname.h
+++ b/Platform/Windows/Interface/SolidSyslogWindowsHostname.h
@@ -5,7 +5,7 @@
 
 EXTERN_C_BEGIN
 
-    void SolidSyslogWindowsHostname_Get(struct SolidSyslogFormatter * formatter);
+    void SolidSyslogWindowsHostname_Get(struct SolidSyslogHeaderField * field, void* context);
 
 EXTERN_C_END
 
diff --git a/Platform/Windows/Interface/SolidSyslogWindowsProcessId.h b/Platform/Windows/Interface/SolidSyslogWindowsProcessId.h
index 81bee13b..58b5f444 100644
--- a/Platform/Windows/Interface/SolidSyslogWindowsProcessId.h
+++ b/Platform/Windows/Interface/SolidSyslogWindowsProcessId.h
@@ -5,7 +5,7 @@
 
 EXTERN_C_BEGIN
 
-    void SolidSyslogWindowsProcessId_Get(struct SolidSyslogFormatter * formatter);
+    void SolidSyslogWindowsProcessId_Get(struct SolidSyslogHeaderField * field, void* context);
 
 EXTERN_C_END
 
diff --git a/Platform/Windows/Source/SolidSyslogWindowsHostname.c b/Platform/Windows/Source/SolidSyslogWindowsHostname.c
index ac4af1c4..e933b122 100644
--- a/Platform/Windows/Source/SolidSyslogWindowsHostname.c
+++ b/Platform/Windows/Source/SolidSyslogWindowsHostname.c
@@ -1,5 +1,5 @@
 #include "SolidSyslogWindowsHostname.h"
-#include "SolidSyslogFormatter.h"
+#include "SolidSyslogHeaderField.h"
 #include "SolidSyslogWindowsHostnameInternal.h"
 
 /* File-local forwarder. Taking the address of an imported Windows API
@@ -20,14 +20,16 @@ enum
     MAX_HOSTNAME_SIZE = 256U
 };
 
-void SolidSyslogWindowsHostname_Get(struct SolidSyslogFormatter* formatter)
+void SolidSyslogWindowsHostname_Get(struct SolidSyslogHeaderField* field, void* context)
 {
     char hostname[MAX_HOSTNAME_SIZE];
     DWORD size = sizeof(hostname);
 
+    (void) context;
+
     if (WindowsHostname_GetComputerNameExA(ComputerNamePhysicalDnsHostname, hostname, &size) != FALSE)
     {
         hostname[sizeof(hostname) - 1U] = '\0';
-        SolidSyslogFormatter_PrintUsAsciiString(formatter, hostname, sizeof(hostname));
+        SolidSyslogHeaderField_PrintUsAscii(field, hostname, sizeof(hostname));
     }
 }
diff --git a/Platform/Windows/Source/SolidSyslogWindowsProcessId.c b/Platform/Windows/Source/SolidSyslogWindowsProcessId.c
index d9a5d8f7..8e29d37b 100644
--- a/Platform/Windows/Source/SolidSyslogWindowsProcessId.c
+++ b/Platform/Windows/Source/SolidSyslogWindowsProcessId.c
@@ -1,5 +1,5 @@
 #include "SolidSyslogWindowsProcessId.h"
-#include "SolidSyslogFormatter.h"
+#include "SolidSyslogHeaderField.h"
 #include "SolidSyslogWindowsProcessIdInternal.h"
 
 #include <stdint.h>
@@ -17,7 +17,8 @@ static DWORD WINAPI WindowsProcessId_CallGetCurrentProcessId(void)
     return GetCurrentProcessId();
 }
 
-void SolidSyslogWindowsProcessId_Get(struct SolidSyslogFormatter* formatter)
+void SolidSyslogWindowsProcessId_Get(struct SolidSyslogHeaderField* field, void* context)
 {
-    SolidSyslogFormatter_Uint32(formatter, (uint32_t) WindowsProcessId_GetCurrentProcessId());
+    (void) context;
+    SolidSyslogHeaderField_Uint32(field, (uint32_t) WindowsProcessId_GetCurrentProcessId());
 }
diff --git a/Tests/Bdd/Targets/BddTargetAppNameTest.cpp b/Tests/Bdd/Targets/BddTargetAppNameTest.cpp
index 94ad9a00..6c7aeebf 100644
--- a/Tests/Bdd/Targets/BddTargetAppNameTest.cpp
+++ b/Tests/Bdd/Targets/BddTargetAppNameTest.cpp
@@ -1,5 +1,6 @@
 #include "BddTargetAppName.h"
 #include "SolidSyslogFormatter.h"
+#include "SolidSyslogHeaderFieldPrivate.h"
 #include "CppUTest/TestHarness.h"
 
 enum
@@ -12,10 +13,12 @@ TEST_GROUP(BddTargetAppName)
 {
     SolidSyslogFormatterStorage storage[SOLIDSYSLOG_FORMATTER_STORAGE_SIZE(FORMATTER_BUFFER_SIZE)];
     struct SolidSyslogFormatter* formatter = nullptr;
+    struct SolidSyslogHeaderField field{};
 
     void setup() override
     {
         formatter = SolidSyslogFormatter_Create(storage, FORMATTER_BUFFER_SIZE);
+        SolidSyslogHeaderField_FromFormatter(&field, formatter, FORMATTER_BUFFER_SIZE);
     }
 
     [[nodiscard]] const char* formatted() const
@@ -29,55 +32,55 @@ TEST_GROUP(BddTargetAppName)
 TEST(BddTargetAppName, BackslashSeparatorExtractsBaseName)
 {
     BddTargetAppName_Set("dir\\binary");
-    BddTargetAppName_Get(formatter);
+    BddTargetAppName_Get(&field, nullptr);
     STRCMP_EQUAL("binary", formatted());
 }
 
 TEST(BddTargetAppName, ForwardSlashSeparatorExtractsBaseName)
 {
     BddTargetAppName_Set("/usr/local/bin/example");
-    BddTargetAppName_Get(formatter);
+    BddTargetAppName_Get(&field, nullptr);
     STRCMP_EQUAL("example", formatted());
 }
 
 TEST(BddTargetAppName, NoSeparatorReturnsWholeArgument)
 {
     BddTargetAppName_Set("example");
-    BddTargetAppName_Get(formatter);
+    BddTargetAppName_Get(&field, nullptr);
     STRCMP_EQUAL("example", formatted());
 }
 
 TEST(BddTargetAppName, MixedSeparatorsUseRightmost)
 {
     BddTargetAppName_Set("C:\\msys64\\home/user/example");
-    BddTargetAppName_Get(formatter);
+    BddTargetAppName_Get(&field, nullptr);
     STRCMP_EQUAL("example", formatted());
 }
 
 TEST(BddTargetAppName, ExeExtensionIsStripped)
 {
     BddTargetAppName_Set("app.exe");
-    BddTargetAppName_Get(formatter);
+    BddTargetAppName_Get(&field, nullptr);
     STRCMP_EQUAL("app", formatted());
 }
 
 TEST(BddTargetAppName, UpperCaseExeExtensionIsStripped)
 {
     BddTargetAppName_Set("APP.EXE");
-    BddTargetAppName_Get(formatter);
+    BddTargetAppName_Get(&field, nullptr);
     STRCMP_EQUAL("APP", formatted());
 }
 
 TEST(BddTargetAppName, ExeWithPathSeparatorIsStripped)
 {
     BddTargetAppName_Set("C:\\bin\\SolidSyslogBddTarget.exe");
-    BddTargetAppName_Get(formatter);
+    BddTargetAppName_Get(&field, nullptr);
     STRCMP_EQUAL("SolidSyslogBddTarget", formatted());
 }
 
 TEST(BddTargetAppName, NonExeExtensionIsKept)
 {
     BddTargetAppName_Set("data.txt");
-    BddTargetAppName_Get(formatter);
+    BddTargetAppName_Get(&field, nullptr);
     STRCMP_EQUAL("data.txt", formatted());
 }
diff --git a/Tests/Bdd/Targets/BddTargetServiceThreadTest.cpp b/Tests/Bdd/Targets/BddTargetServiceThreadTest.cpp
index 1a1ebf2d..dc645efa 100644
--- a/Tests/Bdd/Targets/BddTargetServiceThreadTest.cpp
+++ b/Tests/Bdd/Targets/BddTargetServiceThreadTest.cpp
@@ -76,7 +76,8 @@ TEST_GROUP(BddTargetServiceThread)
         buffer = SolidSyslogPosixMessageQueueBuffer_Create(SOLIDSYSLOG_MAX_MESSAGE_SIZE, 10);
         store  = SolidSyslogNullStore_Get();
 
-        SolidSyslogConfig config = {buffer, sender, nullptr, nullptr, nullptr, nullptr, store, nullptr, 0};
+        SolidSyslogConfig config =
+            {buffer, sender, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, store, nullptr, 0};
         solidSyslog = SolidSyslog_Create(&config);
     }
 
diff --git a/Tests/CMakeLists.txt b/Tests/CMakeLists.txt
index a7c01ceb..5f38639a 100644
--- a/Tests/CMakeLists.txt
+++ b/Tests/CMakeLists.txt
@@ -51,6 +51,7 @@ set(TEST_SOURCES
     SolidSyslogNullSdTest.cpp
     SolidSyslogSdValueTest.cpp
     SolidSyslogSdElementTest.cpp
+    SolidSyslogHeaderFieldTest.cpp
     SolidSyslogNullSenderTest.cpp
     SolidSyslogNullStoreTest.cpp
     SolidSyslogNullSecurityPolicyTest.cpp
diff --git a/Tests/SolidSyslogHeaderFieldTest.cpp b/Tests/SolidSyslogHeaderFieldTest.cpp
new file mode 100644
index 00000000..cc69d73f
--- /dev/null
+++ b/Tests/SolidSyslogHeaderFieldTest.cpp
@@ -0,0 +1,120 @@
+#include <stdint.h>
+
+#include "CppUTest/TestHarness.h"
+#include "SolidSyslogFormatter.h"
+#include "SolidSyslogHeaderField.h"
+#include "SolidSyslogHeaderFieldPrivate.h"
+
+enum
+{
+    TEST_BUFFER_SIZE = 64
+};
+
+#define CHECK_FIELD(expected) STRCMP_EQUAL(expected, SolidSyslogFormatter_AsFormattedBuffer(formatter))
+
+// clang-format off
+TEST_GROUP(SolidSyslogHeaderField)
+{
+    SolidSyslogFormatterStorage storage[SOLIDSYSLOG_FORMATTER_STORAGE_SIZE(TEST_BUFFER_SIZE)];
+    struct SolidSyslogFormatter* formatter = nullptr;
+    struct SolidSyslogHeaderField field{};
+
+    void setup() override
+    {
+        formatter = SolidSyslogFormatter_Create(storage, TEST_BUFFER_SIZE);
+    }
+
+    void fromFormatter(size_t maxLength)
+    {
+        SolidSyslogHeaderField_FromFormatter(&field, formatter, maxLength);
+    }
+};
+
+// clang-format on
+
+TEST(SolidSyslogHeaderField, PrintUsAsciiOfEmptyStringWritesNothing)
+{
+    fromFormatter(32);
+    SolidSyslogHeaderField_PrintUsAscii(&field, "", 32);
+    CHECK_FIELD("");
+}
+
+TEST(SolidSyslogHeaderField, PrintUsAsciiAppendsPrintableAscii)
+{
+    fromFormatter(32);
+    SolidSyslogHeaderField_PrintUsAscii(&field, "host01", 32);
+    CHECK_FIELD("host01");
+}
+
+TEST(SolidSyslogHeaderField, PrintUsAsciiSubstitutesControlCharacter)
+{
+    fromFormatter(32);
+    SolidSyslogHeaderField_PrintUsAscii(&field, "a\tb", 32);
+    CHECK_FIELD("a?b");
+}
+
+TEST(SolidSyslogHeaderField, PrintUsAsciiSubstitutesSpace)
+{
+    fromFormatter(32);
+    SolidSyslogHeaderField_PrintUsAscii(&field, "a b", 32);
+    CHECK_FIELD("a?b");
+}
+
+TEST(SolidSyslogHeaderField, PrintUsAsciiStopsAtNul)
+{
+    fromFormatter(32);
+    SolidSyslogHeaderField_PrintUsAscii(&field, "ab\0cd", 5);
+    CHECK_FIELD("ab");
+}
+
+TEST(SolidSyslogHeaderField, PrintUsAsciiTruncatesAtMaxLength)
+{
+    fromFormatter(32);
+    SolidSyslogHeaderField_PrintUsAscii(&field, "abcdef", 3);
+    CHECK_FIELD("abc");
+}
+
+TEST(SolidSyslogHeaderField, PrintUsAsciiTruncatesAtFieldWidth)
+{
+    fromFormatter(3);
+    SolidSyslogHeaderField_PrintUsAscii(&field, "abcdef", 32);
+    CHECK_FIELD("abc");
+}
+
+TEST(SolidSyslogHeaderField, PrintUsAsciiBudgetIsSharedAcrossCalls)
+{
+    fromFormatter(4);
+    SolidSyslogHeaderField_PrintUsAscii(&field, "ab", 32);
+    SolidSyslogHeaderField_PrintUsAscii(&field, "cdef", 32);
+    CHECK_FIELD("abcd");
+}
+
+TEST(SolidSyslogHeaderField, Uint32AppendsDigits)
+{
+    fromFormatter(32);
+    SolidSyslogHeaderField_Uint32(&field, 1234);
+    CHECK_FIELD("1234");
+}
+
+TEST(SolidSyslogHeaderField, Uint32OfZeroAppendsZero)
+{
+    fromFormatter(32);
+    SolidSyslogHeaderField_Uint32(&field, 0);
+    CHECK_FIELD("0");
+}
+
+TEST(SolidSyslogHeaderField, Uint32IsDroppedOnceFieldWidthIsExhausted)
+{
+    fromFormatter(2);
+    SolidSyslogHeaderField_PrintUsAscii(&field, "ab", 32);
+    SolidSyslogHeaderField_Uint32(&field, 9);
+    CHECK_FIELD("ab");
+}
+
+TEST(SolidSyslogHeaderField, Uint32ExhaustsBudgetSoLaterAppendsAreDropped)
+{
+    fromFormatter(3);
+    SolidSyslogHeaderField_Uint32(&field, 123);
+    SolidSyslogHeaderField_PrintUsAscii(&field, "x", 32);
+    CHECK_FIELD("123");
+}
diff --git a/Tests/SolidSyslogMessageFormatterTest.cpp b/Tests/SolidSyslogMessageFormatterTest.cpp
index af010cec..b40411fd 100644
--- a/Tests/SolidSyslogMessageFormatterTest.cpp
+++ b/Tests/SolidSyslogMessageFormatterTest.cpp
@@ -43,9 +43,12 @@ TEST_GROUP(SolidSyslogMessageFormatter)
         context = {
             MessageFormatterTestClock,
             StringFake_GetHostname,
+            nullptr,
             StringFake_GetAppName,
+            nullptr,
             StringFake_GetProcessId,
             nullptr,
+            nullptr,
             0
         };
         message = {SOLIDSYSLOG_FACILITY_LOCAL0, SOLIDSYSLOG_SEVERITY_INFORMATIONAL, nullptr, nullptr};
diff --git a/Tests/SolidSyslogPoolTest.cpp b/Tests/SolidSyslogPoolTest.cpp
index 95b023f7..5135520a 100644
--- a/Tests/SolidSyslogPoolTest.cpp
+++ b/Tests/SolidSyslogPoolTest.cpp
@@ -40,7 +40,7 @@ TEST_GROUP(SolidSyslogPool)
     {
         fakeSender = SenderFake_Create();
         buffer = SolidSyslogPassthroughBuffer_Create(fakeSender);
-        config = {buffer, fakeSender, nullptr, nullptr, nullptr, nullptr,
+        config = {buffer, fakeSender, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr,
                   SolidSyslogNullStore_Get(), nullptr, 0};
     }
 
diff --git a/Tests/SolidSyslogPosixMessageQueueBufferTest.cpp b/Tests/SolidSyslogPosixMessageQueueBufferTest.cpp
index 26c3109d..0b49000f 100644
--- a/Tests/SolidSyslogPosixMessageQueueBufferTest.cpp
+++ b/Tests/SolidSyslogPosixMessageQueueBufferTest.cpp
@@ -188,7 +188,8 @@ TEST(SolidSyslogPosixMessageQueueBuffer, ServiceSendsMessageWrittenViaLog)
 {
     struct SolidSyslogSender* fakeSender = SenderFake_Create();
     SolidSyslogStore* nullStore = SolidSyslogNullStore_Get();
-    SolidSyslogConfig config = {buffer, fakeSender, nullptr, nullptr, nullptr, nullptr, nullStore, nullptr, 0};
+    SolidSyslogConfig config =
+        {buffer, fakeSender, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullStore, nullptr, 0};
     struct SolidSyslog* solidSyslog = SolidSyslog_Create(&config);
 
     SolidSyslogMessage message = {SOLIDSYSLOG_FACILITY_LOCAL0, SOLIDSYSLOG_SEVERITY_INFORMATIONAL, nullptr, nullptr};
diff --git a/Tests/SolidSyslogTest.cpp b/Tests/SolidSyslogTest.cpp
index b20533ed..499163c1 100644
--- a/Tests/SolidSyslogTest.cpp
+++ b/Tests/SolidSyslogTest.cpp
@@ -112,7 +112,7 @@ TEST_GROUP(SolidSyslog)
         metaSdCounter = nullptr;
         metaSd        = nullptr;
         timeQualitySd = nullptr;
-        config = {buffer, nullptr, nullptr, StringFake_GetHostname, StringFake_GetAppName, StringFake_GetProcessId, store, nullptr, 0};
+        config = {buffer, nullptr, nullptr, StringFake_GetHostname, nullptr, StringFake_GetAppName, nullptr, StringFake_GetProcessId, nullptr, store, nullptr, 0};
         solidSyslog = SolidSyslog_Create(&config);
         message = {SOLIDSYSLOG_FACILITY_LOCAL0, SOLIDSYSLOG_SEVERITY_INFORMATIONAL, nullptr, nullptr};
     }
@@ -478,7 +478,8 @@ TEST(SolidSyslogTimestamp, TimestampAppearsInCorrectMessageFieldPosition)
 TEST(SolidSyslog, ServiceSendsMessageReadFromBuffer)
 {
     SolidSyslogBuffer* fakeBuffer = BufferFake_Create();
-    SolidSyslogConfig serviceConfig = {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, store, nullptr, 0};
+    SolidSyslogConfig serviceConfig =
+        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, store, nullptr, 0};
 
     SolidSyslog_Destroy(solidSyslog);
     solidSyslog = SolidSyslog_Create(&serviceConfig);
@@ -500,7 +501,7 @@ TEST(SolidSyslog, ServiceSendsBufferedMessageWithNullStore)
     SolidSyslogBuffer* fakeBuffer = BufferFake_Create();
     SolidSyslogStore* nullStore = SolidSyslogNullStore_Get();
     SolidSyslogConfig serviceConfig =
-        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, nullStore, nullptr, 0};
+        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullStore, nullptr, 0};
 
     SolidSyslog_Destroy(solidSyslog);
     solidSyslog = SolidSyslog_Create(&serviceConfig);
@@ -522,7 +523,7 @@ TEST(SolidSyslog, ServiceSendsFromStoreWhenHasUnsent)
     SolidSyslogBuffer* fakeBuffer = BufferFake_Create();
     SolidSyslogStore* fakeStore = StoreFake_Create();
     SolidSyslogConfig serviceConfig =
-        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
+        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
 
     SolidSyslog_Destroy(solidSyslog);
     solidSyslog = SolidSyslog_Create(&serviceConfig);
@@ -545,7 +546,7 @@ TEST(SolidSyslog, ServiceMarksSentAfterSuccessfulSend)
     SolidSyslogBuffer* fakeBuffer = BufferFake_Create();
     SolidSyslogStore* fakeStore = StoreFake_Create();
     SolidSyslogConfig serviceConfig =
-        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
+        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
 
     SolidSyslog_Destroy(solidSyslog);
     solidSyslog = SolidSyslog_Create(&serviceConfig);
@@ -566,7 +567,7 @@ TEST(SolidSyslog, ServiceDoesNotMarkSentOnSendFailure)
     SolidSyslogBuffer* fakeBuffer = BufferFake_Create();
     SolidSyslogStore* fakeStore = StoreFake_Create();
     SolidSyslogConfig serviceConfig =
-        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
+        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
 
     SolidSyslog_Destroy(solidSyslog);
     solidSyslog = SolidSyslog_Create(&serviceConfig);
@@ -588,7 +589,7 @@ TEST(SolidSyslog, ServiceWritesBufferMessageToStore)
     SolidSyslogBuffer* fakeBuffer = BufferFake_Create();
     SolidSyslogStore* fakeStore = StoreFake_Create();
     SolidSyslogConfig serviceConfig =
-        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
+        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
 
     SolidSyslog_Destroy(solidSyslog);
     solidSyslog = SolidSyslog_Create(&serviceConfig);
@@ -614,7 +615,7 @@ TEST(SolidSyslog, ServiceSendsStoreMessageNotBufferMessage)
     SolidSyslogBuffer* fakeBuffer = BufferFake_Create();
     SolidSyslogStore* fakeStore = StoreFake_Create();
     SolidSyslogConfig serviceConfig =
-        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
+        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
 
     SolidSyslog_Destroy(solidSyslog);
     solidSyslog = SolidSyslog_Create(&serviceConfig);
@@ -641,7 +642,7 @@ TEST(SolidSyslog, ServiceDoesNotBypassToSenderWhenNonTransientStoreRejectsWrite)
     SolidSyslogBuffer* fakeBuffer = BufferFake_Create();
     SolidSyslogStore* fakeStore = StoreFake_Create();
     SolidSyslogConfig serviceConfig =
-        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
+        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
 
     SolidSyslog_Destroy(solidSyslog);
     solidSyslog = SolidSyslog_Create(&serviceConfig);
@@ -664,7 +665,7 @@ TEST(SolidSyslog, ServiceDoesNotSendWhenStoreReadFails)
     SolidSyslogBuffer* fakeBuffer = BufferFake_Create();
     SolidSyslogStore* fakeStore = StoreFake_Create();
     SolidSyslogConfig serviceConfig =
-        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
+        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
 
     SolidSyslog_Destroy(solidSyslog);
     solidSyslog = SolidSyslog_Create(&serviceConfig);
@@ -687,7 +688,7 @@ TEST(SolidSyslog, ServiceDoesNotMarkSentWhenSendingFromBuffer)
     SolidSyslogBuffer* fakeBuffer = BufferFake_Create();
     SolidSyslogStore* fakeStore = StoreFake_Create();
     SolidSyslogConfig serviceConfig =
-        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
+        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
 
     SolidSyslog_Destroy(solidSyslog);
     solidSyslog = SolidSyslog_Create(&serviceConfig);
@@ -780,7 +781,7 @@ TEST(SolidSyslog, ServiceDoesNothingWhenStoreIsHalted)
     SolidSyslogBuffer* fakeBuffer = BufferFake_Create();
     SolidSyslogStore* fakeStore = StoreFake_Create();
     SolidSyslogConfig serviceConfig =
-        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
+        {fakeBuffer, fakeSender, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, fakeStore, nullptr, 0};
 
     SolidSyslog_Destroy(solidSyslog);
     solidSyslog = SolidSyslog_Create(&serviceConfig);
@@ -801,7 +802,8 @@ TEST(SolidSyslog, ServiceDoesNothingWhenStoreIsHalted)
 TEST(SolidSyslog, LogAfterDestroyAndRecreateWithNullFunctionsProducesNilvalues)
 {
     SolidSyslog_Destroy(solidSyslog);
-    SolidSyslogConfig nilConfig = {buffer, nullptr, nullptr, nullptr, nullptr, nullptr, store, nullptr, 0};
+    SolidSyslogConfig nilConfig =
+        {buffer, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, store, nullptr, 0};
     solidSyslog = SolidSyslog_Create(&nilConfig);
     Log();
     CHECK_TIMESTAMP_IS_NILVALUE();
@@ -840,7 +842,7 @@ TEST_GROUP(SolidSyslogLifecycle)
 
     [[nodiscard]] SolidSyslogConfig validConfig() const
     {
-        return {buffer, sender, nullptr, nullptr, nullptr, nullptr, store, nullptr, 0};
+        return {buffer, sender, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, store, nullptr, 0};
     }
 };
 
diff --git a/Tests/SolidSyslogWindowsHostnameTest.cpp b/Tests/SolidSyslogWindowsHostnameTest.cpp
index 3e38c86d..6677b745 100644
--- a/Tests/SolidSyslogWindowsHostnameTest.cpp
+++ b/Tests/SolidSyslogWindowsHostnameTest.cpp
@@ -1,5 +1,6 @@
 #include "CppUTest/TestHarness.h"
 #include "SolidSyslogFormatter.h"
+#include "SolidSyslogHeaderFieldPrivate.h"
 #include "SolidSyslogWindowsHostname.h"
 #include "SolidSyslogWindowsHostnameInternal.h"
 
@@ -38,6 +39,7 @@ TEST_GROUP(SolidSyslogWindowsHostname)
 {
     SolidSyslogFormatterStorage storage[SOLIDSYSLOG_FORMATTER_STORAGE_SIZE(FORMATTER_BUFFER_SIZE)];
     struct SolidSyslogFormatter* formatter = nullptr;
+    struct SolidSyslogHeaderField field{};
 
     void setup() override
     {
@@ -45,6 +47,7 @@ TEST_GROUP(SolidSyslogWindowsHostname)
         fakeReturnValue = TRUE;
         UT_PTR_SET(WindowsHostname_GetComputerNameExA, FakeGetComputerNameExA);
         formatter = SolidSyslogFormatter_Create(storage, FORMATTER_BUFFER_SIZE);
+        SolidSyslogHeaderField_FromFormatter(&field, formatter, FORMATTER_BUFFER_SIZE);
     }
 
     const char* formatted() const
@@ -57,21 +60,21 @@ TEST_GROUP(SolidSyslogWindowsHostname)
 
 TEST(SolidSyslogWindowsHostname, WritesFakeHostnameIntoFormatter)
 {
-    SolidSyslogWindowsHostname_Get(formatter);
+    SolidSyslogWindowsHostname_Get(&field, nullptr);
     STRCMP_EQUAL("winhost", formatted());
 }
 
 TEST(SolidSyslogWindowsHostname, WritesNothingWhenApiFails)
 {
     fakeReturnValue = FALSE;
-    SolidSyslogWindowsHostname_Get(formatter);
+    SolidSyslogWindowsHostname_Get(&field, nullptr);
     STRCMP_EQUAL("", formatted());
 }
 
 TEST(SolidSyslogWindowsHostname, EmptyHostnameProducesEmptyString)
 {
     fakeHostname = "";
-    SolidSyslogWindowsHostname_Get(formatter);
+    SolidSyslogWindowsHostname_Get(&field, nullptr);
     STRCMP_EQUAL("", formatted());
 }
 
@@ -83,6 +86,6 @@ TEST(SolidSyslogWindowsHostname, HostnameTooLongForBufferProducesEmptyString)
     memset(longName, 'x', 260);
     longName[260] = '\0';
     fakeHostname = longName;
-    SolidSyslogWindowsHostname_Get(formatter);
+    SolidSyslogWindowsHostname_Get(&field, nullptr);
     STRCMP_EQUAL("", formatted());
 }
diff --git a/Tests/SolidSyslogWindowsProcessIdTest.cpp b/Tests/SolidSyslogWindowsProcessIdTest.cpp
index 8f9b4787..7327de45 100644
--- a/Tests/SolidSyslogWindowsProcessIdTest.cpp
+++ b/Tests/SolidSyslogWindowsProcessIdTest.cpp
@@ -1,5 +1,6 @@
 #include "CppUTest/TestHarness.h"
 #include "SolidSyslogFormatter.h"
+#include "SolidSyslogHeaderFieldPrivate.h"
 #include "SolidSyslogWindowsProcessId.h"
 #include "SolidSyslogWindowsProcessIdInternal.h"
 
@@ -22,12 +23,14 @@ TEST_GROUP(SolidSyslogWindowsProcessId)
 {
     SolidSyslogFormatterStorage storage[SOLIDSYSLOG_FORMATTER_STORAGE_SIZE(FORMATTER_BUFFER_SIZE)];
     struct SolidSyslogFormatter* formatter = nullptr;
+    struct SolidSyslogHeaderField field{};
 
     void setup() override
     {
         fakePid = 4321;
         UT_PTR_SET(WindowsProcessId_GetCurrentProcessId, FakeGetCurrentProcessId);
         formatter = SolidSyslogFormatter_Create(storage, FORMATTER_BUFFER_SIZE);
+        SolidSyslogHeaderField_FromFormatter(&field, formatter, FORMATTER_BUFFER_SIZE);
     }
 
     const char* formatted() const
@@ -40,20 +43,20 @@ TEST_GROUP(SolidSyslogWindowsProcessId)
 
 TEST(SolidSyslogWindowsProcessId, WritesFakePidAsDecimal)
 {
-    SolidSyslogWindowsProcessId_Get(formatter);
+    SolidSyslogWindowsProcessId_Get(&field, nullptr);
     STRCMP_EQUAL("4321", formatted());
 }
 
 TEST(SolidSyslogWindowsProcessId, WritesZeroWhenPidIsZero)
 {
     fakePid = 0;
-    SolidSyslogWindowsProcessId_Get(formatter);
+    SolidSyslogWindowsProcessId_Get(&field, nullptr);
     STRCMP_EQUAL("0", formatted());
 }
 
 TEST(SolidSyslogWindowsProcessId, WritesMaxDwordValueAsDecimal)
 {
     fakePid = 0xFFFFFFFFU;
-    SolidSyslogWindowsProcessId_Get(formatter);
+    SolidSyslogWindowsProcessId_Get(&field, nullptr);
     STRCMP_EQUAL("4294967295", formatted());
 }
diff --git a/Tests/StringFake.c b/Tests/StringFake.c
index 85fd6bb1..a6a89236 100644
--- a/Tests/StringFake.c
+++ b/Tests/StringFake.c
@@ -2,9 +2,9 @@
 
 #include <string.h>
 
-#include "SolidSyslogFormatter.h"
+#include "SolidSyslogHeaderField.h"
 
-struct SolidSyslogFormatter;
+struct SolidSyslogHeaderField;
 
 static const char* fakeHostname;
 static const char* fakeAppName;
@@ -22,9 +22,10 @@ void StringFake_SetHostname(const char* hostname)
     fakeHostname = hostname;
 }
 
-void StringFake_GetHostname(struct SolidSyslogFormatter* formatter)
+void StringFake_GetHostname(struct SolidSyslogHeaderField* field, void* context)
 {
-    SolidSyslogFormatter_PrintUsAsciiString(formatter, fakeHostname, strlen(fakeHostname));
+    (void) context;
+    SolidSyslogHeaderField_PrintUsAscii(field, fakeHostname, strlen(fakeHostname));
 }
 
 void StringFake_SetAppName(const char* appName)
@@ -32,9 +33,10 @@ void StringFake_SetAppName(const char* appName)
     fakeAppName = appName;
 }
 
-void StringFake_GetAppName(struct SolidSyslogFormatter* formatter)
+void StringFake_GetAppName(struct SolidSyslogHeaderField* field, void* context)
 {
-    SolidSyslogFormatter_PrintUsAsciiString(formatter, fakeAppName, strlen(fakeAppName));
+    (void) context;
+    SolidSyslogHeaderField_PrintUsAscii(field, fakeAppName, strlen(fakeAppName));
 }
 
 void StringFake_SetProcessId(const char* procId)
@@ -42,7 +44,8 @@ void StringFake_SetProcessId(const char* procId)
     fakeProcessId = procId;
 }
 
-void StringFake_GetProcessId(struct SolidSyslogFormatter* formatter)
+void StringFake_GetProcessId(struct SolidSyslogHeaderField* field, void* context)
 {
-    SolidSyslogFormatter_PrintUsAsciiString(formatter, fakeProcessId, strlen(fakeProcessId));
+    (void) context;
+    SolidSyslogHeaderField_PrintUsAscii(field, fakeProcessId, strlen(fakeProcessId));
 }
diff --git a/Tests/StringFake.h b/Tests/StringFake.h
index 90b8b2e5..5b862483 100644
--- a/Tests/StringFake.h
+++ b/Tests/StringFake.h
@@ -3,17 +3,17 @@
 
 #include "ExternC.h"
 
-struct SolidSyslogFormatter;
+struct SolidSyslogHeaderField;
 
 EXTERN_C_BEGIN
 
     void StringFake_Reset(void);
     void StringFake_SetHostname(const char* hostname);
-    void StringFake_GetHostname(struct SolidSyslogFormatter * formatter);
+    void StringFake_GetHostname(struct SolidSyslogHeaderField * field, void* context);
     void StringFake_SetAppName(const char* appName);
-    void StringFake_GetAppName(struct SolidSyslogFormatter * formatter);
+    void StringFake_GetAppName(struct SolidSyslogHeaderField * field, void* context);
     void StringFake_SetProcessId(const char* procId);
-    void StringFake_GetProcessId(struct SolidSyslogFormatter * formatter);
+    void StringFake_GetProcessId(struct SolidSyslogHeaderField * field, void* context);
 
 EXTERN_C_END
 
diff --git a/Tests/StringFakeTest.cpp b/Tests/StringFakeTest.cpp
index 2dd61d2f..386070ac 100644
--- a/Tests/StringFakeTest.cpp
+++ b/Tests/StringFakeTest.cpp
@@ -1,4 +1,5 @@
 #include "SolidSyslogFormatter.h"
+#include "SolidSyslogHeaderFieldPrivate.h"
 #include "StringFake.h"
 #include "CppUTest/TestHarness.h"
 
@@ -14,10 +15,12 @@ TEST_GROUP(StringFake)
 {
     SolidSyslogFormatterStorage storage[SOLIDSYSLOG_FORMATTER_STORAGE_SIZE(TEST_BUFFER_SIZE)];
     SolidSyslogFormatter* formatter;
+    struct SolidSyslogHeaderField field{};
 
     void setup() override
     {
         formatter = SolidSyslogFormatter_Create(storage, TEST_BUFFER_SIZE);
+        SolidSyslogHeaderField_FromFormatter(&field, formatter, TEST_BUFFER_SIZE);
         StringFake_Reset();
     }
 };
@@ -26,7 +29,7 @@ TEST_GROUP(StringFake)
 
 TEST(StringFake, ReturnsEmptyStringAfterReset)
 {
-    StringFake_GetHostname(formatter);
+    StringFake_GetHostname(&field, nullptr);
     STRCMP_EQUAL("", SolidSyslogFormatter_AsFormattedBuffer(formatter));
     LONGS_EQUAL(0, SolidSyslogFormatter_Length(formatter));
 }
@@ -34,7 +37,7 @@ TEST(StringFake, ReturnsEmptyStringAfterReset)
 TEST(StringFake, ReturnsConfiguredHostname)
 {
     StringFake_SetHostname("MyHost");
-    StringFake_GetHostname(formatter);
+    StringFake_GetHostname(&field, nullptr);
     STRCMP_EQUAL("MyHost", SolidSyslogFormatter_AsFormattedBuffer(formatter));
     LONGS_EQUAL(6, SolidSyslogFormatter_Length(formatter));
 }
diff --git a/docs/rfc-compliance.md b/docs/rfc-compliance.md
index 00eb2d94..c9c0ea78 100644
--- a/docs/rfc-compliance.md
+++ b/docs/rfc-compliance.md
@@ -23,10 +23,10 @@ Status key:
 | 6.2.5 | PROCID — max 128 chars, PRINTUSASCII | Supported | Truncated to 128. Non-PRINTUSASCII bytes substituted with `?` |
 | 6.2.6 | MSGID — max 32 chars, PRINTUSASCII | Supported | Truncated to 32. Non-PRINTUSASCII bytes substituted with `?` |
 | 6.3 | STRUCTURED-DATA — SD-ELEMENTs or NILVALUE | Supported | Extensible via `SolidSyslogStructuredData` vtable |
-| 6.3.3 | SD-PARAM value escaping (`]`, `\`, `"`) | Supported | `SolidSyslogFormatter_EscapedString` — RFC 3629 UTF-8 validated, ill-formed input substituted per-byte with U+FFFD (Unicode §3.9); `OriginSd` escapes software, swVersion, enterpriseId, and each ip via this primitive; `MetaSd` escapes language via the integrator's `SolidSyslogStringFunction` callback that wraps the same primitive. SD-NAME / SD-ID syntax validation only matters once callers can supply names — landed under [E14](https://github.com/DavidCozens/solid-syslog/issues/64) (Custom Structured Data); the standard SDs (meta / timeQuality / origin) use compile-time-constant names |
+| 6.3.3 | SD-PARAM value escaping (`]`, `\`, `"`) | Supported | `SolidSyslogFormatter_EscapedString` — RFC 3629 UTF-8 validated, ill-formed input substituted per-byte with U+FFFD (Unicode §3.9); `OriginSd` escapes software, swVersion, enterpriseId, and each ip via this primitive; `MetaSd` escapes language via the integrator's `SolidSyslogSdValueFunction` callback streaming into a `SolidSyslogSdValue`, which applies the same escaping. SD-NAME / SD-ID syntax validation only matters once callers can supply names — landed under [E14](https://github.com/DavidCozens/solid-syslog/issues/64) (Custom Structured Data); the standard SDs (meta / timeQuality / origin) use compile-time-constant names |
 | 6.3.3 | timeQuality SD — tzKnown, isSynced, syncAccuracy | Supported | `SolidSyslogTimeQualitySd` |
 | 6.3.4, 7.2 | origin SD — software, swVersion, enterpriseId, ip | Supported | `SolidSyslogOriginSd` covers all four §7.2 parameters. `software`, `swVersion`, and `enterpriseId` are static strings supplied via `SolidSyslogOriginSdConfig`; each is escaped per §6.3.3 via `SolidSyslogFormatter_EscapedString` and pre-formatted at Create time into an internal storage buffer. `ip` is repeatable per RFC 5424 §7.2 and sourced via two callbacks (`SolidSyslogOriginIpCountFunction`, `SolidSyslogOriginIpAtFunction`) so multi-homed hosts can reflect runtime address changes; the library asks for a count then loops 0..N-1 framing each `ip="…"` token (with a leading space) while the integrator's at-callback writes one escaped IP value per call (this avoids a formatter rewind primitive). All four parameters are independently optional — a NULL field or NULL callback omits the corresponding parameter from the SD-ELEMENT. Per-IP value capped at 64 chars via `_EscapedString`; no library-side cap on the IP count (bounded by `SOLIDSYSLOG_MAX_MESSAGE_SIZE`). Bare `[origin]` with no parameters is RFC-legal (§7.2 marks all params OPTIONAL, no SHOULD enforcement) and is what the library emits when the integrator wires nothing |
-| 6.3.5, 7.3 | meta SD — sequenceId, sysUpTime, language | Supported | `SolidSyslogMetaSd` covers all three IANA-registered parameters. `sequenceId` (§7.3.1) sourced via an injected `SolidSyslogAtomicCounter`. `sysUpTime` (§7.3.2 / RFC 3418 `TimeTicks`) sourced via a `SolidSyslogSysUpTimeFunction` callback returning `uint32_t` hundredths; reference platform integrations are `SolidSyslogPosixSysUpTime` (`clock_gettime(CLOCK_BOOTTIME)`) and `SolidSyslogWindowsSysUpTime` (`GetTickCount64`), with the cast to `uint32_t` providing RFC 3418's natural wrap. `language` (§7.3.3 / BCP 47) sourced via a `SolidSyslogStringFunction` callback that writes through `SolidSyslogFormatter_EscapedString` to satisfy SD-PARAM-VALUE escaping per §6.3.3. All three independently optional — a NULL field in `SolidSyslogMetaSdConfig` omits that parameter from the SD-ELEMENT |
+| 6.3.5, 7.3 | meta SD — sequenceId, sysUpTime, language | Supported | `SolidSyslogMetaSd` covers all three IANA-registered parameters. `sequenceId` (§7.3.1) sourced via an injected `SolidSyslogAtomicCounter`. `sysUpTime` (§7.3.2 / RFC 3418 `TimeTicks`) sourced via a `SolidSyslogSysUpTimeFunction` callback returning `uint32_t` hundredths; reference platform integrations are `SolidSyslogPosixSysUpTime` (`clock_gettime(CLOCK_BOOTTIME)`) and `SolidSyslogWindowsSysUpTime` (`GetTickCount64`), with the cast to `uint32_t` providing RFC 3418's natural wrap. `language` (§7.3.3 / BCP 47) sourced via a `SolidSyslogSdValueFunction` callback streaming into a `SolidSyslogSdValue`, which applies SD-PARAM-VALUE escaping per §6.3.3. All three independently optional — a NULL field in `SolidSyslogMetaSdConfig` omits that parameter from the SD-ELEMENT |
 | 6.3.5, 7.3.1 | meta SD — sequenceId wraps at 2147483647 to 1 | Supported | `SolidSyslogAtomicCounter` wraps via CAS-loop in [1, 2³¹ - 1]; never returns 0; never above max. AtomicCounter is a vtable abstraction — concrete impls are `SolidSyslogStdAtomicCounter` (C11 `<stdatomic.h>` + `atomic_compare_exchange_strong_explicit`) on POSIX/clang/gcc/modern MSVC, and `SolidSyslogWindowsAtomicCounter` (`volatile LONG` + `InterlockedCompareExchange`) on legacy MSVC. The integrator picks one at setup time by calling the relevant platform's `_Create`; CMake's `HAVE_STDATOMIC_H` / `HAVE_WINDOWS_INTERLOCKED` checks gate which platform sources are compiled. sequenceId is assigned at the point of message raise (application-layer originator), preserving end-to-end loss-detection across the internal buffer / store-and-forward / transport pipeline. Trade-off: under concurrent raise from multiple threads, a small reorder window may occur in transmitted IDs (adjacent IDs may invert, since buffer/transport scheduling between raise and wire is not under library control). All IDs remain unique and non-zero — SIEMs performing gap detection identify message loss correctly; SIEMs requiring strict monotonic ordering should sort by timestamp |
 | 6.4 | MSG — UTF-8 preferred | Supported | RFC 3629 UTF-8 validated at the formatter primitives (`SolidSyslogFormatter_BoundedString`), with ill-formed input substituted per-byte with U+FFFD (Unicode §3.9). MSG is prefixed with the §6.4 UTF-8 BOM (`%xEF.BB.BF`) unconditionally; if the caller's body already begins with a BOM it is stripped so the wire frame contains exactly one ([S12.13](https://github.com/DavidCozens/solid-syslog/issues/219)). Truncation preserves codepoint boundaries at both layers: the formatter clips at `SOLIDSYSLOG_MAX_MESSAGE_SIZE` without splitting a codepoint ([S12.10](https://github.com/DavidCozens/solid-syslog/issues/121)), and on UDP the sender walks back over any partial codepoint when the kernel reports `EMSGSIZE` for the path MTU ([S12.12](https://github.com/DavidCozens/solid-syslog/issues/210)). TCP/TLS streams fragment transparently at the transport layer and so do not need a path-MTU trim |
 | 8.1 | Message size — max 2048 recommended | Supported | Default `SOLIDSYSLOG_MAX_MESSAGE_SIZE` = 2048, matching the §8.1 SHOULD value. Per-target override via a CMake variable is planned in [E21 #217](https://github.com/DavidCozens/solid-syslog/issues/217) for memory-constrained MCUs |
diff --git a/misra_suppressions.txt b/misra_suppressions.txt
index 3a942a3a..c8b486a6 100644
--- a/misra_suppressions.txt
+++ b/misra_suppressions.txt
@@ -103,19 +103,19 @@ misra-c2012-18.7:Core/Source/SolidSyslogFormatter.c:12
 # D.006 — Rule 11.8: const-strip false-positives + platform-API (Winsock, lwIP)
 # See docs/misra-deviations.md#d006
 misra-c2012-11.8:Core/Source/BlockSequence.c:465
-misra-c2012-11.8:Core/Source/SolidSyslog.c:53
-misra-c2012-11.8:Core/Source/SolidSyslog.c:54
-misra-c2012-11.8:Core/Source/SolidSyslog.c:55
-misra-c2012-11.8:Core/Source/SolidSyslog.c:56
-misra-c2012-11.8:Core/Source/SolidSyslog.c:57
-misra-c2012-11.8:Core/Source/SolidSyslog.c:58
-misra-c2012-11.8:Core/Source/SolidSyslog.c:59
-misra-c2012-11.8:Core/Source/SolidSyslog.c:60
-misra-c2012-11.8:Core/Source/SolidSyslogMessageFormatter.c:58
+misra-c2012-11.8:Core/Source/SolidSyslog.c:65
+misra-c2012-11.8:Core/Source/SolidSyslog.c:66
+misra-c2012-11.8:Core/Source/SolidSyslog.c:67
+misra-c2012-11.8:Core/Source/SolidSyslog.c:68
+misra-c2012-11.8:Core/Source/SolidSyslog.c:69
+misra-c2012-11.8:Core/Source/SolidSyslog.c:70
+misra-c2012-11.8:Core/Source/SolidSyslog.c:71
+misra-c2012-11.8:Core/Source/SolidSyslog.c:72
 misra-c2012-11.8:Core/Source/SolidSyslogMessageFormatter.c:60
 misra-c2012-11.8:Core/Source/SolidSyslogMessageFormatter.c:62
-misra-c2012-11.8:Core/Source/SolidSyslogMessageFormatter.c:64
-misra-c2012-11.8:Core/Source/SolidSyslogMessageFormatter.c:68
+misra-c2012-11.8:Core/Source/SolidSyslogMessageFormatter.c:69
+misra-c2012-11.8:Core/Source/SolidSyslogMessageFormatter.c:76
+misra-c2012-11.8:Core/Source/SolidSyslogMessageFormatter.c:85
 misra-c2012-11.8:Core/Source/SolidSyslogBlockStoreStatic.c:46
 misra-c2012-11.8:Core/Source/SolidSyslogBlockStoreStatic.c:117
 misra-c2012-11.8:Core/Source/SolidSyslogBlockStoreStatic.c:143
@@ -171,7 +171,7 @@ misra-c2012-5.7:Core/Interface/SolidSyslogFormatter.h:14
 misra-c2012-5.7:Core/Interface/SolidSyslogSecurityPolicyDefinition.h:13
 misra-c2012-5.7:Core/Interface/SolidSyslogTimeQuality.h:12
 misra-c2012-5.7:Core/Interface/SolidSyslogTransport.h:5
-misra-c2012-5.7:Core/Source/SolidSyslogMessageFormatter.c:20
+misra-c2012-5.7:Core/Source/SolidSyslogMessageFormatter.c:21
 misra-c2012-5.7:Core/Source/SolidSyslogCircularBuffer.c:19
 misra-c2012-5.7:Core/Source/SolidSyslogCrc16.c:12
 misra-c2012-5.7:Core/Source/SolidSyslogCrc16Policy.c:10