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Smart encoding automatically replaces Unicode characters with visually similar GSM-7 characters. This keeps your messages in the more efficient GSM-7 encoding, reducing segment counts and costs.

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Why use smart encoding

SMS messages using GSM-7 encoding fit 160 characters per segment. When a message contains even one Unicode character outside GSM-7, the entire message switches to UTF-16 encoding, which only fits 70 characters per segment. A single smart quote (") or em dash (—) can double your messaging costs. Example:
MessageEncodingSegmentsCost impact
Hello, how are you? (150 chars)GSM-71Base cost
Hello, how are you? (150 chars with smart quotes)UTF-1633x cost
Hello, how are you? (150 chars, smart encoding ON)GSM-71Base cost

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How it works

When smart encoding is enabled:
  1. Your message text is scanned for Unicode characters.
  2. Characters with GSM-7 equivalents are automatically replaced.
  3. The final encoding type (GSM-7 or UTF-16) is determined after transformation.
  4. Segment count is recalculated based on the transformed message.
  5. The API response includes smart encoding metadata.

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API response metadata

When you send a message with smart encoding enabled, the API response includes metadata about the transformation: 
{
  "data": {
    "id": "...",
    "encoding": "GSM-7",
    "parts": 1,
    "smart_encoding": {
      "smart_encoding_applied": true,
      "final_encoding": "gsm7",
      "segment_count": 1,
      "character_count": 155,
      "replaced_character_count": 3,
      "length_change": 2
    }
  }
}
FieldDescription
smart_encoding_appliedWhether any characters were replaced.
final_encodingThe encoding used after transformation (gsm7 or ucs2).
segment_countNumber of segments after smart encoding.
character_countMessage length after transformation.
replaced_character_countNumber of unique characters that were substituted.
length_changeDifference in length (positive if message grew, e.g., … → ...).
The parts field in the API response reflects the segment count after smart encoding is applied, so you see the actual billing impact.

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Enable smart encoding

Enable smart encoding on your messaging profile via the API or portal. 
curl -X PATCH https://api.telnyx.com/v2/messaging_profiles/{id} \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -d '{
    "smart_encoding": true
  }'

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Character substitutions

Smart encoding replaces 200+ Unicode characters with GSM-7 equivalents. The tables below show all supported substitutions grouped by category.

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Quotation marks

UnicodeGlyphDescriptionReplacement
U+00AB«Left-pointing double angle quotation mark”
U+00BB»Right-pointing double angle quotation mark”
U+201C”Left double quotation mark”
U+201D”Right double quotation mark”
U+02BAʺModifier letter double prime”
U+02EEˮModifier letter double apostrophe”
U+201F‟Double high-reversed-9 quotation mark”
U+275D❝Heavy double turned comma quotation mark ornament”
U+275E❞Heavy double comma quotation mark ornament”
U+301D〝Reversed double prime quotation mark”
U+301E〞Double prime quotation mark”
U+FF02＂Fullwidth quotation mark”
U+201E„Double low quotation mark”

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Apostrophes and single quotes

UnicodeGlyphDescriptionReplacement
U+2018’Left single quotation mark’
U+2019’Right single quotation mark’
U+02BBʻModifier letter turned comma’
U+02C8ˈModifier letter vertical line’
U+02BCʼModifier letter apostrophe’
U+02BDʽModifier letter reversed comma’
U+02B9ʹModifier letter prime’
U+201B‛Single high-reversed-9 quotation mark’
U+FF07＇Fullwidth apostrophe’
U+00B4´Acute accent’
U+02CAˊModifier letter acute accent’
U+0060`Grave accent’
U+02CBˋModifier letter grave accent’
U+275B❛Heavy single turned comma quotation mark ornament’
U+275C❜Heavy single comma quotation mark ornament’
U+0313̓Combining comma above’
U+0314̔Combining reversed comma above’
U+FE10︐Presentation form for vertical comma’
U+FE11︑Presentation form for vertical ideographic comma’

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Dashes and hyphens

UnicodeGlyphDescriptionReplacement
U+2014—Em dash-
U+2013–En dash-
U+23BC⎼Horizontal scan line-7-
U+23BD⎽Horizontal scan line-9-
U+2015―Horizontal bar-
U+FE63﹣Small hyphen-minus-
U+FF0D－Fullwidth hyphen-minus-
U+2010‐Hyphen-
U+2022•Bullet-
U+2043⁃Hyphen bullet-

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Slashes and division

UnicodeGlyphDescriptionReplacement
U+00F7÷Division sign/
U+00BC¼Vulgar fraction one quarter1/4
U+00BD½Vulgar fraction one half1/2
U+00BE¾Vulgar fraction three quarters3/4
U+29F8⧸Big solidus/
U+0337̷Combining short solidus overlay/
U+0338̸Combining long solidus overlay/
U+2044⁄Fraction slash/
U+2215∕Division slash/
U+FF0F／Fullwidth solidus/

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Backslashes

UnicodeGlyphDescriptionReplacement
U+29F9⧹Big reverse solidus\
U+29F5⧵Reverse solidus operator\
U+20E5Combining reverse solidus overlay\
U+FE68﹨Small reverse solidus\
U+FF3C＼Fullwidth reverse solidus\

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Underscores

UnicodeGlyphDescriptionReplacement
U+0332̲Combining low line_
U+FF3F＿Fullwidth low line_
U+2017‗Double low line_

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Vertical lines

UnicodeGlyphDescriptionReplacement
U+20D2⃒Combining long vertical line overlay|
U+20D3⃓Combining short vertical line overlay|
U+2223∣Divides|
U+FF5C｜Fullwidth vertical line|
U+23B8⎸Left vertical box line|
U+23B9⎹Right vertical box line|
U+23D0⏐Vertical line extension|
U+239C⎜Left parenthesis extension|
U+239F⎟Right parenthesis extension|

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Symbols and punctuation

UnicodeGlyphDescriptionReplacement
U+FE6B﹫Small commercial at sign@
U+FF20＠Fullwidth commercial at sign@
U+FE69﹩Small dollar sign$
U+FF04＄Fullwidth dollar sign$
U+01C3ǃLatin letter retroflex click!
U+FE15︕Presentation form for vertical exclamation mark!
U+FE57﹗Small exclamation mark!
U+FF01！Fullwidth exclamation mark!
U+203C‼Double exclamation mark!!
U+FE5F﹟Small number sign#
U+FF03＃Fullwidth number sign#
U+FE6A﹪Small percent sign%
U+FF05％Fullwidth percent sign%
U+FE60﹠Small ampersand&
U+FF06＆Fullwidth ampersand&
U+2026…Horizontal ellipsis…

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Commas

UnicodeGlyphDescriptionReplacement
U+201A‚Single low-9 quotation mark,
U+0326̦Combining comma below,
U+FE50﹐Small comma,
U+3001、Ideographic comma,
U+FE51﹑Small ideographic comma,
U+FF0C，Fullwidth comma,
U+FF64､Halfwidth ideographic comma,

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Parentheses

UnicodeGlyphDescriptionReplacement
U+2768❨Medium left parenthesis ornament(
U+276A❪Medium flattened left parenthesis ornament(
U+FE59﹙Small left parenthesis(
U+FF08（Fullwidth left parenthesis(
U+27EE⟮Mathematical left flattened parenthesis(
U+2985⦅Left white parenthesis(
U+2769❩Medium right parenthesis ornament)
U+276B❫Medium flattened right parenthesis ornament)
U+FE5A﹚Small right parenthesis)
U+FF09）Fullwidth right parenthesis)
U+27EF⟯Mathematical right flattened parenthesis)
U+2986⦆Right white parenthesis)

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Brackets

UnicodeGlyphDescriptionReplacement
U+2774❴Medium left curly bracket ornament{
U+FE5B﹛Small left curly bracket{
U+FF5B｛Fullwidth left curly bracket{
U+2775❵Medium right curly bracket ornament}
U+FE5C﹜Small right curly bracket}
U+FF5D｝Fullwidth right curly bracket}
U+FF3B［Fullwidth left square bracket[
U+FF3D］Fullwidth right square bracket]

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Asterisks

UnicodeGlyphDescriptionReplacement
U+204E⁎Low asterisk*
U+2217∗Asterisk operator*
U+229B⊛Circled asterisk operator*
U+2722✢Four teardrop-spoked asterisk*
U+2723✣Four balloon-spoked asterisk*
U+2724✤Heavy four balloon-spoked asterisk*
U+2725✥Four club-spoked asterisk*
U+2731✱Heavy asterisk*
U+2732✲Open center asterisk*
U+2733✳Eight spoked asterisk*
U+273A✺Sixteen pointed asterisk*
U+273B✻Teardrop-spoked asterisk*
U+273C✼Open center teardrop-spoked asterisk*
U+273D✽Heavy teardrop-spoked asterisk*
U+2743❃Heavy teardrop-spoked pinwheel asterisk*
U+2749❉Balloon-spoked asterisk*
U+274A❊Eight teardrop-spoked propeller asterisk*
U+274B❋Heavy eight teardrop-spoked propeller asterisk*
U+29C6⧆Squared asterisk*
U+FE61﹡Small asterisk*
U+FF0A＊Fullwidth asterisk*

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Math and comparison

UnicodeGlyphDescriptionReplacement
U+02D6˖Modifier letter plus sign+
U+FE62﹢Small plus sign+
U+FF0B＋Fullwidth plus sign+
U+FE64﹤Small less-than sign<
U+FF1C＜Fullwidth less-than sign<
U+0347͇Combining equals sign below=
U+A78A꞊Modifier letter short equals sign=
U+FE66﹦Small equals sign=
U+FF1D＝Fullwidth equals sign=
U+FE65﹥Small greater-than sign>
U+FF1E＞Fullwidth greater-than sign>
U+2039‹Single left-pointing angle quotation mark>
U+203A›Single right-pointing angle quotation mark<

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Periods and colons

UnicodeGlyphDescriptionReplacement
U+3002。Ideographic full stop.
U+FE52﹒Small full stop.
U+FF0E．Fullwidth full stop.
U+FF61｡Halfwidth ideographic full stop.
U+02D0ːModifier letter triangular colon:
U+02F8˸Modifier letter raised colon:
U+2982⦂Z notation type colon:
U+A789꞉Modifier letter colon:
U+FE13︓Presentation form for vertical colon:
U+FF1A：Fullwidth colon:
U+204F⁏Reversed semicolon;
U+FE14︔Presentation form for vertical semicolon;
U+FE54﹔Small semicolon;
U+FF1B；Fullwidth semicolon;
U+FE16︖Presentation form for vertical question mark?
U+FE56﹖Small question mark?
U+FF1F？Fullwidth question mark?

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Fullwidth digits

UnicodeGlyphDescriptionReplacement
U+FF10０Fullwidth digit zero0
U+FF11１Fullwidth digit one1
U+FF12２Fullwidth digit two2
U+FF13３Fullwidth digit three3
U+FF14４Fullwidth digit four4
U+FF15５Fullwidth digit five5
U+FF16６Fullwidth digit six6
U+FF17７Fullwidth digit seven7
U+FF18８Fullwidth digit eight8
U+FF19９Fullwidth digit nine9

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Fullwidth and small capital letters

Fullwidth uppercase (U+FF21–U+FF3A):
UnicodeGlyphReplacement
U+FF21–U+FF3AＡ–ＺA–Z
Fullwidth lowercase (U+FF41–U+FF5A):
UnicodeGlyphReplacement
U+FF41–U+FF5Aａ–ｚa–z
Small capital letters:
UnicodeGlyphDescriptionReplacement
U+1D00ᴀLatin letter small capital AA
U+0299ʙLatin letter small capital BB
U+1D04ᴄLatin letter small capital CC
U+1D05ᴅLatin letter small capital DD
U+1D07ᴇLatin letter small capital EE
U+A730ꜰLatin letter small capital FF
U+0262ɢLatin letter small capital GG
U+029CʜLatin letter small capital HH
U+026AɪLatin letter small capital II
U+1D0AᴊLatin letter small capital JJ
U+1D0BᴋLatin letter small capital KK
U+029FʟLatin letter small capital LL
U+1D0DᴍLatin letter small capital MM
U+0274ɴLatin letter small capital NN
U+1D0FᴏLatin letter small capital OO
U+1D18ᴘLatin letter small capital PP
U+0280ʀLatin letter small capital RR
U+A731ꜱLatin letter small capital SS
U+1D1BᴛLatin letter small capital TT
U+1D1CᴜLatin letter small capital UU
U+1D20ᴠLatin letter small capital VV
U+1D21ᴡLatin letter small capital WW
U+028FʏLatin letter small capital YY
U+1D22ᴢLatin letter small capital ZZ

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Greek letters

Greek capital letters that visually resemble Latin letters are substituted:
UnicodeGlyphDescriptionReplacement
U+0391ΑGreek capital letter AlphaA
U+0392ΒGreek capital letter BetaB
U+0395ΕGreek capital letter EpsilonE
U+0397ΗGreek capital letter EtaH
U+0399ΙGreek capital letter IotaI
U+039AΚGreek capital letter KappaK
U+039CΜGreek capital letter MuM
U+039DΝGreek capital letter NuN
U+039FΟGreek capital letter OmicronO
U+03A1ΡGreek capital letter RhoP
U+03A4ΤGreek capital letter TauT
U+03A7ΧGreek capital letter ChiX
U+03A5ΥGreek capital letter UpsilonY
U+0396ΖGreek capital letter ZetaZ

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Special language support

UnicodeGlyphDescriptionReplacement
U+00C7ÇLatin capital letter C with cedillaÇ (GSM-7 native)

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Tildes and circumflex

UnicodeGlyphDescriptionReplacement
U+02C6ˆModifier letter circumflex accent^
U+0302̂Combining circumflex accent^
U+FF3E＾Fullwidth circumflex accent^
U+1DCD᷍Combining double circumflex above^
U+02DC˜Small tilde~
U+02F7˷Modifier letter low tilde~
U+0303̃Combining tilde~
U+0330̰Combining tilde below~
U+0334̴Combining tilde overlay~
U+223C∼Tilde operator~
U+FF5E～Fullwidth tilde~

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Whitespace characters

These characters are replaced with a standard space or removed:
UnicodeDescriptionReplacement
U+00A0No-break space(space)
U+2000En quad(space)
U+2001Em quad(space)
U+2002En space(space)
U+2003Em space(space)
U+2004Three-per-em space(space)
U+2005Four-per-em space(space)
U+2006Six-per-em space(space)
U+2007Figure space(space)
U+2008Punctuation space(space)
U+2009Thin space(space)
U+200AHair space(space)
U+200BZero width space(removed)
U+202FNarrow no-break space(space)
U+205FMedium mathematical space(space)
U+3000Ideographic space(space)
U+FEFFZero width no-break space(removed)
U+2028Line separator(removed)
U+2029Paragraph separator(removed)
U+2060Word joiner(removed)

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Control characters

These control characters are removed or transformed:
UnicodeDescriptionReplacement
U+0009Tab7 spaces
U+0000Null(removed)
U+0003End of text(removed)
U+0004End of transmission(removed)
U+0010Escape(removed)
U+0011Device control one(removed)
U+0012Device control two(removed)
U+0013Device control three(removed)
U+0014Device control four(removed)
U+0017End of transmission block(removed)
U+0019End of medium(removed)
U+0080C1 control codes(removed)
U+008DReverse line feed(removed)
U+0090Device control string(removed)
U+009BControl sequence introducer(removed)
U+009FApplication program command(removed)
Tab characters (U+0009) are converted to 7 spaces, which can significantly increase message length and affect segment count.

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Edge cases

Smart encoding handles several edge cases:

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Message length increases

Some substitutions increase message length. For example:
  • Horizontal ellipsis (…) becomes three periods (...) — adds 2 characters.
  • Tab (U+0009) becomes 7 spaces — adds 6 characters.
  • Vulgar fractions like ½ become 1/2 — adds 2 characters.
The segment count is calculated after these replacements, so a message near the 160-character limit may become multi-part after transformation.

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Mixed replaceable and non-replaceable characters

If your message contains both replaceable Unicode characters and non-replaceable ones (like emojis), the replaceable characters are still substituted. However, the non-replaceable characters will still cause UTF-16 encoding.

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Extended GSM-7 characters

The characters ~^|\{}[] are part of the GSM-7 extended set and count as 2 characters each when calculating segment length. Smart encoding accounts for this when determining final segment count.

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Zero-width characters

Zero-width characters (like U+200B zero-width space) are removed entirely from the message.

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Empty message after transformation

If your message consists entirely of zero-width or control characters that get removed, the API will return an error. Messages cannot be empty after smart encoding transformation.

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Limitations

  • Smart encoding applies to SMS only. MMS and RCS use UTF-8 encoding by default.
  • Not all Unicode characters have GSM-7 equivalents. Emojis and non-Latin scripts will still trigger UTF-16 encoding.
  • Substitutions may slightly alter the appearance of your message. Review the character tables above to understand what changes will occur.

Message encoding

Learn about GSM-7, UTF-16, and segment calculations.

Messaging Profiles API

API reference for updating messaging profiles.